catalytic conversion of alkylaromatics to aromatic nitriles

Catalytic conversion of alkylaromatics to aromatic nitriles

Stobbelaar, P.J.

DOI:10.6100/IR538872

Published: 01/01/2000

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Citation for published version (APA):Stobbelaar, P. J. (2000). Catalytic conversion of alkylaromatics to aromatic nitriles Eindhoven: TechnischeUniversiteit Eindhoven DOI: 10.6100/IR538872

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https://research.tue.nl/en/publications/catalytic-conversion-of-alkylaromatics-to-aromatic-nitriles(a0d2dc32-f1b1-42fe-af54-3f51e35a63f6).html

Catalytic Conversion of Alkylaromatics Catalytic Conversion of Alkylaromatics Catalytic Conversion of Alkylaromatics Catalytic Conversion of Alkylaromatics to Aromatic Nitrilesto Aromatic Nitrilesto Aromatic Nitrilesto Aromatic Nitriles

ProefschriftProefschriftProefschriftProefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de

Rector Magnificus, prof.dr. M. Rem, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op

dinsdag 28 november 2000 om 16.00 uur

door

Pieter Johannes StobbelaarPieter Johannes StobbelaarPieter Johannes StobbelaarPieter Johannes Stobbelaar

geboren te Driebergen-Rijsenburg

Dit proefschrift is goedgekeurd door de promotoren: prof.dr. R.A. van Santen en prof.dr. B.K. Hodnett CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Stobbelaar, Pieter J. Catalytic conversion of alkylaromatics to aromatic nitriles / by Pieter J. Stobbelaar. - Eindhoven : Technische Universiteit Eindhoven, 2000. - Proefschrift. - ISBN 90-386-2612-6 NUGI 813 Trefwoorden: katalytische oxidatie ; ammoxidatie / heterogene katalyse ; zeolieten / overgangsmetaalverbindingen ; molybdeenverbindingen Subject headings: catalytic oxidation ; ammoxidation / heterogeneous catalysis ; zeolites / transition metal compounds ; molybdenum compounds The work described in this thesis has been carried out at the Schuit Institute of Catalysis (part of NIOK: the Netherlands School for Catalysis Research), Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, The Netherlands. Financial support has been supplied by the European Community under the Industrial & Materials Technologies Programme (Brite-EuRam III). Printed at Universiteitsdrukkerij, Eindhoven University of Technology.

Contents

Chapter 1: Nitrile formation and conversion reactionsChapter 1: Nitrile formation and conversion reactionsChapter 1: Nitrile formation and conversion reactionsChapter 1: Nitrile formation and conversion reactions 1111 Abstract 1 1. Aromatic nitriles: Production and applications 1 1.1 Ammoxidation reactions 1 1.2 (Potential) Applications of nitriles 3 1.3 Aromatic nitriles as intermediates in selective oxidation reactions 5 2. Scope of research 7 References 9

Chapter 2: Toluene ammoxidation mechanismChapter 2: Toluene ammoxidation mechanismChapter 2: Toluene ammoxidation mechanismChapter 2: Toluene ammoxidation mechanism 11111111 Abstract 11 1. Main reaction steps during toluene ammoxidation 11 2. Toluene activation 12 2.1 Hydrocarbon rupture 12 2.2 Effect of substituents on the aromatic ring 13 2.3 Effect of the catalyst basicity on the ammoxidation of

alkylaromatics 15

2.4 Nature of aromatic reaction intermediate 18 3. Ammonia activation 23 4. Catalyst reoxidation 27 5. Toluene ammoxidation reaction schemes 29 5.1 The propylene ammoxidation mechanism 29 5.2 The ammoxidation of toluene 32 6. Conclusions 35 References 36

Chapter 3: Chapter 3: Chapter 3: Chapter 3: Screening of new toluene ammoxidation catalystsScreening of new toluene ammoxidation catalystsScreening of new toluene ammoxidation catalystsScreening of new toluene ammoxidation catalysts 41414141

Abstract 41 1. Introduction 41 2. Experimental methods 43 2.1 Catalyst preparation and characterization 43 2.2 Catalyst testing 45 3. Results and discussion 46 3.1 Catalyst screening 46 3.2 Catalyst deactivation 50 3.2.1 Performance of ion-exchanged catalysts 52

Contents

3.2.2 Performance of catalysts prepared by CVD of metal carbonyls 52 3.2.3 Performance of NaY based impregnated catalysts 53 3.2.4 Performance of γ-alumina supported catalysts 55

3.3 Benzonitrile selectivity 57 3.4 Temperature influence 60 3.5 Nitroxidation of toluene 61 4. Conclusions 64 References 64

Chapter 4: Faujasite encaged metal oxide toluene ammoxidation catalysts Chapter 4: Faujasite encaged metal oxide toluene ammoxidation catalysts Chapter 4: Faujasite encaged metal oxide toluene ammoxidation catalysts Chapter 4: Faujasite encaged metal oxide toluene ammoxidation catalysts prepared from metal carbonyl precursorsprepared from metal carbonyl precursorsprepared from metal carbonyl precursorsprepared from metal carbonyl precursors

67676767

Abstract 67 1. Introduction 67 2. Materials and methods 71 2.1 Catalyst preparation 71 2.2 Catalyst characterization 72 2.2.1 Determination of the catalyst composition 72 2.2.2 X-Ray Photoelectron Spectroscopy 73 2.2.3 Transmission Electron Microscopy 73 2.2.4 Temperature Programmed Oxidative Decarbonylation 73 2.3 Catalytic tests 74 2.3.1 2-Methyl-3-butyn-2-ol decomposition 74 2.3.2 Toluene ammoxidation 74 3. Results and discussion 75 3.1 Thermal activation of intra-zeolite Mo(CO)6 75 3.2 XPS analysis of Mo(CO)6/NaY and MoOx/NaY 80 3.3 Dispersion of molybdenum oxide clusters in NaY 84 3.4 Mo(CO)6 interaction with the faujasite lattice 87 3.5 Introduction of other transition metal carbonyls by CVD 92 3.5.1 Introduction of V(CO)6 into NaY 92 3.5.2 Introduction of Mn2(CO)10 into NaY 94 3.5.3 Introduction of Co(NO)(CO)3 into NaY 96 3.6 Catalytic activity in the ammoxidation of toluene 97 3.7 The effect of the Lewis acidity and basicity on the ammoxidation

of toluene over MoOx/Y 99

4. Conclusions 100 References 101

Contents

Chapter 5: The effect of molybdenumChapter 5: The effect of molybdenumChapter 5: The effect of molybdenumChapter 5: The effect of molybdenum oxide reducibility on the ammoxidation of oxide reducibility on the ammoxidation of oxide reducibility on the ammoxidation of oxide reducibility on the ammoxidation of toluenetoluenetoluenetoluene

105105105105

Abstract 105 1. Introduction 105 1.1 Preparation methods of supported Mo catalysts 106 1.2 Notation of different Mo species 107 1.3 Molybdate surface species 108 1.4 Characterization of Mo surface species 110 1.5 Molybdate and Mo oxide reduction 112 2. Materials and methods 114 2.1 Catalyst preparation 114 2.2 Catalyst characterization 114 2.2.1 Diffuse reflection UV-Vis spectroscopy 114 2.2.2 Temperature Programmed Reduction 115 2.2.3 Raman Spectroscopy 115 2.2.4 Transmission Electron Microscopy 115 2.2.5 X-Ray Diffraction 116 2.2.6 X-Ray Photoelectron Spectroscopy 116 2.2.7 Hydrogen–deuterium exchange reactions 116 2.3 Ammoxidation of toluene 117 3. Results and discussion 117 3.1 Addition of a second metal to Mo/Al 117 3.2 Variation of the molybdenum oxide loading 120 3.3 DR-UVVis Spectroscopy 121 3.4 Reduction of Mo/Al catalysts 124 3.5 Hydrogen-deuterium exchange over Mo/Al catalysts 126 3.6 Transmission Electron Microscopy on Mo/Al samples 131 3.7 In situ treatment of Mo/Al 131 4. Conclusions 137 References 138

SummarySummarySummarySummary 143143143143

SamenvattingSamenvattingSamenvattingSamenvatting 147147147147

DankwoordDankwoordDankwoordDankwoord 151151151151

Curriculum VitaeCurriculum VitaeCurriculum VitaeCurriculum Vitae 153153153153

1

Chapter 1 Nitrile formation and conversion reactions

Abstract The background of the research project is described. Ammoxidation of alkylaromatics is a simple gas-phase reaction that yields aromatic nitriles. These nitriles have versatile applications, mainly as raw material in the polymer industry. Additionally, alkylaromate ammoxidation can be applied in the production of selective oxidation products since nitriles can be converted by hydrolysis and hydrogenation reactions towards acids, aldehydes, amines and amides. This two-step approach cleanly yields the oxygenate without production of harmful side products. The project focuses on the ammoxidation of toluene. For this reaction the development of new faujasite-based catalysts was performed. Additionally, a comparison with more conventional γ-alumina supported molybdenum oxide catalysts has been made.

1. Aromatic nitriles: Production and applications

1.1 Ammoxidation reactions Aromatic nitriles can be formed by reacting an aromatic hydrocarbon with ammonia and oxygen. The simplest example is benzonitrile production from toluene, as shown in Equation 1.1.

The reaction of a reducible hydrocarbon with ammonia and oxygen are referred to as ammoxidation reactions. Alkenes, alkanes and aromatics are used most often in ammoxidation reactions. The catalysts that are active in ammoxidation reactions consist mainly of mixed oxides containing variable-valence transition metals. For the ammoxidation of propylene bismuth-molybdate based systems are applied industrially on a large scale [1]. The ammoxidation of propylene is well developed and is commercially applied by Sohio since the early sixties

CN + 3 H2OCH3 NH3++ 3/2 O2 (1.1)

Chapter 1

2

[2]. The annual world production amounts to 4600 ktons [3]. Until recently the production of acrylonitrile from propane could only be performed at very high temperatures (750 – 1000 °C) [4]. In the late eighties propylene ammoxidation to acrylonitrile has been patented frequently, for example by BP America (previously SOHIO) [5]. Recently, a large variety of new catalysts have been developed for the ammoxidation of propane. Mostly vanadium antimony oxide [6] systems are reported, though also molybdenum based multi-component catalysts are frequently patented [7]. Pilot-plant studies have been performed already [8] and commercial production of acrylonitrile from propane was announced [9]. The feedstock price of propane is significantly lower than that of propylene, but the acrylonitrile yields are markedly lower because of the poor acrylonitrile selectivity [10,11]. This lower acrylonitrile yield per mole of feedstock delays commercial production of acrylonitrile from propane. To date acrylonitrile production from propane has not started yet [12]. Aromatic ammoxidation reactions are performed mostly over vanadia based catalysts [13]. Several companies practice commercial ammoxidation of alkylated aromatics [14]. Showa Denko converts p-xylene and m-xylene to the corresponding di-nitriles, terephthalonitrile and isophtalonitrile. Also Mitsubishi Gas Chemical ammoxidizes m-xylene to isophtalonitrile on a commercial scale. They operate two plants in the USA and in Japan. BASF and Japan Catalytic Chem. Ind. produce phthalonitrile from o-xylene. Phthalonitrile is applied as an important precursor in the manufacturing of phthalocyanine dyes. Typically, ammoxidation reactions are performed at temperatures between 400 and 500 °C. For propane ammoxidation the reaction temperature may be somewhat higher, because the dehydrogenation of propane needs higher temperatures to occur. During ammoxidation the catalysts is reduced by ammonia and the hydrocarbon. It is generally accepted that lattice oxygen reoxidizes the catalyst during ammoxidation. The oxygen insertion step and catalysts reoxidation can be performed in separate reactors [15]. Under these conditions the production of nitriles from hydrocarbons is referred to as oxidative ammonolysis. In this way explosion hazards can be eliminated, since the hydrocarbon mixture and oxygen are not mixed together in one reactor.

Introduction and background

3

1.2 (Potential) Applications of nitriles The most well-known ammoxidation reaction is the ammoxidation of propylene to form acrylonitrile. Acrylonitrile is basically used for the production of acrylic fibers, which can be used for manufacturing of clothing and carpets [16]. Worldwide, about 65 % of the acrylonitrile production is consumed for this purpose. Another important use of acrylonitrile can be found in the resin production, of which acrylonitrile-butadiene-styrene (ABS) and styrene-acrylonitrile (SAN) are the main applications. An extensive growth in the application of these resins has occurred during the past decade. The largest increase among the end uses of acrylonitrile, however, has come from adiponitrile, which is used as a precursor for hexamethylenediamine (HMD) by Monsanto [17]. HMD is used for the production of nylon-6,6. Recently, also the large-scale production of caprolactam from adiponitrile was reported [18]. Caprolactam is used as precursor for nylon-6, which can be produced in the same production site. Other applications of acrylonitrile are also found in the polymer industry. Catalytic hydrogenation of nitriles may result in several products. Among these, amines, imines, aldehydes, amides and alcohols are the most important products. The main product depends on the catalyst, substrate and reaction conditions [19]. Aromatic nitriles find diverse applications, for example as dyes and in pesticide and fungicide production but also in various nylons and polyurethane foams. Benzonitrile is used as a precursor for resins and coatings. Benzonitrile is also used as an additive in fuels and fibers. Table 1.1 lists the main application of some relatively simple substituted aromatic nitriles. As already discussed nitriles derived from xylenes are primarily used as precursors for the corresponding di-acids, for ultimate use in esters and polyesters.

Chapter 1

4

Table 1.1: Table 1.1: Table 1.1: Table 1.1: Applications of substituted benzonitriles.

CompoundCompoundCompoundCompound ApplicationApplicationApplicationApplication RemarksRemarksRemarksRemarks

2-chlorobenzonitrile azo dyes intermediate in the production of 2-amino-5-nitrobenzonitrile

4-chlorobenzonitrile red pigment for plastics

2,6-dichlorobenzonitrile herbicide for fruit and vine cultivation

intermediate in the production of 2,6-difluorobenzonitrile and 2,6-dichlorothiobenzamide

2,6-difluorobenzonitrile insecticides intermediate 2-chloro-4-nitro-benzonitrile

azo dyes intermediate

4-chloro-2-nitro-benzonitrile

azo dyes intermediate in the production of 2-amino-4-chlorobenzonitrile

2-amino-5-nitro-benzonitrile

azo dyes intermediate

4-hydroxybenzonitrile herbicides intermediate in the production of 3,5-dibromo- and 3,5-diiodo-4-hydroxy-benzonitrile

Data from [20].

The applications of more complex substituted aromatic and hetero aromatic nitriles were described by Grasselli et al. [21]. For example, high performance polymers are formed from atroponitrile. Related substituted aromatic aldehydes such as atropaldehyde and cinemaldehyde, which can be produced by direct gas-phase oxidation of the substrate, are used as flavors or perfumes in different products. In Vitamin B complex nicotinamide (niacinamide) and nicotinic acid (niacin) are used. These products are formed from nicotinonitrile, which can be obtained readily by ammoxidation of 3-methylpyridine over phosphorous molybdate-vanadate catalysts. Additionally, fungicides can be prepared from heteroaromatic nitriles such as 4-cyanothiazole. In Scheme 1.1 some examples are summarized of aromatic nitriles and their applications. Also the most commonly used catalysts are indicated for each example.


5

Scheme 1.1:Scheme 1.1:Scheme 1.1:Scheme 1.1: Applications of several ammoxidation reactions.

1.3 Aromatic nitriles as intermediates in selective oxidation reactions During the eighties ammoxidation reactions were frequently investigated, especially the ammoxidation of alkylaromatics. The ammoxidation of toluene to form benzonitrile was often used as a model reaction for other alkylaromatics such as p-xylene. Showa Denko and Lummus ammoxidize xylenes to mono- and di-nitriles. In the Lummus process aromatic nitriles are prepared via an ammonolysis reaction. Xylene reacts with ammonia and lattice oxygen to form the aromatic nitrile. Gas-phase oxygen is used afterwards to regenerate the catalyst [15]. As described earlier by these authors terephthalic acid can be produced via hydrolysis of the nitriles [22]

Application in Vitamin B complex:Application in Vitamin B complex:Application in Vitamin B complex:Application in Vitamin B complex:

PVMoOx

N

CH3

N

CN

+ 3 H2O+ NH33/2 O2+

Application as intermediate in fungicide production:Application as intermediate in fungicide production:Application as intermediate in fungicide production:Application as intermediate in fungicide production:

S

NCH3

+ NH33/2 O2+

S

NNC

+ 3 H2Ovarious catalysts

Application as monomers for high performance polymers:Application as monomers for high performance polymers:Application as monomers for high performance polymers:Application as monomers for high performance polymers:

CH2 CH3 CH2 CN

+ 3 H2O+ NH33/2 O2+

USb4.6Ox

Application in resin and coating production:Application in resin and coating production:Application in resin and coating production:Application in resin and coating production:

CN + 3 H2OCH3+ NH3

3/2 O2+V2O5

Chapter 1

6

produced in this oxidative ammonolysis reaction. The conversion of alkylaromatics to oxidized products such as terephthalic acid is usually performed by direct oxidation reactions. Since the performance of alkylaromatic autoxidation reactions is relatively simple, because ring oxidation does not occur, these reactions are performed basically in the liquid phase [23]. The oxidizability of alkylaromatic hydrocarbons by liquid-phase autoxidation reactions decreases significantly in the order tertiary > secondary > primary benzylic C-H bonds [24]. Therefore, liquid-phase autoxidations have somewhat limited applications. Especially for primary alkylaromatics such as toluene, it is not possible to achieve high selectivity to hydroperoxide at reasonable high reaction rates. Since the oxidizability of toluene is about five orders of magnitude lower than the oxidizability of benzaldehyde [25] production of benzaldehyde by autoxidation is not possible. Nevertheless, terephthalic acid can be produced in high yields by liquid-phase direct oxidation using a Co/Mn/Br-acetic acid catalyst [26]. Though the yield of terephthalic acid by the conventional liquid-phase process is high, future regulations may restrict the application of this reaction, since the process conditions, which require the highly corrosive bromine–acetic acid environment, are reprehensible from environmental perspective. On the other hand due to the low solubility of terephthalic acid in the solvent, most of it precipitates as it forms. Separation of the product from the solvent is easy and the production process will only be changed if future legislation so obliges. Based on the atom utilization concept described by Sheldon and Dakka [27], in general gas-phase oxidations are preferred over liquid-phase oxidation processes. Moreover, the use of gas-phase oxygen as oxidant is highly desirable since besides the oxidation product only water is produced. The Environmental Quotient (EQ), which is defined by the amount of waste per kilogram of product multiplied by an unfriendliness quotient (Q) is as low as possible for oxidation reactions. In this respect aromatic nitriles can be used as intermediates in selective oxidations. According to Equation 1.1 the aromatic nitrile is manufactured with high atom utilization; only water (having a low Q value) is formed as side product. Conversion of the aromatic nitrile in a second step cleanly yields the oxidation product. Up to now the only industrially important manufacturing process for benzaldehyde is the hydrolysis of benzal chloride or the partial oxidation of


7

toluene [28]. The first route is highly productive and high benzonitrile selectivity is obtained (> 95%). However, for each molecule of benzaldehyde one hydrogen chloride molecule is produced as a side product. Direct selective oxidation of toluene is a clean route. To date, however, only moderate benzaldehyde yields are obtained. Very recently the group of Centi developed a bulk-type Fe-Mo-Ce-oxide catalyst that produces in high yield (50-55 mol%) 3-fluorobenzaldehyde from 3-fluorotoluene [29]. Via classical organic chemistry aldehydes can be formed from nitriles by performing a reduction with di-isobutylaluminumhydride [30]. Chatterjee et al. [31] produced benzaldehyde from benzonitrile over platinum and ruthenium loaded acidic zeolites with high selectivity by vapour-phase reductive hydrolysis. This reaction can also be performed in the liquid phase using Raney nickel [32] though a sulphuric acid or formic acid medium has to be applied in this case. Also nickel and iron precipitated on alumina catalysts have been described in literature for the liquid-phase reductive nitrile hydrolysis [33]. By hydrogenation aromatic nitriles can also be converted to aromatic amines [14]. The production of benzamide can be performed selectively over hydrotalcite-like catalysts as will be reported by Sychev et al. [34]. In our group theoretical work related to aromatic nitrile conversion, was carried out by Barbosa et al. [35], who studied the hydrolysis of acetonitrile over protonic zeolite catalysts.

2. Scope of research The research described in this thesis was aimed at the development of new, selective and clean processes for alkylaromatic side chain oxidation. A gas-phase process was chosen for the conversion of the alkylaromatic side-chain oxidation, based on the poor opportunities for liquid-phase processes. Catalyst leaching complicates severely the possibilities of liquid-phase heterogeneously catalyzed processes. Moreover, the higher cost of the oxidant does not favor the economics of the process. Therefore, a two-step vapour-phase process was studied, in which an alkylaromatic substrate is converted by ammoxidation to an aromatic nitrile. In a second step this aromatic nitrile is subjected to a hydrolysis reaction to form the oxygenate. This reaction pathway cleanly yields oxygenated aromatics, as sketched in Scheme 1.2.

Chapter 1

8

Scheme 1.2:Scheme 1.2:Scheme 1.2:Scheme 1.2: Ammoxidation and sequential hydrolysis to cleanly produce oxygenated aromatic hydrocarbons.

For the ammoxidation reaction toluene was chosen as substrate, based on the relative simplicity of the molecule. An elementary screening study was performed in order to check the feasibility of faujasite-based catalysts for the ammoxidation reaction. The performance of zeolite Y encaged

molybdenum oxide nanoclusters was compared to that of γ-alumina supported molybdenum oxide. The properties of the latter catalysts were studied in great detail by both in situ and ex situ characterization techniques. This thesis focuses on the ammoxidation of toluene. Theoretical work on nitrile hydrolysis was performed by Barbosa [36]. The hydrolysis of

Aldehyde formation:Aldehyde formation:Aldehyde formation:Aldehyde formation:

Overall:Overall:Overall:Overall:

CN +CH3 NH3++ 3/2 O2 3 H2O

CH3 O2+ +

O

HH2O

CN H2O+ +H2

O

H+ NH3

Overall:Overall:Overall:Overall:

CH3 NH3+ + H2O2

2 1/2 O2 CNH2

O

+ 4 H2O+

CN + CNH2

OOH-

O2+ + H2OH2O22

Amide formation:Amide formation:Amide formation:Amide formation:

CN +CH3 NH3++ 3/2 O2 3 H2O


9

benzonitrile was studied in the National Technical University of Ukraine, Kiev by Prihod’ko [37].

References 1. J.F. Brazdil, Acrylonitrile, in: Kirk-Othmer Encyclopedia of Chemical

Technology, Vol 1. Wiley New York, 4th edition, pp. 352-369. 2. J.D. Idol, US Patent 2904580, J.D. Idol, 1959. 3. http://www.chemweek.com/productfocus/1996/acrylonitrile.html 4. P.W. Langvardt, Acrylonitrile, in: Ullmann’s Encyclopedia of Industrial

Chemistry, 6th (electronic) edition, 1999. 5. E.g. A.T. Gutmann, R.K. Grasselli, J.F. Brazdil, US Patent 4746641,

(1988). 6. G. Centi, F. Marchi, S. Perathoner, J. Chem. Soc. Faraday Trans., 92,

(1996), 5141-5149. 7. H. Midorikawa, N. Sugiyama, H. Hinago, US Patent 6973186, 1999. H. Kazuyuki, S. Komada, US Patent 5907052, 1999. K. Aoki, US Patent 5780664, 1998. H. Midorikawa, K. Someya, K. Aoki, O. Nagano, US Patent 5658842,

1997. H. Midorikawa, K. Someya, US Patent 5663113, 1997. R. Canavesi, F. Ligorati, R. Ghezzi, US Patent 4609635, 1986. 8. P. Fairley, Chem. Week, 160(36), (1998), 45. 9. P. Layman, Chem. Eng. News, 73, (1995), 13-15. 10. B.K. Hodnett, Heterogeneous Catalytic Oxidation, John Wiley & Sons,

Chichester, 2000, pp. 240-263. 11. K. Weissermel, H-J. Arpe, Industrial Organic Chemistry, 3rd edition,

VCH, Weinheim, 1997, pp. 303-310. 12. R.K. Grasselli, Ammoxidation, in: Handbook of Heterogeneous

Catalysis, Eds. G. Ertl, H. Knözinger, J. Weitkamp, Vol. 5, VCH, Weinheim, 1997, pp. 2302-2326.

13. R.G. Rizayev, E.A. Mamedov, V.P. Vislovskii, V.E. Sheinin, Appl. Catal. A, 83, (1992), 103-140.

14. K. Weissermel, H-J. Arpe, Industrial Organic Chemistry, 3rd edition, VCH, Weinheim, 1997, pp. 385-403.

15. M.C. Sze, A.P. Gelbein, Hydrocarbon. Proc., 1976, 103-106. 16. J.F. Brazdil, Acrylonitrile, In: Kirk-Othmer Encyclopedia of Chemical

Technology, 4th edition, Vol. 1, pp. 352-369. 17. M.M. Baizer, C.R. Campbell, R.H. Fariss, R. Johnson, US Patent

3193480, 1965. 18. Chemical Market Reporter, 254, (1998), 5. 19. G.V. Smith, F. Notheisz, Heterogeneous Catalysis in Organic

Chemistry, Academic Press, San Diego, 1999, p. 71-79. 20. P. Pollak, G. Romeder, F. Hagedorn, H-P. Gelbke, Nitriles, in:

Ullmann’s Encyclopedia of Industrial Chemistry, 6th (electronic) edition, 1999.

21. R.K. Grasselli, J.D. Burrington, R. Di Cosimo, M.S. Friedrich, D.D. Suresh, in: Heterogeneous Catalysis and Fine Chemicals, Eds. M. Guisnet, J. Barrault, C. Bouchoulle, D. Duprez, C. Montassier, G.

Chapter 1

10

Pérot, Stud. Surf. Sci. Catal., Vol. 41, Elsevier Science Publishers B.V., Amsterdam, 1988, pp. 317-326.

22. A.P. Gelbein, M.C. Sze, R.T. Whitehead, Hydrocarbon Processing, (1973), 209-215.

23. R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, 1981, pp. 315-339.

24. J.A. Howard, Adv. Free Radical Chem., 4, (1972), 49-173. 25. R.A. Sheldon, Liquid Phase Autoxidations, in: Catalytic Oxidation,

Principles and Applications, (Eds. R.A. Sheldon, R.A. van Santen), World Scientific, London, 1995, pp. 150-174.

26. W. Partenheimer, Catal. Today, 23, (1995), 69-158. 27. R.A. Sheldon, J. Dakka, Catal. Today, 19, (1994), 215-246. 28. F. Brühne, E. Wright, Benzaldehyde, in: Ullmann’s Encyclopedia of

Industrial Chemistry, 6th (electronic) edition, 1999. 29. G. Centi, Private communications. 30. T.W.G. Solomons, Organic Chemistry, 5th edition, John Wiley & Sons

Inc., New York, 1992, pp. 686-687. 31. A. Chatterjee, R.A. Skaikh, A. Raj, A.P. Singh, Catal. Lett., 31, (1995),

301-305. 32. P. Tinapp, Chem. Ber., 102, (1969), 2770-2776. B. Staskun, O.G. Backeberg, J. Chem. Soc., (1964), 5880-5881. T. Es, B. Staskun, J. Chem. Soc., (1965), 5775-5777. 33. Z. Bodnar, T. Mallat, J. Petro, J. Mol. Catal., 70, (1991), 53-64. 34. R. Prihod’ko, I. Kolomitsyn, M. Sychev, P.J. Stobbelaar, R.A. van

Santen, Micr. Mesop. Mat., to be published. 35. L.A.M.M. Barbosa, R.A. van Santen, J. Catal., 191, (2000), 200-217. 36. L.A.M.M. Barbosa, Theoretical Study of Nitrile Hydrolysis by Solid

Acid Catalysts, PhD Thesis, Eindhoven University of Technology, 2000. 37. R. Prihod’ko, Synthesis and Characterization of some Heterogeneous

Catalysts for Fine Organic Chemistry, PhD Thesis, National Technical University of Ukraine, Kiev, in preparation.

11

Chapter 2 Toluene ammoxidation mechanism

Abstract The mechanism of the ammoxidation of toluene is reviewed. Ammoxidation of toluene is mainly studied over vanadia-based catalysts. Although the literature is not consistent with respect to the exact mechanism some general trends can be observed. The rate-determining step is the hydrocarbon activation. Most authors agree on the formation of an oxygenated adsorbed organic intermediate. Toluene is adsorbed on the catalyst surface as a benzyl fragment. This benzyl species is oxygenated to form an adsorbed benzaldehyde surface structure. This structure is sometimes also referred to as benzoate species. Additionally a reaction pathway via sequential dehydrogenation of adsorbed benzyl species to an adsorbed amine and imine is plausible. Oxygen is supplied as surface oxygen, according to a Mars-Van Krevelen like mechanism. The exact nature of the nitrogen insertion site is studied less extensively. The amount of ammonia plays a decisive role in the catalyst oxidation state. Strong ammonia adsorption leads to an inactive catalyst, whereas weak ammonia adsorption leads to combustion reactions.

1. Main reaction steps during toluene ammoxidation Several groups have studied toluene ammoxidation in the past years. Rizayev et al. [1], concentrating on Russian literature, have reviewed the ammoxidation of simple alkylaromatics over vanadium oxide based catalysts in 1992, but no literature overview exists that also discusses other catalyst systems. In addition, toluene ammoxidation over VPO-based catalysts as investigated intensively during the last five years by the group of Lücke et al. [2,3] was not included in this review. Recently Centi et al. [4] discussed in more detail ammonia activation with respect to the ammoxidation of alkylaromatic compounds. This section discusses in a more extensive manner the role of the several reaction steps in the ammoxidation of toluene.

Chapter 2

12

In toluene ammoxidation reactions three important processes occur: 1. Toluene activation, during which the methyl group has to be

dehydrogenated. 2. Ammonia activation, leading to the formation of the nitrogen insertion

species. 3. Reoxidation of the catalysts by consumption of gas-phase oxygen. Scheme 2.1 summarises these steps in toluene ammoxidation.

Scheme 2.1:Scheme 2.1:Scheme 2.1:Scheme 2.1: Fundamental steps during toluene ammoxidation

It is generally agreed that activation of the methyl group is the rate-determining step during toluene ammoxidation [E.g. 5,6]. The nature of this activated species, however, is still under debate. According to Golodets [7] partial oxidation reactions occur on oxide catalysts by a mechanism of alternating reduction and oxidation of the catalyst surface. Total oxidation reactions, on the other hand, proceed via both redox and associated mechanisms. This is also true for ammoxidation reactions. In this chapter the literature on the role of each of these three steps in the ammoxidation of toluene is reviewed.

2. Toluene activation

2.1 Hydrocarbon rupture The pathway of hydrocarbon activation has been studied by several groups, mostly by applying kinetic studies or IR Spectroscopy. If the nitrile production occurs along the side chain three basic types of C-H activation must be considered: 1. Heterolytic rupture, producing a carbocation and an H--ion. This

possibility is believed to occur over acidic catalysts. When this pathway of C-H rupture occurs, the H--ion binds to the acid centre to give hydrogen, which is oxidised to water in the presence of oxygen. This pathway, however, was never proven experimentally.

C6H5CH3+ (O) (I) (1)

NH3+ (I) C6H5CN + ( ) (2)

1/2O2 + ( ) (O) (3)

Toluene ammoxidation mechanism

13

2. Heterolytic rupture, producing a H+ ion and a carbanion. This mechanism is plausible over sufficiently strong basic sites. The hydrocarbon acts as an acid when this C-H rupture mechanism applies to the reaction.

3. Homolytic rupture. A hydrocarbon radical and a hydrogen radical are formed. Contrary to the two heterolytic C-H rupture mechanisms the presence of electron donating or electron withdrawing side groups on the benzene ring should have little influence on the activity or selectivity in the ammoxidation of toluene.

2.2 Effect of substituents on the aromatic ring To examine in more detail these types of C-H rupture several groups have studied the effect of electron donating and withdrawing side-groups on the aromatic ring. The addition of an electron-withdrawing group (especially in the para position) to the aromatic ring would increase the reactivity of the methyl group if C-H rupture occurs according to heterolytic rupture via the formation of a carbocation. In this case the formation of a carbanion would be favoured if an electron-donating group is attached to the aromatic ring. The effect of substituents on the aromatic ring can be divided into an inductive effect, in which charges are stabilized by the aromatic ring and a resonance effect, which applies to groups that contain a lone pair of electrons. Generally, the resonance effect is much stronger than the inductive effect. Moreover, the resonance effect is directed to the substituent position. Electron donating groups such as -NH2 stabilize cation formation only in the ortho- and para-position. Table 2.1 lists the most important substrates used in ammoxidation reactions.

Table 2.1: Table 2.1: Table 2.1: Table 2.1: Substituent effect of the most important alkylaromatic ammoxidation substrates

Electron withdrawingElectron withdrawingElectron withdrawingElectron withdrawing Electron donating Electron donating Electron donating Electron donating

NO2 CH3 CN, CHO, COOH C2H5

Inductive effectInductive effectInductive effectInductive effect

OH C6H5

Halogens NH2 OCH3

Resonance effectResonance effectResonance effectResonance effect

OH

Data from Morrison and Boyd [8].

Chapter 2

14

Over titania-supported vanadia catalysts the ammoxidation activity is increased when a substituted toluene is applied as substrate, both with electron withdrawing as with electron donating substituents as found by Busca et al. [9]. The relative alkylaromatic ammoxidation rates are listed in Table 2.2.

Table 2.2: Table 2.2: Table 2.2: Table 2.2: Ammoxidation rates over substituted alkylaromatic substrates

SubstrateSubstrateSubstrateSubstrate RelatiRelatiRelatiRelative ammoxidation rateve ammoxidation rateve ammoxidation rateve ammoxidation rate

Toluene 1.00 m-Xylene 1.12 p-Methoxytoluene 1.27 p-Chlorotoluene 1.42 p-Xylene 1.45

Activities measured at T = 300 ° C over a V-Ti-O catalyst [9].

These data support the occurrence of homolytic C-H rupture over catalysts that have well defined redox properties. Cavani et al. [6] reported the ammoxidation activity of a series of para-substituted alkylaromatics over V-Ti-O. Compared to toluene they found a higher activity towards the nitrile product for all substrates applied, irrespective the electron donating or withdrawing properties of the substituents. This does support a homolytic C-H rupture mechanism. The same group found similar ammoxidation activity with respect to toluene ammoxidation when a methyl group was situated in the meta-position [10]. The resonance effect predicts a strong difference between the ortho- and para-position on the one hand and the meta-position on the other hand. Differences that were found in selectivity towards the nitrile products could be explained well by steric effects. Earlier steric hindrance was found by Chmyr et al. [11] who found lower nitrile yields when the aromatic ring was substituted in the 2 and 6 positions. The methyl group was less accessible for reaction in this case as a result of the presence of these chloro substituents in the 2 and 6 position. 3,5 Chloro substitution protected ring oxidation without decreasing the ammoxidation activity. Cavani et al. [6], however, found significantly lower selectivities towards alkylaromatic nitriles when a strong electron-donating group (methoxy) was attached to the aromatic ring in the para-position. This significantly lower nitrile production could not be explained by the homolytic rupture mechanism proposed. The higher electron density in this case led to a more


15

pronounced attack by electrophilic centres such as O2- or O-. This

electrophilic attack in general leads to degradation of the aromatic ring for hydrocarbon oxidation reactions [12]. As a result the nitrile selectivity and thus nitrile yield is decreased. When weaker electron donating groups were applied the selectivity was similar to that in toluene ammoxidation.

Similar effects were reported by Lücke and Martin [13] over vanadium phosphate (VPO) catalysts. The differences in nitrile yield, however, varied to a greater extend and seem to

indicate an ionic mechanism rather than homolytic C-H rupture. These authors found also significant differences for the different substituent positions in the aromatic ring. Figure 2.1 shows the effect of the different ring positions of the chloro group. As would be expected from a heterolytic C-H rupture mechanism the nitrile yield differs significantly. The lowest yield was obtained over m-chlorotoluene as expected from theory assuming heterolytic rupture with formation of a carbocation. This carbocation is stabilized only in the ortho- and para-position according to the resonance effect. The higher nitrile yield for the para-substituted toluene cannot be explained by the resonance effect. Possibly steric effects account for the higher nitrile yield.

2.3 Effect of the catalyst basicity on the ammoxidation of alkylaromatics It is found that the ammoxidation rate increases with decreasing C-H dissociation energies, as shown by Suleimanov et al. [14] using V-Sb-O catalysts. This observation is explained by heterolytic C-H rupture with the

formation of a proton and a carbanion. The rate of α-hydrogen exchange correlates well with the alkylaromatic ammoxidation rate. This supports a heterolytic C-H rupture mechanism, in which anion-like hydrocarbon species are formed as shown in Figure 2.2.

