delineating similarities and dissimilarities in the use of metal organic frameworks and zeolites as...

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Delineating similarities and dissimilarities in the use of metal organicframeworks and zeolites as heterogeneous catalysts for organic reactions

Amarajothi Dhakshinamoorthy,a,b Mercedes Alvaro,b Avelino Corma*a and Hermenegildo Garcia*a,b

Received 2nd March 2011, Accepted 12th April 2011DOI: 10.1039/c1dt10354g

This perspective article is aimed at providing a comparison of similarities and dissimilarities between thecatalytic properties of zeolites and metal organic frameworks (MOFs). In the first part of the paper, wecomment the general characteristics of MOFs with relevance to catalysis, making emphasis of how theproperties of MOFs can serve to compliment those of zeolites as catalysts. The lower chemical andthermal stability of MOFs compared to zeolites is commented and correlated to the requirements forsome liquid-phase reactions conducted under mild conditions. In the second part, we discuss thebehaviour of zeolites and MOFs for four types of general organic reactions (acid catalysed, basecatalysed, oxidation and hydrogenations). Particular attention is paid to provide critical comments onhow MOFs could be adapted by design or can be modified by post-synthetic treatments to give wellperforming catalysts.

1. General introduction

1.1 Structure of metal organic frameworks (MOFs)

Metal organic frameworks are porous crystalline materials whoseframework is constituted by metal ions or metal ion clustersoccupying nodal framework positions coordinated to di-, tri- ormultipodal organic ligands.1–6 These porous crystalline materialsare synthesised by reaction of metal salts with organic linkers hav-ing nitrogen or oxygen as donating atoms to form different porouscrystalline materials with very high specific surface area.5,7 Themost common organic linkers are rigid aromatic polycarboxylicacids, such as terephthalic acid, 4,4¢-biphenyldicarboxylic acid and1,3,5-benzenetricarboxylic acid.1,4

The important points in the above definition are the existenceof porosity in the material and a strong metal–ligand interactionthat should provide stability to the solid. This widely accepteddefinition of MOFs leaves out related materials in where therecould be also metal ligand interaction but lack porosity. The poresystem allows mass transfer from the exterior of the crystal tothe interior of the solid, rendering accessible internal sites andincreasing considerably the surface area of the material. Also, thisdefinition of MOFs exclude those types of solids, even containingmetal ions and organic components, but where the interactionsmaintaining the structure are not strong coordinative metal ligandbonds but of other type such as p–p stacking, hydrogen bonds orweak dipole forces.

aInstituto Universitario de Tecnologıa Quımica CSIC-UPV Av. De losNaranjos s/n, 46022, Valencia, Spain. E-mail: [email protected] de Quımica, Universidad Politecnica de Valencia, Av. De losNaranjos s/n, 46022, Valencia, Spain. E-mail: [email protected]

1.2 Structural relationship between MOFs and zeolites

Among the various materials discovered until now, zeolites arefound to be ideal heterogeneous catalysts for a large number ofgas-phase reactions.8–10 This is because of their unique structural,thermal and chemical stability that are superior over othermaterials.11 The excellent performance of zeolites as heterogeneouscatalysts in many organic reactions arises from the selectivitytowards the target product that can be frequently achieved withthese solids. Very often this selectivity derives from the fact thatthe reaction occurs in a confined space with limited dimensionsand, therefore, only those substrates, transition states or productsthat can fit inside the pores can react or can be formed. Thiscontrol caused by reaction confinement has been termed as “shapeselectivity” to indicate that the only reason that justifies reactivityor selectivity is the size and shape of the substrates and transitionstates with respect to the zeolite pore dimensions.12 One illustrativeexample of such shape selectivity is toluene disproportionationusing ZSM-5 as catalysts to obtain benzene and p-xylene (eqn (1)).In this case, thermodynamic equilibrium predicts a distributionof xylene isomers including ortho and meta. These isomers are,however, not formed simply because they cannot fit inside thepores when medium pore size ZSM-5 is used as catalysts. Besidesshape selectivity, selectivity using microporous silicates can alsoderive from the fine tuning of the nature and density of active sitesto achieve exclusively the desired catalytic properties.13,14

(1)

6344 | Dalton Trans., 2011, 40, 6344–6360 This journal is © The Royal Society of Chemistry 2011

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While significant advances have been made on the rational-ization of zeolite synthesis, the synthesis of MOFs allows abetter prediction of the resulting crystal structure based on theknowledge of the metal clusters and organic ligands participatingin the synthesis.3,7,15

The coordination geometry around the metal nodes and theorientation of the donor sites of the organic linker determine openframeworks with very large porosity in the range from about 0.5to 2.5 nm. The IUPAC defines as microporous/mesoporous solidsas those in which the dimensions of the pore is below 2 nm orbetween 2 and 10 nm, respectively.11 Some of these MOFs areamong crystalline materials with the lowest framework densities(smallest number of atoms per volume unit) and the highest porevolume.4 It is not then surprising that a number of recent catalyticstudies using MOFs have attempted to establish a parallelismbetween the behaviour of MOFs and zeolites.16–19 Indeed, in afirst approximation, one can easily recognise the existence ofmany properties of zeolites in MOFs, particularly in referenceto microporosity, crystal structure and even nature of someactive sites. However there are important fundamental differencesbetween them in aspects as important for catalysis as the diffusionof reactants and, especially, adsorption properties. Therefore,we would like to discuss in this manuscript on characteristicsthat become, not only relevant, but determinant for achievingsuccessful catalysts.

The purpose is to trigger further research trying to settle on afirm ground if MOFs can be useful catalysts, as zeolites are, andfor which reactions and under which conditions they should beemployed.

1.3 Zeolites as heterogeneous catalysts

The use of zeolites as heterogeneous catalysts started in the 1960ssoon after the first synthesis of these materials made them widelyavailable.8,9 The vast number of studies in which zeolites havebeen employed as catalysts in gas and liquid-phase reactions, andthe exhaustive information about the composition, structure andlocation of the active sites have accumulated an impressive amountof information.20 Just to give an idea, the number of patents onzeolites disclosed in 2009 were still more than 800 according to theSciFinder Scholar Database. Although there are many successfulindustrial applications of zeolites as solid catalysts, includinglarge-scale petrochemical processes and treatment of car exhaustgases, there are still many processes in fine chemicals and biomasstransformations that will benefit of using heterogeneous catalystswith properties similar to zeolites.21–29 Nevertheless, in the case ofliquid-phase reactions with complex organic substrates, one mustbe aware of potential diffusion limitations of the reactants and/orproducts through the micropores of zeolites. Thus, much workon the preparation of zeolites with smaller crystals, mesoporesformed in the crystals hierarchical organization, or the synthesisof extra large pore zeolites is being carried out to tackle diffusionproblems occurring in conventional zeolite micropores.14

1.4 Scope and purpose of this review

MOFs offer a considerably structural diversity in terms of theconstituent metal and organic building blocks.6 Very importantly,there is the possibility to predict the MOF structure upon

consideration of the primary building blocks.2,3 In addition, thesynthesis of these materials allows a large flexibility with respectto the metal salts and the organic linkers that can be employed.

