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Chemical reactivity of cation-exchanged zeolites Citation for published version (APA): Pidko, E. A. (2008). Chemical reactivity of cation-exchanged zeolites. Eindhoven: Technische Universiteit Eindhoven. https://doi.org/10.6100/IR632877 DOI: 10.6100/IR632877 Document status and date: Published: 01/01/2008 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 21. May. 2020

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Page 1: Chemical reactivity of cation-exchanged zeolitesCHEMICAL REACTIVITY OF CATION‐EXCHANGED ZEOLITES PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit

Chemical reactivity of cation-exchanged zeolites

Citation for published version (APA):Pidko, E. A. (2008). Chemical reactivity of cation-exchanged zeolites. Eindhoven: Technische UniversiteitEindhoven. https://doi.org/10.6100/IR632877

DOI:10.6100/IR632877

Document status and date:Published: 01/01/2008

Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)

Please check the document version of this publication:

• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publication

General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.

If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverne

Take down policyIf you believe that this document breaches copyright please contact us at:[email protected] details and we will investigate your claim.

Download date: 21. May. 2020

Page 2: Chemical reactivity of cation-exchanged zeolitesCHEMICAL REACTIVITY OF CATION‐EXCHANGED ZEOLITES PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit
Page 3: Chemical reactivity of cation-exchanged zeolitesCHEMICAL REACTIVITY OF CATION‐EXCHANGED ZEOLITES PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit

CHEMICAL REACTIVITY OF CATION‐EXCHANGED ZEOLITES 

PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op donderdag 13 maart 2008 om 16.00 uur door Evgeny Alexandrovich Pidko geboren te Moskou, Rusland

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Dit proefschrift is goedgekeurd door de promotoren:

prof.dr. R.A. van Santen en prof.dr. V.B. Kazansky

Copromotor: dr.ir. E.J.M. Hensen

Evgeny A. Pidko

A catalogue record is available from the Eindhoven University of Technology Library

ISBN: 978-90-386-1210-2

Copyright © 2008 by Evgeny A. Pidko

The work described in this thesis has been carried out at the Schuit Institute of Catalysis within the Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, The Netherlands. This research was partially supported by the National Computing Facilities Foundation (NCF), which provided computational facilities, with financial support from The Netherlands Organization for Scientific Research (NWO).

Cover design: Tom Bongers (Creanza Media Eindhoven B.V.) and Evgeny A. Pidko

Printed at the Univesiteitsdrukkerij, Eindhoven University of Technology

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V  

CONTENTS

CHEMICAL REACTIVITY OF CATION‐EXCHANGED ZEOLITES 

Chapter 1 Introduction ............................................................................................... 1 

Chapter 2 Confined space-controlled olefin–oxygen charge transfer in zeolites ... 15

Chapter 3 Molecular recognition of N2O4 on alkali-exchanged low-silica zeolites X .............................................................................................. 25

Chapter 4 The interplay of bonding and geometry of the adsorption complexes of light alkanes within cationic faujasites .................................................. 43

Chapter 5 Molecular and dissociative adsorption of ethane on zinc and cadmium ions in ZSM-5 zeolite ............................................................................. 61

Chapter 6 Catalytic dehydrogenation of light alkanes over zinc cations in Zn/ZSM-5 zeolite ................................................................................... 79

Chapter 7 Ethane dehydrogenation over reduced extra-framework gallium cations in ZSM-5 zeolite .................................................................................... 95

Chapter 8 Dehydrogenation of light alkanes over isolated gallyl ions in ZSM-5 zeolite .................................................................................. 113

Chapter 9 Multinuclear gallium-oxo cations in high-silica zeolites ..................... 127

Summary ........................................................................................................................ 143

Samenvatting ................................................................................................................. 147

Резюме ............................................................................................................................ 153

Acknowledgements ...................................................................................................... 159

List of publications ....................................................................................................... 161

Curriculum Vitae ........................................................................................................... 163

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VI  

 

"H-how can you look for a solution, where it d-does not exist? It's s-some sort of n-nonsense.”

"Excuse me, Feodor, but it's you who are reasoning strangely. It's nonsense to look for a solution if it already exists.”

Arkadi and Boris Strugatsky “Monday begins on Saturday”

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CHAPTER 1  

INTRODUCTION  

 

 

1.1. Zeolites

eolites appeared on the scientific stage in the middle of 18 century, when the Swedish scientist Axel Frederik Cronstedt reported on an interesting behavior of a new mineral. Upon rapid heating the material came to life. Its particles began to “dance” and gas bubbles were released. Combining Greek words zein, "to boil",

and lithos, "a stone", he called the mineral zeolite [1]. The researcher could not foresee that what he discovered and named would later be so widely applied to many important processes such as gas separation, softening of water, catalysis in petroleum and fine chemistry.

Zeolites are crystalline microporous alumino-silicate solids containing cavities and channels of a molecular size with an overall composition similar to that of quartz or sand (SiO2) but with some of the silicon atoms in the framework replaced by aluminum. The resulting negative charge on the framework is compensated by an exchangeable cation. These positive extra-framework ions are rather loosely held and can readily be exchanged for others in a contact solution or via other chemical treatments.

The zeolite framework is built of silicon- or aluminum-occupied oxygen tetrahedra. The central atom of these tetrahedra is generally called a T-atom. The arrangement of the T-atoms follows the Löwenstein rule [2] that states that only Si-O-Si and Si-O-Al linkage are allowed, while Al-O-Al moieties cannot occur. It follows that the molar silicon to aluminum ratio (Si/Al) in the resulting solid must be larger than or equal to one.

Topologically, the zeolite framework can be considered as four-connected nets, where each vertex (T-site) is connected to its four closest neighbors via oxygen bridges (Figure 1.1). One can imagine an unlimited number of possible zeolitic structures. However, only a limited fraction of these are of interest, because they exhibit important properties, with even smaller portion being amenable to synthesis. Currently more than 150 zeolite types have been synthesized and 48 naturally occurring zeolites are known. The latter are rarely pure and are contaminated to varying degrees by other minerals, metals, quartz or other

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zeolites. For this reason, natural zeolites are excluded from many important commercial applications, while synthetic zeolites have found large scale applications as ion-exchangers and catalysts.

Zeolites and related materials are widely applied in different technological fields. They act as efficient heterogeneous catalysts, mainly as solid acids, as adsorbents, and as molecular sieves in gas separation and purification. Such a broad spectrum of applications results from the possibility to rather easily tune the chemical and physical properties of the material by choosing the particular zeolite structure (topology) with required type and size of cavities and/or channels, and by introducing extra-framework species into the zeolite matrix. Depending on the type of species introduced, the physicochemical properties and the reactivity of the resulting material can vary drastically. Therefore, a deep understanding of their role in different chemical process is crucial for the rationalization of the behaviors of these materials, which can form the basis for the design of improved and novel systems.

1.2. Extra-framework species in zeolites

The chemical reactivity of zeolites is usually associated with the existence of the exchangeable cations. If the compensation of the framework charge due to the isomorphous substitution of silicon with aluminum is provided by a proton, these protons sit on bridging oxygen atoms connecting framework silicons and aluminums. The resulting bridging hydroxyl groups exhibit pronounced Brønsted acidic properties. The strength of the thus formed solid acid depends on various factors such as the zeolite topology, the Si/Al ratio, the existence of additional extra-framework species, etc, and can be in some cases even as high as that of classic superacids [3-5]. The form of zeolites, in which the

Figure 1.1. Possible zeolite structures formed from SiO4 or AlO4 tetrahedra. 

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negative framework charge is compensated by protons, is usually called the “hydrogen form”. The existence of the acid sites of a variable strength in the solid governs the field of applications of the hydrogen forms of zeolites that is acid catalysis [4,5].

Exchanging the Brønsted acid protons by metal cations, one can further influence chemical properties of zeolites. Introduction of metal ions such as zinc, cadmium, or gallium ions creates new Lewis acid sites within the zeolite host and opens a wide field of applications of these microporous materials in the reactions catalyzied by Lewis acids [4-7]. On the other hand the incorporation of cations of variable valence such as Cu+, Co2+, Fe2+, Mo6+, or V5+ enhances the activity of zeolites in redox reactions [4]. Stabilization of rather inert alkaline and alkali-earth cations in zeolite results in an enhancement of the basic properties of lattice oxygens [8]. In addition the charged species in the zeolite usually produce a rather strong electrostatic field that, in principle, can polarize molecules confined in the microporous matrix resulting in their activation [9,10].

The chemical properties of the extra-framework cations in zeolite depend strongly on the structure and topology of the zeolitic site accommodating the exchangeable species, as well as on the nature and structure of the cation itself. The zeolite framework can be viewed as a ligand that stabilizes the exchangeable cations, thereby influencing their chemical properties [11].

Univalent cations usually sit in zeolitic rings and coordinate to 2-3 lattice oxygen atoms

Figure 1.2. Compensation of the negative framework charge in cation sites of the zeolite with 5 

or 6 T‐atoms: a bivalent M2+ cation in (a) a five‐ring and (b) a six‐ring; (c) a univalent (M2+‐OH‐)+ 

ion  stabilized  in a  five‐ring;  (d) a cationic  (M2+‐O2‐‐M2+)2+ complex  stabilized by  two negative 

framework charges, and a less conventional alternative structure (e), where the charge in two 

adjacent five‐rings is compensated in a charge‐alternating manner by a bivalent M2+.  

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attached to [AlO2]– framework units. Such structures are easily realized within zeolites independently of their Si/Al ratio, because one cation compensates for the charge of a single aluminum-occupied oxygen tetrahedron. In contrast, stabilization of multivalent cations in zeolites requires the existence of two or more closely located framework anionic sites. Thus, dependent on the Si/Al ratio and, hence, on the distance between framework Al ions, charge-compensation may be realized in various manners. For example, bivalent cations can be stabilized in the vicinity of two closely located [AlO2]– units (Figure 1.2 (a) and (b)). Such sites are usually named conventional ion-exchangeable sites. This type of charge-compensation is generally accepted and is typical for low-silica zeolites, where almost all cation sites contain two or more aluminums. When direct charge-compensation is more difficult, as for instance in high-silica zeolites, alternatively multinuclear cationic complexes such as oxo- or hydroxo-substituted species (Mg-OH+ [12], Zn-O-Zn2+ [13], GaO+ [14], etc) can be formed (Figure 1.2 (c), (d)). In addition, it has been proposed recently [15] that bivalent mononuclear cations can, in principle, compensate for the charge of two distant [AlO2]– framework units (Figure 1.2 (e)). According to this model a bivalent cation is located in the vicinity of one aluminum-occupied oxygen tetrahedron with additional coordination to neighboring basic oxygens of neutral [SiO2] units, whereas the charge of the distantly placed anionic site is compensated indirectly. Depending on the type of charge-compensation, the chemical properties and catalytic reactivity of the extra-framework cationic species, and hence, of the cation-exchanged zeolite can vary substantially. However, due to a complex nature and composition, as well as inhomogeneity of most of the zeolite-based catalysts, the structure and properties of the exchangeable intrazeolite species are often unknown.

In summary, extra-framework cations modify the chemical and catalytic properties of microporous zeolites profoundly. Compared to our understanding of Brønsted acidity in zeolites and the resulting catalytic reactivity, our knowledge of the structure and reactivity of the modifying metal-containing cationic species is much more limited. Atomistic simulations of the stability of extra-framework cations in zeolites, their complexes, and their chemical reactivity are therefore instrumental to understand their catalytic properties and to improve the performance of the corresponding catalysts.

1.3. Structural features of zeolites

Besides extra-framework species, the reactivity of zeolites is also dependent on the topological arrangement of the structure forming tetrahedra. It is well known that the selectivity of catalytic reactions over acidic zeolites often depends strongly on the size and shape of the zeolite channels. This is called shape selectivity [16]. Usually three types of shape selectivity are distinguished. The first two types, i.e. reactant and product shape selectivities, are based on the principle of molecular exclusion or molecular sieving. Selectivity in the former case is achieved by selecting a zeolite structure that can differentiate molecules of different size. Only those molecules, which dimensions are smaller than the pore entrances, can reach the internal active sites and be converted to the

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Figure  1.4.  Structure  of mordenite.  The 

highlighted  region  is  the  low‐symmetry 

smaller  unit  cell  used  in  DFT 

calculations.  

 

Figure  1.3.  Structure  of  ZSM‐5  zeolite.  The 

highlighted  part  of  the  zeolite  shows  the  cluster 

model  used  in DFT  calculations. Only  the  silicon‐

aluminum framework is shown, while oxygens are 

omitted for clarity. 

desired products, whereas the larger molecules remain intact. Product selectivity, on the other hand, occurs when only certain product molecules formed in the micropore space can leave the zeolite crystals through the pores. The product molecules that remain in the zeolite undergo secondary reactions and subsequently desorb or slow down the reaction and in some cases even deactivate the catalyst. The third type of shape selectivity is transition-state selectivity that refers to steric constraints imposed by the zeolite structure on the transition state for the formation of certain product molecules. Although reactant and product molecules may freely diffuse into and out of the zeolite, the effective diameter of the channel system may hinder the formation of the bulky transition states or reaction intermediates, resulting in selective formation of only a fraction of possible reaction products. It should be noted that in all these cases the reaction selectivity is controlled only by the zeolite topology, while the nature of the active sites as well as their intrazeolite arrangement is assumed not to affect the selectivity of the catalytic process.

In the present work four types of zeolites of three different framework types are considered: ZSM-5 (MFI framework, Figure 1.3), mordenite (MOR framework, Figure 1.4) and zeolites X and Y (faujasite (FAU) framework, Figure 1.5). This selection is based on the fact that the above microporous materials are among the most widely used zeolites and are highly important for various chemical processes. In addition a vast majority of available experimental studies, needed to support the computational modeling, is devoted to the investigation of properties of these zeolite types.

The ZSM (Zeolite Socony Mobil) family zeolites have no natural analogues and have been synthesized for the first time by the Mobil Oil company in the 1970s [17]. Different types of ZSM zeolites (ZSM -4, -5, -8, -11, -12, -22, etc) can be prepared by varying the conditions of zeolite synthesis (temperature, ratio of components of the reaction mixture, type of template, etc). One of the most widely used zeolites is ZSM-5. Its unit cell is orthorhombic (Pnma, a = 20.07 Å, b = 19.92 Å, c = 13.42 Å) and has an ideal formula in the sodium form of NanAlnSi96-nO192·16H2O [18]. The atomic ratio of silicon to aluminum

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Figure  1.5.  Structure  of  faujasite.  Left  part  shows  the  periodic 

structure (view along [110] plane). The light‐gray boxes represent 

the  cubic  Fd⎯3m  unit  cells;  the  darker  boxes  show  a  smaller 

rhombohedral cell used in periodic DFT calculations. 

(Si/Al ratio) varies from 12 to ∞. Formation of the crystal lattice of ZSM-5 results in an intracrystalline system of intersecting channels, which have ten-membered ring openings. One channel system runs parallel to the a axis (view along [100], Figure 1.3) of the unit cell. It is sinusoidal and has elliptical openings of 5.1x5.5 Å. The other channels are straight, parallel to the b axis (view along [010], Figure 1.3) and have openings of 5.3x5.6 Å. The channel intersections have a critical dimension of nearly 9 Å.

Mordenite is a mineral zeolite, which has an ideal formula in the sodium form of Na8Al8Si40O96·16H2O (Si/Al=5). The crystal structure of mordenite has been reported for the first time by Meier in 1961 [19]. The unit cell of mordenite is orthorhombic (Cmcm, a = 18.3 Å, b = 20.5 Å and c = 7.5 Å) with a unidimensional system of channels (Figure 1.4). The principal sorption channels are formed by twelve-membered rings of [TO2] tetrahedra. The passageways are elliptical in shape and have openings of 6.5x7.0 Å.

Synthetic zeolites Y and X are the analogous of the naturally occurring mineral faujasite. The crystal structure has been reported for the first time by Bergerhoff et al in 1958 [20]. The unit cell is cubic (Fd⎯3m, a = b = c = 24.7 Å) [21]. An ideal formula of faujasite is [Ca,Mg,Na2]29Al58Si134O384·16H2O. Zeolite Y has essentially the same framework structure as zeolite X but differs in its Si/Al ratio, which varies from 2.2 to 3.0, as in natural faujasite, while this ratio in zeolite X is 1.1–1.5. The faujasite framework (Figure 1.5) may be described as a diamond array of cuboctahedral aluminosilicate units. This structural unit is usually referred to as the “sodalite unit” (cage), because of its occurrence in the mineral sodalite. Each sodalite unit is connected at its 6-memebred rings to four other sodalite units via distorted hexagonal prisms. This arrangement of structural units results in the formation of large absorption cages called “supercages” with diameter of about 13.7 Å. The faujasite unit cell consists of eight sodalite cages, and 16 hexagonal prisms. The supercages meet at windows of approximately 7.4 Å in diameter formed by 12-membered rings shared between them. One notes that the active sites located within the sodalite cages

or hexagonal prisms are often inaccessible for the molecules adsorbed to the zeolite, because of the small size of the windows, while only those located within the channel system formed by the supercages are accessible and can directly influence the reactivity of the microporous material.

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1.4. Computational methods in zeolite science

Computational methods are now widely and extensively used in the chemical, physical, biomedical and engineering sciences in assisting the interpretation of experimental data and increasingly in predicting of the behavior of matter at the atomic level. They have a long and successful history of application in solid state and material science for modeling structural and dynamic properties of the bulk and surfaces of solids, and for understanding chemical reactivity, in which they are playing an increasingly important role. Their application to zeolite sciences developed strongly in the 1980s, with initial successes in modeling structure and sorption, and with an emerging capability in quantum mechanical methods.

Atomistic simulation methods can be divided into two very broad categories. The first is based on the use of interatomic potentials (force fields). These methods are usually empirical and no attempt to solve the Schrödinger equation is made. Instead the system of interest is described with mathematical functions, which express the energy as a function of nuclear coordinates. These may then be used to calculate structures and energies by means of minimization methods, to calculate ensemble averages using Monte Carlo simulations, or to model dynamical processes such as molecular diffusion via molecular dynamics simulations. The second category does not use empirical potentials to describe interactions between atoms but rather computes the electronic structure. On the basis of the quantum-mechanical description of the electrons of the chemical system, the solution to the multi-electron Schrödinger equation is approximated. Such methods are essential for the description of the elementary steps underlying catalysis, that is the making and breaking of chemical bonds. Hartree Fock (HF), Density Functional Theory (DFT), and post-Hartree Fock ab initio approaches have been commonly used in modeling zeolites, although DFT methods have been dominating the literature in the past decade.

This thesis is mainly aimed at the investigation of the chemical bonding and reactivity of the exchangeable cations or other cationic complexes encapsulated into the microporous zeolite matrix. These properties are directly related to the properties of the electrons of the chemical system, and moreover, can be described only when the electronic structure is taken into account. Here quantum mechanical methods play the pivotal role to calculate the electronic structure from which the most energetically stable geometry and the transition state for a particular reaction as well as their properties can be calculated. Such understanding is essential to interpret spectroscopic and catalytic data.

The goal of quantum-chemical methods is to predict the structure, energy and properties of a system containing many particles. In order to calculate the electronic states of the system, quantum chemical methods attempt to solve Schrödinger equation, Ĥ Ψ   E Ψ, where Ψ is the wave function and E is the energy of the N-particle (electrons and nuclei) system. Ĥ is the Hamiltonian operator, which is comprised of the kinetic and potential energy operators acting on the overall wave function of the system. The exact solution for this equation can be found only for a very limited number of systems, and thus, a number

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of approximations are required to solve it for larger systems. Below we present a simple overview of the most important approximations and of the most important limitations of the methods when applied to zeolite chemistry. More detail and in-depth discussion on the electronic structure calculations can be found in a number of very good references [22].

There are two important approximations to solve the Schrödinger equation for a multi-body system. The first is the Born-Oppenheimer approximation [23], which assumes uncoupling the electron motion from the nuclear motion since the mass of the latter species is much greater than the electron mass. The electronic wave function can then be solved separately from the nuclear one for a fixed set of nuclear positions. The second approximation is usually done in order to take into account repulsion between electrons. Since the electron-electron interaction cannot be directly solved, it has been common practice to consider each electron moving in a field of the other electrons of the system. The solution then requires convergence of the electronic structure via an iterative scheme. This is known as the self-consistent field approximation [22].  

Electronic structure methods can be categorized as ab initio wave functions-based, density functional, and semiempirical methods. All of them can be applied to obtain insight into various issues concerning zeolite chemistry. Wave function methods start with the Hartree-Fock (HF) solution and use well-prescribed methods that can be used to increase its accuracy. One of the deficiencies of the HF theory is that it does not treat dynamic electron correlation, which refers to the fact that the motion of electrons is correlated so as to avoid one another. The neglect of this effect can cause very serious errors in the calculated energies, geometries, vibrational, and other properties.

There are numerous so-called post-Hartree-Fock methods for treating correlated motion between electrons. One of the most widely used approaches is based on the definition of the correlation energy as a perturbation. In other words, the configurational interactions are treated as small perturbations to the Hamiltonian. Using this expansion the HF energy is equal to the sum of the zero and first order terms, whereas the correlation energy appears only as a second order term. The second order Møller-Plesset perturbation theory (MP2) typically recovers 80-90% of the correlation energy, while MP4 provides a reliably accurate solution to most system. Due to the extremely high computational costs, the application of the post-HF methods in zeolite science is mainly limited to the MP2 method.

A more attractive method is density functional theory (DFT). DFT is “ab initio” in the sense that it is derived from the first principles and does not usually require adjustable parameters. These methods formally scale with increase in the number of basis functions (electrons) as N3 and thus permit more realistic models compared to the higher-level post-HF methods, which usually scale as N5 for MP2 and up to N7 for such methods as MP4 and CCSD(T). On the other hand the theoretical accuracy of DFT is not as high as the higher level ab initio wave function methods.

The application of DFT is attributed to the work of Hohenberg and Kohn [24], who formally proved that the ground-state energy for a system is a unique functional of its

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electron density. Kohn and Sham [25] extended the theory to practical applications by showing how the energy can be portioned into kinetic energy of the motion of the electrons, potential energy of the nuclear-electron attraction, electron-electron repulsion, which involves with Coulomb as well as self interactions, and exchange correlation that covers all other electron-electron interactions. The energy of an N-particle system can then be written as

   (1.1) 

Kohn and Sham demonstrated that the N-particle system can be written as a set of n-electron problems (similar to the molecular orbitals in wave function methods) that could be solved self-consistently in a manner that is similar to the SCF [25].

Although DFT is in principle an exact approach, a number of assumptions and approximations have to be made usually due to the fact that the exact expression for the exchange correlation energy is not known. The most basic one is the local density approximation (LDA), which assumes that the exchange-correlation per electron is equivalent to that in a homogeneous electron gas, which has the same electron density at a specific point r. The LDA is obviously an oversimplification of the actual density distribution and usually leads to overestimation of calculated bond and binding energies. The non-local gradient corrections to LDA functional improve the description of electron density. In this case the correlation and exchange energies are functionals of both the density and its gradient. The gradient corrections take on various different functionals such as P86 [26], B88 [27], PW91 [28], PBE [29], etc. However, the accuracy of these is typically less than what can be expected from high level ab initio methods.

One notes that the Hartree-Fock (HF) theory provides a more exact match of the exchange energy for single determinant systems. Thus, numerous hybrid functionals have been recently developed where the exchange functional is a linear combination of the HF exchange and the correlation (and exchange) calculated from LDA theory. The geometry and energetics calculated within this approach (B3LYP and B3PW91 [30], MPW1PW91 [31], PBE0 [32], etc) are usually in a good agreement with experimental results and those obtained by using post-HF methods. On the other hand, hybrid functionals still fail in describing of chemical effects mainly associated with the electron-electron correlation such as dispersion and other weak interactions [33,34].

As it was mentioned above, the energy in the DFT methods is formally a function of the electron density. However, in practice the density of the system ρ(r) is written as a sum of squares of the Kohn-Sham orbitals:

∑| |     (1.2) 

This leads to another approximation in both DFT and wave function-based methods that consists of the representation of each molecular orbital by a specific orthonormal basis set. The true electron structure of the model can in principle be mathematically represented by

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an infinite number of basis functions. However, due to computational limitations, in practice these functions are truncated and described by a finite number of basis sets resulting in some loss in accuracy. A wide range of different basis sets currently exists and the choice for a certain one strongly depends on the solution method used, the type of the problem considered, and the accuracy required. These functions can take on one of several forms, including Slater-type functions, Gaussian functions, and plane waves. The basis sets build using these functions can be either full-electron basis sets, describing both core and valent electrons of the atoms, or so-called effective core potential (ECP) ones. In the latter case it is assumed that the core electrons do not influence significantly the electronic structure and the properties of atoms, and therefore, are being replaced with an approximate pseudopotential. Such a simplification is useful for the description of heavy atoms, because it decreases the number of basis functions, and correspondingly, the computational time without dramatic loss of accuracy.

The approximations done in order to solve Schrödinger equation by different quantum chemical methods as well as the use of finite basis set for the description of molecular orbitals are not the only factors leading to limited accuracy. When modeling zeolites, one can seldom take into account all of the atoms of the system. Typically, a limited subset of the atoms of the zeolite is used to construct an atomistic model. The size of the model used to describe reaction environment can be critical for obtaining of reliable results. A minimum requirement to the zeolite model is that it involves the reactive site or the adsorption site together with its environment, which gives rise to a so-called cluster approach. Here only a part of the zeolite containing finite number of atoms is considered, while the influence of the rest of the atoms of the zeolite lattice is neglected. Although this approach results in some los of “model” accuracy, it can be very useful for the analysis of different local properties of zeolites such as elementary reaction steps, adsorption, etc. The current progress in computational chemistry also made it possible to use rather efficiently periodic boundary conditions in DFT calculations of solids. This allows theoretical DFT studies of structure and properties of some zeolites with relatively small unit cells using an experimental crystal structure of the zeolite as a model.

In this thesis various physicochemical properties of very different zeolites are investigated by means of quantum-chemical modeling. Thus, the actual choice of the zeolite model and the theoretical method was mainly conditioned by a reasonable compromise between the accuracy needed and the computational time required to calculate particular molecular properties. In addition, the availability of theoretical methods (time-dependent DFT, post-Hartree-Fock methods, etc.) in a combination with the model limitations caused by the available quantum-chemical programs could influence the particular choice of the computational methodology. The reliability of the results obtained by a chosen method was usually tested by recalculation of the selected results at a higher level of theory and/or by comparison with the respective available experimental data.

The choice between the periodic and the cluster modeling approach in this work was

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mainly conditioned by the expected locality of the active site. For example, in the extreme case of the low-silica zeolite X modified with alkaline cations, most of the exchangeable cations of the faujasite supercage can be involved in the interaction with the adsorbed molecules. Therefore, in this case the cluster modeling approach would require a very large cluster model with the number of atoms even exceeding that in the rhombohedral periodic unit cell of faujasite. Thus, the periodic unit cell as a zeolite model becomes the natural choice. On the other hand, in the case of cation-exchanged high-silica zeolites the active site is usually expected to be rather local. Therefore, taking into account the small size of the reagents considered in this work, a reliable description of the active site together with the reaction environment could be done using a relatively small cluster model. Such a choice dramatically reduces the required computational time without a significant loss in the model accuracy, as compared to the case of the periodic model. This allows of a detailed and comprehensive analysis of various reaction paths possible over the active site of high-silica zeolite. At the same time, the application of a rather small cluster model makes it possible to use the higher level theoretical methods to improve the method accuracy.

1.5. Scope of the thesis

This thesis deals with theoretical investigations of various aspects of chemical reactivity of cation-exchanged zeolites. Despite a wealth of literature in the field of zeolites modified with metal ions, there is still a lack of clear understanding of the role of the extra-framework species in catalytic reactions. The main goal of this thesis is to develop a deeper understanding of the structural and chemical properties of extra-framework cationic species in zeolites as well as of the mechanisms of chemical transformations catalyzed by such species.

This work is divided into two parts. The first part (Chapters 2 – 4) focuses on investigations of the chemical properties of low-silica zeolites modified with typical hard Lewis acids such as alkaline and alkali-earth cations. Although these cations are rather inert, their high density in the zeolite can cause important chemical properties of the microporous matrix such as enhanced basicity of the framework and strong electrostatic field in the zeolite cages. More reactive soft Lewis acids such as Zn-, Cd- and Ga-cations stabilized in high-silica zeolites are discussed in the second part of the thesis (Chapter 5 –

9). In this case, special attention is devoted to the mechanism of C–H activation as well as to the stability and structure of the intrazeolitic cationic species.

Confinement and molecular recognition effects due to the specific arrangement and the size of the exchangeable cations in the zeolite matrix are discussed by examples of (i) photo-oxidation of alkenes with molecular O2, and of (ii) N2O4 disproportionation in alkali-earth and alkaline-exchanged faujasites in Chapters 2 and 3, respectively. An attempt is made to separate effects of basicity of the framework, Lewis acidity of the

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exchangeable cations, and the electrostatic field in the zeolite cage for the respective reactions.

Chapter 4 reports a combined FT-IR spectroscopic and quantum chemical investigation of adsorption of light alkanes on magnesium- and calcium-exchanged Y-zeolites. In this chapter an attempt is made to understand how a light alkane molecule is adsorbed in a cation-exchanged zeolite cage and what the nature of the adsorption complex is. Based on the results of ab initio calculations (DFT and MP2) and topological analysis of the electron density distribution function in the framework of quantum theory of atoms in molecules, it is discussed how the geometry of the light alkane adsorption complexes depends on the extra-framework cation, which factors determine the particular adsorption fashion, and how this relates to the intermolecular interactions realized within the adsorption complex.

Adsorption properties of more reactive zinc and cadmium cations in high-silica ZSM-5 zeolites are studied in Chapter 5. The reactivity of different charge-compensating species toward heterolytic dissociative adsorption of light alkanes is investigated by means of DFT calculations. The results obtained are used to identify the factors, which control the activation of light alkanes over Zn- and Cd-exchanged ZSM-5. The influence of the perturbations of the adsorbed molecules due to interaction with the exchangeable cations on their subsequent chemical activation is discussed.

Chemical activation of light alkanes over soft Lewis acids in high-silica zeolites is further investigated in Chapters 6-8, which report comprehensive quantum chemical investigations of catalytic dehydrogenation of ethane over different cationic species in ZSM-5 zeolites modified with zinc and gallium. Various reaction paths for catalytic dehydrogenation of ethane, as a model reaction, are computed and analyzed. Chapter 9 examines the possibilities for formation of bi- and multinuclear cationic extra-framework species in high-silica zeolites modified with gallium. The concept of indirect charge-compensation is further developed in these chapters. It will be shown that such molecular understanding can help in the design of improved catalysts.

References 

1 Cronstedt, A.F. Kungliga Svenska Vetenskapsakademiens Handlingar, Stockholm, 1756, 17, 120‐123. 2 Löwenstein, W. Am. Mineral. 1954, 39, 92. 3 Mirodatos, A.; Barthomeuf, D. J. Chem. Soc., Chem. Commun. 1981, 39. 4 Corma, A. J. Catal. 2003, 216, 298. 5 Stoeker, M. Microporous and Mesoporous Mater. 2005, 82, 257. 6 Hagen, A.; Roessner, F. Catal. Rev. 2000, 42, 403. 7 Serykh, A.I. Microporous and Mesoporous Mater. 2005, 80, 321.  8 Barthomeuf, D. Catal. Rev. 1996, 38, 521. 9 Blatter, F.; Sun, H.; Vasenkov, S.; Frei, H. Catal. Today 1998, 41, 297. 10 Frei, H. Science 2006, 313, 209. 11 Klier, K. Langmuir 1988, 4, 13. 12 Ward,  J.W.  J. Catal. 1968, 10, 34; Uytterhoeven,  J.B.; Schoonheydt, R.; Liengme, B.V.; Keith Hall, W.  J. 

Catal. 1969, 13, 425. 

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13 Penzien,  J.; Abraham, A.; van Bokhoven,  J.A.;  Jentiys, A.; Müller, T.E.; Sievers, C.; Lercher,  J.A.  J. Phys. Chem. B 2004, 108, 4116. 

14 Dooley, K.M.; Chang, C.; Price, G.L. Appl. Catal. A: General 1992, 84, 17; Abdul Hamid, S.B.; Derouane, E.G.; Mériaudeau,  P.;  Naccache,  C.  Catal.  Today  1996,  31,  327;  Kazansky,  V.B.;  Subbotina,  I.R.;  van Santen, R.A.; Hensen, E.J.M. J. Catal. 2005, 233, 351. 

15 Kazansky, V.B.; Serykh, A.I. Phys. Chem. Chem. Phys. 2004, 6, 3760; Kazansky, V.; Serykh, A. Microporous and Mesoporous Mater. 2004, 70, 151; Kazansky, V.B.; Serykh, A.I.; Pidko, E.A. J. Catal. 2004, 225, 369. 

16 Chen, N.Y.; Degnan, Jr., T.F.; Morris Smith, C. Molecular Transport and Reaction in Zeolites. Design and Application of Shape Selective Catalysts, VCH Publishers, Inc., New York, 1994; Degnan, Jr., T.F. J. Catal. 2003, 216, 32. 

17 Arguer,  R.S.; Olson, D.H.;  Landolt, G.R. G.B.  Patent,  1969,  1161974; Argauer,  R.S.;  Landolt, G.R. U.S. Patent, 1972, 3702886; Dwyer, F.G.; Jenkins, E.E. U.S. Patent, 1976, 3941871. 

18 Olson, D.H.; Kokotailo, G.T.; Lawton, S.L.; Meier, W.M. J. Phys. Chem. 1981, 85, 2238. 19 Meier, W.M. Z. Kristallogr. 1961, 115, 439. 20 Bergerhoff, G.; Baur, W.H.; Nowacki, W. N. Jb. Miner. Mh. 1958, 193. 21 Baur, W.H. Am. Mineral. 1964, 49, 697. 22 Jensen,  F.  Introduction  to  Computational  Chemistry, Wiley‐Interscience, New  York,  1999;  Leach, A.R. 

Molecular Modeling:  Principles  and  Applications,  Pearson  Education,  Harlow,  1996;  Foresman,  J.B.; Frish, A. Exploring Chemistry with Electronic Structure, 2nd ed., Pittsburg, PA Gaussian, 1996; Parr, R.G.; Yang, W. Density Functional Theory of Atoms  in Molecules, Oxford University Press, New York, 1989; Young,  D.C.  Computational  Chemistry:  A  Practical  Guide  for  Applying  Techniques  to  Real‐World Problems, Wiley‐Interscience, New York, 2001. 

23 Born, B.; Oppenheimer, J.R. Ann. Phys. 1927, 79, 361. 24 Hohenberg, P.; Kohn, W. Phys. Rev. 1964, 136, B864. 25 Kohn, W.; Sham, L. Phys. Rev. 1965, 140, A1133. 26 Perdew, J.P. Phys. Rev. B 1986, 33, 8822. 27 Becke, A.D. Phys. Rev. A 1988, 38, 3098 28 Perdew, J.P.; Chevary, J.A.; Vosko, S.H.; Jackson, K.A.; Pederson, M.R.; Singh, D.J.; Fiolhais, C. Phys. Rev. 

B 1992, 46, 6671. 29 Perdew, J.P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. 30 Becke, A.D. J. Chem. Phys. 1993, 98, 5648. 31 Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664. 32 Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158. 33 Johnson, E.R.; DiLabio, G.A. Chem. Phys. Lett. 2006, 419, 333. 34 Zhao, Y.; Truhlar, D.G. J. Chem. Theory Comput. 2005, 1, 415.  

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CHAPTER 2  

CONFINED SPACE‐CONTROLLED OLEFIN ‐ OXYGEN CHARGE TRANSFER IN ZEOLITES 

 

 

 

DFT calculations on the initial charge-transfer step in the photo-oxidation of alkenes in cationic zeolites are presented. The used model system represents a part of Y-zeolite supercage containing two SII sites occupied by alkali-earth cations with 2,3-dimethyl-2-butene (DMB) and O2 adsorbed on them. It is found that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of a [DMB+·O2

–] charge-transfer state, whereas the relative orientation and the distance between the DMB and O2 molecules are the most important factors. On the basis of these results the photo-oxidation considered is due to a confinement effect, in which the adsorbed reagents are oriented in a suitable “pre-transition state” configuration.

 

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2.1. Introduction

Recent photochemical studies of alkene-oxygen gas mixtures loaded in alkaline- and alkali-earth-exchanged zeolites have revealed that selective partial oxidation of unsaturated hydrocarbons can be induced by visible light [1-9]. The UV-Visible spectroscopic studies [6,10,11] have shown that a weak continuous absorption tail in the visible extending into the red spectral region appears only when both olefin and O2 are present in the zeolite. This absorption is responsible for the light-induced oxidation of olefins.

The corresponding alkene·O2 contact charge-transfer bands in the liquid phase [12] and in a solid oxygen matix [13,14] are well established and lie in the UV spectral range. For instance, the continuous absorption band attributed to the olefin·O2 charge transfer in the case of 2,3-dimethyl-2-butene trapped in solid O2 has been detected at about 380 nm [13,14]. On the other hand, the same molecules loaded in NaY zeolite exhibit a charge-transfer band at 750 nm [10]. It has been proposed that the interaction of the alkene·O2 contact pair with the strong electrostatic field of the cation-exchanged zeolite upon coadsorption results in a very strong stabilization of the charge-transfer state. Such stabilization is thought to cause the large red shift of alkene·O2 contact charge-transfer transitions from the UV range into the visible range.

The excitation of the alkene·O2 contact pair results in the formation of an [alkene+·O2–]

charge-transfer state. When the hydrocarbon molecule is coordinated to a positively charged cation, this excited state will be strongly destabilized. On the other hand, one expects a very strong stabilization of the charge-transfer state when the O2 molecule is adsorbed by the cation, while the hydrocarbon is located elsewhere far from the positively charged cationic adsorption sites and, at the same time, in the vicinity of the adsorbed dioxygen. This picture is not easily reconciled with the experimental fact that the adsorption of alkenes by the cation-exchanged zeolites is much stronger than O2 adsorption and, hence, the alkene will replace the adsorbed oxygen.

To clarify this, a density functional theory (DFT) study of a model system containing 2,3-dimethyl-2-butene and O2 adsorbed in the Ca-, Mg- and Sr-exchanged supercage of faujasite is performed. The calcium form of zeolite is chosen as a main object for the investigation, since the ionic radius of Ca2+ is similar to that of Na+, which has been used in the experimental studies. In addition, the cluster that models the FAU zeolite exchanged with bivalent cations can be chosen smaller, which reduces computational requirements.

2.2. Computational details

The quantum chemical calculations were carried out within the density functional theory (DFT) using the Gaussian 03 [15] program at the B3LYP/LanL2DZ level [16]. Earlier, the combination of the modest LanL2DZ basis set with the hybrid B3LYP functional was reported to provide reasonably accurate results in calculations of photochemical and adsorption properties [17-19]. The energy and the oscillator strength of the electron

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Figure 2.1. Optimized structures and selected bond lengths (Å) of (a) MgZ, (b) CaZ and (c) SrZ 

cluster models. All of the interatomic distances presented are in angstroms . 

excitations were estimated at the same computational level as the geometry optimization using the time-dependent DFT method that is implemented in the Gaussian 03 program package.

The M2Al4Si12O20H24 clusters, where M is Mg, Ca, and Sr, shown in parts (a)-(c) of Figure 2.1, respectively, were chosen for the model DFT calculations. In the following these structures will be designated as MZ (M = Mg, Ca, or Sr). The cluster represents a part of the wall of the faujasite supercage, containing two 6T rings (SII sites) with a corresponding alkaline-earth cation in each 6T ring, connected via three adjacent 4T silicon rings. To stabilize exchangeable alkaline-earth cations, each 6T ring contains two aluminum atoms. The current MZ cluster was chosen because of the necessity of the existence of two adsorption sites in the zeolite model, while the use of the faujasite unit cell as a model was not possible due to the limitations of application of the time-dependent DFT method for the systems with periodic boundary conditions using the available quantum-chemical software.

The starting geometry of the clusters corresponds to the lattice of FAU zeolite according to X-ray diffraction (XRD) data [20]. Dangling bonds are terminated by H atoms located 1.4 Å from each terminal Si atom and 1.5 Å from each terminal Al atom oriented in the direction of the next T-site. Full geometry optimization was performed for the cluster models with adsorbed DMB and O2 molecules and for the clusters themselves, while the positions of boundary H atoms were fixed according to the initial coordinates. Partial optimization of the DMB·O2 complex with a fixed distance between one of the O atoms and a carbon from the C=C bond was performed in order to compare computational results with those obtained experimentally [14] for the complex stabilized in a solid O2 matrix.

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 Figure 2.2. Coadsorption of 2,3‐dimethyl‐2‐butene and O2 on (a) MgZ, (b) CaZ, and (c) SrZ. All of 

the interatomic distances presented are in angstroms. 

Table 2.1. Correcteda (∆EBSSE) and uncorrected (∆E) for the BSSE energies (kJ/mol) of individual 

adsorptionb and coadsorptionc of O2 and DMB on MgZ, CaZ and SrZ clusters. 

  MgZ  CaZ  SrZ   ∆E  ∆EBSSE  ∆E  ∆EBSSE  ∆E  ∆EBSSE 

DMB/MZ  63  44  81  60  86  65 O2/MZ  31  15  31  14  28  9 [DMB∙O2]/MZ  91  55  110  72  108  70 a ΔEBSSE = ΔE – EBSSE b ΔE = –{E([molecule]/MZ) – (E(molecule) + E(MZ))} c ΔE = –{E([DMB∙O2]/MZ) – (E(DMB)+ E(O2) + E(MZ))}  

The computed adsorption energies were corrected for basis set superposition error (BSSE) (EBSSE) using the counterpoise method [21]. The spin state of O2 was assumed to be triplet in all of the computations presented bellow.

2.3. Results

Figure 2.2 (b) shows the optimized structure of 2,3-dimethyl-2-butene and dioxygen embedded in the CaZ cluster ([DMB·O2]/CaZ). The most important interatomic distances are also displayed in Figure 2.2. Coadsorption of these molecules to a single exchangeable cation is energetically unfavourable due to the stronger adsorption of DMB (60 kJ/mol),

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Table  2.2.  The  optimized  C∙∙∙O  distances  (r)  and  the  estimated  energies  (E)  and  oscillator 

strengths (f) of the charge transfer in the DMB⋅O2 complex. 

  rC…O, Å  E, eV  f [DMB∙O2]/MgZ  3.957/3.807  0.80  0.007 [DMB∙O2] from MgZa  3.957/3.807  1.25  0.005 [DMB∙O2]/CaZ  2.877/2.860  1.67  0.078 [DMB∙O2] from CaZa  2.877/2.860  1.90  0.057 [DMB∙O2]/SrZ  3.782/3.754  1.21  0.008 [DMB∙O2] from SrZa  3.782/3.754  1.41  0.007 

DMB∙O2 gas phase; partially optimizedb  2.150/2.655 2.69  0.099 

4.22  0.070 a  Data  for  the  free  DMB∙O2  complex  with  the  geometry  obtained  from  the  optimization  of  the [DMB∙O2]/MZ (M = Mg, Ca or Sr). b Partially optimized gas phase DMB∙O2 complex with the constrained C∙∙∙O distance of 2.15 Å. 

 Figure 2.3. Shape of the calculated orbitals  involved 

in the DMB⋅O2 charge transfer, respectively, bonding 

(a)  and  antibonding  (b)  molecular  orbitals  of  the 

DMB⋅O2 contact complex 

which suppresses that of O2 (14 kJ/mol). The energies of either individual adsorption or coadsorption of DMB and O2 are listed in Table 2.1.

It is found that the DMB molecule is coordinated with the C1–C2 double bond to the Ca2+ cation (Ca2), whereas the O2 molecule is end-on adsorbed to another cation (Ca1). The changes, which occur in the geometry of the adsorbed molecules due to interaction with the exchangeable cations, are insignificant. This is consistent with the mainly electrostatic nature of the interaction of either DMB or O2 molecules with the alkaline-earth cations stabilized in the zeolitic cavity. For the adsorption complex, in which the DMB is coordinated to one Ca2+ cation and the O2 molecule to the other, the computed energy of intermolecular DMB⋅O2 charge transfer is equal to 1.67 eV (745 nm). The oscillator strength of it is f = 0.078 (Table 2.2). The shapes of molecular orbitals involved in this electron excitation process are shown in Figure 2.3. One can see the interaction of the highest occupied molecular orbital (HOMO) of the DMB with the lowest unoccupied molecular orbital (LUMO) of the dioxygen molecule. The orbitals involved in the charge-transfer process are, respectively, the bonding and antibonding molecular orbitals of a DMB·O2 molecular complex formed in the zeolitic cage. They represent a linear combination of the occupied πβ-orbital of the DMB molecule and the unoccupied π*β-orbital of the O2 molecule. The finite value of the oscillator strength is due to their small but significant overlap.

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To investigate the effect of the electrostatic field in the zeolite cage on the stabilization of the charge-transfer state, the single-point time-dependent DFT calculation was performed for a free DMB·O2 complex with exactly the same geometry as in the [DMB·O2]/CaZ system. In the absence of the zeolite cage, the computed charge-transfer energy (Table 2.2) slightly increases and becomes equal to 1.90 eV (653 nm), whereas the oscillator strength is only slightly lowered (f = 0.057).

Hence, it can be concluded that the low energy intermolecular charge transfer is due to the specific orientation between the DMB and O2 molecules in the ground state resulting from their coadsorption to the exchangeable cations. To support this hypothesis, coadsorption by Mg2+ and Sr2+ stabilized in the same cluster model was also investigated. The optimized structures and the most important interatomic distances for the [DMB·O2]/MgZ and [DMB·O2]/SrZ are presented in parts (a) and (c) of Figure 2.2, respectively. The energies of individual adsorption and coadsorption of DMB and O2 by these clusters are listed in Table 2.1. Similar to the above-discussed case of CaZ model, coadsorption of 2,3-dimethyl-2-butene and dioxygen molecules by an isolated cation is not likely to occur, because of much stronger interaction of the alkene with the cation (Table 2.1) due to the higher basicity of the alkene molecule as compared to that of the O2. Similar trends in the coordination of the DMB and O2 molecules to the cations are observed. The alkene is adsorbed to the cation with the C=C bond, and the oxygen is adsorbed in an end-on fashion.

In spite of similarities in the adsorption fashions, in the case of the MgZ model, both the adsorbed DMB and O2 molecules are located closer to the cations in the cluster and the distance between them is significantly larger. On the other hand, in the case of the [DMB·O2]/SrZ structure, the DMB molecule is coordinated to both strontium ions (Sr1 and Sr2), while dioxygen is forced out from the cluster model, and again the distance between the adsorbed molecules is significantly increased as compared to that in the case of the CaZ. These effects are due to different ionic radii of the considered alkaline-earth cations (Sr > Ca > Mg) and, hence, to the different space between the adsorbed molecules. Besides this, a strongly different relative orientation between the alkene and O2 is detected in the case of the magnesium containing cluster model. Instead of the end-on coordination of the O2 to the C=C bond of the DMB adsorbed to CaZ and SrZ, the interatomic contacts between the O2’ atom from the dioxygen and H-atoms from the methyl groups of the DMB molecule are detected when coadsorbed to MgZ (Figure 2.2).

The computed properties of the intermolecular DMB⋅O2 charge transfer for the [DMB·O2]/MgZ and [DMB·O2]/SrZ are listed in Table 2.2. One can see that the different relative configuration of the O2 molecule to the alkene strongly influences both the energy of the charge transfer and the oscillator strength. The increase of the distance between the DMB and O2 molecules results in substantial decrease of the oscillator strength and, hence, of the probability of the intermolecular charge transfer. Surprisingly, increased separation of the adsorbed molecules from each other also results in significant decrease of the energy

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 Figure 2.4. Partially optimized structure 

of the  isolated DMB⋅O2 complex with a 

fixed C1‐O1 distance at 2.15 Å. 

of the corresponding electron excitation. Most likely, it is connected to the fact that with a larger intermolecular separation no DMB·O2 complex is formed and, therefore, the ground state lies higher in energy. One should also note that a pure charge transfer between the DMB and O2 coadsorbed on MgZ is not detected, because the excitation process additionally involves lone pairs of the basic oxygens of the cluster (O5 and O6). Nevertheless, in the absence of the zeolite framework, in all considered cases, the corresponding charge-transfer band is blue-shifted and the oscillator strength slightly decreases (Table 2.2). The large difference between the charge-transfer energy in the free DMB·O2 complex and that embedded in the MgZ cluster is most likely the result of the artifact connected with the involvement of the cluster model in the electron excitation process.