��

��

��

0

25

50

75

100

4-C l-4-C l-4-C l-4-C l-

to luenetoluenetoluenetoluene

3-C l-3-C l-3-C l-3-C l-


2-C l-2-C l-2-C l-2-C l-


Nit

rile

yie

ld [

%]

Nit

rile

yie

ld [

%]

Nit

rile

yie

ld [

%]

Nit

rile

yie

ld [

%]

Figure 2.1:Figure 2.1:Figure 2.1:Figure 2.1: Substituent effect in ammoxidation over VPO catalysts. Data from Lücke et al. [13].

Chapter 2

16

Figure 2.2:Figure 2.2:Figure 2.2:Figure 2.2: Heterolytic CH rupture [14].

The interaction of the catalyst surface and the hydrocarbon thus was seen as an acid-base interaction. B- is the basic site, which can be formed by surface oxygen ions (O2-) or nucleophilic forms of adsorbed nitrogen species, such as NH2

- or NH2-. The hydrocarbon interacts with the surface as an acid [15]. The rate of isotope exchange of the hydrogen in the hydrocarbon was taken as a first approximation to estimate the hydrocarbon acidity. The data were measured in solution at low

temperature. Also, it must be noted that the rates of α-hydrogen exchange do not correlate exactly with the ammoxidation rates reported in case the methyl group is changed to ethyl or an isopropyl groups. Both substitutions

lead to a severe decrease of the α-hydrogen exchange rate, whereas the ammoxidation rate is slightly higher for ethylbenzene and significantly lower for i-propylbenzene. The order expected purely based on the rates of

α-hydrogen exchange is C6H5CH3 > C6H5CH2CH3 > C6H5CH(CH3)2. The high reactivity of ethylbenzene, therefore, is surprising; bearing in mind the fact that benzonitrile forms from ethylbenzene and from i-propylbenzene via the formation of styrene, which is converted to benzonitrile in a consecutive reaction step. Assuming heterolytic C-H rupture, not only adjusting the substrate would influence the ammoxidation rate, but also the acidity or basicity of the catalyst. When a carbanion is formed as intermediate, stronger basic centres of the catalyst would increase the ammoxidation rate. An investigation of the influence of the ammonia partial pressure on the ammoxidation of toluene showed that the increase in the ammonia concentration led to an increase of the reaction rate over V-Sb-Bi-O catalysts. It was shown that the number of basic sites was increased and the number of acidic sites decreased at the same time [16]. Other work by the same group showed that the introduction of small amounts of alkaline metals or alkaline-earth metals to a V-Sb-Bi-O catalyst led to increase of the toluene ammoxidation rate (referred to as rate of oxidative ammonolysis) [17]. Doping of the catalyst by these compounds led to increase of the basicity and decrease of the acidity as measured by adsorption of benzoic

R Cδ − δ +

H

A+ B - A + B -R C - H +


17

acid and butylamine. At low dopant amounts the increase of the reaction rate is linear with the increase of the basicity as shown in Figure 2.3. Stronger increase of the oxygen nucleophilicity would lead to lower activity, since surface dehydroxylation then becomes too slow [18].

Figure 2.3:Figure 2.3:Figure 2.3:Figure 2.3: Relation between acidity and basicity of the catalyst and the toluene ammoxidation rate. Data from Guseinov et al. [16].

Addition of K2O or BaO did not lead to observable changes in the oxygen bond energy with the catalyst neither to changes in the oxidation state of the catalyst. Only the concentration of basic sites of the catalyst increased [19]. Therefore, heterolytic C-H bond breaking is probable over alumina supported V-Sb-Bi-O catalysts. A negatively charged benzyl intermediate is

proposed to form after this heterolytic α-hydrogen abstraction [20]. The first step in the toluene ammoxidation thus writes as shown in Scheme 2.2.

Scheme 2.2: Scheme 2.2: Scheme 2.2: Scheme 2.2: Toluene ammoxidation over V-Sb-O catalysts according to the group of Rizayev [20].

By performing pulse experiments Niwa et al. [21] also found evidence for heterolytic C-H rupture. They observed nitrile production when toluene was pulsed after ammonia treatment of the V2O5/Al2O3 catalyst, or when they pulsed sequentially ammonia and toluene. When toluene was admitted to the catalyst prior to the ammonia pulse no nitrile production was observed. These authors, however, supported a mechanism in which

6

7

8

9

2 2.5 3 3.5 4

A cid ity /b as ic ity [m olA cid ity /b as ic ity [m olA cid ity /b as ic ity [m olA cid ity /b as ic ity [m ol ad sad sad sad s /m/m/m/m 2222 ]]]]

Ra

te [

Ra

te [

Ra

te [

Ra

te [·1

01

01

01

0-1

5-1

5-1

5-1

5 m

ol

mo

l m

ol

mo

l to

lto

lto

lto

l/(m

/(m

/(m

/(m

2 222·s

)] s)]

s)]

s)]

Basicity[C6H5COOH/m2]

Acidity[C4H9NH2/m2]

6

7

8

9

2 2.5 3 3.5 4

A cid ity /b as ic ity [m olA cid ity /b as ic ity [m olA cid ity /b as ic ity [m olA cid ity /b as ic ity [m ol ad sad sad sad s /m/m/m/m 2222 ]]]]

Ra

te [

Ra

te [

Ra

te [

Ra

te [·1

01

01

01

0-1

5-1

5-1

5-1

5 m

ol

mo

l m

ol

mo

l to

lto

lto

lto

l/(m

/(m

/(m

/(m

2 222·s

)] s)]

s)]

s)]

Basicity[C6H5COOH/m2]

Acidity[C4H9NH2/m2]

H-CH2HN2-........

O2-Mn+ Mn+

H2N- -CH2

Mn+O2-Mn+

H2N• •CH2

O2-M(n-1)+ M(n-1)+

Chapter 2

18

benzoate ions were formed, rather than amine-like intermediates. Ammonium ions were formed upon interaction of the hydrocarbon with the catalyst.

2.4 Nature of aromatic reaction intermediate The nature of the intermediate during toluene ammoxidation is under debate since the first experiments were described in literature. Many studies have been described to elucidate in more detail the nature of the alkylaromatic intermediates formed, mostly based on infrared spectroscopy. Due to the presence of many components that are infrared active, spectral bands greatly overlap. The interpretation of the infrared spectra under ammoxidation conditions, therefore, is very difficult. In particular, the presence of water, one of the reaction products, complicates data interpretation. The fact that species have been detected by IR does not mean a priori that these species are involved in the mechanism. Nevertheless, many attempts were made to describe the nature of the intermediate based at least partly on infrared spectroscopy.

Table 2.3: Table 2.3: Table 2.3: Table 2.3: IR experiments by Azimov et al. [20]

SpeciesSpeciesSpeciesSpecies Band Band Band Band [cm[cm[cm[cm----1111]]]]

AssignmentAssignmentAssignmentAssignment Intensity behaviour Intensity behaviour Intensity behaviour Intensity behaviour upon thermal treatment.upon thermal treatment.upon thermal treatment.upon thermal treatment.

Adsorbed benzyl fragments

1430 δ(CH2) Increases at 20-100 ° C

Benzylimine 1660 ν(C=N) Increases at 150-175 ° C Decreases at T>200 ° C

Coordinated benzonitrile

2280 ν(C≡N) Increases at 150-250 ° C Decreases at T>300 ° C

H-bound benzonitrile 2240 ν(C≡N) Only at T>250 ° C

Benzamide ion 1430 νs(CON) Increases at 150-200 ° C

Benzamide ion 1565 νas(CON) Shift towards νas(COO) at T>250 ° C

Benzoate ion 1420 νs(COO) Increase at T>250

Benzoate ion 1540 νas(COO)

Spectroscopic evidence for the heterolytic C-H rupture mechanism supported by the group of Rizayev was found by Azimov et al. [22]. They detected benzyl-species when V-Sb-Bi-O was treated with an ammonia-oxygen mixture after toluene adsorption. Treatment in either oxygen or


19

ammonia alone did not lead to the presence of these benzyl species, indicating that the degree of surface reduction determines the presence of benzyl species. Table 2.3 summarises the results of the IR experiments related to the ammoxidation of toluene over alumina supported V-Sb-Bi-O, as reported by the group of Rizayev [20,22]. Toluene was adsorbed and heated under NH3/O2 to 400 ° C. The increase of imine (150-175 ° C) and amine (150 ° C and higher) structures at increasing temperature may indicate an amine pathway. However, the intensity of the benzoate bands also increased above 250 ° C. This was explained by the increase of total combustion reactions at higher temperature. The formation of benzoate species was also reported by Haber and Wojciechowska [23]. They performed toluene ammoxidation over MgF2 supported vanadia catalysts. After adsorption of toluene at 400 ° C they observed IR bands at 1410 cm-1 and 1550 cm-1, assigned to benzoate-like species. These intermediates were only found over a freshly calcined catalyst. When the catalyst was treated in NH3 no bands that could be assigned to benzoate species were observed. These benzoate intermediates were believed to react very fast with ammonia. No distinction was made between a benzoate ion and a benzoate complex based on the experiments. The benzoate species that were proposed have been sketched in Figure 2.4.

Figure 2.4:Figure 2.4:Figure 2.4:Figure 2.4: Benzoate intermediates as proposed in [23].

Niwa et al. found the formation of surface benzoate ions when they adsorbed xylenes [21] or toluene [24] on vanadia by IR (bands at 1550 cm-1 and 1436 cm-1). After adsorbing p-xylene and p-tolualdehyde they found very similar IR bands. The assignment of the bands was confirmed by adsorption of deuturated p-xylene. A decrease in the wavenumbers by ca. 20 cm-1 was observed. When they contacted the catalyst with ammonia the

VOO

C

V O V

O O

C

Benzoate ion Benzoate complex

Chapter 2

20

intensity of these bands decreased and benzonitrile was formed. They found similar results when adsorbing substituted toluene on this catalyst or when using titania-supported vanadia as catalyst. The occurrence of an oxygen containing intermediate is reported most frequently in the literature. Besides benzoate ions the same group detected more stable intermediates containing carbonyl groups [25]. The carbonaceous intermediate was supposed to be stabilised on the support material as benzoate (from toluene adsorption [26]) or methylbenzoate (from xylene [21]) species. The greater stability of the carbonyl-like structures, however, does not suggest transformation of these carbonyl groups to benzoate intermediates. The occurrence of aldehyde-like intermediates, on the other hand, is consistent with the so-called aldehyde mechanism. This mechanism can be generalised to the following reaction steps:

Scheme 2.3: Scheme 2.3: Scheme 2.3: Scheme 2.3: Aldehyde pathway for toluene ammoxidation.

The exact form of the nitrogen containing intermediate that is formed upon ammonia adsorption will be discussed in Section 3 and the role of gaseous oxygen in Section 4. Evidence for benzoate and benzaldehyde-like reaction intermediates was also reported by Busca et al. [9]. Using titania-supported vanadia catalysts they found initial adsorption of toluene at room temperature as benzyl species, as shown by IR. Ring vibrations were observed at 1604 cm-1 and 1494 cm-1. Upon heat treatment the aromatic ring was kept intact, but the methyl group transformed into a carboxylate ion as evidenced by the occurrence of strong absorption bands in the 1600-1500 cm-1 region and in

the 1450-1400 cm-1 region. A band present at 3070 cm-1 (νCH) and two sharp bands near 1600 cm-1 were used to identify the adsorbed toluene species as benzoate ion [27]. When ammonia was present the intensity of

O2 + 2 * 2 Oads

NH3 + Oads NHads + H2O

C6H5CH3 + * C6H5CH3,ads

C6H5CHOads + NHads C6H5CN + H2O

+ +C6H5CH3,ads 2 Oads C6H5CHOads H2O


21

this band strongly decreased, whereas the band was strong during toluene/oxygen adsorption. This indicates that the benzoate-species is involved only in the oxidation of toluene. The spectra obtained were very similar to those reported by Van Hengstum et al. [28] who studied toluene oxidation over V-Ti-O. It was proposed that this benzoate ion was involved in oxidation of the methyl group or even in complete oxidation of the aromatic ring, since no nitrogen insertion species was present in this experiment. When adsorbed toluene was heated in oxygen containing conditions benzaldehyde was formed upon heating. This was evidenced by adsorption of benzaldehyde itself, which gave a very similar IR spectrum. Though the mechanism of toluene ammoxidation is often assumed to be very similar to that of selective toluene oxidation these experiments did not prove that benzaldehyde-like intermediates are really involved in the ammoxidation reaction as intermediate. In fact, when coadsorption of toluene and ammonia was performed no clear evidence could be found for the formation of a benzaldehyde-like intermediate. The authors [9] assign the obtained IR spectrum to adsorbed benzylamine, but the spectrum is very complicated. On the other hand when benzaldehyde was adsorbed on an ammonia covered surface the bands assigned to coordinated ammonia (1610 cm-1 and 1230 cm-1) decrease with a corresponding increase of a band at 2270 cm-1, which is assigned to coordinated benzonitrile, showing that benzaldehyde can be converted by ammonia to benzonitrile. Flow reactor studies by the same authors [5,10] show that using benzaldehyde as feedstock under ammoxidation conditions leads to formation of benzonitrile in high yields, as shown in Table 2.4. Formation of benzonitrile from the reaction of benzaldehyde with ammonia and oxygen was also found in an IR study by Murakami et al [26].

Table 2.4: Table 2.4: Table 2.4: Table 2.4: Ammoxidation reaction of possible reaction intermediates

SubstrateSubstrateSubstrateSubstrate Substrate conversionSubstrate conversionSubstrate conversionSubstrate conversion [mol%][mol%][mol%][mol%]

Benzonitrile selectivityBenzonitrile selectivityBenzonitrile selectivityBenzonitrile selectivity [mol%][mol%][mol%][mol%]

Toluene 61 85 Benzaldehyde 100 95 Benzoic acid 100 54 Benzylamine 100 70

V-Ti-O catalyst; T= 310 ° C [10].

Chapter 2

22

The data reported in Table 2.4 suggest the feasibility of benzylamine and benzaldehyde as possible reaction intermediates. The fact that benzoate intermediates were detected by IR under reaction conditions probably relates to proceeding of unselective total oxidation reactions. The selectivity to benzonitrile from benzoic acid feedstock is significantly lower than the selectivity to benzonitrile when feeding toluene. Benzaldehyde and benzylamine were detected under toluene ammoxidation conditions at low residence times and low partial pressures of ammonia (0.025 atm) and oxygen (0.003 atm). Benzylamine intermediates were not found at low residence times by Otimari et al. [29], who used similar catalysts. These authors applied higher oxygen partial pressures in their experiments. They found the presence of benzaldehyde in the reaction mixture under these conditions. Benzaldehyde reaction intermediates are observed in toluene ammoxidation reactions by several other authors, using different catalysts. Over SAPO and VAPO catalysts Kulkarni et al. [30] detected benzaldehyde as reaction product in low yields. They did not find the presence of benzylamine or benzoic acid. Over Cu/ZSM-5 Kim et al. [31] found the formation of benzonitrile from benzaldehyde in high yields. In the absence of ammonia benzene was formed, in similar amounts as during toluene oxidation. These authors propose a mechanism in which a benzaldehyde-like cation acts as the selective intermediate towards benzonitrile as well as the intermediate for benzene production. This benzaldehyde-like cation is formed from a benzyl-cation, which is formed upon toluene adsorption. The authors, however, did not take into account the formation of combustion products, which were produced in significant amounts. Moreover, the presence of small amounts of benzaldehyde in the reaction mixture could possibly be caused by the occurrence of mild toluene oxidation to benzaldehyde. More extensively Martin et al. [32] studied the ammoxidation of toluene over VPO catalysts by FT-IR and TAP experiments. By TAP they measured the transient responses of benzaldehyde and benzonitrile. They demonstrated that benzaldehyde evolved as first reaction product. At longer contact times benzonitrile was found as the main product. It was found that similar IR spectra were obtained when feeding toluene and ammonia as feedstock as when feeding benzaldehyde and ammonia as feedstock. Therefore, it can be concluded that benzaldehyde leads to the formation of benzonitrile over VPO catalysts. It was found that the amount


23

of activated ammonia controlled the formation of benzonitrile. If the amount of activated ammonia was low high benzaldehyde selectivities were found, whereas higher amounts of activated ammonia led to higher yields of benzonitrile [33]. Besides benzylamine and benzaldehyde–like structures additionally also benzylimine was reported as reaction intermediate in the ammoxidation of toluene. The group of Rizayev found benzylimine-like species by spectroscopic investigations [22,34] in the ammoxidation of toluene over V-Sb-Bi-O catalysts. Amine-like species were not detected, because of their high reactivity. Benzylimine, however, is much more reactive under ammoxidation conditions than benzyl amine is. Formation of imines could occur, but is probably not involved as rate determining step during toluene ammoxidation. Benzylimine intermediates were also reported by Den Ridder [35] who reports the formation of benzylimine from benzaldehyde intermediates. It was found that a homogeneous reaction of benzaldehyde to produce benzylimine could occur at temperatures below 125 ° C. It should be noted though, that the ammoxidation reaction temperature is significantly higher.

3. Ammonia activation Ammonia plays multiple roles in the ammoxidation of toluene. It is the source of nitrogen atoms, it also reduces the catalyst and/or it is adsorbed on the catalyst blocking sites that otherwise could have weakly bound oxygen, which could lead to total oxidation. Guseinov et al. [36] found in the absence of ammonia mainly the occurrence of total oxidation reactions over V-Sb-O catalysts. Admission of ammonia to the toluene/oxygen mixture led to the production of benzonitrile (selectivity of 95%). Additionally, the total conversion of toluene was increased significantly. This was explained by the formation of new basic sites upon ammonia adsorption [20,37]. They found by infrared spectroscopy that Lewis acid sites were blocked by ammonia and partially dehydrogenated ammonia species such as NH and NH2 were formed [20,22 ,38]. Contrary, Busca et al. [9] and Niwa et al. [21,24] did not detect partially dehydrogenated ammonia species on titania-supported or alumina-supported vanadia catalysts respectively. Niwa et al. [21] explained ammoxidation of toluene by the formation of NH4

+ species

Chapter 2

24

interacting with toluene along the methyl group. They found indirect proof for the formation of NH4

+ by the decrease of the 3500 cm-1 OH band upon ammonia adsorption. Recently Centi and Perathoner [4] reviewed the role of ammonia adspecies in several reactions, among which was the toluene ammoxidation reaction. They described four main ways in which ammonia can be bound to the metal oxide surface, based on IR experiments [39]: 1. Bonding via a hydrogen atom to a surface oxygen or the oxygen of a

surface OH-group 2. Bonding via the nitrogen atom to a hydrogen atom from a surface OH

group (Brønsted acid site), to form an ammonium ion 3. Coordination to an electron deficient metal atom (Lewis acid site) 4. Dissociative chemisorption of the ammonia atom to form a NH2 or NH

species and a (or two) OH group(s) with a surface oxygen atom Centi et al. [40] studied the formation of NH4

+-ions (“Type 2 species”) vs. the NH3 coordinated to Lewis acid sites by IR, using a V-Sb-O catalyst. They found a decrease of both species as function of temperature. At temperatures from T= 200 ° C and higher no NH4

+-ions were present at the surface, whereas NH3 coordinated to the Lewis site was still present up to temperatures of 400 ° C. This study was executed in vacuo, thus excluding the influence of gas-phase ammonia, water and other compounds, which are present under normal ammoxidation reaction conditions. They also showed that the presence of other substances could influence the nature of ammonia adspecies. The involvement of NH4

+-ions in the ammoxidation of toluene thus cannot be excluded from the reaction mechanism a priori. For example at room temperature ammonia adsorbs mainly as NH4

+-ions on VPO, as shown by Busca et al. [41]. Upon heating they also observe decrease of the relative amount “Type 2 species” and increase of the relative amount of “Type 3 species”. Water presence, on the other hand, leads to the transformation of NH3 coordinated to the Lewis acid sites to NH4

+. The formation of metal-imido (e.g.. Mo=NH) and metal-amino (e.g. Sb-NH-Sb) species was discussed extensively for the (amm)oxidation of propylene over Bi-Mo and Fe-Sb catalysts by the group of Grasselli [42,43]. These metal-imido and metal-amino species have been accepted quite


25

generally as the only surface species responsible for ammonia insertion into olefins. Also for the ammoxidation of toluene dehydrogenated ammonia species were reported in the literature. For example Andersson et al. [44,45] indicated the formation of V=NH and Cu=NH species, using vanadium-oxide and barium-cuprate catalysts for the ammoxidation of toluene. Spectroscopic evidence for the presence of these metal-imido and metal-amino species to be the reactive ammonia species under reaction conditions, however, is scarce. Therefore, attention should be given to the nature of the ammonia adspecies under reaction conditions and the participation of other forms of activated ammonia should not be excluded. For example the presence of NH4

+-ions influences the catalyst acid-base properties. Since no clear evidence is available to answer the question whether homolytic or heterolytic C-H rupture occurs this can play a main role in the ammoxidation of toluene. The chemisorption of ammonia can lead directly to modification of the Brønsted as well as the Lewis acidity of the catalyst. Also the nucleophilic character of the oxygen surface sites is increased by the NH3 adsorption properties. Indirectly, also other effects can play a role. For example the reduction of acidity by ammonia adsorption can lead to easier reduction of products or intermediates with a basic character such as nitriles (or alkenes). Too strong Lewis acid sites, additionally, can lead to total oxidation reactions. Toluene ammoxidation reactions showed an optimum in ammonia concentration with respect to the benzonitrile selectivity [5,45], indicating the effect of change of acid-base properties over vanadia catalysts. Similar relations were found for the ammoxidation of propane to acrylonitrile. Centi and Perathoner [46] describe a mechanism for this reaction over VPO pre-adsorbed with ammonia in the presence of oxygen. At high NH3 coverage the propylene yield was high and the acrylonitrile yield moderate. No combustion products were produced. At low NH3 coverages hardly any other product than COx was produced. NH3 coverages in between led to optimum acrylonitrile yield. It was concluded that propane is converted via two pathways. Propylene is formed by oxidative dehydrogenation and is strongly co-ordinated to the Lewis acid sites. In the absence of oxygen a propylene amine intermediate is formed. This intermediate can react with amido-like (NH2

-), which is formed by dissociatively adsorbed ammonia and slowly forms acrylonitrile. In the presence of oxygen an acrylate intermediate is formed as sketched in Figure 2.5. This intermediate reacts

Chapter 2

26

faster with a nearby NH4+-ion to give

water and acrylonitrile as reaction products. In the case ammonia occupies all sites chemisorption of oxygen is inhibited and the acrylate intermediate cannot be formed. This is the situation at very high ammonia coverages. Where no nearby NH4

+-sites are present, the acrylate intermediate reacts to carbon oxides [46].

Niwa et al. [24] find evidence for the presence of ammonium cations under toluene ammoxidation conditions. They observe by IR the consumption of OH bands after admission of ammonia. Similarly, they observe the formation of benzonitrile. The OH bands were believed to originate from the support. Ammonium cations were also reported by other groups [47] when ammonia was adsorbed on titania supported vanadia monolayer catalysts. When co-adsorption of toluene and ammonia was performed Busca at el. [9] indeed detected the formation of ammonium cations by infrared spectroscopy. Toluene adsorbed similarly as on clean vanadia surfaces. The concentration of the ammonium cations was found to decrease slightly when co-adsorption of toluene and ammonia was performed at higher temperature (470 K). The intensity of the band assigned to chemisorbed ammonia (1230 cm-1) was strongly reduced after toluene adsorption at 570 K. New bands were observed at 3390 (a shoulder), 3260 and 1642 cm-1. These bands were assigned to stretching and deformation of NH2 groups. The authors conclude that ammonia coordinated to Lewis acid sites (IR bands at 1230 and 1610 cm-1) is involved in the formation of benzonitrile. As will be discussed in Section 5.2 the authors [9] support a mechanism in which adsorbed benzyl radicals are stabilized as benzylamine intermediates, which react with ammonia coordinated to Lewis acid sites (see Scheme 2.8). Besides selective insertion of N atoms into the hydrocarbon to produce benzonitrile also ammonia combustion occurs. Since the ammonia partial pressure was found to have an important effect on the production of benzonitrile it is important to avoid ammonia combustion as much as

Figure 2.5: Figure 2.5: Figure 2.5: Figure 2.5: Active site during propane ammoxidation over VPO

C CCH

H

O

O O

V V

O

O P

H

NH4+


27

possible. Based on a kinetic evaluation Cavalli et al. [10] describe the role of ammonia in terms of (1) increase of the benzonitrile selectivity at increasing ammonia concentration and (2) decrease of the activity at increasing ammonia concentration. The increase of selectivity was found to correlate well with the amount of ammonia available in the reaction mixture. Part of the ammonia was combusted to N2 and N2O. The decrease of the activity was explained by a competitive adsorption effect: Ammonia competes with toluene for the same active sites. Too high ammonia concentrations, therefore, lead to depletion of the active sites. The active site for toluene ammoxidation was found to be V(IV). The concentration of V(IV) correlates linearly with the activity in toluene ammoxidation [48]. V(V) sites on the other hand catalyse the combustion of ammonia. A third role of ammonia is to stabilise the intermediate of toluene activation. This explains that V/TiO2 catalysts are unselective in toluene oxidation [49].

4. Catalyst reoxidation With respect to the oxygen inserting species in the alkylaromatic (amm)oxidation mechanism a Mars–Van Krevelen mechanism [50] is generally accepted. The hydrocarbon is oxygenated by lattice oxygen. In the presence of ammonia an aromatic nitrile is produced from the oxygenated intermediate as described above in Section 2.4. In a separate reaction step the catalyst is reoxidized by gaseous oxygen. The mechanism is drawn schematically in Figure 2.6.

Figure 2.6: Figure 2.6: Figure 2.6: Figure 2.6: Consumption of lattice oxygen during toluene ammoxidation

CH3

[Intermediate]

CN

NH3

1/2O2

O

-3 H2O

Chapter 2

28

Several groups have provided experimental evidence for the occurrence of a Mars–Van Krevelen mechanism in alkylaromate ammoxidation. Pulse experiments by Murakami et al. [51] indicate the consumption of surface oxygen when producing benzonitrile over vanadia and over alumina supported vanadia catalysts. They pulsed toluene–ammonia–toluene–ammonia etc. and measured the benzonitrile production. Benzonitrile is formed after pulsing ammonia, but the formation of benzonitrile strongly decreases after a series of pulses. Admission of air and toluene to the catalyst can reactivate the alumina supported vanadia catalyst. Toluene thus forms an oxygen containing intermediate that reacts with gaseous ammonia to form benzonitrile. The intermediate formed is stable on alumina-supported vanadia, but reacts on unsupported vanadia to carbon oxides. Niwa et al. [52] ascribe the more facile reoxidation to faster oxygen diffusion through the thin vanadia layer on alumina, compared to bulk vanadia. Similar mechanisms were suggested for the ammoxidation of toluene over SiO2-Al2O3, SiO2-TiO2 and ZrO-SiO2. During the course of reaction the initially very low activity of the catalysts is increased. This is explained by the formation of a reactive carbonaceous layer on the catalyst [25]. Similar pulse experiments were performed by the group of Rizayev [53]. They also found benzonitrile production after the ammonia pulse during the pulse sequence: toluene–ammonia. The surface is reoxidized by gaseous oxygen, but this surface reoxidation is not rate-determining. Haber et al. [23] used pulse experiments over V/MgF catalysts. They found that toluene/ammonia pulses led to the production of benzonitrile. After a number of sequential toluene/ammonia pulses, however, the activity of the catalyst dropped. Reoxidation by gaseous oxygen led to regain of the activity. Also in flow experiments evidence for the occurrence of a Mars–Van Krevelen mechanism was found. Benzonitrile was produced over V/Bi catalysts when toluene and ammonia was fed to the catalyst. The benzonitrile yield dropped as a function of time on stream, but could be returned to its initial value after reoxidation of the catalyst in gaseous oxygen [54]. The importance of oxygen in the feedstock, however, is not only limited to reoxidation -and thus regeneration- of the active sites. Martin et al. [55]


29

showed that for VPO catalysts the vanadium oxidation state has a very delicate role. Using TAP they performed sequential pulse reactions of a typical ammoxidation mixture. In between the ammoxidation feedstock pulses the catalyst oxidation state was adjusted by either pulsing oxygen or ammonia. CO2 was formed predominantly over oxidized sites. The total activity of the catalyst was found to correlate with the oxidation state as well. As a result optimum benzonitrile yield is obtained at intermediate oxidation state. Indeed very small differences in the bulk oxidation state were detected by titration. Therefore the concentration of oxygen in the feedstock could have a significant influence on the ammoxidation reaction. The optimum in nitrile production at intermediate V oxidation state is very similar to the optimum in acrylonitrile yield from propane ammoxidation found by Centi and Perathoner [46]. As already discussed in Section 3 the oxidation state was also influenced by the surface ammonia concentration.

5. Toluene ammoxidation reaction schemes

5.1 The propylene ammoxidation mechanism Many reaction schemes for the ammoxidation of toluene proposed in literature are based upon the ammoxidation of propylene. This process has been patented already in the 1950s by SOHIO [56] and Distillers [57]. The process is applied industrially on a large scale. Nowadays, over 5.000.000 tons of acrylonitrile is produced via the SOHIO Process [58]. Because of its industrial importance [59] this reaction has been studied in great detail. General agreement in literature exists about the mechanism. The early stages of research were summarized in 1970 [60], but a more complete review was published in 1981 [42]. The reactions of propylene oxidation to acrolein and propylene ammoxidation to acrylonitrile occur simultaneously over bismuth molybdate catalysts. Most experimental evidence for the mechanism, however, is deduced from propylene oxidation reactions. Oxygen insertion occurs via a Mars-Van Krevelen-type mechanism as shown by Keulks [61] by 18O labeling experiments. The first step of the

reaction is the abstraction of a α-hydrogen in order to form an allylic intermediate, as was shown by deuterium and 14C-labeling experiments [62,63]. This intermediate then undergoes abstraction of a second hydrogen atom and insertion of oxygen or nitrogen (while abstraction of a third

hydrogen takes place) to form acrolein resp. acrylonitrile. α-Hydrogen

Chapter 2

30

abstraction is the rate determining step of the reaction, as was shown by measurement of the kinetic isotope effect after deuterium labeling [63,64].

Scheme 2.4: Scheme 2.4: Scheme 2.4: Scheme 2.4: O-allyl and N-allyl surface species in the (amm)oxidation of propylene (from [66]).

+ NH3

- H2O

MoO3

O

O

Mo

O

O

HN

Mo

O

O

Mo

OO

H

O

Mo

OHN

H

Oxidation Ammoxidation

O

H+ MoO OH

[O]

MoN

OHO

H

O

CN + MoO2 + H2O

MoN OH

OH


31

Reoxidation of the catalyst lattice occurs at a higher rate [65]. By directly reacting the allylic intermediates, which were formed by reacting

azopropene, Burrington and Grasselli [66] showed the formation of a π-

allyl surface complex, which is quickly converted to a σ-O-allyl species (allyl molybdate ester) upon interaction with propylene. Davydov et al. [67]

found infrared evidence for the presence of σ-allyl and π-allyl complexes in the oxidation of propylene over copper, chromium and molybdenum based catalysts. Allyl radicals are not involved in the reaction mechanism as selective intermediates, as shown by quantum chemical calculations [68].

Formation of a σ-O-allyl complex via a π-allyl complex seems to be more probable in the oxidation of propylene. In a later publication, Burrington et al. [69] described a more complete reaction scheme for the oxidation of propylene. By substituting the Mo=O bond by a Mo=NH bond nitrile

formation was explained. As a result σ-N-allyl is formed instead of a σ-O-allyl intermediate. This reaction scheme is shown in Scheme 2.4.

Scheme 2.5: Scheme 2.5: Scheme 2.5: Scheme 2.5: Generalized alkene ammoxidation reaction scheme [58].

In a later review the propylene ammoxidation reaction scheme was generalized. Bimetallic (or multi-component) catalysts show much higher

NH3

H2O

CH2 CH

CH3

3 M [ ]

2 [ ] [ ]

[O]2-2 [O]2-

M1

OM2

O

M2

NHM1O

H

H

M1

OM2

NHM1 M2

[ ] [ ]

CH2 CH

CN

+ 2 H2O

CH

CH3CH3

M1

OHM2

NH

Active siteActive siteActive siteActive site

O21½

Reoxidation siteReoxidation siteReoxidation siteReoxidation site

Ammoxidation siteAmmoxidation siteAmmoxidation siteAmmoxidation site

Reduced siteReduced siteReduced siteReduced site

Allylic surface complexAllylic surface complexAllylic surface complexAllylic surface complex

Chapter 2

32

acrylonitrile yields. Therefore, Scheme 2.4 is too simple. A generalized propylene ammoxidation scheme for bimetallic metal oxides is described by Grasselli [58] and is shown in Scheme 2.5. This reaction scheme can also be applied to monometallic metal oxides as shown for antimonate catalysts [43].

5.2 The ammoxidation of toluene For the ammoxidation of toluene over molybdenum oxide catalysts Scheme 2.4 rewrites to Scheme 2.6 [70].

Scheme 2.6: Scheme 2.6: Scheme 2.6: Scheme 2.6: Toluene ammoxidation over supported MoO3 catalysts [70].

Sanati and co-workers [71] describe a reaction mechanism that was based on DRIFTS experiments on V-Ti-O at temperatures not higher than 300 ° C. They propose a reaction mechanism that involves the presence of all hydrocarbon intermediates discussed before. Toluene adsorbs and reacts with adsorbed or lattice oxygen to form a benzyl fragment and a surface OH species. Further reaction of the benzyl fragments with an oxygen atom

C

H

Mo

N O

OO

OH

[O]CN + MoO2

+ H2O

Mo

NH O

OO

O

CH2

•s-complex

••

C

H

Mo

N OH

OO

OH

CH3

Mo

O O

OO

O

CH2

MoO3 + H2ONH3 Mo

O NH

OO

O

CH2


33

leads to formation of a C6H5-CH2O species, which in turn reacts with the OH species to form water and a benzaldehyde like species. This benzaldehyde-like species can desorb or can react with an oxygen atom to form adsorbed benzoic acid that can also desorb. If the benzaldehyde-like fragment reacts with two oxygen atoms an adsorbed benzoate species and a hydroxyl group is formed. Ammonia reacts as an adsorbed amine (NH2) species, which is formed by reaction with oxygen, again producing a hydroxyl group. Reaction of the adsorbed benzaldehyde with the adsorbed NH2 group forms a C6H5-CH(NH2)-O species, which is converted to an adsorbed imine (again with the formation of a hydroxyl group). This adsorbed imine reacts with oxygen to benzonitrile and water. The benzylimine species, however, was not observed in the IR spectra. The other pathway discussed is a reaction of the benzoate intermediate with the NH2 group that was formed by ammonia adsorption to form benzonitrile as sketched in Scheme 2.7.

Scheme 2.7: Scheme 2.7: Scheme 2.7: Scheme 2.7: Benzonitrile formation from benzoate-like intermediate according to Sanati et al. [71].

As already discussed in Section 2.4 benzaldehyde intermediates are not the only reaction intermediates observed the in literature. Cavalli et al. [5] propose another mechanism, which involves the presence of an amine like intermediate. This so-called amine mechanism, or dehydrogenation mechanism involves the presence of amine-like intermediates, which were believed to be the species responsible for benzonitrile formation. This

(NH2)C

O

O

ads

C

O

O

δ+[-NH2] +

(NH2)C

O

O

ads

CHN

-O

+ ads

ads

[-OH]

CHN

-Oads

NC

ads

+ ads[-OH]

Chapter 2

34

mechanism consists of stepwise dehydrogenation of the substrate forming amine- and imine-like intermediates. A generalized form of the mechanism was discussed, as show in Scheme 2.8.

Scheme 2.8: Scheme 2.8: Scheme 2.8: Scheme 2.8: Reaction mechanism according to Cavalli et al. [10]

As discussed earlier these authors neglected the reaction pathway via benzoic acid. Based on the high benzonitrile yields when the amine or aldehyde were fed they proposed that both pathway 3 and pathway 4 are possible over titania-supported vanadia. In both pathways the highly reactive radical is attacked by adsorbed ammonia or oxygen. By infrared spectroscopy it was shown that the intensity of the bands assigned to ammonia coordinated to Lewis acid sites decreased and coordinated benzonitrile was formed upon benzaldehyde adsorption on an ammonia-covered V-Ti-O catalyst. This was also observed upon toluene adsorption on an ammonia-covered catalyst. Pathway 3, therefore, seems plausible. On Reaction pathway 2 seems to be less probable, since carbene and imine bi-radicals were not detected in any spectroscopic investigations. The mechanism shows similarities with the reaction mechanism for propylene ammoxidation over Bi-Mo-O catalysts, as was described by Grasselli et al. [72]. Total combustion products can be formed from all intermediates proposed, but not from benzonitrile [73]. Ammonia stabilizes the reaction intermediate. Combustion of ammonia to nitrogen can occur over different sites. This separate reaction leads to lower benzonitrile yields. A mechanism similar to pathway 3 was proposed by the group of Martin and Lücke [2] for the ammoxidation of toluene to benzonitrile over VPO catalysts. They detected the presence of benzaldehyde as short-living reaction intermediate and found also IR evidence for the presence of benzoate surface species. Benzyl amine was not detected as intermediate. This species was believed to react rapidly to benzoate and ammonium by hydrolysis. The reaction mechanism is shown in Scheme 2.9. Toluene is believed to adsorb as benzaldehyde-like intermediate on the catalyst. This benzaldehyde-like species could be converted to a benzoate-like species by the nearby oxygen atoms attached to the vanadium centres. This reaction,

CH..