Due to the large porosity of MOFs and their large metal ioncontent, one possible application of MOFs could be in the fieldof heterogeneous catalysis.16,17 In fact, and despite of the relativelyshort time elapsed since MOFs are under the research spotlight,several reviews describing the use of MOFs as catalysts in organicreactions have been published.16,17 The purpose of this review isnot to cover all reactions catalysed by MOFs, but to concentratein a series of them and to compare their catalytic behaviour ofMOFs and zeolites, highlighting the similarities between themand indicating the relative advantages and disadvantages of eachone of the two families of porous materials. In providing thiscomparison one should keep in mind that while zeolites ascatalysts are well-established materials developed for more thanfifty years, it may take still some time until the use of MOFs ascatalysts reaches a certain degree of maturity. We will attempt topresent here some scenarios in which MOFs could be valuableas catalysts, particularly stressing the challenges to be achieved.Research in the use of MOFs as catalysts should stress the uniquefeatures of these materials compared to other solids highlightingthe advantages and the relative performance of MOFs. As wewill comment extensively below, structural stability is one of themajor limitations to be overcome. The final objective should beto delineate for which reactions types and under which conditionsMOFs are advantageous heterogeneous catalysts.

2. General requirements for the use of MOFs asheterogeneous catalysts

One of the major drawbacks for the use of MOFs as catalysts istheir poor thermal and chemical stability.30–34 The limited thermalstability of the MOF crystal structure is certainly a limiting factorfor vapor-phase reactions carried out typically at temperaturesabove 200 ◦C. Thermogravimetric analyses under air showsthat most MOFs undergo a massive decomposition starting attemperatures above 300 ◦C. However, thermogravimetry tends tooverestimate the decomposition temperature since this techniqueis performed under temperature scanning, instead of keepingconstant the temperature at a certain value and determining thestability of the material over the time at this fixed temperature.Also some solvents, reagents and products could largely favorthe structural collapse of MOFs at lower temperatures than thatdeduced by thermogravimetry. Under these conditions, zeolites,metal oxides and other inorganic solids have proved to be goodheterogeneous catalysts. Oil refining and petrochemical processare examples for which catalyst thermal stability is a crucialfactor.22,35–37 At the present it is difficult to imagine that theperformance of MOFs can surpass or even be close to that ofzeolites for this type of application.

Nevertheless, MOFs can compete or even overcome the activityof zeolites in liquid-phase reactions, particularly those carriedout under mild conditions. In contrast to vapor-phase, liquid-phase reactions are typically carried out at temperatures below200 ◦C and the need of high thermal stability is not a mandatoryrequirement, provided that thermal regeneration of the catalystis not required. The fact that thermal stability is not a necessary

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 6344–6360 | 6345

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Table 1 Comparison of some structural and catalytically relevant properties for zeolites and MOFs

Properties Zeolites MOFs

Lewis acidity Sites due to metal ions in the framework or exchangeablepositions

Coordination positions not compromised with theframework

Brønsted acidity Bridging Si(OH)/Al hydroxyl groups Introduced by post-synthetic modifications or functionalgroup present in the ligands not compromised in thestructure

Surface area (BET) Around 200–500 m2 g-1 Up to 5000 m2 g-1

Pore volume Around 0.1–0.5 cm3 over 1 cm3

Thermal stability Mostly stable above 450 ◦C Not stable above 300 ◦CChemical stability Stable to solvents, acids, oxidizing and reducing agents Limited choice of solvents. Unstable to acids and bases.Diffusion High for substrates whose size is smaller than the pore

dimensions. Branched substrates diffuse slowerStrongly influenced by the polarity of linkers

Basicity Arises from the framework oxygens Introduced by post-synthetic modifications or functionalgroups present in linkers

Metal site density Low percentage of metal framework. Typically they areunstable.

High percentage of metal frameworks that could act asLewis acid or redox sites

Framework defects Plays an important role in many reactions Expected to play a minor role. Framework defects easyto be introduced through the mixed ligand approach

Active site environment Mostly hydrophilic but can also be made hydrophobic Considerably hydrophobicReactivation By thermal treatment Thermal treatment not valid for catalyst regeneration

requirement in catalysis has been clearly illustrated in the field ofenzymatic catalysis and biocatalysis in general that are promisingto perform many liquid-phase organic transformations under mildreaction conditions.38–43 In this sense, MOFs can be considered, interms of stability, intermediate materials between robust zeolitesand labile biomolecules.

3. Similarities between zeolites and MOFs

Although zeolites and MOFs have very different compositions,both types of materials possess some similar properties whichcan be advantageously used to develop selective gas adsorbents,stationary phases for liquid chromatography, sensors and catalysisamong other applications.18,32,44–46 Characteristic features andproperties of these materials are summarized in Table 1.

4. Liquid-phase vs. gas-phase organic reactions

The catalytic activity of microporous solids in liquid-phasereactions can be controlled by substrate and product diffusioninside the micropore system. In contrast, due to the higher reactiontemperature, diffusion limitations are generally less importantfor vapor-phase reactions. Other factors such as distribution ofactive sites on a high surface area and density and activity ofthe sites are equally important in gas- and liquid-phase catalysis.Considering the large surface area and porosity as well as thefact that the structure is constituted by nodal metals, MOFs canhave advantages over zeolites and non-porous amorphous oxidesunder conditions in which the structural stability of MOFs is notcompromised. Scheme 1 summarizes a list of parameters that canhave relevance in catalysis comparing zeolites and MOFs.