Hashimoto and Akimoto [14] estimated a distance of 4.1 Å between the positive (DMB) and negative ends (O2) of the dipole moment of the DMB·O2 charge-transfer complex in a solid O2 matrix. One notes that this distance is realized when the shortest distance between one of the carbons (C1) of the C=C bond of DMB and the interacting O1 atom equals 2.15 Å (Figure 2.4). In this case, two charge-transfer absorption bands with rather high values of the oscillator strength are computed: 2.69 eV (462 nm, f = 0.099) and 4.22 eV (294 nm, f = 0.070). The latter value very well agrees with that reported in [14] (4.32 eV). On the other hand the experimentally observed charge-transfer absorption spectrum [14] exhibits a very broad absorption band overlapping both values. It is also noticeable that the model used does not take into account additional interactions of the complex with the other surrounding dioxygen molecules of the O2 matrix, which can influence the geometrical and electronic properties of the contact complex.

2.4. Discussion

A very strong decrease of the energy for intermolecular charge transfer between branched alkenes and oxygen embedded in alkaline-earth zeolites compared to the free state has been detected experimentally [6,10,11]. Indeed, UV-Visible spectroscopy of alkali- and alkaline-earth zeolite Y loaded with alkenes and O2 have revealed a visible absorption tail to be attributed to the hydrocarbon·O2 contact charge-transfer transition [10]. On the other hand, UV charge-transfer bands for similar contact complexes are well known in solid O2 matrixes [14]. It has been suggested that the interaction of the strong electrostatic field in cation-exchanged zeolites with the large dipole generated upon excitation of the hydrocarbon·O2 to the charge-transfer state leads to the stabilization of the excited state by 1.5-3 eV. This results in a strong red shift of the absorption from the UV region into the visible spectral region.

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Indeed, the interaction of an O2 molecule with a zeolitic cation strongly increases its electron affinity, but on the other hand, an adsorption of the hydrocarbon molecule to the exchangeable cations would lead to simultaneous increase of its ionization potential. Both of these effects cancel each other. It is known that alkenes are adsorbed on zeolites significantly stronger than oxygen. Thus, if one supposes that only the hydrocarbon molecules are adsorbed on the cation sites of zeolite and the O2 molecules are located elsewhere, the electrostatic field will be directed opposite to the direction of the charge transfer. Therefore, the observed absorption band should be blue-shifted compared to that for the gas phase.

The results of DFT model calculations presented above show that the estimated energies and the probabilities of the charge transfer between 2,3-dimethyl-2-butene and dioxygen molecules with the same geometries are very close both in the presence and in the absence of the zeolite cage. It is evident that the major factor in the shift of the experimental absorption band is the relative geometry of the molecules. The specific relative orientation, which is due to adsorption to the closely located cation sites, results in the formation of a molecular complex with the excitation energy of visible light. Absorption bands in the absence of the zeolite matrix have been experimentally detected in a matrix of solid oxygen [13,14]. In this case the average distance between the hydrocarbon and oxygen molecules should be rather small and, in accordance with the presented theoretical results, this leads to a strong blue shift of the absorption band to UV range and to a significant increase of the probability of the electron transition. The model DMB·O2 complex (Figure 2.4) has a distance of about 4.1 Å between the positive and negative ends of the dipole, which are located at the opposite to O2 part of the alkene and at the non-interacting oxygen atom (O2), respectively. Such contact complex shows a charge-transfer band of 4.22 eV. This value well agrees with that obtained experimentally (4.32 eV, Ref. [14]).

The specific orientation of the C=C double bond of the hydrocarbon to the O2 molecule results in the formation of a molecular complex with an overlap of the πβ-orbital of the DMB molecule and the π*β -orbital of the O2 molecule. The optimum configuration is found in the case of coadsorption of these molecules on calcium-exchanged faujasite. The adsorption of the DMB and O2 on the nearest exchangeable cations of the zeolite Y supercage results in their confinement in specific orientation that is suitable for a rather effective overlap of corresponding HOMOs and LUMOs and, hence, for the intermolecular charge transfer. One notes that the C–O distances (2.877 and 2.860 Å) in the [DMB·O2]/CaZ are significantly lower than the sum of corresponding van der Waals radii (3.1 Å, Ref. [22]). On the other hand, when due to steric factors such a suitable configuration between the adsorbed molecules cannot be realized (as is found for [DMB·O2]/MgZ and [DMB·O2]/SrZ), the molecular complex is not formed and the effective charge transfer can not be observed. Thus, one can expect a much lower activity of MgY and SrY zeolites in the photo-oxidation of 2,3-dimethyl-2-butene in comparison with that of CaY.

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2.5. Conclusions

The role of the zeolite in the photo-oxidation of alkenes with molecular oxygen is the complexation of the hydrocarbon and O2 to the extraframework cations, resulting in confinement of these molecules with a specific relative orientation. This leads to the formation of a π-π intermolecular complex. The interaction between alkene and oxygen in this complex occurs with a finite overlap of the corresponding π and π* molecular orbitals. The formation of such a complex results in a significant transition moment of the intermolecular charge transfer. The zeolite matrix stabilizes the reagents in a suitable “pre-transition state” configuration. The role of the electrostatic field of the zeolite is only indirect.

References 

1 Frei, H. Science 2006, 313, 209. 2 Blatter, F.; Frei, H. J.  Am. Chem. Soc. 1994, 116, 1812. 3 Blatter, F.; Sun, H.; Vasenkov, S.; Frei, H. Catal. Today 1998, 41, 297. 4 Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1996, 118, 6873. 5 Sun, H.; Blatter, F.; Frei, H. Chem. Eur. J. 1996, 118, 6873. 6 Tang, S. L. Y.; McGarvey, D. J.; Zholobenko, V. L. Phys. Chem. Chem. Phys. 2003, 5, 2699. 7 Myli, K.B.; Larsen, S.C.; Grassian, V.H. Catal. Lett. 1997, 48, 199. 8 Larsen, R.G.; Saladino, A.C.; Hunt, T.A.; Mann, J.E.; Xu, M.; Grassian, V.H.; Larsen, S.C. J. Catal. 2001, 204, 

440. 9 Xiang, Y.; Larsen, S.C.; Grassian, V.H. J. Am. Chem. Soc. 1999, 121, 5063. 10 Blatter, F.; Moreau, F.; Frei, H. J. Phys. Chem. 1994, 98, 13403. 11 Vasenkov, S.; Frei, H. J. Phys. Chem. B 1997, 101, 4539. 12 Coomber, J.W.; Hebert, D.M.; Kummer, W.A.; Marsh, D.G.; Pitts, Jr., J.N. Environ. Sci. Technol. 1971, 4, 

1141. 13 Hashimoto, S.; Akimoto, H. J. Phys. Chem. 1986, 90, 529. 14 Hashimoto, S.; Akimoto, H. J. Phys. Chem. 1987, 91, 1347. 15 Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. 

A.,  Jr.;  Vreven,  T.;  Kudin,  K.  N.;  Burant,  J.  C.; Millam,  J. M.;  Iyengar,  S.  S.;  Tomasi,  J.;  Barone,  V.; Mennucci, B.; Cossi, M.;  Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,  X.;  Knox,  J.  E.;  Hratchian,  H.  P.;  Cross,  J.  B.;  Bakken,  V.;  Adamo,  C.;  Jaramillo,  J.;  Gomperts,  R.; Stratmann,  R.  E.;  Yazyev,  O.;  Austin,  A.  J.;  Cammi,  R.;  Pomelli,  C.;  Ochterski,  J.  W.;  Ayala,  P.  Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al‐Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;  Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,  J. A. Gaussian 03,  revision B.05; Gaussian, Inc.: Pittsburgh PA, 2003. 

16 Becke, A.D. Phys. Rev. 1988, A38, 3098; Becke, A.D. J. Chem. Phys. 1993, 98, 1372; Becke, A.D. J. Chem. Phys. 1993, 98, 5648. 

17 Ermakov, A.I.; Mashutin, V.Y.; Vishnjakov, A.V. Int. J. Quant. Chem. 2005, 104, 181. 18 Brocławik, E.; Borowski, T. Chem. Phys. Lett. 2001, 339, 433. 19 Song, X.; Liu, G.; Yu, J.; Rodrigues, A.E. J. Mol. Struct. (TEOCHEM) 2004, 684, 81. 20 Olson, D.J. J. Phys.Chem. 1970, 74, 2758. 21 Simon, S.; Duran, M.; Dannenberg, J. J. J. Chem. Phys. 1996, 105, 11024. 22 Bondi, A. J. Phys. Chem. 1964, 68, 441.  

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CHAPTER 3  

MOLECULAR RECOGNITION OF N2O4 ON ALKALI‐EXCHANGED LOW‐SILICA ZEOLITES X  

 

 

 

Adsorption and disproportionation of dinitrogen tetraoxide on sodium, potassium and rubidium exchanged zeolites X with Si/Al ratio of 1.18 are studied using density functional theory calculations with periodic boundary conditions. It is found that the stabilization and activation of most of the N2O4 isomers confined in the zeolitic cage does not follow the differences in Lewis acidity of the extra-framework cations. This is also observed for the energetics of the N2O4 disproportionation reaction resulting in a spatially separated NO+···NO3

– ion pair. The reaction energy increases in the row NaX < RbX < KX. The strength of perturbations and, therefore, the low-frequency shift of the N–O stretching frequency of the adsorbed NO+ cations correlate well with the basicity of the framework oxygens (RbX > KX > NaX). However, this factor is not the relevant reactivity parameter for the N2O4 disproportionation in the cationic zeolites. The higher activity for the disproportionation as well as the stronger molecular adsorption of N2O4 on RbX and KX zeolites as compared to that on NaX is ascribed to the features analogous to the molecular recognition characteristics of supramolecular systems. The steric properties of the zeolite cage and the mobility of the extra-framework cations induced by adsorption are essential to shape the optimum configuration of the active site for N2O4 disproportionation.

 

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3.1. Introduction

Typically, zeolites with alkali and alkaline-earth cations compensating for the negative charge of the framework exhibit well pronounced basic properties [1-4]. The presence of such rather inert species enhances the electron density of the framework oxygens, which can act as basic sites. The extra-framework cations, on the other hand, produce rather strong electrostatic field in the zeolite cage, and can in principle polarize molecules confined in the microporous matrix resulting in their activation. Usually these two effects, i.e. presence of rather strong basic sites and the electrostatic field due to the exchangeable cations, are considered to account for the chemical reactivity of alkali- and alkaline-earth exchanged low-silica zeolites [1-6].

In the previous chapter the promotion of selective oxidation of hydrocarbons (HC) with O2 over cation-exchanged low-silica faujasite-type zeolites have been discussed. In contrast to the earlier propositions on the dominating role of the electrostatic field stabilizing the [HC+···O2

–] charge-transfer state [6,7], it has been shown that the reactivity in this case is mainly due to the steric properties of the zeolite cage. The zeolite matrix prearranges the reactants in a specific orientation suitable for the subsequent chemical activation. These conclusions cohere well with the recent finding of Lahr et al. [8]. A detailed investigation of H2 and CO oxidation over various ion-exchanged X and Y zeolites did not show any correlation between the reactivity and the expected electrostatic field within the zeolite cage. 

 

Scheme 3.1: Relative energies  (B3LYP/6‐31G(d)) and geometrical parameters  (Å) of  the selected 

N2O4  isomers  in  gas  phase  (from  Ref.  13).  The  geometrical  parameters  of  the D2h  and  Cs N2O4 

isomers calculated at PW91/PAW level of theory with periodic boundary conditions are presented 

as well for comparison. 

Another interesting chemical property of the alkali-exchanged zeolites is the promotion of the disproportionation of nitrogen dioxide at low temperatures [9-12]. Indeed, the gas-phase reaction is unfavorable (Scheme 3.1). Formation of a separated NO3

–···NO+ ion pair requires energies as high as 205 kJ/mol [13]. Such endothermic reactions can be substantially stabilized in a polar media, e.g. in solution via solvation. Adsorption measurements, theoretical modeling, and extensive reaction chemistry using cationic faujasites indicate that the intracrystalline void space shows high polarity [14]. Different endothermic reactions are strongly facilitated in zeolite pores. The interaction between the occluded molecules and zeolite framework displays the characteristics of a strong

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electrolyte or solvent [15,16]. In particular, the high concentration of framework aluminum and extra-framework cations in low-silica zeolites leads to strongly pronounced “solvent effects”. These are similar to the effect of polar solvents in solutions, with specific features due to the local environment and confined space in the zeolite pores [14].

The disproportionation of nitrogen dioxide observed on alkali-exchanged zeolites X and Y has been associated with the basic properties of the microporous materials [12,17]. NO+ adsorbed on the negatively charged lattice oxygens has been proposed as a new probe molecule for zeolite basicity associated to the negative charge around oxygen atoms [1]. The stretching frequency of the adsorbed NO+ has been related to the oxygen basicity that increases along with the size of the exchangeable cation.

In addition, Li et al. [11] proposed possible reasons for the superior performance of zeolite-based catalysts over those supported on conventional oxides in the NOx reduction. Among other reasons, the higher activity of the former systems has been attributed to the promotion of disproportionation reactions due to the existence of rather mobile Lewis acid sites, which are extra-framework alkali- and alkaline-earth cations.

One notes that the reactivity of the alkali- and alkaline-earth exchanged zeolites is determined by a combination of Lewis acidity of the exchangeable cations and the basicity of the framework oxygens. These factors cannot be easily separated. Herein a periodic density functional theory (DFT) study of adsorption and disproportionation of N2O4 on sodium-, potassium-, and rubidium-exchanged low-silica zeolites X is presented in an attempt to separate the effects of zeolitic cations and framework oxygens.

3.2. Computational details

3.2.1. Methodology

Periodic density functional theory (DFT) as implemented in Vienna Ab Initio Simulation Package [18,19] was used to identify equilibrium structures and their energetics as well as to calculate vibrational frequencies of N2O4 adsorption complexes within alkali-exchanged zeolite X. The calculations were performed using the Perdew–Wang (PW91) form of the generalized gradient approximation for the exchange and correlation energies [20]. The projected augmented waves (PAW) method was used to describe electron-ion interactions [21,22]. A plane wave basis set was employed for valence electrons. The energy cut-off was set to 400 eV. A modest Gaussian smearing was applied to band occupations around the Fermi level (σ = 0.1 eV) and the total energies were extrapolated to σ → 0. The Brillouin zone sampling was restricted to the Γ-point [23].

Cell parameters were initially optimized for each of the zeolites. The obtained parameters were then used in all calculations. Full geometry optimizations were performed for each structure with the fixed cell parameters using a conjugated gradient algorithm. Convergence was assumed to be reached when the forces on each atom were below 0.05 eV/Å.

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Figure  3.1.  Part  of  the  structure  of 

cation‐exchanged faujasite 

Table 3.1. Optimized structural properties of alkaline‐exchanged zeolites X. 

  NaX  KX  RbX Unit cell dimensions:  a=b=c=17.775 Å 

α=β=γ=60.00° a=b=c=18.033 Å α=β=γ=60.00° 

a=b=c=18.186 Å α=β=γ=60.00° 

Selected interatomic distances:     M(SI’)‐O  2.17‐2.41 Å  2.56‐2.78 Å  2.70‐2.97 Å M(SII)‐O  2.23‐2.46 Å  2.70‐2.88 Å  2.84‐3.04 Å M(SIII)‐O  2.39‐2.63  2.76‐3.26 Å  2.94‐3.32 Å 

Vibrational frequencies of the adsorbed N2O4 species were calculated using the finite difference method as implemented in VASP. Small displacements (0.02 Å) of atoms from the N2O4 species and of the zeolitic ions involved in direct interaction with the N2O4 were used for the estimation of numerical Hessian matrix. The rest of the zeolitic atoms were kept fixed to their equilibrium positions.

The stabilities of the adsorption complexes discussed below were evaluated based on the relative energies (∆E) of the respective structures. These energies were calculated as

∆E = E(N2O4·zeolite) – E(zeolite) – E(N2O4), where E(N2O4·zeolite) is the calculated total electronic energy of the adsorption complex, E(zeolite) is the total energy of the optimized zeolite structure, and E(N2O4) is the total energy of the isolated N2O4 molecule.

3.2.2. Models

A crystallographic unit cell of faujasite has Fd⎯3m symmetry and contains 576 atoms [24-27]. Obviously the DFT study involving such a large unit cell is computationally prohibitive. It is, however, possible to define a smaller rhombohedral cell with 144 atoms (48 silicon and 96 oxygen atoms, see Figure 1.5 in section 1.3 of this thesis). The latter unit cell was used in our calculations. To model low-silica X zeolite 22 silicon atoms in this cell were replaced by aluminium atoms following the Löwenstein rule [28]. The resulting Si/Al ratio of the zeolite model was equal to 1.18. To compensate the thus formed negative charge of the lattice, 22 alkaline cations were introduced into the structure. A complete and selective ion-exchange of the zeolite was assumed. The chemical composition of the thus constructed zeolite unit cells was M22Al22Si26O96, where M is Na, K, and Rb for NaX, KX, and RbX zeolite, respectively. The uniform initial distribution of the extra-framework cations was used for all three zeolites. Initially, 8 cations were located at SI’ sites, 8 cations - at SII and 6 were placed at SIII sites (Figure 3.1).

The optimized geometry parameters of the NaX, KX, and RbX model zeolites are listed

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in Table 3.1. Introduction of the larger cations into the zeolite results in an increase of the cell parameters of the crystal. One notes that the difference is larger between the NaX and KX as compared to that between the KX and RbX. This correlates very well with the differences in ionic radii of the respective cations (0.95, 1.33, and 1.48 Å for Na, K, and Rb, respectively [29]).

The calculated interatomic distances involving the extra-framework cations show similar trends. The shortest cation–oxygen bonds are detected for the cations located inside the sodalite cages and the longest bonds are formed between lattice O atoms and SIII cations. Unfortunately, the experimental structural data are available only for the sodium [24,25]

and potassium [26,27] forms of faujasites. For NaX zeolite Na–O bond lengths of 2.24-2.36 Å, 2.34 Å, and 2.41-2.45 Å have been reported in Ref. [24] for SI’, SII, and SIII sites, respectively. In the case of potassium-exchanged zeolite the respective distances are 2.51-2.63 Å for SI’, 2.92-3.014 Å for SII, and 2.66-2.93 Å for K+ located at SIII sites [27]. One can see that the calculated values reasonably well agree with the experimental data.

The molecular N2O4 isomers (D2h and Cs) were represented within the same periodic code and level of theory as the periodic structures by surrounding of the N2O4 molecule with vacuum in a 20 x 20 x 20 Å3 cubic supercell.

3.3. Results

3.3.1. Adsorption of N2O4 dimer

Molecular adsorption precedes the disproportionation of the NO2 dimer, which is the N2O4 molecule (D2h), in the cage of the cation-exchanged zeolite X. N2O4 is a weak base. Therefore, one expects that bonding of the dimer to the zeolitic cations is mainly due to the induced polarization of the adsorbed species in the field of the zeolite cage with small charge donation to the exchangeable cations. Two different accessible adsorption sites can be distinguished for each of the zeolites (NaX, KX, and RbX), which are the cations at the SII and at the SIII extra-framework positions (Figure 3.1). Thus, two different situations were considered for each of the zeolites, where the primary interaction takes place between the N2O4 molecule and the exchangeable cations at either the SII or SIII positions.

Calculated results for the structures of the adsorption complexes of dinitrogen tetraoxide with zeolites NaX, KX, and RbX are shown in Figure 3.2. The corresponding adsorption energies and the N–O stretching frequencies are listed in Table 3.2. Independently of the cationic form of the zeolite and of the type of the adsorption site, the N2O4 molecule coordinates to the exchangeable cation via an end-on fashion by two O atoms from one of the NO2 moieties. Such an interaction results in a significant elongation of the N–O bonds directly interacting with the adsorption site (Figure 3.2). In addition, the N–N bond length decreases upon adsorption reflecting its strengthening.

For all of the considered cationic forms of X zeolites, N2O4 adsorption to the SIII cation is slightly more favorable than the adsorption on the SII site. This is, most likely, due to

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Figure 3.2. Optimized  structures  (only a part of  the periodic  structure  is  shown) and  selected 

interatomic distances (Å) of the adsorption complexes of N2O4 on (a) SII and (b) SIII sites of NaX; 

(c) SII and (d) SIII site of KX; (e) SII and (f) SIII site of RbX. 

Table  3.2.  Adsorption  energies  (∆E,  kJ/mol)  and  N–O  stretching  frequencies  (cm–1)  of  the 

adsorption complexes of N2O4 in cation‐exchanged X zeolite.  

  NaX  KX  RbX   SII  SIII  SII  SIII  SII  SIII ∆E  –10  –11  –10  –15  –21  –26 ν(N–O)             

b2u  1778  1796  1732  1731  1696  1695 b3g  1754  1732  1707  1695  1670  1665 ag  1404  1396  1264  1261  1237  1230 b1u  1264  1272  1234  1234  1216  1216  

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the lower coordination saturation of the exchangeable cation at the four-membered ring of the SIII site. The stronger adsorption to the SIII cations is accompanied with the stronger perturbations of the adsorbed molecules. Indeed, both the elongation of the N–O bonds and the shortening of the N–N bond are the most pronounced in this case (Figure 3.2).

One expects the strength of induced polarization of the zeolitic cations to decrease in the series Na+ > K+ > Rb+, because of the decrease of the “charge-to-radius” ratio. As a result the N2O4 adsorption energies should also decrease along with the increase of the ionic radius of the exchangeable cations, i.e. NaX > KX > RbX. In contrast to such expectations, the opposite trend is observed. The highest adsorption energies are calculated for the RbX, while the N2O4 adsorption on NaX is the weakest (Table 3.2).

One of the reasons for this observation is the following: in addition to the primary interactions between the adsorbed molecule and the cations at the sites SII or SIII, some weaker interatomic contacts with the neighboring extra-framework cations can occur (Figure 3.2). One notes that with the increase of the ionic radius of the exchangeable ion the number of these secondary interactions increases in parallel. The respective cation···O2NNO2 distances decrease. Indeed, in the case of the sodium form of zeolite X, the cations neighboring the adsorption site are located at a rather long distance from the adsorbed molecule and, therefore, cannot interact efficiently with it. These Na+ ions can only weakly polarize the adsorbed N2O4. On the other hand, the larger potassium ions at the positions, neighboring the adsorption site, form contacts (3.390–3.848 Å), which are comparable with those (3.024–3.150 Å) involved in the primary N2O4···cation interaction (Figure 3.2 (c) and (d)). In the case of RbX, the “primary” and the “secondary” interactions are already characterized by very similar Rb···O2NNO2 distances. Moreover, in this case the adsorbed N2O4 interacts not only with the cations nearest to the initial adsorption site but with almost all Rb+ in the supercage of zeolite X (Figure 3.2 (e) and (f)). Thus, in spite of the weaker individual interatomic contacts between the adsorbed molecule and the extra-framework cations, the number of the contacts increases with the increase of the ionic radius. This results in the unexpected enhancement of the N2O4 adsorption energy in the row: NaX < KX < RbX.

The cations stabilized at the SII sites of faujasite are better shielded by the surrounding framework oxygens as compared to those at the SIII sites. This leads to the observed stronger adsorption of N2O4 on the SIII cations. For the small sodium ions this effect is not important due to a significant shielding of Na+ both at the SII and SIII sites. However, in the case of K+ the difference in the adsorption energies becomes more pronounced. Indeed, a larger cation can be still rather well shielded by the lattice oxygens of the zeolitic six-membered ring, while K+ at the SIII site is coordinatively unsaturated. Further increase of the ionic radius in the case of RbX results in equalization of the properties of the exchangeable ions at the SII and SIII extra-framework positions.

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3.3.2. Adsorption of O2NONO isomer

In the first step of the N2O4 disproportion event the N–N bond breaks and the NO2 moiety distant to the adsorption site rotates. This results in a formation of a contact NO3

δ– NOδ+ ion pair. This is a Cs conformer O2NONO bound with two oxygen atoms of the NO3 fragment to the alkaline cation and with a nitrogen atom from the NO moiety coordinated to the framework oxygens. This structure has been proposed earlier [17] to be formed upon adsorption of N2O4 on cation-exchanged faujasites. Although the NO3 and NO moieties are still covalently bound in the O2NONO species in the gas phase, it has been shown in Ref. [30] that this isomer already displays a large degree of ionic bonding character relative to the symmetric N2O4 (D2h) isomer. It has been also suggested that the thermal disproportionation of dinitrogen tetraoxide proceeds via formation of the Cs isomer. Therefore, one expects that interaction of the negatively charged NO3

δ– moiety with the extra-framework cations and simultaneous coordination of the NOδ+ to the basic framework oxygens would facilitate the decomposition of the contact ion pair and, as a result, would lead to the disproportionation products.

Similar to the above case, the adsorbed O2NONO species can coordinate through the NO3

δ– moiety to the exchangeable cation located at either SII or SIII site, and through the NOδ+ to the neighboring framework oxygens. The initial guess structures for these adsorption complexes were constructed as follows: the starting geometry of the O2NONO molecule corresponded to that of the Cs gas-phase isomer. The negatively charged part was located at the same cation···ON distances as calculated for the case of adsorption of the symmetric N2O4 (D2h) isomer. The distance between the nitrogen atom from the NOδ+ moiety and basic lattice oxygens was kept at 3Å, which is the sum of van der Waals radii of N and O atoms [29].

Figure 3.3 shows optimized structures of the thus constructed adsorption complexes of “O2NONO” in NaX, KX, and RbX zeolites. The structure of the O2NONO isomer remains similar to the proposed one only in the case of adsorption to the SII cations of NaX zeolite (Figure 3.3 (a)). One should note that this is a rather unstable configuration (∆E = +18 kJ/mol). In this structure N–O bonds coordinated to Na+ are elongated by about 0.03 Å, while the one directed away from Na+ is shortened by more than 0.15 Å as compared to the geometry parameters computed for the gas-phase isomer (PW91/PAW, Scheme 3.1). The NOδ+ moiety is 1.993 Å distant from the nitro moiety. That is 0.321 Å larger than the respective interatomic distances in the gas-phase isomer. These strong perturbations reflect an increase of the ionic bonding character in the O2NONO isomer adsorbed on the SII site of NaX.

Adsorption of O2NONO on the SIII site of NaX zeolite (Figure 3.3 (b)) leads to a rather stable structure (∆E = –45 kJ/mol), and, at the same time, results in a more pronounced separation of the nitro- and nitroso-moieties (r(ON···ONO2) = 2.190 Å). The higher stability of this structure is most likely due to a more effective interaction of the nitro group with Na+ at the SIII site as well as due to a more favorable coordination of NO+ to

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Figure  3.3. Optimized  structures  (only  a  part  of  the  periodic model  is  shown)  and  selected 

interatomic distances (Å) of the adsorption complexes of the initially O2NONO isomer on (a) SII 

and (b) SIII sites of NaX; (c) SII and (d) SIII site of KX; (e) SII and (f) SIII site of RbX. 

Table 3.3. Adsorption energies (∆E, kJ/mol) and selected vibrational frequencies (cm–1) of the 

adsorption complexes formed via confinement of O2NONO in cation‐exchanged X zeolite.  

  NaX  KX  RbX   SII  SIII  SII  SIII  SII  SIII ∆E  +18  –45  –65  –65  –78  –80 Frequencies:             ν(N‐O)NOδ+  2010  2017  1977  1982  1942  1975 ν(NO3)assym’  1487  1436  1354  1369  1355  1367 ν(NO3) assym’’  1254  1259  1332  1322  1324  1315 ν(NO3) sym  985  1018  1047  1042  1035  1039  

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the framework oxygen atoms of the SIII’ site that prevents direct repulsion with the surrounding Na+.

In the case of potassium- and rubidium-exchanged X zeolites adsorption complexes involving a well distinguishable O2NONO moiety are not found. Geometry optimization resulted in disproportionation of the adsorbed molecule and formation of a space-separated NO+···NO3

– ion pair (Figure 3.3 (c-f)). Such adsorption complexes are rather stable (Table 3.3). Independently of the initial geometry and of the type of the main adsorption site (SII or SIII), geometry optimization resulted in formation of NO3

– species coordinated to three exchangeable cations and of NO+ bound via the nitrogen atom to the lattice oxygen ions of the SIII’ site. Two different types of coordination of the nitrosonium cation to the basic framework oxygens are found, i.e. mono- (Figure 3.3 (c) and (e)) and bidentate (Figure 3.3 (d) and (f)). In the former case the shortest distance between the NO3

– and NO+ moieties is realized (r(ON···ONO2) = 2.890 and 3.370 Å, respectively for KX and RbX). That is however too long to be considered as an interatomic contact of a significant energy.

The adsorption energies do not dependent strongly on the type of NO+ coordination to the zeolite framework (Table 3.3). Thus, the rather high stability of the discussed adsorption complexes on KX and RbX zeolites is mainly due to an effective interaction of NO3

– with three extra-framework cations.

The selected results of the vibrational analysis for the adsorption complexes are presented in Table 3.3. The basicity of lattice oxygen anions increases along with the increase of the ionic radius of the exchangeable cation (NaX < KX < RbX). As a result the red shift of the N–O stretching vibration (ν(N-O)NOδ+) of the adsorbed nitrosonium cation increases in this row. This reflects the stronger interaction of the NO+ species with the zeolite framework. This conclusion is also supported by the fact that the interatomic distances ON···Oz between the NO+ species and zeolitic O atoms decrease with the increase of the ionic radius of the extra-framework cations. Simultaneously, the corresponding N–O bond elongates. This reflects the stronger perturbation of the adsorbed NO+ cation.

The calculated frequencies of the N–O stretching vibrations of the NO3 moiety (ν(NO3), Table 3.3) coordinated to the extra-framework cations show that, similar to the above discussed adsorption of symmetric D2h N2O4, the strength of perturbation does not correlate with the expected Lewis acidity strength of the exchangeable cations. The weakest red shift of ν(NO3) and the smallest geometrical perturbations of the adsorbed nitro group are observed for NaX zeolite, where the NO3 species is coordinated only to one sodium ion. Therefore, the perturbations of the adsorbed NO3

– species are mainly controlled by the number of the intermolecular contacts formed upon adsorption rather than by the strength of the individual contact interaction.

3.3.3. Adsorption of the NO+···NO3– ion pair

Three different conformations of the NO+···NO3– ion pair confined in the cation-

exchanged zeolites X were considered. In all cases the nitro-group is coordinated to the

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largest possible number of neighboring cations. The NO+ cation is initially located either at the SIII site of faujasite (structure I), i.e. in the center of the four-membered ring next to the SII site, or at the SIII’ site nearest (II) or next nearest (III) to the six-membered ring of faujasite. The optimized structures of these adsorption complexes on NaX, KX, and RbX are shown in Figures 3.4, 3.5, and 3.6, respectively. Relative energies and selected vibrational frequencies for the respective complexes are summarized in Table 3.4.

Structure I is found to be the least stable ion pair, whatever the cation. The nitrosonium cation in this structure is coordinated to two framework oxygens of the four-membered ring (r(ON···Oz) = 2.135 and 2.215 Å for NaX; 2.078 and 2.134 Å for KX; 2.073 and 2.096 Å for RbX), with significantly longer distances to the other zeolitic oxygens (2.847, 2.667, and 2.641 Å, respectively for the structure I on NaX, KX, and RbX zeolite). NO+ at the SIII site is significantly perturbed. The N–O bond is the longest found among the systems considered. The strong perturbations are also evident from the calculated ν(N-O)NOδ+ frequencies, which are very strongly red-shifted (Table 3.4). The shortening of the ON···Oz interatomic distances together with the enhancement of the perturbations of the adsorbed NO+ species observed along with the increase of the ionic radius of the exchangeable alkaline ions are due to the increase of the basicity of the framework oxygen ions. Thus, one can conclude that location of the nitrosonium cation at the SIII site of zeolite X provides a very effective binding of NO+ to the zeolite. Moreover, the binding enhances simultaneously with the increase of the ionic radius of the exchangeable cations.

On the other hand, such a localization of NO+ results in a strong displacement of the original alkaline cation located at this site towards the distant SII cation. This results in a strong repulsion between these likely charged species. At the same time, because of such geometry changes, the NO3

– ion coordinates to only two alkaline cations, although as it has been shown above, the most stable structures are formed when a three-fold coordination is realized. Thus, in spite of the very effective interaction of the nitrosonium cation with the basic framework oxygens, the overall energetics of structures I are unfavorable due to the unfavorable distribution of the extra-framework cations and the resulting lower efficiency of the stabilization of NO3

–.

Structures II and III are the most stable among the N2O4 isomers confined in the zeolites (Table 3.4). The nitro group in these complexes coordinates to three extra-framework alkaline cations. Two cations (the SII and the initially SIII neighboring to the NO+ ion) coordinate each to two oxygens atoms, while the SIII cation distant to the NO+ is bound to only one oxygen atom of the adsorbed NO3

–. Such coordination provides an efficient stabilization of the negatively charged nitro-group. NO+ is stabilized at the SIII’ sites next (II, Figures 3.4-3.6) or next-nearest (III, Figures 3.4-3.6) to the six-membered ring (SII site) of the faujasite supercage. To minimize repulsion of the like charges, the original SIII alkaline cation migrates from the initial cation site to the SIII’ position opposite to that occupied by the NO+. This, in turn, allows coordination of this alkaline cation to the nitro group providing additional substantial stabilization of the system.

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Figure  3.4.  Optimized  structures  and  selected  geometrical  parameters  (Å)  of  the  adsorption 

complexes of the NO+∙∙∙NO3– product of N2O4 disproportionation on NaX. 

 

 

 

Figure  3.5.  Optimized  structures  and  selected  geometrical  parameters  (Å)  of  the  adsorption 

complexes of the NO+∙∙∙NO3– product of N2O4 disproportionation on KX. 

 

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 Figure  3.6. Optimized  structures  and  selected  geometrical  parameters  (Å)  of  the  adsorption 

complexes of the NO+∙∙∙NO3– product of N2O4 disproportionation on RbX. 

Table 3.4. Adsorption energies (∆E, kJ/mol) and selected vibrational frequencies (cm–1) of the 

NO+∙∙∙NO3– product of N2O4 disproportionation confined in cation‐exchanged X zeolite.  

  NaX  KX  RbX   INa  IINa  IIINa  IK  IIK  IIIK  IRb  IIRb  IIIRb ∆E  +18  –69  –65  +9  –91  –90  –9  –82  –78 Frequencies:                   ν(N–O)NOδ+  1955  2017  2023  1881  1991  1991  1863  1972  1991 ν(NO3)assym’  1402  1391  1386  1368  1370  1358  1363  1359  1353 ν(NO3) assym’’  1314  1351  1351  1328  1336  1341  1322  1329  1329 ν(NO3) sym  1040  1044  1047  1039  1039  1038  1038  1039  1036  

The most stable structures are formed on KX zeolite (IIK and IIIK, Figure 3.5). The corresponding adsorption complexes on RbX zeolite (IIRb and IIIRb, Figure 3.6) are only slightly (by ~10 kJ/mol) less stable (Table 3.4). This is in agreement with the lower Lewis acidity of the larger cations. However, in the case of NaX, these structures show lower stability. This is, most likely, due to the fact that stabilization of the NO3

– moiety with three Na+ requires much larger displacements of the small sodium cations from their equilibrium positions as compared to the case of larger K+ or Rb+ ions. Moreover, such displacements in the case of NaX zeolite require more energy due to the stronger binding of the harder sodium ions to the zeolite framework. Both these effects destabilize the system.

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 Figure 3.7. Comparison of the experimental [12] and calculated ν(N–O)NOδ+ (a) and ν(NO3)assym (b)  stretching  frequencies  (cm–1)  for  the  stable  adsorption  complexes  on  alkali‐exchanged  X zeolites. Arrows in (b) correspond to the frequency range where the asymmetric N‐O stretching vibrations of NO3

‐ adsorbed on Na, K and Rb exchanged X zeolites were observed [12]. 

Structures IIRb and IIIRb (Figure 3.6) show relative energies (–82 and –78 kJ/mol, respectively (Table 3.4)) very similar to those computed for the structures discussed above (Figure 3.3 (e) and (f)) resulted from O2NONO adsorption on the SII and SIII cations of RbX (–78 and –80 kJ/mol, respectively (Table 3.3)). Indeed, in spite of the different coordination of NO+ to zeolitic oxygens and some slight differences in interatomic distances, these systems show one general feature: bidentate coordination of the NO3

– ion to Rb+ at SII and SIII sites along with additional monodentate coordination to another SIII cation. In the case of KX zeolite, such stable coordination environment of the nitro anion is realized only for structures IIK and IIIK (Figure 3.5), whereas the complexes resulting from the adsorption of O2NONO isomer (Figure 3.3 (c) and (d)) involve less interatomic contacts between the exchangeable alkaline ions and the NO3

– moiety. This results in the less effective stabilization of the nitro group and, hence, in a lower (by ~15 kJ/mol, Tables 3.3 and 3.4) stability of the latter structures.

The calculated N–O stretching frequencies for complexes II and III (Table 3.4) show trends similar to those discussed in Section 3.2. ν(N–O)NOδ+ stretching frequencies correlate well with the basicity of the cation-exchanged zeolites. The increasing basicity of the zeolite framework (NaX < KX < RbX) is accompanied with the increase of the low-frequency shift of the ν(N–O)NOδ+ stretch. The same tendency is observed for the calculated N–O stretching bands of the NO3

– anion. Although because of the deficiencies of the method used, the calculated frequencies are somewhat different from those observed experimentally [12], the general trend is reproduced well (Figure 3.7).

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3.4. Discussion

Gas-phase N2O4 disproportionation is an unfavorable process. The O2NONO isomer and the NO+···NO3

– ion pair are less stable by 45 and by 205 kJ/mol, respectively, than the D2h symmetric isomer [13]. The DFT calculations presented above show that formation of even the least stable structures within the zeolite matrix is significantly preferred over the gas phase isomerization. Indeed, N2O4 isomerization results in formation of rather polar structures, which are stabilized in the cage of the cation-exchanged zeolite. The microporous matrix shows characteristics of polar solvents, which facilitates the charge separation.

Molecular adsorption of a nonpolar N2O4 on the alkali-exchanged X zeolites is relatively weak. Bonding within the adsorption complexes can be well described as induced polarization of the adsorbed molecules in the field of the exchangeable cations. Strength of such interactions depends on the size of the alkali ion. The smaller the cation is, the stronger its polarizing ability and Lewis acidity are. As a result one expects a decrease of the N2O4 adsorption energies simultaneously with the increase of the ionic radius of the zeolitic cations. However the computational results show the opposite trend (Table 3.2). Upon adsorption, a high density of extra-framework cations in the zeolite X leads to interaction of N2O4 with several exchangeable ions. In the case of NaX zeolite the smaller sodium ions are located too far to form significant interatomic contacts with the adsorbed molecule and can only slightly polarize it. On the other hand, when the ionic radius of the zeolitic cations increases, numerous addition contacts are formed between the N2O4 molecule confined in the supercage of zeolite X and the accessible extra-framework cations (Figure 3.2). Despite the weaker individual contacts, the overall interaction energy increases in the row NaX < KX < RbX. These are the features analogous to the molecular recognition characteristics formulated for supramolecular systems [31].

At the first stages of the disproportionation reaction the nonpolar symmetric molecule rearranges to form an O2NONO isomer that involves a rather ionic bond between the O2NO and the NO moieties. Already in this structure the former fragment bears an excessive negative charge, while the latter fragment is charged positively [30]. Therefore, interaction of each fragment with the corresponding zeolitic counterion facilitates cleavage of this contact ion pair. The supercage of the cation-exchanged zeolite X provides numerous positively charged sites, i.e. alkali cations, which can accommodate the O2NO moiety, while the electron-deficient NO fragment can be well stabilized via interaction with the basic framework oxygen atoms. The stronger such interactions are, the easier the cleavage of the contact ion pair is.

In the case of NaX, the framework oxygen atoms have the lowest basicity and, hence, bind NOδ+ weaker. At the same time due to the smaller size, the supercage sodium cations cannot provide the appropriate configuration to stabilize the negatively charged NO3

δ– fragment. On the other hand, the larger potassium and rubidium cations shape the optimum configuration of the faujasite supercage providing a more efficient stabilization of NO3

δ–.

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This along with the enhanced basicity of the framework O atoms facilitates the N2O4 disproportionation. As a result, a rather unstable adsorption complex involving a well distinguished O2NONO isomer is found only in the case of NaX zeolite, while the contact ion pair confined within KX and RbX is completely cleaved resulting in a more stable isomer (Figure 3.3).

For the structures that result from adsorption of the O2NONO isomer a trend in the adsorption energies similar to that for the N2O4 adsorption is found. The interaction energy increases along with the increase of the size of the zeolitic cations. The lower adsorption energies calculated for the case of NaX zeolite are due to the factors described above. However the weaker adsorption on KX as compared to that on RbX is due to a less favorable coordination of the nitro-group to the exchangeable cations (Table 3.3, Figure 3.3). Indeed, further structural rearrangement of these adsorption complexes in KX leads to formation of more stable (Table 3.4) structures IIK or IIIK involving distant NO3

– and NO+ ions (Figure 3.5), despite the lower framework basicity of potassium-exchanged zeolite X. In this case the extra-framework rubidium and potassium ions provide similar configurations stabilizing the nitrate ion, because of their close ionic radii. Thus, the strength of the individual intermolecular contact becomes important.

This shows the dominant effect of the exchangeable cations on the reactivity of the zeolite toward N2O4 decomposition. The role of the framework basicity is only indirect. The red shift of the stretching frequencies of NO+ bound to the zeolite framework correlate well with the strength of the corresponding interaction and, accordingly, with the basicity of the zeolite lattice. This effect has been experimentally reported by Marie et al [12]. The perturbations of the N–O stretching frequencies of the adsorbed NO3

– ions reflect the strength of the interaction of the adsorbed nitro anion with the exchangeable cations. One notes a deviation between the values of ν(N–O)NOδ+ reported in Ref. [12] and those presented above. The systematic error due to deficiencies of the method and anharmonicity effects can be suppressed by using a multiplication coefficient. The calculated ν(N–O)NOδ+ and ν(NO3)assym reasonably well agree with those observed experimentally (Figure 3.7 (a) and (b), respectively). However the calculated values do not correspond precisely to those reported in Ref. [12]. It illustrates that the theoretical model of a completely ion-exchanged zeolite X probably does not perfectly match with the experimentally used partially exchanged samples. One also notes that the experimental IR spectra of N2O4 adsorbed on cationic X zeolites show rather broad bands corresponding to the N–O stretching vibrations. This coheres well with the current theoretical finding.

One notes that the strongest perturbations of NO+, i.e the strongest elongation of the N–O bond and the strongest red shift of ν(N–O)NOδ+ band, are observed in the cases of the least stable structures INa, IK and IRb. The low stability (Table 3.4) of those is due to an unfavorable coordination of the NO3

– ion to the exchangeable cations (Figures 3.4-3.6). Moreover, as could be seen from the data presented in Tables 3.3-3.4 and Figures 3.3-3.6, although different coordination of the nitrosonium cation to the zeolitic framework can

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apparently affect its vibrational properties, the variation of the relative energy depending on this factor is very low. The stability of the adsorption complexes is mainly determined by the interactions of the negatively charged nitro group with the exchangeable cations.

3.5. Conclusions

Molecular adsorption and disproportionation of dinitrogen teraoxide on completely exchanged NaX, KX, and RbX zeolites are investigated using the periodic plane-wave DFT method. The molecular N2O4 adsorption is a precursor for the subsequent isomerization and dissociation leading to formation of spatially separated NO3

–···NO+ ion pair. The calculated N2O4 adsorption energies increase along with the increase of the ionic radii of the extra-framework zeolitic cations (NaX < KX < RbX). The increase of the size of the exchangeable cations reduces the space in the faujasite supercage and makes possible formation of multiple interatomic contacts between the adsorbed molecule and the extra-framework cations located in the supercage of low-silica zeolite X. This results in an increase of the overall adsorbate — adsorbent interaction, in spite of the weaker individual contacts. A similar effect is also observed for the adsorption of other N2O4 isomers.

N2O4 disproportionation in cation-exchanged zeolite X results in formation of NO3–

anion bound to exchangeable cations and NO+ attached to Lewis basic lattice oxygen anions. The computational results presented show that the stability of such structures is mainly controlled by the interactions between the negatively charged nitro group and the extra-framework alkaline cations. The role of the interactions between the nitrosonium cation and the basic sites of the zeolites is only minor. The N2O4 disproportionation over alkali-exchanged low-silica faujasites is mainly due to the cooperative effect of the extra-framework cations as well as due to the confinement effect of the zeolite matrix. This can be considered as the analogue of molecular recognition phenomena in organic supramolecular systems. 

References 

1 Barthomeuf, D. Catal. Rev. 1996, 38, 521. 2 Barthomeuf, D. J. Phys. Chem. B 2005, 109, 2047. 3 Corma, A. J. Catal. 2003, 216, 298. 4 Davis, R. J. Catal. 2003, 216, 396. 5 Lefferts, L.; Seshan, K.; Mojet, B.; van Ommen, J. Catal. Today 2005, 100, 63.  6 Blatter, F.; Sun, H.; Vasenkov, S.; Frei, H. Catal. Today 1998, 41, 297. 7 Blatter, F.; Frei, H. J. Am. Chem. Soc. 1993, 115, 7501. 8 Lahr, D.G.; Li, J.; Davis, R.J. J. Am. Chem. Soc. 2007, 129, 3421. 9 Chao, C.C.; Lunsford, J. J. Am. Chem. Soc. 1971, 93, 71. 10 Bentrup, U.; Brückner, A.; Richter, M.; Fricke, R. Appl. Catal. B 2001, 32, 229. 11 Li, M.; Yeom, Y.; Weitz, E.; Sachtler, W.M.H. J. Catal. 2005, 235, 201. 12 Marie, O.; Malicki, N.; Pommier, C.; Massiani, P.; Vos, A.; Schoonheydt, R.; Geerlings, P.; Henriques, C.; 

Thibault‐Starzyk, F. Chem. Commun. 2005, 1049. 13 McKee, M.L. J. Am. Chem. Soc. 1995, 117, 1629. 

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14 Rabo,  J.A.;  Gajda,  G.J.  Acid  Function  in  Zeolites:  Recent  Progress.  In  Guidelines  for  Mastering  the Properties of molecular Sieves: Relationship between the Physicochemical Properties of Zeolitic Systems and Their Low Dimensionality; Barthomeuf, D.; Deurane, E.G.; Hölderich, W., Ed.; NATO ASI Series B221: Plenum Press, New York, 1990; pp 273‐297. 

15 Rabo, J.A.; Angell, C.L.; Kasai, P.H.; Schomaker, V. Discuss. Faraday Soc. 1966, 41, 328. 16 Rabo, J.A. Catal. Rev. 1981, 23, 1981. 17 Vos,  A.M.;  Mignon,  P.;  Geerlings,  P.;  Thibault‐Starzyk,  F.;  Schoonheydt,  R.A.  Microporous  and 

Mesoporous Mater. 2006, 90, 370. 18 Kresse, G.; Furthmüller, J. Comp. Mat. Sci. 1996, 6, 15. 19 Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. 20 Perdew, J.P.; Chevary, J.A.; Vosko, S.H.; Jackson, K.A.; Pederson, M.R.; Singh, D.J.; Fiolhais, C. Phys. Rev. B 

1992, 46, 6671. 21 Blöchl, P.E. Phys. Rev. B 1994, 50, 17953. 22 Kresse, G.; Joubert, J. Phys. Rev. B 1999, 59, 1758. 23 Monkhorst, H.J.; Pack, J.D. Phys. Rev. B 1976, 13, 5188. 24 Hriljac, J.A.; Eddy, M.M.; Cheetham, A.K.; Donohue, J.A.; Ray, G.J. J. Solid State Chem. 1993, 106, 66. 25 Olson, D.H. Zeolites 1995, 15, 439. 26 Zhu, L.; Seff, K. J. Phys. Chem. B 2000, 104, 8946. 27 Lim, W.T.; Choi, S.Y.; Choi, J.H.; Kim, Y.H.; Heo, N.H.; Seff, K. Microporous and Mesoporous Mater. 2006, 

93, 234. 28 Löwenstein, W. Am. Mineral. 1954, 39, 92. 29 Lide, D.R. (Ed.), CRC Handbook of Chemistry and Physics 80th ed., CRC Press, New York, 1999. 30 Wang, X., Qin, Q.‐Z.; Fan, K. J. Molec. Struct. (Theochem) 1998, 432, 55. 31 Lehn, J.‐M. Angew. Chem. Int. Ed. (Engl.) 1988, 27, 89. 