1

2 34CH3 CH2

.CHO COOH CN

CH NHCH2NH2


35

however, was not included in the mechanism according to Martin et al. [2]. The benzaldehyde intermediate reacts to an benzyl-imine intermediate with desorption of water. Similar to the mechanism for propane ammoxidation over the same catalyst the first step is oxygen addition to an organic intermediate, which is attacked by adsorbed ammonium anions in a subsequent reaction step.

Scheme 2.9: Scheme 2.9: Scheme 2.9: Scheme 2.9: Reaction mechanism of toluene ammoxidation over VPO catalysts according to Lücke et al. [2].

6. Conclusions Up to now, there is no general agreement in literature considering the exact reaction mechanism for the ammoxidation of toluene. Many authors, though, agree on the presence of an oxygenated adsorbed organic intermediate as a first reaction step. Oxygen is supplied from the catalyst

PO

O

O

P

VO

P

OOO

VO

PP

P

O

O

OV

O

OOO

VNH2

O

NH4

P

O

OO

VO

OO

O

VH4N

O

NH2

P

O

O

OV

O

OO

O

VH4N

HCH2

O OV

O

OO

O

VH4N

O

H

HO O

VO

OOO

VOH

NH

O OV

O

OOO

VH4N

NH3

CH3

+O2

-H2O-H2O

CN

-H2O

+O2/NH3

+O2

Chapter 2

36

surface, implying a redox sequence according the Mars and Van Krevelen mechanism. The benzaldehyde- or benzoate-like oxygenated species then reacts with a nitrogen-insertion site to produce benzonitrile. The exact nature of the nitrogen-insertion site is presently not known precisely. Because of the lack of exact information about the reaction mechanism and about the rate limiting steps, it is presently not known what defines the optimal catalyst properties. The fact that most active catalysts consist of two- or multi-compound systems suggests that hydrocarbon activation and nitrogen insertion occur on different catalytic sites.

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3-8. 56. J.D. Idol, US Patent 2904580, 1959. 57. J.L. Barclay, J.B. Bream, D.J. Hadley, D.G. Stewart, Brit. Patent

876446, 1959. J.L. Barclay, J.B. Bream, D.J. Hadley, D.G. Stewart, US Patent

3152170, 1959. 58. R.K. Grasselli, Ammoxidation, in: Handbook of Heterogeneous

Catalysis, Eds. G. Ertl, H. Knözinger, J. Weitkamp, Vol. 5, VCH, Weinheim, 1997, pp. 2302-2326.

59. J. Haber, in: New Developments in Selective Oxidation by Heterogeneous Catalysis, Eds. P. Ruiz, B. Delmon, Stud. Surf. Sci. Catal., Vol. 72, Elsevier Science Publishers BV, Amsterdam, 1992, pp. 279-304.

60. J.L. Callahan, R.K. Grasselli, E.C. Milberger, H.A. Strecker, Ind. Eng. Chem. Prod. Res. Dev., 9(2), (1970), 134-142.

61. G.W. Keulks, J. Catal., 19, (1970), 232-235.


39

62. W.M.H. Sachtler, N.H. de Boer, in: Proc. 3rd Int. Congr. Catal., Eds. W.M.H. Sachtler, G.C.A. Schuit, P. Zwietering, North Holland Publishing Company, Amsterdam, 1965, 252-263.

63. C.R. Adams, T.J. Jennings, J. Catal., 2, (1963), 63-68. 64. C.R. Adams, T.J. Jennings, J. Catal., 3, (1964), 549-558. 65. J.F. Brazdil, D.D. Suresh, R.K. Grasselli, J. Catal., 66, (1980), 347-367. 66. J.D. Burrington, R.K. Grasselli, J. Catal., 59, (1979), 79-99. 67. A.A. Davydov, V.G. Mikhaltchenko, V.D. Sokolovskii, G.K. Boreskov,

J. Catal., 55, (1978), 299-313. 68. A.N. Orlov, S.G. Gagarin, Kinet. and Catal., 15, (1974), 1308-1312. 69. J.D. Burrington, C.T. Kartisek, R.K. Grasselli, J. Catal., 63, (1980), 235-

254. 70. J. Haber, M. Wojciechowska, J. Catal., 110, (1988), 23-36. 71. M. Sanati A. Andersson, J. Mol. Catal., 81, (1993), 51-62. 72. R.K. Grasselli, J.D. Burrington, J.F. Brazdil, Faraday Discuss. Chem.

Soc., 72, (1981), 203-223. 73. P. Cavalli, F. Cavani, I. Manenti, F. Trifirò, Ind. Eng. Chem. Res., 26,

(1987), 639-647.

41

Chapter 3 Screening of new toluene ammoxidation catalysts

Abstract A broad range of new alkylaromatic ammoxidation catalysts was prepared. Zeolite NaY or γ-alumina was used as matrix for different transition metal oxides. Toluene ammoxidation was used as test reaction. Zeolite samples were prepared from NaY by means of incipient wetness impregnation, ion exchange or chemical vapour deposition of metal carbonyls. All γ-alumina samples were prepared by means of incipient wetness impregnation. The influence of dopants was studied for vanadia based NaY catalysts. The selectivity towards benzonitrile increased; however, the catalyst activity was less after dopant addition. γ-Alumina supported molybdenum oxide showed high benzonitrile yields. High temperature ammonia treatment increases the benzonitrile yield significantly, indicating the importance of nitrogen containing species on the catalyst surface. The benzonitrile yield could be improved by doping this catalyst with vanadia. The stability of the catalyst samples under ammoxidation conditions was found to be an important parameter. Copper exchanged NaY catalysts were found to have high benzonitrile yields as well, but their activity decreases drastically during the first hours on stream. By performing the conversion of toluene to benzonitrile in the presence of NO and oxygen it was shown that nitrile formation does not occur from ammonium nitrate or ammonium nitrite intermediates, where presence of these intermediates might be proposed from selective catalytic reduction of NO.

1. Introduction Aromatic nitriles are widely used as solvents in organic reactions. In addition the nitrile group can be easily converted by means of hydrogenation, hydration or hydrolysis reactions to amines, amides, imines and other functional groups [1]. These functional groups are found in many intermediates in polymer and pharmaceutical industry [2]. Current commercial catalysts for aromatic (di-)nitrile production are based on supported V oxide, combined with other metal oxides [3]. For toluene ammoxidation similar catalysts have been patented. V is combined with elements such as Mo, Sb, Cr, Ti, Bi, P and other components [4,5]. Both

Chapter 3

42

supported and unsupported oxides are used. Usually alumina supported catalysts show good performances. Other supports such as zeolites, TiO2, SiO2, MgF2 have also been studied [6-10]. Numerous patents have been published in which the ammoxidation activity of mixed transition metal oxide is claimed; see for example [4,11]. A combination of V2O5 and Pt on SiC has been patented [12] also. Benzonitrile yields up to 90 percent are attainable.

V based catalysts are most widely reported to catalyse alkylaromatic ammoxidation, due to their high activity. Several groups have studied toluene ammoxidation over vanadia supported on titania [10,13], alumina [14,15], or zirconia [16]. For Mo oxide also the use of MgF as support has been reported [8]. Like for unsupported mixed oxide catalysts the benzonitrile selectivity can be increased by addition of Sb to alumina supported V catalysts [17]. Alkylaromatic ammoxidation over VPO catalysts has recently been studied in detail by the group of Martin [18,19]. The ammoxidation of alkylaromatics over zeolite-based catalysts has been studied less extensively, although copper loaded ZSM-5 [20-22] and other zeolites [23] have been used by several authors for the ammoxidation of different alkylaromatic substrates. Benzonitrile yields of over 80 % can be reached via ammoxidation of toluene in the presence of a substantial amount of water in the reactor feed. The application of other zeolite based metal oxide catalysts is reported less frequently, though earlier NaX zeolites containing Zn or Ag were reported to have moderate nitrile selectivity for toluene ammonolysis, i.e. when nitrile formation occurs in an ammonia flow and catalyst reoxidation occurs separately [6]. However, when hydrocarbon and oxygen is passed over the catalyst simultaneously, these zeolites as well as CrNaX, FeCrNaX, FeMnNaX give very poor yields to aromatic nitriles [2]. V-zeolites [24,25] and -zeotypes [26] have also been used for the ammoxidation of toluene and xylenes respectively. Zeolite based transition metal oxide catalysts generally yield lower amounts of selective oxidation products than alumina supported metal oxides do. Recently, however, Fe-Mo oxides stabilized in pentasil-type zeolites have shown high yields towards aldehyde reaction products in the oxidation of p-xylene [27]. Benzaldehyde has been proposed as selective surface intermediate for the production of benzonitrile from toluene by several groups [7,13,20,26,28]. The performance of these catalysts in alkylaromatic oxidations indicates the possibility for the use of zeolite-based catalysts to

Screening of new toluene ammoxidation catalysts

43

achieve high yields in selective oxidation reactions. In addition, Li and Armor recently reported highly active and selective zeolite based catalysts for the ammoxidation of ethane and other small hydrocarbons [29]. The research described in this chapter is aimed at the development of improved catalysts for ammoxidation of alkylaromatics. The vapour phase ammoxidation of toluene has been used as a test reaction. The use of zeolite-based catalysts has not been reported extensively so far. Therefore, in this research zeolite NaY was loaded with transition metals by means of ion exchange, chemical vapour deposition (CVD) or incipient wetness

impregnation. For comparison, γ-alumina supported toluene ammoxidation catalysts were prepared. Since NH3 activation plays a key role in ammoxidation reactions [14,18] activation of the catalysts was performed not only under oxidizing conditions, but also in NH3 atmospheres to produce MoxN catalysts. The effect of the zeolite matrix has been examined as well as dopant addition.

2. Experimental methods

2.1 Catalyst preparation and characterization Three different preparation methods were applied for the introduction of

transition metals into NaY and γ-alumina (Al): Ion-exchange (ie), impregnation (im) and metalcarbonyl sublimation (s). Catalysts are denotated according to the format Metal 1-Metal 2(preperation method, wt% Metal 1,wt% Metal 2)/support. The catalyst loadings are expressed as wt% of metal, unless otherwise mentioned. Conventional ion exchange was used to prepare Co(ie)/NaY, Cu(ie)/NaY and NH4(ie)/NaY. Nitrate salts were added to a NaY/water slurry and stirred overnight at room temperature. The applied NaY batch (PA 73022) was supplied by Akzo. The unit cell composition was Na55(AlO2)55(SiO2)137) as confirmed by A.A.S. After centrifuging and thorough washing, the

samples were dried at 110 ° C. A 250-425 µm sieve fraction was used for the catalytic tests.

All impregnated samples were prepared by incipient wetness impregnation. V(im)/NaY and V(im)/Al were prepared by dissolving NH4VO3 in water. The solution was heated until all NH4VO3 had dissolved. For the

Chapter 3

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preparation of promoted V(im)/NaY catalysts sequential impregnation was applied using solutions containing SbCl3, K2CO3, Bi(NO3)3∙ 5H2O and (NH4)2HPO4 respectively. In between the impregnation steps the catalyst precursors were dried at 110 ° C in order to remove the solvent. In the case

of the γ-alumina supported sample, NH4VO3 was added to a 2 M oxalic acid solution. Prior to impregnation the samples were pelleted, crushed and

sieved into a 250-425 µm sieve fraction. The supports were heated to 400 ° C in ambient air before reaction. V containing catalysts were prepared by sequential impregnation, due to the low solubility of NH4VO3. In between the two impregnations the catalyst precursors were heated to 110 ° C in order to remove the solvent from the pores. Mn(im)/Al was prepared from a manganese(II) nitrate solution, Mo/Al was prepared from (NH4)6Mo7O24 as described by Peeters et al. [30]. The catalyst was activated by heating at 500 ° C for one hour in flowing dry air (60 ml/min). Bi-metallic catalysts

were prepared by incipient wetness co-impregnation. γ-Alumina was supplied by Akzo (surface area of 205 m2/, pore volume 0.55 ml/g).

Mo(im)/Al and commercial MoO3 were used as precursors to prepare bulk and supported molybdenum nitride (MoxN and MoxN(im)/Al). The precursors were heated in a fixed bed reactor to 360 ° C at a heating rate of 10 ° C per minute in an NH3(1 vol%)/He mixture. The heating rate was then reduced to 1 ° C per minute and the sample was heated to 700 ° C. After keeping the sample at this temperature for one hour the sample was allowed to cool to room temperature. Finally the sample was passivated at room temperature in an O2(1 vol%)/He flow (20 ml/min) for several hours. Co(s)/NaY, Mo(s)/NaY, Mn(s)/NaY, V(s)/NaY and W(s)/NaY were prepared by chemical vapour deposition by exposing dehydrated NaY (250-

425 µm) to Co(CO)3NO, Mo(CO)6, Mn2(CO)10, V(CO)6 or W(CO)6 vapour respectively, in a N2 atmosphere. The samples were brought into glass ampoules, which were evacuated (p = 1∙ 10-2 mbar). Ampoules were then heated to 60 ° C or 120 ° C, depending on the volatility of the metal carbonyl used. Previous experiments with Mo(CO)6 have shown that during this treatment the metal carbonyl migrates into and is stabilised by the zeolite supercages [31]. The catalysts were then transferred to a fixed bed reactor, without exposure to ambient air and heated under controlled conditions. In all cases the metal loading was two metal atoms per zeolite


45

supercage; this corresponds to the saturation limit for Mo(CO)6 on zeolite NaY [32].

Table 3.1: Table 3.1: Table 3.1: Table 3.1: Toluene ammoxidation catalysts.

CatalystCatalystCatalystCatalyst LoadingLoadingLoadingLoading1 1 1 1 [wt%][wt%][wt%][wt%] ActivationActivationActivationActivation

Co(ie,4.4)/NaY 4.4 None Cu(ie,4.3)/NaY 4.3 None NH4(ie)/NaY 2 None Mo(im,9.8)/Al 9.8 Calcination at 500 ° C in He/O2 V(im,1.9)/NaY 1.9 None V(im,1.7)/Al 1.72 None V(im,4.4)/Al 4.42 None Mn(im,3.0)/Al 3.02 Calcination at 500 ° C in He/O2 Co(s,4.5)/NaY 4.5 Co(NO)(CO)3 introduction at 50 ° C.

Oxidation in He/O2. T= 400 ° C. Mo(s,11.4)/NaY 11.4 Mo(CO)6 introduction at 60 ° C.

Oxidation in He/O2. T= 400 ° C. Mn(s,3.5)/NaY 3.52 Mn2(CO)10 introduction at 120 ° C.

Oxidation in He/O2. T= 400 ° C. V(s,4.2)/NaY 4.2 V(CO)6 introduction at 60 ° C.

Oxidation in He/O2. T= 400 ° C. W(s,18)/NaY 18 W(CO)6 introduction at 60 ° C.

Oxidation in He/O2. T= 400 ° C. 1 determined by A.A.S. using a Perkin Elmer 3030 Atomic Absorption

Spectrophotometer. 2 loading was not determined.

2.2 Catalyst testing Toluene ammoxidation was performed at atmospheric pressure between 300

° C and 460 °C in a quartz glass fixed bed reactor (4 mm internal diameter). In all cases a plug flow regime was ensured. The amount of catalyst used

was between 0.2 and 0.3 g (250-425 µm particles). Thermal mass flow controllers were used to control all gas flows. He was used as an inert gas. Toluene vapour was introduced to the system by saturating a part of the He carrier gas at 9.4 ° C and atmospheric pressure. Weight Hourly Space Velocities (WHSV) between 0.7 and 1.0 gtoluene/(gcat∙ hr) were used. The molar toluene: NH3: O2-ratio (T:N:O) was 1: 8: 13 and the total gasflow was 110 Nml/min, unless mentioned otherwise. Similarly, toluene nitroxidation reactions were performed by using NO instead of NH3. All lines

Chapter 3

46

downstream of the reactor were thermostatted at 200 °C to prevent condensation. The organic compounds were analysed using an HP 5980 gas chromatograph, equipped with a 50 m HP-5 column and a flame ionisation detector. CO, CO2, NH3 and H2O were detected by non-dispersive infrared spectroscopy using a Fischer-Rosemound NGA-2000 MLT 4.2 analyser platform. The analyser was equipped with a paramagnetic cell to estimate the O2 concentration. Occasionally detection of inorganic products was performed by GLC using a Carboplot P7 column (25 m). Prior to analysis all organic products were removed by passing the gas stream through a trap containing 1-hexanol. NH3 was removed using a trap containing 2 M H2SO4. Qualitative analysis was also performed by quadruple mass spectrometry. Occurrence of homogeneous gas-phase reactions was found to be negligible under the experimental conditions applied. Conversion (X), selectivity (S) and yield (Y) calculations are based on the molar amount of toluene fed to the reactor. This amount was examined by GLC prior to performance of the catalytic reactions. The production rate of benzonitrile is defined as the molar amount of benzonitrile produced per gram of catalyst per second.

3. Results and discussion

3.1 Catalyst screening Since the objective of the experiments described in this Chapter was to screen new catalysts for the ammoxidation of toluene a V(im)/Al catalyst was also prepared. Though vanadia catalysts suffer from low selectivity and undesired ammonia decomposition vanadia based catalysts are traditionally used for the toluene ammoxidation reaction [33]. Since the reaction conditions reported in literature differ slightly the comparison based on toluene conversion, benzonitrile selectivity and benzonitrile yield is complicated. Therefore, the catalytic performance of this prepared V(im,1.7)/Al was taken as a reference for the performance of the newly prepared catalysts. The results are shown in Figure 3.1. From this figure it is clear that the selectivity to benzonitrile over V(im,1.7)/Al is relatively stable over the whole temperature range. The activity of this catalyst, however, is low. Only at elevated temperature high activities were obtained. However, the benzonitrile production rate, which amounts to 0.35 gbenzonitrile/(gcat∙ hr), is comparable to that reported in literature. For example Azimov et al. [14]


47

find toluene ammoxidation rates up to 37 µmol/(m2∙ hr) for toluene ammoxidation over V-Sb-Bi-O/Al at 360 ° C. Assuming a surface area of 80 m2/g, on a gram basis benzonitrile productivities up to 0.27 gbenzonitrile/(gcat∙ hr) were achieved.

Figure 3.1:Figure 3.1:Figure 3.1:Figure 3.1: The ammoxidation of toluene over V(im,1.7)/Al as a function of temperature. WHSV = 0.8; T:N:O= 1: 8: 13.

It must be noted that the catalytic performance of supported vanadia catalysts generally strongly depends on the loading. Cavalli et al. [34] found higher benzonitrile yields upon increase of the V loading for V/Ti. When the monolayer loading was reached no further increase of the benzonitrile yield was observed. Contrary Sanati et al. [10] ascribe the activity of V/Ti catalysts to the presence of V5+-species located on top of a vanadia monolayer. V loadings even higher than the monolayer coverage therefore are needed for the production of the most active toluene ammoxidation catalyst. Increase of the V loading led to increase of the activity as was expected from literature [10,34]. The catalytic results at 360 and 380 ° C for two vanadia

loaded γ-alumina catalysts are shown in Table 3.2. Two drawbacks, however, must be noticed. Though 55 % yield can be obtained over

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V(im,4.4)/Al at 360 ° C, temperature increase leads to severe benzonitrile selectivity decrease, and thus to reduction of the benzonitrile yield. Secondly, the preparation of vanadia catalysts is somewhat complex, since the solubility of NH4VO3, the catalyst precursor, is low. Therefore, sequential impregnation steps are necessary to reach V loadings higher than 2 wt%.

Table 3.2:Table 3.2:Table 3.2:Table 3.2: The effect of V loading and reaction temperature on the toluene ammoxidation over V(im)/Al catalysts.

CatalystCatalystCatalystCatalyst TTTT [° C][° C][° C][° C]

X(Toluene)X(Toluene)X(Toluene)X(Toluene) [mol%][mol%][mol%][mol%]

S(Benzonitrile)S(Benzonitrile)S(Benzonitrile)S(Benzonitrile) [mol%][mol%][mol%][mol%]

Y(Benzonitrile)Y(Benzonitrile)Y(Benzonitrile)Y(Benzonitrile) [mol%][mol%][mol%][mol%]

V(im,1.7)/Al 360 14 74 10 V(im,1.7)/Al 380 22 74 16 V(im,4.4)/Al 360 65 85 55 V(im,4.4)/Al 380 95 35 33

WHSV = 0.8; T:N:O = 1: 8: 13.

Table 3.3 shows the effect of the support on the vanadia performance in the ammoxidation of toluene at 400 ° C.

Table 3.3: Table 3.3: Table 3.3: Table 3.3: Effect of the support for vanadia catalysts on the performance in toluene ammoxidation at 400 ° C.

CatalystCatalystCatalystCatalyst X(Toluene)X(Toluene)X(Toluene)X(Toluene) [mol%][mol%][mol%][mol%]

S(Benzonitrile)S(Benzonitrile)S(Benzonitrile)S(Benzonitrile) [mol%][mol%][mol%][mol%]

V(im,1.9)/NaY 90 55 V(im,1.5)/ZSM-5 17 55 V(im,1.7)/Al 30 75

WHSV = 0.8; T:N:O = 1: 8: 13.

At low V loadings NaY is superior to the other supports that were applied. High conversion levels could be obtained. The benzonitrile selectivity, however, was lower than that over V(im,1.7)/Al. Nevertheless, since the benzonitrile selectivity can be increased by promoter addition [4,17], faujasite based catalysts show high potential for the ammoxidation of alkylaromatics. High activities can be achieved at relatively low temperatures. Moreover, due to the pore dimensions of the zeolite matrix, size exclusion effects can be obtained. For example Beschmann et al. [20] showed that only p-xylene was converted when a mixture of o-, m- and p-xylene was fed to a Cu/ZSM-5 catalyst. Due to its larger pore size


49

compared to ZSM-5, faujasite based catalysts can potentially be applied for selective conversion of more complex alkylaromatic molecules. In the remainder of this chapter the screening of a wide range of catalysts for their ability to ammoxidize toluene will be described. The conversion and selectivity were measured in order to determine the benzonitrile production rate. The catalysts were evaluated at high conversion levels to find the most productive samples. Catalyst deactivation was observed for most catalysts. Therefore, initial data and data measured after 1000 minutes on stream are shown. The activity of some catalysts increased slightly during the first 100 minutes on stream. The maximum conversion level obtained for those catalysts is shown between brackets. Generally the selectivity to benzonitrile remained constant as a function of time on stream for the catalysts investigated. The results of the toluene ammoxidation screening tests are shown in Table 3.4

Table 3.4: Table 3.4: Table 3.4: Table 3.4: Toluene ammoxidation results at 400 ° C.

X [mol%]X [mol%]X [mol%]X [mol%] S [mol%]S [mol%]S [mol%]S [mol%] RRRRpppp [ [ [ [µµµµmole/g∙ s]mole/g∙ s]mole/g∙ s]mole/g∙ s] CatalystCatalystCatalystCatalyst initialinitialinitialinitial finalfinalfinalfinal initialinitialinitialinitial finalfinalfinalfinal initialinitialinitialinitial finalfinalfinalfinal

NaY 22.5 (26.3) 22.5 62 62 0.34 (0.40) 0.34 Al 5.4 5.41 62 621 0.08 0.08 Co(ie,4.4)/NaY 98 43.72 35 572 1.06 0.77 Cu(ie,4.3)/NaY 100 872 0 742 0 1.79 NH4(ie)/NaY 98 37.52 37 572 1.01 0.59 Co(s,4.5)/NaY 39.0 36.31 76 761 0.61 0.57 Mo(s,11.8)/NaY 42.5 30 45 49 0.62 0.48 Mn(s,3.4)/NaY 100 100 0 1.7 0 0.05 V(s,4.2)/NaY 62 (66) 62 48 59 0.67 (0.71) 0.83 W(s,18)/NaY 41 25.2 45 45 0.57 0.35 Mn(im,4.2)/NaY 100 991 0 41 0 0.10 V(im,1.9)/NaY 97 95.8 54 54 1.41 1.39 Cu(im,3.2)/Al 67 67 1 1 0.02 0.02 Mo(im,9.8)/Al 94.6 92 80 80 1.67 1.62

Mn(im,3.0)/Al 76 751 23 231 0.37 0.36

V(im,4.4)/Al 73 66 86 86 1.50 1.35 WHSV = 0.8 – 1.0; T:N:O= 1: 8: 13 End time is after 1000 minutes on stream, except 1 400 min; 2 800 min As can be seen the benzonitrile production rates obtained after 1000 minutes on stream were of the order of 1 µmole/g∙ s. This rate is comparable to that reported earlier over VSb/Al (V/Sb = 0.2) [14]. The main side reaction observed in our experiments was total combustion to CO2. Additionally a small amount of CO was observed. In some cases some

Chapter 3

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dark coloured higher molecular weight compounds were observed on the catalyst bed or at the reactor outlet.

3.2 Catalyst deactivation

Figure 3.2: Figure 3.2: Figure 3.2: Figure 3.2: Toluene ammoxidation over NaY. T= 400 ° C; WHSV = 0.78; T:N:O= 1: 8: 13.

Figure 3.2 shows the change of conversion and selectivity with time on stream for toluene ammoxidation over zeolite NaY. After a short period of activation the catalyst deactivated continuously as a function of time on stream. After reaction some dark brown products were found both on the catalyst and on the wall of the condenser. Coking was also observed by Niwa et al. [9]. These authors proposed that carbonaceous material was responsible for the activity in the ammoxidation of toluene. The surface of the SiO2-Al2O3 applied by Niwa et al. was covered with carbonaceous material. This layer showed absorption bands belonging to carbonyl or carboxyl groups in the IR spectrum. When ammonia was pulsed on this layer benzonitrile was formed. Therefore toluene could be oxidized on surface oxides, followed by stabilization as adsorbed benzoate ion to produce benzonitrile. In performing pulse experiments they found benzonitrile formation only when the pulse sequence toluene-ammonia was applied [15], implying that ammonia reacts with the adsorbed alkylaromate compound. Other experiments performed by this group showed that

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unsupported vanadia catalysts did not yield nitrile formation [35]. It was concluded, based on IR experiments [15] that a methyl benzoate surface species was formed initially from xylene. This species was stabilized after migration to the alumina support and nitrile products are formed by

reaction of this methyl benzoate species, which was stabilized on the γ-alumina support. The formation of the methyl benzoate species was assumed to occur on coke containing V sites.

Indeed the γ-alumina support does not show significant activity for the formation of benzonitrile as shown in Figure 3.3. The benzonitrile selectivity was in the order of 60 %. No activating effect was observed after the start of the reaction.

Figure 3.3: Figure 3.3: Figure 3.3: Figure 3.3: Toluene ammoxidation over γ-alumina. WHSV = 0.75; T:N:O= 1: 8: 13.

The initial increase in activity for NaY in our experiments may be similarly explained by the formation of carbonaceous deposits on the catalyst surface. Eventually increase of the coke amount could have caused blocking of the zeolite pores, resulting in decreased ammoxidation activity after a longer time on stream. Similar deactivation behaviour was observed by Ramachandra Rao et al. [36], who found a 50 % reduction in the ammoxidation activity after 10 hours on stream during the ammoxidation of 3-picoline over a VAPO catalyst. The pore dimensions of this catalyst are similar to that of zeolite NaY. This suggests that modification of NaY

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52

should not only be directed at increasing the benzonitrile yield over freshly prepared catalysts, but also at minimising coke forming sites.

3.2.13.2.13.2.13.2.1 Performance of ionPerformance of ionPerformance of ionPerformance of ion----exchanged catalystsexchanged catalystsexchanged catalystsexchanged catalysts Toluene conversion can be increased significantly by introduction of metal sites by means of ion exchange, as shown in Figure 3.4 and Table 3.4. The selectivity towards benzonitrile is initially considerably lower than that of NaY, but after a period of about two hours on stream the benzonitrile selectivity equals the selectivity obtained when using NaY. Cu(ie,4.3)/NaY shows an increase in benzonitrile selectivity up to 74 %. However, the deactivation that occurs as a function of time on stream is even more pronounced compared to NaY. After about 600 minutes on stream the activity of the Co(ie,4.4)/NaY and NH4(ie)/NaY does become stable. Since the brown products can still be observed on the ion-exchanged catalysts after reaction, we can assume that coking again causes the deactivation. This means that the catalyst activity can be improved by ion exchange with transition metal ions, but this modification does not prevent the formation of carbonaceous deposits completely.

Figure 3.4: Figure 3.4: Figure 3.4: Figure 3.4: Performance of ion exchanged NaY. T= 400 ° C; WHSV= 0.8, T:N:O= 1: 8: 13.

3.2.23.2.23.2.23.2.2 Performance of catalysts prepared by CVD of metal carbonylsPerformance of catalysts prepared by CVD of metal carbonylsPerformance of catalysts prepared by CVD of metal carbonylsPerformance of catalysts prepared by CVD of metal carbonyls The performance of the catalysts that were prepared by means of CVD is shown in Figure 3.5. These results show that the activity of the NaY zeolite can be significantly improved by introducing Mn. However, this leads to a

0

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0 200 400 600 800

T im e o n stream [m in]T im e o n stream [m in]T im e o n stream [m in]T im e o n stream [m in]

Co

nv

ers

ion

[m

ole

%]

Co

nv

ers

ion

[m

ole

%]

Co

nv

ers

ion

[m

ole

%]

Co

nv

ers

ion

[m

ole

%]

NaY

NH4(ie)/NaY

Co(ie,4.4)/NaY

Cu(ie,4.3)/NaY

0

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0 200 400 600 800

T im e o n stream [m in]T im e o n stream [m in]T im e o n stream [m in]T im e o n stream [m in]

Co

nv

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ion

[m

ole

%]

Co

nv

ers

ion

[m

ole

%]

Co

nv

ers

ion

[m

ole

%]

Co

nv

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ion

[m

ole

%]

NaY

NH4(ie)/NaY

Co(ie,4.4)/NaY

Cu(ie,4.3)/NaY


53

dramatic decrease in benzonitrile selectivity. For this catalyst only COx was observed in the product stream. The introduction of V doubles the activity where the selectivity remains at the same level compared to NaY. Introduction of Mo and especially W leads to only a slight increase in activity, whereas the benzonitrile selectivity is slightly lowered. After an initial period of serious deactivation the stability of the catalysts that were prepared by CVD of metal carbonyl molecules seems to be improved as compared to the NaY catalyst. However, since the conversion level over Mn(s,3.4)/NaY was 100% no judgement can be made about the deactivation over this catalyst. Though the introduction of metal oxide centres into the pores of faujasite catalysts can yield very specific catalytic sites, which can be controlled accurately, the toluene ammoxidation activity is generally low.

Figure 3.5: Figure 3.5: Figure 3.5: Figure 3.5: Performance of catalysts prepared by CVD. T= 400 ° C; WHSV= 0.8, T:N:O= 1: 8: 13.

3.2.33.2.33.2.33.2.3 Performance of NaY based impregnated catalystsPerformance of NaY based impregnated catalystsPerformance of NaY based impregnated catalystsPerformance of NaY based impregnated catalysts As shown in Figure 3.6 the ammoxidation activity is greatly improved by impregnating NaY with a NH4VO3 solution. A low V loading can improve the activity to a conversion level over 95 %. This result is contrary to Cavani et al. [24] who found a low reaction rate in p-xylene ammoxidation over a NH4VO3/NH4Y catalyst. Contrary to the V(im,1.9)/NaY catalyst described here, their catalyst was prepared by wet impregnation of a NH4Y zeolite. The activity of the V(im,1.9)/NaY is not significantly decreased upon time on stream, although some higher molecular weight compounds condensed at the reactor outlet.

NaYW(s,18)/NaYMo(s,11.8)/NaY

V(s,6.4)/NaY

Mn(s,3.4)/NaY

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0 200 400 600 800 1000Tim e on stream [m in]Tim e on stream [m in]Tim e on stream [m in]Tim e on stream [m in]

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[m

ole

%]

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nv

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on

[m

ole

%]

Co

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on

[m

ole

%]

Co

nv

ersi

on

[m

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%]

NaYW(s,18)/NaYMo(s,11.8)/NaY

V(s,6.4)/NaY

Mn(s,3.4)/NaY

0

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Co

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[m

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%]

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nv

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on

[m

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%]

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[m

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%]

Co

nv

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on

[m

ole

%]

Chapter 3

54

Though V(im,1.9)/NaY has the highest activity among all the catalysts

screened, the selectivity is not very high, it does not exceed 60% at 400 °C. Therefore, the addition of dopants was examined to improve the benzonitrile selectivity over this catalyst. Table 3.5 summarises the results obtained for single doped V(im)/NaY catalysts.

FigFigFigFigure 3.6: ure 3.6: ure 3.6: ure 3.6: Performance of V(im,1.9)/NaY. T= 400 ° C; WHSV= 0.9, T:N:O= 1: 8: 13.

Table 3.5: Table 3.5: Table 3.5: Table 3.5: Performance of two component catalysts.

CatalystCatalystCatalystCatalyst Me(2)/VMe(2)/VMe(2)/VMe(2)/V----ratio ratio ratio ratio [mol/mol][mol/mol][mol/mol][mol/mol]

X(toluene) X(toluene) X(toluene) X(toluene) [mol%][mol%][mol%][mol%]

S(benzonitrile) S(benzonitrile) S(benzonitrile) S(benzonitrile) [mol%][mol%][mol%][mol%]

V(im,1.9)/NaY - 90 55 V-Sb(im,1.5-3.4)/NaY 0.9 70 80 V-K(im,1.1-1.4)/NaY 0.2 70 55 V-P(im,2.4-1.3)/NaY 0.9 85 45 V-Bi(im,1.5-1.6)/NaY 0.3 60 60

T=400°C; WHSV=0.9; T:N:O= 1: 8: 13.

All the catalysts listed in Table 3.5 were prepared by sequential incipient wetness impregnation. Sb is the best dopant. The selectivity of V-Sb(im,1.5-

3.4)/NaY increases to 80% at 400°C. When decreasing the temperature to

380°C, the selectivity can reach 90% with an overall conversion of 51%. However, except for V-Bi(im,1.5-1.6)/NaY the stability of promoted V(im)/NaY catalysts is decreased slightly compared to V(im,1.9)/NaY. Figure 3.7 shows the activity as a function of time on stream of this series of

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nv

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[m

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on

[m

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on

[m

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%]

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[m

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%]

NaY

V(im,1.9)/NaY

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[m

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[m

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%]

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[m

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NaY

V(im,1.9)/NaY


55

doped V(im)/NaY catalyst. For comparison the non-promoted V(im,1.9)/NaY catalyst has been plotted in Figure 3.7 as well.

Figure 3.7: Figure 3.7: Figure 3.7: Figure 3.7: Toluene ammoxidation over doped V(im)/NaY catalysts. T= 400 ° C; WHSV = 0.9; T:N:O= 1: 8: 13.

3.2.43.2.43.2.43.2.4 Performance of Performance of Performance of Performance of γγγγ----alumina supported catalystsalumina supported catalystsalumina supported catalystsalumina supported catalysts

Figure 3.8 shows the activity of some γ-alumina supported catalysts as a function of time on stream.

FiFiFiFigure 3.8: gure 3.8: gure 3.8: gure 3.8: Performance of γ-alumina supported catalysts. WHSV = 0.8; T:N:O= 1: 8: 13. T= 400 ° C, except V(im,4.4)/Al: T= 360 ° C.