Provided that MOFs maintain their structural integrity andthere is no leaching of active species to the solvent at the reactiontemperature, intraparticle diffusion becomes one of the generalphenomena controlling the activity in porous solids. In thisregard, MOFs having pore sizes up to 2.5 nm should exhibitadvantages over zeolites for liquid-phase reactions when largerpore dimensions than those found in classical large pore zeolites

Scheme 1 Advantages of MOFs in heterogeneous catalysis in comparisonwith zeolites.

are required.11 Large pore zeolites with 12 ring pore aperturescan reach pore diameters up to 0.74 nm,10,11,22 while, for extralarge aluminosilicates the pore diameter can go up to 1 nm.47,48

Very recently extra large pore zeolites with pore diameters upto 2.0 nm have been synthesised that while catalytically active,they may exhibit stability problems when in the presence ofmoisture.47

It has been shown that MOFs have a large degree of frameworkflexibility, adapting their structure to the presence of guests.44,49,50

In this sense, MOFs have been termed as “soft porous materials”to indicate that internal pressure or external conditions caninfluence the structure of MOF.51 It has been described thatMOFs can breath in the sense that the cell parameters can expandto accommodate adsorbed guests and then relax when they are


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removed.52 In contrast to this property, zeolites are more rigidmaterials, since the average pore diameter is almost constant inthe temperature range up to 700 ◦C.53

5. Metal ions as active sites

MOFs contain a large percentage of metal ions that can be poten-tial active sites when they have available coordination positions.In addition, MOFs can also be prepared in a larger variety ofmetal centers than zeolites. Indeed, in the case of zeolites, metalscan occupy framework or extra-framework positions. When inframework positions, the transition metal is replacing a siliconatom. The difference in ionic radii and properties of transitionmetals with respect to Si4+, makes metal isomorphic replacementdifficult and only a small weight percentage of transition metalscan be introduced by direct synthesis. Moreover, and due to therelatively lower stability of some metals in framework positions,attention should be paid to the possibility that the heteroatommigrates from framework to extra-framework positions uponthermal treatment and annealing. This heteroatom migration willtypically form some metal oxide inside of the micropores, causingin some cases, a reduction of surface area and the evolution ofthe original sites to aged species with different (typically lower)catalytic activity upon use. In any case, MOFs should also be testedfor structural integrity under conditions in which heteroatommigration occurs in zeolites.

An alternative possibility to incorporate a transition metalinside a zeolite can be as a charge-balancing cation occupying extraframework positions. However, particularly if the transition metalhas multiple charge, they can undergo hydrolysis even under mildheating, this leading to the formation of cationic metal hydroxideand protons. These metal hydroxide cations may eventually tend toripe forming metal oxide clusters inside the zeolite. Therefore, ionexchanged transition metals are also not stable in zeolites undermoderate calcination and these sites may evolve and age upon use,forming metal oxide clusters. In addition, non-framework metalions tend to leach from the solid to the solution in the presence ofpolar liquids.

In contrast to this, there are several transition-metal MOFs thathave shown to be stable under liquid-phase reaction conditions.54,55

Thus, these MOFs combine high surface area and porositytogether with a high density of single site metal ions or metalion clusters that can be stable under some reaction conditions.

6. MOF deactivation and catalytic stability

An ideal heterogeneous catalyst should exhibit high activity andselectivity and should not deactivate over the course of thereaction. Obviously, it is difficult to fully fulfil simultaneously thesethree conditions and solid catalysts deactivate after sufficientlylong reaction operation time. While, reusability tests to prove thatMOFs do not deactivate upon reuse are frequently performed, wehave to point out that the number of recycles and the accumulatedturnover number (TON) do not ensure the use of MOFs for longtime operation. The TON indicates how many moles of productare formed as an average per active site. Highly stable catalysts aszeolites in some cases have TON above 100 000 indicating that oneactive site promotes the formation of many molecules of product

before becoming deactivated. It remains to be seen whether suchhigh values can be achieved for MOF catalysed reactions.

In this regard, the figure of merit should be the maximumcatalyst productivity or kg of product produced per kg of catalyst.Thus, highly stable catalysts that do not become easily deactivatedduring the reaction exhibit high productivity and many kg ofproduct can be produced per kg of catalyst. For instance, in thedecomposition of phenol by hydrogen peroxide in the presence ofgold supported catalysts, 120 000 kg of phenol can be decomposedin water by kg of gold.56 Again, there is a poverty of productivitydata using MOFs as catalysts and these numbers are needed inorder to determine the economics of the process and the possibilityto develop industrial processes based on MOF catalysis. Certainly,a catalyst that can be successfully regenerated is a more desired onefrom the process as well as from the economic point of view. Then,in the case of MOFs, they can be of limited use unless a sufficientlyhigh TON is obtained before the catalyst becomes deactivated.Indeed, compared to zeolites, MOFs have the disadvantage thattheir limited thermal and chemical stability preclude combustion,pyrolysis or thermal treatments as valid procedures to reactivatedeactivated catalysts. However, in the case of MOFs, selectiveextraction of the poison or desorption under mild temperaturesunder reduced pressure may be viable alternatives to regeneratethe catalyst.

Other possible causes of deactivation can be the collapse ofthe structure. Severe structural damages are difficult to reverseand generally imply that the material is irreversibly deactivated. Ifthis occurs, only recovery of the metal and organic ligand willbe possible. Therefore the aim in the synthesis of MOFs hasto be to produce catalysts that allow a sufficient large TON sothe catalyst could be replaced by a fresh one instead of beingregenerated.

In order to study the catalyst deactivation, samples of de-activated MOFs should be obtained from experiments withlarge substrate excess and scrutinized for the primary causes ofdeactivation using textural, microscopy and spectroscopic tools.Isothermal gas adsorption would report of the blocking of themicropore system, while powder XRD of the deactivated catalystcould serve to determine the integrity of the crystal structure.FT-IR spectroscopy can be used to determine the presence oforganic poisons and the creation of new functional groups in thelinker. One approach that has been used in the case of zeolites todetermine the nature of the poisons is dissolving the solid with HFsolutions and proceeding to extraction and analysis of the organicmatter present in the resulting liquor.57 This methodology basedon dissolution of the solid framework and analysis of the organiccompounds could be even more simply applied to the case ofMOFs, since these solids can be redissolved more easily (no needof HF) and the resulting liquor submitted to analysis that caneasily differentiate the organic linker from the poison. Electronmicroscopy can also be applied to establish whether or not themorphology of the MOF particles has changed during the reactionand the formation of micro-/nanoparticles on the solid. When aMOF is being used as support of metal nanoparticles, electronmicroscopy can be used to assess variations of the particle sizeand aggregation of metal with the concomitant loss of catalyticactivity. Caution has to be taken to avoid damage of the MOFstructure by the highly energetic electron beams used in high-resolution electron microscopes.


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It can be assumed that the extended methodology used toassess the life cycle of solid catalysts will be increasingly appliedto the catalysis by MOFs because stability and productivity arecertainly the critical points to be evaluated when using MOFs asheterogeneous catalysts.