 

 

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CHAPTER 4  

THE INTERPLAY OF BONDING AND GEOMETRY OF THE ADSORPTION COMPLEXES OF LIGHT ALKANES WITHIN CATIONIC FAUJASITES  

 

 

 

A FT-IR spectroscopic study of methane, ethane, and propane adsorption on magnesium and calcium forms of zeolite Y reveals different vibrational properties of the adsorbed molecules depending on the exchanged cation. This is attributed to different adsorption conformations of the hydrocarbons. Two-fold η2 coordination of light alkanes is realized for MgY, whereas in the case of CaY zeolite quite different adsorption modes are found, which involve more C–H bonds in the interaction with the extra-framework cation. The topological analysis of the electron density distribution function of the adsorption complexes in the framework of quantum theory of atoms in molecules shows that when a hydrocarbon coordinates to the exchangeable Mg2+ ions, van der Waals bonds between H atoms of the alkane and basic lattice oxygens of the zeolite significantly contribute to the overall adsorption energy. In the case of CaY zeolite such interactions are less important. It is found that, due to the much smaller ionic radius of the Mg2+ ion as compared to that of the Ca2+, the former ions are significantly shielded with the surrounding oxygens ions of the zeolitic cation site. This results in a small electrostatic contribution to the stabilization of the adsorbed molecules. In contrast, for CaY zeolite the stabilization of alkanes in the electrostatic field of the partially shielded Ca2+ cation significantly contributes to the adsorption energy. The preferred conformation of the adsorbed alkanes is controlled by the bonding within the adsorption complexes that, in turn, strongly depends on the size and location of the cations in the zeolite cavity.

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4.1. Introduction

The initial coordination of the reagents to the exchangeable cations is important both for gas separation and for their chemical activation in zeolites. It has been shown in Chapter 2 that in the case of photo-initiated oxidation of branched alkenes the main role of the cations is to prearrange reactants in the zeolite cavity via adsorption so that they can be activated in the subsequent reaction steps. On the other hand, for the thermal oxidation of propane the lower activity of the magnesium exchanged Y zeolite as compared to that of CaY has been attributed by Xu et al. [1] to different adsorption properties of the corresponding ions stabilized at the zeolitic cation sites. Obviously, depending on the adsorption fashion and the bonding realized within the resulting complex the properties and the reactivity of the adsorbed molecules may vary significantly.

However, there is only limited information concerning activation of alkane molecules loaded into the zeolite. Most of the studies reported [2-8] deal mainly with methane adsorption on cationic or hydrogen forms of zeolites, because of the ease of interpretation of the spectroscopic data due to the high symmetry and the small size of the molecule. Upon adsorption, the C–H breathing (ν1) mode of CH4 is no longer symmetry-forbidden and therefore becomes active in the IR spectrum. The values of the red shift of the corresponding band have been correlated to the polarizing ability of the exchangeable cations [4,8].

The adsorption of heavier alkanes by zeolites has been studied to a much lesser extent, because it is very difficult to separate the IR spectrum of the hydrocarbon-cation adsorption complexes from those formed due to the physical adsorption of the hydrocarbon on zeolite walls [4]. Ethane adsorption on ZnZSM-5 [9] or CuZSM-5 [10] is very strong and results in formation of two-fold coordinated (η2) C2H6 to the exchangeable cations. Such a strong interaction results in a significant perturbation of the adsorbed molecules that influences the respective infrared spectrum. The vibration corresponding to the fully symmetrical C–H stretching (ν1) of the gaseous C2H6 has been found to be the most sensitive to the nature of the adsorption center. Recently, it has been shown in Ref. [11] by means of a combined DRIFTs and theoretical study that, depending on the nature of the cation in the zeoltic adsorption site different adsorption geometries of ethane can be realized, and hence different vibrational properties of the adsorbed alkane are observed. In addition, IR absorption studies have shown that propane also adsorbs on alkaline-earth exchanged Y zeolite and polarizes at the supercage cation sites [12,13]. However, for light alkanes in cationic zeolites, the exact adsorption geometries as well as their chemical bonding features are not well understood yet.

In this chapter a combined spectroscopic and computational study of methane, ethane, and propane adsorption on magnesium- and calcium-exchanged zeolites Y is reported. In addition a detailed analysis of the nature of chemical bonding of light alkanes to zeolitic cations is provided. The Atoms in Molecules theory proposed by Bader [14] is applied. Using this approach one can reveal not only covalent interactions but also van der Waals

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Figure  4.1.  Structure  of  the  faujasite 

supercage  and  a  MZ  (M  is  Mg  or  Ca) 

cluster model used in the computations. 

and hydrogen bonds between the interacting species and determine their relative contribution to the total adsorption energy.

4.2. Experimental and computational details

The magnesium and calcium exchanged Y zeolites were prepared by triple wet ion exchange of the sodium from of Y zeolite (Akzo Nobel, Si/Al = 2.5) with a 0.1M aqueous solution of, respectively, magnesium chloride or calcium nitrate (Merck) for 20h at 363K under stirring. After the third ion-exchange the material was washed three times with distilled water, filtered, and dried at 363K overnight. The chemical composition of the resulting zeolites was analyzed by X-ray fluorescence. The M2+/Al ratio was 0.34 for MgY and 0.45 for CaY zeolite. Powder X-ray diffraction showed no collapse of the structures.

FT-IR measurements were performed for the self-supported wafers pressed from the zeolite powder (30 mg) using a Bruker Vector 22 FT-IR spectrometer with a mercury cadmium telluride detector. A miniature cell, equipped with NaCl transparent windows, which can be evacuated to pressures below 10-7 mbar, was used for the activation of zeolites and for the spectroscopic studies. The samples were activated in a vacuum (<10–7 mbar) at 773 K (ramp 10 K/min) for 2 hours, subsequently cooled to 473 K (dwell 10h), and cooled to room temperature. The spectra of adsorbed alkanes (CH4, C2H6, and C3H8) were recorded at room temperature and at different equilibrium pressures. The resulting FT-IR spectra were corrected both for absorption by the activated zeolite and for the gaseous hydrocarbon. The experimental part was done in collaboration with the group of Prof.dr.ir. Leon Lefferts (University of Twente) [15].

The quantum-chemical calculations were carried out within a density functional theory (DFT) or a Hartree-Fock method followed by a second-order Møller-Plesset correlation energy correction involving all electrons of the model (MP2(FULL)) using the Gaussian 03 program [16]. In the former case, the hybrid B3LYP [17] functional was used. Earlier, the hybrid B3LYP method was reported to provide excellent descriptions of various molecular properties and particularly of geometries and vibrational properties of different compounds [18]. The calculations were performed using the standard full-electron 6-31G(d,p) basis set for all atoms of the cluster modeling the adsorption site and of the adsorbed hydrocarbon molecule. The full-electron basis set as well as the full-electron MP2 calculations were employed to calculate the properties of the electron density distribution functions more accurately.

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The modeling of light alkane adsorption to cationic sites of faujasite was performed using a MAl2Si4O6H12, where M is Mg or Ca, cluster model representing a six-member ring (SII extra-framework position) from the wall of the faujasite supercage containing an extra-framework cation (Figure 4.1). Below, these structures are designated as MgZ and CaZ, respectively. To stabilize the exchangeable bivalent cation the zeolitic ring contained two Al atoms. To saturate dangling bonds at the border of these clusters hydrogen atoms were used. By analogy with Refs. [10,11], special restrictions were imposed on the procedure of optimization of their positions. At first, the structure of the initial zeolite cluster was constrained according to the X-ray diffraction data [19]. Only the lengths of the Si–H and Al–H bonds were then optimized, while the positions of the other atoms as well as the directions of the chemical bonds were fixed according to the crystallographic data. The extra-framework cation was allowed to move freely. The obtained positions of boundary atoms were fixed in all subsequent calculations, while those of the remaining atoms of the cluster were optimized. The optimization of the positions of boundary H-atoms as well as the full subsequent optimization of the cluster and the adsorption complexes was carried out at both DFT and MP2 (FULL) levels. The vibrational analysis of the resulting structures was done only at the DFT level of theory. Analysis of the topology of electron density distribution ρ(r) functions was carried out using the AIM2000 program [20].

4.3. Results and Discussion

4.3.1. FT-IR spectroscopy Figure 4.2 (a) and (b) show the difference FT-IR spectra of methane adsorbed at room

temperature and at different equilibrium pressures on MgY and CaY zeolites, respectively. The spectra were corrected for the presence of gas-phase methane. Because of the imperfection of the correction a sharp minimum at 3016 cm–1(the Q-branch) and the P- and R-branches at 2950-3100 cm–1 of the vibration-rotational manifold for the ν3 C–H stretching vibration of gaseous methane are observed in both cases. In addition, the bands from adsorbed methane at 2864 cm–1 for MgY and 2867 cm–1 for CaY are clearly visible in the spectrum. By analogy with previous studies [2-8], these bands are attributed to the ν1 symmetric C–H stretching vibration. This vibration is IR inactive for the gaseous methane, and the appearance of the corresponding adsorption bands is due to perturbation of CH4 molecules with the adsorption sites. A shoulder at 2840 cm–1 and a band at 2898 cm–1 in the case of CaY are most likely due to either ν1 vibration of methane adsorbed to some weaker interacting sites or combination vibrations of the adsorbed CH4 [6,7]. In the case of MgY, one can see an intense band at ~3000 cm–1 corresponding to the ν3 asymmetric C–H stretching vibration of the adsorbed CH4, which overlaps strongly with the residual bands from the gaseous methane. Surprisingly, for the CaY zeolite, no band of remarkable relative intensity, which could be attributed to such vibration, is observed. The positions of the bands attributed to the ν1 C–H stretch of the adsorbed methane are very close both for MgY and for CaY zeolite. There is only a slight difference in the peak position. The main difference between spectra presented in Figure 4.2 parts (a) and (b) is in the band

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Figure  4.2. Difference  FT‐IR  spectra  of  CH4  adsorbed  at  room  temperature  and  at  different 

equilibrium pressures by (a) MgY and (b) CaY zeolites. 

 Figure  4.3.  Schematic  representation  of 

possible  conformations of  light alkanes  to 

the exchanged cation (M2+  is Mg2+ or Ca2+; 

R is H, CH3,or C2H5).

intensities. Indeed, the IR spectra of methane adsorbed on CaY are about 1 order of magnitude higher in intensity as compared to those for MgY.

Two following η2 and η3 alternative adsorption modes can be proposed for methane coordinated to the zeolitic cations (Figure 4.3 (a) and (b), respectively). The two-fold (η2) coordination leads to a decrease of the symmetry of adsorbed molecule from Td to C2v and, hence, all four C–H stretching vibrations become IR active. In contrast, when CH4 coordinates to the cation with three H atoms, the symmetry changes to C3v and the resulting IR spectrum contains only three C–H stretching bands, one of which corresponds to doubly degenerate vibration (e).

Corrected for gaseous alkane FT-IR spectra of ethane adsorbed on MgY and CaY zeolites are shown in Figure 4.4 parts (a) and (b), respectively. The spectra in both cases strongly differ in the position of the absorption bands, in their relative intensities, and even in the number of observed C–H stretching bands. In the former case the spectrum consists of at least five rather narrow bands with maxima at 2995, 2960, 2902, 2858, and 2814 cm–1 (Figure 4.4 (a)), whereas only four less resolved absorption bands at 2988, 2961, 2903, and 2848 cm–1 are detected for ethane adsorbed on CaY zeolite (Figure 4.4 (b)). Note that the gas-phase ethane (D3d) gives only two absorption bands with the maxima at 2994 (ν7 (eu)) and 2954 cm–1 (ν5 (a2u)) [21]. The other two vibrations are active only in the Raman spectrum, occurring at 2963 (ν10 (eg)) and 2899 cm–1 (ν1 (a1g)). The larger number of IR bands observed in the case of MgY zeolite can be attributed to a stronger symmetry decrease as compared to that for ethane adsorbed on CaY, which leads not only to activation of the symmetry-forbidden

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Figure 4.4. Difference  FT‐IR  spectra of C2H6 adsorbed at  room  temperature and at different 

equilibrium pressures by (a) MgY and (b) CaY zeolites. 

vibrations (ν10 and ν1) but also to the removal of degeneracy of the ν7 (eu) and ν10 (eg) C–H stretching modes. On the other hand, in the case of ethane adsorption on CaY, the IR spectra are not well resolved. All of the bands detected are rather broad and overlap each other. Thus, one cannot exclude that these spectra represent a superposition of different adsorption modes of ethane to the exchangeable Ca2+ cations.

Similar to the above-discussed methane adsorption, it is possible to propose η2 and η3 alternative C2H6 adsorption modes (Figure 4.3 (a) and (b), respectively). The η3 coordination resulting in the C3v symmetry of the C2H6 moiety leads to the IR spectrum containing four C–H stretching bands, whereas for the η2 coordination (Cs symmetry), all six C–H stretching vibration are nondegenerate and IR active. In this connection, it appears that the η2 adsorption mode is realized in the case of MgY zeolite, and that the η3 coordination of ethane is more likely a candidate for the adsorption complexes on CaY.

The low-frequency band in the spectra presented in Figure 4.4 parts (a) and (b) is by about 40 cm–1 stronger red-shifted for ethane adsorbed on MgY as compared to that for CaY zeolite. This reflects a stronger perturbation of the adsorbed molecules upon the interaction with the exchangeable magnesium ions. This band has been attributed previously to a C–H vibration analogous to the ν1 (a1g) [4,9-11]. The high-frequency bands are perturbed much weaker. Moreover, their positions are very close for both zeolites. Thus, the corresponding C–H stretching vibrations involve mainly displacements of the H atoms, which do not interact directly with the cation. A more detailed assignment of the detected absorption bands is provided below based on the results of DFT calculations.

Upon propane adsorption, the IR spectrum of C–H stretching vibrations shows six bands at 2978, 2951, 2906, 2887, 2864, and 2792 cm–1 for MgY (Figure 4.5 (a)) and only four bands with maxima at 2974, 2954, 2889 and 2839 cm–1 for CaY (Figure 4.5 (b)). Because of the C2v symmetry of the free C3H8 molecule, all of its C–H stretching vibrations are, in principle, IR active except the a2 mode (stretching nonsymmetrical vibration of CH3 groups). Although for adsorbed propane all of the vibrations should be IR active, most of

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Figure 4.5. Difference  FT‐IR  spectra of C3H8 adsorbed at  room  temperature and at different 

equilibrium pressures by (a) MgY and (b) CaY zeolites. 

them have similar or close frequencies and cannot be resolved. When the hydrocarbon coordinates to a cation, the mostly perturbed vibrations are those involving the atoms directly interacting with the adsorption site. Therefore, depending on the conformation, different IR bands become better resolved, influencing the number of the absorption bands in the spectrum. Thus, by analogue with the case of ethane adsorption, one can propose that such a striking difference in the IR spectra presented in Figure 4.5 (a) and (b) is due to different conformations of propane realized upon adsorption to magnesium- and calcium-exchanged faujasite.

In contrast to the case of ethane adsorption where the low-frequency band dominates the IR spectrum independently of the zeolite (Figure 4.4), in the case of C3H8 adsorption the intensity of the low-frequency C–H stretching band is comparable to the high-frequency one for CaY (Figure 4.5 (a)) and much lower for MgY zeolite (Figure 4.5 (b)). This reflects the strong contribution of the vibrations of the nonperturbed parts of the adsorbed C3H8 to the IR spectrum.

Adsorption of propane to a single cation is now considered. For this case four alternative modes of coordination of C3H8 to the exchanged cation can be proposed (Figure 4.3). The η3 mode corresponds to the alkane coordinated through three H atoms of one of the methyl groups of propane. The η2-C1 and η2-C2 are those where only two H atoms from, respectively, methyl and methylene group are coordinated to the cation. The η3 and η2-C1 adsorption modes result in Cs symmetry of the C3H8 moiety, and hence, all of the C–H stretching vibrations become active. In the case of η2-C2 adsorption mode the symmetry remains C2v and the a2 vibration is symmetry forbidden. Another adsorption mode corresponding to C2v symmetry is the η4 where four H atoms (two hydrogens from each methyl group) are involved in the interaction (Figure 4.3 (c)).

All of the spectra discussed so far show one striking feature. The overall intensity of the C–H stretching bands of alkanes adsorbed to MgY zeolite is significantly lower than that in the case of CaY. On the other hand, the polarizing ability as well as the Lewis acidity of

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Figure 4.6. B3LYP/6‐31G(d,p) and MP2(FULL)/6‐31G(d,p) (in parentheses)  predicted structures 

and C–H bond  lengths  (Å) of  (a) η2‐CH4/MgZ, (b) η2‐C2H6/MgZ, (c) η2‐C1‐C3H8/MgZ, (d) η2‐C2‐

C3H8/MgZ,  (e)  η3‐CH4/CaZ,  (f)  η3‐C2H6/CaZ,  (g) η

2‐C2H6/CaZ,  (h)  η3‐C3H8/CaZ,  (i)  η

2‐C3H8/CaZ, 

and (j) η4‐C3H8/CaZ. 

the bare Mg2+ ions is significantly higher than that of Ca2+, because of the higher charge-to-radius ratio. Thus, one expects a more effective polarization of the alkane molecule by the Mg2+, which, in turn, leads to higher interaction energies and higher alkane loading. In addition, the stronger polarization of the adsorbed molecules leads to an increase of their dipole moment. Both of these effects would result in an increase of the intensity of the corresponding IR bands. The spectroscopic results presented above show the opposite

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trend. A possible explanation could be that the occupancy of the accessible cationic sites in the supercage of faujasite is lower in MgY zeolite. However, on the basis of the experimental results one does not expect more than a twice lower occupancy of the supercage cation sites for the MgY as compared to that for the CaY zeolite, and, obviously, this supposition cannot entirely explain the 1 order of magnitude lower intensities of the C–H stretching vibrations of alkanes loaded onto magnesium-exchanged zeolite.

In summary, adsorption of light alkanes to MgY and CaY zeolites results in rather different IR spectra in the region of C–H stretching vibrations. This is, most likely, due to different adsorption modes realized depending on the exchanged cation. In addition, the unexpectedly low intensity of the C–H stretching bands is observed for alkanes loaded onto MgY. To clarify the observed peculiarities, the systems discussed were investigated by means of quantum-chemical calculations.

4.3.2. Quantum-chemical modeling

Calculated results for the structures of the adsorption complexes of light alkanes on MgZ and CaZ cluster models are shown in Figure 4.6. One notes that, although the optimized geometry parameters (interatomic distances) obtained at the B3LYP/6-31G(d,p) level (Figure 4.6) differ from those computed within the MP2(FULL)/6-31G(d,p) method (Figure 4.6, values in parentheses), for both cases the shape of the optimized adsorption complexes is very similar. The geometry of the MgZ and CaZ does not considerably alter upon adsorption of the alkane. The interatomic distances between the Mg or Ca ion and the lattice oxygen atoms increase by less than 0.01 Å.

Light alkanes coordinate to the MgZ cluster with two hydrogen atoms independently of the hydrocarbon chain length resulting in formation of the η2-CH4/MgZ, η2-C2H6/MgZ, η2-C1-C3H8/MgZ, and η2-C2-C3H8/MgZ complexes (Figure 4.6 (a-d)). The local minima for other adsorption modes such as η3- for all of the discussed molecules or η4- for propane are not found. The geometry optimization of such initial structures leads to formation of the complexes presented. The C–H bonds directly interacting with the exchanged cation are slightly (by ~0.006 Å for DFT and by ~0.008 Å for MP2 optimized complexes) elongated as compared to the values computed for the gas-phase alkanes, whereas the rest of the C–H bonds remain intact.

Adsorption on the CaZ site involves more C–H bonds in the interaction with the cation. The η3-conformation is found for all molecules. The corresponding η3-CH4/CaZ, η3-C2H6/CaZ, and η3-C3H8/CaZ are shown in Figure 4.6 parts (e), (f), and (h), respectively. In the two former complexes, the alkane molecule can freely rotate around the axis connecting Ca and C atoms. The calculated rotational barrier does not exceed 0.5 kJ/mol for both the η3-CH4/CaZ and the η3-C2H6/CaZ complexes. An increase of the hydrocarbon chain length results in additional possible adsorption modes. For ethane adsorption also the η2-C2H6/CaZ complex (Figure 4.6 (g)) is realized. Unlike the corresponding η2-C2H6/MgZ structure, an additional interaction between one of the C–H bonds of the β-methyl group

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Table  4.1.  Adsorption  energies  (kJ/mol) 

for  the  complexes  of MgZ  and  CaZ with 

light  alkanes  calculated  at  different 

computational level. 

  Eads,DFT  Eads,MP2 η2‐CH4/MgZ  18.2  35.5 η2‐C2H6/MgZ  19.8  43.6 η2‐C1‐C3H8/MgZ  21.0  49.3  η2‐C2‐C3H8/MgZ  21.7  51.5  

η3‐CH4/CaZ  22.7  31.3 

η3‐C2H6/CaZ  24.8  36.8 η2‐C2H6/CaZ  23.8  39.6 η3‐C3H8/CaZ  34.2  50.1  η2‐C3H8/CaZ  24.5  44.1 η4‐C3H8/CaZ  35.1  61.8 

and the adsorption site occurs. The η2-C3H8/CaZ complex (Figure 4.6 (i)) and the η4-C3H8/CaZ (Figure 4.6 (j) are found for propane adsorption. The other adsorption modes similar to those found for the MgZ are not realized. Geometry optimization of the corresponding models for CaZ excluded them.

Upon alkane adsorption on the CaZ cluster, also only C–H bonds directly interacting with the adsorption site elongate (by ~0.004-0.005 Å for DFT and by ~0.005-0.006 Å for MP2 optimized complexes; Figure 4.6). The slightly smaller elongation in comparison with that found in the case of MgZ is attributed to a lower Lewis acidity of Ca2+ ion and, hence, to a less effective charge donation from the hydrocarbon to the extra-framework cation.

The calculated adsorption energies are listed in Table 4.1. One observes that the DFT method gives significantly lower values for the adsorption energies (Eads,DFT) as compared to those obtained (Eads,MP2) within MP2. This is due to an underestimation of the van der Waals interactions. Moreover, the values of Eads,DFT show a slightly stronger interaction of alkanes with the CaZ cluster, whereas the opposite trend is observed in the MP2 adsorption energies (Eads,MP2). This suggests the stronger contribution of the dispersive bonds in the adsorption complexes formed with the MgZ model.

Different geometries of the C2H6/CaZ and C3H8/MgZ complexes exhibit very similar adsorption energies (the deviation does not exceed 5 kJ/mol). Thus, all of these adsorption modes are possible. On the other hand, when propane coordinates to CaZ, the η4-C3H8/CaZ mode is the most stable. The adsorption energies (Eads,MP2) for the other conformations are about 20% lower. Thus, the population of this structure is expected to be higher than the populations of η2-C3H8/CaZ and η3-C3H8/CaZ. Nevertheless, because the energy difference for these structures is very low, the experimental separation of different C3H8 adsorption modes is impossible under the chosen conditions.

To support the theoretical modeling of the adsorption modes, the vibrational analysis of the corresponding complexes was performed. The calculated frequencies and intensities of the C–H stretching vibrations as well as the assignment of them to the “perturbed” fundamentals of the corresponding gaseous alkanes are listed in Table 4.2. One should realize that the values presented correspond to vibrations of the single isolated adsorption complex, whereas under the experimental conditions the adsorbed alkanes are present in a conformational equilibrium. Also the method and the model used do not take into account different factors (electrostatic field due to the zeolite cavity, high density of the exchanged cations, etc.), which can affect the exact values of intensities and frequencies of the

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    Tab

le 4.2. C

alculated with

in DFT m

etho

da IR

 frequ

encies

a  (cm

‐1) and the correspo

nding intensities (km

/mol) 

of th

e C–

H stretching vibrations fo

r the adsorptio

n complexes con

side

red. 

            η2‐CH4/MgZ              η

3 ‐CH

4/Ca

Z              η

2 ‐C 2H6/MgZ             η

3 ‐C 2H6/Ca

Z             η2‐C

2H6/Ca

Z  

ν Int.  

          ν         Int.  

      ν 

     Int.   

  ν         Int.  

 ν         Int.  

  ν 3’ 

3018

 2.4 

ν  3’  

3018

 0.9 

ν  7’ 

2990

 15

.7 ν  

7’ 

2996

 17

.4 

ν  7’ 

2999

 11

.6 

ν  3’’ 

3000

 2.2 

ν  3’’ 

2980

 6.4 

ν  7’’ 

2985

 27

.9 ν  

7’’ 

2996

 17

.7 

ν  7’’ 

2981

 16

.5 

ν  3’’’ 

2963

 0.5 

ν  3’’’ 

2975

 9.9 

ν  10’ 

2960

 3.4 

ν  5 

2922

 20

.7 

ν  10’ 

2957

 16

.3 

ν  1 

2860

 6.2 

ν  1 

2868

 13

.9 ν  

5 29

15 15

.7 ν  

10’ 

2918

 10

.5 

ν  10’’ 

2909

 15

.9 

  

  

  

ν  10’’ 

2903

 5.6 

ν  10’’ 

2909

 17

.3 

ν  1 

2887

 29

.3 

  

  

  

ν  1 

2838

 40

.5 ν  

1 28

41 54

.2 

ν  5 

2850

 36

.9 

    η2‐C1‐C 3H8/MgZ 

η2‐C2‐C 3H8/MgZ 

η3‐C

3H8/Ca

Z η2‐C

3H8/Ca

Z η4‐C

3H8/Ca

Z ν 

Int.  

 ν 

Int.  

 ν 

Int.  

 ν 

Int.  

 ν 

Int.  

  2974

 37

.4 

 29

81 

30.7 

 29

77 

30.1 

 29

86 

32.4 

 29

73 

11.7 

2968

 43

.2 

 29

78 

34.7 

 29

71 

45.4 

 29

82 

20.1 

 29

72 

14.8 

2964

 13

.5 

 29

78 

3.8 

 29

46 

3.4 

 29

82 

3.7 

 29

59 

33.4 

2944

 1.7 

 29

77 

13.7 

 29

20 

40.6 

 29

67 

24.3 

 29

15 

29.0 

2910

 28

.7 

 29

08 

12.3 

 29

08 

9.2 

 29

10 

15.0 

 29

09 

0.4 

2898

 13

.3 

 29

07 

18.7 

 29

06 

6.7 

 29

03 

11.4 

 29

07 

22.1 

2897

 3.6 

 28

53 

10.5 

 29

00 

14.0 

 28

38 

30.7 

 28

53 

75.3 

2828

 35

.6 

 28

06 

70.2 

 28

35 

55.5 

 27

98 

106.8 

 28

47 

7.6 

a  The DFT com

puted freq

uencies were scaled

 by factor 0.953 

vibrations. However, it has been previously shown [10,11], that the spectra simulated using this approach agree well with the experiment and reproduce all qualitative features of the infrared spectra of hydrocarbons adsorbed on cation-exchanged zeolites.

For adsorbed methane, the calculated C–H stretching frequencies (Table 4.2) well agree with the experimental data (Figure 4.2). Interaction with the zeolitic adsorption site produces the strongest effect on the symmetrical ν1 mode of CH4, while the perturbation of the other vibrations is much slighter. In addition to the highest low-frequency shift, this vibration exhibits the highest intensity. The experimentally observed slightly lower red-shift of the ν1 band for methane adsorbed to CaY as well as the overall lower intensity of the spectrum in case of MgY are well reproduced by the present theoretical calculations.

In contrast to the experimental observations, the calculated intensity of the ν3 vibration

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for methane adsorbed on CaY is high (Table 4.2). The computed intense ν3’’ and ν3’’’ bands correspond to the e vibration for the C3v symmetry of the CH4···Ca moiety. The e vibration has a transition dipole moment only in a plane perpendicular to the C3 component (axis connecting Ca and C atoms). Free rotation of the CH4 molecule in this complex around the C3 axis averages this dipole and suppresses the corresponding IR bands.

The decrease of symmetry of the ethane molecule from D3d to Cs due to formation of the η2-C2H6/MgZ adsorption complex results in the appearance of all six C–H stretching vibrations in the calculated IR spectrum. Despite this the positions of the two high-frequency bands at 2990 and 2985 cm–1 due to the ν7 (eu) vibrations are very close. Because the main contribution to these vibrations is provided by the displacements of H atoms of the β-methyl group, the influence of the adsorption site on them is only indirect. Therefore, one does not expect these bands to be resolved in the spectrum. On the other hand, the resolution of the vibrations corresponding to the ν10 (eg) fundamental is more pronounced due to the more significant contribution of the C–H bonds directly coordinated to the cation. The symmetric C–H vibration (ν1 (a1g)) is the most activated one. This results in the strongest red shift and the highest relative intensity of the corresponding band.

The symmetry of the C2H6 moiety for η3-C2H6/CaZ complex is very close to C3v. This results in a not well-resolved spectrum, in which a high-frequency band (2996 cm–1) corresponding to the double degenerate (ν7 (eu)) asymmetric C–H vibration and the low-frequency band with a maximum at 2841 cm–1 from the symmetric one (ν1 (a1g)) dominate the infrared spectrum. In contrast, the most strongly perturbed and dominating vibration in the case of η2-C2H6/CaZ corresponds to the ν5 (a2u), which can be described as a symmetric C–H vibration for each of the methyl groups. Because of the very similar frequency of the mostly perturbed bands and their high relative intensity, one expects that when both adsorption modes are present, the low-frequency band dominates the resulting infrared spectrum of ethane adsorbed on CaY. This conclusion agrees well with the experimental results presented in section 4.3.1 (Figure 4.4 (b)).

Because of the large number of atoms, the assignment of the vibrations for propane molecule is a very difficult task even for the gas phase [21]. Adsorption to the zeolitic cations leads to a further complication of this problem. Coordination of C3H8 to the MgZ cluster with the methyl or methylene group results in the appearance of a set of high-frequency intense bands in the range of 2970–2980 cm–1 corresponding mostly to different asymmetric C–H vibrations. The summed intensity of these bands (164 km/mol) is significantly higher than that of the strongly perturbed low-frequency bands: 35.6 km/mol for the band at 2828 cm–1 of the η2-C1-C3H8/MgZ, and 70.2 km/mol for the band at 2806 cm–1 of the η2-C2-C3H8/MgZ, which correspond to the symmetric C–H vibrations of the interacting methyl (ν3 (a1)) and methylene group (ν1 (a1)), respectively. These findings cohere well with the spectroscopic results, showing domination of the high-frequency band in the IR spectra of propane adsorbed on MgY zeolite (Figure 4.5 (a)).

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Similarly, for the C3H8 adsorption to the CaZ cluster, the high-frequency C–H vibrations form a group of bands at 2970-2986 cm–1, corresponding mainly to the asymmetric C–H stretches. The strongest low-frequency shifts are found for the symmetric vibration of the CH3 group interacting with the Ca2+ (2835 cm–1) in the η3-C3H8/CaZ complex, and in the case of the η2-C3H8/CaZ for the symmetric vibration of the CH2 group, which at the same time is asymmetric for the methyl groups (2798 cm–1). An interesting feature is observed for the η4-C3H8/CaZ adsorption complex. In this case, the strongest red shift is computed for the C–H vibration that is symmetric within each of the methyl groups, but asymmetric as a whole. However, the relative intensity of this vibration is low, whereas the C–H breathing mode shows the highest intensity, and despite the slightly smaller red shift, is the mostly activated one due to the interaction with the exchangeable cation.

The positions of the high-frequency peaks are rather similar for all of the complexes of propane on CaZ. The resulting band in the overall IR spectrum would be rather narrow and intense due to their summation. On the other hand, despite the higher intensity, the mostly perturbed vibrations significantly differ in frequencies depending on the adsorption mode. The resulting overall spectra of propane adsorbed to CaY should contain a broad band with the intensity comparable to that of the high-frequency IR band.

Thus, the computed vibrations for the adsorption complexes considered agree well with the experimental results discussed in section 4.3.1. The unexpectedly low intensities of the spectra of hydrocarbons adsorbed to MgY zeolite cohere well with the finding that the overall calculated intensities are somewhat lower for the adsorption complexes with the MgZ cluster as compared to those for the CaZ (Table 4.2). However, based on the results provided so far an explanation for the different adsorption modes realized as a function of the exchanged cation cannot yet be given. To clarify this, a topological analysis of the electron density distribution function ρ(r) of the adsorption complexes was performed.

4.3.3. Topological analysis of the electron density ρ(r)

Ab initio (MP2) calculations provide the total electron density distribution functions ρ(r) for the adsorption complexes that can be used to perform a topological analysis, and hence, to define bonds between the hydrocarbon and the adsorption site. There are critical points where the gradient of the electron density ∇ρ(r) vanishes. Besides the maxima associated with the atom positions, there are also saddle points where the curvature of the ρ(r) is positive in one direction and negative in the two others. Saddle points are located somewhere between the nuclei. According to the Atoms in Molecules theory [14], the existence of such a critical point can be used as evidence of bonding between two atoms. “Bonding paths” are curves of the maximum electron density that connect two interacting atoms. The saddle points are located at these curves and, accordingly, usually termed “bond critical point”. The results of the topological analysis of the ρ(r) for bond critical points and bonding paths are shown in Figure 4.7.

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Figure 4.7. The results of the topological analysis of the electron density distribution function 

for bond critical points  (CP(3,–1)) and bonding paths of  (a) η2‐CH4/MgZ,  (b) η2‐C2H6/MgZ,  (c) 

η2‐C1‐C3H8/MgZ,  (d)  η2‐C2‐C3H8/MgZ,  (e)  η3‐CH4/CaZ,  (f)  η3‐C2H6/CaZ,  (g) η

2‐C2H6/CaZ,  (h) 

η3‐C3H8/CaZ, (i) η2‐C3H8/CaZ, and (j) η

4‐C3H8/CaZ adsorption complexes. 

All of the bonds that one would expect within the cation site of the zeolite and within the hydrocarbons are found. Most interesting are the bonds revealed between the adsorbed molecules and the adsorption site. In the complexes considered, a bond connecting one of the carbon atoms of the alkane and the cation is detected. The only exception is the η4-C3H8/CaZ where two carbon atoms are bound with the calcium ion. Such interaction corresponds to a charge-donation from the hydrocarbon to the extra-framework cation. In addition, for the magnesium-exchanged zeolite a number of bonds connecting H atoms of the adsorbed molecule and basic zeolitic oxygens are detected (Figure 4.7 (a)-(d)). Such contacts are also found for the η2-C2H6/CaZ and for all of the complexes of C3H8 with CaZ (Figure 4.7 (g)-(j)). These interactions correspond to weak hydrogen bonds.

One notes that due to the larger ionic radius, the Ca2+ ion points outward from the cation site, while the smaller Mg2+ is located within the plane of the cluster model of the faujasite SII site. As a consequence, when the hydrocarbon coordinates to the MgZ, it is located closer to the framework oxygen atoms and stabilizing C–H···O contacts can be formed. η3 coordination of an alkane to the MgZ would lead to formation of additional C–H···O interactions, but the distance between the exchanged cation and the adsorbed alkane would

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Table 4.3. Bond lengths and electron density ρ(rc) at the corresponding critical point (3,–1) for the interactions between the alkane and the adsorption site. 

  C∙∙∙cation contact  C–H∙∙∙O contacts   Bond length, Å  ρ(rc), a.u.  Bond length, Å  ρ(rc), a.u. η2‐CH4/MgZ  2.556  0.0128  2.613; 2.579  0.0089; 0.0097 η2‐C2H6/MgZ  2.594  0.0138  2.616; 2.603; 

2.788; 2.722 0.0091; 0.0096; 0.0092; 0.0065 

η2‐C1‐C3H8/MgZ  2.610  0.0137  2.573; 2.722;  2.846 

0.0098; 0.0100; 0.0052 

η2‐C2‐C3H8/MgZ  2.582  0.0150  2.711; 2.601; 2.916; 2.637 

0.0103; 0.0096;0.0043; 0.0094  

η3‐CH4/CaZ  2.847  0.0121  —  — η3‐C2H6/CaZ  2.835  0.0126  —  — η2‐C2H6/CaZ  2.854  0.0125  2.867  0.0056 η3‐C3H8/CaZ  2.836  0.0128  2.868  0.0050 η2‐C3H8/CaZ  2.854  0.0130  2.736  0.0066 η4‐C3H8/CaZ  3.009; 3.022  0.0100; 0.0097  2.903; 2.906  0.0053; 0.0052  

increase in parallel, resulting in a significant weakening of the overall interaction. This is the reason why only two-fold coordination to the Mg2+ site is realized instead of the three-fold one that is favorable for the Ca2+ site. It is also noticeable that when the alkane interacts with free Ca2+ or Mg2+ ions, in both cases the η3 adsorption mode is the most stable [22], because it provides the more effective electrostatic interaction between the interacting species.

The lengths of the intermolecular bonds and the electron densities at the corresponding critical points ρ(rc) are summarized in Table 4.3. These contacts are so-called “closed-shell’ types of interactions, because the values of ρ(rc) are relatively low and ∇2ρ(rc) is positive [14]. This corresponds to hydrogen bonding and van der Waals interactions. Hydrogen bonds have typical values of electron density between 0.004 and 0.035 a.u. [23]. The ρ(rc) values found for the classical van der Waals complexes are also in this range (for instance, ρ(rc) in Ar···HF complex is equal to 0.0077 a.u. [14]). In the present case, the bonds between the hydrogen atoms of alkanes and the zeolite oxygen atoms are also within this range (Table 4.3), and, further, the CH···O distances are comparable to those found for intermolecular CH···O bonds [23].

To estimate the strength of interatomic interactions, the correlation between the potential energy density (V(rc)) at the corresponding critical point and the contact energy

)(21

cc rVE = is used [24]. The smallest individual contact energies (4.0-8.0 kJ/mol) are

computed for the C–H···O interactions. Despite the rather small value of the energy of an individual contact, in the case of MgZ model, they significantly contribute to the adsorption energy. For the η2-CH4/MgZ and η2-C2H6/MgZ, their contribution equals 15.2 and 28.0 kJ/mol, respectively. These values agree very well with the adsorption heats experimentally obtained for methane and ethane loaded onto a pure silica MCM-41 [25],

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 Figure  4.8. Adsorption  energies  calculated  at  the MP2(FULL)/6‐31G(d,p)  level  of  theory  ( , 

Eads,MP2); total energy of all intermolecular contacts ( , EΣC) within the adsorption complexes; 

total energy  of interaction of an alkane with the CaZ including the “nondirected” electrostatic 

term  ( , EEl.Stat.+ΣC); energy of  the contact  ( , ECP(C∙∙∙M)) between a carbon atom of an alkane 

and an exchanged cation. 

where this type of interaction is predominant. The energy of the contact between the carbon atom of alkane and the exchanged cation (ECP(C···M)) is slightly higher for MgZ than for CaZ. This finding supports the above conclusion on the stronger charge donation from the hydrocarbon to the magnesium cation. The C···Mg and C···Ca contact energies (ECP(C···M)) only slightly depend on the alkane or on the adsorption mode (Figure 4.8).

The summed energies of all of the intermolecular contacts (EΣC) in the adsorption complexes with the MgZ site cohere well with the adsorption energies (Eads,MP2) obtained from the MP2 ab initio calculations (Figure 4.8). For the CaZ model, this approach gives a significant underestimation. Indeed, in the latter case the larger calcium ion is only partially shielded by the surrounding negatively charged framework oxygens of the SII site, and hence, it produces a significant nondirected electrostatic field, which additionally stabilizes the adsorbed molecules. To check this hypothesis, single-point energy calculations were performed for the alkane complexes at the CaZ but with the adsorption site replaced with a point charge of +1e– corresponding to a Mulliken charge on the Ca2+ ion in the adsorption complexes. The stabilization energies (EEl.Stat.) obtained via this procedure are then added to the respective EΣC. The total interaction energies (EEl.Stat.+ΣC), which include terms due to “directed” electrostatic interactions (EΣC), that is, van der Waals, hydrogen bonds, and charge donation, as well as to the “nondirected” ones (EEl.Stat.), that is, the interaction with the cation represented by a point charge, are found to be in excellent agreement with the corresponding Eads,MP2 energies (Figure 4.8). One could expect a stronger induced polarization of the alkane adsorbed to the MgZ because of the

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larger “charge-to-ionic radius” ratio for Mg2+ as compared to that for Ca2+. However, the strong shielding of the former ion with the lattice oxygen ions of the zeolitic cation site leads to the absence of any significant contribution of the “nondirected” electrostatic interaction. This conclusion coheres well with the fact that the values of the calculated adsorption energies are comparable for MgZ and CaZ cluster models, while interaction of a hydrocarbon with the free Mg2+ ion is substantially stronger than that with the free Ca2+. Obviously, a strong shielding of the smaller magnesium ions in MgY zeolite also leads to the weaker polarization of the adsorbed alkanes as compared to that for the CaY, resulting in lower intensity of the corresponding bands in the infrared spectrum.

Summarizing the foregoing results, the magnesium cation is strongly shielded by the surrounding zeolitic oxygens, and hence, significant stabilization due to weak hydrogen bonds between the H atoms of the alkane and O atoms of the cation site occurs. The preferred conformation of the adsorbed alkane is to a significant extent controlled by the resulting steric constraints, which lead to a longer hydrocarbon–cation bond. On the other hand, the larger Ca2+ cations at the SII site of faujasite are only slightly shielded, and hence the interaction of the adsorbed molecules with the nondirected electrostatic field dominates. In this case the adsorption modes are similar to those observed in the case of interaction with the free nontransition metal cations [22]. Thus, for the even larger ionic radii of Sr2+ and Ba2+, one expects conformations of the alkanes adsorbed to the corresponding cationic faujasites to be similar to those found for CaY.

4.4. Conclusions

Adsorption of light alkanes to magnesium- and calcium-exchanged zeolite Y is investigated by means of FT-IR spectroscopy, quantum-chemical calculations, and topological analysis of the electron density distribution functions in the framework of quantum theory of atoms in molecules. Different conformations of light alkanes are realized depending on the cationic form of the zeolite, resulting in remarkable differences in the corresponding vibrational spectra. Light alkanes (CH4, C2H6, and C3H8) coordinate to the Mg2+ adsorption site with only two C–H bonds, whereas in the case of CaY zeolite the η3 coordination for CH4 and C2H6, and the η4 coordination for C3H8 are preferred. Although, the DFT C–H stretching frequencies agree well with the experimental results, the DFT method gives significantly lower values for the adsorption energies as compared to MP2. This is shown to be due to the underestimation of the dispersive bonds, which significantly contribute to the adsorption energy in the particular case and, at the same time, do not influence strongly vibrational properties of the adsorbed molecules.

It is found that due to the much smaller ionic radius of the Mg2+ ion as compared to that of Ca2+, the former ions are significantly shielded with the surrounding oxygens of the zeolitic cation site. Thus, when a hydrocarbon coordinates to the exchangeable Mg2+ ions, numerous van der Waals bonds between H atoms of the alkane and basic oxygens of the zeolite are formed. These intermolecular contacts significantly contribute to the overall

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adsorption energy, whereas in the case of CaY zeolite such interactions play only an indirect role. On the other hand, the strong shielding of the Mg2+ at the zeolitic cation site results in a small electrostatic contribution to the stabilization of the adsorbed molecules, while for CaY zeolite the stabilization of alkanes in the electrostatic field of the partially shielded Ca2+ cation dominates the adsorption energy. Thus, the preferred conformation of the adsorbed alkanes is controlled by the bonding within the adsorption complexes, which, in turn, strongly depends on the size and location of the cations in the zeolite cavity. 

References 

1 Xu, J.; Mojet, B. L.; Ommen, J. G. v.; Lefferts, L. J. Phys. Chem. B 2005, 109, 18361. 2 Cohen de Lara, E.; Seloudoux, R. J. Chem. Soc. Faraday Trans. 1983, 79, 2271. 3 Cohen de Lara, E.; Kahn, R.; Seloudoux, R. J. Chem. Phys. 1985, 83, 2646. 4 Khodakov, A.Yu.; Kustov, L.M., Kazansky, V.B.; Williams, C. J. Chem. Soc., Faraday Trans. 1993, 89, 1393. 5 Huber, S.; Knoezinger, H. Chem. Phys. Lett. 1995, 244, 111. 6 Kazanskii, V.B.; Serykh, A.I.; Bell, A.T. Kinetics and Catalysis 2002, 43, 453. 7 Kazanskii, V.B.; Serykh, A.I.; Bell, A.T. Catal. Lett. 2001, 77, 215. 8 Kazansky, V.B.; Serykh, A.I.; Pidko, E.A. J. Catal. 2004, 225, 369. 9 Kazansky, V.B.; Pidko, E.A. J. Phys. Chem. B 2005, 109, 2103.  10 Pidko, E.A.; Kazansky, V.B. Phys. Chem. Chem. Phys. 2005, 7,1939. 11 Pidko, E.A.; Kazanskii, V.B. Kinetics and Catalysis, 2005, 46, 407. 12 Xu, J.; Mojet, B. L.; Ommen, J. G. v.; Lefferts, L. Phys. Chem. Chem. Phys. 2003, 5, 4407. 13 Xu, J.; Mojet, B. L.; Ommen, J. G. v.; Lefferts, L. J. Phys. Chem. B 2004, 108, 15728. 14 Bader, R.F.W. Atoms in Molecules. A Quantum Theory; Clareondon Press: Oxford, 1990. 15 Pidko, E.A.; Xu, J.; Mojet, B.L.; Lefferts, L.; Subbotina, I.R.; Kazansky, V.B.; van Santen, R.A. J. Phys. Chem. 

B 2006, 110, 22618. 16 Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. 

A.,  Jr.;  Vreven,  T.;  Kudin,  K.  N.;  Burant,  J.  C.; Millam,  J. M.;  Iyengar,  S.  S.;  Tomasi,  J.;  Barone,  V.; Mennucci, B.; Cossi, M.;  Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,  X.;  Knox,  J.  E.;  Hratchian,  H.  P.;  Cross,  J.  B.;  Bakken,  V.;  Adamo,  C.;  Jaramillo,  J.;  Gomperts,  R.; Stratmann,  R.  E.;  Yazyev,  O.;  Austin,  A.  J.;  Cammi,  R.;  Pomelli,  C.;  Ochterski,  J.  W.;  Ayala,  P.  Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al‐Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;  Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,  J. A. Gaussian 03,  revision B.05; Gaussian, Inc.: Pittsburgh PA, 2003. 

17 Becke, A.D. Phys. Rev. 1988, A38, 3098; Becke, A.D. J. Chem. Phys. 1993, 98, 1372; Becke, A.D. J. Chem. Phys. 1993, 98, 5648. 

18 Backer,  J.; Muir, M.;  Andzelm,  J.;  Scheiner,  A.  in:  Laird,  B.B.;  Ross,  R.B.;  Ziegler,  T.  (Eds.)  Chemical Applications of Density‐Functional Theory, ACS Symposium Series, vol. 629, American Chemical Society, Washington, DC, 1996. 