Deactivation is generally less severe compared to the zeolite-based catalysts. The performance of the catalysts was compared at 400 ° C, except for V(im,4.4)/Al. At higher temperature the selectivity towards benzonitrile drastically decreased to 30 mol%. Therefore, the performance was measured

0

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0 200 400 600 800 1000

Tim e on stream [m in]Tim e on stream [m in]Tim e on stream [m in]Tim e on stream [m in]

Co

nv

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on

[m

ole

%]

Co

nv

ersi

on

[m

ole

%]

Co

nv

ersi

on

[m

ole

%]

Co

nv

ersi

on

[m

ole

%]

NaY

Mo(im,9.8)/Al

V(im,4.4)/Al

MoN(im,9.8)/Al

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Co

nv

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on

[m

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%]

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nv

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on

[m

ole

%]

Co

nv

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on

[m

ole

%]

Co

nv

ersi

on

[m

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%]

NaY

Mo(im,9.8)/Al

V(im,4.4)/Al

MoN(im,9.8)/Al

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0 200 400 600 800


Co

nv

ersi

on

[m

ol%

]C

on

ver

sio

n [

mo

l%]

Co

nv

ersi

on

[m

ol%

]C

on

ver

sio

n [

mo

l%]

V(im,1.9)/NaYV-P(im,2.4-1.3)/NaYV-K(im,1.1-1.4)/NaYV-Sb(im,1.5-3.4)/NaYV-Bi(im,1.5-1.6)/NaY

0

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on

[m

ol%

]C

on

ver

sio

n [

mo

l%]

Co

nv

ersi

on

[m

ol%

]C

on

ver

sio

n [

mo

l%]

V(im,1.9)/NaYV-P(im,2.4-1.3)/NaYV-K(im,1.1-1.4)/NaYV-Sb(im,1.5-3.4)/NaYV-Bi(im,1.5-1.6)/NaY

Chapter 3

56

at 360 ° C for this catalyst. The activity of the γ-alumina supported metal oxide catalysts generally equals or exceeds that measured on the most active

zeolite-based catalysts. The benzonitrile selectivity was also higher on γ-alumina supported catalysts. The highest yield of benzonitrile is obtained

over γ-alumina supported Mo oxide.

When this catalyst was applied as precursor for the production of γ-alumina supported Mo nitride the activity and the benzonitrile selectivity could be increased slightly. The higher activity obtained for the toluene ammoxidation reaction supports the importance of N containing intermediates in the mechanism of alkylaromatic ammoxidation reactions. Martin et al. [18] also stressed this in a study using isotopic NH3 labelling in a TAP apparatus. These authors performed the ammoxidation of toluene

over an α-(NH4)2[(VO)3(P2O7)2] catalyst. The benzonitrile that was formed initially contained only N atoms originating from the catalyst. In a later stage of the reaction also N originating from NH3 was inserted into the alkylaromatic compound.

Figure 3.9: Figure 3.9: Figure 3.9: Figure 3.9: The effect of calcinations on the ammoxidation of toluene over Mo(10.5)/Al; WHSV = 0.6; T:N:O= 1: 5: 8.

Since Mo nitrides are not stable in an oxygen-containing atmosphere it is doubted that structural formation of surface MoxN has occurred. The improvement of catalyst performance may have been caused by heat

60

70

80

90

100

0 20 40 60 80 100T oluene co nv ersion [m o l% ]T oluene co nv ersion [m o l% ]T oluene co nv ersion [m o l% ]T oluene co nv ersion [m o l% ]

Ben

zo

nit

rile

sel

ecti

vit

y

Ben

zo

nit

rile

sel

ecti

vit

y

Ben

zo

nit

rile

sel

ecti

vit

y

Ben

zo

nit

rile

sel

ecti

vit

y

[mo

l%]

[mo

l%]

[mo

l%]

[mo

l%]

N o pre-treatment

Calcination at T=700 °C

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360 380 400 420 440 460Tem perature [°C]

X(T

olu

ene)

[m

ol%

]

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100

0 20 40 60 80 100T oluene co nv ersion [m o l% ]T oluene co nv ersion [m o l% ]T oluene co nv ersion [m o l% ]T oluene co nv ersion [m o l% ]

Ben

zo

nit

rile

sel

ecti

vit

y

Ben

zo

nit

rile

sel

ecti

vit

y

Ben

zo

nit

rile

sel

ecti

vit

y

Ben

zo

nit

rile

sel

ecti

vit

y

[mo

l%]

[mo

l%]

[mo

l%]

[mo

l%]

N o pre-treatment

Calcination at T=700 °C

0

20

40

60

80

100

360 380 400 420 440 460Tem perature [°C]

X(T

olu

ene)

[m

ol%

]


57

treatment. Indeed, high temperature treatment slightly increased the selectivity towards benzonitrile, as shown in Figure 3.9. The activity was not significantly changed upon heat treatment, as shown in the inset. Mo based catalysts such as the well-known Bi-Mo oxide alkene ammoxidation catalyst yield a large amount of hydrocarbon decomposition products [37]. The selectivity for nitrile formation usually increases when V is added to the catalyst, which was observed for V promoted Ce containing Bi-Mo oxide catalysts [38]. As already discussed, Mo(im)/Al shows a fairly toluene ammoxidation activity. The benzonitrile selectivity, which does not exceed 70%, needs to be improved. As shown in Table 3.6 increasing the temperature can improve slightly the benzonitrile selectivity for this type of catalyst, unlike the performance of V based catalysts. This may indicate the different mechanism of side reaction for these two catalysts.

Table 3.6: Table 3.6: Table 3.6: Table 3.6: Effect of promoters Mo(im,11)/Al.

CatalystCatalystCatalystCatalyst Mo loading Mo loading Mo loading Mo loading [mol%][mol%][mol%][mol%]

Dopant loading Dopant loading Dopant loading Dopant loading [mol%][mol%][mol%][mol%]

T T T T

[[[[°°°°C]C]C]C]

XXXX [mol%][mol%][mol%][mol%]

SSSS [mol%][mol%][mol%][mol%]

380 50 70 Mo(im,11)/Al 11.6 0 400 91 76 380 65 75 Mo-V(im,11-0.4)/Al 11.6 0.8 400 100 77

Mo-V(im,11-0.8)/Al 11.5 1.6 380 95 82 380 88 83 Mo-V(im,11-1.6)/Al 11.4 3.1 400 100 76 380 51 80 Mo-La(im,11-2.4)/Al 11.7 1.8 400 93 85

Mo-Ni(im,11-0.8)/Al 11.6 1.4 380 51 75

WHSV = 0.8 gtoluene/(gcat∙ hr); T:N:O= 1: 8: 13.

A number of secondary components were tested on their promoter effect in the Mo(im,11)/Al. The results of Table 3.6 show that La addition can increase the selectivity from 76% to 85% while the activity remains unchanged. V addition can greatly increase the activity of the catalyst but has less effect on the selectivity.

3.3 Benzonitrile selectivity Table 3.7 shows the selectivity towards benzonitrile at 20 % conversion. Though in several cases the amount of combustion reactions increased significantly all preparation methods applied in this research can produce

Chapter 3

58

more selective catalysts for the ammoxidation of toluene compared to NaY, depending on the transition metal introduced. With respect to benzonitrile selectivity all preparation methods applied can lead to high benzonitrile selectivities. Ion exchange of Cu, impregnation of V and deposition of Co into NaY lead to the most selective catalysts.

Table 3.7: Table 3.7: Table 3.7: Table 3.7: Benzonitrile selectivity at toluene iso-conversion (20 mol%).

CatalystCatalystCatalystCatalyst Benzonitrile selectivity Benzonitrile selectivity Benzonitrile selectivity Benzonitrile selectivity [mol%][mol%][mol%][mol%]

NaY 63 NH4(ie)/NaY Not measured Co(ie,4.4)/NaY 59

IonIonIonIon----exchanged exchanged exchanged exchanged catalystcatalystcatalystcatalyst

Cu(ie,4.3)/NaY 86

Co(s,4.5)/NaY 95 Mo(s,11.8)/NaY 50 Mn(s,3.4)/NaY 60 V(s,4.2)/NaY 65

Carbonyl based Carbonyl based Carbonyl based Carbonyl based catalystscatalystscatalystscatalysts

W(s,18)/NaY 45 V(im,1.9)/NaY 82 Mo(im,10)/Al 73

Impregnated Impregnated Impregnated Impregnated catalystscatalystscatalystscatalysts

V(im,1.7)/Al 74

WHSV = 0.8 gtoluene/(gcat∙ hr); T:N:O= 1: 8: 13.

Figure 3.10: Figure 3.10: Figure 3.10: Figure 3.10: Conversion–selectivity plot for V(im,1.7)/NaY and Cu(ie,4.3)/NaY. WHSV = 0.8; T:N:O= 1: 8: 13.

As discussed in earlier sections high benzonitrile selectivities could not be obtained at high conversion levels for all catalysts. Generally the change in

0

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0 20 40 60 80 100

C o n v ers io n [m o l% ]C o n v ers io n [m o l% ]C o n v ers io n [m o l% ]C o n v ers io n [m o l% ]

Se

lec

tiv

ity

[m

ol%

]S

ele

cti

vit

y [

mo

l%]

Se

lec

tiv

ity

[m

ol%

]S

ele

cti

vit

y [

mo

l%]

Cu(ie,4.3)/NaY

V(im ,1.7)/NaY

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C o n v ers io n [m o l% ]C o n v ers io n [m o l% ]C o n v ers io n [m o l% ]C o n v ers io n [m o l% ]

Se

lec

tiv

ity

[m

ol%

]S

ele

cti

vit

y [

mo

l%]

Se

lec

tiv

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[m

ol%

]S

ele

cti

vit

y [

mo

l%]

Cu(ie,4.3)/NaY

V(im ,1.7)/NaY


59

benzonitrile selectivity with toluene conversion is complex, as shown in Figure 3.10. A volcano shape relation between conversion and selectivity is found. At low conversion levels low selectivities are obtained. Increase of the conversion level leads to increase of the benzonitrile selectivity over a broad range. At higher conversion the benzonitrile selectivity drops again. The complexity of the benzonitrile selectivity behaviour with the conversion level can be understood well based on the reaction network of toluene ammoxidation. Scheme 3.1 sketches the most important reactions during toluene ammoxidation. Changes in the reactant concentrations influence not only the toluene ammoxidation reaction itself, but also the combustion of ammonia.

Scheme 3.1: Scheme 3.1: Scheme 3.1: Scheme 3.1: Most important processes during toluene ammoxidation.

A change in the benzonitrile selectivity was observed as a function of time on stream for the ion exchanged catalysts as well as for the V(s,4.2)/NaY catalyst. Initial benzonitrile values were low, as shown in Table 3.4. For Co(ie,4.4)/NaY and NH4(ie)/NaY the increase in benzonitrile selectivity during time on stream can possibly be related to the catalyst deactivation. Total combustion probably occurs via a consecutive reaction over these catalysts. A lower conversion level leads to higher benzonitrile selectivity because consecutive combustion reactions are reduced. For V(s,4.2)/NaY and Cu(ie,4.3)/NaY the increase in benzonitrile selectivity could possibly be related to the presence of water. Water addition to the reactant feed in toluene ammoxidation increases significantly the benzonitrile selectivity [36,39]. During the ammoxidation or deep oxidation

NH3

+ O2

- H2ON2, NOx

CH3 CN+ NH3, O2

- H2O

CO, CO2

(- HCN, N2, NOx)- H2O

+ O2+ O2

- H2O

Chapter 3

60

of toluene water is generated in a substantial amount. These water molecules could possibly cause the increase of benzonitrile selectivity for V(s,4.2)/NaY and Cu(ie,4.3)/NaY. This, however, does not explain why selectivity changes during time on stream were not observed for all catalysts used. Thus the beneficial effect of water relates strongly to the catalyst composition. It has been proposed for V catalysts that adsorbing water prevents oxygen from binding weakly to the active sites of the catalysts. This weakly bound oxygen leads to total combustion [2 and references therein, 39]. Water may also suppress ammonia decomposition, leading to a decrease in total oxidation reactions.

3.4 Temperature influence

Figure 3.11: Figure 3.11: Figure 3.11: Figure 3.11: Toluene ammoxidation over NaY and Al based catalysts. Benzonitrile yield as a function of T. WHSV = 0.8, T:N:O= 1: 8 :13.

Figure 3.11 shows the effect of the reaction temperature on the benzonitrile yield during toluene ammoxidation over several catalysts. The reaction temperature was varied between 300 ° C and 460 ° C. Increasing the reaction temperature increases the catalyst activity, but in general the selectivity is decreased due to the increase of total combustion. Though the catalytic activity varied from catalyst to catalyst the catalytic activity at 400 ° C could be taken as reference for comparison between the different catalysts. For Cu(ie,4.3)/NaY and for Mo(im,10)/Al higher reaction temperatures can be applied without selectivity loss. The temperature at which the maximum yield of benzonitrile is achieved for Mn(s,3.4)/NaY is significantly lower, at 400 ° C this catalyst leads to combustion reactions.

0

20

40

60

80

300 340 380 420 460 500

Tem perature [°C ]Tem perature [°C ]Tem perature [°C ]Tem perature [°C ]

Yie

ld [

mo

l%]

Yie

ld [

mo

l%]

Yie

ld [

mo

l%]

Yie

ld [

mo

l%]

Mo(im,10)/Al

Mn(s,3.4)/NaY

Co(s,4.4)NaY

Cu(ie,4.3)/NaYMo(s,11.8)/NaY

V(im,1.9)/NaY

V(s,6.4)/NaY

V(im,1.7)/Al


61

3.5 Nitroxidation of toluene As discussed extensively in Chapter 2 the exact nature of the nitrogen insertion site for toluene ammoxidation is not known. Surface NH [40] species, formed from dissociatively adsorbed ammonia, or NH4

+-ions [41] are proposed by most authors as reactive N insertion species. Another possible way of ammonia activation is described by Centi and Perathoner [42]. Upon interaction with Brønsted acid sites NH3 adsorbs on CuO as NH4

+. In the presence of NO, NH4NO2 and NH4NO3 surface intermediates can be formed. Besides from other N producing intermediates, N2 and N2O can be formed from these structures upon desorption as shown in Scheme 3.2.

Scheme 3.2: Scheme 3.2: Scheme 3.2: Scheme 3.2: Surface NH4NO2 and NH4NO3 in the conversion of NO and NH3 in the presence of O2 over Cu/Al. Adapted from Centi and Perathoner [43].

The selective catalytic reduction of NO by hydrocarbons has been studied extensively during the last ten years. Though most research is focused only on the inorganic reaction products several groups have found nitriles in the SCR product mixture [44] in the presence of hydrocarbons. A broad overview of reaction data shows that every metal oxide that is active in selective hydrocarbon oxidation catalysis can act as active component in the SCR reaction [45]. Ammonia combustion occurs as side reaction during toluene ammoxidation. Since at reaction temperatures around 400 ° C NO is formed in this reaction [46] it seems plausible to consider also NO as reaction intermediate during toluene ammoxidation. Indeed it has been found by several groups that the reaction between toluene and NO yields benzonitrile formation. This reaction is referred to as nitroxidation in the

Cu O

NO NH4

H+

NH3

N2 H2O+

N2O H2O+

+

Cu O

NH4NO2

+-

Cu O

NH4NO3+-

O

O2

Cu O

NO

Cu ONO

Chapter 3

62

following. Alkylaromate nitroxidation is performed in the absence of oxygen at reaction temperatures around 450 ° C.

As shown in Equation 3.1 also N2 is formed in this reaction. For the nitroxidation reaction mostly PbO catalysts are applied [47,48], but also chromate [49] and NiO [48,50] catalysts have been reported in the literature. The nitroxidation of toluene produces benzonitrile with high selectivity. This reaction, however, leads to reduction of the catalyst [48]. Based on this catalyst reduction it is not clear whether nitriles can be produced in steady state or only during catalyst reduction. Moreover, reduction of the metal oxides applied (NiO and PbO) is difficult. Therefore, a redox mechanism as proposed by the authors seems not very probable. Figure 3.12 shows the results of the nitroxidation of toluene in the presence of gaseous oxygen over Cu(ie,4.3)/NaY. The catalytic performance was compared to the ammoxidation of toluene using the same conditions. In this reaction NH3 is present instead of NO. Though the activity towards toluene conversion is comparable for both reactions significant amounts of benzonitrile can only be achieved under ammoxidation conditions. In the presence of oxygen toluene is combusted mainly to CO and CO2.

Figure 3.12: Figure 3.12: Figure 3.12: Figure 3.12: Toluene nitroxidation and ammoxidation over Cu(ie,4.3)/NaY. WHSV = 0.84; T:N:O= 1: 5: 8.

Also other catalysts show very low benzonitrile selectivities as is shown in Table 3.8. Though the reaction is not optimised towards temperature or

0

20

40

60

80

100

400 420 440 460

T e m pe ra tu re [°C ]T e m pe ra tu re [°C ]T e m pe ra tu re [°C ]T e m pe ra tu re [°C ]

Co

nv

ers

ion

, S

ele

ctiv

ity

C

on

ve

rsio

n,

Se

lect

ivit

y

Co

nv

ers

ion

, S

ele

ctiv

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C

on

ve

rsio

n,

Se

lect

ivit

y

[mo

l%]

[mo

l%]

[mo

l%]

[mo

l%]

X (am m oxidation)

X(nitroxidation)

S(am m oxidation)

S(nitroxidation)

(3.1)CH3+ NO3/2 CN + 3/2 H2O 1/4 N2+


63

reactant concentration the benzonitrile production from NO is significantly lower than from NH3. Only Mo(s,11.8)/NaY shows benzonitrile yields that are not negligible.

Table 3.8: Table 3.8: Table 3.8: Table 3.8: Toluene nitroxidation in the presence of oxygen at 400 ° C.

CatalystCatalystCatalystCatalyst Toluene conversion [mol%]Toluene conversion [mol%]Toluene conversion [mol%]Toluene conversion [mol%] Benzonitrile Selectivity [mol%]Benzonitrile Selectivity [mol%]Benzonitrile Selectivity [mol%]Benzonitrile Selectivity [mol%]

Cu(ie,4.3)/NaY 37 3 Mn(s,3.4)/NaY 100 0 Mo(s,11.8)/NaY 75 21 Mo(im,9.8)/Al 24 22

WHSV = 0.8 gtoluene/(gcat∙ hr); Toluene : NO : O2 = 1 : 5 : 8. If the oxygen concentration is optimised with respect to benzonitrile selectivity, still only 30 % benzonitrile selectivity is obtained as shown in Figure 3.13.

Figure 3.13: Figure 3.13: Figure 3.13: Figure 3.13: Toluene nitroxidation over Mo(s,11.8)/NaY as a function of O2 concentration. T = 420 ° C; WHSV = 0.8; Toluene: NO= 1: 5.

From the nitroxidation results it is clear that benzonitrile formation from NO can occur. Benzonitrile yields over catalysts that are active in toluene ammoxidation are very low. Those catalysts that are very selective in toluene ammoxidation do not show significant benzonitrile production from NO. In the presence of oxygen mainly toluene combustion occurs. Similarly Pajonk found that those catalysts active in toluene nitroxidation did not show very high benzonitrile yields [48].

0

20

40

60

80

100

5 : 1.7

5 : 3.3

5 : 55 : 6

.75 : 8

.35 : 1

0

5 : 11.7

5 : 13.3

N O : ON O : ON O : ON O : O 2222 m o la r ra tio (T o lu en e = 1 ) m o la r ra tio (T o lu en e = 1 ) m o la r ra tio (T o lu en e = 1 ) m o la r ra tio (T o lu en e = 1 )

Co

nv

ers

ion

, S

ele

cti

vit

y

Co

nv

ers

ion

, S

ele

cti

vit

y

Co

nv

ers

ion

, S

ele

cti

vit

y

Co

nv

ers

ion

, S

ele

cti

vit

y

[mo

l%]

[mo

l%]

[mo

l%]

[mo

l%]

X (To luene)

S(Benzo nitrile)

Chapter 3

64

4. Conclusions The toluene ammoxidation can be catalysed by zeolitic and by γ-alumina supported metal oxide catalysts. Among the zeolite-based samples, both incipient wetness and CVD methods can be applied. Although the benzonitrile yield can be greatly improved by using ion exchange techniques, the initial stability during time on stream is dramatically lowered for Cu(ie,4.3)/NaY and Co(ie,4.4)/NaY. In spite of this deactivation, still high benzonitrile yields can be obtained over Cu(ie,4.3)/NaY after reaching steady state conditions. Mn(s,3.4)/NaY and V(im,1.9)/NaY showed relatively high benzonitrile yields under optimum

conditions (T= 360 ° C). Better stability is obtained over γ-alumina supported catalysts. Especially Mo(im,9.8)/Al shows good performance, mainly because of the higher selectivity towards benzonitrile at high conversion levels. The benzonitrile yield can be improved by high temperature pre-treatment of Mo(im,9.8)/Al. The benzonitrile yield over this catalyst remains high over a broad temperature range for this catalyst. V addition can increase the activity without loss of benzonitrile selectivity. For V(im)/NaY catalyst the selectivity can be increased by doping the catalyst with a second metal, but the activity is decreased significantly upon dopant addition. Surface NH4NO2 or NH4NO3 are not selective intermediates for the ammoxidation of toluene, as shown by the low benzonitrile selectivities in the reaction of toluene, NO and oxygen.

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J.S. Yoo, J.A. Donohue, M.S. Kleefish, P.S. Lin, S.D. Elfine, Appl. Catal. A., 105, (1993), 83-105.

J.S. Yoo, Catal. Today, 41, (1998), 409-432. 28. A. Martin, H. Berndt, M. Lücke, M. Meisel, Topics in Catal., 3, (1996),

377-386. 29. Y. Li, J.N. Armor, Chem. Commun., 20, (1997), 2013-2014.

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30. I. Peeters, A.W. Denier van der Gon, M.A. Reijme, P.J. Kooyman, A.M. de Jong, J. van Grondelle, H.H. Brongersma, R.A. van Santen, J. Catal., 173, (1998), 28-42.

31. H. Koller, A.R. Overweg, R.A. van Santen, J.W. de Haan, J. Phys. Chem. B., 101, (1997), 1754-1761.

H. Koller, A.R. Overweg, L.J.M. van de Ven, J.W. de Haan, R.A. van Santen, Microp. Mat., 11, (1997), 9-17.

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67

Chapter 4 Faujasite encaged metal oxide toluene ammoxidation

catalysts prepared from metal carbonyl precursors

Abstract Metal oxide loaded faujasite catalysts were prepared by deposition of metal carbonyl compounds into the pores of a faujasite zeolite host. Mo(CO)6, V(CO)6, Mn2(CO)10 and Co(CO)3NO catalyst precursors were used as metal guests. Removal of the CO ligands under oxidative conditions led to the oxidation of the transition metal. It was indicated by XPS that the resulting Mo species remained in the faujasite supercage. Transmission Electron Microscopy indicated that the Mo oxide clusters were well dispersed throughout the NaY zeolite host. The cluster diameter was limited by the size of the faujasite supercage, which is 13.4 Å. The introduction of Mn2(CO)10 led to the deposition of manganese oxide clusters on the exterior zeolite surface due to the low volatility of this compound. After contacting the V(CO)6 guest into the NaY host immediate and uncontrolled oxidation takes place as soon as the catalyst precursor is exposed to an oxygen-containing environment, as was evidenced by mass spectrometry. For Mo loaded NaY it was shown by XPS that the electrons of the carbonyl group interact with the Na+-ion. The effect of the faujasite extra-framework cation on the Mo(CO)6 guest was examined by ion exchange of Na+ with a series of alkali ions. It was shown by temperature programmed desorption that the host–guest interaction between Mo(CO)6 and the alkali cation is strongest for the least basic faujasite sample. For this purpose the basicity was successfully determined using the decomposition reaction of 2-methyl-3-butyn-2-ol. Toluene ammoxidation over alkali-exchanged catalysts showed increased toluene conversion at higher acidity. The benzonitrile selectivity on the other hand is enhanced at higher catalyst basicity, indicating formation of the selective intermediate by heterolytic C-H rupture with formation of a carbanion.

1. Introduction The introduction of high valent metal ions into zeolites via ion exchange is impossible, due to insufficient stabilization of highly charged cations inside the zeolite lattice. Application of conventional impregnation techniques also is not preferable in most cases, because of the restricted size of the zeolite pores. During impregnation oxy-anionic or neutral complexes are

Chapter 4

68

formed. These complexes cannot penetrate the zeolite pores in the presence of water [1]. Additionally the presence of a significant amount of moisture in the zeolite pores complicates well-defined incipient wetness impregnation. Sublimation of volatile metal carbonyl compounds offers the possibility to introduce these metals in a straightforward manner. The process to produce heterogeneous catalysts by this technique requires two basic steps: 1) volatilization and deposition of the organometallic compound inside the zeolite cavities, and 2) decomposition of the entrapped structure. The first step can be performed by applying a static method in which the zeolite host is mixed with stoichiometric amounts of the metal carbonyl. Upon heating the metal carbonyl compound sublimes and disperses throughout the zeolite pores. Also a flow method can be applied to introduce the metal carbonyl. By this flow method a metal carbonyl containing vapor flow is admitted to the zeolite matrix [2]. Only the static method of metal carbonyl introduction was applied in the experiments described in this chapter. This method can be controlled more accurately with respect to catalysts loading. The zeolite matrix can be loaded only with the saturation loading of the metal carbonyl compound when the flow is applied. Additionally, the time of metal carbonyl vapor exposure until saturation is reached is quite long, approximately 10 hours [3]. Mo(CO)6 uptake in static experiments, on the other hand, occurs within 20 minutes [4]. The equilibrium metal carbonyl loading equals two metal hexacarbonyl molecules per supercage, as shown by gravimetric analysis by several groups [3,5-9]. It is generally accepted that this saturation loading applies to all metal hexacarbonyls. Özkar et al. [10] indeed found the same saturation loading of 2 metal hexacarbonyl complexes per supercage for Cr(CO)6, W(CO)6 and Mo(CO)6. The metal loading can be increased by repeating metal carbonyl introduction after decarbonylation. As was shown by Asakura et al. [3,11] two Mo(CO)6 molecules are introduced into NaY in each additional Mo(CO)6 deposition sequence up to a loading of eight Mo atoms per faujasite supercage. On the other hand Yong and Howe report Mo loadings up to 10 Mo atoms per supercage [12]. Activation of this metal carbonyl loaded zeolite can occur in different ways, in order to produce intra-zeolite, highly dispersed (zero-valent) metal sites, metal oxides or metal sulfides, depending on the treatment. This activation

Faujasite encaged metal oxide catalysts prepared from metal carbonyl precursors for the ammoxidation of toluene

69

treatment can occur thermally or by means of UV irradiation. The latter involves lower temperatures needed for catalyst activation. In contrast to ion-exchange techniques with subsequent reduction by hydrogen, the production of highly dispersed zero-valent metal sites is not accompanied by the generation of protons, which would lead to the formation of a bi-functional catalyst. Recently, the absence of acidic protons has been shown elegantly by Ugo et al., who found a 100 % selective methylcyclopentane ring opening reaction using a real mono-functional Pd-Y catalyst prepared by this technique [13]. In literature the formation of intra-zeolite metal or metal sulfide catalysts has been studied most frequently. Zeolite encaged metal catalysts show promising performance in hydrogenation reactions [14,15]. Zeolite encaged molybdenum sulfide catalysts have been studied very extensively for hydrodesulfurization reactions [9,16-18]. To date metal oxides occluded in zeolite pores on the other hand have been studied mainly for the production of well-defined semi-conductors [19]. Therefore, the research described in this chapter focuses on the preparation of intra-zeolite metal oxides. The properties of the zeolite host can be adjusted with respect to pore structure and size. Moreover, the acid-base properties of the zeolite host can be effectively tuned by means of exchange of the Na+-ions ions. Therefore, these systems are very interesting from the fundamental point of view. Since the introduction of metal carbonyl vapor into the pores of faujasite catalyst is well controlled these catalysts can be characterized quite well. Uniform distribution of the metal carbonyl vapor is generally achieved and the motion of these metal carbonyl compounds has been described well in the literature. The introduction of metal carbonyl compounds can be applied to several transition metals, which (potentially) act as catalysts in a wide range of chemical reactions. The most frequently used metal carbonyl compound is Mo(CO)6, due to its relative ease of handling and low price; see for example [12,17,20,21]. Also Fe(CO)5, Cr(CO)6, Co2(CO)8, Co(CO)3NO, Ni(CO)4 and W(CO)6 have been used formerly by several groups [22-26]. Few articles have been published about the use of Mn2(CO)10 precursors for the production of Mn loaded zeolite Y [13,27].

Chapter 4

70

Based on pore dimensions, zeolite Y has been used as metal carbonyl host most frequently. The kinetic diameters of metal hexa-carbonyls are in the range of 5.3-5.5 x 7.4-7.6 Å, just fitting into the pores of zeolite Y [28]. The structure of faujasite contains supercages, which have an approximate diameter of 13.4 Å. A part of the faujasite structure, containing one supercage is sketched in Figure 4.1. Three different cation positions are indicated with SI, SI’ and SII. The SII cations are located in the

supercage. Mo(CO)6 is stabilized in these supercages as was shown by several authors using different characterization techniques. [4,18,29-31] Although other zeolites contain large cages that might be able to stabilize the metal carbonyl compound, the cage entrance is too small to enable diffusion of the metal carbonyl complex. Therefore, the amount of Mo(CO)6 introduced in these zeolites is significantly smaller; the Mo(CO)6 uptake is lower than 1 Mo atom per unit cell [32]. Nevertheless, Zeolite X, ZSM-5, Zeolite L and Mordenite have been used also as metal carbonyl hosts, but not as frequently as Zeolite Y. The first publications that deal with metal carbonyl introduction onto a catalyst support describe introduction of Mo(CO)6 onto alumina [33]. Metal hexacarbonyl introduction to silica supports appeared to be only possible for very low metal loadings [34], whereas other supports (MgO, TiO2) have been used successfully [35]. Gallezot et al. [36] were the first to adsorb metal carbonyl substrates onto a zeolitic support. They used a HY support for adsorption and decomposition of Mo(CO)6, Re2(CO)10 and Ru3(CO)12. Adsorption occurred by sublimation of the metal carbonyl at moderate temperatures (60 to 120 ° C, depending on the metal carbonyl used). Decarbonylation was performed by heating the samples in a closed system. All ligands were removed gradually at temperatures below 300 ° C for Mo(CO)6.

Figure 4.1: Figure 4.1: Figure 4.1: Figure 4.1: Structure of Zeolite Y.

SII

SI

SI’

hexagonal prismsodalite cage

or β-cage

supercage

SII

SI

SI’

hexagonal prismsodalite cage

or β-cage

supercage


71

The acidity and basicity of oxidation catalysts can influence severely the selectivity in vapour phase mild oxidation reactions. Generally acidic sites enhance the formation of deeper oxidation products such as acids and anhydrides [37]. Ion exchange of Na+-ions for other alkali ions into MoNaY offers the unique possibility to examine the influence of the catalyst acidity and basicity on the toluene ammoxidation reaction. The research described in this chapter focuses on the preparation and characterization of different zeolite encaged metal oxides. The acidity of the zeolite NaY host was varied by exchange of Na+ for a series of alkali cations. Alcohol decomposition reactions were applied to determine the acid-base properties of these catalysts.

2. Materials and methods

2.1 Catalyst preparation For all experiments zeolite NaY (Akzo, unit cell: Na55(AlO2)55(SiO2)137) was used. In some cases the Na+-ions were exchanged with other alkali cations by ion exchange. For this purpose alkali-metal nitrates were used. Batches of 10 gram NaY were suspended in 600 ml 1 M alkali-metal nitrate solution. Ion exchange was performed during 24 hours at 60 ° C while continuously stirring the suspension. After separation by centrifuging at 4800 rpm ion exchange was repeated twice using fresh metal nitrate solutions, according to the same procedure. After the third ion exchange sequence the samples were washed three times and dried in ambient air at 60 ° C. For the exchange of Na+ by H+, NH4NO3 was applied. After finishing ion exchange, the resulting NH4/Y was treated in N2 flow (300 ml/min) in order to remove NH3. The heating rate was 5 ° C per minute. The temperature was kept at 110 ° C for one hour in order to remove adsorbed water. After heating the catalyst at 510 ° C for 60 minutes the temperature was reduced to 250 ° C and the flow was changed to N2/O2 (80%/20%, 300 ml/min). Then the temperature was increased with 5 ° C per minute to 500 ° C and kept at this temperature for 20 minutes to remove NH3 traces. Co, Mn, Mo and V were introduced into the zeolite matrix by deposition of metal carbonyl vapors. Co(NO)(CO)3, Mn2(CO)10, Mo(CO)6 and V(CO)6

Chapter 4

72

were applied as transition metal sources. Prior to deposition of the metal carbonyl vapor the zeolite matrix was pelleted, crushed and sieved into a

sieve fraction of 250-425 µm. Batches of zeolite (1 gram) were carefully dehydrated under continuous evacuation (p= 1∙ 10-2 mbar). The temperature was raised with 1 ° C per minute to 400 ° C. The temperature was kept at 400 ° C overnight, still under dynamic vacuum. The dehydrated zeolite was then mixed with the metal carbonyl in nitrogen atmosphere, without exposure to ambient air in between. The maximum amount of metal carbonyl was kept equivalent to two transition metal atoms per supercage, the saturation loading for NaY zeolites [12,38] in order to exclude deposition at the outer zeolite surface. The metal carbonyl was sublimated and diffused through the zeolite pores by heating the obtained Mx(CO)y/NaY mixture in a closed ampoule (which was kept under static vacuum; p= 1∙ 10-2 mbar) to 60 ° C. This ensured introduction of Mo(CO)6 into the zeolite supercages as described by Koller et al. [39]. The catalyst was then cooled to room temperature and transferred to a tubular reactor, still without exposing the sample to ambient air. CO ligands were subsequently removed from the catalyst precursor by heat treatment as described in Section 2.2.4. The catalysts and catalyst precursors will be denoted according to the following notation. Zeolite encaged metal carbonyls (Mex(CO)y) will be indicated as Mex(CO)y/NaY, zeolite encaged decarbonylated zero-valent metal as Me0/NaY and zeolite encaged oxidized metal as MeOx/NaY. The catalysts will be indicated simply as Me/NaY when their oxidation state is not discussed.

2.2 Catalyst characterization

2.2.12.2.12.2.12.2.1 Determination of the catalystDetermination of the catalystDetermination of the catalystDetermination of the catalyst composition composition composition composition

The alkali-metal and transition-metal content of the catalysts was determined by Atomic Absorption Spectroscopy, using a Perkin-Elmer 3030 Atomic Absorption Spectrophotometer. Destruction of the catalyst by hydrochloric acid was used to dissolve the alkali metals. Prior to the analysis the catalysts were destroyed and dissolved in sulfuric acid. The Mo content was analyzed using the standard addition method.


73

2.2.22.2.22.2.22.2.2 XXXX----Ray Photoelectron SpectroscopyRay Photoelectron SpectroscopyRay Photoelectron SpectroscopyRay Photoelectron Spectroscopy

X-Ray Photoelectron Spectroscopy (XPS) was applied to determine the catalyst surface composition. A VG ESCALAB 200 spectrometer equipped

with an Al Kα X-ray source and a hemispherical analyzer was applied for the XPS experiments. Prior to recording of the spectra the samples were ground and pressed on an indium film placed on an iron stub. The binding energies were corrected for charging assuming a binding energy of 102.8 eV for the Si 2p peak [22,40]. Charging was on the order of 6 eV. The data were analyzed by a standard fit routine using a non-linear Shirley background subtraction and a Gauss-Lorentzian curve-fit function. Mo(CO)6/NaY catalysts precursors were transferred to the XPS chamber without exposure to ambient conditions by mounting the sample on the stub in a nitrogen filled glove box. The surface amounts of Na, Al and Si were estimated using the atomic sensitivity factors determined by Wagner et al. [41].

2.2.32.2.32.2.32.2.3 Transmission Electron MicroscopyTransmission Electron MicroscopyTransmission Electron MicroscopyTransmission Electron Microscopy

Transmission Electron Microscopy (TEM) was performed using a Philips CM 30 ST electron microscope, equipped with a LaB6 filament as electron source, operated at 300 kV. The catalyst was mounted on a microgrid, carbon polymer supported on a copper grid, by suspending the powdered catalyst in ethanol and placing a few droplets of this suspension on the grid. Drying was performed prior to the experiment under ambient conditions in order to remove the ethanol. Several grains of the sample were analysed in order to obtain a representative image of the sample, without focusing on possible artefacts present. EDX analysis was performed using a LINK EDX system by turning the electron beam towards the detector. This enabled us to obtain information about the chemical composition of a small part of the TEM image.

2.2.42.2.42.2.42.2.4 Temperature Programmed Oxidative DecarbonylationTemperature Programmed Oxidative DecarbonylationTemperature Programmed Oxidative DecarbonylationTemperature Programmed Oxidative Decarbonylation

Decarbonylation of the Mex(CO)y/NaY precursors was performed under well-controlled conditions. 1 Gram batches of the catalyst precursor were taken and heated while flowing a He or He/O2 gas flow (80%/20%, 60 Nml/min) through them. The heating rate was 5 ° C per minute. When activation was performed in He, the resulting Me0/NaY was oxidized by

Chapter 4

74

subsequently flowing He/O2 (80%/20%, 60 Nml/min) through it at a heating rate of 5 ° C/min. During the activation treatment the evolving gases were analysed using a Balzers QMG-420 quadruple mass spectrometer operated at an ionisation potential of 70 eV and an inlet pressure of 1.0∙ 10-5 mbar. Prior to some of the experiments the CO signal was calibrated using a He/CO feedstock with known CO concentration in order to quantify the amount of evolved CO. To compensate for variations of the inlet pressure a known large quantity of He was added to the reactor effluent. In all cases the mass spectrometer signals were normalized to He.