In the next sections will focus on the comparison of the catalyticbehaviour of zeolites and MOFs for a few important generalreactions such as those catalysed by acid and basic sites and redoxcenters.

7. Lewis/Brønsted acid-catalysed reactions

Lewis acid catalysis has been of great interest in organic synthesis.58

While many different types of Lewis acid-promoted reactionshave been developed, and many have been applied at industrialscale, there is still a considerable interest in developing newheterogeneous Lewis/Brønsted acid catalysts, particularly if theycan be reused. Characterization of the population and strengthdistribution of the Lewis/Brønsted acid sites by physical and/orspectroscopic techniques is necessary to gain insight into the cat-alytic properties of a material. However, when the characterizationtechniques widely used in zeolites are attempted to be applied toMOFs, some problems arise that make some of them of low use.For instance, the use of basic probe molecules can be limited. Thethermo-programmed ammonia desorption cannot be a suitabletechnique due to the possibility that ammonia adsorption destroysthe structural integrity of the solid and also because MOFs do notstand the high temperatures that may be required for desorption.Also, the pyridine titration procedure requires to monitor thecharacteristic aromatic stretching band of pyridinium and pyridineLewis adduct by IR at around 1550 and 1450 cm-1, respectively, afact that is not possible due to the presence of intense bands of theorganic ligand at this region.

7.1 Test reactions for Lewis acid sites

In view that the conventional techniques for acid site character-ization such as pyridine and ammonia adsorption/desorptionmonitored by IR spectroscopy or thermogravimetry are notappropriate for MOFs, and until instrumental techniques basedon adsorption/desorption of probe molecules are universallyimplemented, the use of test reactions could be a convenient wayto determine acidity in MOFs. In this regard, the rearrangement ofthe ethylene acetal of 2-bromopropiophenone is a very informativetest reaction based on the analysis of the product distribution.59,60

This reaction discriminates among Brønsted and hard or softLewis acids.59,60 Each of this type of sites gives a specific productas indicated in the Scheme 2 and the overall substrate conversiongives a quantitative assessment of the acid site strength.

By using the cyclic acetal of 2-bromopropiophenone as a testsubstrate, it was demonstrated that the active sites in Cu3(BTC)2

(BTC = 1,3,5-benzenetricarboxylate) are hard Lewis acid sites.61

In the case of zeolites, it has been determined that their protonicform has Brønsted acid sites, while the nature of the transition-metal determines the hardness/softness nature for ion exchangedzeolites.59

The acidity of Cu3(BTC)2 has resulted in a highly selective Lewisacid catalyst for the isomerization of terpene derivatives, such asthe rearrangement of a-pinene oxide to campholenic aldehyde

Scheme 2 Rearrangement of cyclic ethylene acetal of 2-bromopropiophe-none used as a test reaction to discriminate the nature of acid sites on solidcatalysts.

(Scheme 3) and the cyclization of citronellal to isopulegol.61 Inaddition, the rate of the reaction on the conversion of a-pineneoxide to campholenic aldehyde depended on the solvent nature(Fig. 1), catalyst synthesis procedure and also different catalystpre-treatments.

Scheme 3 Isomerization of a-pinene oxide to campholenic aldehyde usingCu3(BTC)2.

Fig. 1 Conversion of a-pinene oxide vs. time with Cu3(BTC)2 indifferent solvents. EtOAc = ethyl acetate, DCE = dichloroethane; MeOH =methanol; CH3CN = acetonitrile; PhCH3 = toluene (taken from ref. 61).

Recently, Liu and Zhong have taken two model MOFs, namelyCu3(BTC)2 and Cu-MIPT (MIPT = 5-methylisophthalate), asprobe materials to study its Lewis acidic nature by DFT theoreticalcalculations.62 The strength of these sites was estimated as afunction of parameters such as geometry of the site, natural bondorbital charge, and vibrational frequency as well as the adsorptionenergy of the probe CO molecule. The results show that bothMOFs have Lewis acid sites, and the strength of the Lewis acid sitesin Cu3(BTC)2 is stronger than that in Cu-MIPT. Fig. 2 presents


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Fig. 2 The structures of the CO molecule adsorbed on a clusterof Cu3(BTC)2: (a) adsorption through carbon; (b) adsorption throughoxygen. (c) optimized structure of the CO molecule adsorbed on the clusterof Cu-MIPT through the carbon atom (taken from ref. 62).

three models of the optimized geometry of CO adsorbed on theLewis acid sites of Cu3(BTC)2 and Cu-MIPT.

Further, these calculations and experimental results have ledto propose that the presence of electronegative elements in theorganic linker and the use of a metal cation with more vacantd orbitals, could be a way to increase the Lewis acidity of theresulting material.62 This study constitutes a nice example of howthe combination of theoretical calculations and measurements canserve to propose novel MOFs that can be suitable as solid Lewisacids. The synthesis and catalytic test of the predicted MOFs isa work worth to be surveyed in order to validate the proposalssince it will constitute a clear realization of MOF versatility interms of design and structural diversity that can be used to adoptthe material for specific use in catalysis. This methodology forMOFs will parallel, then, the strategies used in zeolites to tuneacidity by controlled introduction of different TIII atoms, acidcharge-compensating cations or the generation of extra frameworkaluminium as Lewis acid sites.11

7.2 Formation of acetals

A new In(OH)L (L = C17F6O4H8) MOF was obtained us-ing 4,4¢-(hexafluoroisopropylidene)bis(benzoic acid) (H2hippb) asligand.63 The structure was described as thick layers with the bentgeometry of the ligand leading to the formation of square-shapedchannels, which run inside the framework layers (Fig. 3).

Thermally stable In(OH)L (L = C17F6O4H8), MOF has beenproved to be an efficient heterogeneous catalyst for acetalizationof benzaldehydes with trimethylorthoformate63 at 70 ◦C in 15 minexhibiting TOF as high as 1200 h-1 (Scheme 4). Interestingly,in a similar fashion, a modified series of In(OH)L MOFs weresynthesised including different mole fraction of pyridine as anadditional linker. The modified In-MOFs with 0.5 mole fractionof pyridine showed a lower TOF of 480 h-1. The influence of thepresence of pyridine on the catalytic activity supports that the

Fig. 3 Polyhedral representation of layer structure of In(III)-hippb MOF(taken from ref. 63).