19 Olson, D.J. J. Phys.Chem. 1970, 74, 2758. 20 Biegler‐König, F.; Schönbohm, J.; Bayles, D. J. Comp. Chem. 2001, 22, 545; Biegler‐König, F.; Schönbohm, 

J. J. Comp. Chem. 2002, 23, 1489 21 Herzberg,  G.  Molecular  Spectra  and  Molecular  Structure,  vol.  2:  Infrared  and  Raman  Spectra  of 

Polyatomic Molecules; Van Nostrand: New York, 1947. 22 Makhaev, V.D. Russ. Chem. Rev. 2003, 3, 257. 23 Koch, U.; Popelier, P. L. A. J. Phys. Chem. 1995, 99, 9747. 24 Espinosa, E.; Molins, E.; Lecomte, C. Chem. Phys. Lett. 1998, 285, 170. 25 He, Y.; N.A. Seaton, N.A. Langmuir 2006, 22, 1150. 

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CHAPTER 5  

MOLECULAR AND DISSOCIATIVE ADSORPTION OF ETHANE ON ZINC AND CADMIUM IONS IN ZSM‐5 ZEOLITE 

 

 

 

Molecular and dissociative adsorption of ethane on Zn/ZSM-5 and Cd/ZSM-5 zeolites is studied by means of DFT calculations. Three types of cationic extra-framework sites are considered for Zn/ZSM-5: Zn2+ ions stabilized at conventional ion-exchange sites (Zn Zs), Zn2+ ions stabilized at cation sites with distantly placed framework aluminum ions (Zn Zd), and binuclear [ZnOZn]2+ cations (ZnOZn). For Cd/ZSM-5 zeolite only the mononuclear Cd Zs and Cd Zd sites are discussed. Interaction of C2H6 with the mononuclear sites in both zeolites results in strongly perturbed molecular adsorption, whereas coordination of ethane to binuclear ZnOZn sites is very weak. No correlation between the perturbations of the adsorbed molecules and their subsequent heterolytic dissociation is found. Despite of much stronger perturbations of ethane adsorbed to Cd2+ cations in ZSM-5 zeolite, their reactivity in ethane dissociation is predicted to be of similar magnitude as that of extra-framework Zn2+ cations.

 

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5.1. Introduction

Unlike the hard Lewis acids such as Mg2+ and Ca2+ cations stabilized in low-silica zeolite Y (Chapter 5), which are rather inert and cannot be directly involved in the chemical activation of hydrocarbon molecules, soft Lewis acid sites, such as zinc-, cadmium-, and gallium-containing extra-framework species stabilized in zeolites, are able to activate light alkanes. Similar to the alkaline-earth cations, Zn2+ and Cd2+ are characterized by their closed-shell electronic structure (d10s0). In addition, the ionic radii of these ions are very close to those of Mg2+ and Ca2+, respectively. However, due to the softer nature of zinc and cadmium ions, zeolites modified by these cations usually exhibit much higher reactivity toward activation of C–H bonds of alkanes [1-3]. In addition, it has been shown [4] that, whereas adsorption of various probe molecules to Mg2+ ions in zeolites is mainly due to polarization effects with some influence of charge-donation to the cation, the interaction with zinc ions additionally involves back-donation of electrons to the adsorbed molecule from d-orbitals of the cation. This results in an increased interaction of hydrocarbons with Zn2+ and in a significant activation of the adsorbed molecule.

High-silica zeolites modified with zinc are known to be effective catalysts for promoting dehydrogenation and aromatization of light alkanes [1-3]. The reaction mechanism is thought to consist of a complex scheme involving dehydrogenation, oligomerization, and ring-closure steps. The modifying cations play a key role in the dehydrogenation of paraffins, whereas the Brønsted acid protons catalyze oligomerization of the resulting olefins and their subsequent aromatization [5-11]. Numerous experimental [5-15] and theoretical [16-28] studies have been devoted to the investigation of the active sites and the mechanism of catalytic dehydrogenation of light alkanes over zinc-exchanged ZSM-5 zeolties (Zn/ZSM-5). Despite that, the structure of the active intrazeolite Zn species and, accordingly the mechanism of hydrocarbon activation have not been fully elucidated.

Modification of zeolites with zinc ions can in principle result in the formation of various cationic species depending on the type of zeolite, its Si/Al ratio, the method of preparation of the catalyst, and the conditions of the subsequent thermochemical activation. Zn/ZSM-5 prepared using conventional techniques (incipient wetness impregnation, ion exchange in aqueous solution) can, therefore, contain (i) isolated Zn2+ ions stabilized in cation sites of the zeolite, (ii) binuclear [ZnOZn]2+ species, and (iii) intrazeolite clusters of zinc oxide. Hence, comparison of the activity of these species for the dehydrogenation of alkanes is important to understand the catalytic properties of zinc-exchanged high-silica zeolites.

Most studies have indicated the predominance of isolated Zn2+ species in the cationic positions of zeolites [6-9,11-14]. Using in situ Zn K-edge X-ray absorption technique Biscardi et al. [7] have shown that zinc in the Zn/ZSM-5 zeolite (Si/Al=14.5) prepared via the conventional ion exchange technique is present as isolated Zn-Ox species located at the zeolitic exchange sites coordinating to four oxygen nearest neighbors and without next nearest Zn neighbors. Computational studies [18-21,26] on the location and stabilization of

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Zn2+ ions in ZSM-5 report a preferential 4-fold coordination of Zn2+ to the lattice oxygen anions.

Binuclear [ZnOZn]2+ extra-framework species have been also suggested to account for the experimentally observed overexchange phenomena in Zn/ZSM-5 [7]. Recently, Penzien et al. [29] have identified such species in high-silica zeolite BEA modified with zinc. These species have been theoretically investigated using either the cluster [17,23] or the periodic [24,25] approach. It has been shown that the [ZnOZn]2+ cations are rather reactive toward alkane and hydrogen dissociation. The initial heterolytic dissociation of ethane on Zn2+ and [ZnOZn]2+ ions stabilized in cluster models representing different conventional cation sites of ZSM-5 zeolite has been investigated by Shubin et al. [19]. A correlation between the stability and the initial reactivity of the zinc-site has been observed. It has been proposed that binuclear species or small intrazeolite zinc oxide particles may be important for the dehydrogenation ability of Zn/ZSM-5.

Recently a new and completely anhydrous route for the preparation of well-defined Zn cationic species via a high temperature reaction of parental HZSM-5 with zinc vapor has been proposed [30-32]. This technique results in a complete and selective substitution of zeolitic protons with bivalent zinc cations. The resulting Zn/Al ratio is 1/2. Due to the lack of oxygen source, formation of any oxygen-containing intrazeolite zinc species is not possible. It has been assumed that bivalent Zn2+ cations can be stabilized at one anionic site of the zeolite with a distantly located second charge-compensating [AlO2]– framework unit. The indirect charge compensation of Zn2+ ions at such cationic sites results in a significant enhancement of their Lewis acidity.

Adsorption of H2 [33-37], CH4 [31], and C2H6 [11,32] by Zn/ZSM-5 have been studied using diffuse-reflectance infrared spectroscopy (DRIFTS). Large red shifts of H–H and C–H stretching vibrations have been observed for the adsorbed molecules. In addition, after a moderate elevation in temperature their heterolytic dissociation has been detected [31-35]. This has been explained by a promoted activation of adsorbed molecules on the above-mentioned strong Lewis acid active sites. This assumption has been subsequently supported by quantum-chemical calculations [21,22,27]. Formation of strongly activated molecular adsorption complexes, as well as their subsequent dissociation, has been observed for Zn/ZSM-5 zeolites independently of the method of preparation. Therefore, it has been suggested that the high activity of Zn2+ ions in high-silica zeolites is due to their unique location resulting in very strong polarization of the adsorbed light alkanes, which, in turn, facilitates the heterolytic C–H bond cleavage. The properties of Zn2+ ions stabilized in the conventional ion-exchangeable sites have been assumed to be similar to those for zinc ions stabilized in low-silica Y zeolite [32].

Another related and very interesting material is Cd/ZSM-5 that is the most active of the cationic zeolites for hydration of acetylene [38]. In addition, similar to Zn/ZSM-5 it exhibits a remarkable activity in C2H6 dehydrogenation [39]. However, no direct comparison of reactivity of these catalysts has been reported before.

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Low-temperature DRIFTS study of H2 adsorption on CdZSM-5 has revealed that different cationic species are being formed within the zeolite depending on the method of preparation [40]. It has been also found that H2 molecules heterolytically dissociate on this catalyst. The proposed active sites are the isolated Cd2+ cations stabilized at distant anionic sites in ZSM-5. One should note that Cd2+ and Zn2+ ions have very similar electronegativities, while the ionic radius of cadmium is by 0.2 Å larger compared to bivalent zinc [41]. A larger cation is less shielded by the surrounding lattice oxygens. It has been show in Chapter 4 that this results in a stronger electrostatic field of the exchanged cation and, hence, leads to stronger polarization of the adsorbed molecules (see Chapters 2 – 4). Thus, one expects that Cd2+ ions exchanged to ZSM-5 zeolite would stronger perturb adsorbed alkanes and show enhanced catalytic properties compared to the Zn/ZSM-5.

In spite of wealth experimental and theoretical data available on the state and properties of zinc ions in ZSM-5 zeolite, no clear conclusion on this issue could have been drawn yet. The nature of the active sites in Zn/ZSM-5 zeolite is still debatable [15,28]. Moreover, the influence of different charge-compensation schemes on the reactivity of the exchangeable cations in high-silica zeolites has not been investigated before. To address this, below a comparative DFT study of molecular and dissociative adsorption of ethane on extra-framework zinc- and cadmium-sites in ZSM-5 zeolite is presented. The investigation of the influence of the size of the cations stabilized in zeolites on their adsorption properties and reactivity is continued. 

5.2. Computational details

Ethane activation over Zn/ZSM-5 and Cd/ZSM-5 zeolite was studied using Zn(Cd)Al2Si6O9H14 and Zn2OAl2Si6O9H14 cluster models (Figure 5.1), which represent two adjacent five-membered rings from the wall of the straight channel of ZSM-5 zeolite. Aluminum atoms were placed in T12 and T8 lattice positions [42] to model the charge-alternating cation site Zd with distantly separated [AlO2]– framework units (Figure 5.1 (a), (c) and (d)). The next nearest T12 and T6 lattice positions were occupied with aluminum atoms to model a conventional ion-exchange site Zs (Figure 5.1 (b) and (e)). The distance between two aluminum ions was 8.14 Å and 4.84 Å for the Zd and Zs cluster models, respectively. Note that the T12 site is located at the cross-section of the straight and the sinusoidal channel of ZSM-5. Accordingly, the cations stabilized in the vicinity of this site have the highest accessibility. Therefore, the charge-compensating cation was placed in the five-membered ring containing aluminum at the T12 position. The starting geometry of the clusters corresponded to the lattice of the ZSM-5 zeolite according to X-ray diffraction data [43]. Hydrogen atoms were used to saturate the dangling Si–O bonds at the periphery of the cluster. Special restrictions on the positions of the boundary H atoms were imposed during the procedure of optimization, as described in Chapter 4.

Density functional theory (DFT), with the B3LYP [44] hybride exchange-correlation functional, was used to perform the quantum-chemical calculations. The B3LYP method

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was shown before to describe very well various reaction profiles, and in particular, geometries, heats of reactions, activation energies, and vibrational properties of different molecules [45]. Geometry optimization and saddlepoint searches were performed using the Gaussian 03 program [46]. The 6-31G(d,p) basis set was used for zinc ions, while the full-electron DGDZVP basis set [47] was used for cadmium. The ethane molecule and the zeolitic oxygen atoms were described by the 6-311G(d,p) basis set. Al and Si atoms of the zeolite framework, as well as the boundary hydrogen atoms, were treated by the D95-Dunning/Huzinaga basis set [48]. Such a combination has been shown to be a successful compromise for studies of reactivity of cation-exchanged zeolites [21,22,27]. The applicability of the chosen theoretical method was additionally verified by recomputing the adsorption energy for ethane coordinated to the Zn Zd and Zn Zs clusters at the higher B3LYP/6-311++G(d,p) level of theory. The difference in the thus computed C2H6 adsorption energies deviated by less than 2 kJ/mol from the value calculated using the basis set combination described above.

All energies obtained from the DFT calculations used for the estimation of the reaction heats and activation barriers were corrected for the zero-point energy obtained from frequency calculations performed on all of the optimized structures. Although the use of basis sets of different quality for different atoms of the model can cause relatively high basis set superposition error (BSSE), the contribution of the BSSE to the reaction energies and activation energies is expected to be small and similar for all the structures discussed and, hence, not to affect the conclusions drawn. Thus, the energetics reported were not corrected for BSSE.

The nature of the stationary points was tested by analyzing the analytically calculated harmonic normal modes. During the normal-mode analysis, constraints on the boundary H atoms result in imaginary frequencies (negative force constants). These imaginary frequencies can be suppressed by removing the contributions from the constrained atoms to the Hessian matrix for all the reaction intermediates. All of the transition states showed only a single imaginary frequency corresponding to the eigenvector along the reaction path. The assignment of the transition state structure to a particular reaction path was tested by perturbing the structure along the reaction path eigenvector in both directions of the products and the reactants with subsequent geometry optimization of the resulting models.

5.3. Results

5.3.1. Structure of active sites

Figure 5.1 displays the optimized structures of isolated Zn2+ and Cd2+ cations stabilized at the Zd and Zs models, and of the binuclear ZnOZn site. The corresponding optimized M–O bond lengths (where M is Zn or Cd) are summarized in Table 5.1. The structural parameters show that in all of the mononuclear sites the exchanged cation coordinates to four adjacent zeolitic oxygen atoms. Lattice oxygen atoms bound to Al atoms are more basic compared to those from the [SiO2] framework units. This results in slightly shorter

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Figure 5.1. Optimized structures of (a) ZnZd, (b) ZnZs, (c) ZnOZn, (d) CdZd, and (e) CdZs clusters. 

Table 5.1. Selected interatomic distances (Å) for the Zn and Cd containing adsorption sites. 

          ZnOZn   Zn Zd  Zn Zs  Cd Zd  Cd Zs Zn1  Zn2 M‐O1  1.974  2.057  2.229  2.283 Zn‐O1/O6 2.043  3.150 M‐O2  1.917  1.930  2.194  2.194 Zn‐O2/O7 2.025  2.071 M‐O3  3.204  3.271  3.177  3.329 Zn‐O3/O8 3.487  2.048 M‐O4  2.018  1.958  2.314  2.208 Zn‐O4/O9 2.392  3.027 M‐O5  2.079  2.028  2.316  2.272 Zn‐O5 2.897  2.244           Zn‐O10 1.822  1.831  

M–O1 and M–O2 distances for the ZnZd and CdZd structure. Although in the Zs site all of the O atoms coordinated to the exchanged cation are equally basic (ZnZs and CdZs), slightly shorter M–O bonds are formed with O2 and O4 atoms most likely due to the steric properties of the cation site. For the ZnOZn structure, each 5T ring of the model is occupied by Zn2+ cations connected via the extralattice oxygen atom O10. The optimized Zn-O interatomic distances are very close for both zinc ions reflecting their similar chemical properties. For this reason, ethane activation was studied only on one of the Zn atoms, namely Zn1 in Figure 5.1 (c).

5.3.2. Molecular adsorption

Ethane adsorbed on the zeolitic cation is a precursor for its subsequent heterolytic C–H bond cleavage. Whatever the cation, C2H6 coordinates to the adsorption sites with two hydrogen atoms from one methyl group (η2-coordination) as shown in Figure 5.2. The strongest adsorption (53 kJ/mol) is computed for the ZnZd site, while coordination of C2H6 to the ZnZs site is weaker by a factor of 2 (26 kJ/mol). Although significantly weaker bonding of ethane to the larger Cd2+ cations than to the smaller Zn2+ cations is expected, very similar adsorption energies are computed for the interaction of ethane with the Cd-containing adsorption sites (49 and 27 kJ/mol, respectively for C2H6 adsorption on CdZd

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Figure  5.2.  Optimized  structures  of  ethane  adsorption  complex with  (a)  ZnZd,  (b)  ZnZs,  (c) 

ZnOZn, (d) CdZd, and (e) CdZs site. 

Table 5.2. Selected geometry parameters [interatomic distances (Å) and H1‐C1‐H2 bond angle 

(°)] for the C2H6a adsorption complexes on Zn and Cd containing adsorption sites. 

  C2H6/Zn Zd  C2H6/Zn Zs C2H6/Cd Zd C2H6/ CdZs  C2H6/ ZnOZnM–O1  2.015  2.089 2.251 2.302 2.044 M–O2  1.952  1.960 2.211 2.207 2.048 M–O4  2.112  1.995 2.373 2.231 2.534 M–O5b  2.131  2.059 2.364 2.299 1.828 M–C1  2.387  2.577 2.712 2.793 2.863 M–H1  2.087  2.416 2.385 2.461 2.657 M–H2  1.955  1.966 2.216 2.332 2.291 C1–H1  1.105  1.095 1.103 1.101 1.092 C1–H2  1.115  1.112 1.119 1.110 1.101 ∠H1‐C1‐H2  114.9  111.9 114.0 112.7 109.3 a The calculated C–H bond length in the gas phase C2H6 is equal to 1.094 Å; the H‐C‐H bond angle is equal to 107.5°. b Zn1‐O10 distance for the ZnOZn 

and CdZs, Figure 5.2). Similar to the above-discussed adsorption of light alkanes on MgY and CaY zeolites (Chapter 4), the larger cation in this case is weaker shielded with the lattice oxygens atoms of the cation site. This results in the enhancement of the polarizing ability of the extra-framework cadmium cation. The stronger adsorption in the case of Zd site is due to a higher Lewis acidity of the cations at the site with indirect charge-compensation involving distantly placed anionic sites, compared to the case when aluminum atoms are located at the next nearest lattice positions (Zs).

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Figure 5.3. Calculated IR frequencies (ν, cm–1) of C–H stretching vibrations with their intensities 

(Int., km/mol) and  simulated  IR  spectra  (with all DFT computed  frequencies  scaled by  factor 

0.964) for adsorption complexes of ethane with (a) the zinc‐sites [(1) Zn Zd; (2) Zn Zs; (3) ZnOZn] 

and  (b)  the  cadmium‐sites  [(1) Cd Zd;  (2) CdZs] of ZSM‐5  zeolite.  IR  spectrum of  free ethane 

molecule calculated at the B3LYP/6‐311G(d,p) level is presented for comparison [(4) in (a), and 

(3) in (b)]. Artificial line broadening was used for visualization of the calculated spectra. 

The four-fold coordination of the isolated Cd and Zn cations to the lattice oxygen atoms remains unchanged after ethane adsorption. Although the coordination of C2H6 to the zeolitic cations does not dramatically alter the structure of the adsorption site, for the mononuclear sites the M–O distances (M is Zn or Cd) become somewhat longer and the exchangeable cation slightly moves towards the adsorbed molecule (Table 5.2). More important are changes observed in the geometry of the C2H6 species. The C–H bonds directly interacting with the cation are significantly elongated in comparison with the corresponding bonds of gas-phase ethane. In addition, the respective H1–C1–H2 bond angles increase remarkably upon adsorption. (Table 5.2). One notes that similar to the trends observed in the adsorption energies, the perturbations of the geometry of the adsorbed ethane molecule are very similar for the complexes on Zn and Cd cations stabilized at the respective cation sites. The effective transfer of electron density from ethane to the ZnZd and ZnZs sites equals 0.115 and 0.052 electrons, respectively, while the charge redistribution within the adsorption complexes with Cd-containing sites is significantly stronger. The effective electron transfer equals 0.173 and 0.124 electrons, respectively for the C2H6/CdZd and C2H6/CdZs complexes.

In the case of the binuclear ZnOZn species charge compensation is much more pronounced, since an extra-framework oxygen atom (O10) connects zinc ions in the complex. It results in very weak (11 kJ/mol) ethane adsorption (C2H6/ZnOZn). In contrast to the case of adsorption on the mononuclear sites, practically no charge transfer is observed for the C2H6/ZnOZn adsorption complex. This is due to the more effective

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Figure 5.4. The shape of the C–H 

vibration mostly perturbed due to 

interaction with  the  zeolitic  Zn2+ 

or Cd2+ cation. 

shielding of the zinc cation by the bridging O10 oxygen atom. In this case the main contribution to the adsorption energy is provided by induced polarization of C2H6, whereas charge transfer dominates bonding in the adsorption complexes with the isolated Zn2+ and Cd2+ stabilized at Zd and Zs sites. Thus, there is a qualitative correlation between ethane adsorption energies, the elongation of the interacting C–H bonds and the Lewis acidity of the cation.

Coordination of ethane to the positively charged adsorption sites and the resulting charge redistribution within the adsorption complex also affects its vibrational properties. The computed infrared spectra of C–H stretching vibrations of the C2H6/ZnZd, C2H6/ZnZs, and C2H6/ZnOZn complexes in comparison with the gas-phase ethane are shown in Figure 5.3 (a). The IR spectra of C2H6/CdZd and C2H6/CdZs are shown in Figure 5.3 (b). These spectra contain a number of slightly perturbed IR bands with frequencies close to those calculated for the gas-phase ethane (2920-3020 cm–1). These bands correspond mainly to the displacements of H atoms from the C–H bonds, which do not interact directly with the adsorption site. In addition, for each of the complexes one IR band (at 2725, 2746, and 2858 cm–1 for the ZnZd, ZnZs, and ZnOZn sites, respectively, and at 2677 and 2771 cm–1 for CdZd and CdZs, respectively) is strongly red-shifted and exhibits a higher relative intensity as compared to the other C–H stretching bands (Figure 5.3). This band corresponds to the perturbed symmetric C–H stretch that originates from the ν1 (a1g) mode with the main contribution of the displacements of the H1 and H2 atoms (Figure 5.4). In the cases of ethane coordination to ZnZd, CdZd, and CdZs very weak absorption bands at 2850, 2853, and 2878 cm–1, respectively, correspond to the asymmetric vibration of the H1–C1–H2 moiety originating from the ν10 (eg) vibrational mode of gaseous ethane. The weak interaction of C2H6 with the ZnOZn site results in only a slight perturbation of the adsorbed molecules (Figure 5.3 (a), spectrum 3). On the other hand, the higher Lewis acidity of the low-coordinated zinc and cadmium ions at Zs and Zd sites leads to more pronounced perturbations of the C2H6 species.

In summary, the C2H6 interaction energy with the zinc-sites in ZSM-5 decreases significantly in the order ZnZd, ZnZs, ZnOZn. Adsorption energies on cadmium sites are very similar to those on the respective zinc sites. Coordination of ethane to a [ZnOZn]2+ cation is very weak and should be considered as physical adsorption. On the other hand, the interaction of C2H6 with the isolated bivalent cations stabilized at the cation sites of ZSM-5 is stronger and mainly consists of charge transfer from the adsorbed molecule to the cation. This results in significant geometry changes and charge redistribution in the adsorbed C2H6 species. Both these effects

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 Figure 5.5. Activation of ethane over mononuclear Zn2+ sites in ZSM‐5 zeolite. 

Table  5.3.  Selected  interatomic  distances  (Å)  of  the  intermediates  and  transition  state 

structures involved in chemical activation of ethane over ZnZd and Zn Zs sites. 

  Zn Zd  Zn Zs 

 C2H6/ZnZd  TS1  TS1’  II  II’ 

C2H6/ ZnZs  TS1  TS1’  II  II’ 

Zn‐O1  2.015  1.930 2.097  1.957 2.086 2.089 2.048 2.500 2.049  2.787Zn‐O2  1.952  1.977 1.979  2.082 1.946 1.960 1.977 2.029 2.042  2.074Zn‐O4  2.112  2.856 2.285  4.323 3.155 1.995 2.573 2.073 3.589  2.052Zn‐O5  2.131  2.668 2.410  3.299 3.283 2.059 2.128 2.106 2.431  2.251Zn‐C1  2.387  2.016 2.074  1.940 1.942 2.577 2.069 2.106 1.955  1.978O6‐H1  2.640  —  1.238  —  0.968 2.704 — 1.173 —  0.975O4‐H2  2.725  1.153 —  0.973 — 2.922 1.191 — 0.968  — C1‐H1  1.105  1.108 1.490  1.093 — 1.095 1.106 1.571 1.095  — C1‐H2  1.115  1.588 1.104  —  1.094 1.112 1.513 1.103 —  1.095 

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Figure 5.6. C2H6 activation over binuclear ZnOZn sites and mononuclear Cd2+ in ZSM‐5 zeolite. 

Table  5.4.  Selected  interatomic  distances  (Å)  of  the  intermediates  and  transition  state 

structures involved in chemical activation of ethane over ZnOZn, CdZd, and CdZs sites. 

  ZnOZn    CdZd  CdZs 

 C2H6/ ZnOZn  TS1  II 

  C2H6/ CdZd  TS1  II 

C2H6/ CdZs  TS1  TS1’  II  II’ 

M‐O1  2.049  2.013  2.039    2.251 2.371 2.430 2.302 2.289 2.487  2.215  2.737M‐O2  2.048  2.065  2.206    2.211 2.221 2.235 2.207 2.234 2.262  2.403  2.343M‐O4  2.534  3.007  3.534    2.373 2.505 2.954 2.231 2.757 2.292  4.911  2.306M‐O5a  1.828  1.977  2.223    2.364 2.552 3.111 2.299 2.386 2.445  3.561  2.568M‐C1  2.863  2.225  1.975    2.712 2.329 2.204 2.793 2.359 2.354  2.202  2.227O6‐H1b  2.488  1.268  0.960    2.717 1.220 0.969 2.866 — 1.158  0.968  0.975O4‐H2  —  —  —    3.215 — — 3.450 1.216 —  —  —C1‐H1  1.092  1.397  —    1.103 1.515 — 1.101 1.103 1.599  1.093  —C1‐H2  1.101  1.105  1.095    1.119 1.102 1.092 1.110 1.489 1.101  —  1.094a Zn1‐O10 bond for the ZnOZn  b O10‐H1 distance for the ZnOZn 

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influence the vibrational spectrum of the adsorbed species leading to a strong red shift of some of the absorption bands as well as an increase of their relative intensities. However, the differences in the simulated spectra of C2H6/ZnZd and C2H6/ZnZs are very slight. On the other hand, in the case of cadmium sites, the effect of the charge compensation is more pronounced due to the weaker shielding of the larger cation with the lattice oxygens. The red shift of the low-frequency C–H vibration of ethane adsorbed to Cd2+ at the charge-alternating site Zd is larger by ~100 cm–1 than that for the C2H6/CdZs complex. In agreement with the earlier proposal [32,49], it is found that the C–H stretching vibration that originates from the fully symmetric ν1 (a1g) mode of C2H6 is the most sensitive to the nature of the adsorption site. The appearance of the low-frequency C–H vibration is evidence for a significant weakening of the corresponding chemical bonds. Therefore, a facile dissociation of C2H6 adsorbed to Cd/ZSM-5 zeolite can be expected.

5.3.3. Heterolytic C–H cleavage

According to the previous results [18,31,32], heterolytic dissociative adsorption of ethane results in the formation of a hydroxyl group and an alkyl fragment bound to the exchanged cation. Two different reaction paths are, in principle, possible over the isolated Zn2+ and Cd2+ sites (Zn(Cd)Zd and Zn(Cd)Zs), which differ in the mechanism of abstraction of a proton from C2H6 involving either O4 or O6 lattice oxygen. However, in the case of CdZd model, the geometry optimization of the product of the reaction path resulting in a Brønsted acid site at the O4 atom leads to an improbable structure. Cadmium ion migrates out of the cation site model resulting in formation of artificial bonds with the atoms at the periphery of the cluster. This is due to the very strong repulsion between the likely charged species (proton and a rather large and diffuse cadmium cation) located in one zeolitic ring. Only one pathway for heterolytic ethane dissociation over the ZnOZn is found, where hydrogen atom (H1) is abstracted by the bridging oxygen (O10) resulting in destruction of the [ZnOZn]2+ dimer and formation of [Zn–C2H5]+ and [Zn–OH]+ species.

The energetics of the ethane dissociation over the ZnZs and ZnZd sites are shown in Figure 5.5, while those for the ZnOZn site and Cd2+ ions at the Zd and Zs models are presented in Figure 5.6. The geometry parameters of the intermediates and transition state structures involved in dissociative ethane adsorption are summarized in Tables 5.3 and 5.4. The dissociative adsorption of C2H6 on Zn2+ and Cd2+ ions at either the Zs or Zd sites is endothermic (∆E = 59–142 kJ/mol, depending on the active site), while C–H bond cleavage over the ZnOZn is an exothermic process (IIZnOZn, ∆E = –67 kJ/mol). In addition, the latter process exhibits the lowest activation energy (TS1ZnOZn, ∆E≠ = 77 kJ/mol). C–H bond cleavage over the bare Zn2+ or Cd2+ ions in either the Zs and Zd clusters results in the loss of the stable 4-fold coordination of the exchangeable cation to the zeolitic oxygens (Tables 5.3 and 5.4). Formation of the new M–C bond cannot compensate for the energy loss from the breaking of strongly stabilizing M–O bonds (M is Zn or Cd).

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In the case of Zs cation site the reaction paths invloving abstraction of a proton by the oxygen atom (O4), to which the exchangeable cation is attached (C2H6/ZnZs→TS1ZnZs→IIZnZs and C2H6/CdZs→TS1CdZs→IICdZs), are both thermodynamically and kinetically the preferred paths, whatever the cation. The activation energy and the enthalpy of this reaction are lower than those computed for the reaction paths (C2H6/ZnZs→TS1’ZnZs→IIZnZs and C2H6/CdZs→TS1’CdZs→IICdZs) leading to formation of a Brønsted acid site (O6-H1) in the empty zeolitic ring (Figures 5.5 and 5.6).

   In the case of the charge-alternating cation site (Zd), the most favorable reaction path is the C–H bond cleavage resulting in formation of [M–C2H5]+ and [H+] species attached, respectively, to different zeolitic rings (C2H6/ZnZd→TS1’ZnZd→II’ZnZd and C2H6/CdZd→TS1’CdZd→II’CdZd). This path leads to the formation of the most stable products II’ (∆E = 59 kJ/mol and 61 kJ/mol, respectively, for the ZnZd and CdZd site.) and shows the lowest activation energy (TS1’ZnZd, ∆E≠ = 117 kJ/mol; TS1’CdZd, ∆E≠ = 113 kJ/mol) among C2H6 dissociation pathways over the isolated Zn2+ and Cd2+ sites.

Another possible reaction path (IZnZd→TS1ZnZd→IIZnZd) gives a less stable product IIZnZd containing two neighboring positively charged species, while, as it has been mentioned above, this reaction path was not identified for the case of the CdZs site due to the same reasons. Note that in the case of Zd site the newly formed O4–H2 or O6–H1 bonds are rather weak due to the low basicity of the oxygen atoms from the silicon-occupied oxygen tetrahedrons. In contrast, the O4–H2 bond formed at the anionic site of the zeolite due to the C–H activation over the ZnZs or CdZs site (I→TS1→II) is much stronger as compared to the O6–H1 (I→TS1’→II’). The energy gain due to the formation of a stronger chemical bond compensates more effectively for the energy loss due to the decrease of the coordination of the exchangeable cation and repulsion of the likely charged species. This, in turn, results in higher stability of II as compared to II’ for both ZnZs and CdZs sites.

5.4. Discussion

In the present study three different possible types of charge compensation are considered for zinc species in Zn/ZSM-5: isolated Zn2+ stabilized at either charge-alternating (Zd) or conventional ion-exchange cation site (Zs), and oxygen-bridged dimeric [ZnOZn]2+ cation. In addition two sites of Cd/ZSM-5 are discussed, which are isolated Cd2+ cations at either Zd or Zs cation site. C2H6 specifically adsorbs on the mononuclear site independently of the exchanged cation and of the type of the cation site, whereas ethane coordination to the [ZnOZn]2+ cation is very weak (∆Eads = 11 kJ/mol) and corresponds to physical adsorption. The calculated ethane adsorption energy on ZnZd is almost twice as high as the corresponding value for the ZnZs model (53 and 26 kJ/mol, respectively). In spite of the expected lower Lewis acidity of Cd2+ ions than that of Zn2+, very similar adsorption energies are computed for the cadmium-sites. This is due to a weaker shielding of the

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Figure 5.7. Diffuse‐reflectance infra‐red spectra of ethane adsorbed at relatively low pressure on 

(a)  Zn/ZSM‐5  [32]  and  (b)  ZnY  [49]  zeolites  prepared  via  high‐temperature  reaction  of  the 

hydrogen form of the respective zeolite with zinc vapor. 

larger Cd2+ ion with the lattice oxygen atoms of the cation site. This effect is similar to that discussed in Chapter 4.

The strong interaction is accompanied with large changes in the geometric and charge properties of the adsorbed species. As a result the calculated infrared spectra of the C–H stretching vibrations significantly differ from those of the gas-phase ethane as well as of the weak adsorption complex with ZnOZn. These spectra contain a strongly red-shifted and intense IR band corresponding to the vibration originated from the ν1 (a1g) stretching mode of gaseous ethane.

Recently a detailed experimental study of molecular and dissociative adsorption of ethane on Zn/ZSM-5 zeolite has been reported [32]. The catalyst has been prepared via a high-temperature reaction of zinc vapor with the hydrogen form of the zeolite. This method results in complete and selective substitution of zeolitic protons with isolated Zn2+ cations. Adsorption of light alkanes on Zn/ZSM-5 results in a very strong red shift of one C–H stretching band as well as in increase of its relative intensity (Figure 5.7 (a)). It has been concluded that stabilization of bivalent Zn2+ cations at [AlO2]– framework unit is possible with distant placing of the second charge-compensating aluminum ion. Coordination of an alkane molecule to the thus-stabilized extra-framework cation results in a strong and selective polarization of the interacting C–H bonds. This along with the high Lewis acidity of the exchangeable cation results in the increase of the relative intensity and strong red shift of the respective infrared stretching band. The strong molecular C2H6 adsorption has been shown to be a precursor to the subsequent dissociative adsorption of ethane. It has been proposed also that adsorption of ethane on zinc ions stabilized in the conventional ion-exchange sites results in similar perturbations and, hence, in similar vibrational properties of the adsorbed molecules to those observed in the case of low-silica zeolites modified with zinc (Figure 5.7 (b)).

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In contrast, our calculations show that molecular C2H6 adsorption on Zn2+ ions stabilized in either charge-alternating (Zd) or conventional ion-exchange sites (Zs) results in very similar infrared spectra, containing a very intense and low-frequency vibrational band. The frequency of the corresponding vibration is only by ~20 cm–1 lower for the ZnZd site, while the overall infrared spectrum is very different from that observed in the ZnY zeolite (Figure 5.7 (b)) [49]. Therefore, the strength of perturbations of the molecules adsorbed on Zn2+ ions stabilized in zeolites mainly depends on the geometry of the cation site rather than on the relative location of the anionic sites. The shape and geometry of cation site influence the ligand field stabilizing the exchangeable cation. This, in turn, affects strength of donor-acceptor interactions between the adsorbate and the extra-framework cation. Moreover, it has been shown in Chapter 4 that depending on the shape of the cation site and the size of the exchanged cation the effective electrostatic field of the cation can be significantly varied. This would result in different polarization and redistribution of charges within the adsorbed molecule.

Indeed, the larger cadmium ions stabilized in the five-membered ring of ZSM-5 zeolite are less effectively shielded by the surrounding lattice oxygen ions. This results in enhancement of the polarization ability of the extra-framework cations. Therefore, vibrational properties of adsorbed ethane become very sensitive to the type of the charge-compensation of the exchanged cation, i.e. relative position of the anionic sites of the zeolite. Depending on the local environment of Cd2+ in the zeolite, the adsorption complexes exhibit very different vibrational and chemical properties. However, cadmium-exchanged ZSM-5 zeolite is expected to exhibit similar reactivity as Zn/ZSM-5 reactivity for ethane activation. Using infrared spectroscopy of light alkanes adsorption as molecular probe one can distinguish different location of Cd2+ in ZSM-5, while in the case of Zn/ZSM-5 zeolite this is unlikely.

According to the experimental findings [32], adsorption of ethane on Zn/ZSM-5 zeolite at relative high equilibrium pressure results in the appearance of an intense and low-frequency IR band with a maximum at 2740 cm–1. Decrease of the equilibrium pressure above the sample leads to a further red shift of this band down to 2727 cm–1. On the other hand the opposite trend has been observed upon dissociative adsorption of ethane. The position of this band shifts to the higher frequencies. Thus, the experimental data clearly show an inhomogeneity of C2H6 species adsorbed on Zn/ZSM-5, which differ in both adsorption energies and in chemical reactivity, and at the same time, being characterized by very similar infrared spectra.

This coheres very well with the calculated results presented here. Indeed, heterolytic dissociation of ethane over zinc ions stabilized at the cation site with distant anionic [AlO2]– framework units is a more favorable process than the respective reaction over Zn2+ at the conventional ion-exchanged site. At the same time molecular C2H6 adsorption is significantly stronger on the ZnZd sites and is predominant at lower pressures, explaining the experimentally observed red shift of the low-frequency band upon the decrease of the

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equilibrium pressure above the zeolite. Dissociative ethane adsorption takes place at the same ZnZd sites resulting in occupation of the respective adsorption sites as well as in a shift of the low-frequency infrared band toward higher frequencies.

In contrast to the assumption made in Refs. [11,31,32,50], the calculations presented show no direct correlation between the reactivity of the zeolitic cations and the perturbations of ethane adsorbed on the respective sites. Moreover, the shape of the mostly perturbed vibration does not always correspond to the reaction coordinate. This is, most likely, due to the fact that the transition state involved in the heterolytic C–H cleavage is a so-called “late transition state” [4]. Taking into account the very similar computed reactivity of ZnZd and CdZd sites, one can conclude that the reactivity in this case is mainly controlled by the energy of the strongest bond formed due to the reaction (OH), while the energetics of the metal-carbon bond has only a small influence. This explains the absence of the correlation between the perturbation of the adsorbed ethane and subsequent dissociation.

5.5. Conclusions

Molecular and dissociative adsorption of ethane on various zinc and cadmium containing cationic species stabilized in ZSM-5 zeolite are investigated using DFT calculations. Despite the very weak interaction with the molecular C2H6, binuclear ZnOZn sites are found to be the most reactive for the initial heterolytic dissociation of ethane. C–H bond cleavage over mononuclear sites is less favored.

Cadmium and zinc cations stabilized at the charge-alternating cation sites (Zd) show the strongest molecular adsorption and the highest reactivity among the mononuclear sites considered. Initial proton abstraction is preferred on the O-sites neighboring the ring where the cation is located rather than to the oxygen atoms of the same zeolitic ring. On the other hand, in the case of the conventional ion-exchange site (Zs), initial proton abstraction on the oxygen atom neighboring the exchanged cation is favored. In this case the energy gain due to the formation of a hydroxyl group at the more basic site of aluminum-occupied oxygen tetrahedron dominates the energy loss due to the repulsion of the likely-charged species closely located in one zeolitic ring.

No correlation between the perturbations of the adsorbed molecules and their subsequent heterolytic dissociation is found. In spite of much stronger perturbations of the adsorbed C2H6 to Cd2+ ions at the Zd site, their reactivity is the same as of zinc cations. The reactivity of the extra-framework cations for heterolytic C–H bond cleavage is controlled by the stability of the reaction products rather than by the properties of the initial state.

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Chem. B 2004, 108, 4116. 30 Kazansky, V.B.; Serykh, A.I. Microporous and Mesoporous Materials 2004, 70, 151. 31 Kazansky, V.B.; Serykh, A.I.; Pidko, E.A. J. Catal. 2004, 225, 369. 32 Kazansky, V.B.; Pidko, E.A. J. Phys. Chem. B 2005, 109, 2103. 33 Kazansky, V.B.; Borovkov, V. Yu.; Serykh, A.I.; van Santen, R.A.; Anderson, B.G. Catal. Lett. 2000, 66, 39. 34 Kazansky, V.B.; Serykh, A.I.; van Santen, R.A.; Anderson, B.G. Catal. Lett. 2001, 74, 55. 35 Kazansky, V.B.; Serykh, A.I.; Anderson, B.G.; van Santen, R.A. Catal. Lett. 2003, 88, 211. 36 Kazasnky, V.B. J. Catal. 2003, 216, 192. 37 Kazansky, V.B.; Borovkov, V. Yu.; Serykh, A.I.; van Santen, R.A.; Stobbelar, P. Phys. Chem. Chem. Phys. 

1999, 1, 2881. 38 Kallo, D.; Onyestyak, Gy. in: Delman, B.; Fróment, G.F. (Eds.), Catalyst Deactivation , Stud. Surf. Sci. and 

Catal., Vol. 34, Elsevier, Amsterdam, 1987, p. 605. 39 Bandiera, J.; Ben Taarit, Y. Appl. Catal. A: General 1997, 152, 43. 40 Serykh, A.I. Microporous and Mesoporous Materials 2005, 80, 321. 41 Lide, D.R. (Ed.), CRC Handbook of Chemistry and Physics 80th ed., CRC Press, New York, 1999. 42 Lermer, H.; Draeger, M.; Steffen J.; Unger, K.K. Zeolites 1985, 5, 131. 43 Olson, D.H.; Kokotailo, G.T.; Lawton, S.L.; Meier, W.M. J. Chem. Phys. 1981, 85, 2238. 44 Becke, A.D. Phys. Rev. 1988, A38, 3098; Becke, A.D. J. Chem. Phys. 1993, 98, 1372; Becke, A.D. J. Chem. 

Phys. 1993, 98, 5648. 45 Backer,  J.; Muir, M.;  Andzelm,  J.;  Scheiner,  A.  in:  Laird,  B.B.;  Ross,  R.B.;  Ziegler,  T.  (Eds.)  Chemical 

Applications of Density‐Functional Theory, ACS Symposium Series, vol. 629, American Chemical Society, Washington, DC, 1996. 

46 Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.,  Jr.;  Vreven,  T.;  Kudin,  K.  N.;  Burant,  J.  C.; Millam,  J. M.;  Iyengar,  S.  S.;  Tomasi,  J.;  Barone,  V.; Mennucci, B.; Cossi, M.;  Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,  X.;  Knox,  J.  E.;  Hratchian,  H.  P.;  Cross,  J.  B.;  Bakken,  V.;  Adamo,  C.;  Jaramillo,  J.;  Gomperts,  R.; 

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Stratmann,  R.  E.;  Yazyev,  O.;  Austin,  A.  J.;  Cammi,  R.;  Pomelli,  C.;  Ochterski,  J.  W.;  Ayala,  P.  Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al‐Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;  Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,  J. A. Gaussian 03,  revision B.05; Gaussian, Inc.: Pittsburgh PA, 2003. 

47 Godbout, N.; Salahub, D.R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70, 560. 48 Dunning, Jr., T.H.; Hay, P.J. in Schaefer III, H.F. (Ed.), Modern Theoretical Chemistry, Vol. 3, Plenum, New 

York, 1976, p. 1‐28. 49 Pidko, E.A.; Kazansky, V.B. Kinet. Catal. 2005, 46, 407. 50 Kazansky, V.B.; Subbotina, I.R.; Pronin, A.A.; Schlogl, R.; Jentoft, F.C. J. Phys. Chem. B 2006, 110, 7975.   

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CHAPTER 6  

CATALYTIC DEHYDROGENATION OF LIGHT ALKANES OVER ZINC CATIONS IN ZN/ZSM‐5 ZEOLITE 

 

 

 

The mechanism of alkane dehydrogenation over Zn/ZSM-5 zeolite is studied using the DFT cluster modeling approach. Three types of active sites, discussed in the previous chapter, are considered here as well: Zn2+ ions stabilized either at conventional ion-exchange sites (ZnZs) or at cation sites with distantly placed aluminum ions (ZnZd), and binuclear [ZnOZn]2+ cations (ZnOZn). A comparison of the computed energetics of various reaction paths for ethane dehydrogenation indicates that the catalytic reaction proceeds most easily over the ZnZd sites. The enhanced Lewis acidity of these sites facilitates the heterolytic C–H bond cleavage. The most favorable proton-accepting sites are not those of the framework ring to which Zn2+ is attached, but lattice oxygen anions of a neighboring ring. After the heterolytic C–H bond cleavage step, the zinc-alkyl group and acidic proton recombine via a cyclic transition state resulting in formation of an alkene and H2 in one-step. This process is thermodynamically and kinetically preferred over the consecutive mechanism for the isolated Zn2+ sites. The activation barrier for the one-step elimination reaction strongly depends on the relative position of the reacting zinc-alkyl and framework-attached H+ ions. Despite initial heterolytic C–H bond dissociation being strongly favored on the [ZnOZn]2+ cations, the activation energy for the subsequent decomposition of the resulting products is high.

 

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Scheme 6.1: Possible reaction paths for 

ethane dehydrogenation. 

6.1. Introduction

In the previous chapter the initial activation of C–H bonds over the extra-framework zinc and cadmium cations in ZSM-5 zeolites have been discussed. It has been shown that the exchangeable cadmium cations show similar reactivity to that of zinc-sites. Despite the very weak interaction with molecular ethane, the highest reactivity toward heterolytic dissociation of C–H bond has been found for the binuclear [ZnOZn]2+ cations stabilized in ZSM-5 zeolite. However, in practice the dehydrogenation of light alkanes takes place at rather high temperature, and in spite of the difference in the energetics of the initial C–H bond activation, the reaction rate can be determined by one of the subsequent reaction steps. Therefore, in order to compare the catalytic activity of the different zinc sites and to clarify the preferred reaction paths, one must investigate the energetics of the complete catalytic cycle.

The first theoretical study of the reaction paths of ethane dehydrogenation on a Zn2+ cation stabilized in a four-membered zeolitic ring has been reported by Frash and van Santen [1]. It has been concluded that the so-called “alkyl” path (Rδ-–Hδ+) of ethane activation is strongly favored over the “carbenium” mechanism (Rδ+–Hδ-). A three-step mechanism has been proposed that included (i) heterolytic C–H bond cleavage, resulting in formation of a zinc-ethyl group and a Brønsted acid proton, (ii) subsequent elimination of molecular ethylene via abstraction of a hydrogen attached to β-carbon of the grafted C2H5 fragment, and (iii) hydrogen desorption via recombination of the acidic proton and the hydride ion attached to zinc. Although the initial heterolytic dissociation of ethane on Zn2+ and [ZnOZn]2+ cations stabilized in extended cluster models representing different cation sites of ZSM-5 zeolite has been reported later [2-5], a complete investigation of the catalytic light alkane dehydrogenation over such sites has not been reported before.

This chapter continues the DFT study of the nature of the active site and the mechanism of the catalytic dehydrogenation of light alkanes over zinc-modified ZSM-5 zeolites. Ethane dehydrogenation is chosen as a model reaction. Two different reaction paths are considered (Scheme 6.1). The initial step in both pathways consists of the dissociation of molecularly adsorbed ethane (I) via the alkyl mechanism. The resulting intermediate II decomposes via elimination of ethylene (II→III→IV) with subsequent hydrogen recombination from intermediate IV (outer cycle in Scheme 6.1), similar to the mechanism proposed in Ref. [1]. One could also imagine that the reaction proceeds via an

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alternative route (inner cycle in Scheme 6.1) that is a one-step process resulting in desorption of H2 and formation of a molecular complex V of ethylene with the active site. Both mechanisms are studied for three different representations of the active sites in Zn/ZSM-5: binuclear [ZnOZn]2+ ion (ZnOZn), isolated Zn2+ stabilized in the conventional ion-exchange site of the zeolite (ZnZs), or in the cation site with distantly separated framework aluminums(ZnZd).  

This work is focused on the investigation of elementary steps of the catalytic cycle involving elimination of the products of ethane dehydrogenation and on the energetics of the full catalytic cycles for various reaction mechanisms involving three types of Zn cations.

6.2. Computational details

The same cluster models (ZnZd, ZnZs, and ZnOZn) and the quantum-chemical methods as described in Chapter 5 were employed in this study. All energies obtained from the DFT calculations used for the estimation of the reaction heats and activation barriers were corrected for the zero-point energy obtained from frequency calculations performed on all the optimized structures. The frequency calculations provided the thermochemical analysis using the ideal gas approximation at a pressure of 1 atm and a temperature of 298.15 K. The values of ∆Gº reported for selected reactions were calculated at these conditions.