2.3 Catalytic tests

2.3.12.3.12.3.12.3.1 2222----MethylMethylMethylMethyl----3333----butynbutynbutynbutyn----2222----ol decompositionol decompositionol decompositionol decomposition

To probe the catalyst acid-base features, decomposition of 2-methyl-3-butyn-2-ol (MBOH) and 2-propanol was performed in a pulse micro reactor

at 150 ° C and 300 ° C. 0.5 µl Pulses were admitted to the catalyst and the reaction products were analysed using a 3 meter Porapak Q GC column. The catalysts were activated in situ at 350 ° C under an Ar flow (30 ml/min) prior to performance of the alcohol decomposition reactions. First order rate constants were calculated as described elsewhere [42]. As will be described in more detail in Section 3.1.4 the yield of 3-methyl-3-buten-1-yne (mbyne) was measured to probe the catalyst acidity. The dehydration function (DHD-value) is defined as the yield of mbyne/total conversion of MBOH. On basic sites, the molecule decomposes to acetone and acetylene. The dehydrogenation function (DHG-value) is defined similarly as the yield of acetone and acetylene/total conversion of MBOH.

2.3.22.3.22.3.22.3.2 Toluene ammoxidationToluene ammoxidationToluene ammoxidationToluene ammoxidation

The ammoxidation reaction of toluene was performed using a single-pass tubular reactor (4 mm internal diameter) operated under plug flow conditions. NH3, O2 and He (as an inert diluent) were controlled using Brooks Thermal Mass-flow Controllers. Part of the He flow was saturated with toluene (p.a.) using a three-step saturator that was kept at 9.4 ° C. No purification of the gases was found necessary. All lines after the catalyst bed were kept at a temperature of 200 ° C in order to prevent condensation of products. Detection of the organic products was performed by on-line gas chromatography, using a Hewlett Packard 5890 series II GLC,


75

equipped with a 50 m HP-5 column and a flame ionisation detector. Conversions, selectivities and benzonitrile production rates were calculated, based on the toluene inlet signal, which was measured before starting the reaction. Toluene conversion levels and selectivities towards organic products could be analysed accurately by using this method, irrespective of a lack of carbon balance.


3.1 Thermal activation of intra-zeolite Mo(CO)6 The decarbonylation of the Mo(CO)6/NaY catalyst precursor in He flow is plotted in Figure 4.2. The CO ligands gradually desorb from the catalyst precursor during heating. The main desorption peak is observed at 237 ° C. Mo(CO)6 melts between 147 and 149 ° C. The boiling point is 155 ° C [43]. The processes observed during heating are usually observed as sublimation, based on the small temperature window in which melting and boiling occur. The high CO desorption temperature that is observed in our experiments clearly shows that Mo(CO)6 is stabilized by the zeolite host. During preparation of the NaY-Mo(CO)6 mixture a slow color change from clear white to pale yellow was observed. This indicates that some decarbonylation occurred during preparation of the catalyst precursor. During decarbonylation the sample colour changes from pale yellow to black, indicating the removal of all CO ligands. Small amounts of H2 are observed between 200 ° C and 250 ° C, indicating the consumption of traces of OH-groups present in the faujasite structure. The amount of H2, however, is very small. Earlier, the reaction of adsorbed Mo(CO)6 with surface hydroxyl groups was reported for alumina supports by Brenner and Burwell Jr. [44]. These authors found that hydroxylated alumina was able to accommodate more Mo(CO)6 than dehydroxylated alumina. Nakamura et al. [45] proved that metallic Mo could be obtained by loading completely

dehydroxylated alumina. More recently the interaction of σ-OH with adsorbed Mo(CO)6 was described by Jaenicke and Loh, who report lower stability of adsorbed Mo(CO)6 in the presence of surface hydroxyls [46]. For faujasite based catalysts it was reported by several authors that surface hydroxyl groups of HY are able to oxidize Mo. The average Mo oxidation number increases with increasing concentration of hydroxyl groups [15] in the host structure. XPS experiments verified the increase of the Mo

Chapter 4

76

oxidation number [7]. The slightly lower temperature of the main H2 peak compared to the main CO desorption peak agrees with this. Thermal desorption occurs during TPD, but also trace OH-groups oxidize the zero-valent Mo atom.

Figure 4.2:Figure 4.2:Figure 4.2:Figure 4.2: Temperature programmed decarbonylation of Mo(CO)6/NaY. Temperature ramp 5 ° C/min; He flow (40 Nml/min).

Since the faujasite host structure is in the sodium form, the H2 amount in our experiments was very low, more than 100 times smaller than the amount of desorbed CO. Therefore, the reaction of hydroxyl groups with adsorbed Mo(CO)6 can be neglected in our experiments. CO2 production was observed in two discrete desorption steps at 80 ° C and 237 ° C. Formation of CO2 in the absence of O2 can be explained by occurrence of the CO disproportionation reaction (Equation 4.1). For Pt/SiO2 catalysts this reaction occurs on small Pt particles [47].

Since the dispersion of the metal carbonyl guests in the zeolite matrix is very high, possibly this reaction also occurs on Mo(CO)6/NaY during decarbonylation. Also for other supported metal carbonyl complexes the occurrence of CO disproportionation was observed [48]. As was verified by MS analysis O2 was not present in the gas mixture applied during decarbonylation. Therefore, this CO2 production cannot be related to oxidation of the CO ligands. Similar to the amount of H2 observed in our experiments the CO2 evolution can be neglected in further discussion, since the amount was less than 1 percent compared to CO.

0 100 200 300 400 500

Tem perature [°C]

No

rma

lise

d M

S i

nte

nsi

ty [

a.u

.]N

orm

ali

sed

MS

in

ten

sity

[a

.u.]

No

rma

lise

d M

S i

nte

nsi

ty [

a.u

.]N

orm

ali

sed

MS

in

ten

sity

[a

.u.] m/e = 28: CO

m/e = 2: H2 (·100)

m/e = 44: CO2 (·100)

0 100 200 300 400 500

Tem perature [°C]

No

rma

lise

d M

S i

nte

nsi

ty [

a.u

.]N

orm

ali

sed

MS

in

ten

sity

[a

.u.]

No

rma

lise

d M

S i

nte

nsi

ty [

a.u

.]N

orm

ali

sed

MS

in

ten

sity

[a

.u.] m/e = 28: CO

m/e = 2: H2 (·100)

m/e = 44: CO2 (·100)

C + CO2CO2 (4.1)


77

The desorption profile in Figure 4.2 differs significantly from that reported by Okamoto et al. [20,49]. These authors find two CO desorption peaks at 100 ° C and 266 ° C respectively upon temperature programmed decarbonylation in vacuo. The evolution of other molecules was not reported. Based on the desorption profile measured and on infrared data these authors propose formation of a Mo(CO)3 subcarbonyl species as first step during decarbonylation. However, they report a ratio of 1.3 to 1.4 for the peak areas of the low temperature and the high temperature desorption peak. Assuming complete removal of all six CO ligands this result would indicate “Mo(CO)2.5” formation after the first decarbonylation step. Our result indicates a rather slow, but continuous removal of small amounts of CO up to 200 ° C. Consecutively, the majority of the CO ligands desorbs at 237 ° C. The formation of a stable Mo(CO)3 subcarbonyl species, therefore, is not very likely. During decarbonylation a gradual color change from pale

yellow to black was observed over the catalyst bed in the direction of the gas flow. Figure 4.3 sketches the gradual colour change over the catalyst bed.

The absence of a second CO desorption peak may be explained by the experimental conditions. In comparison with experiments using catalysts in the form of thin wafers we have used a relatively large amount of catalyst. During temperature programmed desorption the He flow becomes partly saturated with the desorbing CO. As the proposed subcarbonyl Mo species can be re-saturated with six carbonyl ligands upon CO addition [10,39] this CO presence possibly leads to inhibition of formation of the highly reactive subcarbonyl species. At higher temperature all resulting CO ligands are irreversibly removed, leading to a quite broad desorption peak of CO. Quite similar to our experiments Asakura et al. [50] report a single CO desorption peak at around 200 ° C. The amount of desorbed CO was 5.5 moles per mole Mo, indicating incomplete decarbonylation. Further heating of the Mo(CO)6/NaYprecursor, however, did not lead to further evolution of CO. When the sample was exposed to O2 at 400 ° C after decarbonylation, the remaining CO ligand was desorbed as CO2. Temperature Programmed Oxidation (TPO) of decarbonylated Mo0/NaY shows that in addition to

Figure 4.3: Figure 4.3: Figure 4.3: Figure 4.3: Colour change over the catalyst bed during decarbonylation in He.

Helium Helium, CO

pale yellow black

Chapter 4

78

the consumption of O2, the remaining CO ligands are removed as CO and CO2 at temperatures between 25 and 300 ° C. TPO results are plotted in Figure 4.4.

Figure 4.4: Figure 4.4: Figure 4.4: Figure 4.4: TPO of Mo0/NaY. He/O2 flow (80%/20%; 60 Nml/min).

The CO and CO2 production observed during oxidation can also result from carbon fragments adsorbed on the catalyst surface. As was explained above, C was probably formed during decarbonylation according to the Boudart disproportionation reaction. CO2 production indeed was observed during decarbonylation in He. The C formed was oxidized by O2 as shown in Figure 4.5. The C oxidation probably was catalyst by Mo, since the reaction occurred at relatively low temperature. The Boudart reaction has been proposed also by Brenner and Hucul to account for CO2 production

during decarbonylation of W(CO)6/γ-alumina catalysts [51]. These authors

examined a broad range of W(CO)6/γ-alumina catalysts and found similar broad CO desorption patterns as we did for Mo(CO)6/NaY. Figure 4.5 shows the oxidative decarbonylation of Mo(CO)6/NaY. During heating of the catalyst precursor only the evolution of CO was observed. The signal at m/e= 44 was assigned to the formation of CO2 at the mass spectrometer filament, as was confirmed by calibration experiments using CO. Simultaneously with the evolution of CO from the catalyst consumption of O2 was observed. Mo is oxidized during decarbonylation. Mo oxidation was confirmed by the colour change of the sample. A white colour was assigned to oxidized Mo. Asakura et al. [3] observe CO desorption at 473 K when applying heat treatment in vacuo. The sample

0 100 200 300 400 500T em peratu re [ °C ]T em peratu re [ °C ]T em peratu re [ °C ]T em peratu re [ °C ]

No

rma

lise

d M

S I

nte

nsi

tyN

orm

ali

sed

MS

In

ten

sity

No

rma

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d M

S I

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nsi

tyN

orm

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MS

In

ten

sity

O2

CO (•10) CO2 (•10)


79

colour turned from pale yellow to black, as was expected since Mo is in valence state zero in Mo(CO)6. Oxidation of the Mo thus facilitates the decarbonylation of Mo(CO)6.

Figure 4.5: Figure 4.5: Figure 4.5: Figure 4.5: Temperature Programmed Oxidative Decarbonylation of Mo(CO)6/NaY. He/O2 flow (80%/20%; 60 Nml/min).

The Temperature Programmed Decarbonylation (TPOD) experiments that were described above can be summarized in Scheme 4.1.

Scheme 4.1: Scheme 4.1: Scheme 4.1: Scheme 4.1: Decarbonylation and oxidation of NaY encaged Mo(CO)6.

When decarbonylation occurs in an inert atmosphere, NaY encaged molybdenum oxide is prepared according to steps 1-3.

1) Mo(CO)6 is mixed together with the NaY host and diffuses throughout the pores.

2) Upon heating in He, the CO ligands are removed and a zero-valent Mo0/NaY precursor is formed.

3) By heating this Mo0/NaY precursor in O2 to at least 300 ° C the formation of MoOx/NaY completes.

0.0

0.2

0.4

0.6

0 100 200 300 400 500T im e [m in ]T im e [m in ]T im e [m in ]T im e [m in ]

Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

-0.2

-0.1

0.0

0.1

Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

m /e = 28

m /e = 32

m /e = 44

0.0

0.2

0.4

0.6


Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

-0.2

-0.1

0.0

0.1

Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

Inte

nsi

ty [

a.u

.]In

ten

sity

[a

.u.]

m /e = 28

m /e = 32

m /e = 44

Mo(CO)6/NaY O2, 100 °C MoOx/NaY 6 CO+ (4)

MoNaY MoOx/NaYO2, 300 °C (3)

(1)Mo(CO)6 + NaY Mo(CO)6/NaYmixing

(2)Mo(CO)6/NaY Mo(CO)x/NaY Mo0/NaY + (6 [-x]) COHe, 100 °C

(6-x) CO

He, 240 °C

Chapter 4

80

When O2 is present during decarbonylation, zeolite encaged molybdenum oxide is formed according to reaction steps 1 and 4. The Mo(CO)6/NaY precursor that is formed in step 1 is converted to zeolite encaged molybdenum oxide in a single step. Decarbonylation and oxidation of Mo(CO)6/NaY can be achieved at temperatures below 100 ° C in this case. The amount of CO that evolved from the Mo(CO)6/Y catalyst precursor was quantified for some alkali-exchanged catalysts during TPOD. Table 4.1 shows the results.

Table 4.1: Table 4.1: Table 4.1: Table 4.1: TPOD of Mo(CO)6/Y catalyst precursors. Amount of CO produced.

Amount of CO [mmol]Amount of CO [mmol]Amount of CO [mmol]Amount of CO [mmol] CatalystCatalystCatalystCatalyst

Expected from Mo(CO)Expected from Mo(CO)Expected from Mo(CO)Expected from Mo(CO)6666 loading loading loading loading MeasuredMeasuredMeasuredMeasured

Mo(CO)Mo(CO)Mo(CO)Mo(CO)6666/CsY/CsY/CsY/CsY 4.6 5.0

Mo(CO)Mo(CO)Mo(CO)Mo(CO)6666/RbY/RbY/RbY/RbY 5.2 5.4

Mo(CO)Mo(CO)Mo(CO)Mo(CO)6666/NH/NH/NH/NH4444YYYY 7.0 6.4

Within the experimental error the amount of CO measured indicates complete decarbonylation. The experimental error is estimated at around 10 %, since electrostatic charging in the glove box severely complicated accurate sample weighing. For the NH4/Y host loss of NH3 ligands was observed at temperatures between 360 ° C and 490 ° C. Small amounts of NH3 were removed as N2 at 100 ° C. The CO signal was corrected for this N2 production.

3.2 XPS analysis of Mo(CO)6/NaY and MoOx/NaY The Mo(CO)6/NaY catalyst precursor and the decarbonylated and oxidized MoOx/NaY sample were analyzed by XPS. Some evaporation and premature decarbonylation of the Mo(CO)6/NaY catalyst precursor was observed during introduction of the sample into the vacuum chamber, as indicated by the strong increase of the pressure in the vacuum chamber. Decomposition of bulk Mo(CO)6 is known to occur when exposed to vacuum [52]. Mo(CO)6 sublimates readily in the range of 50-90 ° C at 1 Torr [43]. The high vacuum applied in the XPS experiments may have led to further decrease of the sublimation temperature. Mass spectrometry analysis showed the evolution of CO during the introduction of the sample


81

into the vacuum chamber. After sample introduction the sample was allowed to stabilize and the XP spectrum was measured.

Figure 4.6: Figure 4.6: Figure 4.6: Figure 4.6: XP Spectrum of 12 wt% loaded Mo(CO)6/NaY.

Figure 4.6 shows the Mo(3d) peak of the Mo(CO)6/NaY catalyst precursor. The spectrum was fitted with two Mo(3d) peaks. A correct fit could not be obtained when only one Mo(3d) peak was applied for fitting. This indicates that Mo exists in at least two different states, although also the low peak intensity in combination with the broad lines complicates determination of the exact binding energy. The binding energies found for the two Mo(3d) doublets were 230.2 and 233.4 eV. As can be seen in Table 4.2 the intensity of the low binding energy peak was around 14 times larger than the high binding energy peak. This small high-energy peak can be attributed to the formation of oxidized Mo during the evaporation and premature decarbonylation, which was observed during introduction into the vacuum. The highly reactive decarbonylated Mo0/NaY species is probably oxidized by trace OH-groups of the NaY metal carbonyl host. Evidence for the presence of two Mo states can also be found in the literature, though the authors do not always ascribe the XP spectrum to two Mo states [49]. The poor quality of the Mo XP spectra further complicates peak assignment. Komatsu et al. qualitatively conclude to a the shift of the Mo(3d) peak to higher binding energies when the catalyst is briefly (4 minutes) exposed to air [7].

224228232236240

B in d ing E n ergy [eV ]B in d ing E n ergy [eV ]B in d ing E n ergy [eV ]B in d ing E n ergy [eV ]

XP

S I

nte

nsi

tyX

PS

In

ten

sity

XP

S I

nte

nsi

tyX

PS

In

ten

sity

Peakfit

Eb= 230.2 eV

Eb= 233.4 eV

Chapter 4

82

Table 4.2: Table 4.2: Table 4.2: Table 4.2: XPS Binding energies of Mo/NaY catalysts and precursors.

SampleSampleSampleSample Mo(3d)Mo(3d)Mo(3d)Mo(3d) Al(2p)Al(2p)Al(2p)Al(2p) Na(1s)Na(1s)Na(1s)Na(1s) Peak 1Peak 1Peak 1Peak 1 Peak 2Peak 2Peak 2Peak 2

Mo(CO)Mo(CO)Mo(CO)Mo(CO)6666/NaY 12 wt%/NaY 12 wt%/NaY 12 wt%/NaY 12 wt% 230.2 (93%) 233.4 (7 %) 74.6 1071.3

MoMoMoMoOOOOxxxx/NaY 5 wt%/NaY 5 wt%/NaY 5 wt%/NaY 5 wt% 232.2 (83%) 233.1 (17%) 74.5 1072.2

MoOMoOMoOMoOxxxx/NaY 12 wt%/NaY 12 wt%/NaY 12 wt%/NaY 12 wt% 231.9 (41%) 233.3 (59%) 74.6 1072.3

Binding energies were corrected for charging by normalization to the Si(2p) peak at 102.8 eV.

Table 4.3: Table 4.3: Table 4.3: Table 4.3: Comparison of Binding energies (BE) of metals and metal carbonyls....

MetalMetalMetalMetal BEBEBEBEaaaa [eV] [eV] [eV] [eV] Metal carbonylMetal carbonylMetal carbonylMetal carbonyl BEBEBEBEaaaa [eV] [eV] [eV] [eV] Shift [eV]Shift [eV]Shift [eV]Shift [eV]

CoCoCoCo 778.3 Co(CO)Co(CO)Co(CO)Co(CO)3333NONONONO 780.7 2.4 CrCrCrCr 574.4 Cr(CO)Cr(CO)Cr(CO)Cr(CO)6666 576.3 1.9 FeFeFeFe 707.0 Fe(CO)Fe(CO)Fe(CO)Fe(CO)5555 709.6 2.6 MnMnMnMn 639.0 MnMnMnMn2222(CO)(CO)(CO)(CO)10101010 641.6 2.6 NiNiNiNi 852.7 Ni(CO)Ni(CO)Ni(CO)Ni(CO)4444 854.8 2.1 MoMoMoMo 228.0 Mo(CO)Mo(CO)Mo(CO)Mo(CO)6666/NaY/NaY/NaY/NaY 230.2b 2.2

a data from [54] and references therein except b b this work The binding energy of the main peak, which amounted at 230.2 eV is assigned to Mo(CO)6. Schwartz and Hercules report a binding energy of 226.6 eV for the Mo(3d) peak [53] in bulk Mo(CO)6. These authors report a value of 226.1 eV for metallic Mo. Except for these quite old literature values, to our knowledge no other XP spectra of bulk Mo(CO)6 have been reported in the literature, probably due to premature decarbonylation of the sample in the sample XPS chamber. If we compare the binding energy found for the main peak it is around 2 eV higher than that of zero-valent Mo [54]. This increase of the binding energy is explained by the strong electron withdrawing effect of the carbonyl groups present in the molecule. For other, more stable, carbonyl compounds similar shifts in binding energies were reported, as shown in Table 4.3. On the other hand, Andersson and Howe [22] report a binding energy of 227.8 eV for Mo(CO)6/NaY recorded at –100 ° C. Upon decarbonylation in vacuo at 400 ° C they find a shift in binding energy to 229.2, which was explained by accidental oxidation during transport to the XPS chamber. The Mo(3d) peak fit, however, was not completely correct. An interfering signal at


83

lower binding energy was observed as well. This peak was not included in defining the reported binding energy.

Figure 4.7: Figure 4.7: Figure 4.7: Figure 4.7: XP Spectrum of 12 wt% loaded MoOx/NaY.

The Mo(3d) XP Spectrum of the decarbonylated MoOx/NaY sample is shown in Figure 4.7. Again at least two Mo states were observed, since the spectrum could not be fitted when only one Mo doublet was applied. The binding energies were found at significantly higher values compared to the catalyst precursor. The Mo(3d) doublets were found at 231.9 and 233.3 eV respectively. These values show that Mo is in a high oxidation state. The binding energy for bulk MoO3 is 232.6 eV [54 and references therein]. The high binding energy value observed in our experiments is consistent with the binding energy reported by other authors [7,20,22]. This shift to higher binding energy is explained by the electron deficiency in the zeolite host [55,56]. Table 4.4 shows the Mo:Si atomic ratio and the Mo:Na atomic ratio as determined by XPS. These ratios are very close to the atomic ratios as calculated from bulk composition for MoOx/NaY. No indication was observed for possible high concentrations of Mo at the external surface of the faujasite lattice. For the Mo(CO)6/NaY the Mo concentration measured by XPS was much smaller. This is explained well by the evaporation of Mo(CO)6 in the vacuum chamber as discussed above. The actual amount of Mo in the sample, therefore, is much lower than that

228230232234236238240

C orrected B inding En ergy [eV]C orrected B inding En ergy [eV]C orrected B inding En ergy [eV]C orrected B inding En ergy [eV]

XP

S I

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nsi

tyX

PS

In

ten

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XP

S I

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nsi

tyX

PS

In

ten

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Eb= 231.9 eV

Eb= 233.3 eV

Peakfit

228230232234236238240

C orrected B inding En ergy [eV]C orrected B inding En ergy [eV]C orrected B inding En ergy [eV]C orrected B inding En ergy [eV]

XP

S I

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nsi

tyX

PS

In

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XP

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PS

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Eb= 231.9 eV

Eb= 233.3 eV

Peakfit

Chapter 4

84

expected from the amount of Mo(CO)6 initially mixed together with the zeolite host. Since chemical analysis of the bulk could not be performed after performing the XPS experiment the bulk Mo concentration is strongly overestimated.

Table 4.4: Table 4.4: Table 4.4: Table 4.4: Relative XPS intensities energies of Mo/NaY catalysts.

SampleSampleSampleSample Mo:SiMo:SiMo:SiMo:Si Mo:NaMo:NaMo:NaMo:Na BulkBulkBulkBulk XPSXPSXPSXPS BulkBulkBulkBulk XPSXPSXPSXPS

Mo(CO)Mo(CO)Mo(CO)Mo(CO)6666/NaY 12 wt%/NaY 12 wt%/NaY 12 wt%/NaY 12 wt% 0.12 0.03 0.29 0.03 MoOMoOMoOMoOxxxx/NaY 5 wt%/NaY 5 wt%/NaY 5 wt%/NaY 5 wt% 0.05 0.05 0.12 0.09 MoOMoOMoOMoOxxxx/NaY 12 wt%/NaY 12 wt%/NaY 12 wt%/NaY 12 wt% 0.13 0.18 0.32 0.31

XPS surface Mo:Si and Mo:Na atomic ratios determined according to [41].

3.3 Dispersion of molybdenum oxide clusters in NaY As was shown in Sections 3.1.1 and 3.1.2 Mo(CO)6/NaY catalyst precursors can be decarbonylated at low temperature. No indication of MoOx clustering by sintering was given. XPS experiments indicated high Mo dispersion, since the Mo:Si ratio was close to the ratio expected from the bulk composition of the catalyst.

Figure 4.8: Figure 4.8: Figure 4.8: Figure 4.8: TEM image of 12 wt% loaded MoOx/NaY.

Transmission Electron Microscopy (TEM) experiments were performed in order to verify the Mo dispersion. Figure 4.8 shows the d-spacings of the


85

zeolite crystal, showing that the catalyst structure is not damaged during the electron microscopy experiment.

Figure 4.9: Figure 4.9: Figure 4.9: Figure 4.9: TEM image of 6 wt% loaded Mo(CO)6/NaY.

Figure 4.10: Figure 4.10: Figure 4.10: Figure 4.10: TEM image of 6 wt% loaded MoOx/NaY.

Figure 4.9 shows the TEM image of Mo(CO)6/NaY. No Mo oxide clusters were observed. This means that no Mo oxide clusters with a size larger

Chapter 4

86

than approximately 5 Å are present, since the detection limit of the microscope is estimated at 5 Å. After oxidative decarbonylation metal oxide clusters have formed as is indicated in Figure 4.10. The Mo clusters are well dispersed throughout the sample as was verified by EDX analysis. The Mo concentration was the same throughout the sample. Segments of the TEM images that had an apparent higher concentration of TEM-visible MoOx clusters did not show higher Mo concentrations than the bulk. The size of the MoOx clusters was estimated by measuring the diameter of the MoOx clusters visible in the microscope.

Figure 4.11: Figure 4.11: Figure 4.11: Figure 4.11: The MoOx cluster size of MoOx/NaY estimated from TEM.

Figure 4.11 shows the distribution of the MoOx cluster diameter. Since only MoOx clusters having a diameter larger than approximately 5 Å could be visualized by TEM, the method overestimates the cluster size. Therefore, Figure 4.11 can only be taken as a rough estimate for the upper limit of the MoOx cluster size. For this reason no attempts were made to give a statistical evaluation of the MoOx cluster size measured by TEM. However, from the XP Spectra in combination with the TEM experiments and the low decarbonylation temperature it is clear that the MoOx clusters formed are well dispersed. No indication was found for the presence of high concentrations of MoOx clusters at the exterior zeolite surface. The fact that two different Mo oxidation states were observed by XPS and the presence of MoOx clusters larger than 14 Å for the calcined sample, however, suggests that Mo is possibly deposited partly on the external zeolite surface. NH3 treatment after calcination does not lead to growth of the average

0

5

10

15

20

5 7 9 12 14 16 18 21 23 25 28

Particle diameter [Ångstrom]Particle diameter [Ångstrom]Particle diameter [Ångstrom]Particle diameter [Ångstrom]

Fre

qu

ency

[-]

Fre

qu

ency

[-]

Fre

qu

ency

[-]

Fre

qu

ency

[-]

MoOx/NaY

MoOx/NaY afterammonia treatment


87

MoOx cluster size. TEM does not show any indication for the presence of significant amounts of Mo containing clusters on the external zeolite surface after NH3 treatment either. The diameter of the MoOx clusters does not exceed 14 Å after NH3 treatment and the Mo concentration at the edges of crystals does not differ from the bulk Mo concentration as was verified by EDX.

3.4 Mo(CO)6 interaction with the faujasite lattice The binding energy of the Na(1s) peak at 1071.3 eV is about 1.1eV lower than expected from the literature [40]. This shift can be explained well by the interaction of the Na+-ion with the Mo(CO)6 guest. Donation of electrons from the carbonyl ligands to the Na+-ion occurs and results in

lower Na(1s) binding energies. The interaction of adsorbed Mo(CO)6 with Na+ was discussed earlier by Andersson and Howe. They compared NaX and NaY encaged Mo(CO)6. The Mo(3d) binding energy of the NaX encaged Mo(CO)6 sample was slightly (0.4 eV) higher than that of NaY encaged Mo(CO)6. This can be explained by the higher density of Na+-ions and thus lower electron density around the Mo containing

clusters in NaX than in NaY. More convincingly, 23Na-NMR experiments showed changes in the NMR shift attributed to the SII Na+-ions. W(CO)6 guest molecules interact with the Na+-ions of NaY as was shown by Ozin and co-workers [57]. When performing 23Na DOR-NMR experiments these authors found an increase of the intensity of the shoulder at –16 ppm upon increase of the W loading of the zeolite host. This increase can be explained by an increased contribution of SII Na+-ions interacting with the W(CO)6 guest. The interaction of Mo(CO)6 guests with the NaY host is quite

Figure 4.12: Figure 4.12: Figure 4.12: Figure 4.12: XP Spectrum of the Na(1s) region. a. Mo(CO)6/NaY, b. MoOx/NaY.

1065107010751080

B in d in g en erg y [eV ]B in d in g en erg y [eV ]B in d in g en erg y [eV ]B in d in g en erg y [eV ]

XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

a

b

1072.3 eV

1071.3 eV

NaY

1065107010751080


XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

a

b

1072.3 eV

1071.3 eV

1065107010751080


XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

a

b

1065107010751080


XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

a

b

1065107010751080


XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

XP

S I

nte

nsi

ty [

a.u

.]X

PS

In

ten

sity

[a

.u.]

a

b

1072.3 eV

1071.3 eV

NaY

Chapter 4

88

similar, as shown by the same group. In our group the molecular motion of Mo(CO)6 in NaY hosts was studied by Koller et al. [39]. They showed by 13C-NMR that Mo(CO)6 interacts with Na+. After oxidative treatment the Mo(3d) binding energies are the same within the experimental error (0.2 eV). Our experiments clearly show that the Na(1s) binding energy is affected by the Mo(CO)6 guest. After decarbonylation the Na(1s) binding energy is almost equal to that reported for NaY that does not contain any transition metal guests. Figure 4.13 shows the temperature programmed desorption profiles of CO from Mo(CO)6 loaded faujasite based catalyst precursors. The NaY mother batch was ion exchanged with different alkali-metal nitrates. Based on the periodic trend the electronegativity of the alkali ions applied increases in the order HY<LiY<NaY<KY<RbY<CsY. Therefore, the strongest interaction of the Mo(CO)6 guest with the alkali cation -and thus the highest decarbonylation temperature- is expected for HY and for LiY.

Figure 4.13: Figure 4.13: Figure 4.13: Figure 4.13: CO evolution during TPOD of Mo(CO)6/Y precursors. Heating rate 5 ° C/min; He/O2 flow (80%/20%; 60 Nml/min).

To explain the deviation from this trend the base strength of the Mo loaded samples was estimated. For this purpose alcohol decomposition reactions were performed. It was already described in literature that the decomposition reaction of 2-methyl-3-butyn-2-ol (MBOH) can be applied to estimate the basic and the acidic function of the catalyst [58]. With good results the basic strength of a wide range of catalysts was evaluated using

0 100 200 300 400 500

T e m p e ra tu re [ °C ]T e m p e ra tu re [ °C ]T e m p e ra tu re [ °C ]T e m p e ra tu re [ °C ]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

M o/N H 4Y

M o/CsY

M o/RbY

M o/KY

M o/N aY

M o/LiY


89

this reaction [59]. Meyer and Hölderich [60] recently evaluated the decomposition of MBOH upon its ability to probe the basic strength of alkali-containing zeolites. They found good correlation between CO2 TPD experiments and MBOH decomposition results in probing the basicity of NaX based catalysts. Catalysed by Lewis acids, 3-methyl-3-butene-1-yn (mbyne) and 3-methyl-2-butenaldehyde (prenal) are formed, where basic sites decompose MBOH into acetone and acetylene (see Scheme 4.2). Additionally amphoteric sites (i.e. a combination of acidic and basic sites) can catalyse the production of 3-hydroxy-2-methyl-2-butanone and 3-methyl-3-buten-2-one [61]. When the selectivity towards acetylene and acetone is high compared to all other reaction products the yield of these reaction products can be taken as a measure for the catalyst basicity. Also strong Lewis centres can be estimated, since Lewis acid sites catalyse the dehydration of MBOH to 3-methyl-3-buten-1-yne [62]. The production of 3-methyl-3-buten-2-on occurs over Brønsted acid sites. Quite similarly 2–propanol decomposition yields propylene over acidic sites and acetone over basic sites [63]. In earlier experiments Lercher et al. [64] however, observed only propene as desorption product. Acetone was formed, but it was trapped in the pores of alkali-exchanged ZSM-5 as was evidenced by infrared spectroscopy. Therefore, the decomposition of MBOH is preferred over 2-propanol decomposition to estimate the basicity of alkali-exchanged zeolites. When performing MBOH decomposition the yields towards the aforementioned products can be taken as a measure for the basic and acidic functions of the catalyst studied. Scheme 4.2 shows the proposed reaction pathways. Performing the decomposition of MBOH, Fouad et al. [65] found infrared evidence for the presence of alcoholate-like species adsorbed on Cs+-promoted MgO. Accordingly, desorption of acetone and acetylene was observed at 70 ° C. For Ba2+-promoted MgO, MBOH adsorbed more strongly to the surface via the acetylene bond. This stronger interaction with the surface leads to surface reactions such as the polymerisation of acetone to di-acetone or mesityl oxide, showing that not only the reaction products mentioned in Scheme 4.2 are important, but also these polymerisation products that may cause a strong deactivating effect in the MBOH decomposition reaction.

Chapter 4

90

Scheme 4.2: Scheme 4.2: Scheme 4.2: Scheme 4.2: Determination of acidic and basic functions by MBOH decomposition.

Figure 4.14 shows the acidic and basic function of alkali-exchanged NaY. The values were corrected for the ion-exchange level, which was determined by Atomic Absorption Spectroscopy, according to Equation 4.2. The exchange level varied between 69 and 91 percent. This is slightly higher than those reported in related experiments [4,18]. The exchange levels are shown in Table 4.4. The DHG-values were obtained similarly.

(4.2)

where: q = fraction of exchanged Na+ DHD = Corrected dehydrydation function DHDexp = measured DHD-value DHDNaY = DHD-value of NaY

CH3

O

CH3

+ CH CH

acetone acetylene

CH3

CH3

OH

H

2-methyl-3-butyn-2-ol(MBOH)

Amphotericpathway

Bronstedacid pathway

Lewis acidpathway

Lewis basicpathway

CH3

CH3

OH

O

CH3

3-hydroxy-3-methyl-2-butanone

CH3

CH2

O

CH3

3-methyl-3-buten-2-one

3-methyl-3-buten-1-yn(mbyne)

3-methyl-2-buten-aldehyde (prenal)

CH3

CH2CH +

CH3

CH3

CH

CH

O

( )DHDq DHD

DHD q DHDNaY

NaY=

•

• − −

1 11exp ( )


91

As discussed earlier these trends are accounted for by the periodic changes of the electronegativity of the alkali cations applied. An increase of the basicity is expected from HY to CsY. The acidic function is in the opposite order. Deactivation was observed only for the HY based catalysts. This led to the slight deviation from the observed trend for these catalysts.

Figure 4.14: Figure 4.14: Figure 4.14: Figure 4.14: Acidic and basic function of alkali-exchanged NaY. The basic and acidic functions are corrected for the alkali ion exchange level according to Equation 4.2 and normalised to NaY = 1.

After introduction of Mo into the supercages of alkali-exchanged faujasite the basicity does not correspond to the periodic trend. For Mo/NaY the basicity is strongly increased compared to NaY, but for the other catalysts generally a lower DHG value was observed. The values in Table 4.5 suggest that the basicity is not solely determined by the alkali ion present in the zeolite lattice, but also by the Mo(CO)6 guest molecule. Table 4.5 shows the results of the MBOH decomposition reaction. The DHG values in Table 4.5 were corrected according to Equation 4.3, which is very similar to Equation 4.2.

(4.3)

Table 4.5: Table 4.5: Table 4.5: Table 4.5: Basic function of Mo encaged faujasite catalysts.

CatalystCatalystCatalystCatalyst Exchange levelExchange levelExchange levelExchange level1111 [%] [%] [%] [%] DHGDHGDHGDHGexpexpexpexp [%] [%] [%] [%] DHGDHGDHGDHG2222 [ [ [ [----]]]]

NaYNaYNaYNaY - 33.1 1 Mo/HYMo/HYMo/HYMo/HY 76 23.4 0.37 Mo/LiYMo/LiYMo/LiYMo/LiY 69 18.3 0.00 Mo/NaYMo/NaYMo/NaYMo/NaY - 59.2 1.79 Mo/KYMo/KYMo/KYMo/KY 91 38.9 1.11 Mo/RbYMo/RbYMo/RbYMo/RbY 72 34.2 0.74 Mo/CsYMo/CsYMo/CsYMo/CsY 72 n.d n.d 1 Exchange levels determined by A.A.S. 2 DHG value corrected for exchange level and normalised to DHGNaY=1

0

0.8

1.6

2.4

3.2

aci

dic

fu

nct

ion

aci

dic

fu

nct

ion

aci

dic

fu

nct

ion

aci

dic

fu

nct

ion

0.6

0.8

1

1.2

1.4

ba

sic

fun

ctio

nb

asi

c fu

nct

ion

ba

sic

fun

ctio

nb

asi

c fu

nct

ion

NaY

LiYKY

RbY

HY

HYNaY

LiYKY

RbY

0

0.8

1.6

2.4

3.2

aci

dic

fu

nct

ion

aci

dic

fu

nct

ion

aci

dic

fu

nct

ion

aci

dic

fu

nct

ion

0.6

0.8

1

1.2

1.4

ba

sic

fun

ctio

nb

asi

c fu

nct

ion

ba

sic

fun

ctio

nb

asi

c fu

nct

ion

NaY

LiYKY

RbY

HY

HYNaY

LiYKY

RbY

( )MoNaYNaY

DHGqDHGDHGq

DHG )1(11 exp −−•

•

=

Chapter 4

92

Figure 4.15: Figure 4.15: Figure 4.15: Figure 4.15: The effect of the basicity on the CO desorption temperature.