Scheme 4 Acetalization of benzaldehyde with trimethylorthoformateusing In-MOF as catalyst.

substrates can easily reach the Lewis acid sites inside the porousmaterials to give high TOF and when pyridine is blocking someof the sites, as determined by crystallography, then the catalyticactivity decreases. In addition, these new Lewis heterogeneouscatalysts exhibited good stability in water and organic solvents,being easily recovered by filtration and reused at least in fourcycles without loss of yield or selectivity.

However, the fact that acetalization is not performed directlywith alcohols and that halogenated solvents were employed inthe process are factors limiting the environmental benignity of theprocess. Formation of methyl acetals using trimethyl orthoformatein aromatic hydrocarbons has also been reported using Brønstedzeolites as catalysts.64 A large number of acetal fragrances havebeen made with zeolites and if made hydrophobic, do not requirethe removal of water during the reaction.65

7.3 Ring opening of epoxides

Ring opening of epoxides by nucleophilic attack is an importantreaction in organic synthesis that finds an ample use in the prepa-ration of pharmaceutical and natural products.66–68 The generalapplicability of this reaction derives from the fact that changingthe nucleophile (alcohols, amines, thiols and halides) results in avariety of divergent products with various functionalities.69 Theclassical approach for the nucleophilic ring opening of epoxidesinvolves the use of homogeneous or heterogeneous Brønstedor Lewis acids in the presence of solvents.69 Clays, aluminaand particularly zeolites have been employed as heterogeneouscatalysts.70,71

Initially, Baiker and co-workers synthesised latent cop-per metal organic framework Cu(bpy)(H2O)2(BF4)2(bpy) fromCu(BF4)2·H2O and 4,4¢-bipyridine (bpy) in aqueous ethanol.72 Alatent MOF is typically a non-porous crystalline solid constitutedby a polymeric metal organic complex that under certain con-ditions or in the presence of some solvents undergo structuralreorganization forming a porous MOF. This latent Cu-MOF has


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been used as catalyst for regioselective ring opening of styreneoxides with methanol under ambient, solvent-free conditions with93% conversion and 95.5% selectivity (Scheme 5).72

Scheme 5 Ring-opening of styrene oxide catalysed by Cu(bpy)-(H2O)2(BF4)2(bpy).

Other epoxides were also tested with this catalyst usingmethanol and aniline as nucleophiles. The authors have observedthat the activity of the catalyst decreased as increasing the stericnature of alcohols, a fact that has been interpreted as indicatingincreasingly difficult access to the internal active sites of thecatalyst as the size of the substrate increase (Scheme 6).

Scheme 6 Schematic representation of latent Cu-MOF used for ringopening of epoxides.

The mechanism of the reaction was studied by attenuatedtotal reflection IR, Raman, EPR and UV/Vis spectroscopictechniques.73 It was found that first, methanol induces reconstruc-tion of the solid which is transformed from a non-porous 3DCu-MOF to a pillared 2D sheets with an open structure, andalso in a minor extent onto soluble multicopper clusters. Afterreconstruction, methanol and styrene oxide can easily access tothe copper ions which act as catalytic sites and efficiently promotethe ring opening reactions when compared with other alcohols.In contrast, this kind of solid reconstruction does not occur withbulkier alcohols such as 2-propanol and 2-methyl-2-propanol andtherefore, only slow transformation of the epoxide was observed,which was attributed to the low external surface area of bulk solidCu-MOF (Fig. 4).73

Recently, an iron-based metal–organic framework, Fe(BTC)[iron(III) linked with 1,3,5-benzenetricarboxylate] has also beenreported as an efficient catalyst for the ring-opening of epoxidewith alcohols and aniline under mild reaction conditions.74 Therate of ring-opening reaction of styrene oxide decreases as thesize of the alcohol is increased suggesting again the location ofthe active sites inside the micropores. Steric encumbrance on thealcohol or impeded diffusion plays an unfavorable role on theconversion. Also, it has been shown that Fe(BTC) is a general

Fig. 4 Ring-opening of styrene oxide with alcohols with latent Cu-MOF(black) and the homogeneous catalyst Cu(BF4)2·H2O (gray). Reactionconditions: 0.11 mmol Cu, 5 mL alcohol, 1.25 mmol styrene oxide, RT,2 h (taken from ref. 73).

catalyst when compared with Cu3(BTC)2 and Al2(BDC)3 (BDC =1,4-benzenedicarboxylate) (Fig. 5). The ring-opening of epoxidesproceeds efficiently with high regioselectively to give good yields.

Fig. 5 Time conversion plot for ring-opening of styrene oxide withmethanol catalysed by (a) Fe(BTC); (b) Cu3(BTC)2 and (c) Al2(BDC)3.Reaction conditions: styrene oxide (2 mmol), catalyst (50 mg), 40 ◦C,methanol (5 mL) (taken from ref. 74).

The regioselectivity in most of the cases studied was complete,rendering the product expected from a Lewis-acid catalysed ring-opening through an SN1 mechanism (Scheme 7). This mechanismwas also supported with an additional experiment with Lewis baseto illustrate the competitive coordination with the active sites.74

The stability of Fe(BTC) in this reaction may be explained by themildness of the reaction conditions and the tolerance of MOFs tomethanol at room temperature. Also, Fe(BTC) exhibited superiorperformance than other related MOFs.

Scheme 7 Ring-opening of styrene oxide catalysed by Fe(BTC) withmethanol.


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In this regard, we have already commented that alcohols ingeneral and methanol in particular are not good solvents toperform acid catalysed reactions using zeolites as solid acids. Thehydrophilicity of conventional acid zeolites show a large preferencefor adsorption of methanol over the substrates and the acid sitesof the zeolites becomes neutralized by the alcohol, reducing theiracid strength.

NaY zeolites have been reported as efficient, recyclable, eco-friendly and industrially applicable catalysts for the epoxide ring-opening reaction under solvent-free conditions.75 The reactionproceeds rapidly to give b-amino alcohol in excellent yield (92%)with 100% regioselectivity (Scheme 8).75 For example, aromaticamine such as aniline showed excellent regioselectivity via pref-erential nucleophilic attack of the amine at the benzylic carbonto give terminal alcohol as major product with styrene epoxide(Major-1A). On the other hand, aliphatic amine selectively attacksthe terminal carbon of the epoxide to give b-amino alcohol as themajor product (Major-2B).

Scheme 8 Ring opening of epoxides catalysed by NaY zeolites.

In a different strategy directed to achieve asymmetric ring-opening in zeolites, metal salen complexes, particularly cobalt(III)has been incorporated inside the pores of the zeolites and relatedaluminosilicates.76 It would be interesting to see if this strategy canalso be used with MOFs and if the functional groups in the linkercan be used to anchor these complexes to the framework.