The notations for the intermediates and transition state structures will be according to the numbering used in Scheme 6.1 with the corresponding active site given in the subscript.

6.3. Results and Discussion

6.3.1. Decomposition of the grafted ethyl species

The catalytic cycle of dehydrogenation is initiated by dissociative adsorption of C2H6 on the conjugate Lewis acid-base pair formed by the extra-framework zinc species and one of the basic lattice oxygen atoms in the case of mononuclear Zn2+ sites or the extra-framework bridging oxygen of ZnOZn site (Chapter 5). Heterolytic dissociation of ethane results in the formation of a hydroxyl group and zinc-alkyl fragment (II).

 

Scheme 6.2: β–H transfer reaction 

The next reaction step consists of decomposition of the [Zn–C2H5]+ species via ethylene desorption. The generally accepted mechanism for this process is the β-scission reaction shown in Scheme 6.2 [1,5,6]. Abstraction of a hydride ion from the β C-atom of the ethyl group by zinc results in the formation of a zinc hydride and molecular ethylene. Stretching of the Znδ+–Cδ– bond of the grafted C2H5 group enhances the polarization of this bond and therefore increases the Lewis acid strength of the Zn cation. The Zn ion polarizes the β C–

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 Figure 6.1. Reaction paths for β–H transfer reactions and subsequent H2 desorption on the ZnZd. 

Table  6.1.  Selected  interatomic  distances  (Å)  of  the  intermediates  and  transition  state 

structures  of  decomposition  of  the  products  of  ethane  dissociatively  adsorbed  on  ZnZd  via 

consecutive C2H4 and H2 desorption 

ZnZd  II  II’  TS2  TS2’  III III’ TS5 IV IV’ TS3  TS3’  V Zn‐O1  1.957  2.086 1.967  2.045  1.975 2.100 1.942 1.937 2.038 1.934  2.117  2.026Zn‐O2  2.082  1.946 2.047  1.957  2.040 1.963 1.998 2.056 1.930 1.947  1.963  1.971Zn‐O4  4.323  3.155 4.463  3.829  3.413 3.879 3.310 4.401 3.097 2.803  2.175  2.140Zn‐O5  3.299  3.283 3.499  3.697  3.279 3.724 3.212 3.323 3.272 2.336  2.259  2181Zn‐C1  1.940  1.942 2.044  2.072  2.317 2.596 2.295 — — —  —  2.278

2.334 Zn‐H3  —  —  1.709  1.658  1.591 1.528 1.661 1.513 1.512 1.635  1.700  —O6‐H1  —  0.968 —  0.968  — 0.965 — — 0.968 —  1.279  —O4‐H2  0.973  —  0.973  —  1.024 — 1.259 0.974 — 1.274  —  —C2‐H3  1.095  1.095 1.643  1.687  — — — — — —  —  —H1(2)‐H3  —  —  —  —  1.446 — 0.988 — — 1.006  1.026  — 

H bond of the ethyl fragment resulting in abstraction of the hydride ion and desorption of ethylene. 

The energetics of this process are presented in Figures 6.1, 6.2, and 6.3 for the products of dissociative C2H6 adsorption (II and II’) on ZnZd, ZnZs, and ZnOZn, respectively. All of these reactions are endothermic and show rather high activation barriers. The energetic parameters for the C2H4 elimination from structures IIZnZd and II’ZnZd (IIZnZd →IIIZnZd and II’ZnZd →III’ZnZd, Figure 6.1) are similar. The calculated activation energies (∆E≠) are equal to 180 and 194 kJ/mol, respectively, for the decomposition of IIZnZd and II’ZnZd. The geometry parameters for the transition states are also very similar (Table 6.1). Thus, in the

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 Figure 6.2. Reaction paths for β–H transfer reactions and subsequent H2 desorption on the ZnZs. 

Table  6.2.  Selected  interatomic  distances  (Å)  of  the  intermediates  and  transition  state 

structures  of  decomposition  of  the  products  of  ethane  dissociatively  adsorbed  on  ZnZs  via 

consecutive C2H4 and H2 desorption 

ZnZs  II  II’  TS2 TS2’ III IV IV’  TS3Zn‐O1  2.049  2.787  1.968 2.056 1.987 2.075 2.775  2.045Zn‐O2  2.042  2.074  2.035 1.954 2.040 2.021 2.060  1.950Zn‐O4  3.589  2.052  4.239 3.265 3.354 3.449 2.036  2.483Zn‐O5  2.431  2.251  3.466 3.591 3.225 2.268 2.205  2.62Zn‐C1  1.955  1.978  2.061 2.134 2.369 — —  —Zn‐H3  —  —  1.699 1.642 1.571 1.533 1.549  1.690O6‐H1  —  0.975  — 0.973 — — 0.975  —O4‐H2  0.968  —  0.968 — 0.991 0.968 —  1.304C2‐H3  1.095  1.095  1.643 1.692 — — —  —H1(2)‐H3  —  —  — — 1.654 — —  0.988 

case of ethane dissociation on the ZnZd site, subsequent ethylene desorption does not depend appreciably on the relative position of the zinc site and the zeolitic Brønsted acid site. On the other hand, in the case of the ZnZs site, the zinc ion from the [Zn–C2H5]+ group in II’ZnZs is coordinated to four zeolitic oxygen atoms (Table 6.2). Two of these coordination bonds (Zn–O4 and Zn–O5) are broken in the corresponding transition state structure (TS2’ZnZs) resulting in a very high activation barrier (257 kJ/mol) for the β-H transfer (Figure 6.2). However, when both the acidic proton and the zinc-alkyl fragment are located in the same ring of the ZnZs (IIZnZs) no additional loss of coordination of zinc atom to O atoms is required (Table 6.2). Both the activation energy and the energy change for the C2H4 elimination in this case (IIZnZs→IIIZnZs: ∆E = 67 kJ/mol, ∆E≠ = 192 kJ/mol, Figure 6.2) are similar to those calculated for the ZnZd site (Figure 6.1).

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 Figure  6.3.  Reaction  paths  for  β‐H  transfer  reactions  and  subsequent  H2  desorption  on  the 

ZnOZn site. 

Table  6.3.  Selected  interatomic  distances  (Å)  of  the  intermediates  and  transition  state 

structures of decomposition of  the products of ethane dissociatively adsorbed on ZnOZn via 

consecutive C2H4 and H2 desorption 

ZnOZn  II  TS2  III IV TS3 TS5 V Zn‐O1  2.039  2.063  2.084 2.024 2.030 2.038  2.038 Zn‐O2  2.206  2.218  2.239 2.159 2.040 2.170  2.150 Zn‐O4  3.534  3.481  3.382 3.483 2.398 3.195  3.003 Zn‐O10  2.223  2.180  2.339 2.201 1.945 1.986  1.846 Zn‐C1  1.975  2.098  2.460 — — 2.387  2.447/2.384Zn‐H3  —  1.741  1.576 1.548 1.912 1.950  — O10‐H1  0.960  0.961  0.961 0.960 1.352 1.392  — C2‐H3  1.097  1.664  — — — — — H1(2)‐H3  —  2.326  2.261 2.737 0.938 0.910  — 

For  the third type of active site (ZnOZn), decomposition of the grafted ethyl group (IIZnOZn→IIIZnOZn, Figure 6.3) is more endothermic (∆E = 99 kJ/mol), while the activation barrier is comparable to the reactions discussed above (∆E≠ = 190 kJ/mol).

All of the above β-H-transfer reactions result in the formation of zinc hydride species with molecularly adsorbed ethylene (III). The strongest binding of C2H4 to [Zn–H]+ ion is found for structures IIIZnZd and IIIZnZs with proximate zinc and acidic proton. The corresponding adsorption energies are equal to 55 and 33 kJ/mol, respectively. The lower adsorption energy for the Zs case is due to the lower Lewis acidity of the zinc species stabilized at this site. In the case of the distant location of the zinc hydride and the acidic proton in structure III’ZnZd the ethylene adsorption is much weaker (16 kJ/mol), whereas no similar adsorption complex  with the zinc-hydride ion at the Zs site (IV’ZnZs) is identified. Most likely, this is due to the higher coordinative saturation of the Zn2+ ion in structures IV’ZnZd and IV’ZnZs.

Although C2H4 can bind in principle to intermediate IVZnOZn, it does not result in any considerable energy change (see IIIZnOZn→ IVZnOZn, Figure 6.3). Most likely, due to the

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very low Lewis acidity of this zinc site, the stabilization of the adsorbed ethylene can only be provided by formation of very weak CH···O interactions with framework oxygen atoms. However, such dispersive interactions are not correctly described within the employed DFT formalism (see Chapter 4).

The energies of C2H4 adsorption to the zinc-hydride fragments correlate very well with the enthalpies of the β-scission reactions: the stronger the ethylene adsorption is, the less endothermic the reaction is. Thus, we conclude that the stability of the zinc-hydride species apparently does not depend on the local surrounding, while the thermodynamics of the β-H transfer is mainly controlled by the strength of the C2H4 interaction with the [Zn–H]+ ion in the respective reaction product.

At the next step of the catalytic cycle, H2 is being removed via recombination of an acidic proton and a hydride ion bound to Zn2+ (IV and IV’). The corresponding reaction paths as well as their energetics are shown in Figures 6.1 – 6.3. Recombination of H2 at ZnZd and ZnZs sites (Figures 6.1 and 6.2) is an easy process and shows activation energies (∆E≠) in the range of 34−41 kJ/mol, depending on the relative stability of the [Zn2+–H–···H+]2+ species. The transition state structure for the H2 recombination from intermediate IV’ZnZs was not located. Dihydrogen desorption from IVZnOZn (Figure 6.3) is expected to be a highly activated process (∆E≠ = 146 kJ/mol) because of the high basicity of the terminal Zn–OH group and strong strains of the resulting ZnOZn site.    

A very interesting peculiarity is observed for H2 recombination from the closely located H+ and [Zn2+–H–]+ species in the intermediate IIIZnZd. Coordination of C2H4 to zinc ion results in loss of the activation barrier for the hydrogen removal (IIIZnZd→VZnZd). Indeed, the charge-donation from adsorbed ethylene to zinc results in weakening of the Zn–H bond. At the same time the O–H bond in this structure is weak due to the rather low basicity of the corresponding O4 atom of the Zd site. Both these factors along with the rather short distance between the H– and H+ species result, already in structure IIIZnZd, in a direct interaction between these two unlikely charged ions. This strongly facilitates the subsequent H2 desorption resulting in formation of a strong π-complex of ethylene with the low-coordinated Zn2+ ion. A similar effect is also observed for the ZnOZn site (IIIZnOZn→ VZnOZn). However, in the latter case this effect is less pronounced due to the significantly weaker bonding of C2H4 to the [ZnOZn]2+ ion.

 Scheme 6.3: One‐step decomposition of the product of C2H6 dissociative adsorption 

Besides consecutive formation of C2H4 and H2, an alternative reaction path leading to closure of the ethane dehydrogenation catalytic cycle is a one-step decomposition of the

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 Figure 6.4. Reaction paths for one‐step decomposition of products of dissociative C2H4 adsorption 

on the zinc‐sites of ZSM‐5 zeolite.  

Table 6.4. Selected interatomic distances (Å) of the intermediates and transition state structures 

involved in the one‐step C2H4 and H2 desorption reaction. 

  ZnZd  ZnZs  ZnOZn   II  II’  TS4  TS4’  V  II II’ TS4 TS4’ V II  TS4  V Zn‐O1  1.957  2.086  1.943  2.064  2.026 2.049 2.787 2.165 2.247 2.146 2.039  2.035  2.038Zn‐O2  2.082  1.946  2.009  1.991  1.971 2.042 2.074 1.995 2.029 1.987 2.206  2.184  2.150Zn‐O4  4.323  3.155  3.668  2.191  2.140 3.589 2.052 2.320 2.053 2.026 3.534  3.210  3.003Zn‐O5a  3.299  3.283  3.387  2.329  2181 2.431 2.251 2.087 2.189 2.077 2.223  1.900  1.846Zn‐C1  1.940  1.942  2.018  2.054  2.278/

 2.3341.955 1.978 2.065 2.074 2.343/

2.342 1.975  2.146  2.447/

2.384 Zn‐H3  —  —  1.862  2.972  —  — — 3.003 3.019 — —  2.988  — O6‐H1b  —  0.968  —  1.496  —  — 0.975 — 1.440 — 0.960  1.381  — O4‐H2  0.973  —  1.038  —  —  0.968 — 1.452 — — —  —  — C2‐H3  1.095  1.095  1.608  1.520  —  1.095 1.095 1.482 1.426 — 1.097  1.858  — H1(2)‐H3  —  —  1.382  0.846  —  — — 0.858 0.871 — —  0.881  — a Zn1‐O10 distance for the ZnOZn; b O10‐H10 distance for the ZnOZn. 

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Table  6.5.  Gibbs  free  energy  changes 

(∆Go298, kJ/mol)  for  the decomposition 

of  ethyl  species  grafted  to  the  active 

sites of Zn/ZSM‐5. 

    ∆Go298 

β–H abstraction ZnZd  IIZnZd → IIIZnZd +55   II'ZnZd → III'ZnZd +83 ZnZs  IIZnZs → IIIZnZs +72   II’ZnZs → IV’ZnZs+C2H4 +51 ZnOZn IIZnOZn → IIIZnOZn  +100 

One‐step reaction ZnZd  IIZnZd → VZnZd + H2   –56   II'ZnZd → VZnZd + H2   –10 ZnZs  IIZnZs → VZnZs+H2   –34   II'ZnZs → VZnZs+H2  –85 ZnOZn IIZnOZn → VZnOZn+H2   +138   

product of C2H6 dissociative adsorption (II) via a cyclic transition state (Scheme 6.3). The corresponding reaction paths are shown in Figure 6.4, while Table 6.4 lists geometry parameters of the intermediates and the transition states involved.

For the isolated Zn2+ sites, independently of the relative location of [Zn–C2H5]+ and H+ species, this process (IIZnZd→VZnZd and II’ZnZd→VZnZd) is characterized by smaller values of the reaction enthalpy and lower activation energies as compared to the corresponding β-scission reactions. Table 6.5 lists the calculated Gibbs energy changes for the respective reactions. The values of ∆Go

298 for the one-step reaction over ZnZd and ZnZs are negative, whereas the corresponding parameters for the β-H transfer reactions are strongly positive.

Formation of cyclic transition states (TS4’ZnZd and TS4’ ZnZs) from the distant H+ and zinc-ethyl groups (II’ZnZd and II’ZnZs) demands less energy (∆E≠ = 145 and 105 kJ/mol, respectively) than from the intermediates containing these species in one zeolite ring (IIZnZd→TS4ZnZd→VZnZd: ∆E≠ = 166 kJ/mol; IIZnZs→TS4ZnZs→VZnZs: ∆E≠ = 155 kJ/mol). This effect is due to steric reasons. When both the proton and zinc-ethyl groups are located closer, stronger deformations are required to form the cyclic transition state structure for the one-step C2H4/H2 elimination. This explains why this reaction path is less favored for the ZnOZn site as compared to the β-H transfer reaction (IIZnOZn→TS4ZnOZn→VZnOZn: ∆E≠ = 253 kJ/mol and IIZnOZn→TS2ZnOZn→IIIZnOZn: ∆E≠ = 190 kJ/mol, respectively).

6.3.2. Catalytic dehydrogenation of ethane

So far results of DFT computations for the different elementary reaction steps, which are parts of the reaction cycle of catalytic C2H6 dehydrogenation, have been discussed. To compare the overall catalytic activity of the different zinc sites and to clarify the preferred reaction paths, the reaction energy diagrams of the catalytic pathways should be analyzed for the various possible reaction path and active sites.

The calculated energetics for catalytic ethane dehydrogenation over ZnZd site are presented in Figure 6.5. It has been shown in Chapter 5 that the initial proton abstraction resulting in formation of a distantly located zinc-alkyl group and Brønsted acid site (I→II’, Figure 6.5 (b)) is favored both kinetically and thermodynamically over the respective pathway resulting in formation of these species in the same zeolitic ring (Figure 6.5 (a)). Indeed the apparent activation energy for the former reaction is lower by 33 kJ/mol, while

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 Figure 6.5. Ethane dehydrogenation over ZnZd with initial proton abstraction to the O4 (a) and 

O6 (b) site.  

the corresponding product II’ZnZd is more stable by 50 kJ/mol as compared to IIZnZd (Figure 6.5 (b) and (a), respectively). However, in practice the dehydrogenation of light alkanes takes place at rather high temperature and, in spite of the difference in the energetics of the initial C–H bond activation, the catalytic process can be driven by the subsequent steps.

Independent of the mechanism of the initial C–H bond cleavage, subsequent one-step decomposition of the zinc-alkyl fragments (II→V+H2 and II’→V+H2, Figure 6.5) is preferred over the consecutive elimination of C2H4 and H2 (II→III and II’→III’). In the case of location of [Zn–C2H5]+ and H+ ions in the same zeolite ring (II) the activation energies for both these processes are very close. Nevertheless, the one-step reaction is

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 Figure 6.6. Ethane dehydrogenation over ZnZs with initial proton abstraction to the O4 (a) and 

O6 (b) site. 

strongly favored thermodynamically (Table 6.5) and results in a stable intermediate VZnZd. For the distantly separated zinc-ethyl fragment and an acidic proton, the one-step formation of C2H4 and H2 shows about a 50 kJ/mol lower activation barrier and also is thermodynamically favorable.

Thus, the most favorable pathway for the catalytic dehydrogenation of ethane over the ZnZd site consists of the following steps: after the molecular adsorption of C2H6 on the extra-framework Zn2+ cation heterolytic C–H bond cleavage takes place. Subsequent decomposition of the grafted alkyl group with simultaneous H2 desorption via the cyclic transition state results in formation of a stable adsorption complex of ethylene with the exchangeable zinc cation. This reaction is the rate-limiting step. Desorption of ethylene

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 Figure 6.7. Ethane dehydrogenation over ZnOZn. 

closes the catalytic cycle. The estimated apparent activation energy for the catalytic cycle over Zn2+ stabilized at the charge-alternating cation site is equal to 153 kJ/mol.

Figure 6.6 shows the pathways for ethane dehydrogenation over the ZnZs site. In contrast to the case of ZnZd, formation of [Zn–C2H5]+ and H+ ions in one zeolitic ring (I→II, Figure 6.6 (a)) via initial C–H bond cleavage is more favored due to higher stability of the proton bound to the aluminum-occupied oxygen tetrahedron. As for the other processes, the trends are the same as those discussed above. The one-step decomposition reactions of intermediates IIZnZs and II’ZnZs (II→V+H2 and II’→V+H2, Figure 6.6) are more favorable than the hydride abstraction reactions (II→III and II’→III’). The height of the activation barrier (TS4ZnZs and TS4’ZnZs) for the former reaction strongly depends on the relative position of the reacting [Zn–C2H5]+ and H+ species, whereas no apparent correlation between the nature of the O-site to which a proton is attached and the energetics of this reaction is observed. However, the apparent activation energies for the catalytic process are very similar for both mechanisms of the initial heterolytic dissociation (Figure 6.6 (a) and (b)). The corresponding calculated values are equal to 226 and 222 kJ/mol, respectively for the IZnZs→IIZnZs and IZnZs→II’ZnZs mechanisms of the initial C–H bond cleavage.

The reaction energy diagrams for catalytic C2H6 dehydrogenation over the binuclear ZnOZn site are shown in Figure 6.7. The high exothermicity of the initial dissociative ethane adsorption (I→II) is due to high reactivity of bridging oxygen and strong steric strains in the [ZnOZn]2+ cations, which are removed via cleavage of one of the Zn–O bonds. The resulting product IIZnOZn is very stable, and therefore, the subsequent transformations demand higher energies. In contrast to the above-discussed active sites, the one-step C2H4 and H2 desorption (II→V+H2) in this case is less thermodynamically and kinetically favorable than the β-H atom abstraction from the grafted ethyl group by zinc

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ion (II→III). The respective activation energies differ by 63 kJ/mol, while the products of the latter reaction (IIIZnOZn) are by 68 kJ/mol more stable than the structure VZnOZn.

The intermediate IIIZnOZn is unstable. Although coordination of ethylene to zinc-hydride ion in IIIZnOZn can in principle promote H2 desorption (TS5ZnOZn), it rather decomposes via reaction IIIZnOZn→IVZnOZn. The catalytic cycle closes via recombination of a hydride ion from Zn–H species and a proton. The apparent activation energy for the lowest-energy pathway of ethane dehydrogenation over ZnOZn is very close to the respective value for the ZnZd site (166 and 153 kJ/mol, respectively). However, the former process proceeds via formation of a very thermodynamically stable intermediate IIZnOZn and a relatively high energy is required to decompose it. Therefore, one can expect that the catalytic cycle will be significantly slowed since the equilibrium will be shifted to the structure IIZnOZn for IIZnOZn↔IIIZnOZn reaction (Table 6.5).

6.3.3. Implications for active sites in Zn/ZSM-5

Three different possible types of charge-compensating zinc species in Zn/ZSM-5 have been considered above: isolated Zn2+ stabilized in either charge-alternating (Zd) or conventional ion-exchange cation site (Zs), and oxygen-bridged dimeric [ZnOZn]2+ cation. Initial C–H bond cleavage over zinc-sites is followed by the decomposition of the zinc-ethyl species and zeolitic Brønsted acid site, which takes place in the case of mononuclear Zn-species via the one-step elimination of molecular ethylene and dihydrogen. This process proceeds via recombination of an H atom from β-methyl group of [Zn–C2H5]+ ion and H+ ion attached to framework oxygen. On the other hand, desorption of ethylene has been observed experimentally simultaneously with formation of zinc-hydride ions [6]. It has been concluded that the catalytic cycle is closed by consecutive desorption of C2H4 and H2. However, both present calculations and recent experimental results [7-9] show that H2 dissociative adsorption on Zn/ZSM-5 is a very easy process. Independent of the cation site, the corresponding activation energies are in the range of 25–75 kJ/mol. Most likely, under the experimental static conditions [6] the one-step decomposition of the products of dissociative ethane adsorption can be accompanied by the H2 dissociation, which explains the experimental observation of zinc-hydrides.

Although there is no experimental evidence of the presence of dimeric [ZnOZn]2+ ions in Zn/ZSM-5 zeolite, these species have been considered as active sites responsible for light alkane dehydrogenation [10]. However, the present study shows that in spite of their high activity for the initial C–H activation, the subsequent chemical transformations over these sites are unfavored compared to the isolated Zn2+ sites.

Recently adsorption properties and reactivity toward C–H bond cleavage of Zn/ZSM-5 zeolites have been investigated in detail by means of diffuse-reflectance infra-red spectroscopy (DRIFTS) [6,11]. The catalyst used has been prepared by a high-temperature reaction of zinc vapor with the parent hydrogen form of zeolite ZSM-5. This method allows selective and quantitative (one zinc atom per two framework aluminums)

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Table  6.6.  Calculated  and  experimental 

frequencies  (ν,  cm–1)  for  the  Zn–H 

stretching vibrations in structures IV 

  ZnZd  ZnZs  ZnOZn  Exp. [6]ν(Zn‐H)  1941 

1946a 1878 1835a 

1835  1934 

a values for structures IV’ 

substitution of zeolitic Brønsted acid sites with isolated Zn2+ cations at the cation sites of ZSM-5. Due to the absence of any source of O2 or water during the preparation of Zn/ZSM-5 zeolite by this method, formation of any oxygen-containing cationic species ([ZnOZn]2+, [Zn–OH]+, etc.) can be ruled out. On the other hand, when traditional techniques such as

incipient wetness impregnation or ion exchange in water solution are used for preparation of zeolite modified with zinc, one cannot exclude formation of such species, as well as small zinc oxide particles or other multinuclear assembles within the zeolite channels. However, comparison of different zinc-exchanged ZSM-5 zeolites prepared using different techniques has shown that the behavior of the adsorbed alkane molecules does not depend on the method of zinc loading into the zeolite [6,11]. Moreover, it has been shown by Heemsoth et al. [12] that the catalysts prepared by the conventional incipient wetness impregnation and by a solid-state reaction between zinc dust and HZSM-5 display no difference in activity or selectivity for ethane dehydrogenation. Thus, one concludes that independent of the method of preparation the nature of the active sites for the C–H activation in the zinc-modified ZSM-5 zeolites is the same [13].

Thus, taking into account the discussion in Chapter 5, we conclude that the binuclear [ZnOZn]2+ ions cannot be considered as the active sites responsible for dehydrogenation of light alkanes over Zn/ZSM-5. Moreover, recent computational studies [14] reported very low stability of such dimeric species independent of the zeolitic cation site.

No experimental data are available on the C2H6 adsorption energies or activation energies for light alkane dehydrogenation over well-defined zinc-exchanged zeolites. Therefore, to support the computational results presented, one should use indirect parameters such as vibrational properties of the intermediates involved in the dehydrogenation reaction for comparison of the experimental and theoretical results. Table 6.6 lists the calculated frequencies of the Zn–H stretching vibrations for the intermediates IV discussed above. The corresponding band at 1934 cm–1 has been experimentally observed after H2 dissociation [9] and as one of the products of the C2H6 transformations [6] over Zn/ZSM-5 zeolite independent of its Si/Al ratio and the method of the preparation of the catalyst. One can see that among the active sites considered, only the frequency of the stretching vibration of the Zn–H moiety formed at the ZnZd site (IVZnZd and IV’ZnZd) cohere well with the experimental value, whereas the stretching bands of zinc hydrides formed at the other sites show significantly lower frequencies.

Summarizing the foregoing discussions, it can be put forward that isolated bivalent zinc cations stabilized at the zeolitic cation sites with distant charge-compensating alumum-occupied oxyegn tetrahedra are the most probable active species for the alkane

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dehydrogenation. The calculated apparent activation energy for this reaction equals 153 kJ/mol.

6.4. Conclusions

The reaction mechanism of ethane dehydrogenation is investigated using DFT calculations for different possible zinc cationic sites in Zn/ZSM-5. The catalytic cycle starts with the heterolytic C–H bond cleavage of the C2H6 molecule strongly adsorbed on Zn2+ ion at zeolitic cation sites with distantly located [AlO2]– framework units. Due to indirect charge-compensation the Lewis acidity of the extra-framework cations is significantly increased leading to the enhanced chemical reactivity. Initial proton abstraction is preferred by the lattice oxygens neighboring the ring where the cation is located rather than by the oxygen atoms of the same zeolitic ring. The resulting product decomposes via elimination of H2 and C2H4 in one step. The activation energy for this reaction is strongly dependent on the relative position of [Zn–C2H5]+ and H+ species. Therefore, the presence of acidic protons in the catalyst can promote the regeneration of the active sites. Subsequent desorption of ethylene from the thus formed molecular complex of C2H4 with Zn2+ ion regenerates the initial active site (ZnZd). The calculated overall activation barrier for the C2H6 dehydrogenation over ZnZd is equal to 153 kJ/mol.

Bivalent zinc ions stabilized at the conventional ion-exchange sites (ZnZs) are less probable active species. The low-energy path for the catalytic reaction over these sites consists of the same elementary steps as in the case of ZnZd. Due to their lower Lewis acidity, the initial heterolytic dissociation of ethane is less favorable. Moreover, subsequent steps of the catalytic cycle exhibit higher activation barriers and reaction energies. The reactivity of the isolated Zn2+ sites depends strongly on the Al distribution in zeolite.

Although binuclear ZnOZn sites show the highest initial activity, heterolytic C2H6 dissociation results in formation of very stable species. Both these effects are strongly interrelated and result from the high basicity of the extra-lattice oxygen and strong steric strain of the active site. Ethylene elimination from [Zn–C2H5···HO–Zn]2+ intermediate faces a high activation barrier of 190 kJ/mol. It is also strongly thermodynamically unfavored. In contrast to the isolated Zn2+ sites, in this case β-H abstraction resulting in formation of zinc-hydride species is preferred over the one-step elimination of C2H4 and H2. 

References 

1 Frash, M.V.; van Santen, R.A. Phys. Chem. Chem. Phys. 2000, 2, 1085. 2 Yakovlev, A.L.; Shubin, A.A.; Zhidomirov, G.M.; van Santen, R.A. Catal. Lett. 2000, 70, 175. 3 Shubin, A.A.; Zhidomirov, G.M.; Yakovlev, A.L.; van Santen, R.A. J. Phys. Chem. B 2001, 105, 4928. 4 Barbosa, L.A.M.M.; Zhidomirov, G.M.; van Santen, R.A. Catal. Lett. 2001, 77, 55. 5 Zhidomirov, G.M.; Shubin, A.A.; Kazansky, V.B.; van Santen, R.A. Theor. Chem. Acc. 2005, 114, 90. 6 Kazansky, V.B.; Pidko, E.A. J. Phys. Chem. B 2005, 109, 2103. 7 Kazansky, V.B.; Borovkov, V. Yu.; Serykh, A.I.; van Santen, R.A.; Anderson, B.G. Catal. Lett. 2000, 66, 39. 8 Kazansky, V.B.; Serykh, A.I.; van Santen, R.A.; Anderson, B.G. Catal. Lett. 2001, 74, 55. 

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9 Kazansky, V.B.; Serykh, A.I.; Anderson, B.G.; van Santen, R.A. Catal. Lett. 2003, 88, 211. 10 Barbosa, L.A.M.M.; van Santen, R.A. Catal. Lett. 1999, 63, 97. 11 Kazansky, V.B.; Serykh, A.I.; Pidko, E.A. J. Catal. 2004, 225, 369. 12 Heemsoth, J.; Tegeler, E.; Roessner, F.; Hagen, A. Microporous and Mesoporous Materials 2001, 46, 185. 13 Hagen, A.; Roessner, F. Catal. Rev. 2000, 42, 403. 14 Aleksandrov, H.A.; Vayssilov, G.N.; Rösch, N. Stud. Surf. Sci. Catal. 2005, 158, 593. 

  

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CHAPTER 7  

ETHANE DEHYDROGENATION OVER REDUCED EXTRA‐FRAMEWORK GALLIUM CATIONS IN ZSM‐5 ZEOLITE  

 

 

 

The stability of various gallium species (Ga+, GaH2+, and GaH2+) as models for the active

sites in reduced Ga/ZSM-5 and the possible reaction paths of alkane dehydrogenation are studied using a DFT cluster modeling approach. In general, alkanes are activated via an “alkyl” mechanism, in which gallium acts as an acceptor of the alkyl group. A comparison of the computed energetics of the various reaction paths for ethane indicates that the catalytic reaction most likely proceeds over Ga+. The initial step of C–H activation is the oxidative addition of an alkane molecule to the Ga+ cation, which proceeds via an indirect heterolytic mechanism involving the basic oxygen atoms of the zeolite lattice. Although the catalytic reaction can also occur over GaH2

+ and GaH2+ sites, these paths are not favored. Decomposition of GaH2

+ leading to formation of Ga+ upon the catalytic cycle is more favorable than regeneration of these sites. The reactivity of GaH2+ ions is strongly dependent on the distance between the stabilizing aluminum-occupied oxygen tetrahedra. In the cases of greater Al–Al distances, the stability of the GaH2+ species is very low, and it decomposes to Ga+ and a Brønsted acid site, whereas when Al atoms are located more closely, the charge-compensating GaH2+ ions are the most stable and, at the same time, show the lowest reactivity toward C–H bond activation.

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7.1. Introduction

Similar to Zn/ZSM-5, gallium-exchanged ZSM-5 zeolites are effective catalysts for promoting the selective conversion of light alkanes to aromatics [1]. It is generally believed that the modifying gallium species are mainly responsible for the non-oxidative dehydrogenation of paraffins [2-4,12-14]. The mechanism of catalytic dehydrogenation of alkanes over high-silica zeolites modified with gallium has been investigated in numerous experimental [2-14] and theoretical [15-20] studies. However, the structure of the active intrazeolite Ga species, and accordingly, the mechanism of hydrocarbon activation have not been fully elucidated.

Extra-framework gallium is usually introduced into zeolites by either conventional ion-exchange technique or solid-state ion exchange. In both cases, gallium is initially deposited on the external surface of the zeolite crystals, because hydrated Ga3+ ions are too bulky to enter the elliptical channels of ZSM-5 [21]. The Ga2O3 species obtained after calcination are reduced during pretreatment with hydrogen or with the hydrocarbon feed to Ga2O species that migrate into the zeolite channels [9,10]. These mobile species react with the zeolitic Brønsted acid protons, resulting in formation of reduced cationic Ga+, GaH2

+, or GaH2+ species bound to zeolite oxygen atoms. The resulting material may contain several types of reduced Ga species besides gallium oxide particles if the reduction process is not complete. The oxide species may include bulkier aggregates on the external surface or smaller ones in the micropore space of the zeolite.

Meitzner et al. [9] have shown using in situ Ga K edge X-ray absorption spectroscopy that in the working catalyst gallium is present in reduced Ga+ form, although the oxidation state changes to 3+ upon cooling to lower temperatures. Similarly, Kazansky et al. [13] have found that Ga+ species are the most stable species at high temperatures, and that oxidative addition of hydrogen to Ga+ leads to GaH2

+ species at lower temperatures. It is important to note that these gallium hydride species are relatively stable and decompose slowly only at higher temperatures [13].

Recently a completely anhydrous route for preparation of well-defined extra-framework cationic Ga species has been reported in Refs. [12-14,22-24]. The method is based on chemical vapor deposition of trimethylgallium to anhydrous HZSM-5 resulting in formation of grafted Ga(CH3)2

+ species. Careful removal of the methyl ligands by reduction leads to GaH2

+ cations, which can then be decomposed at high temperatures to Ga+ cations. When Ga+ ions are predominant in ZSM-5, a stable activity in propane dehydrogenation to propylene is observed [14]. Decreased initial activity is found when the catalyst contains hydrogenated GaH2

+ species. The higher activity at steady state is attributed to the decomposition of less active GaH2

+ cations to Ga+ under the reducing high-temperature reaction conditions.

Most theoretical studies [15,16,19] have been considered GaH2+ as the active site for

alkane dehydrogenation. Frash and van Santen [15] have proposed a three-step mechanism

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including (i) “alkyl” (Rδ––Hδ+) activation of the C–H bond, resulting in the formation of a zeolitic Brønsted acid proton and a rather unstable neutral H2Ga–R species, (ii) the subsequent desorption of molecular hydrogen via recombination of the acidic proton and one of the hydride ions bounded to gallium, and (iii) the decomposition of the resulting [H–Ga–R]+ complex to alkene and GaH2

+. The decomposition step is rate-limiting. Activation energy of 254 kJ/mol has been computed for a model reaction of ethane dehydrogenation. Alternatively, a one-step concerted mechanism over GaH2

+ has been considered by Pereira and Nascimento [19]. The activation energy for this process is substantially higher than the mechanism proposed by Frash and van Santen [15]. Very recently, Joshi and Thomson [20] have proposed the existence of bivalent extra-framework gallium species (GaH2+) close to a pair of framework [AlO2]– units in ZSM-5 zeolite as active sites for alkane dehydrogenation. The overall activation energy for the “carbenium” pathway for ethane activation over GaH2+ species, which are stabilized by two aluminum-occupied oxygen tetrahedra, lie in the range of 260–360 kJ/mol, depending on the cation site at which GaH2+ species are stabilized. Moreover, the choice for a positively charged alkyl group for the initial activation of ethane contrasts with earlier proposals.

The recent experimental results indicating that Ga+ cations are the active sites in Ga/ZSM-5 zeolite [12-14] suggest that the reaction mechanism of alkane activation over reduced Ga cations stabilized in zeolites should be reconsidered. Below a detailed comparative analysis of the reactivity of various cationic species (Ga+, GaH2

+, and GaH+2) is performed by means of DFT cluster calculations. Additionally, the relative stability of various cationic Ga species and the dependence of their chemical properties on the aluminum distribution in ZSM-5 zeolite are discussed.

7.2. Computational details

Dehydrogenation of ethane over Ga/ZSM-5 zeolite, as well as stability of different gallium-containing active sites, was studied using GaHAl2Si6O9H14 and GaH3Al2Si6O9H14 cluster models (Figure 7.1), which were similar to those used for the investigation of chemical properties of ZSM-5 zeolite modified with zinc in Chapters 5 and 6. A detailed description of Zs and Zd models used can be found in Section 5.2. The charge-compensating gallium species were located in the five-ring containing aluminum at the T12 framework position [25]. Hydrogen atoms were used to saturate the dangling Si–O bonds at the periphery of the cluster. The starting geometry of the clusters corresponded to the real lattice of the ZSM-5 zeolite corresponding to X-ray diffraction data [26]. The optimization of the positions of the boundary H atoms was similar to that described in Chapter 4. All of the energies obtained from the DFT calculations used for estimating the reaction heats and activation barriers were corrected for the zero-point energy.

Density functional theory (DFT), with the B3LYP [27] hybrid functional was used to perform the calculations. Geometry optimization and saddlepoint searches were performed using the Gaussian 03 program [28]. The 6-31G(d,p) basis set was used for the gallium

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Figure 7.1. Optimized structures of (a) Ga Zd, (b) GaH2 Zd, (c) GaH Zd, (d) Ga Zs, (e) GaH2 Zs and 

(f) GaH Zs clusters. 

Table 7.1. The optimized bond lengths and interatomic distances (Å), and the relative energies 

of Ga+, GaH2+, and GaH2+ species stabilized in cluster model with distantly separated Al atoms 

(Zd) and with Al atoms located at the next‐nearest positions (Zs).   Ga Zd GaH2 Zd  GaH Zd Ga Zs GaH2 Zs GaH Zs ZnZd 

a  ZnZsa

ΔE, kJ/mol  0  –24b  +88 –105 –115b –207 —  —          Distance:         Ga‐O1  2.123 2.011  1.949 2.098 2.000 1.977 1.917  1.930Ga‐O2  2.130 2.014  1.976 2.192 2.045 2.066 1.974  2.057Ga‐O3  3.792 3.840  2.080 3.349 3.713 2.025 2.079  2.028Ga‐O4  3.802 3.987  2.138 2.937 3.692 2.028 2.018  1.958Ga‐H1  —  1.562  1.540 — 1.566 1.549 —  — Ga‐H2  —  1.564  — — 1.559 — —  — 

a  The geometry parameters for ZnZd and ZnZs models were obtained from Chapter 5  b  ΔE for reaction Ga+Z– + H2 → GaH2

+Z– 

cation and the bridging hydroxyl group, whereas the ethane molecule and the zeolitic oxygen atoms were described by the 6-311G(d,p) basis set. Al and Si atoms of the zeolite framework, as well as the boundary hydrogen atoms, were treated by the D95-Dunning/Huzinaga basis set [29].

7.3. Results

7.3.1. Structure and properties of active sites

Figure 7.1 displays the optimized structures of Ga+, GaH2+, and GaH2+ ions stabilized in

the Zd (distantly separated Al ions) and Zs (Al ions in next-nearest positions) cluster models. Table 7.1 lists the corresponding optimized Ga–O and Ga–H bond lengths and the relative energies. The univalent cations Ga+ and GaH2

+ are located in close vicinity to one of framework anionic [AlO2]– sites (T12) and coordinate to two adjacent oxygen atoms (O1 and O2). The Ga–O bonds (Ga-O1 and Ga-O2) between Ga and lattice oxygens are

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somewhat shorter for the GaH2+ ion than for the Ga+ cation. On the other hand, the Ga–O

distances to the other oxygens (Ga–O3 and Ga–O4) of the cluster are shorter in the latter case. This is most likely due to the repulsive interactions of basic oxygen atoms of the zeolite framework with the negatively charged hydride ions in the GaH2

+ species. On the other hand, weak electrostatic interaction of Ga+ with remote zeolitic oxygens leads to additional stabilization of the cation. This is also supported by the fact that the increased basicity of O3 and O4 atoms due to the introduction of Al atom at the framework T6 position results in a significant shortening of the corresponding Ga–O distances in the Ga Zs cluster. The computed Ga–O and Ga–H distances for GaH2

+ species stabilized in either Zd or Zs clusters agree very well with those reported for the dihydrido-gallyl ion in the extended cluster model containing 11 T atoms [20], corresponding to the cation site located at the cross-section of the straight channel and sinusoidal channel of ZSM-5 zeolite. We infer that the presence of an additional anionic site in our model that is balanced by an acidic proton does not significantly influence the properties of the gallium-containing sites (Ga+ and GaH2

+).

The bivalent GaH2+ ion is preferentially located at the Zd site (Figure 7.1 (c)) near the Al1 atom and at a rather long distance (r(Ga-Al2) = 6.857 Å) from the second anionic site of the cluster. Table 7.1 shows that the GaH2+ ion coordinates to 4 lattice oxygen atoms, two of which are bonded at a somewhat shorter distance (~0.2 Å). This is due to the lower basicity of the oxygen atoms of the silicon-occupied oxygen tetrahedron compared with those of the aluminum-occupied tetrahedron. This is in agreement with the fact that the GaH2+ cation sits almost in the center (r(Ga-Al1) = 2.995 Å and r(Ga-Al2) = 2.957 Å) of the 5T rings containing two anionic sites (GaH Zs structure, Figure 7.1 (f)).

The Lewis acidic properties of the GaH2+ ions are expected to be similar to those of Zn+2 and, accordingly, one expects similar bonding properties of these charge-compensating ions with the zeolite. Indeed, the Zn–O bond lengths (Table 7.1) for zinc stabilized at the similar cation sites cohere well with the geometry parameters of the GaH Zd and GaH Zs clusters, taking into account the slightly smaller radius of the Ga3+ cation.

Desorption of H2 from GaH2+ (7.1) resulting in formation of Ga+ ions is slightly

endothermic (Table 7.1). However, due to increase of entropy the equilibrium of this process at high temperature is strongly shifted toward Ga+ formation (∆Gº823K = –75 and –101 kJ/mol, respectively for Zd and Zs models). This finding agrees well with the experimental observation that Ga+ ions in zeolite are only transformed into GaH2

+ ions at lower temperatures in hydrogen atmosphere [9,13]. Dehydrogenation of these species, resulting in formation of Ga+ ions, has been observed upon high-temperature treatment in inert atmosphere or in vacuum [13].  

GaH2+ Z– Ga+ Z– + H2 (7.1)

Formation of GaH2+ ions from Ga+ and a Brønsted acid proton (7.2) is found to be a strongly endothermic process (∆E = +88 kJ/mol) for the case of a longer Al–Al distance

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(Zd). On the other hand it is a strongly exothermic reaction (∆E = –102 kJ/mol) when two Al atoms are placed at the next-nearest framework positions (Table 7.1).

Ga+ Z– + H+Z- Z– + GaH+2 Z– (7.2)

Therefore, it can be proposed that for relatively long Al–Al distances the equilibrium of reaction (7.2) shifts to the side of the univalent Ga+ ion and the proton. On the other hand, when located in the vicinity of two anionic sites, the bivalent GaH2+ ion is a more probable charge-compensating species.

7.3.2. Ethane dehydrogenation over Ga+

In agreement with recent experimental DRIFTS investigations [12], molecular adsorption of ethane on Ga Zd does not result in any specific interaction between C2H6 and the adsorption site at low temperature. The geometry parameters of both the C2H6 moiety and the cluster model in the adsorption complex are similar to those calculated for the individual fragments. Recent experiments [12] indicate that already at moderate temperature the dehydrogenation of ethane over Ga/ZSM-5 zeolite is initiated by dissociative adsorption of C2H6 on charge-compensating reduced Ga+ species, resulting in the formation of gallium-ethyl hydride according to

Ga+ Z– + C2H6 → [H–Ga–C2H5]+ Z– (7.3)

Reaction (7.3) as written corresponds to an oxidative addition of ethane to Ga+. The activation energy of the direct homolytic cleavage of the C–H bond over the Ga Zd cluster equals 374 kJ/mol (Figure 7.2, route (a), TS1’). This barrier is too high to explain the experimentally observed formation of [H–Ga–C2H5]+ species (II). The computed value for the activation energy is close to the C–H bond energy in molecular ethane (423 kJ/mol [30]). Thus, it appears that for the case of homolytic C–H activation on Ga+-exchanged ZSM-5 the stabilization of the non-polar transition state (TS1’) is very weak, and most likely, the role of the active site is limited to stabilize the products of the homolytic dissociation via chemisorption of C2H5 and H radicals.

Generally, alkanes are activated over transition metal (TM) complexes [31-33] via formation of σ-CH complexes. The bonding of a hydrocarbon molecule with the TM ion in such complexes is described by a synergetic combination of the ligand-to-metal donation from the σ C–H orbitals of the hydrocarbon to the partially-occupied s-orbital of the TM and the metal-to-ligand back donation from the dπ orbital to the C–H σ*-orbitals. Such interactions weaken the C–H bond, which in turn, together with formation of new TM···C and TM···H bonds, significantly facilitates homolytic C–H bond cleavage. However, the fully occupied 3d-orbitals of univalent gallium are too low-energy, and thus, cannot contribute to the back-donation. Moreover, because the 4s-orbital is occupied, it cannot effectively act as an electron-acceptor orbital. Thus, the formation of the σ-C2H6 complex on Ga+ is unlikely, and correspondingly, the energy cost for the homolytic dissociation of ethane over GaZd is very close to that for the gas-phase dissociation.

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Figure 7.2. Homolytic (a) and heterolytic (b) “alkyl” pathways of ethane dehydrogenation over 

Ga Zd site. 

Table  7.2. Optimized  bond  lengths  (Å)  of  the  intermediates  along  homolytic  and  heterolytic 

reaction paths on Ga Zd site. 

  GaZd + C2H6a  TS1’ TS1 I TS2 II  TS3

Ga‐O1  2.123  2.030 3.060 3.188 2.848 2.029  2.130Ga‐O2  2.130  2.047 2.421 2.897 2.227 2.039  2.135Ga‐C1  —  2.204 2.226 2.041 2.017 1.984  2.280Ga‐H1  —  1.804 2.426 2.289 1.792 1.569  1.993O1‐H1  —  — 1.037 1.007 1.257 —  —C1‐H1  1.094  1.834 1.757 — — —  —C2‐H2  —  — — — — 1.094  1.604H1‐H2  —  — — — — 3.361  0.871

Alternatively, the interaction with a soft Lewis acid-base pair consisting of Ga+ and the basic lattice oxygen of the zeolite appears to be responsible for the initial C–H bond cleavage. This reaction path of heterolytic C–H bond activation is shown in Figure 7.2, route (b). The geometry parameters of the intermediates and transition state structures are summarized in Table 7.2. In the initial step, heterolytic dissociation of the C–H bond results in the formation of a neutral Ga–C2H5 species in the vicinity of a zeolitic Brønsted acid site (I). The calculated activation energy (TS1, 210 kJ/mol) is significantly lower than that computed for the homolytic path. The corresponding transition state structure TS1 is characterized by very strong polarization of the reacting C–H bond following the “alkyl” mechanism, i.e. both the positive charge on H atom and the negative charge on carbon strongly increase in comparison with gas phase C2H6.

Intermediate I resulting from heterolytic dissociation of ethane on Ga Zd site is very

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unstable and readily rearranges to stable product II. This reaction proceeds via oxidation of the Ga–C2H5 species by the Brønsted acid site with a low activation barrier (9 kJ/mol, TS2). The ease of formation of complex II explains why only the product of oxidative addition has been observed experimentally [12].

The final step in ethane dehydrogenation over Ga+ is a one-step desorption of H2 and C2H4 molecules via the TS3 structure. This reaction can be described as a “destructive reductive elimination”, resulting in reduction of the Ga3+ ion and regeneration of the initial Ga Zd site. The activation energy for this reaction equals 224 kJ/mol. This value is slightly higher than that calculated for the initial C–H cleavage (210 kJ/mol). The overall activation energy of the ethane dehydrogenation over the reduced dehydrogenated Ga/ZSM-5 zeolite is 233 kJ/mol.

7.3.3. Ethane activation over GaH2+

To compare the reactivities of the various charge-compensating gallium species, the alkyl activation pathway over the GaH2

+ site proposed by Frash and van Santen [15] is also investigated using the present Zd cluster model. The energetics for this mechanism are presented in Figure 7.3, and the geometry parameters of the intermediates and transition state structures are listed in Table 7.3.