To determine the effect of the Mo(CO)6 host on the catalyst basicity Figure 4.15 shows the effect of the DHG value on the CO desorption temperature during oxidative Mo(CO)6/Y decomposition. A good correlation between the basic function and the desorption temperature was observed. The more basic the catalyst is, the lower the CO desorption temperature. This effect can be explained well by the higher electron density of the more Lewis basic catalysts. The electrons of the carbonyl group are stabilized better for the least basic catalysts. This means that the interaction of the Mo(CO)6 guest with the zeolite host takes place basically with the alkali cation, confirming characterization studies as described above.

3.5 Introduction of other transition metal carbonyls by CVD

3.5.13.5.13.5.13.5.1 Introduction of V(CO)Introduction of V(CO)Introduction of V(CO)Introduction of V(CO)6666 into NaY into NaY into NaY into NaY

Figure 4.16 shows the evolution of products during exposure of V(CO)6/NaY to artificial air. The carbonyl groups are immediately removed upon contact with the O2 flow. The decarbonylation is accompanied with uncontrolled oxidation of the vanadium. Heat generation was observed during oxidation, indicating heat transfer limitations. Therefore, the preparation of V/NaY by V(CO)6 deposition cannot be controlled, even under well-defined conditions as were applied in our experiments.

50

70

90

110

130

150

10 20 30 40 50 60 70

B a sic func t io n - D H G va lue [% ]B a sic func t io n - D H G va lue [% ]B a sic func t io n - D H G va lue [% ]B a sic func t io n - D H G va lue [% ]

Td

eso

rpti

on

[°C

]T

de

sorp

tio

n [

°C]

Td

eso

rpti

on

[°C

]T

de

sorp

tio

n [

°C]

Mo/LiY

Mo/RbY

Mo/KY

Mo/NaY

50

70

90

110

130

150

10 20 30 40 50 60 70

B a sic func t io n - D H G va lue [% ]B a sic func t io n - D H G va lue [% ]B a sic func t io n - D H G va lue [% ]B a sic func t io n - D H G va lue [% ]

Td

eso

rpti

on

[°C

]T

de

sorp

tio

n [

°C]

Td

eso

rpti

on

[°C

]T

de

sorp

tio

n [

°C]

Mo/LiY

Mo/RbY

Mo/KY

Mo/NaY


93

Figure 4.16: Figure 4.16: Figure 4.16: Figure 4.16: Room temperature oxidative decarbonylation of V(CO)6/NaY

Figure 4.17: Figure 4.17: Figure 4.17: Figure 4.17: TEM image of VOx/NaY.

TEM confirmed the expected low dispersion of vanadia clusters over the sample. EDX analysis showed that some segments of the sample did contain only vanadium whereas other parts of the sample did not contain any vanadium at all. The TEM image shown in Figure 4.17 illustrates the low vanadia dispersion over the catalysts. The vanadia clusters are significantly larger than 13.4 Å, the size of the supercage of NaY. Some of the vanadia clusters are indicated by arrows.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12Tim e [m in]

No

rma

lise

d I

nte

nsi

ty [

-]

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

No

rma

lise

d I

nte

nsi

ty [

-]

m /e = 28

m /e = 44

m /e = 32

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12Tim e [m in]

No

rma

lise

d I

nte

nsi

ty [

-]

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

No

rma

lise

d I

nte

nsi

ty [

-]

m /e = 28

m /e = 44

m /e = 32

Chapter 4

94

3.5.23.5.23.5.23.5.2 Introduction of MnIntroduction of MnIntroduction of MnIntroduction of Mn2222(CO)(CO)(CO)(CO)10101010 into NaY into NaY into NaY into NaY

The decarbonylation of Mn2(CO)10 occurs less effectively than decarbonylation of Mo(CO)6/NaY catalyst precursors. Since the volatility of Mn2(CO)10 is lower than the volatility of Mo(CO)6 the effect of the preparation temperature on the decarbonylation was examined. Figure 4.18 shows the temperature programmed decarbonylation of a series of 6 wt%. loaded Mn2(CO)10/NaY catalyst precursors. Two main differences can be observed. Firstly the amount of CO evolution differs greatly as function of evaporation temperature. The amounts of CO were not quantified, since no extra He was added to the product mixture after decarbonylation, contrary to the decarbonylation experiments of Mo(CO)6/NaY. The sample that was evaporated at 120 ° C shows the highest amount of CO produced, whereas hardly any CO evolution was observed for the catalyst precursor that was prepared at 160 ° C. Secondly the onset of Mn2(CO)10 decomposition (83 ° C) of Mn2(CO)10/NaY prepared at 60 ° C was significantly higher than that of the other catalyst precursors (43 ° C).

Figure 4.18: Figure 4.18: Figure 4.18: Figure 4.18: Decarbonylation of Mn2(CO)10/NaY (6 wt%). Heating by 5 ° C/min to 100 ° C.

The low amount of CO evolving from the catalyst that was prepared at 160 ° C can be explained by the low thermal stability of Mn2(CO)10. When the Mn2(CO)10/NaY catalyst precursor was heated to 220 ° C the pressure of the capsule containing the powder mixture was so high that the sample was lost during opening of the ampoule. Premature decarbonylation of Mn2(CO)10 caused the high pressure in the capsule. For Mn/NaY prepared

0

1

2

3

4

5


MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

0

25

50

75

100

125

Te

mp

era

ture

[°C

]T

em

pe

ratu

re [

°C]

Te

mp

era

ture

[°C

]T

em

pe

ratu

re [

°C]

T (evap)= 120 °C

T(evap)= 60 °C

T(evap)= 160 °C

Temperature

(right axis)


95

at 160 ° C also premature decarbonylation has taken place, though the total pressure in the capsule was not raised so high that the sample was completely lost. The very low amount of CO produced during decarbonylation evidences the loss of the greater part of the CO ligands from Mn2(CO)10 prior to activation of the catalyst precursor. The high onset temperature of CO production during decarbonylation of the Mn/NaY catalyst that was prepared at 60 ° C can be explained by low volatility of Mn2(CO)10. Heating to 60 ° C was insufficient to transport the Mn2(CO)10 through the pores of the NaY host. Therefore, Mn2(CO)10 probably was deposited mainly on the exterior zeolite surface or as bulk Mn2(CO)10. The onset temperature of CO evolution of bulk Mn2(CO)10 was found to be 79 ° C, which is similar to that of Mn2(CO)10/NaY prepared at 60 ° C.

Figure 4.19: Figure 4.19: Figure 4.19: Figure 4.19: TEM image of 1.7 wt% loaded MnOx/NaY.

For all Mn/NaY catalysts TEM reveals the presence of large manganese oxide clusters. Moreover, the distribution of Mn over the catalyst surface was inhomogeneous, as was examined by EDX analysis. A representative TEM image showing the presence of some large Mn containing clusters is shown in Figure 4.19. The electron microscopy studies are consistent with the information obtained from the temperature programmed desorption

Chapter 4

96

experiments. Low volatility of Mn2(CO)10 leads to deposition of Mn on the exterior of the zeolite structure. Increase of the preparation temperature in principle leads to enhanced diffusivity of the Mn2(CO)10. However, the low temperature stability of the molecule results in deposition of Mn on the external surface of the zeolite surface as well.

3.5.33.5.33.5.33.5.3 Introduction of Co(NO)(CO)Introduction of Co(NO)(CO)Introduction of Co(NO)(CO)Introduction of Co(NO)(CO)3333 into NaY into NaY into NaY into NaY

The introduction of Co via deposition of carbonyl complexes be done by using Co2(CO)8 and via Co(NO)(CO)3. Since it was shown that the use of di-nuclear complexes could lead to uncontrolled catalyst preparation due to lower volatility of the carbonyl compound, Co2(CO)8 was not applied in the research described here. Alternatively Co(NO)(CO)3 was introduced into the supercages of NaY.

Figure 4.20: Figure 4.20: Figure 4.20: Figure 4.20: TPOD of Co(NO)(CO)3/NaY.

As shown in Figure 4.20 decomposition of the molecule occurs stepwise in a rather complex way. At 85 ° C the main desorption peak of CO is observed. At slightly higher temperature some of the CO ligands are removed as CO2. The removal of NO ligands occurs in three steps. NO production is observed at 185 ° C and 310 ° C. The main peak of m/e= 28 at 85 ° C consists not only of CO, but also a part of the NO ligands is desorbed as N2, as was verified by deconvolution of the spectrum using N and C fragments.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 100 200 300 400 500Temperature [°C]Temperature [°C]Temperature [°C]Temperature [°C]

Nor

mal

ised

MS

Sign

al [

-]N

orm

alis

ed M

S Si

gnal

[-]

Nor

mal

ised

MS

Sign

al [

-]N

orm

alis

ed M

S Si

gnal

[-]

m/e= 30m/e= 44

m/e= 28


97

Figure 4.21:Figure 4.21:Figure 4.21:Figure 4.21: TEM image of CoOx/NaY

Though the low temperature desorption peak during TPOD suggests well-controlled Co oxidation, larger CoOx clusters were detected by TEM, as shown in Figure 4.21. Arrows indicate some of the CoOx clusters.

3.6 Catalytic activity in the ammoxidation of toluene

Figure 4.22: Figure 4.22: Figure 4.22: Figure 4.22: Toluene ammoxidation over Co/NaY as function of temperature. WHSV = 0.7; T: N: O= 1: 5: 8.

The benzonitrile yields over zeolite encaged metal oxides prepared from metal carbonyl precursors are generally low, as discussed in Chapter 3. Based on the high selectivity to benzonitrile that can be achieved, however, these catalysts can be of interest at low reaction temperature. In this respect the performance of Co/NaY is most interesting. Figure 4.22 shows the

0

20

40

60

80

100

360 380 400 420 440 460

T em p era tu re [ °C ]T em p era tu re [ °C ]T em p era tu re [ °C ]T em p era tu re [ °C ]

[mo

l%]

[mo

l%]

[mo

l%]

[mo

l%]

Benzonitrileselectivity Toluene

conversion

Benzonitrile yield

0

20

40

60

80

100

360 380 400 420 440 460

T em p era tu re [ °C ]T em p era tu re [ °C ]T em p era tu re [ °C ]T em p era tu re [ °C ]

[mo

l%]

[mo

l%]

[mo

l%]

[mo

l%]

Benzonitrileselectivity Toluene

conversion

Benzonitrile yield

Chapter 4

98

activity in toluene ammoxidation. At increasing temperature the benzonitrile selectivity decreases. An even stronger decrease in the benzonitrile yield was observed for Mn/NaY as was explained in Chapter 3. Moreover, a strong deactivation of the conversions of toluene, ammonia and oxygen occurs as function of time on stream (see Figure 4.23).

Figure 4.2Figure 4.2Figure 4.2Figure 4.23: 3: 3: 3: Deactivation of Mn/NaY during toluene ammoxidation. T= 360 ° C; WHSV = 0.9; T: N: O= 1: 8: 13.

Not only the toluene conversion decreases strongly with time, but also the conversion of oxygen and ammonia. Though ammonia depletion did not occur at any of the temperatures applied the benzonitrile selectivity may have been limited by the ammonia combustion reaction. This reaction leads to the consumption of oxygen. Since all oxygen was consumed at high temperatures and thus high conversion levels, benzonitrile could not be reached. The selectivity towards benzonitrile is proportional to the concentration of oxygen in the reactor as shown in Figure 4.24. Clearly, the strong deactivation observed over Mn/NaY makes it almost impossible to compare the catalytic performance on a more quantitive basis. The initial reaction rate should be compared, but the cause of deactivation should be studied in detail in order to be able to define the initial concentrations of reactants and (intermediate) reaction products.

0

20

40

60

80

100

0 50 100 150 200 250 300T im e o n s t r e a m [m in ]T im e o n s t r e a m [m in ]T im e o n s t r e a m [m in ]T im e o n s t r e a m [m in ]

Co

nv

ers

ion

[m

ol%

]C

on

ve

rsio

n [

mo

l%]

Co

nv

ers

ion

[m

ol%

]C

on

ve

rsio

n [

mo

l%] Toluene

Oxygen

Ammonia

0

20

40

60

80

100

0 50 100 150 200 250 300T im e o n s t r e a m [m in ]T im e o n s t r e a m [m in ]T im e o n s t r e a m [m in ]T im e o n s t r e a m [m in ]

Co

nv

ers

ion

[m

ol%

]C

on

ve

rsio

n [

mo

l%]

Co

nv

ers

ion

[m

ol%

]C

on

ve

rsio

n [

mo

l%] Toluene

Oxygen

Ammonia


99

FiguFiguFiguFigure 4.24: re 4.24: re 4.24: re 4.24: Relation between O2 concentration and benzonitrile selectivity during toluene ammoxidation at 400 ° C.

3.7 The effect of the Lewis acidity and basicity on the ammoxidation of toluene over MoOx/Y

Figure 4.25: Figure 4.25: Figure 4.25: Figure 4.25: Toluene ammoxidation activity over alkali-exchanged Mo/Y as a function of the basicity. WHSV= 0.8; T= 400 ° C; T: N: O= 1: 5: 8.

Figure 4.25 shows the effect of the basicity of 12 wt% loaded Mo/Y catalysts on the toluene ammoxidation activity. Higher basicity of the catalysts leads to higher activity. The benzonitrile selectivity, on the other hand, seems to increase with the Lewis acidity of the catalyst as is indicated by Figure 4.26. This effect might be influenced by the size of the alkali cation. The catalysts that contain larger cations show higher benzonitrile

0

10

20

30

40

50

0 5000 10000 15000 20000 25000 30000 35000 40000

C (OC (OC (OC (O 2222 ) [pp m ]) [pp m ]) [pp m ]) [pp m ]

Se

lec

tiv

ity

[m

ol%

]S

ele

cti

vit

y [

mo

l%]

Se

lec

tiv

ity

[m

ol%

]S

ele

cti

vit

y [

mo

l%]

3

4

5

6

7

8

9

10

0 0.4 0.8 1.2 1.6 2

B as ic fu n ct io nB as ic fu n ct io nB as ic fu n ct io nB as ic fu n ct io n

X(T

olu

ene)

X(T

olu

ene)

X(T

olu

ene)

X(T

olu

ene)

Mo/LiY

Mo/RbY

Mo/KY

Mo/NaY

3

4

5

6

7

8

9

10

0 0.4 0.8 1.2 1.6 2

B as ic fu n ct io nB as ic fu n ct io nB as ic fu n ct io nB as ic fu n ct io n

X(T

olu

ene)

X(T

olu

ene)

X(T

olu

ene)

X(T

olu

ene)

Mo/LiY

Mo/RbY

Mo/KY

Mo/NaY

Chapter 4

100

selectivities. The size of the alkali cation, and thus the size of the zeolite pores, does not seem to influence the rate of toluene activation. Interpretation of the results is rather difficult and preliminary, since few data have been measured and the selectivity towards benzonitrile is rather low over faujasite encaged MoOx catalysts. Nevertheless, the higher activity over the more basic catalysts could be explained by the formation of a carbanion as a first step of toluene activation. Stabilization of the carbanion occurs on the basic centers of the catalyst. Also Guseinov et al. found higher toluene activation rates over V-Sb-Bi-O catalysts during toluene ammoxidation [66]. Since no further characterization on the nature of reaction intermediates was performed conclusive explanations can be made based only on the reaction data given here.

Figure 4.26:Figure 4.26:Figure 4.26:Figure 4.26: Benzonitrile selectivity over alkali-exchanged Mo/Y as a function of acidity. WHSV= 0.8; T= 400 ° C; T: N: O= 1: 5: 8.

4. Conclusions Intra-zeolite molybdenum oxide nanoclusters can be produced by entrapment of Mo(CO)6 in faujasite zeolites. Thermal desorption of the encaged Mo(CO)6 structure in an oxygen containing gas mixture leads to low temperature decarbonylation of Mo(CO)6. During this decarbonylation Mo is oxidized to a 6+-oxidation state. Decarbonylation and oxidation occurs more easily compared to decomposition in helium, followed by a separate oxidation treatment. In this process subcarbonyl species could have been formed, though we did not find direct experimental evidence for this. The subcarbonyl species appear to be very reactive, both towards

0

10

20

30

40

50

0 1 2 3 4 5A cid ityA cid ityA cid ityA cid ity

S(B

enz

on

itri

le)

[mo

l%]

S(B

enz

on

itri

le)

[mo

l%]

S(B

enz

on

itri

le)

[mo

l%]

S(B

enz

on

itri

le)

[mo

l%]

Mo/LiY

Mo/NaY

Mo/RbY

0

10

20

30

40

50

0 1 2 3 4 5A cid ityA cid ityA cid ityA cid ity

S(B

enz

on

itri

le)

[mo

l%]

S(B

enz

on

itri

le)

[mo

l%]

S(B

enz

on

itri

le)

[mo

l%]

S(B

enz

on

itri

le)

[mo

l%]

Mo/LiY

Mo/NaY

Mo/RbY


101

oxidation by oxygen and by recarbonylation using gas phase CO. The Mo(CO)6 structure interacts with the extra-framework cations of the zeolite host. Higher electron density of the cation leads to weaker interaction with the Mo(CO)6 guest. Introduction of Mn, or V by means of metal carbonyl deposition cannot be performed in a reproducible manner. The low volatility of Mn2(CO)10 complicates diffusion through the zeolite pores. This leads to deposition of large manganese oxide clusters on the external zeolite surface. The transport of Mn2(CO)10 can be slightly improved by increase of the preparation temperature. This, however, leads to premature decarbonylation of the Mn2(CO)10 guest. V(CO)6/NaY mixtures are extremely sensitive to oxygen, leading to instantaneous and uncontrolled decarbonylation and oxidation of the V(CO)6 guest. Over Mn/NaY catalysts the ammoxidation of toluene was accompanied with strong deactivation. Lack of available oxygen seemed to govern the low benzonitrile selectivity. Toluene ammoxidation over Mo/NaY was influenced by the acid/base properties of the catalyst. Higher Lewis basicity leads to higher toluene conversion, indicating heterolytic toluene rupture.

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105

Chapter 5 The effect of molybdenum oxide reducibility on the

ammoxidation of toluene

Abstract The effect of molybdenum oxide reduction on the ammoxidation of toluene was studied using γ-alumina supported molybdenum oxide (Mo/Al) catalysts, which were prepared by pore volume impregnation. The reducibility was studied by hydrogen TPR as well as by temperature programmed ammonia decomposition. In situ Raman spectroscopy proved that ammonia reduces molybdenum oxide. Under toluene ammoxidation reaction conditions, however, XPS showed only the presence of Mo6+ at the catalyst surface. It was shown that the benzonitrile production rate is increased at increasing Mo loading. At low loadings only tetrahedrally coordinated isolated surface molybdate is present at the catalyst, as was shown by Raman Spectroscopy and diffuse reflectance UV spectrometry. These isolated molybdates show low toluene ammoxidation activity. Hydrogen–deuterium exchange was used to probe the Mo dispersion. When the Mo loading is increased the activity for the hydrogen exchange reaction strongly decreases. At Mo loadings higher than 8.5 wt% no H-D exchange activity was observed, indicating that only tetrahedrally coordinated surface molybdates are active for the H2-D2 exchange reaction.

The effect of vanadia dopants on the toluene ammoxidation activity of Mo/Al was studied also. As was shown by Raman spectroscopy MoO3-crystallites are formed when Mo/Al is doped with vanadia. XRD showed that the MoO3 clusters have a very small size, since no long range ordering was observed. Transmission Electron Microscopy confirmed the small MoO3 cluster size. MoO3 clusters were not formed when the catalyst contained only Mo, since the loading of the catalyst did not exceed the monolayer capacity of the support. Both benzonitrile selectivity and catalyst activity increased when Mo/Al was doped with vanadia.

1. Introduction Supported molybdenum oxide catalysts have been described in the literature very frequently. Since Mo based catalyst formulations are applied in many types of catalytic reactions, such as selective (amm)oxidation, metathesis and hydrotreating, both the preparation and the surface properties of these catalysts have been intensively studied. Recently, industrial interest for Mo-based ammoxidation catalysts increased, because of the high nitrile yields that can be obtained in alkane ammoxidation [1]. Although ammoxidation reactions usually

Chapter 5

106

involve multi-component catalysts there is still a lack of knowledge about the exact role of Mo in these reactions. Therefore, this study systematically examines the influence of molybdenum oxide reduction and reoxidation during the

ammoxidation of toluene. γ-Alumina was chosen as a support. By varying the Mo loading the properties of the Mo and O containing surface species were varied.

1.1 Preparation methods of supported Mo catalysts The preparation of γ-alumina supported molybdenum oxides (Mo/Al) can be achieved by several methods. In principle, the simplest technique is just making a physical mixture of MoO3 and the support. Upon heating in an O2 containing flow MoO3 spreads over the surface and forms a layer on the alumina surface [2,3], when applying the appropriate Mo:Al2O3-ratio. Recently, the preparation of Mo/Al catalysts by mechanically mixing MoO3 with the support was investigated in more detail [4,5]. It was shown that besides thermal activation, also mechanical

activation by ball milling can spread the MoO3 phase over the γ-alumina support [6]. Though in principle the method is very simple, control of the preparation by this method is not very easy. For example, moisture can play an inhibiting role [5]. Organometallic precursors can be applied to introduce Mo onto a variety of

supports, including γ-alumina. For example allyl-precursors (Mo(η3-C3H5)4 have been described by several authors [7-10]. The preparation using this method is quite complex [11]. Though authors sometimes assign special properties and Mo sites to this preparation method, others compared these catalysts with differently prepared supported molybdenum oxide without finding differences [10-12]. Similarly MoCl5 can be used for the preparation of supported molybdenum oxide catalysts. MoCl5 reacts with the hydroxyl groups of the support, producing so-called grafted Mo and HCl. A washing step is required to remove weakly bound Mo [13]. Other precursors have been used, most notably Mo(CO)6 [14,15]. Though these methods are quite versatile and the Mo oxidation state can be tuned by heat treatment, the maximum Mo loading that can be achieved is rather low. Therefore, in general these preparation methods are not preferred for the

introduction of Mo onto γ-alumina supports. Using ammonium heptamolybdate (AHM) as Mo source the equilibrium adsorption method can be applied [16-19]. This method consists of adsorption of molybdates onto the support at a fixed pH value of the aqueous solution applied. AHM adsorption onto the support takes an extended period of time, usually of the order of days [20]. Therefore, this method is applied basically for studying the

The effect of molybdenum oxide reducibility on the ammoxidation of toluene

107

effect of the surface charge on the Mo surface chemistry. By varying the pH the Mo loading of the catalyst can be controlled accurately [21]. Though catalysts prepared by equilibrium adsorption are very homogeneous [22],

impregnation of AHM solutions into the γ-alumina support has been used most often. Molybdates are attached to the surface OH groups and decompose upon

high temperature. Three different OH-groups can be distinguished in γ-alumina, as determined by infrared spectroscopy [23]. The different OH groups are drawn in Figure 5.1.

Figure 5.1:Figure 5.1:Figure 5.1:Figure 5.1: Surface OH groups present in γ-alumina.

The basic OH groups are involved when AHM is adsorbed onto the γ-alumina surface, as shown by the decrease of only the 3775 cm-1 line upon MoO2(acac)2 adsorption. Upon protonation of the basic sites molybdate adsorption can also

occur reversibly via electrostatic interaction [21]. Additionally, γ-alumina contains coordinatively unsaturated Al3+ sites. These sites are only involved with physisorbed adsorbates [16,18].

1.2 Notation of different Mo species Species containing Mo surrounded by O ligands exist in many different forms, both at the catalyst surface and in the impregnation solution. For the sake of convenience, the way these species will be indicated in this chapter is outlined here. In solution Mo species basically exist as molybdates. They will be referred to as monomolybdate or MoO4

2-, heptamolybdate or Mo7O246-, octamolybdate or

Mo8O264-, or simply as polymolybdate in the case where their degree of

polymerisation is unknown. At the catalyst surface Mo species can exist as molybdates, surrounded by counterions provided by the support surface and also as Mo oxides.

Al

OH

1: basic

Al Al

OH

2: neutral

Al Al

OH

Al

3: acidic

Chapter 5

108

Similarly, the surface molybdates will be notated as surface monomolybdate or surface MoO4

2-, surface heptamolybdate or surface Mo7O246-, surface

octamolybdate or surface Mo8O264-, or simply as surface polymolybdate in the case

where the degree of polymerisation is unknown. Surface Mo oxides are notated as MoO3, MonO(3n-1) and MoO2, dependent on their degree of reduction. When the degree of oxidation is unknown the surface molybdenum oxide will be notated simply as surface Mo oxide or MoOx. Unless otherwise mentioned the Mo loading of the catalyst is always expressed as wt% Mo metal, irrespective of the chemical nature of the Mo species. Based on the difference in structure tetrahedrally coordinated and octahedrally coordinated Mo can be distinguished. Monomolybdates generally have tetrahedral coordination and polymolybdates generally have octahedral coordination.

1.3 Molybdate surface species Many groups have studied the effect of the pH of the aqueous AHM solution. The pH value determines the structure of the molybdate species that are present in the impregnation solution. Equation 5.1 shows the equilibrium between heptamolybdate (Mo7O24

6-) and monomolybdate (MoO42-) species.

According to this equation the MoO42- ions are stable at pH higher than 6.5 [24],

whereas pH values lower than 4.5 lead to complete formation of Mo7O246- [25] in

the impregnation solution. Further decrease of the pH to 1.5 leads to formation of octamolybdate (Mo8O26

4-) [26], or to polymolybdates with an even higher degree of polymerisation. At low Mo loading the impregnation of AHM leads to the formation of monomolybdate surface species only, as was shown by Jeziorowski and Knözinger [27]. These authors impregnated alumina with AHM solutions at pH= 6 and pH= 11. The metal loading was 3 wt%. Only surface monomolybdate was observed by Raman spectroscopy. The presence of only surface monomolybdates at low Mo loading can be explained by the occurrence of an

irreversible reaction of heptamolybdate anions with the basic hydroxyl groups of alumina according to Equation 5.2 [18,28]. During catalyst drying, ammonia desorbs at a lower temperature than water, leading to a decrease in the pH of the solution. Therefore, even at high initial pH values surface monomolybdates and surface polymolybdates can be present as well.

7 MoO42- + 8 H+ Mo7O24

6- + 4 H2O (5.1)

8 Al-OH + Mo7O246- 4 (Al)2MoO4 + 3 MoO4

2- + 4 H2O (5.2)


109

The influence of the solutions pH-value can be explained well by a change in the surface pH at the point of zero charge (PZC), or iso-electric point (IEP). The charge of the surface can be either positive or negative, depending on the pH of the impregnation solution. When the pH value of the solution is above the IEP, the surface becomes negatively charged and

no adsorption of any of the negatively charged molybdate anions occurs. On the other hand, when the pH of the impregnation solution is lower than the IEP, molybdate anions can adsorb on the positively charged alumina surface [29]. By performing equilibrium adsorption on alumina Wang and Hall showed that the equilibrium Mo loading is a function of the pH of the solution [19]. At lower pH the amount of Mo that was adsorbed onto the alumina support was much higher than at higher pH values, which is consistent with this model. Figure 5.2 shows the adsorption curve. The break in the curve is at pH= 6-8, as was expected from the PZC, which is reported to be 6-8 [30] or 9.1 [31], depending on the crystal plane. According to Van Veen et al. [16] molybdates adsorb primarily on the basic Al-OH sites. In a second stage adsorption on the coordinatively unsaturated (cus) Al3+ occurs. They performed equilibrium adsorption experiments at different pH values. As expected from the PZC model a higher number of molybdates adsorbed at the surface at lower pH values. Irrespective of the pH applied it was found that after consumption of all basic Al-OH groups still molybdate adsorption from the liquid phase occurred. It was indicated that the cus Al3+ sites were consumed during the adsorption of additional molybdate. It was shown also that precipitated

molybdates could be formed on γ-alumina [18]. As was discussed above protonated OH groups are involved via electrostatic interaction [21]. Applying a series of 13 wt% Mo loaded alumina catalysts Okamoto et al. [32] did not find the presence of a larger amount of surface polymolybdates at lower pH

Figure 5.2:Figure 5.2:Figure 5.2:Figure 5.2: Molybdate adsorption on Al2O3 [19].

0

2

4

6

8

0 2 4 6 8 10

F inal p H valueF inal p H valueF inal p H valueF inal p H value

Ad

sorb

ed m

oly

bd

enu

mA

dso

rbed

mo

lyb

den

um

Ad

sorb

ed m

oly

bd

enu

mA

dso

rbed

mo

lyb

den

um

[•1

0[•

10

[•1

0[•

10

-20

-20

-20

-20 m

ol/

g m

ol/

g m

ol/

g m

ol/

gca

tca

tca

tca

t] ]]]

Chapter 5

110

values. Contrary, they observed a slight decrease of Mo dispersion when the pH of the impregnation solution was increased from 2 to 8. They also explained this slightly lower dispersion by the IEP of alumina, which was assumed to be 8.8 [33]. During impregnation most of the molybdate anions are in solution, without adsorption to the alumina surface, resulting in agglomerated Mo species during drying. The fraction of adsorbed molybdates during impregnation is higher when the difference between the pH of the solution and the PZC of the support is larger.

To conclude: the exact nature of the Mo species on γ-alumina is still not completely clear. Though in equilibrium adsorption experiments the PZC model explains well the presence of surface monomolybdates (high pH), surface polymolybdates (low pH) and precipitated molybdates on the surface no equilibrium can be reached during incipient wetness impregnation. Since the pores of alumina are exactly filled with the impregnation solution when this method is applied deposition of additional amounts of AHM occurs. Upon calcination Mo originating from deposited AHM could be bound to the alumina support, since usually no significant Mo loss takes place upon catalyst calcination. This means that the Mo loading of the catalyst can be higher than expected from equilibrium adsorption according to the PZC-theory only.

1.4 Characterization of Mo surface species Raman spectroscopy has been applied very frequently to study the Mo surface species, which are formed after calcinations of the impregnated catalysts. As explained above, AHM impregnation followed by calcination can lead to different surface molybdates and molybdenum oxide species. Depending on the Mo loading, generally three different Mo species can be distinguished. Furthermore aluminium molybdate can be formed [34]. At low Mo loadings tetrahedral surface monomolybdates are formed. If the Mo loading is increased, octahedral coordinated molybdates are formed, having Raman shifts in the 940 to 975 cm-1 range [35]. Wang and Hall claim that the intensity ratio of the Raman bands at 950 cm-1 and 970 cm-1 can be taken as a measure for the ratio between the tetrahedral and the octahedral molybdates [36]. However, other work does not suggest any shift in the Mo=O stretching band position between tetrahedral and octahedral coordinated molybdates [37]. Subsurface Al2(MoO4)3 was found to be

formed at Mo loadings near 20 wt% (γ-Al2O3 surface area 240 m2/g) [38]. If the Mo loading is increased even further, MoO3 phases are formed. The loading at which MoO3 formation occurs is generally denoted as monolayer loading. This monolayer loading is calculated by assuming complete coverage of the catalyst


111

surface estimating a size of 0.15 nm2 per surface interacting MoO3 group [39]. No evidence is described in the literature that only MoO3 is formed when the Mo loading exceeds the so-called monolayer coverage. In fact Brown et al. [40] find evidence for the presence of both Al2(MoO4)3 and MoO3 in a 18 wt% loaded Mo/Al catalyst. Nevertheless, it is generally accepted that at Mo loadings lower than the so-called monolayer loading MoO3 is not present. Based on X-ray diffraction data as described by Stencel [41] three distinct molybdenum oxide structures can be distinguished in MoO3.

1) One terminal O atom is bound to one Mo atom 2) Two bridged O atoms are bound to two Mo atoms 3) Three bridged O atoms are bound to three Mo atoms

The three different Mo-O bonds are sketched in Figure 5.3. For the sake of convenience the coordination of only one O atom is sketched in this figure.

Figure 5.3: Figure 5.3: Figure 5.3: Figure 5.3: Different Mo-O bonds in MoO3

The resulting Raman and infrared bands are shown in Table 5.1.

Table 5.1: Table 5.1: Table 5.1: Table 5.1: Position of Raman and infrared bands for MoO3

Assignment (stretch)Assignment (stretch)Assignment (stretch)Assignment (stretch) MoMoMoMo----O bond length [Å]O bond length [Å]O bond length [Å]O bond length [Å] IR [cmIR [cmIR [cmIR [cm----1111]]]] Raman [cmRaman [cmRaman [cmRaman [cm----1111]]]]

I:O-Mo B3u 1.67 998 (1004) 885

I: O-Mo A1g, B1g 1.67 993 (997) II: O-Mo2 B3u 1.73, 2.25 810 (840) II: O-Mo2 B1g 1.73, 2.25 817 (820) III: O-Mo3 B3g 1.95, 1.95, 2.33 664 (668) III: O-Mo3 B1u 1.95, 1.95, 2.33 566 (545)

Experimental data: Nazari et al. [42] and Beattie and Gilson [43] (in brackets).

The interpretation of complex Raman spectra is often based on the strongest Raman lines of the MoO3 spectrum. E.g. if only one strong raman band is observed at 993 cm-1 bridging O atoms are not present in the surface molybdate. Such a Raman spectrum suggests the presence of isolated surface monomolybdates.

I: OMo II: OMo2 III: OMo3

O atomMo atom

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When Al2(MoO4)3 has formed, Raman bands are present at 877, 845, 791 and 321 cm-1 as measured by Brown et al. [40], who recorded the spectra of several molybdate reference salts. The difference between the Raman spectra of tetrahedral and octahedral surface molybdates is less well defined. Both species show bands in the 940 to 990 cm-1 range. Raman spectra of solutions containing polymolybdate anions show a strong line at 943 for Mo7O24

6- and at 965 cm-1 for Mo8O26

4- indicating frequency increase of the bands in this region when the degree of polymerisation increases [26]. Though distinguishing between the exact nature of the surface molybdate species by Raman spectroscopy is difficult, all surface polymolybdate species contain not only a band in the 940 to 990 cm-1 range, but also a less intense band in the 840 to 890 cm-1 range. Additionally in the low frequency range of the spectrum the Mo-O-Mo bending mode can be observed at 220 cm-1. This band dominates the low-frequency portion of the spectrum [10]. The absence of Raman bands in these regions indicates the absence of polymolybdate surface species. The Mo=O bending mode of tetrahedrally coordinated monomolybdate is found around 320 cm-1, as was measured for MoO4

2--solutions [44]. For octahedrally coordinated Mo several bands are found for the Mo=O bending. The 320 cm-1 band is shifted to higher wavenumber, approximately 365 cm-1 [45]. Though the intensity of the low wavenumber bands is rather low, these bands can be used to distinguish between tetrahedrally and octahedrally coordinated Mo in supported Mo catalysts.

1.5 Molybdate and Mo oxide reduction Reduction of surface molybdates can be described by the so-called shear plane

model [46]. This model, which was originally developed for the reduction of bulk MoO3 is also applicable to Bi-Mo catalysts as described by Brazdil et al. [47]. Oxygen vacancies arise during reduction of tetrahedral and octahedral molybdate, as sketched in Figure 5.4 and 5.5. After reduction ordered arrays of oxygen vacancies are formed. The layers,

ReductionReductionReductionReduction ReductionReductionReductionReduction

SurfaceSurfaceSurfaceSurfacereconstructionreconstructionreconstructionreconstruction

Figure 5.4: Figure 5.4: Figure 5.4: Figure 5.4: Mo oxide tetrahedra reduction and reconstruction into shears [46].


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which were initially linked via the corners, are rearranged into edge-linked octahedra. After surface reconstruction these vacancies are filled and the catalyst is reoxidized. The surface reconstruction requires a minimum number of oxygen vacancies, depending on the bulk structure of the metal oxide applied [48]. If this reduction model is applied to supported Mo oxide surface reconstruction cannot occur when the Mo surface species are isolated. This is the case when the Mo loading of Mo/Al is low, as was indicated by UV-VIS and by Raman spectroscopy. Surface polymolybdates that are formed at higher Mo loadings, therefore, are easier to reduce.