7.4 Addition of trimethylsilyl cyanide to aldehydes

Cyanohydrins are important precursors in industry as they providemany possibilities for conversion into various other functionalgroups which are usually synthesised by addition of cyanohydricacid or trimethylsilyl cyanide (TMSCN) to carbonyl groups.77–79

To achieve this transformation, Lewis acids have been used ascatalysts.

Cyanosilylation reaction was reported between aromatic alde-hydes and trimethylsilyl cyanide in benzene as solvent with 92%yield in 45 min using Zn-based MOFs.80 When the aromaticaldehyde was replaced with 1-naphthaldehyde, the yield of theproduct in the latter reaction was poor and this fact was attributedto the small pore dimension of the catalyst and as a resultsubstrates larger than benzaldehyde could not penetrate to reachthe catalytically active sites. However, since one of the potentialadvantages of MOFs is their larger pore size, it would have beenof interest to show that by using larger pore MOFs this limitationcan be overcome. In addition, the low structural stability of theZn-based MOF used makes its use as catalysts unviable.

Kaskel and co-workers have studied the cyanosilylation ofaldehydes with Cu3(BTC)2 as heterogeneous catalyst (5 mol%).33

It has been established that this catalyst possessed well definedaccessible Lewis acidic sites located at the copper ions.33 Using

Cu3(BTC)2 as catalyst, moderate yields of 57% (88.5% selectivity)were achieved in 72 h in pentane at 40 ◦C with a high catalystloading of 5 mol%, referred to the amount of benzaldehyde forthe cyanosilylation (Scheme 9).33 It was observed that Cu3(BTC)2

undergoes some reduction during the catalytic reaction resultingin the framework decomposition at higher reaction temperatures.MIL-101 [Cr(III) linked with 1,4-benzenedicarboxylate] was foundto be superior with respect to Cu3(BTC)2 in terms of percentageconversion of benzaldehyde,33 giving the corresponding adductwith trimethylsilylcyanide with 98.5% yield in 3 h at 100 ◦C.

Scheme 9 Cyanosilylation of benzaldehyde catalysed by MIL-101.

A scandium-based metal organic framework was synthesisedfrom squarate ion, C4O4

2- as a ligand and the correspondingscandium metal salts.81 This material was stable up to 420 ◦C asdemonstrated by the TGA/DTA curves and variable-temperaturepowder X-ray diffraction. The catalytic activity of this materialwas tested for cyanosilylation of benzaldehyde giving 90% conver-sion in 12 h at room temperature.81 Compared to benzaldehyde,acetophenone is less reactive and only 80% conversion in 24 h at40 ◦C is achieved.81

The target of cyanosilylation of aldehydes should be to performthe reaction under conditions as mild as possible, so that usingan enantioselective catalyst asymmetric formation of one of theenantiomers could be achieved. However, zeolites offer a limitedpore size and it has been found that the use of mesoporousaluminosilicates with passivated silanol groups are very appro-priate hosts for chiral vanadyl salen complexes that are highlyefficient for promoting enantioselective addition at temperaturesdown to -20 ◦C.82–84 Thus, the results achieved with MOFs forthis cyanosilylation are still far from optimum and the next stepshould be trying to decrease the reaction temperature (otherwisepoor enantioselectivity will be achieved) and trying to implementasymmetric discrimination in the process.

Towards these goals, Long and co-workers have syn-thesised Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 (H3BTT = 1,3,5-benzenetristetrazol-5-yl) metal organic framework wherein Mn2+

ions exposed on the surface of the framework might serve as Lewisacids (Fig. 6).85 The experimental conditions included a 1 : 2 molarratio of selected aromatic aldehydes and cyanotrimethylsilane indichloromethane at room temperature with 11 mol% of catalystwhich led to a 98% conversion of benzaldehyde after 9 h. Thiscatalytic system was further studied with other aromatic aldehydesin which the molecular size has been varied, confirming the broadapplicability of the catalyst, and the well defined pore dimensions.

7.5 Cycloaddition of CO2 into epoxides

Cycloaddition of CO2 to epoxides to produce five-memberedcyclic carbonates under mild reaction conditions is a processof large importance in the context of so-called CO2 fixation.86–90

CO2 is a very inert molecule but considering its availability andthe incentive for removing it from the atmosphere, it is of clearinterest for developing novel processes based on the use of CO2


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Fig. 6 A portion of the crystal structure of Mn3[(Mn4Cl)3-(BTT)8(CH3OH)10]2 showing the two different types of Mn(II) sites locatedwithin its three-dimensional pore system of 10 A wide channels (takenfrom ref. 85).

as C-1 feedstock.91–93 Typically, most of the reactions involvingCO2 are thermodynamically unfavourable, with the equilibriumshifted towards the reactants.94 However, CO2 fixation to epoxidesis a thermodynamically favourable reaction that can yield highamounts of cyclic carbonates.66,86 The driving force for this reactionis the relaxation of the ring strain of the epoxide that makesthe overall process thermodynamically downhill. In this context,MOF-5 (Zn4O clusters linked with 1,4-benzenedicarboxylate) inthe presence of quaternary ammonium salts (Me4NCl, Me4NBr,Et4NBr, n-Pr4NBr, n-Bu4NBr) has been reported as a hetero-geneous catalytic system for the insertion reaction of CO2 intopropylene oxide to produce propylene carbonate (Scheme 10).95 Itwas found that the addition of quaternary ammonium salts exerts asynergetic effect in promoting the reaction, and n-Bu4NBr was thebetter salt in combination with MOF-5 as heterogeneous catalystat 50 ◦C for 4 h at CO2 pressure 6 MPa. This catalytic systemwas further extended to other epoxides, such as glycidyl phenylether, epichlorohydrin and styrene oxide (Scheme 10). However,since MOF-5 is structurally unstable, it is of interest to expandthis study to other more robust MOFs.

Scheme 10 CO2 insertion into epoxides to form cyclic carbonates usingMOF-5 as catalysts.