Similar to the findings for Ga+, molecular C2H6 adsorption on the GaH2 Zd cluster is very weak and does not result in any significant perturbation of either the adsorbed molecule or the adsorption site. Thus, it is assumed that this molecular adsorption does not influence the subsequent chemical activation of ethane. The mechanism of the initial activation of the hydrocarbon molecule on GaH2

+ ion is similar to the one suggested for the heterolytic dissociation of ethane on Ga Zd. A strong polarization of the reacting C–H bond is observed in the TS4 structure, leading to heterolytic C–H bond splitting with formation of a neutral (H–)2Ga3+–C2H5

– species in the vicinity of a Brønsted acid site (III). The calculated activation energy for this step (193 kJ/mol) is significantly higher than the one (158 kJ/mol) previously reported in Ref. [15] for GaH2

+ species stabilized in a fully relaxed 3T cluster. On the other hand, the present value is slightly lower than the one (203 kJ/mol) reported by Yoshi and Thomson [20] for the dihydrido-gallyl ion stabilized at an extended zeolite cluster containing 11T atoms. The higher activation barrier than that reported by Frash and van Santen [15] is attributed mainly to greater steric hindrance of the interaction of the C–H bond with the Ga···O pair caused by the constrained geometry and the larger size of the Zd model. The difference between the present result and that reported in [20] is also attributed to minor differences in steric properties of the reactive site resulting from size differences of the cluster models.

The H2Ga–C2H5 species formed in the initial step are stabilized by interaction between a hydride ion (H1’) bonded to the gallium and an acidic proton (H1). This results in a strong elongation (by about 0.2 Å) of the O1–H1 bond. The Ga–O2 distance in III (Table 7.3) is significantly shorter than the corresponding distance in I (Table 7.2). This is indicative of

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Figure 7.3. Reaction paths for the “alkyl activation” mechanism of ethane dehydrogenation (a) 

over  GaH2  Zd  and  dehydrogenation  of  these  species  in  the  catalytic  cycle  (b)  resulting  in 

formation of Ga Zd. 

Table  7.3. Optimized  bond  lengths  (Å)  of  the  intermediates  along  ethane  dehydrogenation 

reaction paths on GaH2 Zd site. 

   GaH2 Zd + C2H6  TS4 III TS5 II  TS6  TS3Ga‐O1  2.011  3.029 3.226 3.181 2.029  2.034  2.130Ga‐O2  2.014  2.090 2.196 2.098 2.039  2.159  2.135Ga‐H1’  1.562  1.569 1.660 1.757 —  —  —Ga‐H2’  1.564  1.573 1.576 1.570 1.569  1.554  1.993Ga‐C1  —  2.200 2.005 1.997 1.984  2.234  2.280O1‐H1  —  1.189 1.037 1.237 —  —  —C1‐H1  1.094  1.490 — — —  —  —H1’‐H1  —  3.383 1.267 0.972 —  —  —C2‐H2  —  — — — 1.094  1.681  1.604Ga‐H2  —  — — — 3.134  1.674  —H2’‐H2  —  — — — 3.361  —  0.871

 

the stronger interaction of the trivalent gallium in the H2Ga–C2H5 with the lattice zeolitic oxygens. Due to the latter interaction, the coordination of the Ga3+ ion in III is close to tetrahedral, which is known to be preferred for Ga3+-containing compounds [34]. Thus, both attractive interactions provide additional stabilization of the reactive species III compared to Ga–C2H5 species (I). Formation of an interatomic H1’···H1 contact facilitates H2 recombination and formation of the stable tetrahedral gallium in the [H–Ga–C2H5]+ ion II (III→II+H2, Figure 7.3).

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To close the catalytic cycle and regenerate the intial GaH2+ site, it has been proposed in

[15] that decomposition of II proceeds via abstraction of a hydride ion from the β-position of the C2H5 group by the positively charged Ga atom and simultaneous desorption of ethylene (Figure 7.3, route (a)). The calculated activation energy for this process is 257 kJ/mol, in an excellent agreement with the previously reported values of 258 kJ/mol [20] and 254 kJ/mol [15]. This close agreement indicates that the chemical properties of the extra-framework GaH2

+ species do not apparently depend on the surrounding unless the other atoms of the zeolite are involved in the reaction.

On the other hand, simultaneous desorption of H2 and C2H4 (Figure 7.3, route (b)), which results in formation of the univalent gallium ion (Ga Zd), is about 30 kJ/mol more favorable than the closure of the cycle with regeneration of the GaH2

+ ion. Note that this process is also strongly thermodynamically favored. The calculated Gibbs energy changes (∆Gº823) at the conditions of catalytic reaction are equal to +1 for route (a) and –74 kJ/mol for route (b).

7.3.4. Ethane activation over GaH2+

Recently, Joshi and Thomson [20] have proposed that the GaH2+ species stabilized in the vicinity of two framework Al atoms are the active sites for alkane dehydrogenation in Ga/ZSM-5 catalysts. The initial step of alkane activation over these species has been suggested to be a heterolytic dissociation of the C–H bond via a “carbenium” mechanism, resulting in the formation of a “carbenium” ion attached to the basic oxygen of the zeolite and a hydride ion bonded to gallium (GaH Zd + C2H6 → V, Figure 7.4, route (a)). In view of earlier results [15,35], this mechanism seems disputable. Thus, to clarify the mechanism of C–H activation over GaH2+ species and to compare the reactivity with those of the earlier discussed Ga-sites, the initial activation of ethane over GaH2+ stabilized at the Zd cluster is computed.

The elementary steps and the reaction energy diagrams for the “carbenium” and “alkyl” paths of ethane activation over GaH Zd are shown in Figure 7.4, route (a) and (b), respectively. The geometry parameters of the intermediates and transition state structures involved are summarized in Table 7.4. According to Joshi and Thomson [20], activated molecular adsorption of ethane to GaH2+ ions (structure IV in Figure 7.4) precedes C–H bond cleavage. In contrast to the aforementioned molecular adsorption of C2H6 on Ga Zd and GaH2 Zd, this process leads to remarkable changes of the geometry and charge parameters of both adsorbed C2H6 and the adsorption site. The Ga–O coordination opens (Ga–O3 and Ga–O4 bonds are broken) and the GaH2+ ion partially leaves the cation site (5T ring), so that gallium ion coordinates only to 2 lattice oxygens (O1 and O2). The resulting coordinative unsaturation of the GaH2+ species is partially compensated by interaction with one of the methyl group of ethane. Formation of such an activated adsorption complex is endothermic (GaH Zd + C2H6 → IV, ∆E = +68 kJ/mol, Figure 7.4), because it requires destruction of a stable fourfold coordination of GaH by the lattice

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Figure 7.4. “Alkyl” (a) and “carbenium” (b) activation of ethane over GaH Zd. 

Table 7.4. Optimized bond lengths (Å) of the intermediates involved in the initial activation of 

ethane on GaH Zd site.   GaH Zd + C2H6 IV V TS6  VIGa‐O1  1.949 1.948 2.058 2.000  2.112Ga‐O2  1.976 1.872 1.979 1.924  1.992Ga‐O3  2.080 3.470 3.615 3.400  3.722Ga‐O4  2.138 3.882 4.372 3.349  4.538Ga‐H1’  1.540 1.533 1.552 1.552  1.559Ga‐C1  — 2.566 — 2.145  1.997Ga‐H1  — 2.027 1.571 2.371  —O4‐C1  — 3.295 1.543 2.664  —O4‐H1  — 2.418 — 1.201  0.980C1‐H1  1.094 1.132 3.172 1.505  3.441

 

oxygen atoms. On the other hand, coordination of C2H6 to the cation does not compensate for the energy loss associated with the breaking of two Ga–O bonds (Ga–O3 and Ga–O4). In other words, it is not possible to form a strong adsorption complex of ethane with GaH Zd without a decrease of the effective coordination number of the cation to the basic zeolitic oxygens, as has been observed for the case of zinc ions stabilized in a similar cluster model (Chapter 5). The difference with GaH2+ is that the gallium ion in the initial GaH Zd model is shielded by the attached hydride ion, which hinders the stabilizing charge donation from ethane to the cation. The twofold coordination of GaH2+ ion in IV removes these hindrances. The effective transfer of electron density from ethane to the adsorption site is 0.283 e—. This indicates appreciable Lewis acidity of the ‘activated’ GaH2+ ion in a cationic position with distantly placed aluminum ions. The charge transfer is much larger (by 0.168 e—) than for ethane adsorption on zinc ions (see Chapter 5), indicating a stronger Lewis acidity of low coordinated GaH2+ species.

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Figure  7.5.  Calculated  IR  frequencies  of  C–H 

stretching  vibrations  with  their  intensities  and 

simulated  IR  spectrum  (with  all  DFT  computed 

vibrational  frequencies  scaled  by  factor  0.964)  for 

structure  IV. The shape of  the C–H vibration mostly 

perturbed due to interaction with GaH2+ ion is shown 

by “balls‐and‐sticks” model.  IR spectrum of free C2H6 

molecule  calculated  at  B3LYP/6‐311G(d,p)  level  is 

presented for comparison by the dashed line.

Interaction of C2H6 with the excessively charged low-coordinated GaH2+ ion results in a strong polarization of the adsorbed molecule. The C1–H1 bond is 0.038 Å longer than the corresponding bond in the gas-phase ethane. Such perturbations of the adsorbed molecule affect the vibrational properties of the C2H6 moiety in IV (Figure 7.5). The calculated infrared spectrum of IV in the region of C–H vibrations contains five slightly perturbed bands with frequencies close to those observed in the gas phase, whereas one band is strongly red-shifted (by >300 cm–1) and exhibits a very high relative intensity compared to those of the

other C–H stretching bands. In contrast to the case of ethane adsorption on zinc- and cadmium-modified ZSM-5 zeolites (Chapter 5), this band corresponds mainly to displacements of one hydrogen atom (H1) for the C1–H1 bond directly interacting with the GaH2+ ion. One expects that such a strong perturbation of adsorbed molecule can promote its subsequent chemical activation at higher temperatures following the displacements of atoms of the adsorbed molecule corresponding to the mostly perturbed vibration. Thus, the initial structure for the product of C–H activation of ethane on GaH Zd was built by displacement of the H1 atom by 1 Å in IV following the aforementioned C–H stretch. Geometry optimization of the resulting structure leads to formation of product VI of the “alkyl’ heterolytic C–H bond cleavage. The calculated activation energy for this process (IV → VI, Figure 7.4, route (b)) equals 104 kJ/mol. The corresponding transition state structure TS6 is characterized by an increase of the “alkyl” C1-H1 bond polarization resulting in the formation of a proton (H+) attached to the basic zeolitic O atom and an alkyl (C2H5

–) grafted to the gallium ion. One notes that VI is destabilized due to formation of likely charged ions (H+ and [H–Ga–C2H5]+) in the immediate vicinity of each other. In contrast to the case of ethane activation over Zn- and Cd Zd sites (Chapter 5), an alternative reaction path, involving a proton abstraction by the O-sites neighboring the ring where the exchangeable cation is located, is forbidden due to steric properties of complex IV. Nevertheless, product VI can be additionally stabilized by 195 kJ/mol via proton transfer from O4 to the more basic O7 ion bounded to the Al2 (structure II). It has been shown in Ref. [36] that the activation energy of this process does not exceed 50 kJ/mol.

The product V of “carbenium” activation of C2H6 on GaH Zd is just 14 kJ/mol less stable than structure VI (Figure 7.4 routes (a) and (b), respectively). Unfortunately, the transition

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Table 7.5. Energetics of initial activation of ethane over Ga+ and GaH2+ sites based on Ga Zs and 

GaH Zs models, respectively. 

  ∆E≠, kJ/mol  ∆E, kJ/mol Ga Zs     “Homolytic” path     

Ga+ Z– + C2H6 → [H‐Ga‐C2H5]+Z–  +386  +23 

“Heterolytic” path     Ga+ Z– + C2H6 → Ga‐C2H5∙∙∙H

+Z–  +219  +194 Ga‐C2H5∙∙∙H

+Z– → [H‐Ga‐C2H5]+Z–  —  –171 

GaH Zs     “Alkyl” path     

Z– GaH2+ Z– + C2H6 → [H‐Ga‐C2H5]+Z– + H+Z–  +252  +125 

“Carbenium” path     Z– GaH2+ Z– + C2H6 → [H‐Ga‐H]+Z– + C2H5

+Z–  —  +274  

state structure corresponding to the “carbenium” mechanism of ethane activation on GaH2+ species (IV → V, Figure 7.4, route (a)) proposed by Joshi and Thomson [20] was not located. However, with regard to the above discussion, this process is expected to be less favorable than the “alkyl” mechanism, since reaction IV → V requires very strong “re-polarization” of the electron density of the adsorbed ethane to produce a negative charge on H1 and positive charge on the carbon atom (C1). This does not appear to be facilitating factors for the process, and hence, the “carbenium” pathway will be strongly disfavored as compared to the “alkyl” activation of C2H6 over GaH2+ site.

Thus, the initial activation of C2H6 on GaH Zd results in formation of the intermediate II, and hence, the catalytic cycle follows the reaction pathway described for ethane dehydrogenation over Ga Zd (Section 7.3.2), and GaH2+ is converted to Ga+.

7.3.5. Effect of Al–Al distance on the reactivity of Ga+ and GaH2+ sites

The stability of Ga+ and GaH2+ ions in the Zs cluster model have been compared in Section 7.3.1 with those for the cluster Zd where the framework aluminum ions are separated by a relatively long distance (8.138 Å). Below the computational results of C2H6 activation over these ions compensating for the negative charge in the models where the aluminum ions are placed at the next-nearest framework sites (Ga Zs and GaH Zs; r(Al1–Al2) = 4.837 Å) are presented. The intermediates and transition state structures thus calculated are very similar to those obtained for the Zd model and show similar trends in changes of geometrical parameters upon chemical transformations. The calculated energetics for elementary reaction steps of ethane activation on Ga Zs and GaH Zs are summarized in Table 7.5.

The activation energies and enthalpies for both the homolytic and heterolytic C–H bond cleavage on Ga+ ion in Zs cluster are similar to those computed for the case of distantly placed Al atoms. Thus, it can be concluded that the relative localization of anionic [AlO2]–

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framework units in the zeolite does not significantly affect the reactivity of the charge-compensating univalent gallium ions.

In contrast, the chemical properties of GaH2+ depend strongly on the Al–Al distance in the cluster model. Similar to Joshi and Thomson [20], the activated molecular adsorption complex of ethane on GaH Zs corresponding to the earlier-considered structure IV (Figure 7.4) could not be detected. This is connected with the much stronger binding of GaH2+ ion to basic lattice oxygens of the Zs than to those of the Zd model. Therefore, adsorption of ethane to the low-coordinated gallium ion cannot sufficiently compensate the loss of energy due to Ga–O bond breaking, which is necessary for the formation of the C2H6···GaH2+ activated adsorption complex. This coheres with a higher activation barrier for the “alkyl” C–H bond dissociation. As follows from Table 7.5, the energy cost of the reaction (7.4) is 42 kJ/mol higher for the GaH Zs site than for the GaH Zd site.

Z– GaH+2 Z– + C2H6 → [H-Ga-C2H5]+ Z– + H+ Z– (7.4)

The transition state structure for the “carbenium” initial activation of ethane on GaH Zs was not identified. The enthalpy of this process equals +274 kJ/mol. Therefore, the activation energy is expected to be higher than this value, which already remarkably exceeds the ∆E≠ value calculated for the “alkyl” pathway. Taking into account the results of Section 7.3.4, the latter process is more favorable over GaH2+ ions independent of the distance between the charge-compensating [AlO2]– framework anionic sites.

7.4. Discussion

In the present study, three possible types of charge-compensating gallium species in reduced Ga/ZSM-5, i.e. Ga+, GaH2

+, and GaH2+, are considered. Stability of univalent species (Ga+, GaH2

+) is remarkably higher than that of GaH2+ in the case of low Al density in the zeolite, i.e. in the case of distant location of framework [AlO2]– units. In contrast, when the Al–Al distance is rather small (two aluminums are located at the next-nearest lattice positions), the presence of the bivalent ion is preferred in the immediate vicinity of two anionic sites. The repulsive interaction between two closely situated positively charged univalent species results in destabilization of Ga+ and GaH2

+ at such cation sites.

The fraction of cation sites in high-silica zeolites containing two aluminum atoms at the next-nearest framework positions is small. For instance, in case of a zeolite with Si/Al ratio equal to 25, it does not exceed 30% [37]. Thus, with regard to the results presented in Table 7.1 one can conclude that univalent Ga+ or GaH2

+ ions are the predominant charge compensating species in the reduced high-silica gallium-exchanged zeolites, whereas GaH2+ cations can be found in only a relatively small portion of zeolitic cation sites, which contain two Al atoms.

It is found that at the initial step of ethane activation, all of the considered gallium species act as Lewis acids promoting heterolytic C–H cleavage involving the basic oxygen atoms of the zeolite lattice. For Ga+ and GaH2

+ active sites, this results in the formation of

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unstable neutral Ga–C2H5 and H2Ga–C2H5 species, respectively, as well as a Brønsted acid site that readily either oxidizes Ga–C2H5 species or recombines with one of the hydride ions bounded to H2Ga–C2H5 leading to desorption of hydrogen. In both cases, at the end of the reaction, stable [H–Ga–C2H5]+Z– species (II) are formed.

The catalytic cycle is closed by one-step decomposition of [H–Ga–C2H5]+Z-. Simultaneous desorption of H2 and C2H4 from these species (reaction (7.5)) is the most favorable process. The activation energy for production of Ga+ from [H–Ga–C2H5]+ following reaction (7.5) is 33 kJ/mol lower than that for regeneration of GaH2

+ via desorption of ethylene (7.6). Moreover, under the conditions of the catalytic process, the ∆Gº823 is much lower for reaction (7.5) than for reaction (7.6) (–74 vs. +1 kJ/mol).

[H–Ga–C2H5]+ Z– → Ga+ Z– + C2H4 + H2 (7.5)

[H–Ga–C2H5]+ Z– → GaH2+ Z– + C2H4 (7.6)

Thus, ethane dehydrogenation over dihydrido-gallyl ions leads to their decomposition. The equilibrium concentration of GaH2

+ at high temperature is low and, hence, these sites can not be considered to be responsible for ethane dehydrogenation. This is in agreement with the recent experimental results [14] indicating decomposition of GaH2

+ ions upon the catalytic process.

According to the results presented above on the reactivity of GaH2+ ions, these species cannot play a significant role in dehydrogenation reaction. In addition, the mechanism of ethane activation proposed by Joshi and Thomson [20] is disputable. Those authors have suggested that gallium in such species act as a hydride ion acceptor and the C–H bond polarization follows a “carbenium” mechanism (Cδ+–Hδ–). However, the activated molecular ethane adsorption, which precedes C–H cleavage, results in the opposite Cδ––Hδ+ polarization. Note that this finding is in line with the higher electronegativity of carbon with respect to hydrogen (2.5 vs. 2.1 in the Pauling scale [38]). Thus, dissociative adsorption of ethane should follow the “alkyl” mechanism leading to formation of the intermediate product [H–Ga–C2H5]+Z–.

It is also noticeable that the GaH2+ species are isolobal to Zn2+ ions. This is reflected in similarities of the geometrical properties of the zeolitic cation site containing corresponding species (Table 7.1), as well as of their chemical properties. It has been shown experimentally [39,40] and theoretically [35] that the “alkyl” path of alkane activation over zinc-exchanged zeolites is preferred over the “carbenium” pathway. The same conclusion directly follows from the calculations presented for GaH2+ ions exchanged in zeolite independently on the distance between charge compensating [AlO2]– framework units.

Summarizing the foregoing results, it can be concluded that univalent Ga+ ions at cation sites of high-silica zeolites are the most probable catalytically active species for the dehydrogenation reaction. The apparent activation barrier is estimated to 233 kJ/mol that is

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significantly higher than that calculated for Zn-sites in ZSM-5 (Chapter 6). This coheres well with the experimentally observed higher activity of Zn/ZSM-5 zeolites [12].

7.5. Conclusions

The reaction mechanism of ethane dehydrogenation is investigated using DFT calculations for various reduced gallium-sites in Ga/ZSM-5. The probable catalytic cycle starts with the heterolytic C–H bond cleavage involving exchangeable univalent gallium cation and a basic oxygen atom of the zeolite lattice. The low energy of the d-orbitals of Ga+ and its occupied s-orbital that make them unable to donate or accept electrons, respectively, from the alkane, result in a high barrier for the direct oxidative addition of C2H6. Therefore, heterolytic splitting of C–H bond is more favorable due to the polarization induced by the interaction of the hydrocarbon with the Ga···O Lewis acid-base pair. The resulting product readily rearranges to [H–Ga–C2H5]+, which decomposes by simultaneous desorption of H2 and C2H4 regenerating the initial Ga+ species.

Hydrogenated gallium species (GaH2+Z– and Z–GaH2+Z–) are less likely active sites.

Decomposition of the dihydrido-gallyl ions resulting in univalent gallium ions upon the catalytic cycle is preferred over the reaction path leading to the regeneration of GaH2

+. In addition, the estimated overall activation barrier for the respective catalytic reaction is higher (by 58 kJ/mol) for GaH2

+ sites than for Ga+ sites.

Bivalent GaH2+ cations stabilized at the sites with distant framework [AlO2]– units show significantly higher initial activity for ethane activation along with much lower stability as compared to Ga+ sites. Nevertheless, initial heterolytic cleavage of C–H bond results in formation of [H–Ga–C2H5]+ ions, which preferentially follow the reaction path leading to formation of univalent gallium cations. On the other hand, GaH+2 ions compensating for the charge of two proximate framework [AlO2]– units are the most stable cationic species. However, the initial activation of C–H bond over these sites is strongly disfavored both thermodynamically and kinetically. In contrast, the reactivity of univalent gallium ions in zeolite depends only slightly on the relative position of framework anionic sites.

References 

1 Hagen, A.; Roessner, F. Catal. Rev. 2000, 42, 403. 2 Gnep, N.S.; Doyemet, J.Y.; Guisnet, M. J. Mol. Catal. 1988, 45, 281. 3 Price, G.L.; Kanazirev, V. J. Catal. 1990, 126, 267. 4 Price, G.L.; Kanazyrev, V.; Dooley. K.M.; Hart, V.I. J. Catal. 1998, 173, 17. 5 Kwak, B.S.; Sachtler, W.M.H.; Haag, W.O. J. Catal. 1994, 149, 465. 6 Yao, J.; le van Mao, R.; Dufresne, L. Appl. Catal. A: Gen. 1990, 65, 175. 7 Iglesia, E.; Baumgartner, J.E.; Price, G.L. J. Catal. 1992, 134, 549. 8 Iglesia, E.; Baumgartner, J.E. Catal. Lett. 1993, 21, 55. 9 Meitzner, G.D.; Iglesia, E.; Baumgartner, J.E.; Huang, E.S. J. Catal. 1993, 140, 209. 10 Biscardi, J.A.; Iglesia, E. Catal. Today 1996, 31, 207. 11 Iglesia, E.; Barton, D.G.; Biscardi, J.A.; Gines, M.J.L.; Soled, S.L. Catal. Today 1997, 38, 339. 

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12 Kazansky, V.B.; Subbotina, I.R.; Rane, N.; van Santen, R.A.; Hensen, E.J.M. Phys. Chem. Chem. Phys. 2005, 7, 3088. 

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A.,  Jr.;  Vreven,  T.;  Kudin,  K.  N.;  Burant,  J.  C.; Millam,  J. M.;  Iyengar,  S.  S.;  Tomasi,  J.;  Barone,  V.; Mennucci, B.; Cossi, M.;  Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,  X.;  Knox,  J.  E.;  Hratchian,  H.  P.;  Cross,  J.  B.;  Bakken,  V.;  Adamo,  C.;  Jaramillo,  J.;  Gomperts,  R.; Stratmann,  R.  E.;  Yazyev,  O.;  Austin,  A.  J.;  Cammi,  R.;  Pomelli,  C.;  Ochterski,  J.  W.;  Ayala,  P.  Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al‐Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;  Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,  J. A. Gaussian 03,  revision B.05; Gaussian, Inc.: Pittsburgh PA, 2003. 

29 Dunning, Jr., T.H.; Hay, P.J. in Schaefer III, H.F. (Ed.), Modern Theoretical Chemistry, Vol. 3, Plenum, New York, 1976, p. 1‐28. 

30 Bruice, P.Y. Organic chemistry 2nd ed.; Prentic‐Hall, inc.: New Jersey, 1998. 31 Hall, C.H.; Perutz, R.N. Chem. Rev. 1996, 96, 3125. 32 Jones, W.D. Acc. Chem. Res. 2003, 36, 140. 33 Niu, S.; Hall, M.B. Chem. Rev. 2000, 100, 353. 34 Wilkinson,  G.;  Gillard,  R.D.;  McCleverty,  J.A.  (Eds.),  Comprehensive  Coordination  Chemistry,  Vol.  3, 

Pergamon Press, Oxford, 1987, p. 1601.   35 Frash, M.V.; van Santen, R.A. Phys. Chem. Chem. Phys. 2000, 2, 1085. 36 Shubin, A.A.; Zhidomirov, G.M.; Kazansky, V.B.; van Santen, R.A. Catal. Lett. 2003, 90, 137. 37 Kuroda, Y.; Kumashiro, R.; Itadani, A.; Nagao, M.; Kobayachi, H. Phys. Chem. Chem. Phys. 2001, 3, 1383. 38 Philips, C.S.G.; Williams, R.J.P. Inorganic Chemistry, Oxford University Press, Oxford, 1965, p.114. 39 Kazansky, V.B.; Serykh, A.I.; Pidko, E.A. J. Catal. 2004, 225, 369. 40 Kazansky, V.B.; Pidko, E.A. J. Phys. Chem. B 2005, 109, 2103. 

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CHAPTER 8  

DEHYDROGENATION OF LIGHT ALKANES OVER ISOLATED GALLYL IONS IN ZSM‐5 ZEOLITE  

 

 

 

The catalytic dehydrogenation of ethane over gallyl ions (GaO+) as a model for the active sites in oxidized Ga/ZSM-5 zeolite is studied using a DFT cluster modeling approach. Initial activation of ethane occurs either via heterolytic dissociation of the C–H bond over the gallyl ion following an “alkyl”-type mechanism or via oxidation by the extra-lattice oxygen atom resulting in formation of an adsorbed ethanol molecule. In the latter case, ethanol rapidly reacts with the adsorption site producing a stable intermediate corresponding to the so-called “carbenium” initial activation. Both these paths are kinetically and thermodynamically preferred over the initial C–H activation by the reduced gallium species. However, the subsequent regeneration of the gallyl ion is very difficult. It is shown that the reduction of the gallyl ions upon the catalytic reaction is strongly preferred over the regeneration of the active sites. In addition, possible side reactions involving conversion of ethanol over GaO+ sites are discussed. Although several catalytic cycles can occur over the gallyl ions via a reaction channel involving one-step conversion of additional ethane molecules over the stable intermediates of the primary dehydrogenation path, these sites cannot be responsible for the reasonably stable catalytic activity of the oxidized Ga/ZSM-5 zeolites.

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8.1. Introduction

The modifying gallium cations can be present in the zeolite in different forms. In addition to the various reduced gallium species discussed in Chapter 7, gallyl ions (GaO+) have been proposed to be responsible for the high catalytic activity of Ga/ZSM-5 [1-8].

Interaction of methane with the GaO+ cations in zeolite has been studied by Himei et al. [4] and Broclawik et al. [5] Selected reaction paths of the dehydrogenation of ethane over GaO+ ions in ZSM-5 zeolite has been theoretically investigated by Frash and van Santen [7]. The gallyl ions have been shown to be highly active for the initial C–H bond cleavage [4-7]. However, the subsequent regeneration of the active site has been found to be strongly hampered by a high stability of the thus formed surface species. This does not tally with the recent experimental results [8] showing a high initial catalytic activity of the selectively oxidized Ga/ZSM-5 zeolite.

Indeed, the oxidized form of Ga/ZSM-5, where the main fraction of gallium is assumed to be present in the form of isolated gallyl ions, has been found to be initially the most active for light alkane dehydrogenation [8]. In addition to this, a high activity of the oxidized gallium-containing zeolites in the H/D exchange reaction has been confirmed both experimentally [9] and theoretically [10]. However, the oxidized catalyst deactivates with time on stream. After prolonged reaction, the dehydrogenation activity of oxidized Ga/ZSM-5 becomes equal to that of a catalyst containing predominantly Ga+ ions [8]. This finding has been explained by rapid reduction of the gallyl ions under the reductive conditions of propane dehydrogenation. It has been estimated that on average 1000 molecules of propane are converted per the oxidized gallium site before being reduced to Ga+. Clearly, currently available theoretical results do not satisfactorily agree with such experimental observations.

Herein, the systematic theoretical studies of reactivity of various extra-framework Lewis acid sites in ZSM-5 zeolite are continued. A detailed comparative analysis of the various reaction paths over the oxygenated form of gallium, which is the gallyl GaO+ ion stabilized in ZSM-5 zeolite, is presented below.

8.2. Computational details

In this work the Zd model for cation site (Figure 8.1) and the methods described in Chapter 7 were used.

8.3. Results and Discussion

8.3.1. “Alkyl” path of ethane activation

The gallium and oxygen atoms of the gallyl ion GaO+ represent a Lewis acid-base pair that is able to polarize and cleave the C–H bond of ethane following the alkyl (Cδ––Hδ+) mechanism. This leads to dissociative adsorption of ethane resulting in the formation of an

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 Figure  8.1.  Optimized  structure  of 

GaO+  ion  stabilized  in  the  Zd  cluster 

model representing ZSM‐5 zeolite. 

alkyl fragment (C2H5δ–) and a hydroxyl group

grafted to the gallium cation (IIa in Figure 8.2). This reaction (I+C2H6→TSa1→IIa) is strongly favored both thermodynamically and kinetically. The corresponding activation energy (∆E≠) is equal to 96 kJ/mol, and the reaction enthalpy (∆E) equals –234 kJ/mol.

In addition to the extra-framework oxygen atom of the gallyl ion, one identifies another basic site, that is, the oxygen atom (O2) of the zeolite lattice. Thus, heterolytic cleavage of the C–H bond may also occur over the Lewis acid-base pair formed by the gallium ion and this framework oxygen anion. This competing path (I+C2H6→TSa1*) is reminiscent of the mechanism proposed for the reduced gallium species in ZSM-5 (i.e. Ga+ and GaH2

+ cations, see Chapter 7). The computed activation energy for this reaction path (I+C2H6→TSa1*, Figure 8.2) is significantly higher (by 58 kJ/mol) than that of the reaction I+C2H6→TSa1. The difference lies in the higher basicity of the extra-framework oxygen atom (O). Moreover, this mechanism results in the formation of a highly unstable neutral C2H5–Ga=O species in the vicinity of an acidic proton. These species readily rearrange to intermediate IIa.

Activation of ethane over the gallyl ion is more facile than over reduced Ga+ and GaH2+

species. In the previous chapter the ∆E≠ values of 210 and 193 kJ/mol have been reported for the initial step of C–H cleavage for the latter two species, respectively. Additionally, the dissociation of ethane over the reduced sites is a slightly endothermic process (∆E = 9 and 34 kJ/mol, respectively for Ga+ and GaH2

+), whereas it is strongly exothermic over GaO+ (∆E = –234 kJ/mol). The exothermic nature of reaction I→IIa is due to the formation of a strong covalent hydroxyl bond in the latter reaction mechanism. The high exothermicity and rather low activation energy for the “alkyl” ethane activation over gallyl ions in zeolite clearly show that these species should easily react with ethane.

The resulting intermediate IIa, [HO–Ga–C2H5]+, decomposes via desorption of ethylene (IIa→TSa

β-H→III+C2H6) resulting in formation of the hydroxyhydridegallium ion III. This process is in fact a β-H transfer reaction, where the hydrogen atom at the β-position of the grafted C2H5

– species is abstracted by the gallium cation. This reaction is endothermic (∆E=104 kJ/mol) and faces a rather high activation barrier (∆E≠=203 kJ/mol). However, it is accompanied with a significant entropy increase. As a result, the corresponding value of Gibbs free-energy change at the reaction temperature is rather low (∆G823° = 10 kJ/mol). A similar elementary reaction has been identified in the reaction mechanism of ethane over gallium dihydride species. A reaction enthalpy of 95 kJ/mol and an activation energy of 257 kJ/mol have been computed for the reduced gallium species (Chapter 7). The lower activation energy for the β-H transfer in the present case is likely due to the enhanced Lewis acidity of the gallium ion bound to a hydroxide anion as compared to the case where

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Figure  8.2.  Reaction  paths  for  the  “alkyl  activation” mechanism  of  ethane  dehydrogenation 

over gallyl ion in ZSM‐5 zeolite. 

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Table  8.1.  Selected  interatomic  distances  (Å)  for  the  intermediates  and  transition  state 

structures involved in the “alkyl” path of ethane dehydrogenation. 

  I+C2H6  TS1  TS1*  II  TSβ‐H  TS1‐step  III  TSH2  TSH2O  IV Ga‐O  1.690  1.754  1.702  1.815  1.843  1.732  1.810  1.745  2.008  2.326 Ga‐O1  1.948  1.957  1.988  2.023  2.092  1.987  2.007  1.944  2.040  2.265 Ga‐O2  1.952  1.956  2.832  2.024  2.012  1.978  2.008  1.946  2.005  2.214 Ga‐C1  —  2.301  2.143  1.975  2.109  2.219  —  —  —  — O‐H1  —  1.412  —  0.967  0.964  1.631  0.967  1.493  0.972  0.982 O2‐H1  —  —  1.284  —  —  —  —  —  —  — C1‐H1  1.094  1.359  1.391  —  —  —  —  —  —  — Ga‐H2  —  —  —  —  1.708  —  1.551  1.967  1.645  — C2‐H2  —  —  —  —  1.771  2.037  —  —  —  — H1‐H2  —  —  —  —  —  0.812  —  0.923  —  — H1‐O4  —  —  —  1.939  2.346  —  1.948  —  2.182  1.748 H2‐O  —  —  —  —  —  —  2.974  —  1.488  0.963  

gallium coordinates a hydride anion. The overall result is stronger polarization of the β-CH bond and facilitation of the subsequent hydride abstraction.

The catalytic cycle is closed via recombination of molecular hydrogen from [HO–Ga–H]+ ions III (III→TSH2→I+H2). This reaction can be considered a Brønsted acid-base reaction, where the proton from the HO–(Ga) group attacks the hydride ion resulting in hydrogen desorption and regeneration of the GaO+ site. This reaction is calculated to be highly endothermic (∆E=259 kJ/mol). Moreover, an extremely high activation energy of 328 kJ/mol has to be overcome. Indeed, because of the high basicity of the terminal extra-framework oxygen atom, the dissociation of the H1–O bond in III is very unfavorable.

An alternative route is dissociation of the Ga–OH bond. Subsequent recombination of the hydroxyl group and the hydride ion (III→TSH2O→IV) leads to reduction of [HO–Ga–H]+ to Ga+ via elimination of H2O. In agreement with the assumption made in Ref. [8], this reduction path is preferred over the regeneration of the gallyl ion (I). The calculated enthalpy and the activation energy for the reduction are significantly lower (by 219 and 95 kJ/mol, respectively) than the corresponding values for hydrogen recombination.

The catalytic cycle can be also closed via a one-step reaction path leading to the regeneration of GaO+ ion directly from intermediate IIa (IIa→TSa

1-step→I+C2H4+H2, Figure 8.2) similar to the path proposed for Ga+. In this case, the hydrogen atom (H2) from the β-methyl group coordinates to the proton (H1) from the hydroxide group. However, this reaction proceeds via a very high activation energy barrier (∆E≠=390 kJ/mol), because of the extremely low acidity of the H1 proton. In addition, this process is strongly thermodynamically unfavored even at high temperature (∆G823° = 141 kJ/mol).

In summary, the initial ‘alkyl’ C–H bond cleavage over gallyl ions in ZSM-5 zeolite is a facile process, resulting in the formation of a stable [HO–Ga–C2H5]+ intermediate IIa. This intermediate can in principle decompose to form the gallium hydroxide hydride ion III and

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ethylene. However, subsequent regeneration of structure I is prohibited. This implies that after the initial step of dissociative ethane adsorption the thermodynamic equilibrium will be almost completely shifted to structures IIa and III, whereas formation of structure I will not take place. Instead, reduction of the active site via desorption of water will lead to the formation of Ga+ sites. These findings tally with previous theoretical works [6,7], although the exact energy values significantly vary because of the larger size of the cluster employed in the present work.

8.3.2. “Carbenium” activation of ethane

Alternative to the “alkyl” type mechanism is the polarization of ethane over the Gaδ+–Oδ– Lewis acid-base pair of the gallyl ion so as to lead to formation of a carbenium ion transition state (Cδ+–Hδ–) with an increase of the positive charge on the C1 carbon atom simultaneous with an increase of the negative charge on the H1 hydrogen atom. Such a polarization is followed by heterolytic dissociation of ethane with subsequent formation of the intermediate IIc-2 (Figure 8.3) involving a hydride atom and an alkoxyl group bound to the extra-framework gallium ion. Although this mechanism has been proposed before for ethane activation over GaO+ species, the respective transition state structure has not been detected [7]. Similarly, the transition state corresponding to the direct heterolytic C–H bond cleavage following the “carbenium” polarization mechanism could not be indentified in the present work.

Instead, an indirect route is found that leads to IIc-2. In this case, the extra-framework O atom initially plays the role of the oxidizing agent (Figure 8.3). At the initial step (I+C2H6→TSc1-1→IIc-1) the H1 atom of the ethane molecule coordinates to the basic O atom in the TSc1-1. This results in a significant elongation of the Ga–O and C1–H1 bonds (Table 8.2). Then the remaining ethyl group binds to the oxygen (O) atom resulting in formation of IIc-1, which is a C2H5OH molecule coordinated to the extra-framework Ga+ ion and hydrogen bound (H1···O4) to one of the basic lattice oxygens of the zeolite. Although this reaction is slightly less favorable that the “alkyl” C–H activation, it is strongly exothermic (∆E = –141 kJ/mol) and has a rather low activation energy of 109 kJ/mol.

In the next step, the adsorbed ethanol dissociates over the Ga+ site, and intermediate IIc-2 is formed (IIc-1→TSc1-2→IIc-2). This process is slightly exothermic (∆E = –21 kJ/mol). Although the activation energy for ethanol dissociation is rather high (∆E≠ = 203 kJ/mol), the overall activation barrier for the two-step “carbenium” activation (I+C2H6→IIc-2, Figure 8.3) is moderate (109 kJ/mol). One notes that it is also favored thermodynamically (∆G823°= –38 kJ/mol).

The decomposition of IIc-2 has been proposed by Frash and van Santen [7] to be a one-step process (IIc-2→TSc

1-step→ I+C2H4+H2, dashed lines in Figure 8.3). In this case the O–C1 bond breaks and one β-hydrogen atom (H2) of the ethyl group coordinates to the H1 atom of the Ga–H group. This results in a one-step desorption of molecular hydrogen and

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 Figure  8.3.  Reaction  paths  for  the  “carbenium  activation”  mechanism  of  ethane 

dehydrogenation over gallyl ion in ZSM‐5 zeolite. 

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Table  8.2.  Selected  interatomic  distances  (Å)  for  the  intermediates  and  transition  state 

structures involved in the “carbenium” path of ethane dehydrogenation 

  I+C2H6  TS1‐1  II‐1  TS1‐2  II‐2  TSβ‐H  TS1‐step  III Ga‐O  1.690  1.862  2.317  1.999  1.821  1.816  1.725  1.810 Ga‐O1  1.948  2.102  2.265  2.056  2.009  2.025  1.975  2.007 Ga‐O2  1.952  2.098  2.231  2.044  1.972  2.011  1.971  2.008 Ga‐H1  —  —  —  1.659  1.555  1.560  1.792  1.551 O‐C1  —  2.392  1.443  1.434  1.423  1.964  2.381  — O‐H1  —  1.036  0.979  1.462  —  —  —  — C1‐H1  1.094  1.570  —  —  —  —  —  — O‐H2  —  —  —  —  —  1.310  —  0.967 C2‐H2  —  —  —  —  —  1.357  1.538  — H1‐H2  —  —  —  —  —  —  0.912  — H1(2)‐O4  —  2.755  1.772  —  —  2.588  —  1.948  

ethylene with simultaneous regeneration of the initial GaO+ active site. Similar to the one-step decomposition of IIa, this reaction is not likely to occur. The calculated activation energy is equal to 354 kJ/mol.

On the other hand, ethylene can desorb from IIc-2 via abstraction of a β-hydrogen atom (H2) by the extra-framework O atom (IIc-2→TSβ-H→III+C2H6, solid lines in Figure 8.3). This step is favored both thermodynamically and kinetically over the above-considered one-step process. Most likely this is due to formation of a strong O–H2 bond. This energy gain significantly compensates the energy loss due to decomposition of the alkoxyl species. One can see that the overall activation energy for the dehydrogenation of ethane via “carbenium” mechanism until formation of III is less than 109 kJ/mol. Therefore, one expects generation of these species via this route under the reaction conditions. However, according to the results in section 8.3.1 the subsequent regeneration of the active site via the hydrogen recombination is prohibited.

Although the initial “alkyl” activation of ethane is slightly more favorable as compared to the “carbenium” mechanism, subsequent chemical transformations of the reaction intermediates are easier in the latter case because of their lower stability. The calculated activation barrier for the initial oxidation step of the “carbenium” activation is only slightly higher than that for the heterolytic C–H bond cleavage over the gallyl ion. On the other hand the former reaction is less exothermic. Taking into account that catalytic ethane dehydrogenation takes place at rather high temperatures, one can expect that the oxidative “carbenium” route is the most likely path. However, due to a small difference in energetics of the processes, the “alkyl” activation cannot be excluded as well.

8.3.3. Dehydration of ethanol over GaO+ sites

At the initial step of the “carbenium” mechanism of ethane activation, ethanol and the reduced Ga+ site are formed. Their further interaction faces a rather high activation barrier,

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and thus, desorption of ethylene can take place as a competitive route. Therefore, the interaction of the thus-formed ethanol with another GaO+ site of the zeolite was investigated. The energetics for this process are presented in Figure 8.4, and the geometry parameters of the intermediates and transition state structures are listed in Table 8.3.

Molecular adsorption of ethanol to the gallyl ion (I-1) is rather strong (∆E = –83 kJ/mol). During this process a hydrogen bond is formed between the H1 atom of C2H5OH and the extra-framework oxygen (O) of the gallyl ion. In addition, the negatively charged O’ atom of ethanol coordinates to the positively charged gallium (Table 8.3). Both these interactions result in a significant weakening of the O’–H1 bond and facilitate its heterolytic cleavage. The O’–H1 bond of the ethanol heterolytically dissociates (I-1→TSe1→IIe, Figure 8.4) on the oxo-gallyl species resulting in very stable [HO-Ga-OC2H5]+ species IIe without notable barrier. Further possible chemical transformations are similar to those discussed above for the dehydrogenation of ethane.

Intermediate IIe can decompose in one step via reaction IIe→TSe1-step→IVe+C2H6

resulting in elimination of molecular ethylene and formation of water adsorbed on the gallyl ion GaO+. This reaction proceeds via coordination of a β-hydrogen atom (H2) from the ethoxyl group to the oxygen atom of the O–H1 group (TSe

1-step). This results in elongation of the O’–C1, C2–H2 and Ga–O bonds with simultaneous shortening of the Ga–O’ and formation of a new O–H2 bond (Table 8.3). The one-step reaction is strongly disfavored thermodynamically (∆E=248 kJ/mol, ∆G823°=131 kJ/mol) and shows a high activation barrier (∆E≠ =262 kJ/mol).

Similar to the above-considered ethane activation, consecutive formation of ethylene and water is preferred. The energetics of the β-hydrogen (H2) abstraction by the O’ atom resulting in formation of a [HO–Ga–OH]+ intermediate IIIe (IIe→TSe

β-H→IIIe+C2H4, Figure 8.4) is very similar to that calculated for the IIc-2→TSc

β-H→III+C2H4 reaction (Figure 8.3). This shows that the chemical transformations of the alkoxyl group bound to the extra-framework gallium in ZSM-5 zeolite depend only slightly on other ligands attached to the Ga atom, unless they are directly involved in these transformations (e.g. in the case of one-step decomposition reactions).

The resulting dihydroxylgallium ion IIIe decomposes via water elimination (IIIe→TSe

H2O→IVe). This reaction is also rather unfavorable (∆E=217 kJ/mol; ∆E≠=217 kJ/mol). One can see that the reverse reaction is barrierless. The catalytic cycle for the dehydration of C2H5OH over gallyl ions is closed via water desorption (IVe→I+H2O). The catalytic cycle considered is therefore expected to stop at the step of formation of either intermediate IIe or IIIe.

It has been shown by Rane et al. [8] that ethane dehydrogenation over the oxidized Ga/ZSM-5 zeolite shows a lower olefin selectivity as compared to the reduced catalyst. Moreover, coke formation in this case is also more pronounced. Indeed, formation of very reactive ethanol molecules upon ethane transformation over GaO+ sites, as well as the

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 Figure 8.4. Dehydration of ethanol over gallyl ion in ZSM‐5 zeolite. 

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Table  8.3.  Selected  interatomic  distances  (Å)  for  the  intermediates  and  transition  state structures involved in the ethanol dehydrogenation over GaO+ sites 

  I+ C2H5OH 

I‐1  TS1  II  TSβ‐H  TS1‐step  III  TSH2O  IV 

Ga‐O  1.690  1.710  1.749  1.812  1.810  1.943  1.802  2.072  2.089 Ga‐O1  1.948  1.973  1.948  1.988  2.013  2.014  1.993  1.970  2.024 Ga‐O2  1.952  1.981  1.957  1.963  2.000  1.991  1.991  1.956  1.999 O’‐H1  0.962  0.977  1.111  —  —  —  —  —  — Ga‐O’  —  2.106  2.041  1.802  1.811  1.739  1.805  1.746  1.706 O‐H1  —  2.333  1.472  0.961  0.968  0.977  0.968  0.968  0.983 O’‐C1  1.425  1.453  1.435  1.426  1.905  2.072  —  —  — O’‐H2  —  —  —  —  1.263  1.547  0.962  1.492  2.680 O‐H2  —  —  —  —  —  1.115  —  1.101  0.968 C2‐H2  —  —  —  —  1.392  1.547  —  —  — H1‐O4  —  —  —  —  1.942  1.821  1.970  2.164  1.743  

existence of numerous reaction pathways are in accord with these findings. On the other hand, despite the lower reactivity, the Ga+ sites in ZSM-5 do not produce highly reactive intermediates and do not provide competitive reaction paths resulting in high and stable selectivity to the products of ethane dehydrogenation (Chapter 7).

8.3.4. “Concerted" dehydrogenation over the intermediates of the primary route

The primary ethane dehydrogenation paths over the GaO+ site in ZSM-5 zeolite result in the formation of very stable intermediates, and therefore, subsequent closure of the catalytic cycle is strongly activated as compared to the facile reduction of the gallyl ion to univalent gallium ions. However, it has been experimentally shown [8] that although the activity of the selectively oxidized Ga/ZSM-5 zeolite decreases with time on stream to the same value as for the sample containing only Ga+ sites, several catalytic cycles occur over the oxidized gallium species before their dehydration. Obviously, the reaction paths discussed above cannot explain formation of more than one ethylene molecule per oxidized active site.

Recently, Pereira and Nascimento [11] have proposed a new concerted mechanism to describe the catalytic activity of dihydridegallium ions (GaH2

+) in ZSM-5. It has been shown that the activation barrier in this case is higher than that of the conventional three-step mechanism proposed by Frash and van Santen [7]. On the other hand, the concerted mechanism has an important advantage since it does not require conformational rearrangements of the reacting species. The primary routes of ethane dehydrogenation over GaO+ ions result in formation of intermediates IIc-2 and III, which contain a hydride ion bound to gallium. Thus, secondary reactions over these intermediates involving additional C2H6 molecules may explain the experimentally observed dehydrogenation activity.