Figure 5.5: Figure 5.5: Figure 5.5: Figure 5.5: Mo oxide octahedra reduction and reconstruction into shears [46].

A recent publication clearly showed that well-defined molybdenum suboxides (MonO(3n-1), e.g. Mo4O11) are not formed during MoO3 reduction [49]. MoO3 is converted to MoO2 in one step at temperatures below 420 ° C. During reduction the growth of the crystallite size was observed for all phases present. This explains accelerated MoO3 reduction at higher temperature assuming a nucleation-growth mechanism. At higher temperature the solid-state reaction of MoO3 and MoO2 can result in the formation of Mo4O11 when the hydrogen concentration is high (over 40%). These severe reduction conditions are not applied in selective oxidation reactions. Therefore the formation of long-range ordered structures (e.g. Mo8O23) can be excluded during MoO3 reduction. Shear planes (having short-to-medium-range disorder possibly form from oxygen vacancies as explained above.

ReductionReductionReductionReduction

SurfaceSurfaceSurfaceSurfacereconstructionreconstructionreconstructionreconstructionReoxidationReoxidationReoxidationReoxidation

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2. Materials and methods

2.1 Catalyst preparation Mo/Al catalysts were prepared by incipient wetness impregnation. The pores of γ-alumina (Ketjen, surface area 205 m2/g, pore volume 0.55 ml/g) were exactly filled with aqueous AHM solutions, containing different amounts of AHM in order to vary the Mo loading of the catalyst. In most cases the impregnation solution was kept at pH= 10 by adding NH3, but also some catalysts were prepared using neutral impregnation solutions. After impregnation, the catalyst precursors were heated overnight in ambient air to 110 ° C. In order to gently remove the water the catalyst precursors were kept for two hours at 60 ° C and 80 ° prior to heating to 110 ° C. Calcination was then performed by heating the catalyst with 50 ° C increments from 200 ° C to 400 ° C in static air. The sequence described above was repeated for those catalysts that contained a Mo loading higher than 14 wt%, due to limited solubility of AHM in water. MoV/Al catalysts were prepared by co-impregnation. The impregnate contained not only AHM, but also NH4VO3 in this case. The pH of these impregnates was neutral. Due to the low solubility of NH4VO3, impregnation was performed twice in order to obtain the desired Mo and V loadings. The catalysts are denoted using the format Mo(wt%)V(wt%)/Al. Unless otherwise mentioned catalyst loadings are given as weight percentage of metal. In the case loadings are calculated on a per mole basis, the loadings are given as mole of metal per mole of Al2O3. The metal loading of all catalysts was analysed by Atomic Absorption Spectrometry, using a Perkin-Elmer 3030 Atomic Absorption Spectrophotometer. The standard addition method was applied for the analysis of Mo.

2.2 Catalyst characterization

2.2.12.2.12.2.12.2.1 Diffuse reflection UVDiffuse reflection UVDiffuse reflection UVDiffuse reflection UV----Vis spectroscopyVis spectroscopyVis spectroscopyVis spectroscopy Diffuse reflectance UV-Vis (DR-UVVis) spectra were recorded on a Shimadzu UV-2401 spectrometer, equipped with a Shimadzu ISR-240A integrating sphere. Cuvettes having a path length of 5.0 mm constructed from Suprasil quartz were

used. γ-Alumina shows some absorption at low wavelengths (lower than 500 nm).

Since all Mo oxide bands show intensities in this region γ-alumina was used as reference for all experiments. All samples were ground before the experiment.


115

2.2.22.2.22.2.22.2.2 TeTeTeTemperature Programmed Reductionmperature Programmed Reductionmperature Programmed Reductionmperature Programmed Reduction Temperature Programmed Reduction (TPR) was performed by heating 25 mg of catalyst in an 8 ml/min N2/H2 flow (96%/4%). An equal degree of oxidation was assured for all catalysts by heating the samples in situ to 500 ° C in He/O2 (8 ml/min, 96%/4%) flow prior to TPR experiment. The samples were flushed in He and N2 subsequently to remove all oxygen present at the catalyst after this treatment. H2 and O2 consumption was monitored using thermistor detectors (Gow Mac Instrument). Water formed during reduction was removed using a zeolite 4A molecular sieve before detection of the hydrogen signal. The temperature resolution was 1 ° C.

2.2.32.2.32.2.32.2.3 Raman SpectroscopyRaman SpectroscopyRaman SpectroscopyRaman Spectroscopy Raman Spectroscopy was performed using a single monochromator Renishaw 1000 spectrometer equipped with a cooled CCD detector (200 K) and a holographic Notch filter. The samples were excited with the 514 nm Ar line in an in situ cell (Linkam, TS-1500). The spectral resolution was better than 2 cm-1. The samples were heated in N2 to 500 ° C by 15 ° C per minute prior to recording the Raman signal, in order to remove fluorescence. The Raman shift was recorded after cooling the sample to room temperature. Some in situ analyses were performed while continuously flowing He, O2 and/or NH3. The Raman signal was recorded under He flow at room temperature after treatment at different temperatures and under different flow conditions. UV Raman spectra were recorded using a Jobin-Yvon/Spex T64000 Raman spectrometer. Excitation of the catalysts took place using a Lexel Laser Inc. model 95 ion laser, using the 244 nm line. The spectral resolution was 2.2 cm-1 in this case. The catalyst samples were not pre-treated prior to the experiments, since the use of the UV laser prevented the occurrence of fluorescence in the spectra.

2.2.42.2.42.2.42.2.4 Transmission Electron MicroscopyTransmission Electron MicroscopyTransmission Electron MicroscopyTransmission Electron Microscopy Transmission Electron Microscopy (TEM) was performed using a Philips CM 30 ST electron microscope, equipped with a LaB6 filament as electron source, operated at 300 kV. The catalyst was mounted on a carbon polymer microgrid supported on a copper grid, by placing a few droplets of a suspension containing the powdered catalyst in ethanol and evaporating the ethanol under ambient conditions. Several grains of the sample were analysed in order to obtain a representative image, without focusing on possible local artefacts.

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EDX analysis was performed using a LINK EDX system by turning the electron beam towards the detector. This enabled us to obtain information about the chemical composition of a small part of the TEM image.

2.2.52.2.52.2.52.2.5 XXXX----Ray DiffractionRay DiffractionRay DiffractionRay Diffraction X-Ray powder diffraction (XRD) patterns were recorded on a Rigaku Geigerflex

X-ray powder diffractometer using Cu-Kα radiation. Prior to the experiment the catalysts were grinded and pressed into a sample holder containing vaseline. The applied scanning speed was 1° per minute. Background subtraction was not applied.

2.2.62.2.62.2.62.2.6 XXXX----Ray Photoelectron SpectroscopyRay Photoelectron SpectroscopyRay Photoelectron SpectroscopyRay Photoelectron Spectroscopy X-ray Photoelectron Spectroscopy (XPS) experiments were performed using a VG

ESCALAB 200 spectrometer equipped with an Al Kα X-ray source. A hemispherical analyzer was used for detection at a pass-energy of 20 eV. The catalyst samples were ground and pressed in an indium film placed on an iron

stub. γ-Alumina based samples were corrected for charging assuming a binding energy of 74.4 eV for the Al 2p peak. Charging was usually on the order of 9 eV. The data were analysed by a standard fit routine using a non-linear Shirley background subtraction and a Gauss-Lorentzian curve-fit function. Samples that were transferred to the XPS chamber without exposure to ambient conditions by mounting the sample in the indium film in a N2 filled glove box. For this purpose reaction and pre-treatment of the catalysts was performed in a reactor that could be closed and transported to the glove box preventing exposure of the catalyst to ambient air. The XPS cell could also be closed from ambient air.

2.2.72.2.72.2.72.2.7 HydrogenHydrogenHydrogenHydrogen––––deuterium exchange reactionsdeuterium exchange reactionsdeuterium exchange reactionsdeuterium exchange reactions Hydrogen-deuterium exchange reactions were performed in a recirculation reactor setup equipped with a membrane pump. Oxygen traces were removed from the hydrogen-deuterium exchange gas mixture using a BTS catalyst. After purging the system sampling of 10 ml H2 and 10 ml D2 was performed. The carrier gas applied was Ar. During recirculation a small amount of the reaction mixture was leaked into a quadruple mass spectrometer (Balzers QMG 200M system) equipped with a secondary electron multiplier operated at 1200 V. The total decrease in pressure during a reaction was not larger than 1 %. Typically mass spectra were recorded every 30 seconds. The initial reaction rate was calculated by extrapolation of the


117

signals to t= 0 s. Two hundred milligrams of catalyst were used for the reaction. The reaction temperature was 150 ° C. The circulation flow rate was 60 Nml/min.

2.3 Ammoxidation of toluene Toluene ammoxidation reactions were carried out in a single-pass tubular reactor (4 mm internal diameter) under plug flow conditions. The flows of NH3, O2 and He (as an inert diluent) were controlled using Brooks Thermal Mass-flow Controllers. Part of the He flow was saturated with toluene (p.a.) using a three-step saturator that was kept at a temperature of 9.4 ° C. No purification of the gases was found to be necessary. All lines after the catalyst bed were kept at a temperature of 200 ° C in order to prevent condensation of products. The organic products were analysed by on-line gas chromatography, using a Hewlett Packard 5890 series II GLC, equipped with a 50 m HP-5 column and a flame ionisation detector. Conversions, selectivities and benzonitrile production rates were calculated, based on the toluene inlet signal, which was measured before starting the reaction. Toluene conversion levels and selectivities towards organic products could be analysed accurately by using this method, irrespective of a lack of carbon balance. The molar ratio of toluene: NH3: O2 is represented as T: N: O.


3.1 Addition of a second metal to Mo/Al As is shown before [50] Mo/Al produces benzonitrile in high yields in the ammoxidation of toluene and addition of vanadia dopants leads to increase of the benzonitrile yield. In this section the characterization of MoV/Al is described in more detail. To facilitate interpretation catalysts loadings are expressed as mole percentage of metal in this section. The influence of V addition to Mo/Al catalysts on the toluene ammoxidation is shown in Figure 5.6. Rate constants were calculated per mol of metal assuming first order kinetics in toluene. The toluene ammoxidation kinetics are not well described in the literature. Only empirical rate equations are available. Therefore this assumption is rather speculative. As a rough estimate, however, first order hydrocarbon dependence can be applied [51]. The reaction is assumed to be independent in NH3 and O2 above a minimum concentration of both reactants. Consecutive and side reactions were neglected for the calculation of the rate constants. The Mo(11.6)/Al and V(3.3)/Al catalysts,

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also prepared using neutral impregnation solutions, are included for comparison. These catalysts were. Addition of V to Mo(11.6)/Al leads to a strong increase in toluene ammoxidation activity, with respect to both undoped Mo/Al and undoped V/Al catalysts. As shown before [50], the benzonitrile selectivity is hardly influenced upon V doping.

Figure 5.6: Figure 5.6: Figure 5.6: Figure 5.6: Effect of V addition to Mo/Al on the toluene ammoxidation rate. T: N: O= 1: 8: 13; WHSV = 0.7. Numbers in parentheses are mole percentages of Mo resp. V.

Raman spectroscopy studies were performed on the MoV/Al samples, as shown in Figure 5.7. The spectrum of Mo(11.6)/Al contains a somewhat broadened band at 996 cm-1 only. This band was observed also by Vuurman et al. [52]. The band can be assigned to the stretching vibration of Mo=O groups [27]. Since no other Raman bands are present this Mo=O stretching must originate from an isolated surface monomolybdate [53]. Studying Mo/SiO2 De Boer et al. [54] and Bañares et al. [55] also observed the formation of isolated surface molybdates. They observed a slightly lower frequency (986 cm-1) for the Raman band of dehydrated Mo/SiO2. It is well known that the Mo=O stretching frequency is higher when the sample is dehydrated [56]. Due to interaction with surrounding H2O molecules the Mo=O bond is weakened when the sample is exposed to ambient air. The effect of

dehydration is generally more pronounced for Mo/γ-Al2O3 than for Mo/SiO2 [11]. The broadening of the Mo=O band in our Mo(11.6)Al sample (see Figure 5.7) may indicate the small cluster size of the surface Mo species when no V is present [57]. The absence of Raman bands at around 820 cm-1 (antisymmetric O-Mo-O stretching [27]) and 220 cm-1 (Mo-O-Mo deformation) indicates that surface

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polymolybdates were not formed in significant amounts when only Mo was present [10,54].

Figure 5.7: Figure 5.7: Figure 5.7: Figure 5.7: Raman Spectroscopy for Mo/Al and MoV/Al samples. Numbers in parentheses are mole percentages.

Conversely, for the MoV/Al samples at V loadings higher than 0.8 mol% (higher than 0.4 wt%) the presence of MoO3 was clearly observed. The Mo(11.6)V(0.8)Al spectrum is not plotted since it was highly distorted due to the presence of a strong fluorescence background. No indication for the presence of MoO3 was found for this catalyst, though. For reference also the spectrum of bulk MoO3 was recorded, showing the same bands as the MoV/Al samples. The formation of MoO3 is not expected based on the Mo loading. If we calculate the theoretical monolayer a Mo loading of around 23 mol% is found. Apparently V addition reduces monolayer coverage. As a result MoO3 is formed at lower loadings. Competition between Mo and V for reaction with the alumina hydroxyl groups was also observed by Vuurman et al. [58]. These authors found an increase in polymerised vanadium oxide species when molybdena or tungsten oxide was present. The presence of MoO3 could explain the higher toluene ammoxidation activity of

MoV/Al. The dispersion of the MoO3 clusters over the γ-alumina support is quite good, since XRD did not reveal any peaks that correlate with MoO3 or Al2(MoO4)3 as can be seen from Figure 5.8. The detection limit is determined by

the crystal size, which is estimated at around 4 nm [59]. Only γ-alumina spacings could be clearly identified. Some evidence was found for the formation of Al2V10O28∙ 22H2O (lattice spacings of 10.6 and 7.0 Å). The presence of the strong

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line at a diffraction angle of 47 ° (lattice spacing of 3.1 Å) could not be explained.

This line is present in the pure γ-alumina batch applied. The diffractogram of the γ-alumina support was reported earlier by Peeters et al. [60]. The noisy background level of the spectrum shows the presence of an amorphous fraction in the catalyst.

Figure 5.8: Figure 5.8: Figure 5.8: Figure 5.8: Röntgen diffractogram of Mo(11.4)V(3.1)/Al. * typical γ-Al2O3 lines; •

typical Al2V10O28∙ 22H2O lines.

3.2 Variation of the molybdenum oxide loading Figure 5.9 shows the effect of increasing the Mo loading of Mo/Al on the ammoxidation of toluene.

Figure 5.9: Figure 5.9: Figure 5.9: Figure 5.9: Toluene ammoxidation over Mo/Al as a function of Mo loading. T= 380 ° C; T: N: O= 1: 5: 8; WHSV = 0.8.

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The activity per mole Mo, which was measured under differential conditions, is plotted as a function of Mo loading. When all Mo sites would have similar toluene ammoxidation activities, a constant activity is expected since the activity is expressed per mole Mo. However, after a slight decrease until a Mo loading of around 8 wt% the activity increases with the catalyst loading. This increase was mainly caused by the higher benzonitrile selectivity observed at higher Mo loading. At Mo loadings higher than 8.5 wt%, Mo sites are formed that are significantly more active for the ammoxidation of toluene. With increasing above 8.5 wt% Mo the number of these active clusters increased, leading to higher benzonitrile production rates. As will be explained in great detail in the following sections the Mo surface chemistry is strongly influenced by the Mo loading. At low Mo loading surface monomolybdates are formed, whereas at higher loading surface polymolybdates are present. Recently, Han et al. [61] discussed the relationship between metal oxide cluster size and propylene oxidation activity over silica supported bismuth molybdate catalysts. They found poor acrolein yields over those catalysts that contained highly dispersed metal oxide clusters. Assuming a Mars and Van Krevelen mechanism a rather bulky layer that can play the role of an active oxygen reservoir is required. At low Mo loadings only highly dispersed surface monomolybdates are present, which cannot supply oxygen, as explained in Section 1.5. This could explain the low (amm)oxidation activity of this catalyst. Similarly Iwasawa [7] reports much higher turnover frequencies for propylene oxidation over silica supported surface polymolybdates compared to surface monomolybdates. Also the selectivity to acrylaldehyde is much higher over surface polymolybdates than over surface monomolybdates. For Mo/Al Peeters et al. [60] found the presence of larger MoOx clusters. They performed the oxidative ammonolysis of ethylene to acetonitrile. By hydrogen pre-treatment or by exposure for a longer time to reaction conditions the catalyst was reduced to a MoO2-like structure and loss of dispersion occurred. Compared to fresh catalysts they found a significantly higher activity for those reduced catalysts. This is in accordance with the low toluene ammoxidation activity found in our experiments when the Mo loading is low.

3.3 DR-UVVis Spectroscopy Figure 5.10 shows the DR-UVVis spectra of Mo/Al as function of the Mo loading.

All bands can be ascribed to the ligand to metal charge transfer processes O2- → Mo6+, since Mo has the d0 electronic configuration. The spectra were recorded

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against a γ-alumina blank sample since γ-alumina itself shows some absorbance at lower wavelength as shown in the inset.

Figure 5.10: Figure 5.10: Figure 5.10: Figure 5.10: DR-UVVis spectra of Mo/Al catalysts. The inset shows the γ-alumina spectrum (against a BaSO4 reference).

Those samples having a Mo loading up to 6 wt% show only bands located at around 230 nm and at around 290 nm. These bands are usually ascribed to tetrahedral surface molybdates. Wang and Hall for example found similar bands using molybdate reference salts [19]. Small differences were found since Wang and Halls experiments were performed in solution; the values found in our experiments are slightly higher. Ashley and Mitchell [62] also found a shift to higher wavelength in reflection compared to absorption experiments in solution. For Mo(18)/Al a shoulder appears in the 320-370 nm region of the spectrum. This band was frequently assigned to octahedral surface polymolybdates [2,8,62,63]. Usually, the assignment of the UV-bands to either tetrahedral or octahedral molybdenum oxide is based on comparison with the UV-Vis data of model compounds such as Na2MoO4 (tetrahedral Mo coordination) or (NH4)6Mo7O24 or MoO3 (octahedral Mo coordination), as described formerly by several authors [64]. The spectra of these compounds, however, vary greatly. Jeziorowski and Knözinger [27] were the first to reinterpret the assignment of the UV bands. They ascribed the 270-295 band to ligand to metal charge transfer in the Mo-O-Mo bridge of polymolybdate rather than to tetrahedral monomolybdate. Since the literature reports differences in both the position of the bands as well as their

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widths, it is important to analyse the UV-Vis data carefully. Fournier et al. [65] interpreted DR-UVVis spectra of various polymolybdate salts differing in size, thus in Mo-Mo interaction, and in symmetry. This approach excluded differences in the spectra that could be caused by the comparison of solid samples and solutions, influence of the degree of hydration and the influence of absorption by the support. Especially the high-energy bands, which appear at low wavenumbers, can be influenced strongly by the support. These authors found that the local symmetry (i.e. the symmetry of the inner coordination sphere of the Mo centers) did not have any clear influence on the LMCT bands of the polymolybdate anions. The overall symmetry (i.e. the symmetry of the polymolybdates themselves) did only show a marginal influence on the width of the low energy band at around 350 nm. The main difference of the spectra was ascribed to the effect of the polymolybdate cluster size. Increase of the size led to broadening and a shift of the low energy band to higher wavelengths. This concept can be applied successfully to supported catalysts. When Mo is well dispersed, the interaction between the Mo atoms is low, leading to a small band at low energy. On the other hand, when the dispersion is decreased the surface molybdate clusters grow and have more Mo-Mo interactions. This results in a shift to higher wavenumber as well as to broadening of the low energy band. Broadening of this band can be

clearly observed in Figure 5.10 upon increase of the Mo loading. For the γ-alumina samples described in this chapter also a shift to higher wavenumbers was observed when the Mo loading increases. Additionally, UV Raman spectroscopy clearly shows the development of a shoulder at 820 cm-1 when the Mo loading is increased from 6 to 10 wt%, as shown in Figure 5.11. This shoulder is assigned to formation of surface polymolybdates. The band at 960 cm-1, assigned to (isolated) monomolybdates remains unaltered. The presence of both the 820 cm-1 shoulder and the 960 cm-1 bands proves that both surface polymolybdates and surface monomolybdates are present in the Mo(10)/Al catalyst. In the low wavenumber part of the Raman spectrum a poorly developed band can be observed in the 320-370 cm-1 region. Though the intensity of this band is quite low and the resolution is poor a shift from lower (328 cm-1) to higher (360 cm-1) wavenumber can be seen. This shift does support the presence of polymolybdates in the Mo(10)/Al sample. However, also for Mo(6)/Al this band is broad and besides the peak maximum at 328 cm-1 also a very small peak at 354 cm-1 can be observed. This indicates the presence of some octahedrally coordinated Mo surface sites for this sample as well. This is consistent with the data measured by Li [66], who found the presence of both

Chapter 5

124

tetrahedrally coordinated Mo and octahedrally coordinated Mo at Mo loadings as low as 0.1 wt%.

Figure 5.11: Figure 5.11: Figure 5.11: Figure 5.11: UV Raman spectra of Mo(6)/Al and Mo(10)/Al.

To summarize, the UV-VIS data of Mo/Al support a decrease in dispersion for those catalysts that have a Mo loading of 10 weight percent and higher, though the technique cannot discriminate between the exact nature of the Mo surface species.

3.4 Reduction of Mo/Al catalysts Hydrogen TPR experiments were conducted in order to explain the behaviour of the benzonitrile production rate as a function of Mo loading. In Figure 5.12 the TPR results are shown as a function of Mo loading for Mo/Al catalysts in the 250-650 ° C temperature range. As described in Section 2.2.2, all catalysts were treated in He/O2 flow prior to performance of the TPR experiment. During this treatment O2 consumption was not observed for any of the catalysts, as expected since all catalysts were in a calcined form. The peak maximum of the reduction peak clearly shifts to lower temperature at increasing Mo loading. The decrease of Tmax was strong for Mo loadings lower than approximately 8 wt% and levelled off at higher Mo loading. Park et al. [67] and Van Veen et al. [16] observed a similar behaviour for Mo/Al catalysts that were prepared under acidic and neutral conditions. The shift of Tmax can be explained by the presence of Mo7O24

6- sites at Mo loadings higher than 8.5 weight percent. At low Mo loading only tetrahedral coordinated surface monomolybdate is present. De Beer and Schuit found that these clusters are difficult to reduce [68].

100 300 500 700 900 1100

W avenum ber [cm -1 ]W avenum ber [cm -1 ]W avenum ber [cm -1 ]W avenum ber [cm -1 ]

Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

M o(10)/Al

M o(6)/Al


125

Also TPR and XPS experiments using differently loaded Mo/α-Al2O3 catalysts showed that reduction occurred easier at higher Mo loadings [69].

Figure 5.12: Figure 5.12: Figure 5.12: Figure 5.12: Peak maximum temperature of the main reduction peak during hydrogen TPR of Mo/Al catalysts. Inset shows the temperature profiles.

When NH3 was used as reducing agent the NH3 dissociation reaction took place, producing H2 and N2.

Figure 5.13: Figure 5.13: Figure 5.13: Figure 5.13: Ammonia dissociation over Mo(2.6)/Al.

As an example Figure 5.13 shows the NH3 dissociation over Mo(2.6)/Al. NH3 was consumed for Mo reduction, as was observed by the formation of water

750

800

850

900

0 5 10 15 20M o lybd enum loa d ing [w t% ]M o lybd enum loa d ing [w t% ]M o lybd enum loa d ing [w t% ]M o lybd enum loa d ing [w t% ]

Tre

d [

K]

Tre

d [

K]

Tre

d [

K]

Tre

d [

K]

300 500 700 900 1100T em pera tu re [K ]T em pera tu re [K ]T em pera tu re [K ]T em pera tu re [K ]

TP

R s

ign

al

[a.u

.]T

PR

sig

na

l [a

.u.]

TP

R s

ign

al

[a.u

.]T

PR

sig

na

l [a

.u.]

Increasing Mo loading

750

800

850

900

0 5 10 15 20M o lybd enum loa d ing [w t% ]M o lybd enum loa d ing [w t% ]M o lybd enum loa d ing [w t% ]M o lybd enum loa d ing [w t% ]

Tre

d [

K]

Tre

d [

K]

Tre

d [

K]

Tre

d [

K]

300 500 700 900 1100T em pera tu re [K ]T em pera tu re [K ]T em pera tu re [K ]T em pera tu re [K ]

TP

R s

ign

al

[a.u

.]T

PR

sig

na

l [a

.u.]

TP

R s

ign

al

[a.u

.]T

PR

sig

na

l [a

.u.]

Increasing Mo loading

0.00

0.01

0.02

0.03

775 825 875 925 975

T em p er a t u r e [K ]T em p er a t u r e [K ]T em p er a t u r e [K ]T em p er a t u r e [K ]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

NH3 H2

N2

0.00

0.01

0.02

0.03

775 825 875 925 975

T em p er a t u r e [K ]T em p er a t u r e [K ]T em p er a t u r e [K ]T em p er a t u r e [K ]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

MS

In

ten

sity

[a

.u.]

NH3 H2

N2

Chapter 5

126

during the experiment and the brown colour of the catalyst at the end of the experiment. The water formation could not be quantified, since the amount of water produced was very low and mass interference of water and ammonia occurred. Therefore no attempt was made to quantify the degree of reduction, based on the amount of water produced.

Figure 5.14: Figure 5.14: Figure 5.14: Figure 5.14: NH3 dissociation over Mo/Al catalysts as function of Mo loading.

Similar to the relation between the maximum of the reduction peak in the H2-TPR experiments the relation was plotted between the onset of N2 production and the Mo loading. The onset of N2 production, i.e. the temperature at which the NH3 dissociation reaction started to occur (determined by the temperature at which the m/e=2 signal increased by at least 10 %) was lowered when the Mo loading was increased, as shown in Figure 5.14. The curve had a similar behaviour as was found for the H2-TPR experiments. This means that N activation occurs more easily when the Mo loading is increased. At Mo loadings of 8.5 wt% and higher no significant further decrease in the temperature required for NH3 dissociation into H2 and N2 was observed.

3.5 Hydrogen-deuterium exchange over Mo/Al catalysts To verify the influence of the Mo loading on the dispersion H2-D2 exchange experiments were performed. Known amounts of H2 and D2 were sampled to the catalyst. Subsequently Ar was flowed over the catalyst bed. While continuously measuring the concentration of all H containing molecules present, the reaction

700

750

800

850

900

0 5 10 15 20

M o l o a d i n g [ w t . % ]M o l o a d i n g [ w t . % ]M o l o a d i n g [ w t . % ]M o l o a d i n g [ w t . % ]

Nit

rog

en

on

set

[K]

Nit

rog

en

on

set

[K]

Nit

rog

en

on

set

[K]

Nit

rog

en

on

set

[K]


127

mixture was recirculated through the catalyst bed. H2 and D2 adsorb dissociatively on the catalyst surface. Therefore, consecutive desorption leads to scrambling of the H and D atoms. When no other processes occur than dissociative adsorption and desorption theoretically a mixture consisting of 50 % HD, 25 % D2 and 25 % H2 would be obtained after reaching chemical equilibrium. As shown in Figure 5.15, dissociation of H2 and D2 occurred and a mixture of H2, D2 and HD was formed.

Figure 5.15: Figure 5.15: Figure 5.15: Figure 5.15: H2–D2 exchange over Mo(1.2)/Al at T= 150 ° C.

As was demonstrated for platinum catalysts by Hanson and Boudart, H2 dissociation is structure sensitive [70]. The reaction rate increases with the dispersion. In the experiments reported here the rate of HD formation has been taken as a measure for the dispersion. The initial HD formation rates were measured at 150 ° C as a function of Mo loading for a series of Mo/Al catalysts. The rates were expressed per mole of Mo, to enable easy interpretation of the influence of Mo loading on the Mo dispersion. The initial HD formation and the initial H2 and D2 consumption rates were calculated by interpolation to time is zero, using a fit routine to describe the rates. Figure 5.16 shows the HD formation rate as function of Mo loading. The HD formation rate is high at low Mo loading and quickly decreases when the Mo loading is increased. This can be explained well by a decrease of dispersion upon increasing Mo loading. At low Mo loading well-dispersed tetrahedrally coordinated surface monomolybdates are formed. These sites are highly active for H2 and D2 dissociation and lead to high HD formation rates. When the Mo loading is increased surface polymolybdates are formed. As a consequence, the dispersion of Mo decreases and the HD formation rate decreases. When the Mo loading is higher than 8.5 wt% the HD formation rate becomes close to zero. At this loading the amount of tetrahedrally

0.0

0.2

0.4

0.6

0 200 400 600 800 1000

T im e [m in ]T im e [m in ]T im e [m in ]T im e [m in ]

Mo

le f

ract

ion

Mo

le f

ract

ion

Mo

le f

ract

ion

Mo

le f

ract

ion

HDH2D2

0.0

0.2

0.4

0.6

0 200 400 600 800 1000

T im e [m in ]T im e [m in ]T im e [m in ]T im e [m in ]

Mo

le f

ract

ion

Mo

le f

ract

ion

Mo

le f

ract

ion

Mo

le f

ract

ion

HDH2D2

Chapter 5

128

coordinated surface monomolybdates has strongly decreased as was indicated by the UV Raman spectra discussed in Section 3.3 (Figure 4.11).

Figure 5.16: Figure 5.16: Figure 5.16: Figure 5.16: The HD formation rate over Mo/Al as a function of Mo loading.

To estimate the Mo dispersion at higher loadings, the H2-D2 exchange reaction was also performed at higher temperature. Figure 5.17 shows the temperature programmed H2-D2 exchange reaction over Mo(14)/Al. The HD fraction is plotted as a function of temperature. Also the H2 consumption during TPR is plotted in this figure. Obviously H-D exchange occurs only at temperatures higher than 250 ° C. When further reduction occurs, the HD production increases. At temperatures lower than 250 ° C Mo is in 6+-oxidation state. This shows that for Mo(14.2)/Al only (partly) reduced Mo is able to catalyse the H-D exchange reaction. As shown in Section 3.4 the reduction temperature strongly increases at lower Mo loading. Higher reaction temperatures for the H2-D2 exchange reaction, therefore, could not be applied to compare the dispersion of Mo, since the oxidation state is not the same over the whole range of Mo/Al catalysts. The decrease of the H-D exchange rate with the Mo loading was similar to that observed by Hensen [71] over sulfided Mo/Al catalysts, who showed that high HD formation rates at Mo loadings lower than 5 wt% can be explained by the

presence of basic hydroxyls of the γ-alumina support. As explained in Section 1.1, the basic hydroxyls primarily react with the impregnation solution. At Mo loadings higher than 5 wt% all basic hydroxyls have been consumed. The higher HD formation rates at lower Mo sulfide loading therefore could be explained by the presence of spillover hydrogen, originating from basic hydroxyls. In the case of

0

5

10

15

20

25

30

35

0 4 8 12 16 20

M o load ing [w t% ]M o load ing [w t% ]M o load ing [w t% ]M o load ing [w t% ]

Ra

te [

mo

l/(m

ol

Ra

te [

mo

l/(m

ol

Ra

te [

mo

l/(m

ol

Ra

te [

mo

l/(m

ol M

oM

oM

oM

o•h

r)]

•hr)

]•h

r)]

•hr)

]


129

γ-alumina supported Mo oxide the H-D exchange rates are much higher at low

Mo loading compared to γ-alumina supported Mo sulfide.

Figure 5.17: Figure 5.17: Figure 5.17: Figure 5.17: TPR and Temperature Programmed H2-D2 exchange over Mo(14.2)/Al.

Table 5.2 compares these rates. Note that the same catalyst precursors were applied for both the calcined and sulfided catalysts. Spillover hydrogen alone, therefore, does not explain the decrease of the HD formation rate with increasing Mo loading for the Mo oxide catalysts. Tetrahedrally coordinated surface monomolybdate catalyses H-D exchange. When the Mo loading is increased surface polymolybdates are formed and the HD formation rate decreases.

Table 5.2: Table 5.2: Table 5.2: Table 5.2: H2-D2 exchange rates over γ-alumina supported Mo oxide and sulfide

HD exchange rate [molHD exchange rate [molHD exchange rate [molHD exchange rate [molHDHDHDHD/(mol/(mol/(mol/(molcatcatcatcat∙ hr)]∙ hr)]∙ hr)]∙ hr)] Mo loading [wt%]Mo loading [wt%]Mo loading [wt%]Mo loading [wt%] Mo oxideMo oxideMo oxideMo oxide Mo sulfideMo sulfideMo sulfideMo sulfide1111

1.2 38.7 6.9 2.6 12.4 5.8 4.9 6.8 3.7 6.7 1.6 3.2 8.5 0.9 3.0

10.3 0.3 2.7 14.2 0.1 2.3 16.8 0.0 Not measured

1 Data from Hensen [71].

0.0

0.1

0.2

0.3

200 300 400 500 600 700

Tem pera tu re [°C]Tem pera tu re [°C]Tem pera tu re [°C]Tem pera tu re [°C]

HD

fra

ctio

n [

-]H

D f

ract

ion

[-]

HD

fra

ctio

n [

-]H

D f

ract

ion

[-]

H HHH2 222 c

on

sum

pti

on

c

on

sum

pti

on

c

on

sum

pti

on

c

on

sum

pti

on

[a.u

.][a

.u.]

[a.u

.][a

.u.]

HD production during

H 2 - D 2 exchange

TPR curve

Chapter 5

130

Figure 5.18: Figure 5.18: Figure 5.18: Figure 5.18: Equilibrium fractions of H2, D2 and HD as function of Mo loading of Mo/Al catalysts.

Spillover H atoms could explain the deviation from the equilibrium composition. Figure 5.18 shows the equilibrium fractions of HD, H2 and D2, again plotted against the Mo loading. The equilibrium fractions were obtained after performing the reaction for such a long time that no change was observed in the concentrations of H2, D2 and HD. Usually the time of reaction was in the order of 16 hours before equilibrium was obtained. At Mo loadings up to 7 wt% the H2

fraction is slightly higher than the D2 fraction. This can be explained qualitatively by the participation of spillover H. However, the molar amount of spillover H decreases to zero if the Mo loading is increased from 1.2 wt% to 4.9 wt%, as shown in Figure 5.19. This result is very similar to the result described by Hensen [71].The amount of spillover H

follows from the mass balance, according to Equation 5.3. Gas-phase molecules are notated with the suffix g and the surface hydrogens that are initially present on the alumina surface are indicated as H0.

HD

H HD

H HDD HD

g

g initial

g g

g g equilibrium

=

+

=

++

22

22

2 0

2

2

2

,

,

,

, (5.3)

0.0

0.1

0.2

0.3

0.4

0.5

0 1 2 3 4 5M olybdenum loading [w t.% ]M olybdenum loading [w t.% ]M olybdenum loading [w t.% ]M olybdenum loading [w t.% ]

HHHH0

0

0

0

[m

mo

l/g

[mm

ol/

g[m

mo

l/g

[mm

ol/

gca

tca

tca

tca

t] ]]]

Figure 5.19: Figure 5.19: Figure 5.19: Figure 5.19: Amount of H0 as function of Mo loading of Mo/Al.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14 16 18

M o ly b d en um lo ad ing [w t.% ]M o ly b d en um lo ad ing [w t.% ]M o ly b d en um lo ad ing [w t.% ]M o ly b d en um lo ad ing [w t.% ]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

H2

D2

HD

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12 14 16 18

M o ly b d en um lo ad ing [w t.% ]M o ly b d en um lo ad ing [w t.% ]M o ly b d en um lo ad ing [w t.% ]M o ly b d en um lo ad ing [w t.% ]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

Eq

uil

ibri

um

fra

ctio

n [

mo

l/m

ol]

H2

D2

HD


131

Though the decrease of H0 correlates well with the decrease of basic OH-groups upon increase of the Mo loading, the equilibrium fractions over Mo(6.7)/Al and Mo(8.5)/Al do not equal the values expected from dissociative adsorption, recombination and desorption only. The H0 amount increased to 0.48 and 0.30 mmol/gcat. This can be explained by assuming that increase of the Mo loading leads to clustering of isolated surface monomolybdates to surface polymolybdates, which are less active in hydrogen activation. When surface polymolybdates have formed, those sites that contain spillover H become available for reaction again.