Mixed-linker metal–organic frameworks, Zn4O(BDC)x-(ABDC)3-x (ABDC = 2-amino-1,4-benzenedicarboxylate) hasbeen synthesised by partial substitution of BDC linkers with

ABDC in MOF-5.96 As a result, a number of catalytically activematerials having amino groups can be synthesised using thedesired BDC/ABDC ratio. These mixed-linker MOFs weretested as catalysts for the reaction of propylene oxide and carbondioxide giving 63% yield under optimised reaction conditions.Here again, it was observed that addition of quaternary saltsincreases the catalytic activity of this hybrid material, as alsooccurs with homogeneous catalytic systems.96

Conventional zeolites tend to open the epoxide ring veryeasily due to the presence of adventitious acid sites that affectthe ring opening very rapidly and do not give an opportunityfor CO2 fixation. However, this should not occur if defect freezeolites with Lewis-acid sites (Ti) were used.76,87,97 Hexagonalmesoporous silicas (MCM-41) materials having pore size about3.4 nm can also be suitable for hosting metal complexes, such aschromium(III) salen, that are typically used as Lewis-acid catalystsin homogeneous phase for CO2 insertion to epoxides.76 In addition,these metal salen complexes, when chiral, are able to promotethe enantioselective CO2 insertion of chiral epoxides.66,86 Similarlyto what has been commented earlier, the larger pore dimensionsof MOFs, the presence of metal carboxylates in the nodes andeven the possibility of using a linker having an extra functionalitymay allow devising strategies in MOFs that can be difficult withaluminosilicates. Particularly encouraging is that epoxides arestable under a wide range of conditions in the presence of MOFs.Again, the fact that the use of MOF in catalysis is a relativelyrecent field can explain the still limited work that has been carriedout up to now in this area.

7.6 Claisen–Schmidt condensations

The Claisen–Schmidt condensation is one of the classical C-Cbond forming reaction that starts with available aryl ketonesand renders useful a,b-conjugated aromatic ketones employedas intermediates in a large variety of syntheses including theformation of heterocyclic compounds.69 This reaction is typicallycarried out with homogeneous Lewis acids such as aluminiumchloride or zinc chloride that are destroyed during the reactionworkup in order to separate the reaction products. There hasbeen a continued interest in developing solid Lewis acids thatcan be recovered and reused and do not lead to the formationof metal salts and chlorides as waste. In this context, Claisen–Schmidt reactions have successfully been carried out with zeolitesand other solid inorganic catalysts with either Brønsted or Lewisacidity. Claisen–Schmidt reaction between acetophenone andbenzaldehyde to give selectively chalcone has also been studiedwith Fe(BTC) as a heterogeneous catalyst in toluene.98 It was foundthat Fe(BTC) is also an efficient catalyst to synthesise many otherchalcone derivatives as well in good yields with high selectivity(Scheme 11).98 Comparison of this Fe(BTC) was also made withother MOFs such as Cu3(BTC)2 and Al2(BDC)3 and the formerseems to perform better in terms of percentage conversion. Thisparticular material was used to prepare many chalcone derivativesin moderate to high yields without compromising the structuralstability during the course of the reaction.

We have to point out that when using toluene99 as solvent, for-mation of undesirable byproducts decreased the actual chalconeyield 99 with zeolite. The formation of these byproducts usingzeolites as catalysts contrasts with the previous results commented


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Scheme 11 Synthesis of chalcone using Fe(BTC) as heterogeneouscatalysts in toluene as solvent.

for Fe(BTC) that in the same solvent gives chalcones in higheryield. On the other hand, condensation of aldehydes with aromaticketones when aromatic hydrocarbons were employed as solvents,give rise to a variety of related carbocations inside zeolites thatact as poisons, although the activity could be restored almostcompletely by calcining the zeolite in air.57 It appears that for thistype of aldolic condensations, Fe(BTC) works at least as well oreven better than zeolites.

7.7 Enantioselective Brønsted-acid reactions

Considering the large number of Brønsted-acid catalysed reactionsand the importance of asymmetric catalysis, development ofenantioselective Brønsted-acid catalysts has been a target that hasbeen long pursued both in homogeneous and heterogeneous phasewith limited success and lack of general applicability. Among themajor problems to induce chirality in acid reactions it can bementioned the different sites where the proton is located, its easymobility and fast kinetics of the process lacking activation energy,and the reversibility of the protonation.

In spite of the more than 60 years in zeolite research, oneof the fields in which clearly the present achievements areunsatisfactory is in the field of enantioselective catalysis, includingenantioselective acid catalysed reactions. In contrast, it can beeasily anticipated that enantiomerically pure MOFs obtainedfrom enantiomerically pure organic linkers hold considerablypromise in asymmetric catalysis.100–104 One common characteristicof the asymmetric synthesis such as the low reaction temperatureat which the process must be carried out even increases thecompatibility with MOFs.

Chiral amino acid-based MOFs with the following formulaeCu(asp)L0.5-(guests) (asp = aspartic acid) where L = bipy, bpe,(bpe = 1,2-bis(4-pyridyl)ethylene) and Cu(L-asp)bpe0.5(HCl)(H2O)have been synthesised.105 The latter catalyst has shown to exhibitBrønsted acidity due to the presence of HCl and this materialpresents activity towards methanolysis of cis-2,3-epoxybutane.105

The catalytic reactions chosen were selected considering the lim-ited pore size inherent in these MOFs imposing a starting substratesize restriction as well to the reaction products and correspondingtransition states. When methanolysis by sulfuric acid (and HCl)is carried out in the presence of amino-acid containing MOFsracemic mixture (3% ee) of products was obtained. Using Cu(L-asp)bpe0.5(HCl)(H2O), although the ee generated is still far frombeing useful, a maximum of 17% ee was obtained, opening theway for further improvements and showing the opportunities thatMOFs offer in this field. As commented earlier, enantioselectivityin Brønsted-acid catalysed reactions remains still a challenge.106–110

Therefore although a 17% ee may seem a low value comparedto the ee values that can be achieved for other enantioselectivereactions, it has the importance to be obtained for a Brønsted-acidcatalysis.

8. Base-catalysed condensation reactions

The Knoevenagel condensation of aromatic aldehydes withsubstrates having active methylene groups to give benzylidenederivatives has been a favourite test reaction to assess the strengthof solid bases having sites of weak to medium basicity.111,112

Malonic esters, cyanoacetate and malonodinitrile are commonsubstrates where the acidity of the methylene hydrogens in-creases with the number of cyano substituents.112 In classicalorganic synthesis this condensation reaction can be catalysedusing alkali hydroxides, aliphatic amines or pyridines amongother homogeneous bases,69 and alkaline earth oxides as solidbases.113

Using the yield of the Knoevenagel product, we determinedempirically some years ago that the basicity of alkali metal ion-exchanged faujasites increases with the ionic radius of the alkalimetal ion and with the framework Al content.111 It was foundthat the maximum basic strength of zeolites was between that ofpyridine and piperidine.112 The zeolite basic sites were consideredthe negatively charged framework oxygens associated to the chargecompensating alkali metal cation.114,115

Besides this basic oxygen, zeolites and mesoporous silicascan be covalently functionalized to introduce pendant basicamino groups. Thus, amino-grafted Cs-exchanged NaX zeolite116

and amine-modified pore-expanded MCM-41117 exhibit mod-erate to high yields of 70 and 99%, respectively, with 100%selectivity for the condensation of ethyl cyanoacetate andbenzaldehyde.