The calculated reaction paths for the concerted ethane dehydrogenation pathway over intermediates IIc-2 and III as well as their energetics in comparison with the competing

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 Figure  8.5.  Concerted mechanism  of  ethane  dehydrogenation  over  [H–Ga–OR]+  ions, where R = H, C2H5. 

Table  8.4.  Selected  interatomic  distances  (Å)  for  the  intermediates  and  transition  state structures involved in the ‘concerted’ mechanism of ethane dehydrogenation  

  IIc‐2 + C2H6  TS1C  III + C2H6  TS2C Ga‐O  1.821  1.833  1.810  1.820 Ga‐O1  2.009  2.045  2.007  2.044 Ga‐O2  1.972  2.006  2.008  2.052 Ga‐H1  1.555  2.061  1.551  2.020 Ga‐H1’  —  1.703  —  1.697 C1’‐H1’  1.094  1.455  1.094  1.470 C2’‐H2’  —  1.662  —  1.650 H1‐H2’  —  0.830  —  0.831 O‐C1  1.423  1.422  —  — H2‐O4  —  —  1.948  1.978  

primary dehydrogenation or reduction routes are shown in Figure 8.5. Table 8.4 summarizes the geometry parameters of the reaction intermediates and transition state structures involved in these processes. Following this concerted reaction, one of the hydrogen atoms (H1’) of the second ethane molecule coordinates to the gallium ion, while the hydrogen atom (H2’) from the β-methyl group binds to the hydride ion grafted to Ga (Figure 8.5). This results in one-step formation of ethylene and dihydrogen as well as in regeneration of the active intermediate (IIc-2 or III). The geometry parameters for the corresponding transition state structures TS1C and TS2C are similar (Table 8.4). The activation energy for the direct dehydrogenation reaction depends only slightly on the noninteracting (–OH or –OC2H5) group bound to gallium. The calculated activation

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energies for the concerted ethane dehydrogenation over intermediates IIc-2 and III are equal to 258 and 262 kJ/mol, respectively. These values are somewhat lower than those reported before [11] for the respective process over the GaH2

+ site. Most likely, this is due to a higher Lewis acidity of the gallium ion bound to oxygen atoms in IIc-2 and III compared to that of the GaH2

+ ion.

The activation energies for the concerted C2H6 dehydrogenation paths are very close to those for the corresponding competitive elementary steps of the primary dehydrogenation route (III→TSH2O→IV and IIc-2→TSc

β-H→III+C2H6, Figures 8.2 and 8.3). Therefore, these paths will compete, and several catalytic cycles can in principle occur following the concerted mechanism before the active intermediate transforms according to the primary reaction path. The overall activation energy for ethane dehydrogenation over GaO+ sites in this case will not exceed 150 kJ/mol. This value is about 25% lower than the one calculated for ethane dehydrogenation over the reduced Ga+ sites in ZSM-5 zeolite (Chapter 7). This could explain the very high initial activity of the oxidized Ga/ZSM-5 catalyst. However, with time on stream the active site will rapidly reduce to form a more stable Ga+ ion, since this reaction path is strongly preferred over the regeneration of the gallyl ions. We estimate formation of not more than three ethylene molecules per isolated GaO+ species before being reduced to Ga+. This, however, contradicts the recently reported observation of about 1000 catalytic cycles per oxidized gallium site in Ga/ZSM-5 [8]. On the other hand, the low stability and, therefore, high reactivity of the isolated gallyl ions may cause processes such as oligomerization of the isolated GaO+ species in the oxidized Ga/ZSM-5 zeolite. In this case a wide variety of different gallium-oxide clusters can be generated.

8.4. Conclusions

Different reaction paths for ethane dehydrogenation over the gallium species in oxidized Ga/ZSM-5 (i.e. GaO+ ions stabilized at cationic sites of the zeolite) are studied by means of DFT calculations. Two different reaction paths are found for the initial C–H activation. The first one is the heterolytic dissociation of ethane on the gallyl ion following the “alkyl” (Cδ––Hδ+) polarization mechanism. This reaction path exhibits a rather low activation barrier and results in formation of a very stable intermediate [C2H5–Ga–OH]+ that can be decomposed via ethylene desorption to form a [H–Ga–OH]+ species. Further regeneration of the active site via H2 desorption is strongly disfavored both thermodynamically and kinetically.

Another possible reaction path for ethane dehydrogenation is initiated by partial oxidation of an ethane molecule by the extralattice oxygen atom. The resulting ethanol molecule then interacts with the adsorption site (Ga+) leading to formation of a product of “carbenium” mechanism [H–Ga–O–C2H5]+. The direct dissociative adsorption path for ethane on GaO+ ions following such a polarization mechanism (Cδ+–Hδ–) is not found. Similar to the case of the “alkyl” mechanism, subsequent C2H4 desorption leads to a stable [H–Ga–OH]+ cation.

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Although catalytic cycles of these primary dehydrogenation routes cannot be closed because of formation of very stable grafted intermediates, several catalytic cycles may occur via a concerted dehydrogenation mechanism. Involvement of these secondary reactions over the intermediates formed via the primary route leads to estimation of the overall activation energy for the catalytic ethane dehydrogenation over GaO+ ions below 150 kJ/mol. This value is significantly lower than the overall activation energy calculated for the reduced gallium containing active sites. However, despite this facile pathway gallyl ions reduce very rapidly via water desorption to produce univalent gallium ions. This latter reaction shows the activation energy lower by 95 kJ/mol than the regeneration of the oxygenated active site, and is significantly favored thermodynamically. From the results presented, one cannot expect formation of more than three ethylene molecules per GaO+ site before being reduced to Ga+. Therefore, gallyl ions cannot be considered as catalytically active sites for dehydrogenation of light alkanes. 

References 

1 Dooley, K.M.; Chang, C.; Price, G.L. Appl. Catal. A: General 1992, 84, 17. 2 Abdul Hamid, S.B.; Derouane, E.G.; Mériaudeau, P.; Naccache, C. Catal. Today 1996, 31, 327. 3 Kazansky, V.B.; Subbotina, I.R.; van Santen, R.A.; Hensen, E.J.M. J. Catal. 2005, 233, 351. 4 Himei, H.; Yamadaya, M.; Kubo, M.; Vetrivel, R.; Broclawik, E.; Miyamoto, A.  J. Phys. Chem. 1995, 99, 

12461. 5 Broclawik, E.; Himei, H.; Yamadaya, M.; Kubo, M.; Miyamoto, A.; Vetrivel, R. J. Chem. Phys. 1995, 103, 

2102.  6 Gonzales, N.O.; Chakraborty, A.K.; Bell, A.T. Top. Catal. 1999, 9, 207. 7 Frash, M.; van Santen, R.A. J. Phys. Chem. A 2000, 104, 2468. 8 Rane, N.; Overweg, A.R.; Kazansky, V.B.; van Santen, R.A.; Hensen, E.J.M. J. Catal. 2006, 239, 478. 9 Hensen, E.J.M.; García‐Sánchez, M.; Rane, N.; Magusin, P.C.M.M.; Liu, P.H.; Chao, K.J.; van Santen, R.A. 

Catal. Lett. 2005, 101, 79. 10 Kuzmin, I. V.; Zhidomirov, G.M.; Hensen, E.J.M. Catal. Lett. 2006, 108, 187. 11 Pereira, M.S.; Nascimento, M.A.C. Chem. Phys. Lett. 2005, 406, 446; J. Phys. Chem. B. 2006, 110, 3231. 

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CHAPTER 9  

MULTINUCLEAR GALLIUM‐OXO CATIONS IN HIGH‐SILICA ZEOLITES 

 

 

 

The stability of cationic gallium-oxo complexes in mordenite is studied by means of periodic DFT calculations. It is found that independently of the framework Al distribution the stability of two isolated gallyl (GaO+) ions in mordenite (Si/Al=23) is considerably lower (330-460 kJ/mol) as compared to that of the cyclic Ga2O2

2+ cations stabilized in the immediate vicinity of two framework anionic sites of mordenite. More surprisingly, it is found that such clusters can be present as stable charge-alternating species when favorable tetrahedral environment of Ga ions cannot be achieved along with the direct interaction of the positively charged complex with the anionic framework sites. Oligomerization of four isolated gallyl ions in zeolites with a higher aluminum density (Si/Al=11) results in the formation of cubic Ga4O4

4+ ions, which are remarkably stable when the cluster directly interacts with only two anionic sites, while the remaining two framework [AlO2]– units are significantly distant. Multinuclear gallium-oxo species show remarkable reactivity toward heterolytic C–H bond cleavage of alkanes. At the same time, the lower basicity of the extra-framework bridging oxygen anions as compared to that of the terminal oxygen anions in GaO+ facilitates regeneration of the active site by hydrogen desorption. It is concluded that such multinuclear species can be responsible for the enhanced catalytic activity of oxidized gallium-exchanged high-silica zeolites. These sites, however, tend to decompose via water desorption resulting in less active reduced Ga-species. These theoretical results suggest that the high alkane dehydrogenation activity of oxygenated cationic complexes may be maintained by selective hydrolysis of the reduced extraframework Ga-species. Indeed, experimental catalytic tests show that addition of steam to the hydrocarbon feed substantially increases dehydrogenation activity of Ga/ZSM-5. The rate enhancement is due to increase of the steady-state concentration of the oxygenated multinuclear Ga-species in the catalyst.

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9.1. Introduction

Activation of alkanes over various mononuclear gallium cations stabilized in high-silica zeolites has been discussed in Chapters 7 and 8. The theoretical insights of the reduced active sites (Chapter 7) are well supported by experimental data. Isolated GaO+ sites have been proposed as active species for oxidized Ga cations [1-6]. However, DFT calculations suggest that catalytic light alkane dehydrogenation over such species is not likely to occur (Chapter 8). The initial heterolytic C–H bond cleavage over GaO+ cations is favorable and results in formation of very stable intermediates. Due to a high basicity of the terminal extra-framework oxygen ion in GaO+, subsequent H2 recombination, which is required to regenerate the active site, is strongly disfavored. Rather reduction of the active site via H2O desorption takes place during the catalytic reaction. These results do not agree with the experimentally observed high catalytic activity of the oxidized Ga/ZSM-5 zeolite [6]. It has been suggested in Chapter 8 that the high reactivity of gallyl ions for C–H cleavage is due to the low stability of these species that, in turn, can cause their oligomerization in the zeolite matrix resulting in formation of multinuclear cationic gallium-oxo clusters. To clarify this a periodic DFT study of stability and reactivity of different isomeric (GaO)2

2+ species in high-silica mordenite is performed.

9.2. Computational details

The quantum-chemical calculations were carried out using Vienna Ab Initio Simulation Package [7,8] within DFT with the gradient-corrected PW91 [9] exchange-correlation functional. The projected-augmented wave (PAW) method [10,11] was used to describe electron-ion interactions, and for valence electrons a plane wave basis set was employed. The energy cut-off was set to 400 eV. The Brillouin zone sampling was restricted to the Γ-point [12]. Full geometry optimizations were performed for each structure with the fixed optimized cell parameters using a conjugated gradient algorithm. Convergence was assumed to be reached when the forces on each atom were below 0.05 eV/Å.

A detailed periodic DFT study of ZSM-5 zeolite is currently impossible due to low symmetry and the large size of the unit cell. On the other hand, mordenite zeolite is often used as a general model of high-silica zeolites. Its purely siliceous structure has Cmcm space group symmetry. The orthorhombic cell of mordenite contains 144 atoms (48 Si and 96 O) [13]. It is possible, however, to construct a smaller monoclinic primitive cell of 72 atoms (24 Si and 48 O) [14]. After optimizing volume and shape, the parameters of the primitive cell are the following: a = b = 13.648 Å, c = 7.508 Å, and γ = 97°. To model a high-silica zeolite, a supercell was used. It was constructed by doubling the monoclinic unit cell along the c axis. The resulting cell contained 48 silicon atoms and 96 oxygens. To model gallium-exchanged mordenite two (models I, II and III) or four (models I+II and I+III) framework silicon ions were substituted with aluminums. The above mordenite models differ in framework Al distribution (Figure 9.1). In the case of model I both Al ions were located in one 8-membered ring connecting the main channel with the side-pocket of

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Figure  9.1. Mordenite  unit  cells  (only  silicon‐aluminum  framework  is  shown  for  clarity) with 

different aluminum distribution. Models I, II, and III show Si/Al ratio of 23, while that for I+II and 

I+III equals 11. Subscripts SP and W are used  to differentiate Al  ions  located,  respectively,  in 

side‐pockets of mordenite and those located in smaller 5T or 6T rings from the wall of the main 

channel. 

mordenite. In model II aluminum ions were located in the side-pockets opposite to each other in the main channel. In model III both Al ions were part of 5T and 6T rings of the zeolite wall and were located distantly from the large 8-membered rings. Models I+II and I+III correspond to the lower-silica mordenite unit cell (Si/Al=11) with Al localization similar to that in models I and II in the former case (I+II), and to that of models I and III for the case of I+III model.

9.3. Results and Discussion

9.3.1. Structure and stability

Figure 9.2 summarizes the relative stabilities of various (GaO)22+ isomers stabilized in

mordenite models I, II and III (Si/Al=23). The stability of univalent gallium cations located at the intersection of the main channel and the side-pockets of mordenite (models I and II) does not depend on the position of the framework anionic sites. The difference in the calculated total energy of zeolites I and II containing two Ga+ does not exceed 0.01 kJ/mol. Structure [(Ga+)2]III is however less stable by 9 kJ/mol. This is most likely due to a lower flexibility of the [AlO2]– framework units in small 5T and 6T rings of the main zeolite channel (III) as compared to that of tetrahedra located at the intersection of the mordenite channels. This results in slightly longer Ga–Oz bonds, and therefore, in a weaker interaction between the exchangeable cation and the framework oxygen ions (Figure 9.3).

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Figure  9.2.  Optimized  structures  of  (GaO)22+  isomers  in mordenite.  The  ∆E  values  (kJ/mol) 

correspond  to  the  reaction  energy  for  the  stochiometric oxidation of  two  exchangeable Ga+ 

cations with N2O according to the reaction: [2Ga+] MOR + 2N2O → [(GaO)2

2+] MOR + 2N2. 

Table 9.1. Selected geometry parameters (interatomic distances (Å), bond and dihedral angles 

(°))  in cyclic Ga2O2 core stabilized  in mordenite zeolite. For comparison the respective data  is 

provided for complex 2 [20b].   2  [(Ga2O2)]

I  [(Ga2O2)]II‐W [(Ga2O2)]

II‐SP [(Ga2O2)]III‐W  [(Ga2O2)]

III‐SP

Ga1‐O1  1.848  1.859  1.878 1.870 1.875 1.856 Ga1‐O2  1.854 1.861  1.883 1.874 1.885 1.854 Ga2‐O1  1.854 1.869  1.882 1.853 1.894 1.857 Ga2‐O2  1.848 1.856  1.860 1.842 1.841 1.860 Ga1‐Ga2  2.599 2.501  2.597 2.490 2.471 2.490 Ga1‐O1‐Ga2  89.18 84.59  87.40 83.99 81.92 84.19 Ga1‐O2‐Ga2  89.18 84.28  87.90 84.17 83.07 84.24 Ga1‐O1‐Ga2‐O2  0.00 0.28  –10.96 –1.00 –24.85 –0.28 Ga1‐Al1  —  2.817  2.908 2.808 2.785 5.313 Ga2‐Al2  —  2.831  2.904 5.441 6.285 8.360 

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Figure  9.3.  Optimized  structures  and  selected  geometry  parameters  (Å)  of  (GaO)2  isomers 

stabilized in mordenite. The respective data for 2 is provided for comparison. 

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Nevertheless, these differences are within the accuracy of the method. In agreement with the conclusion in Chapter 7, the properties of univalent gallium ions in zeolite depend only slightly on the aluminum distribution in the framework.

Stoichiometric oxidation of Ga+ sites with nitrous oxide to GaO+ is slightly exothermic ([(GaO+)2], Figure 9.2). Formation of two isolated gallyl ions in a single side-pocket of mordenite I requires a slightly unfavorable coordination of one of the GaO+ (Ga2–O2 in [(GaO+)2]I, Figure 9.3) cations to the zeolite wall. Geometry optimization of a similar starting structure but with both GaO+ species initially coordinated to the oxygen anions (O1z, O2z, O3z and O4z) of one 8-membered ring of model I leads to [(GaOGaO)2+]I, which is more stable by 118 kJ/mol than [(GaO+)2]I.

Because of an almost in-plane location of the GaO+ in the 8-memebered rings of zeolite II, the terminal extra-framework oxygen atoms (O1 and O2) are only 3.1 Å distant from the lattice oxygens of the ring, resulting in their repulsion. This explains the slightly lower stability of [(GaO+)2]II as compared to the structure in model III where such repulsion is absent. Nevertheless, similar to the case of model I, rearrangement of the gallyl ions results in an oxygen-bridged dimer (GaOGaO)2+ in II and III with an energy benefit of 114 and 30 kJ/mol, respectively (Figure 9.2). One notes that due to the large distance between the framework Al ions in III (7.511Å), such dimer cannot interact with both framework [AlO2]– units, and hence, a charge-alternating structure is formed ([(GaOGaO)2+]III). In this configuration one of the positively charged gallium ions (Ga2) coordinates to the O6z and O7z ions from the neutral [SiO2] framework unit (Figure 9.3).

Further stabilization of the extra-framework species is achieved by formation of an almost square-planar (Ga2O2)+2 cation. This further lowers the total energies by 313, 187, and 37 kJ/mol for models I, II, and III, respectively (Figure 9.2). The much higher stability of the cyclic dimer cation in zeolite I is due to the formation of an almost perfect tetrahedral surrounding of both Ga ions in the side-pocket of mordenite ([(Ga2O2)2+]I, Figure 9.3). The lower stability of the cations in zeolites II and III is due to the location of Ga2O2

2+ along the zeolite channel that does not allow the favorable coordination of the Ga ions. The lack of direct charge-compensation in [(Ga2O2)2+]III further decreases its stability. These differences already indicate the importance of optimal geometrical environment of the extra-framework cationic complex.

A more favorable location of the Ga2O22+ ion in models II and III is obtained by

allowing the complex to adopt an almost perfect tetrahedral surrounding of the Ga ions in the side pocket ([(Ga2O2)2+]II-SP and [(Ga2O2)2+]III-SP). The total energies of these structures are lowered by 30 and 49 kJ/mol, respectively, in spite of further separation of the charges. In [(Ga2O2)2+]II-SP one of the Ga3+ ions coordinates to a neutral silicon-occupied oxygen tetrahedron and is located 5.441 Å away from the negative framework charge in the main channel (Figure 9.3, Table 1). Structure [(Ga2O2)2+]III-SP can be considered as an extreme case of charge-alternatation. The charge-compensating Ga2O2

2+ cation in this structure coordinates only two neutral [SiO2] framework units, while the anionic sites are 5.313–

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Figure  9.4.  Experimental  structure  of  Ni(H2O)62+  cation 

with  two glycoluril anions  [16c]. 6 water molecules  form 

octahedral surrounding of Ni2+, whereas bulky anions are 

located outside the first coordination sphere of the cation. 

8.360 Å distant from the cationic complex. The favorability of such an unconventional charge-compensation of the cationic species derives from the domination of energy gain due to the stable tetrahedral surrounding of Ga3+ over energy loss resulting from the lack of a counterion in the first coordination sphere of the metal center. It is important however to realize that the framework oxygen anions bridging lattice silicons are basic.

When both the direct charge-compensation and the favorable geometry of the extra-framework cations can be realized, the stability of the system is the highest ([(Ga2O2)2+]I). On the other hand, the aluminum distribution in zeolites is mostly random [15]. Thus, in general the availability of pairs of anionic sites in close proximity and ideal for formation of a stable [(Ga2O2)2+] cation geometrical environment is unlikely. In such cases, the computational results indicate that the positions of multivalent cations or cationic complexes are not dominated by localized charge-compensation between the cations and the framework charges. One should note that the indirect charge-compensation is not uncommon in for instance coordination compounds, where often a central metal ion is surrounded by neutral ligands, while the charge-compensating anions are located somewhere outside the coordination shell (Figure 9.4) [16].

Although it has been thought that the isolated location of Ga+ sites in high-silica zeolites could lead to stable oxidized sites of low nuclearity [1-6], the present results show that such species easily oligomerize, because in this way a more optimal coordination of the metal ion centers is obtained. Moreover, the formation of multinuclear gallium-oxo species is not surprising considering the known organometallic complexes of Ga. Indeed, a search of the Cambridge Structural Database [17] has revealed 139 different compounds containing a cyclic Ga2O2 moiety, while no crystal structure containing a terminating gallyl moiety has been found.

Most of these multinuclear compounds contain bis-(µ2-OR)-di-Ga moieties (R = H, alkyl) and can be synthesized by a variety of synthetic routes. The most widely used method involves hydrolysis of gallium trialkyls [18]. The unsubstituted Ga2O2 core can be formed only when gallium ions coordinate bulky chelating ligands [19,20]. Further correlation between the chemistry of gallium-exchanged zeolites and conventional coordination chemistry of gallium is derived from the fact that compound 2 can be prepared via stoichiometric oxidation of the Ga+ precursor 1 with N2O according to reaction (9.1) [20b]. Similar reaction was used for

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preparation of the selectively oxidized Ga/ZSM-5 zeolite in Ref. [6].

 

In the current case, the zeolite framework can be considered a relatively rigid polydentate ligand coordinating the exchangeable cations and conditioning their unique properties. Compound 2 is the analogue of a Ga2O2

2+ cation stabilized in zeolite. The structural parameters of the Ga2O2 core in 2 and in the zeolitic structures discussed above are very similar (Table 9.1, Figure 9.3). However, the less symmetric structures [(Ga2O2)2+]II-W and [(Ga2O2)2+]III-W have interatomic distances slightly longer as compared to the organometalic counterpart 2.

In addition to binuclear Ga3+ species, numerous compounds containing gallium-oxo and gallium-hydroxo polyhedron or ring structures with rather high degree of aggregation have been reported [21]. Structurally characterized (GaO)n cages are limited to the silyl substituted species [(tBu)3SiGaO]4 (3), which has an almost perfect cubic Ga4O4 core and an average Ga–O distance near 1.91 Å (Figure 9.5, Table 2) [21b]. To check the possibility of formation of similar clusters in mordenite, the stability of Ga4O4

4+ cation in models I+II and I+III was theoretically investigated. The gallium-oxo cluster was placed at the side-pocket of mordenite containing two framework Al ions, which is reminiscent to the cation site in model I, while the other two Al ions required for the charge-neutrality were placed distantly. The relative arrangement of the latter [AlO2]– framework units corresponded to the above-discussed models II and III, respectively for I+II and I+III. Figure 9.5 shows the optimized structures and the calculated energies of formation of cubic Ga4O4

4+ cation in these models according to reaction (9.2). Table 9.2 lists interatomic distances for the Ga4O4 core in the organometallic complex 3 and for that stabilized in the zeolite matrixes.

   

Unlike the silyl substituted complex 3, the zeolite matrix cannot provide equally favorable coordination of all Ga3+ ions of the Ga4O4 cationic cluster. Independently of the zeolite model, three gallium ions of the cluster are surrounded by five oxygen ions each, two of which belong to the zeolite framework. The remaining Ga3+ is coordinatively unsaturated and binds to three extra-framework O atoms. This results in significant deviation of Ga–O bond lengths compared to that observed in 3 (Table 9.2).

In spite of similar geometries of the gallium-oxo clusters in [(Ga4O4)4+]I+II and [(Ga4O4)4+]I+III, the latter structure shows a 50 kJ/mol lower stability due to a more distant

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Figure 9.5. Optimized structures of cubic Ga4O4 clusters stabilized in mordenite. The ∆E values correspond  to  the  reaction energy  for  the  stochiometric oxidation of  four exchangeable Ga+ 

cations with N2O according to the reaction (9.2). Complex 3 is shown for comparison. 

Table 9.2. Selected interatomic bond lengths (Å) in in organometallic complex 3 [21b] and in 

tetrameric cubic Ga‐Oxo clusters stabilized in models I+II and I+III.  

  3  I+II  I+III 3 I+II  I+III Ga1‐O1  1.901  2.150  2.128 Ga1‐Ga2 2.713 2.810  2.798Ga1‐O2  1.915  1.982  1.990 Ga1‐Ga3 2.705 2.896  2.892Ga1‐O4  1.928  1.921  1.929 Ga1‐Ga4 2.706 2.807  2.818Ga2‐O2  1.912  2.049  2.026 Ga2‐Ga3 2.706 2.745  2.757Ga2‐O3  1.924  2.009  1.995 Ga2‐Ga4 2.719 2.800  2.788Ga2‐O4  1.909  1.947  1.946 Ga3‐Ga4 2.713 2.765  2.791Ga3‐O1  1.928  1.961  1.964 Ga1‐O1z — 1.945  1.935Ga3‐O3  1.915  2.027  2.037 Ga1‐O2z — 1.969  1.966Ga3‐O4  1.901  1.962  1.963 Ga2‐O3z — 1.955  1.985Ga4‐O1  1.909  1.899  1.912 Ga2‐O4z — 1.994  2.001Ga4‐O2  1.924  1.908  1.915 Ga3‐O5z — 1.970  1.979Ga4‐O3  1.912  1.890  1.894 Ga3‐O6z — 2.070  2.084 

location of the charge-compensating framework [AlO2]– units. One notes that the most stable structures in the higher-silica models are those involving cyclic Ga2O2 dimer stabilized in the side-pocket of mordenite. Assuming that the increasing concentration of framework aluminum does not significantly affect the stability of binuclear cationic species, the ∆E values for reaction (9.3) are estimated to 822 and 630 kJ/mol, respectively

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for models I+II and I+III. These values are close to those calculated for the formation of (Ga4O4)4+ species. Formation of two binuclear cations in I+II is only 30 kJ/mol per Ga ion more favorable than the formation of a cubic cation. For model I+III reaction (9.2) is slightly more favorable (6 kJ/mol per Ga ion).

Summarizing the foregoing results, binuclear cyclic Ga2O2 sites in high-silica zeolites should be predominant in the oxidized gallium-containing zeolites with a Ga-content close to the ion-exchange capacity. However, one should not exclude the formation of multinuclear intrazeolite cationic gallium-oxo and -hydroxo clusters with a higher degree of aggregation. Such clusters can be present as the charge-alternating sites when the conventional charge-compensation does not provide favorable coordination of Ga ions. The zeolite in this case acts as a rigid polydentate ligand and the trends observed in conventional coordination chemistry of transition metal (TM) ions can be expected in this case. The formation of multinuclear cationic species may be more general for transition metal ions stabilized in microporous matrixes, especially when the metal content exceeds the ion-exchange capacity.

9.3.2. Reactivity of binuclear sites

In catalysis, the most stable sites often show significantly lower chemical reactivity than their less stable isomers. To check whether this could be the case for the Ga2O2 isomers, the energetics of the most important reaction steps of catalytic ethane dehydrogenation over the most stable structure [(Ga2O2)2+]I were computed. Note, that basicity of the extra-framework oxygen atoms in isolated GaO+ cations is very high (Chapter 8). This leads to a high initial reactivity of these species toward initial activation of hydrocarbons and, at the same time, does not allow regeneration of the active site due to the high stability of the grafted reaction intermediates. In principle, coordination of a terminal oxygen atom to a Lewis acid site (gallium ion) and formation of an oxygen-bridged species must decrease its basicity significantly. This, in turn, will facilitate hydrogen recombination upon catalytic dehydrogenation reaction providing a path for the regeneration of the active site.

According to the results presented in the previous chapters, one can propose the following elementary reaction steps in a simplified scheme of alkane (RH) dehydrogenation over Lewis acid-base pares (Ga3+—O2–) of the extra-framework oxygenated gallium species (GaO+, Ga2O2

2+, etc.) in zeolites

Ga O2- + RH3+ Ga3+ O2-R- H+

(9.4)

Ga3+ O2-H- H+

Ga O2- + H23+ (9.6)

Ga3+ O2-H- H+

Ga+ + H2O (9.7)  

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Table 9.3. Reaction energies  (∆E, kJ/mol) and activation energies  (∆E≠, kJ/mol)  for  the  initial 

heterolytic C–H bond cleavage of ethane (reaction (9.4)), H2 recombination (reaction (9.6)) and 

H2O  desorption  (reaction  (9.7)  over  oxygenated  Ga3+  sites  (GaO+,  Ga2O22+,  and 

[H‐Ga(OH)(O)Ga]2+) in mordenite.  

    ∆E  ∆E≠ GaO+       

GaO+ + C2H6 → [C2H5‐Ga‐OH]+  (9.4)  –258 (‐234)  69   (96) 

[H‐Ga‐OH]+ → GaO+ + H2  (9.6)   288  (259)  326 (328) [H‐Ga‐OH]+ → Ga+ + H2O

  (9.7)   46     (40)  n/a  (233) 

Ga2O22+       

Ga2O22+ + C2H6 → [C2H5‐GaOGa‐OH]

2+  (9.4)  –62  118 [H‐GaOGa‐OH]2+ → Ga2O2

2+ + H2  (9.6)   92  168 [H‐GaOGa‐OH]2+ → GaOGa+ + H2O (9.7)   186  n/a 

[H‐Ga(OH)(O)Ga]2+       

[H‐Ga(OH)(O)Ga]2+ + C2H6 → [H‐Ga(OH)2Ga‐C2H5]2+  (9.4)  –105  74 

[H‐Ga(OH)2Ga‐H]2+ → [H‐Ga(OH)(O)Ga]2+ + H2  (9.6)   142  174 

[H‐Ga(OH)2Ga‐H]2+ → [H‐Ga(OH)Ga]2+ + H2O (9.7)   70  127 

a Values  in parentheses  correspond  to  those  calculated using  the  cluster modeling  approach  for  the respective elementary steps of ethane dehydrogenation over GaO+ ions in ZSM‐5 (Chapter 8). 

Initial heterolytic C–H bond cleavage (reaction (9.4)) over isolated mononuclear gallyl ions in ZSM-5 zeolite is energetically favorable. Subsequent ethylene desorption (9.5) depends only slightly on the nature of the surroundings of the Ga3+ ion. The terminal OH group in the formed Ga(H)(OH)+ cation shows pronounced basic properties. As a result, H2O formation (9.7) is strongly favored energetically over H2 desorption (9.6) in the case of GaO+ site (Chapter 8).

The energetics of the selected elementary steps (9.4), (9.6), and (9.7) of this scheme for the isolated GaO+ site stabilized in the periodic mordenite model II are summarized in Table 9.3. The data presented cohere well with those computed using the cluster modeling approach for ZSM-5 model (Chapter 8), supporting the conclusion of the relatively small influence of the local surrounding on the chemical properties of isolated Ga-sites in high-silica zeolites.

Initial C–H bond cleavage of C2H6 over the dimeric Ga2O22+ site is exothermic (∆E = –62

kJ/mol) and shows a moderate activation barrier (∆E≠ = 118 kJ/mol, Table 9.3). Upon ethane dissociation, the Ga2O2 ring opens. This results in formation of Ga–C2H5 and Ga–OH species each bound to [AlO2]– framework units and connected via a bridging oxygen ion ([C2H5-GaOGa-OH]2+ species). The decomposition of the ring structure allows conservation of the favorable tetrahedral environment of Ga3+ ions. In the next step, the grafted C2H5

– group decomposes via β-H abstraction resulting in formation of ethylene and a hydride ion ([H-GaOGa-OH]2+) bound to Ga (reaction (9.5)). This reaction is endothermic and shows ∆E = 140 kJ/mol.

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H2 recombination (9.6) from [H-GaOGa-OH]2+ ion regenerates the active cyclic Ga2O2 species and closes the catalytic cycle. This reaction shows an activation barrier of 164 kJ/mol. This is twice lower than the value of 326 kJ/mol estimated for the less stable isolated GaO+ site (Table 9.3). Unfortunately, the transition state structure for the competing H2O desorption pathway has not been identified. However, taking into account that the enthalpy of water desorption in this case exceeds the activation barrier for H2 recombination, one can conclude that the latter process is significantly favored over the binuclear Ga2O2 site.

Intermediate [H-GaOGa-OH]2+ can rearrange to a [H-Ga(OH)(O)Ga]2+ cyclic cation. This process competes with H2 recombination (∆E = 61 and 92 kJ/mol, respectively for the rearrangement and H2 recombination reactions). The resulting cationic complex can also be involved in the catalytic dehydrogenation of ethane. The catalytic cycle consists of the elementary steps similar to those discussed above. The coordinative unsaturation of Ga3+ in this structure leads to a significant decrease of the activation barrier, and at the same time, enhances the stability of the product of the initial C–H cleavage [H-Ga(OH)2Ga-C2H5]2+ (Table 9.3). The activation energy in this case is close, while the stability of the reaction products is significantly lower as compared to the respective reaction over the isolated GaO+ site. Formation of the less stable species upon the catalytic process allows subsequent regeneration of the active site. Indeed, moderate barriers are calculated for H2 recombination in the catalytic cycle involving [H-Ga(OH)(O)Ga]2+ species. Water desorption in this case is favored over the regeneration of the active site. However, the difference in the calculated energetics for these competing processes over binuclear site is significantly reduced as compared to the case of mononuclear GaO+ species.

The data presented clearly shows that formation of binuclear oxygenated Ga3+ species in high-silica zeolites provides a path for the active site regeneration that is absent in the case of isolated gallyl ions The decrease of basicity of the bridging extra-framework oxygen atoms can lead to facilitation of the one-step mechanism of H2 and C2H4 desorption over the consecutive one, similar to the case of zinc-exchanged zeolites (Chapter 6). This would increase the lifetime of the oxygenated Ga3+ species upon the catalytic reaction.

Nevertheless, in agreement with the experimental observations [6], these sites decompose upon the catalytic process via H2O desorption resulting in formation of less reactive reduced Ga-sites. The reduction path, therefore, could in principle be suppressed by water addition to the reaction mixture. The results of catalytic activity experimental measurements [22] of propane conversion as a function of the steam partial pressure over initially Ga+ ions stabilized by ZSM-5 zeolite are summarized in Table 9.4. Clearly, co-feeding of water leads to a substantial increase of the propane conversion. The conversion passes a maximum at pH2O of 0.3 kPa. Catalyst performance in the presence of steam (0.3 kPa) is found to be stable over a period of at least 4 h (ca. 2000 turnovers). The very different product composition between Ga+/ZSM-5 in the absence of steam and HZSM-5 points to a change in the mechanism from protolytic hydrocarbon activation involving C–H

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Table 9.4. Conversion (X, %) and molar hydrocarbon product distribution (%) during reaction 

of propane over Ga/ZSM‐5  as  a  function of  the water partial pressure  (PH2O,  kPa)  and over 

parent HZSM‐5 (50 mg catalyst, P = 1 atm, T = 823 K, WHSV = 11.8 gC3H8/gzeolite∙h) [22].  

PH2O   X   CH4  C2H4  C3H6  C4H8  BTXa  H2b 

Ga/ZSM‐5 0.00  7  5  9  80  2  3  1.06 0.01  14  7  15  65  4  7  1.10 0.05  16  8  16  63  3  7  1.16 0.3  19  10  19  58  2  7  1.19 0.5  16  11  19  61  2  4  1.11 1  14  11  19  62  2  4  1.08 4  11  11  21  65  1  2  0.98 

HZSM‐5 0.00  5  42  42  14  —  —  0.24 

a benzene, toluene, xylenes  b moles of H2 per mole converted C3H8 

and C–C bond cleavage for HZSM-5 to dominant C–H activation over the Lewis acidic Ga-sites [6].

Addition of water to the feed strongly increases the dehydrogenation activity. The main product remains propylene although with increasing water content somewhat increased amounts of methane and ethylene are formed. The formation of methane and ethylene is due to protolytic cracking of propane. It points to formation of Brønsted acid protons due to hydrolysis of extra-framework species [5]. More ethylene than methane is formed due to additional proton-catalyzed oligomerization/cracking reactions of propene. The carbon selectivity to dehydrogenated products remains well over 85%. The increased dehydrogenation activity of Ga/ZSM-5 over HZSM-5 is obvious from the increased amount of hydrogen formed. No carbon oxides are detected in the reactor effluent. The activity decrease at higher pH2O is likely due to complete hydrolysis of the active intrazeolitic Ga sites. 27Al NMR measurements indicate that prolonged steam exposure does not lead to massive redistribution of tetrahedral aluminium species. The promoting effect of water is due to the formation of reactive partly hydrolyzed gallium species such as [H-GaOGa-OH]2+ and [H-Ga(OH)2Ga-H]2+ discussed above.

These theoretical insights imply that H2 desorption should be rather facile from the hydroxylated Ga clusters in high-silica zeolites. To verify this, a temperature-programmed desorption experiment was carried out over Ga+/ZSM-5 in an atmosphere of 0.3 vol.% H2O in He [22]. Hydrogen evolution is observed around 600 K. In a similar experiment for HZSM-5 zeolite, no hydrogen is formed. The desorption energy estimated from the H2 desorption maximum gives a value of 140±12 kJ/mol. This result agrees very well with the computed value for H2 recombination from the fully hydroxylated cluster [H-Ga(OH)2Ga-H]2+ (∆E = 142 kJ/mol, Table 9.3).

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9.4. Conclusions

The stability of isomeric binuclear Ga3+ oxygenated extra-framework species with the general formula Ga2O2

2+ in high-silica mordenite is investigated using periodic DFT calculations. It is concluded that the isolated gallyl ions cannot be formed in high-silica zeolites. GaO+ cations tend to oligomerize resulting in formation of oxygen-bridged Ga3+ pairs. Independently of framework aluminum distribution in the zeolite, cyclic Ga2O2 dimers are the most stable cations. The location and stability of such cationic clusters is mainly controlled by the favorable geometrical environment of the Ga3+ ions, while the effect of the direct interaction with the framework anionic sites is less important. This leads to formation of stable charge-alternating structures when the conventional charge-compensation does not provide favorable tetrahedral coordination of Ga ions. In addition, it is found that cubic Ga4O4

4+ ions directly interacting with only two [AlO2]– framework units, while the remaining two anionic sites are significantly remote, also show remarkable stability. Therefore, it is reasonable to assume that multinuclear oxygen-bridged cations with indirect charge-compensation could be also typical for other zeolites modified with transition-metal ions.

Binuclear gallium-oxo cationic species show significant reactivity toward heterolytic C–H cleavage. Catalytic dehydrogenation of ethane over the Ga2O2 dimer and related sites proceeds via initial heterolytic dissociation by the Lewis acid-base pair formed by the Ga3+ and the basic extra-framework oxygen atom followed by olefin desorption and hydrogen recombination. The last two steps may take place in a concerted manner. The decreased basicity of the extra-framework oxygen and the stable tetrahedral coordination of Ga3+ in the dimeric species facilitate H2 desorption. However, the oxygenated species decompose upon the catalytic process via water desorption resulting in less reactive reduced sites. It is suggested that addition of H2O to the hydrocarbon feed can increase the steady-state concentration of the binuclear gallium-oxo and –hydroxo species in the catalyst. The catalytic experiments support well this proposal. A substantial increase in the dehydrogenation activity of Ga+/ZSM-5 zeolite is observed upon water co-feeding. Continuous addition of water is required to maintain a high steady-state concentration of the reactive oxygenated extra-framework species. 

References and notes 

1 Dooley, K.M.; Chang, C.; Price, G.L. Appl. Catal. A: General 1992, 84, 17. 2 Himei, H.; Yamadaya, M.; Kubo, M.; Vetrivel, R.; Broclawik, E.; Miyamoto, A.  J. Phys. Chem. 1995, 99, 

12461. 3 Broclawik, E.; Himei, H.; Yamadaya, M.; Kubo, M.; Miyamoto, A.; Vetrivel, R. J. Chem. Phys. 1995, 103, 

2102.  4 Abdul Hamid, S.B.; Derouane, E.G.; Mériaudeau, P.; Naccache, C. Catal. Today 1996, 31, 327. 5 Kazansky, V.B.; Subbotina, I.R.; van Santen, R.A.; Hensen, E.J.M. J. Catal. 2005, 233, 351. 6 Rane, N.; Overweg, A.R.; Kazansky, V.B.; van Santen, R.A.; Hensen, E.J.M. J. Catal. 2006, 239, 478. 7 Kresse, G.; Furthmüller, J. Comp. Mat. Sci. 1996, 6, 15. 8 Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. 

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9 Perdew, J.P.; Chevary, J.A.; Vosko, S.H.; Jackson, K.A.; Pederson, M.R.; Singh, D.J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. 

10 Blöchl, P.E. Phys. Rev. B 1994, 50, 17953. 11 Kresse, G.; Joubert, J. Phys. Rev. B 1999, 59, 1758. 12 Monkhorst, H.J.; Pack, J.D. Phys. Rev. B 1976, 13, 5188. 13 Alberti, A.; Davoli, P.; Vezzalini, G. Z. Kristallogr. 1986, 175, 249. 14 Grybos, R; Hafber, J.; Benco, L.; Toulhoat, H. J. Phys. Chem. C 2007, 111, 6454. 15 Feng, X.; Hall, W.K. Catal. Lett. 1997, 46, 11. 16 Examples of crystal structures of Mg2+, Ni2+ and Zn2+ complexes with indirect charge‐compensation can 

be found in (a) Meyer, G.; Nolte, M; Berners, R. Z. Anorg. Allg. Chem. 2006, 632, 2184. (b) Mastropietro, T.F.; Armentano, D.; Marino, N.; De Munno, G. Cryst. Grow. & Des., 2007, 7, 609.  (c) Chikunov,  I.E.; Kravchenko,  A.N.;  Belyakov,  P.A.;  Lyssenko,  K.A.;  V.V.  Baranov;  Lebedev,  O.V.;  Makhova,  N.M. Mendeleev Commun. 2004, 14, 253. (d) Sun, R.; Li, Y.; Pan, Y. Cryst. Grow. & Des., 2007, 7, 890. 

17 The  data  were  recovered  from  the  November  2006  update  release  of  the  CSD  (version  5.28).  No additional constrains were imposed on the required moiety. 

18 Baron, A.R. Comments Inorg. Chem. 1993, 14, 123. 19 Baker, R.J.; Jones, C.; Kloth, M. Dalton Trans. 2005, 2106. 20 (a) Hardman, N.J.; Eichler, B.E.; Power, P.P. Chem. Commun. 2000, 1991. (b) Hardman, N.J.; Power, P.P. 

Inorg. Chem. 2001, 40, 2474. 21 (a) Hodge, P.; Piggott, B. Chem. Commun. 1998, 1933. (b) Wiberg, N; Amelunxen, K.; Lerner, H.‐W.; Noth, 

H.; Ponikwar, W.; Schwenk, H. J. Organomet. Chem. 1999, 574, 246. (c) Nichols, P.J.; Papadopoulos, S.; Raston, C.L. Chem. Commun. 2000, 1227. (d) Linti, G.; Li, G.; Pritzkow, H. J. Organomet. Chem. 2001, 626, 82. (b) Albrecht, M.; Dehn, S.; Frönlich, R. Angew. Chem. Int. Ed. 2006, 45, 2792. (e) Papaefstathiou, G.S.; Manessi, A.; Raptopoulou, A.T.; Terzis, A.; Zafiropoulos, T.F. Inorg. Chem. 2006, 45, 8823. 

22 Ga/ZSM‐5  was  prepared  according  to  Ref.  [6].  In  Ga/ZSM‐5  the  Brønsted  protons  have  been quantitatively  replaced by Ga+  ions. Propane  conversion was  carried out  in a  single‐pass atmospheric quartz reactor by passing a mixture of C3H8 (5 kPa) in He at a WHSV = 11.8 gpropane.gcatalyst

‐1.h‐1 at 823 K. Water was added via a well‐thermostated saturator. Further experimental details can be  found  in  (a) Hensen, E.J.M.; Pidko, E.A.; Rane, N.; van Santen, R.A. Angew. Chem. Int. Ed. 2007, 46, 7273. (b) Rane, N. Hydrocarbon Conversion over Bronsted and Lewis Acidic Zeolites, Ph.D. thesis, Eindhoven, 2007.  

  

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SUMMARY CHEMICAL REACTIVITY OF CATION‐EXCHANGED 

ZEOLITES 

Zeolites modified with metal cations have been extensively studied during the last two decades because of their wide application in different technologically important fields such as catalysis, adsorption and gas separation. Contrary to the well-understood mechanisms of chemical reactions catalyzed by Brønsted acid sites in the hydrogen forms of zeolites, the nature of chemical reactivity, and related, the structure of the metal-containing ions in cation-exchanged zeolites remains the subject of intense debate.

In this thesis, the chemical properties of zeolites modified with hard Lewis acids such as alkaline- and alkali-earth cations (Chapters 2 – 4) and with soft Lewis acids such as Zn-, Cd- and Ga-cations (Chapters 5 – 9) are discussed. Special attention is paid to the mechanism of chemical transformations promoted by such exchangeable species and, accordingly, their role in these processes.

Low-silica zeolites modified with alkaline- and alkali-earth cations are rather inert materials. However, it has been experimentally found that they can efficiently promote photo-oxidation of unsaturated hydrocarbons with molecular oxygen. The details of this reactivity are not clear. Chapter 2 reports DFT calculations on the initial charge-transfer step for alkene photo-oxidation in zeolite Y modified with alkali-earth cations (Mg, Ca, and Sr). The photo-oxidation of 2,3-dimethyl-2-butene (DMB) with O2 has been used as a model reaction. It is predicted that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of the charge-transfer state, while the relative orientation and the distance between the adsorbed alkene and oxygen molecules are the critical factors. A high density and specific location of the exchangeable cations in the zeolite matrix determines a specific confinement of the adsorbed reagents in a suitable “pre-transition state” configuration. The optimum configuration of co-adsorbed DMB and O2 molecules is identified for CaY zeolite. A significantly lower activity of SrY and MgY in the photo-oxidation of 2,3-dimethyl-2-butene-2 in comparison with that of CaY is predicted.

Another interesting property of low-silica zeolites modified with alkaline cations is their ability to promote N2O4 disproportionation under very mild conditions. Chapter 3 presents periodic DFT calculations on N2O4 disproportionation in Na-, K-, and Rb-exchanged low-silica zeolite X. The disproportionation reaction results in rather polar NO+···NO3

– species, which are effectively stabilized by the cage of cation-exchanged zeolite. NO+ binds to the basic framework oxygens, and NO3

– anion coordinates to the exchangeable cations. Although the binding energy of NO+ ion to the zeolite is influenced by the basicity of the

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framework, the theoretical results show that the overall disproportionation reaction is mainly controlled by the interactions between the negatively charged nitro group and the extra-framework cations. The role of the interaction between the nitrosonium cation and basic sites of the zeolite is only of minor importance. The function of the microporous matrix is to facilitate the charge separation in a fashion similar to that of a polar solvent. It is concluded that steric properties of the zeolite cage, the cooperative effect of the extra-framework cations as well as their mobility induced by adsorption are essential to form the optimum configuration of the active site for N2O4 disproportionation.

Chapter 4 reports a combined infrared spectroscopic and computational study of light alkane adsorption to alkali-earth exchanged zeolite Y. Although these materials do not catalyze C–H or C–C bond cleavage, they can be successfully used as model adsorbents to investigate the factors influencing structural and electronic properties of the resulting adsorption complexes. The experimental IR spectra of the C–H stretching vibrations of the adsorbed hydrocarbons differ strongly for MgY and CaY zeolites. On the basis of ab initio MP2 and DFT calculations it is found that different geometries of the light alkane adsorption complexes are realized depending on the cation in the adsorption site. Topological analysis of the electron density distribution function in the framework of quantum theory of atoms in molecules is applied to investigate the bonding of the adsorption complexes. It is found that numerous van der Waals bonds between the H atoms of the alkane and basic oxygens of the zeolite are formed, when a hydrocarbon coordinates to Mg2+ ions. These intermolecular contacts significantly contribute to the overall adsorption energy, whereas they play only an indirect role in the adsorption of light alkanes on CaY. On the other hand, in the case of CaY the stabilization of alkanes in the electrostatic field of the partially shielded Ca2+ cation dominates the adsorption energy. It is concluded that the dominance of a particular type of intermolecular interactions is dependent on the properties of the adsorption site. The type of intermolecular interactions determines the final conformation of light alkanes adsorbed to the cation-exchanged zeolite Y.