3.6 Transmission Electron Microscopy on Mo/Al samples Transmission Electron Microscopy (TEM) experiments were performed on fresh Mo/Al samples. MoOx clusters were not observed by TEM, even not at a Mo loading of 17 wt%. Since MoOx clusters can only be detected when their size is larger than ca. 1 nm this means that in all cases the size of the MoOx clusters was smaller than ca. 1 nm. EDX analysis showed the presence of Mo throughout the whole sample. No inhomogeneities were observed for any of the catalysts. On the other hand some small clusters were detected on the MoV/Al samples. Additionally, the sample with the highest toluene ammoxidation activity, Mo(11)V(0.8)/Al, displayed many Mo containing structures. The clusters could not be convincingly demonstrated to be MoO3 clusters, however, since they were to small to perform electron diffraction. The MoO3 clusters as were observed by Raman Spectroscopy (see Figure 5.7), therefore, must be highly dispersed over the surface, as was also concluded also from the XRD results described in Section 3.2.

3.7 In situ treatment of Mo/Al The lower reducibility of Mo/Al samples that have a Mo loading below 10 wt% may explain the lower toluene ammoxidation. In situ Raman Spectroscopy was applied to examine the effect of thermal treatment using various gas flows on the chemistry of surface molybdates. Figure 5.20 shows the Raman spectra of Mo(11)/Al, which are all recorded at room temperature after treating the catalyst in situ applying the indicated conditions. Subsequently the catalyst was calcined at 700 ° C, treated in He/NH3 at 360 ° C and 700 ° C respectively, passivated in artificial air and treated in an NH3 and O2 containing flow at 450 ° C. Finally recalcination of the catalyst was performed at 700 ° C.

Chapter 5

132

Figure 5.20: Figure 5.20: Figure 5.20: Figure 5.20: In situ Raman Spectroscopy of Mo(11)/Al.

Freshly calcined Mo(11)/Al shows two Raman bands at 860 cm-1 and at 990 cm-1. The 860 cm-1 band can be assigned to the asymmetric Mo-O-Mo bending and the 990 cm-1 band to the Mo=O stretching mode. Isolated surface monomolybdates as well as surface polymolybdates therefore are present at the surface of Mo(11)/Al as discussed in Section 3.2. After cooling of the sample in He and heating in He/NH3 by 10 ° C per minute to 360 ° C both bands are still present, although their intensities were decreased and their frequency was decreased by 15 cm-1. This decrease upon exposure to NH3 was also found by Stencel et al. [56] and is similar to the decrease in frequency upon hydration caused by the decrease of the Mo=O bond strength due to adsorption of H2O. Further heating of the sample in He/NH3 by 1 ° C per minute to 700 ° C did remove the Mo-O bands completely. No other bands appeared during treatment in He/NH3 flow or after subsequent cooling to room temperature, in the presence of a He/O2 flow. Heating the sample in a flow that contained both NH3 and O2 by 5 ° C per minute to 450 ° C led to the reappearance of the Raman bands that were present initially. Further oxidation in He/O2 to 700 ° C did not change the Raman spectrum significantly. These observations indicate that a redox mechanism is operative on Mo/Al catalysts. Oxidation occurs by O2 and reduction of Mo occurs by NH3.

200 400 600 800 1000

Ram

an I

nten

sity

[A

.U.]

Ram

an I

nten

sity

[A

.U.]

Ram

an I

nten

sity

[A

.U.]

Ram

an I

nten

sity

[A

.U.]

Wavenumber [cmWavenumber [cmWavenumber [cmWavenumber [cm-1-1-1-1]]]]

He/O2 (700 °C)

He/NH3 (700 °C)

He/O2 (700 °C)

He/NH3 (360 °C)

He/O2 (25 °C)

He/NH3/O2 (450 °C)

200 400 600 800 1000

Ram

an I

nten

sity

[A

.U.]

Ram

an I

nten

sity

[A

.U.]

Ram

an I

nten

sity

[A

.U.]

Ram

an I

nten

sity

[A

.U.]

Wavenumber [cmWavenumber [cmWavenumber [cmWavenumber [cm-1-1-1-1]]]]

He/O2 (700 °C)

He/NH3 (700 °C)

He/O2 (700 °C)

He/NH3 (360 °C)

He/O2 (25 °C)

He/NH3/O2 (450 °C)


133

Figure 5.21: Figure 5.21: Figure 5.21: Figure 5.21: Raman spectra of Mo(2.6)/Al during NH3-O2 cycles. Oxidation in O2/Ar (20/80, 100 ml/min); NH3 treatment in NH3/Ar (5/95, 100 ml/min).

Figure 5.22: Figure 5.22: Figure 5.22: Figure 5.22: Raman spectra of Mo(8.5)/Al during NH3-O2 cycles. Oxidation in O2/Ar (20/80, 100 ml/min); NH3 treatment in NH3/Ar (5/95, 100 ml/min).

200 400 600 800 1000 1200W avenum ber [cmW avenum ber [cmW avenum ber [cmW avenum ber [cm -1-1-1-1 ]]]]

Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Calcined at 700 °C

NH3 treated at 400 °C

Re-calcined at 300 °C


Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Calcined at 700 °C


Re-calcined at 300 °C

0

500

1000

1500

2000

2500

3000


Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Calcined at 700 °C


Recalcined at room temperature

0

500

1000

1500

2000

2500

3000


Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Ra

ma

n i

nte

nsi

ty [

a.u

.]R

am

an

in

ten

sity

[a

.u.]

Calcined at 700 °C


Recalcined at room temperature

Chapter 5

134

Figure 5.23: Figure 5.23: Figure 5.23: Figure 5.23: Raman spectra of Mo(16.8)/Al during NH3/O2 cycles. Oxidation in O2/Ar (20/80, 100 ml/min); NH3 treatment in NH3/Ar (5/95, 100 ml/min).

Irrespective of the Mo loading redox cycles were observed for Mo/Al. Figures 5.21-5.23 show the effect of reduction by NH3 and sequential reoxidation in artificial air on the Raman spectra of Mo(2.6)/Al, Mo(8.5)/Al and Mo(16.8)/Al. For Mo(2.6)/Al and Mo(8.5)/Al similar decreases in frequency of the high-wavenumber band were observed, indicating weakening of the Mo=O band upon ammonia adsorption. Ammonia adsorption, however, does not lead to complete removal of the Raman bands. Therefore Mo still is in oxidised form. For Mo(16.8)/Al such a frequency decrease was not observed. For this catalyst a strong decrease of the intensity of all bands was observed. This could have been caused by the more easy reduction of the catalyst, as was observed in the H2-TPR and NH3 dissociation experiments. It should be mentioned though that Mo reduction also leads to blackening of the catalyst. Though the black colour strongly supports Mo reduction it also leads to a decrease of the Raman intensities. For Mo(16.8)/Al recalcination at 300 ° C is sufficient to re-oxidize the catalyst to its original state, as is shown in Figure 5.23. After (re)calcinations, the Raman spectrum basically shows the presence of aluminium molybdate and MoO3. For Mo(2.6)/Al and Mo(8.5)/Al this temperature does not seem sufficient to completely reoxidise the catalyst. Strong fluorescence, however, prevented us from determining by Raman spectroscopy the temperature necessary for complete Mo reoxidation.

200 400 600 800 1000 1200W aven u m be r [cmW aven u m be r [cmW aven u m be r [cmW aven u m be r [cm -1-1-1-1 ]]]]

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.]R

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.]R

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[a

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Calcined at 700 °C


Recalcined at 300 °C



200 400 600 800 1000 1200W aven u m be r [cmW aven u m be r [cmW aven u m be r [cmW aven u m be r [cm -1-1-1-1 ]]]]

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Calcined at 700 °C






135

Figure 5.24: Figure 5.24: Figure 5.24: Figure 5.24: XP Spectra of Mo(16.8)/Al after calcination, toluene oxidation and toluene ammoxidation.

To check whether Mo/Al was in a reduced state upon NH3 treatment quasi in situ XPS experiments were performed using a Mo(16.8)/Al sample. Various treatments were applied at 400 ° C. After two hours the catalyst was cooled to room temperature in a He flow. Without exposure to ambient air the catalyst was transferred to the XPS equipment. Figure 5.24 shows the XP Spectra after calcination in artificial air, toluene oxidation and toluene ammoxidation conditions. All spectra could be fitted correctly using only one Mo 3d doublet, indicating the presence of only one Mo oxidation state. The binding energy was

232.8 eV (± 0.1 eV). This value, which is slightly higher than that of bulk MoO3 [59,72], corresponds to Mo(VI) [73,74], as was expected since the spectra did not change compared to the calcined sample. The increase of the binding energy compared to bulk MoO3 is caused by the interaction of the molybdate with the support [75]. Though Figure 5.24 suggests that the catalyst surface is unaltered upon toluene (amm)oxidation, reduction of the molybdenum oxide is expected during reaction as explained earlier [76]. Toluene is adsorbed in an oxygenated form, mostly referred to as the aldehyde-like intermediate. Oxidation of toluene occurs by lattice oxygen according to a Mars–Van Krevelen mechanism. The reduced catalyst is reoxidized by gaseous oxygen.

228231234237240Binding Energy [eV]

Toluene ammoxidationToluene oxidationCalcination

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Figure 5.25: Figure 5.25: Figure 5.25: Figure 5.25: Mo 3d peaks of calcined and subsequently NH3 treated Mo(16.8)/Al.

Figure 5.25 shows that the catalyst indeed can be partly reduced when the catalyst is contacted with NH3 at 400 ° C. Compared to completely oxidised Mo(16.8)/Al a shift of 1.3 eV to lower binding energy was observed. According to Haber et al.

[46], who found a shift of 1.4 eV in Mo 3d binding energy upon reduction of MoO3 this can be explained with a reduction of Mo6+ to Mo4+. Haber et al. found two doublets for fresh MoO2. A similar spectrum was measured by Peeters et al. [59]. The highest value for the Mo(IV) binding energy accounts for “isolated” Mo, whereas a lower value was found for Mo4+ ions paired in clusters of edge sharing octahedra. These nuclei can be considered to have an apparent oxidation state of 2+. This apparent oxidation state correlates linearly with the 3d binding energy values, found by these authors. Grünert et al. [77] confirmed this correlation based on thermal decomposition of MoO3 in the XPS chamber. The Mo 3d binding energies decrease proportionally with the Mo valency by a factor of 0.8 eV. However, at this relatively high Mo loading at least two different Mo species are present (isolated tetrahedral surface monomolybdate and octahedral surface polymolybdates), as was shown by Raman spectroscopy (see Figure 5.23). XPS cannot distinguish between these two types of Mo species, although the chemical environment is not the same. This means that a linear relationship between the Mo oxidation state and the binding energy measured by XPS could be affected by changes in the structure of the (6+) molybdate species. Other authors prefer to assign the value for the binding energy to Mo(V) [20,72,78]. Poulston et al. [79] also used XPS to study the reduciblity of ammoxidation catalysts. These authors found that Bi-Mo oxide was reduced by NH3 at a temperature of 347 ° C. Besides Mo6+ they found the presence of Mo 3d peaks at Eb = 229.0 eV after reduction by NH3. This peak, which was ascribed to Mo4+ was stable, even after annealing the

224228232236240244

B in din g E n e rgy [e V ]B in din g E n e rgy [e V ]B in din g E n e rgy [e V ]B in din g E n e rgy [e V ]

XP

S I

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.]X

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XP

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Peak 1: 232.8 eV

X P spectrum

Peak 2: 231.5 eV


137

catalyst at 547 ° C. The reduction of Bi-Mo oxide and Fe-Sb oxide was found to be easier than reduction of VSbO4, USb3O10 and Bi4V2O11. This order correlates well with the activity generally found in ammoxidation reactions. To summarize, Mo/Al is in fully oxidized state after toluene ammoxidation and toluene oxidation reactions. Our Raman and XPS results show that NH3 is capable to reduce Mo from a hexavalent to a lower oxidation state. Reoxidation of the Mo occurs by O2 present in the ammoxidation feedstock.

4. Conclusions A redox mechanism applies to the ammoxidation of toluene over γ-alumina supported molybdenum oxide. Reduction of Mo is facilitated when the Mo loading is increased. The minimum reduction temperature is obtained when the Mo loading is higher than 8 weight percent. Surface polymolybdates have formed at this loading. The ammoxidation activity increases when the Mo loading is increased to values higher than 8 wt%. Surface polymolybdates therefore are more reactive towards the ammoxidation of toluene. It was shown by TPR and NH3 dissociation that reduction of surface polymolybdates occurs more easily than the reduction of isolated monomolybdate, which is tetrahedrally coordinated. This indicates that the rate of toluene ammoxidation is determined by the Mo reducibility. This is consistent with the fact that toluene activation as oxidised species (adsorbed benzaldehyde or adsorbed benzoate) is rate determining in the toluene ammoxidation reaction. Hydrogen–deuterium exchange reactions confirm this; high exchange rates are obtained over surface monomolybdates. At increasing Mo loading the H-D exchange rate decreases and becomes close to zero when the Mo loading is higher than 8.5 wt%. At this Mo loading the amount of surface monomolybdates is close to zero. For vanadia containing Mo/Al catalysts the benzonitrile yield is significantly higher than for the corresponding vanadia-free catalysts. It was shown that addition of small amounts of V dopants leads to the formation of crystalline MoO3 clusters. These clusters are believed to lead to high benzonitrile yields, since their reducibility is high.

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Summary

Alkylaromatics oxidation is performed nowadays as a liquid phase reaction, catalysed by metal oxide salts dissolved in acidic solution. Although this process yields high amounts of the oxygenate, the reaction environment is highly demanding on both the equipment and the natural environment. The high demand on the natural environment is expected to lead to more stringent legislation. Therefore, development of new, clean and selective alkylaromatic oxidation processes is of great importance. In this respect it is important to minimize the amount of side- and by-products and to exclude the use of harmful solvents. The application of heterogeneously catalysed vapour phase direct alkylaromatics oxidation in principle satisfies all these demands. However, it is difficult to exclude the formation of by-products due to the higher reactivity of the oxygenate compared to the substrate. This generally leads to over-oxidation and the production of CO2. Alkylaromatic ammoxidation -the reaction of alkylaromatic compounds with ammonia and oxygen to form alkylaromatic nitriles- can be applied as first step of a highly selective route towards alkylaromatic oxygenates. The aromatic nitrile can be converted in a second reaction by hydrolysis. Dependent on the conditions of this second reaction step aromatic aldehydes, amines, amides or acids can be produced. In this second reaction step ammonia is regained so this reaction route cleanly yields the alkylaromatic oxygenate. This thesis focuses on the alkylaromatic ammoxidation. Toluene has been chosen as substrate, since its relative simplicity ensures a comprehensive catalytic study. The reaction is performed in the vapour phase at temperatures between 300 and 460 ° C. Generally high selectivity towards benzonitrile can be achieved, especially compared to aldehydes and alcohols, which are less stable towards combustion. A broad range of zeolite Y based catalysts was developed for the ammoxidation of toluene. Due to the well-defined pore structure of the zeolite, NaY based catalysts offer the unique possibility to introduce transition metals by several methods. Moreover, the acid-base properties of the zeolite matrix can be adapted easily. Therefore, zeolite based catalysts potentially offer an advantage catalysts compared to mixed oxide catalysts, which are currently applied for alkylaromatic ammoxidation reactions.

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Metal oxides were introduced into the matrix of zeolite Y by means of ion exchange, chemical vapour deposition and incipient wetness impregnation. Also

the catalytic performance of γ-alumina supported metal oxide catalysts was explored. Since the stability of the catalysts was found to be an important parameter, catalytic studies were executed over a relative long period, usually

about 15 hours. The catalytic performance of γ-alumina supported molybdenum oxide was superior over all other catalysts examined. At lower temperature, however, equal benzonitrile yields were obtained over Cu/NaY catalysts that were prepared by ion exchange.

High temperature ammonia treatment of γ-alumina supported molybdenum oxide increases significantly the benzonitrile selectivity at complete toluene conversion, indicating the importance of the nitrogen containing surface species. The role of the nitrogen insertion site was examined in more detail using NO instead of NH3 as nitrogen source. Mainly combustion of toluene occurs when the nitroxidation reaction is performed. Though benzonitrile formation can occur by this so-called nitroxidation reaction, NH4NO3 or NH4NO2 surface species cannot be considered as the selective nitrogen insertion sites. Metal oxide encaged NaY catalysts prepared by metalcarbonyl deposition were used as model catalysts based on their well-defined structural properties. Mo(CO)6 can be introduced into the supercages of the zeolite Y, unlike to V(CO)6, Mn2(CO)10 and Co(NO)(CO)3. Transmission Electron Microscopy (TEM) and X-Ray Photoelectron Spectroscopy (XPS) analyses showed that Mo(CO)6 is dispersed homogeneously throughout the zeolite pores. Detailed temperature programmed decarbonylation studies showed complete decarbonylation of the Mo(CO)6 catalyst precursor. In the presence of oxygen, thermal activation ensures low temperature Mo oxidation. All molybdenum oxide clusters were in the 6+-oxidation state as was indicated by XPS. The dispersion of the resulting molybdenum oxide clusters is high as was confirmed by TEM analysis. The approximate molybdenum oxide cluster size did not exceed 14 Å, thus suggesting the presence of intra-zeolite molybdenum oxide clusters. XPS experiments were applied to examine the interaction between the Mo(CO)6 guest and the zeolite host. As was shown by the low Na (1s) binding energy of Mo(CO)6/NaY, Mo(CO)6 interacts with the Na+-cation. The Na (1s) binding

Summary

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energy increases by 1 eV when oxidative decarbonylation is performed. To study in greater detail the interaction of the zeolite cation with the Mo(CO)6 guest the Na+-ions were exchanged with a series of other alkali ions. Based on the electronegativity difference of the cations the basicity of the zeolite host is varied. By performing the decomposition of 2-methyl-3-butyn-2-ol the acid-base properties of the resulting catalysts were probed. It was found by performing control experiments on the alkali exchanged zeolite host materials that this decomposition reaction probes well the amount of Lewis acidity and basicity. Further proof of the interaction of the encaged Mo species with the zeolite cations was found by the effect of the zeolite basicity on the decomposition of the encaged Mo(CO)6. Lower decarbonylation temperatures were found when the Lewis basicity of the zeolite host is higher. Compared to the less basic cations the electron-rich Mo(CO)6 guest is less stable, which leads to lower decarbonylation temperatures. Preliminary catalytic tests on the effect of the Lewis acid/base properties showed an increase of the catalyst activity with increasing basicity, confirming formation of a carbanion as the first step in toluene activation, as was proposed in the literature.

γ-Alumina supported molybdenum oxide catalysts were studied in great detail. The nature of the molybdenum surface species was varied by varying the Mo loading. A combination of Temperature Programmed Reduction, in situ and ex situ Raman Spectroscopy, X-Ray Photoelectron Spectroscopy, X-Ray diffraction, Transmission Electron Microscopy, diffuse reflectance ultraviolet spectroscopy and hydrogen-deuterium exchange test reactions was applied to study the

morphology and the reduction behaviour of γ-alumina supported molybdenum

oxide. At low Mo loading tetrahedrally coordinated Mo is present on the γ-alumina support as surface monomolybdate. The reducibility of Mo is low for these catalysts. At the increase of the Mo loading polymolybdates start to form. These polymolybdate clusters have octahedral Mo coordination. The reduction temperature of these catalysts is lowered by approximately 140 ° C compared to surface monomolybdates. Only tetrahedrally coordinated Mo seems to catalyse the hydrogen–deuterium exchange reaction. At Mo loadings higher than 8.5 wt% no H-D exchange activity was observed. A small amount of hydrogen is present at the support. This hydrogen participates in the H-D exchange reaction by spillover.

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In situ Raman spectroscopy indicated that Mo is reduced by NH3 at the

ammoxidation reaction temperature for γ-alumina supported molybdenum oxide, when the loading is higher than 10 wt%. At this Mo loading no tetrahedrally coordinated monomolybdate is present at the catalyst surface. This result was confirmed by quasi in situ XPS. The Mo oxidation state is 4+ after NH3 treatment at 400 ° C. When the loading is lower than 10 wt% the Raman active Mo-O bands are shifted to lower wavenumber, which is very similar to the Mo-O shift to lower wavenumber observed for hydrated supported molybdenum oxide catalysts. This shows that the Mo-O bond is weakened by the presence of ammonia. Reoxidation in oxygen recovers the initial oxidation state. Since the ammoxidation of toluene occurs via a redox mechanism the catalyst reducibility strongly influences the benzonitrile production rate. At molybdenum oxide loadings below 8.5 wt% low benzonitrile production rates are observed. This can be understood from the fact that reduction by ammonia does not occur for these catalysts. When the molybdenum loading is increased over 10 wt% the toluene ammoxidation activity strongly increases, as expected from the higher molybdenum reducibility at higher Mo loading. Similarly, the low reducibility of intra-zeolite molybdenum oxide could explain the low activity of NaY encaged molybdenum oxide. This, however, was not extensively studied in this thesis. Addition of small amounts of vanadia dopants leads to increase of the ammoxidation activity. Raman Spectroscopy showed that MoO3 clusters are formed upon V doping. The presence of MoO3 is unexpected, since the mono-layer coverage of molybdenum oxide (plus vanadium oxide) is significantly higher

than was applied. The MoO3 clusters were highly dispersed over the γ-alumina surface, since no MoO3 was observed by XRD. Transmission Electron Microscopy confirmed the high dispersion, since no TEM-detectable MoO3-clusters were observed.

147

Samenvatting

De oxidatie van alkylaromaten wordt heden ten dage uitgevoerd als een vloeistoffase proces, gekatalyseerd door metaalzouten in een zure oplossing. Alhoewel in het algemeen hoge opbrengsten van het gewenste oxidatie product worden behaald, stellen deze processen hoge eisen aan zowel de apparatuur als aan het natuurlijk milieu. Naar verwachting zal in de toekomst strengere wetgeving hogere eisen gaan stellen aan deze processen, wat het belang van de ontwikkeling van nieuwe, schone en selectieve alkylaromaat oxidatie processen onderstreept. Hiertoe is het van groot belang om vorming van bijproducten en van schadelijke oplosmiddelen te voorkomen. In principe voldoen heterogeen gekatalyseerde gasfase alkylaromaat oxidatie processen aan de bovenstaande eisen. Omdat het oxidatieprodukt doorgaans reactiever is dan de reactant, het alkylaromaat, is het echter niet eenvoudig om de vorming van bijproducten te minimaliseren. Met name over-oxidatie van het substraat tot CO2 treedt doorgaans in hoge mate op. Een selectief twee-staps proces voor de vorming van alkylaromaat oxidatieproducten is alkylaromaat ammoxidatie – de reactie van een alkylaromaat met ammoniak en zuurstof tot een alkylaromatisch nitril – gevolgd door hydrolyse van het gevormde nitril. Afhankelijk van de exacte condities van de hydrolyse reactie kunnen op deze wijze aromatische aldehydes, amines, amides of zuren worden geproduceerd. In deze tweede processtap wordt ammoniak gevormd, zodat het oxidatieprodukt op zeer schone wijze wordt gevormd. Daar de ammoxidatie reactie eenvoudiger verloopt dan directe zijketen oxidatie speelt de vorming van CO2 een minder grote rol in dit proces. Dit proefschrift richt zich op de ammoxidatie van alkylaromaten. Tolueen is gekozen als substraat, aangezien de relatieve eenvoud van dit substraat uitvoering van een katalytische studie mogelijk maakt. De reactie wordt uitgevoerd in de gasfase bij temperaturen tussen 300 en 460 ° C. In het algemeen kan een hoge selectiviteit naar benzonitril worden verkregen, met name in vergelijking met de directe oxidatie van tolueen tot benzaldehyde en benzylalcohol. Een groot aantal katalysatoren gebaseerd op zeoliet Y zijn ontwikkeld voor de ammoxidatie van tolueen. Met zijn goed gedefinieerde poriestructuur vormt zeoliet NaY een uniek dragermateriaal voor de introductie van katalytisch actieve

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overgangsmetalen. Deze overgangsmetalen kunnen middels verschillende methoden in de porië n van de zeoliet matrix worden gebracht. Ook kunnen de zuur-base eigenschappen op eenvoudige wijze worden gevarieerd. Dit biedt een voordeel ten opzichte van gemengde oxiden, welke thans voor de ammoxidatie van tolueen worden gebruikt. Metaaloxides werden in de matrix van zeoliet NaY ingebracht middels ion

wisseling, metaalcarbonyl depositie en porievolume impregnatie. Ook werden γ-alumina gedragen metaaloxide katalysatoren bereid middels porievolume impregnatie. Aangezien in het algemeen de katalysator stabiliteit een belangrijke rol speelt tijden de ammoxidatie van tolueen over de gebruikte katalysatoren, zijn de katalytische tests uitgevoerd gedurende een relatief lange tijd; doorgaans in de

orde van 15 uur. De opbrengst aan benzonitril was het hoogst over γ-alumina gedragen molybdeenoxide. Bij lagere temperatuur kunnen over koper uitgewisselde NaY katalysatoren gelijke opbrengsten worden verkregen.

Wanneer γ-alumina gedragen molybdeenoxide bij hoge temperatuur voorbehandeld wordt met ammoniak wordt een significant hogere benzonitril selectiviteit verkregen bij volledige tolueen omzetting. Dit duidt op het belang van stikstof houdende oppervlakte structuren in de ammoxidatie van tolueen. De rol van het stikstof insertiecentrum is onderzocht door het gebruik van NO als stikstof bron in plaats van ammoniak. Tijdens deze zogenaamde nitroxidatie reactie trad voornamelijk tolueen verbranding op. Ondanks het feit dat een kleine hoeveelheid benzonitril gevormd werd kan hieruit geconcludeerd worden dat ammonium-nitraat of –nitriet oppervlakte centra uitgesloten kunnen worden als selectieve stikstof insertie centra. Op basis van hun goed gedefinieerde structuur zijn metaaloxide houdende NaY katalysatoren bereid door middel van depositie van metaalcarbonylen. Mo(CO)6 kan succesvol in de superkooien middels deze techniek. Dit in tegenstelling tot de introductie van V(CO)6, Mn2(CO)10 en Co(NO)(CO)3. Transmissie Electron Microscopie (TEM) en Röntgen Foto-elektron Spectroscopie (XPS) analyses tonen aan dat Mo(CO)6 homogeen gedispergeerd is in de zeolite structuur. Gedetailleerde temperatuur geprogrammeerde experimenten toonden tevens aan dat het Mo(CO)6 volledig ontdaan kan worden van alle CO liganden. Met XPS werd aangetoond dat, in de aanwezigheid van zuurstof, het in de superkooi aanwezige molybdeen volledig wordt geoxideerd tot een 6+-oxidatie-

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toestand. Ook van deze molybdeenoxide clusters is de dispersie hoog, zoals met TEM werd onderzocht. De molybdeenoxide clustergrootte was niet hoger dan 14 Å. Dit suggereert dat de clustergrootte wordt gelimiteerd door de grootte van de superkooien van de zeoliet. Met XPS werd tevens de gast-gastheer interactie van de Mo(CO)6 met NaY onderzocht. De lage Na(1s) bindingsenergie na Mo(CO)6-introductie duidt op een directe interactie van de Mo(CO)6-gast met de NaY-gastheer. Na oxidatieve Mo(CO)6 decarbonylering stijgt de Na(1s) weer met 1 eV naar de in de literatuur gerapporteerde waarde. Om de interactie van de kationen van de zeoliet en de Mo(CO)6-gast op meer gedetailleerde wijze te onderzoeken zijn de Na+-ionen uitgewisseld met andere alkali ionen. Op basis van de elektronegativiteit wordt op deze manier de basiciteit van de zeoliet gevarieerd. De zuur-base eigenschappen van de katalysatoren werden onderzocht door uitvoering van de ontleding van 2-methyl-3-butyn-2-ol. Controle experimenten toonden aan dat middels deze reactie de Lewis aciditeit en basiciteit correct kan worden gemeten. Deze ontledingsreactie bewees de interactie van de Mo(CO)6-gast met de kationen van de zeolite. Hoe hoger de basiciteit van de zeoliet is, hoe zwakker de interactie met de het Mo(CO)6-gastmolecuul. Ten gevolge van deze zwakkere interactie vindt Mo(CO)6 decarbonylering bij lagere temperatuur plaats. Katalytische tests duiden op een hogere katalysator activiteit bij hogere basiciteit. Dit is in overeenstemming met de vorming van een carbanion als eerste tolueen activeringsstap, zoals ook in de literatuur wordt voorgesteld.

Zeer gedetailleerde studies werden uitgevoerd naar γ-alumina gedragen molybdeenoxide katalysatoren. Door de molybdeen belading te varië ren werden de molybdeen en zuurstof bevattende oppervlakte structuren gevarieerd. Een combinatie van temperatuur geprogrammeerde reductie, in situ en ex situ Raman spectroscopie, XPS, Röntgen diffractie (XRD), Transmissie Electron Microscopy (TEM), diffuse reflectie ultraviolet spectroscopie en waterstof–deuterium

uitwisselingsreacties werd gebruikt om het reductiegedrag en de morfologie van γ-alumina gedragen molybdeenoxide te onderzoeken. Bij lage molybdeen beladingen is tetraedisch omringd molybdeen aanwezig als monomolybdaat oppervlaktestructuur. Deze structuur reduceert bij hoge temperatuur. Wanneer de molybdeen belading wordt verhoogd worden poly-molybdaat oppervlakte structuren gevormd. Deze structuren hebben octaedrische molybdeen coördinatie.

Samenvatting

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Ten opzichte van oppervlakte monomolybdaten is de reductie temperatuur met ongeveer 140 ° C verlaagd. De waterstof–deuterium uitwisselingsreactie lijkt alleen te worden gekatalyseerd door tetraedisch gecoördineerd molybdeen; bij molybdeen beladingen hoger dan 8.5 gewichtsprocent was er geen waterstof–deuterium uitwisselingsactiviteit. Een kleine hoeveelheid waterstofatomen is aanwezig op het oppervlak van de alumina drager. Deze waterstofatomen nemen deel aan het H-D uitwisselingsproces via een spillover proces. In situ Raman spectroscopie experimenten toonden aan dat molybdeen onder reactiecondities kan worden gereduceerd door ammoniak, wanneer de molybdeen belading hoger dan 10 gewichtsprocent is. Quasi in situ XPS bevestigde deze resultaten. Na ammoniak behandeling is het molybdeen in de 4+-oxidatie toestand. Wanneer de belading lager is dan 10 gewichtsprocent zijn de Mo-O Raman banden verschoven naar lager golfgetal. Deze verschuiving wordt ook waargenomen voor gehydrateerd alumina gedragen molybdeenoxide. Dit suggereert dat de Mo-O binding wordt verzwakt door ammoniak adsorptie. Reoxidatie door middel van zuurstof herstelt de oorspronkelijke oxidatie toestand. Daar de ammoxidatie van tolueen een redox proces is, wordt de benzonitril productie sterk beï nvloed door de reduceerbaarheid van de katalysator. Bij molybdeen beladingen onder de 8.5 gewichtsprocent wordt een lage benzonitril productiesnelheid waargenomen. Deze katalysatoren kunnen bij de heersende reactietemperatuur niet door ammoniak worden gereduceerd. Wanneer de molybdeen belading hoger is dan 10 gewichtsprocent wordt, in overeenstemming met de eenvoudigere reductie een hogere benzonitril productiesnelheid gemeten. De lage reduceerbaarheid van molybdeen oxide in NaY zou de gemeten lage ammoxidatie activiteit kunnen verklaren. Hier is echter geen gedetailleerd onderzoek naar gedaan.

Wanneer kleine hoeveelheden vanadium worden toegevoegd aan γ-alumina gedragen molybdeen oxide, wordt een hogere ammoxidatie activiteit waargenomen. Raman spectroscopie toonde voor deze katalysatoren de vorming van MoO3 aan. De aanwezigheid van dit MoO3 kan niet worden verwacht op basis van de belading, omdat de monolaag bezetting bij een hogere belading wordt bereikt. De MoO3 clusters werden niet waargenomen met behulp van XRD, of TEM. Dit duidt op een hoge dispersie van de MoO3 clusters.

151

Dankwoord

Aan bijna alle dingen komt een eind, zo ook aan het schrijven van een proefschrift. Dit zou echter niet het geval zijn geweest zonder de steun en hulp van een groot aantal mensen. Rutger, bedankt voor het vertrouwen dat je me altijd hebt gegeven in de uitvoering van mijn onderzoek. Ik heb de vrijheid die je me gaf in het onderzoek en zeker ook in de samenwerking met de verschillende onderzoekspartners zeer prettig gevonden. Joop was altijd in staat om mij binnen de vrijheid van het onderzoek het hoofdpad weer te laten vinden, ook wanneer ik een doodlopend zijpad ingeslagen was. Kieran, I have appreciated very much your help, both in helping me understanding better the oxidation catalysis and in accurately correcting the manuscript during writing of my thesis. During your stay in Eindhoven you have motivated me to a great extent in enjoying the PhD research project. Rob, dank je voor de zeer grondige wijze waarop je mijn manuscript hebt gelezen en de nuttige suggesties die je hierbij hebt gegeven. De prettige wijze waarop jij correcties en verbeteringen voorstelt heb ik zeer fijn gevonden. San, van harte bedankt voor de buitengewoon secure manier waarop jij mijn manuscript hebt gelezen. Jouw correcties waren voor mij erg waardevol. Ook Hans wil ik graag van harte bedanken voor alle correcties en suggesties. Mijn collega’s wil ik graag bedanken voor alle tips bij zowel praktische als inhoudelijke problemen en natuurlijk voor alle gezelligheid. Met name wil ik Alina, Annemieke, Bruce, Darek, Frank, Imre, Marco, Mayela en Noud hierbij noemen. Lu Gang greatly helped me in the early stage of my research. Darek, Niels, Jeroen en Maarten dank ik voor hun enthousiasme tijdens het uitvoeren van hun researchstage. Betlem and Gemma, thanks for the enthusiasm you put in the work during your stay in Eindhoven. This was of great value for me. Ook buiten de metaalgroep heb ik op de TUE veelvuldig bij een groot aantal mensen kunnen aankloppen voor hun hulp. Emiel, bedankt voor al je hulp, niet in de laatste plaats bij de H-D uitwisselingsreacties. Roelant, jouw mening over mijn proef-proefschrift was zeer stimulerend. Arian en Marcel dank ik voor de hulp met het fijne werk aan de carbonyl katalysatoren in de glove-box. Ook Tiny en Leon kwamen hier graag een kijkje nemen, maar hen dank ik, samen met Peter, meer voor alle waardevolle hulp bij de XPS experimenten. Wout, jou bedank ik voor

Dankwoord

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alle hulp en meedenken wanneer ik weer eens met een groot of klein verzoek bij je kwam. Natuurlijk wil ik binnen SKA ook het secretariaat bedanken. Charlotte, Edith, Ine, Ingrid en Joyce, het was altijd leuk om even binnen te lopen voor een vraag, verzoek of zomaar voor een praatje. I would like to thank all partners of the EC-Brite project. I have really enjoyed our half-yearly meetings. Manolo, thank you very much for you contribution to my thesis, especially on the Raman and IR experiments. A great deal of the thesis could not have been written without your help. I really appreciated our collaboration and your hospitality during my stays in Madrid. Misha, many thanks for our very pleasant collaboration. Together with Roma you really have done everything one can imagine to help me in my research. Unfortunately I could not visit Kiev yet to see your laboratory during my PhD research. Patricia Kooyman (National Centre for High Resolution Electron Microscopy), jouw TEM metingen zijn zeer waardevol geweest voor mijn onderzoek. Ik heb onze samenwerking altijd zeer prettig gevonden. Mijn ouders wil ik graag bedanken voor de stimulans die zij mij hebben gegeven om deze promotie te volbrengen. Janneke, ten tijde van het schrijven van het proefschrift was je mijn vriendin, maar bij het verschijnen van het proefschrift ben jij mijn vrouw! Dank je wel voor al de liefde en steun die jij me gaf en die jij me geeft. Jij geeft mij de zekerheid dat toch niet aan alle dingen een eind komt. Eindhoven, 7 september 2000.

153

Curriculum Vitae

Pieter Stobbelaar werd op 4 februari 1971 geboren te Driebergen-Rijsenburg. Hij behaalde het Atheneum B diploma in 1989 aan het Eindhovens Protestants Lyceum. Aansluitend startte hij de studie Scheikundige Technologie aan de Technische Universiteit Eindhoven. Na een bedrijfsstage bij Raychem in Kessel-Lo (België ) studeerde hij in september 1995 af bij prof.dr. R.A. van Santen op het onderzoek getiteld “Hydroisomerisation of n-hexane”. Hij startte zijn promotieonderzoek bij de capaciteitsgroep Anorganische Chemie en Katalyse, geleid door prof.dr. R.A. van Santen, in oktober 1995. Het promotieonderzoek werd uitgevoerd in samenwerking met zeven onderzoeksgroepen in het kader van een EG consortium. De resultaten van het onderzoek zijn beschreven in dit proefschrift. Na voltooiing van het promotieonderzoek vervolgde hij in november 2000 zijn loopbaan als development engineer bij Central Development Lamps, Philips Lighting.

catalytic conversion of alkylaromatics to aromatic nitriles

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