From the results in Chapters 2 – 4 an interesting effect is noted: although the smaller exchangeable cations are expected to bind molecules stronger and exhibit higher reactivity as compared to their larger counterparts because of the increased hardness of such cations, the calculations indicate that the properties of the metal ions stabilized in the zeolite matrix do not follow these trends. Indeed, when stabilized at zeolitic cation site, the larger ions are significantly coordinatively unsaturated. This leads to an enhancement of the adsorption properties of the larger cations in spite of their expected lower Lewis acidity.

Molecular and dissociative adsorption of light alkanes on the more reactive high-silica zeolite ZSM-5 modified with zinc and cadmium is investigated in Chapter 5. Adsorption of ethane on coordinatively unsaturated soft Lewis acid sites (Zn2+ and Cd2+) in ZSM-5 zeolite results in stronger changes of the geometry and charge parameters of the adsorbed molecules as compared to the case of adsorption on MgY and CaY. It is found that the degree of the effective shielding of the exchangeable cations by the surrounding oxygen

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ions is an important factor that influences the perturbations of molecularly adsorbed ethane. The C2H6 binding energy does not apparently depend on the type of the cation (Zn or Cd), whereas the nature of the charge compensation of the cations is important. On the other hand, heterolytic dissociative adsorption is mainly controlled by the basicity of the proton-accepting oxygen-site (O-site) and the steric properties of the dissociation products, which determine their stability. As a result, no apparent correlation between the perturbations of the adsorbed molecules and their heterolytic dissociation is observed.

Chapters 6 to 8 report cluster DFT calculations of the various potential reaction paths of catalytic dehydrogenation of light alkanes over zinc- and gallium-exchanged high-silica zeolites. The mechanism of the catalytic reaction and the most probable active site are identified. In addition, an attempt is made to understand the factors, which determine the catalytic activity of different intrazeolite cationic species as well as the preference for a particular reaction path. The theoretical results form a basis for interpreting the experimental catalytic data.

Catalytic dehydrogenation of ethane over various zinc species in Zn/ZSM-5 zeolite is investigated in Chapter 6. It is shown that isolated Zn2+ stabilized at the cation sites with distantly placed anionic [AlO2]– framework units are the most probable active species. A novel mechanism of ethane dehydrogenation is proposed. It involves decomposition of the products of dissociative ethane adsorption (Z–Zn2+-C2H5

–···H+Z–) via one-step desorption of ethylene and hydrogen. This path is strongly favored for the isolated Zn2+ sites as compared to the conventional mechanism involving consecutive desorption of the dehydrogenation products. Similar to the initial heterolytic C-H bond cleavage, the basicity of the O-sites is a determinative factor for the particular reaction mechanism.

In the case of Ga-exchanged ZSM-5 zeolite (Chapter 7), univalent gallium cations are the most probable active sites for reduced catalysts. Hydrogenated extra-framework species decompose rapidly toward Ga+ cations during the catalytic reaction. Initial oxidative addition of C2H6 to Ga+, which has been observed experimentally before, is shown to proceed via an indirect route involving heterolytic C-H cleavage over the Lewis acid-base pair formed by the Ga+ cation and a framework oxygen anion. The direct route is strongly disfavored due to the electronic properties of univalent gallium. C2H4 and H2 desorption in one step closes the catalytic cycle. Although this reaction is reminiscent to that proposed for Zn/ZSM-5, it strongly differs in nature and is controlled by the properties of the Ga site.

It has been observed experimentally that the catalytic activity of ZSM-5 zeolite predominantly containing Ga+ ions can be remarkably enhanced after selective oxidation with N2O. The higher activity of the resulting material has been attributed to formation of extra-framework GaO+ ions. However, a detailed investigation of various possible reaction paths over isolated gallyl ions in ZSM-5 zeolite (Chapter 8) shows that ethane interacts with these species stoichiometrically, because of the extremely low stability of these sites. Indeed, the unfavorable tridentate coordination of gallium along with the high basicity of the extra-framework terminal oxygen ion in GaO+ leads to a rapid heterolytic dissociation

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of C2H6 molecules. The resulting products are very stable, and the closure of the catalytic cycle is not likely to occur. It is concluded that the isolated gallyl ions cannot be considered as catalytically active sites for light alkane dehydrogenation.

The very low stability of GaO+ species, on the other hand, can cause their oligomerization in the zeolite micropores, resulting in formation of various multinuclear cationic gallium-oxide clusters (Chapter 9). Periodic DFT calculations show that formation of cyclic Ga2O2

2+ dimers is strongly favored independently of the aluminum distribution in the high-silica zeolite. Moreover, oligomers with a higher degree of aggregation can be in principle formed in oxidized Ga-exchanged zeolites. The zeolite lattice plays the role of a chelating ligand which stabilizes the (GaO)n cationic cluster. Parallels between conventional coordination chemistry and chemistry of high-silica zeolites modified with gallium are drawn. It is shown that the location and stability of such cationic clusters is mainly controlled by the favorable geometrical environment of the Ga3+ ions, while the effect of the direct interaction with the framework anionic sites which compensates for the positive charge of the extra-framework species is less important. In spite of higher stability, binuclear sites are shown to be active for alkane activation. The lower basicity of the extra-framework oxygen ions provides a path for the closure of the catalytic cycle. However, these sites still tend to reduce upon light alkane dehydrogenation via water desorption, resulting in formation of less reactive reduced Ga-species.

The mechanistic insight provided by the quantum-chemical calculations suggests that the reduction path can be suppressed by addition of water to the hydrocarbon feed. This would lead to an increased steady-state concentration of reactive oxygenated Ga-species in the catalyst. The experimental catalytic tests (Chapter 9) indeed show significant enhancement of the dehydrogenation activity of Ga+ sites in ZSM-5 upon water co-feeding. Continuous addition of water is required to maintain a high steady-state concentration of the reactive oxygenated extra-framework species in the zeolite and leads to high and stable activity of the catalyst.

Thus, it is shown that the reactivity of low-silica zeolites modified with rather inert alkaline- and alkali-earth cations derives mainly from the properties of the confined space of the zeolite cages. The high density and the specific arrangement of the exchangeable cations in the microporous matrix lead to optimum configuration of the adsorbed reagents and consequently to their chemical activation. On the other hand, the active sites in high-silica zeolites modified with softer Zn, Cd, and Ga cations are rather local and usually directly involved in catalytic transformations of the reagents. The chemical reactivity of these system derives from the properties of the Lewis acid-base conjugate pair, which in turn are controlled by the topology of the zeolite cation site accommodating the extra-framework species as well as by the type of charge compensation and the nature of the cation. Both the Lewis acidity of the extra-framework cationic species and the properties of the conjugate basic sites are important for their activity. An optimum must be found in the Lewis acid-base properties of the zeolite active site to achieve a high catalytic activity.

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SAMENVATTING CHEMISCHE REACTIVITEIT VAN KATION 

UITGEWISSELDE ZEOLIETEN 

Zeolieten verrijkt met metal kationen zijn gedurende de laatste twee decennia intensief bestudeerd. Dit vanwege hun brede toepassing in verschillende belangrijke gebieden van de technologie, zoals katalyse, adsorptie en gas scheiding. In tegenstelling tot de goed begrepen mechanismen van chemische reacties die gekatalyseerd worden voor Brønsted zure centra in de waterstof vorm van zeolieten, is de aard van de chemische reactiviteit en daarmee de structuur van de metaalionen in kation uitgewisselde zeolieten, nog steeds onderwerp van intens debat.

De chemische eigenschappen van zeolieten, die behandeld zijn met harde Lewis zuren, zoals alkaline en alkali kationen (Hoofdstuk 2 – 4), evenals die behandelt met zachte Lewis zuren zoals, Zn-, Cd- en Ga-kationen (Hoofdstuk 5 – 9) worden besproken in dit proefschrift. Er zal speciaal aandacht besteedt worden aan het mechanisme van chemische transformaties bevorderd door zulke uitgewisselde entiteiten, en daarmee, diens rol in deze processen.

Zeolieten met een lage verhouding silica, die verrijkt zijn met alkanine en alkali kationen zijn redelijk inert. Ondanks het feit dat het experimenteel is vastgesteld dat ze foto-oxidatie van onverzadigde koolhydraten met moleculair zuurstof efficiënt bevorderen, zijn de details van deze reactiviteit niet duidelijk. Hoofdstuk 2 toont DFT berekeningen van de initiële ladingsverdeling stap voor de alkeen foto-oxidatie in de zeoliet Y gemodificeerd met alkali kationen (Mg, Ca en Sr). De foto-oxidatie van 2-3-dimethyl-2-buteen (DMB) met O2 is gebruikt als model reactie. Er is voorspeld dat het elektrostatische veld van de zeoliet kooi slechts een kleine rol speelt in de stabilisatie van het ladingsoverdracht stadium, terwijl de relatieve oriëntatie en afstand tussen de geadsorbeerde alkeen en zuurstof moleculen wel kritische factoren zijn. Een hoge dichtheid en de specifieke locatie van de uitgewisselde kationen in de zeoliet matrix bepalen een specifieke pakking van de geadsorbeerde reagentia in een goede “pre-transitie-stadium” configuratie. De optimale configuratie van de co-geadsorbeerde DMB en O2 moleculen is bepaald voor de CaY zeoliet. Een significant lager activiteit van SrY en MgY in de foto-oxidatie van 2,3-dimethyl-2-buteen is voorspeld in vergelijking met de CaY.

Een andere interessante eigenschap van zeolieten met een lage verhouding silica, die verrijkt zijn met alkaline kationen, is dat hij in staat is N2O4 disproportienatie onder milde condities te promoten. Hoofdstuk 3 toont periodieke DFT berekeningen van N2O4 disproportienatie door Na-, K- en Rb-verrijkt laagsilicaat zeoliet X. De disproportienatie

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reactie resulteert in aanzienlijke polaire NO+···NO3– entiteiten, weke zeer effectief

gestabiliseerd wordt door de kooi van het kation uitgewisseld zeoliet. NO+ gaat een verbinding aan met de basische roosterzuurstof anionen en het NO3

– anion met het uitgewisselde kation. Ondanks dat de bindingsenergie van het NO+ ion met de zeoliet wordt beïnvloed door het basische gedrag van de roosterstructuur, tonen theoretische berekeningen dat de totale disproportienering reactie wordt gecontroleerd door interacties tussen de negatief geladen nitro-groep en de buiten-roosterstructuur kationen. De rol van de interactie tussen het nitrosonium kation en de basische sites van de zeoliet is slechts van kleine invloed. De functie van de microporeuze matrix is het faciliteren van de ladingssplitsing, op dezelfde manier als een polair oplosmiddel. De conclusie is dat de sterische eigenschappen van de zeoliet kooi, het samenwerkende effect van de buiten-roosterstructuur kationen en de mobiliteit geïnduceerd door adsorptie essentieel zijn voor de optimale configuratie van de actieve site voor N2O4 disproportienering.

Hoofdstuk 4 behandelt een gecombineerd infrarood (IR) spectroscopie en theoretische onderzoek van de adsorptie van lichte alkanen in alkali uitgewisselde zeoliet Y. Ondanks dat deze materialen geen katalysator zijn voor C–H of C–C splitsing, zijn ze succesvol gebruikt als model adsobents om de invloed van structuur en elektronische eigenschappen van de adsorptiecomplexen te onderzoeken. De experimentele IR spectra van de C–H rekkingvibraties van de geadsorbeerde koolwaterstoffen verschillen sterk tussen MgY en CaY zeolieten. Door middel van ab initio MP2 en DFT berekeningen is bepaald dat de verschillende geometriën van lichte alkaan adsorptie complexen afhankelijk zijn van het kation in de adsorptiesite. Topologische analyse van de elektron-dichtheidsverdelings-functie in kwantumtheorie van atomen in moleculen is gebruikt om de bindingen van het adsorptiecomplex te onderzoeken. Het blijkt dat, op het moment dat het koolwaterstof coördineert met de Mg2+ ionen, er veel Van de Waals verbindingen ontstaan tussen de H atomen van het alkaan en de basische zuurstoffen van de zeoliet. Deze intermoleculaire interacties hebben een significante contributie tot de totale adsorptie energie, terwijl ze slechts een indirecte rol spelen in de adsorptie van lichte alkanen in CaY. Daarbij, in het geval van CaY de stabilisatie van alkanen in het elektrostatische veld van gedeeltelijk afgeschermde Ca2+ kation heeft een dominante rol in de adsorptie energie. De conclusie is dat de dominante rol van een specifieke inter-moleculaire interactie afhankelijk is van de eigenschappen van de adsorptie site. Het soort inter-moleculaire interactie bepaald de uiteindelijk conformatie van de geadsorbeerde lichte alkanen in het kation uitgewisseld zeoliet Y.

De resultaten van Hoofdstukken 2–4 laten een interessant effect zien. Er wordt verwacht dat de kleinere uitwisselbare kationen moleculen sterker binden en een hogere reactiviteit laten zien, in vergelijking met hun grotere tegenhangers, vanwege hun vergrote hardheid van zulke kationen. Maar de berekeningen laten zien dat de eigenschappen van de metaal ionen gestabiliseerd in de zeoliet matrix deze trend niet volgen; wanneer gestabiliseerd in de kation-site van de zeoliet zijn de grotere ionen significant coördinatief onverzadigd. Dit leidt tot een

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versterking van de adsorptie eigenschappen van de grotere kationen, ondanks hun verwachte lagere Lewis zuurheid.

Moleculaire en dissociatieve adsorptie van lichte alkanen op de reactievere hoog-silica zeoliet ZSM-5 gemodificeerd met zink en cadmium wordt bestudeerd in Hoofdstuk 5. Adsorptie van ethaan op coördinatief onverzadigde zachte Lewis zure sites (Zn2+ en Cd2+) in ZSM-5 zeolieten resulteren in grote veranderingen van geometriën en ladingsparameters van de geadsorbeerde moleculen in vergelijking met MgY en CaY. Het blijkt dat de mate van effectieve afscherming van de uitwisselbare kationen door de onliggende zuurstof ionen een belangrijke factor is, dit beïnvloed de perturbaties van moleculair geadsorbeerd ethaan. De C2H6 bindingsenergie is niet aanwijsbaar afhankelijk van het type kation (Zn of Cd), maar wel de wijze van ladingscompensering van de kationen is belangrijk. Aan de andere kant, de heterolytische dissociatieve adsorptie is voornamelijk beïnvloed door de basisiteit van de protonaccepterende zuurstofsite (O-site), en de sterische eigenschappen van de dissociatie producten, welke de stabiliteit bepaalt. Er is geen duidelijke correlatie te vinden tussen de perturbatie van de geadsorbeerde moleculen en hun heterolytische dissociatie.

Hoofdstukken 6 tot 8 behandelen cluster DFT berekeningen van verschillende potentiële reactiepaden van katalytische dehydrogenatie van lichte alkanen door zink- en gallium-uitgewisselde hoogsilica zeolieten. Het mechanisme van de katalytische reactie en de waarschijnlijkste actieve sites zijn bepaald. Hieraan toegevoegd wordt er een poging ondernomen te begrijpen welke factoren katalytische activiteit van verschillende intrazeoliet entiteiten beïnvloeden, evenals de voorkeur voor een bepaald reactiepad. De theoretische resultaten vormen de basis voor de interpretatie van de experimentele katalytische data.

Katalytische dehydrogenering van ethaan door verschillende zink-entiteiten in Zn/ZMS-5 zeolieten wordt onderzocht in Hoofdstuk 6. Het is aangetoond dat geïsoleerd Zn2+ gestabiliseerd in de kation site met een op afstand geplaatst anionisch [AlO2]– rooster species de waarschijnlijkste actieve entiteit is. Een nieuw mechanisme voor ethaan dehydrogenatie wordt voorgesteld. Deze behandelt de decompositie van de producten van dissociatieve ethaan adsorptie (Z–Zn2+-C2H5

–···H+Z–) via een-stap desorptie van ethyleen en waterstof. Dit pad is waarschijnlijker voor de geïsoleerde Zn2+ sites in vergelijking tot het conventionele mechanisme, waarbij opeenvolgende desorptie van dehydrogenering producten. Vergelijkbaar met de initiële heterolytische C-H verbreking, is de basisiteit van de O-sites een bepalende factor voor dit reactie mechanisme.

In het geval van Ga-uitgewisselde ZSM-5 zeolieten (Hoofdstuk 7), zijn univalent gallium kationen de waarschijnlijkste actieve site voor de gereduceerde katalysatoren. Gehydrogeneerde buiten-roosterstructuur entiteiten vallen tijdens de katalytische reactie snel uit elkaar tot Ga+ kationen. Zoals eerder experimenteel is aangetoond, wordt er getoond dat de eerste oxidatieve toevoeging van C2H6 aan Ga+ verloopt via een indirecte weg. Hierbij vindt er heterolytische splitsing van C-H plaats door het Lewis base-zuur paar

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dat door de Ga+ kation en het rooster zuurstof anion wordt gevormd. De directe weg is het minst favoriet, vanwege de elektronische eigenschappen van univalent gallium. Ondanks dat C2H4 en H2 desorptie in een stap de katalytische cirkel sluit, en deze reactie doet terugdenken aan het mechanisme van Zn/ZSM-5, verschilt deze in type en wordt deze reactie beïnvloed door de eigenschappen van de Ga-site.

Experimenteel is er aangetoond dat de katalytische activiteit van ZSM-5 zeolieten met voornaamlijk Ga+ ionen aanzienlijk versterkt kan worden na selectieve oxidatie met N2O. De hogere activiteit van het eindproduct kan worden toegedicht tot de vorming van buiten-roosterstructuur GaO+ ionen. Desalniettemin toont een gedetailleerd onderzoek van verschillende mogelijke reactiepaden door geïsoleerde gallyl ionen in het ZSM-5 zeolieten (Hoofdstuk 8) aan, dat ethaan een stoichiometrische interactie aangaat met deze species, vanwege de extreem lage stabiliteit van deze sites. De minder favoriete tridentale coördinatie van gallium, samen met de extreme basisiteit van het buiten-roosterwerk terminaal zuurstof ion in GaO+ leidt tot een snelle heterolytische dissociatie van C2H6 moleculen. De resulterende producten zijn erg stabiel, en het sluiten van de katalytische cirkel zal minder waarschijnlijk plaatsvinden. De conclusie is dat de geïsoleerde gallyl ionen niet gezien moeten worden als de katalytische actieve sites voor de dehydrogenering van lichte alkanen.

De erg lage stabiliteit van GaO+ species kunnen leiden tot de oligomerisatie in de microporiën van zeolieten. Dit zal dan resulteren in de vorming van verschillende multi-nucleaire kationische gallium-oxide clusters (Hoofdstuk 9). Periodieke DFT berekeningen laten zien dat de vorming van cyclisch Ga2O2

2+ dimeren is sterk favoriet. Dit onafhankelijk van de aluminium distributie in de hoog-silica zeoliet. Daarbij kunnen oligomeren met een grotere aggregatie in principe gevormd worden in geoxideerd Ga-uitgewisselde zeolieten. De roosterstructuur van de zeoliet speelt de rol van chelaterende ligand, welke het (GaO)n kationisch cluster stabiliseert. Er kunnen parallellen getrokken worden tussen conventionele coördinatie chemie en de chemie van hoog-silica zeolieten verrijkt met gallium. Er wordt getoond dat de locatie en stabiliteit van zulke kationische clusters voornamelijk bepaald worden door de meer geliefde geometrische omgeving van de Ga3+ ionen, terwijl het effect van de directe interactie met de rooster anionische site, welke de positieve lading van de buiten-roosterwerk entiteiten compenseren, minder belangrijk is. Ondanks de hogere stabiliteit, blijken de bi-nucleaire sites actief in de activering van alkaan. De lagere basisiteit van het buiten-roosterwerk zuurstof ionen leveren de reactiepad voor het sluiten van de katalytische cirkel. Desondanks neigen deze sites te reduceren bij de dehydrogenatie van lichte alkanen door water desorptie. Dit resulteert dan in minder reactieve, gereduceerde Ga-species.

Het mechanistisch inzicht verkregen door de kwantumchemische berekeningen veronderstellen dat het reductiepad kan worden onderdrukt door de toevoeging van water in de koolwaterstof toevoer. Dit zou leiden tot een verhoogde steady-state concentratie van het reactieve zuurstofverzadigde Ga-species in de katalysator. De experimentele

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katalytische testen (Hoofdstuk 9) tonen daadwerkelijk een significante verbetering van de dehydrogenatie activiteit van Ga+ sites van ZSM-5 door het bijvoegen van water. Continue toevoeging van water is nodig voor het behoud van de hoge steady-state concentratie van de reactieve, hoog zuurstofverzadigde buiten-roosterwerk entiteiten in de zeoliet. Dit leidt tot een hoge en continue actieve katalysator.

Er is aangetoond dat de reactiviteit van laag-silica zeolieten, verrijkt met redelijk inerte alkaline- en alkali kationen, voornamelijk kan worden afgeleid van de eigenschappen van de afzonderlijke zeoliet kooien. De hoge dichtheid en de specifieke positie van het uitgewisselde kation in de microporeuze matrix leidt tot de optimale configuratie van de geadsorbeerde reagentia en daaropvolgend hun chemische activering. Daarentegen zijn de actieve sites in de hoog-silica zeolieten verrijkt met zachtere Zn, Cd, en Ga kationen voornamelijk een lokaal fenomeen, en een integraal onderdeel van de katalytische transformaties van de reagentia. De chemische reactiviteit van deze systemen kunnen worden afgeleidt van de eigenschappen van de Lewis base-zuur paar. Deze wordt op zijn beurt weer beïnvloed door de topologie van de zeoliet-kationische site, welke de buiten-roosterwerk entiteit accommodeert, en de wijze van ladingscompensering en de eigenschappen van het kation. Voor de activiteit zijn de Lewis zuurheid van het buiten-roosterwerk kation en de eigenschappen van de conjugerende base site van belang. Een optimum moet gezocht worden in de Lewis base-zuur eigenschappen van de actieve site van de zeoliet, om een hoge katalytische activiteit te verkrijgen.

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РЕЗЮМЕ РЕАКЦИОННАЯ СПОСОБНОСТЬ КАТИОННЫХ 

ФОРМ ЦЕОЛИТОВ 

Химия цеолитов, модифицированных катионами металлов, остается предметом интенсивных научных исследований на протяжении последних двух десятилетий. Стабильный интерес к данному классу пористых материалов обусловлен их широким применением в различных технологически важных областях, таких как катализ, адсорбция и разделение газов. В отличие от детально изученных механизмов химических реакций, катализируемых Бренстедовскими кислотными центрами водородных форм цеолитов, природа реакционной способности и, соответственно, структура металсодержащих ионов в катионных формах цеолитов, по сей день, все еще недостаточно хорошо изучена.

В данной диссертации обсуждаются химические свойства цеолитов, модифицированных жесткими кислотами Льюиса, такими как щелочные и щелочноземельные катионы (Главы 2 – 4), а также мягкими кислотами Льюиса — катионами цинка, кадмия и галлия (Главы 5 – 9). Особое внимание уделено изучению механизмов химических превращений, промотированных подобными центрами и, соответственно, роли катионов в этих процессах.

Низкокремниевые цеолиты, модифицированные катионами щелочных или щелочноземельных металлов, являются сравнительно малоактивными материалами. Несмотря на это, данные системы проявляют достаточно высокую активность в селективном фотоокислении ненасыщенных углеводородов молекулярным кислородом, природа которой не до конца ясна. В ходе изучения данного процесса (Глава 2) проведены квантово-химические расчеты начального этапа (переноса заряда) фотоокисления алкенов в цеолите Y, модифицированном ионами щелочноземельных металлов (Mg, Ca и Sr). В качестве модельной реакции рассмотрено фотоокисление 2,3-диметил-2-бутена (ДМБ). Показано, что наиболее важными факторами в стабилизации комплекса с переносом заряда являются относительная ориентация и расстояние между адсорбированными молекулами алкена и O2, тогда как электростатическое поле цеолитных пор играет лишь незначительную роль. Высокая плотность, а также специфическая локализация внерешеточных катионов в цеолите обуславливает выгодную ориентацию реагентов, в так называемой, конфигурации «пред-переходного состояния». Оптимальная конфигурация адсорбированных ДМБ и О2 обнаружена лишь в случае цеолита CaY. Исходя из полученных данных, можно предполагать, что цеолиты SrY и MgY будут

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обладать существенно меньшей активностью в реакциях фотоокисления 2,3-диметил-2-бутена по сравнению с CaY.

Другим интересным свойством низкокремниевых целитов, модифицированных катионами щелочных металлов, является их способность промотировать диспропорционирование N2O4 при комнатной температуре. В Главе 3 обсуждаются результаты периодических расчетов методом функционала плотности (DFT) реакции диспропорционирования N2O4 в низкокремниевых цеолитах Х, модифицированных катионами натрия, калия и рубидия. Эта реакция приводит к образованию полярных NO+···NO3

– комплексов, которые могут быть эфективно стабилизированы в цеолитной матрице. Катион нитрозония (NO+) связывается с основными кислородами решетки, а NO3

– анион координируется внерешеточными катионами. Несмотря на то, что энергия связи катиона NO+ с цеолитом напрямую зависит от основности решетки, теоретические результаты показывают, что протекание реакции диспропорционирования, как и ее энергетика, обусловлены межатомными взаимодействиями между отрицательно-заряженной нитро-группой и внерешеточными катионами. При этом роль взаимодействий между катионом нитрозония и основными центрами цеолита незначительна. Основная функция микропористой матрицы заключается в облегчении разрушения тесной ионной пары, что аналогично действию полярных растворителей. Показано, что для создания оптимальной конфигурации активного центра при диспропорционировании N2O4

чрезвычайно важны стерические свойства пор цеолита, эффект согласованности катионов, а также их мобильность, обусловленная адсорбцией.

Адсорбция легких алканов на цеолитах Y, модифицированных ионами щелочноземельных металлов, была изучена методами ИК спектроскопии и квантово-химических расчетов (Глава 4). Несмотря на то, что рассматриваемые катионные формы цеолитов Y не являются катализаторами реакций разрыва С–С и С–Н связей, они могут быть успешно использованы в качестве модельных адсорбентов для изучения факторов, влияющих на структурные и электронные свойства адсорбционных комплексов легких углеводородов с внерешеточными катионами цеолитов. Экспериментальные ИК спектры в области С–Н валентных колебаний алканов, адсорбированных на цеолитах MgY и CaY, существенно различаются. На основе неэмпирических MP2 и DFT расчетов показано, что геометрия адсорбционных комплексов зависит от природы адсорбционного центра. Для изучения химических связей в адсорбционных комплексах использован топологический анализ функции распределения электронной плотности в рамках квантовой теории «Атомы в Молекулах». Обнаружено, что при координации углеводородов на катионах магния образуется множество ван-дер-ваальсовых связей между Н-атомами алкана и основными кислородами решетки цеолита. Эти межмолекулярные контакты вносят существенный вклад в энергию адсорбции на цеолите MgY и играют лишь незначительную роль в случае адсорбции легких алканов на цеолите CaY. С другой стороны, в случае CaY стабилизация алканов в

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электростатическом поле частично экранированных внерешеточных катионов Ca2+ является доминирующей составляющей энергии адсорбции. Показано, что преобладание определенного типа межмолекулярных взаимодействий определяется свойствами адсорбционного центра. Тип межмолекулярных взаимодействий, в свою очередь, определяет предпочтительную конформацию адсорбционного комплекса углеводорода в катионных формах цеолита Y.

Результаты, представленные в Главах 2 – 4, позволяют проследить интересный эффект: хотя предполагается, что катионы с меньшим радиусом образуют более прочные связи с адсорбированными молекулами, а также то, что они более реакционно-способны по сравнению с катионами того же ряда, с большим ионным радиусом, квантово-химические расчеты показывают, что свойства катионов, стабилизированных в цеолитной матрице, не следуют этому правилу. Действительно, в случае стабилизации в катионной позиции цеолита, катионы с больший ионным радиусом в значительной степени координационо ненасыщенны, что обуславливает усиленение их адсорбционных свойств, несмотря на меньшую предполагаемую Льюисовскую кислотность.

Молекулярная и диссоциативная адсорбция легких алканов на более реакционно-способных высококремниевых цеолитах ZSM-5, модифицированных ионами цинка и кадмия, рассмотрены в Главе 5. Адсорбция этана на координационно ненасыщенных мягких кислотах Льюиса (Zn2+ и Cd2+) приводит к более существенным изменениям в геометрии и атомных зарядах адсорбированных молекул, по сравнению с вышерассмотренным случаем адсорбции на MgY и СаY. Показано, что степень эффективного экранирования внерешеточных катионов ионами кислорода цеолитной решетки является важным фактором, определяющим степень возмущения молекулярно адсорбированного этана. Энергия адсорбции этана не зависит от природы катиона (Zn или Cd), в то время как наиболее значимым является тип компенсации заряда катиона. С другой стороны, гетеролитическая диссоциативная адсорбция в основном контролируется основностью сопряженного основного центра и стерическими свойствами продуктов диссоциации. Таким образом, можно заключить, что характер возмущений адсорбированных молекул и их последующая гетеролитическая диссоциацией напрямую не связаны.

В Главах 6 – 8 представлены результаты DFT расчетов возможных путей каталитического дегидрирования легких алканов на высоко-кремниевых цеолитах, модифицированных цинком и галлием. Для каждого из катализаторов установлены механизм каталитической реакции и наиболее вероятный активный центр. Также установлены факторы, определяющие каталитическую активность различных внутрицеолитных катионных центров, а также предпочтительность определенного реакционного пути. Полученные теоретические результаты создают основу для интерпретации экспериментальных каталитических данных.

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Так в Главе 6 представлены результаты теоретического исследования каталитического дегидрирование этана на различных цинк-содержащих центрах в Zn/ZSM-5 цеолите. Показано, что наиболее вероятными активными центрами. являются изолированные катионы Zn2+, компенсирующие заряд существенно удаленных друг от друга анионных центров [AlO2]– решетки цеолита. Впервые предложен механизм дегидрирования этана, в котором каталитический цикл замыкается посредством одностадийного разложения продуктов диссоциативной адсорбции этана (Z–Zn2+-C2H5

–···H+Z–) и десорбции этилена и водорода. Подобный реакционный путь существенно более предпочтителен по сравнению с классическим механизмом, в котором десорбция продуктов дегидрирования осуществляется последовательно. Определяющим фактором для осуществления данного механизма реакции является основность О-центров, что согласуется с начальным гетеролитическим разрывом С-Н связи.

В случае ZSM-5 цеолитов, модифицированных галлием, (Глава 7) показано, что одновалентные катионы галлия являются наиболее вероятными активными центрами в востановленных катализаторах. Гидрированные внерешеточные центры быстро разлагаются в ходе каталитической реакции, приводя к образованию ионов Ga+. Показано, что инициирующее каталитический цикл окислительное присоединение С2Н6 к Ga+, обнаруженное экспериментально, протекает по непрямому реакционному пути, включающему гетеролитический разрыв С-Н связи на Льюисовской кислотно-основной паре, образованной катионам Ga+ и кислородом решетки. Электронные свойства одновалентного галлия обсулавливают высокий энергетический барьер для прямого окислительного присоединения. На следующей стадии каталитический цикл замыкается посредством одностадийной десорбции этилена и водорода. Несмотря на то, что эта реакция аналогична предложенной в случае Zn/ZSM-5, ее природа существенно отличается. Энергетика этого процесса контролируется свойствами катиона галлия.

Ранее было показано экспериментально, что каталитическая активность ZSM-5 цеолита, преимущественно модифицированного Ga+ ионами, может быть значительно повышена при селективном окислениии катализатора в атмосфере N2O. Было предложено, что повышенная активность таким образом полученного катализатора обусловлена образованием внерешеточных галлил GaO+ ионов. Однако детальное изучение возможных реакционных путей дегидрирования этана на ионах GaO+ в ZSM-5 цеолите (Глава 8) показало, что, благодаря чрезвычайно низкой стабильности этих центров, их взаимодействие с молекулами этана является стехиометрическим. Действительно, невыгодная тридентантная координация галлия, совместно с высокой основностью терминального атома кислорода в GaO+,обуславливает быструю гетеролитическую диссоциацию молекул этана. Продукт этого процесса чрезвычайно стабилен, и, как следствие, замыкание каталитического цикла маловероятно. Доказано, что изолированные галлил ионы не

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могут рассматриваться в качестве каталитически активных центров дегидрирования легких алканов.

Низкая стабильность ионов GaO+, с другой стороны, может обуславливать их олигомеризацию в порах цеолита и приводить к образованию разнообразных заряженных кластеров оксида галлия (Глава 9). С помощью периодических DFT расчетов показано, что образование циклических Ga2O2

2+ димеров термодинамически выгодно, независимо от распеределения алюминия в решетке высококремниевого цеолита. Более того, олигомеры с более высокими степенями агрегации могут в принципе образовываться в окисленных формах цеолитов, модифицированных галлием. Решетка цеолита играет роль хелатного лиганда, стабилизирующего катионный (GaO)n кластер. Проведены параллели между химией координационных соединений и химией галлий-модифицированных высококремниевых цеолитов. Показано, что локализация и стабильность подобных катионных кластеров в основном контролируется возможностью образования выгодной тетраэдрической координации Ga3+ катионов, в то время как эффект прямой компенсации положительного заряда цеолитными анионными центрами менее важен. Также продемонстрировано, что несмотря на высокую стабильность, двухядерные центры активны в реакциях активации алканов. Низкая основность внерешеточных мостиковых атомов кислорода обуславливает возможность замыкания каталитического цикла. Однако при каталитическом дегидрировании легких алканов подобные биядерные центры могут востанавливаться посредством десорбции воды, приводя к образованию менее реакционных восстановленных галлиевых центров.

Сведения о механизме каталитической реакции, полученные посредством квантово-химических рассчетов, показывают, что реакционный путь, приводящий к восстановлению активных центров, может быть существенно подавлен при добалении воды к исходному углеводороду. Это приводит к повышению равновесной концентрации активных кислород-содержащих галлиевых центров в катализаторе. Экспериментальные каталитические исследования (Глава 9) действительно показали, что при добавлении каталитических количеств воды к реагентам наблюдается заметное усиление дегидрирующей активности Ga+-центров в ZSM-5 цеолите. Для поддержания высокой равновесной концентрации активных кислород-содержащих внерешеточных центров в цеолите необходимо непрерывное добавление воды, что позволяет добиться высокой и стабильной активности катализатора.

Таким образом, в представленной диссертации на основе как квантово-химических, так и экспериментальных (ИК-спектроскопия) исследований показано, что реакционная способность низкокремниевых цеолитов, модифицированных сравнительно интертными катионами щелочных и щелочноземельных металов, обусловлена преимущественно свойствами замкнутого пространства цеолитных пор.

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Высокая плотность и специфическое расположение внерешеточных катионов в микропористой матрице определяет оптимальную конфигурацию адсорбированных реагентов и, следовательно, приводит к их химической активации. С другой стороны, активные центры в высококремниевых цеолитах, модифицированных более мягкими катионами цинка, кадмия и галлия, более локализованы и обычно напрямую вовлечены в каталитические превращения. Реакционная способность последних определяется свойствами Льюисовских кислотно-основных сопряженных пар, которые, в свою очередь, диктуются топологией цеолитной катионной позиции, содержащей внерешеточный катион, а также типом компенсации заряда и природой катиона. Определяющую роль в активности каталитического центра играют льюисовская кислотность внерешеточного катиона, также как и свойства сопряженного основного центра. Для достижения высокой каталитической активности должно быть достигнуто оптимальное соотношение в свойствах Льюисовской кислотно-основной пары цеолитного активного центра.

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ACKNOWLEDGEMENTS First and foremost I would like to thank my supervisor Prof. Rutger van Santen for giving me an opportunity to work in his group. His knowledge, curiosity, valuable insights, patience and tolerance to “odd ideas” were a very strong motivation force. I am very grateful for his careful guidance, for giving me an opportunity to work with so many different scientific topics and to collaborate with so many interesting people, for all the inspiring discussions, and important, not very pleasant sometimes though, for his criticism. I would also extend my gratitude to Dr. Emiel Hensen who was always ready to help me during these three years of my research work in Eindhoven. His readiness to accept new ideas (sometimes nonsense though) and to bring even fresher ones (even late in the night by e-mail), many fruitful discussions, his help in structuring manuscripts and this dissertation into what they are... and his continuous support are highly acknowledged.

Special thanks to Prof. Vladimir Kazansky (Moscow) who awoke my interest in science, who taught me to be skeptical and, at the same time, ready for new results and theories. Many thanks to Prof. Boris Shelimov (Moscow) for my choice of physical chemistry as the main research topic and for being my first teacher in science. I am grateful to Prof. Georgy Zhidomirov (Moscow and Novosibirsk) for introducing me to the world of quantum chemistry, and for many helpful and inspiring discussions. Many thanks to Dr. Christian Müller for bringing phosphabenzenes into my life. This was a very inspiring and fruitful collaboration. I would like to thank Dr. Tonek Jansen for shedding light on theory behind computations and for his help during these three years.

Of course, since I became a computational chemist here in Eindhoven, I was totally dependent on experimentalists to provide me with the results to be rationalized or to be used as a support for the calculations. Thus, Dr. Emiel Hensen and Dr. Christian Müller must be acknowledged again. Special thanks to the newly-made Dr. Neelesh Rane for the experimental contribution to my thesis. His enthralling results were very important to guide my calculations on gallium and to keep me interested in this topic. I would also like to thank Dr. Barbara Mojet, Dr. Jiang Xu and Prof. Leon Lefferts from University of Twente for the successful collaboration and many fruitful discussions about adsorption and oxidation of light alkanes in zeolites. I am grateful to Prof. Johannes Lercher and Dr. Carsten Sievers (Münich, although Carsten works in States now…) for many interesting discussions and a nice collaboration, although we could not understand completely lanthanum in zeolites. My gratitude goes to Prof. Robert Schoonheydt and Dr. Pierre Mignon (Leuven) for the successful collaboration on N2O4 disproportionation. Besides brain power, there was also a lot of computational power involved in my work. Thus, NWO-NCF is acknowledged for providing computational facilities. I am grateful to Joost, Erik S. and Bouke for maintaining the local computational resources.

I would also like to thank some people in Moscow, who were continuously adding fuel to

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the fire of research when I was on holydays there. These are a newly-made professor — Kostya Lyssenko, and fresh doctors — Denis Golovanov, Ivan Fedyanin and Ivan Glukhov from INEOS RAS. Thank you guys for showing me how one must work and publish results. Dr. Irina Subbotina and Natalia Sokolova from IOC RAS are gratefully acknowledged for all their help, support and advices related both to work and to life.

It was my pleasure to work and not only work in the SKA group together with so many interesting personalities, fine researchers and good colleagues. I am grateful to all SKA members for their cooperation, helpful discussions, assistance, kindness and friendship. Elize and Marion are acknowledged for their help in administrative matters, for not being annoyed too much and responding to all my “small questions”.

I have entirely enjoyed the atmosphere provided at SKA. Many thanks to Dilip, Neelesh, Pieter vG., Michel, Emiel H., Gabriela, Volcan, Alessandro, Arijan for the energy and inspiration supply I found with their help on the 3rd floor. I would also like to thank Laura, Michèle, Christian, Jos, Jarno, Erik A., Maria, Katharina, Daniel, Nollaig, Mabel, Gijsbert, Gilbère, Jarl Ivar, Patrick, Bart for the access to the inspiration supply on the 4th floor.

I would like to thank the (ex)members of the theory group, Chrétien, Velitchka, Peter V., Joost, Cristina, Tonek, Willy, Bouke, Ojwang, Thuat, Sharan, Bartek, Luis, Olus, Shu- Xia, Sander, Dani for the nice discussions on both scientific and non-scientific subjects, for creating joyful atmosphere in our part of the building and more important outside the university. Special thanks-complaints to Joost, who was too good as a system administrator making it extremely difficult to work without him. Bouke is acknowledged for translating my complicated Dutch mails, although it was still difficult to understand them even after the translation. My special thanks to Cristina, who was always ready to listen to my complaints and other nonsense I was talking about instead of working. I would like to thank Pieter vG for the loads of fun we had together at the conferences… Indeed, what can be better than a good cigar with a glass of beer (although wine would have been better) in the night at a beach at Côte d'Azur?

I am thankful to Tiny, Peter T., Pieter M., Dieter, Jeroen, Prabashini, Denzil, Han Wei, Svetlana vB., Paul, Davy, Emiel vK., Freek, Barry for many nice discussions and loads of fun on different occasions. I also extend my gratitude to all the people who were not mentioned above but helped and supported me in one way or another during my work in SKA.

Last but certainly not least, everything I am now and everything I could achieve in my life I owe to my bellowed parents. I am deeply indebted to my parents, my brother and my wife who provided me with continuous moral support and love. Любимые мои родители! Все, что у меня есть, и все, что я есть, — благодаря вам! Спасибо вам огромное за все, что вы для меня сделали и делаете! Thank you very much for everything you’ve done for me! I could not have survived so far away from home without your love and support!

Евгений Александрович Пидько

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LIST OF PUBLICATIONS 

1. E.A. Pidko, R.A. van Santen: Confined space-controlled olefin–oxygen charge transfer in zeolites. J. Phys. Chem. B 2006, 110, 2963–2967. (Chapter 2)

2. E.A. Pidko, V.B. Kazansky, E.J.M. Hensen, R.A. van Santen: A comprehensive density functional theory study of ethane dehydrogenation over reduced extra-framework gallium species in ZSM-5 zeolite. J. Catal. 2006, 240, 73–84. (Chapter 7)

3. E.A. Pidko, R.A. van Santen: The conformations of alkanes adsorbed on zeolitic cations. ChemPhysChem 2006, 7, 1657 – 1660 (Chapter 4)

4. E.A. Pidko, J. Xu, B.L. Mojet, L. Lefferts, I.R. Subbotina, V. B. Kazansky, R.A. van Santen: Interplay of bonding and geometry of the adsorption complexes of light alkanes within cationic faujasites. Combined spectroscopic and computational study. J. Phys. Chem. B 2006, 110, 22618–22627 (Chapter 4)

5. E.A. Pidko, R.A. van Santen: Activation of light alkanes over zinc species stabilized in ZSM-5 zeolite: a comprehensive DFT study. J. Phys. Chem. C 2007, 111, 2643–2655. (Chapters 5 and 6)

6. E.A. Pidko, R.A. van Santen: Activation of light alkanes over Cd2+ ions in ZSM-5 zeolite: a theoretical study. Mend. Commun. 2007, 17, 68–70. (Chapter 5)

7. R.A. van Santen, W.K. Offermans, K. Malek, E.A. Pidko: Computational modeling of catalytic reactivity. Molec. Simul. 2007, 33, 327–336.

8. C. Müller, D. Wasserberg, J.J.M. Weemers, E.A. Pidko, S. Hoffmann, M. Lutz, A.L. Spek, S.C.J. Meskers, R.A.J. Janssen, R.A. van Santen, D. Vogt: Donor-functionalized polydentate pyrylium salts and phosphinines: synthesis, structural characterization and photophysical properties. Chem. Eur. J. 2007, 13, 4548–4559

9. E.A. Pidko, E.J.M. Hensen, R.A. van Santen: Activation of alkanes over GaO+ in GaZSM-5. J. Phys. Chem. C 2007, 111, 13068–13075. (Chapter 8)

10. E.J.M. Hensen, E.A. Pidko, N. Rane, R.A. van Santen: Water-promoted hydrocarbon activation catalyzed by binuclear gallium sites in ZSM-5 zeolite. Angew. Chem. Int. Ed. 2007, 46, 7273–7276. (Chapter 9)

11. E.J.M. Hensen, E.A. Pidko, N. Rane, R.A.van Santen: Modification of Brønsted acidity of zeolites by Ga+, GaO+ and AlO+: comparison for alkane activation. Stud. Surf. Sci. Catal. 2007, 170, 1182–1189

12. C. Müller, E.A. Pidko, D. Totev M. Lutz, A.L. Spek, R.A. van Santen, D. Vogt: Atropisomeric phosphinines: design and synthesis. Dalton Trans. 2007, 5372–5375.

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13. E. Hensen, N. Rane, E. Pidko, R. van Santen: A combined experimental and computational study of alkane activation over Ga+, GaO+, GaH2

+ and H+ ions in ZSM-5. Stud. Surf. Sci. Catal. 2008, accepted for publication.

14. E.A. Pidko, P. Mignon, P. Geerlings, R.A. Schoonheydt, R.A. van Santen: A periodic DFT study of N2O4 disproportionation on alkali-exchanged zeolites X. J. Phys. Chem. C 2008, accepted for publication (Chapter 3).

15. E.A. Pidko, E.J.M. Hensen, G.M. Zhidomirov, R.A. van Santen: Non-localized charge compensation in zeolites: a periodic DFT study of cationic gallium-oxide clusters in mordenite. Submitted for publication (Chapter 9).

16. P. Mignon, E.A. Pidko, R.A. van Santen, P. Geerlings, R.A. Schoonheydt: Understanding the reactivity and basicity of zeolites. A periodic DFT study of the N2O4 disproportionation over alkaline cation exchanged zeolite Y. Submitted for publication.

17. E.A. Pidko, R.A. van Santen: “Computational approach in zeolite science” in: Zeolite Chemistry and Catalysis: An integrated Approach and Tutorial. Eds.: E.G. Derouane, A.W. Chester. Submitted for publication.

 

During M.Sc. study in Moscow:

18. V.B. Kazanskii, E.A. Pid’ko: Diffuse reflectance IR spectra of molecular hydrogen and deuterium adsorbed on zinc oxide” Kin. Catal. 2002, 43, 567–572.

19. V.B. Kazansky, A.I. Serykh, E.A. Pidko: DRIFT study of molecular and dissociative adsorption of light paraffins by HZSM-5 zeolite modified with zinc ions: methane adsorption. J. Catal. 2004, 225, 369–373.

20. V.B. Kazansky, E.A. Pidko: A new insight in the unusual adsorption properties of Cu+ cations in Cu-ZSM-5 zeolite. Catal. Today 2005, 110, 281–293.

21. E.A. Pidko, V.B. Kazanskii: IR spectra of ethane adsorbed on the hydrogen, sodium, and zinc forms of a Y-type zeolite: interpretation using ab initio quantum chemical calculations. Kinet. Catal. 2005, 46, 407–413.

22. V.B. Kazansky, E.A. Pidko: Intensities of IR stretching bands as a criterion of polarization and initial chemical activation of adsorbed molecules in acid catalysis. Ethane adsorption and dehydrogenation by zinc ions in ZnZSM-5 zeolite. J. Phys. Chem. B 2005, 109, 2103–2108.

23. E. Pidko, V. Kazansky: σ-Type ethane adsorption complexes with Cu+ ions in Cu(I)-ZSM-5 zeolite. Combined DRIFTS and DFT study. Phys. Chem. Chem. Phys. 2005, 7, 1939–1944.

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Curriculum Vitae 

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CURRICULUM VITAE 

Evgeny A. Pidko was born on the 13th of November 1982 in Moscow, USSR. After his graduation from high school in 1999, he continued studying at the Higher Chemical College of the Russian Academy of Sciences (HCC RAS), Moscow. The same year he joined the Laboratory for Radiospectroscopic and Optical Methods for Investigations of the Mechanism of Heterogeneous Catalysis of the N.D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences, Moscow, Russia, as a research assistant, where he investigated photo-chemical reactions on silica-supported vanadium oxide under the supervision of Prof. Boris N. Shelimov. From 2001 till 2004, he worked under the supervision of Prof. Vladimir B. Kazansky in the same laboratory. His research was aimed at the experimental investigation of adsorption properties of different oxide- and zeolite-based systems using infrared spectroscopy. In the summer of 2004, he graduated from the HCC RAS with honor. His master thesis was entitled “Adsorption of light alkanes on zeolites modified with zinc”. In January 2005, he moved to the Netherlands to start his doctoral research under the supervision of Prof. Rutger A. van Santen at the department of chemical engineering and chemistry of the Eindhoven University of Technology. His research was mainly focused on the development of understanding of chemical reactivity of zeolites modified with metal ions at molecular level. The most important results of this research are described in this thesis.