Metal organic frameworks with non-coordinated amino groups,e.g. IRMOF-3 [Zn4O clusters linked with 2-aminoterephthalicacid] and amino-functionalized MIL-53 [Al(III) linked with 2-aminoterephthalic acid] are stable solid basic catalysts for theKnoevenagel condensation of ethyl cyanoacetate and ethyl ace-toacetate with benzaldehyde.118 Diethylformamide exchanged withIRMOF-3 exhibited activities that are at least as high as the mostactive zeolite or functionalised mesoporous solid basic catalystsreported, with 100% selectivity to the condensation product.118

The performance of amino-containing MOFs could be explainedbased on a large number of sites per unit weight. However one hasto take into account that activity should be preferably calculated bycatalyst unit volume (not by weight) especially when the catalystshave large differences in density. When this is considered, thecatalytic activity of zeolites compete favorably with the basicMOFs reported up to now for the Knoevenagel reaction describedabove.

Coordinatively unsaturated Zn(II)-based MOFs have also beenreported for the Knoevenagel reaction between aromatic aldehydesand malononitrile in dichloromethane as solvent in 80% yield(Scheme 12).80

Scheme 12 Knoevenagel condensation catalysed by Zn-MOF.

From the available data on this test reaction it can be saidthat MOFs may exhibit higher intrinsic activity than zeolites per


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unit weight. However, as has been commented earlier, besidescatalyst density consideration, it should be clarified which is themaximum productivity and the relative stability of MOFs for thereactions. These points have not yet been sufficiently addressed.In addition, considering the low stability of Zn MOFs, thismaterial must be carefully surveyed for structural integrity andstability, since otherwise its use as catalyst appears as not relevant.Concerning catalysts for basic reactions it should be noted that incondensation reactions, basic sites are frequently consumed andthere is a considerably depletion of basic sites during the reaction.Therefore, it is often necessary to replenish somehow the basicsites, typically treating the solid with a soluble base. It remainsto be seen if Zn MOFs or even more stable MOFs can standthese regeneration treatments with base maintaining the crystalstructure integrity.

Another point that requires study is if basic sites strongerthan amines can be created in MOFs, expanding the range ofreactions that can be subject to catalysis by MOFs. In zeolites, thenegative framework oxygens generate basic centres of a strengthintermediate between that of metal hydroxides (NaOH) and thatof metal oxides (MgO). However, in the case of MOFs the fact thatthe metal nodes do not contain basic sites and the organic linkerscan only introduce as substituents organic bases, typically amines,leads to expected low basicity. Strategies such as incorporation ofsuper nucleophilic organocatalysts appended to the frameworkor the use of the internal voids of MOFs to host strongbasic sites could probably be used to increase basicity in thesematerials.

Luminescent MOFs, [Eu(pdc)1.5(dmf)]·(DMF)0.5(H2O)0.5 (pdc =pyridine-3,5-dicarboxylate, dmf = dimethylformamide in latticepositions, DMF = removable dimethylformamide) with Lewis-basic pyridyl sites has been synthesised and used as sensors ofvarious metal ions possessing Lewis-acid nature. The responseof this material arises from the formation of an adduct betweenthe basic pyridyl moiety and the metal ion as Lewis acid.This is an interesting example of the use of Lewis basicity notfor catalysis but for sensing.119 Fig. 7 shows the structure of[Eu(pdc)1.5(dmf)]·(DMF)0.5(H2O)0.5.

Fig. 7 Crystal structure of [Eu(pdc)1.5(dmf)]·(DMF)0.5(H2O)0.5, viewedalong the a axis indicating immobilized Lewis basic pyridyl sites orientedtowards pore centers (taken from ref. 119).

9. Oxidation reactions

Oxidation of organic functional groups plays an important role inthe synthesis of many organic compounds which find interest inacademic as well as in the industrial community. In particular,selective oxygen transfer to olefins still remains an importantresearch area in industrial and synthetic chemistry, since epoxidesare widely used for the preparation of diols, epoxy resins ascomponents in paints, surfactants, as well as intermediates ina large number of organic syntheses. Hence, various catalyticsystems have been developed to effect the epoxidation of alkenesusing a range of oxidizing reagents including peroxides andperacids. However, the finding of heterogeneous catalysts thatare able to effect alkene epoxidation using molecular oxygen asoxidant still remains a challenge. A target for this process that willconstitute a significant achievement will be to reach a conversionof 10% with more than 95% selectivity towards epoxide. In themeantime, the use of hydroperoxides instead of oxygen as oxidizingreagents seems more realistic. Recently some hybrid organic–inorganic catalysts have been presented that are able to carryout the epoxidation of olefins with air with good selectivities andmedium enantioselectivities.101,120,121

The copper-based metal organic framework[Cu(H2btec)(bipy)]m (H4btec = 1,2,4,5-benzenetetracarboxylicacid; bipy = 2,2¢-bipyridine) (see Fig. 8) has been hydrothermallysynthesised and tested as a catalyst for the oxidation ofcyclohexene and styrene, with tert-butyl hydroperoxide asoxidant.122 The catalytic activity (24 h and 75 ◦C) found for[Cu(H2btec)(bipy)]m shows a high conversion of cyclohexene(64.5%), and a lower one for styrene (23.7%). It was, howeverobserved that the oxidation of styrene leads to the quantitativeformation of benzaldehyde (maximum of 74%) with 6%conversion at 30 ◦C. On the other hand, oxidation of cyclohexenewith tert-butyl hydroperoxide occurs in almost quantitative yield,rendering oxidation of cyclohexene epoxide accompanied bycyclohexanone and cyclohexenone. The high turnover frequencyvalues measured for cyclohexene epoxidation indicate that thecatalysts synthesised in this work, not only have a high activity andselectivity for epoxidation reactions, but are also very efficient.Although the present catalytic system could achieve high substrateconversion compared to related catalytic systems, it is necessaryto determine also the selectivity of the tert-butylhydroperoxideconsumed in the oxidation process and the stability of the catalystupon reuse.

Fig. 8 Packing scheme of [Cu(H2btec)(bipy)]m (taken from ref. 122).

Zeolites, on the other hand, have shown excellent activities andselectivities to perform epoxidations, as well as other oxidations


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using hydrogen peroxide and organic peroxides as oxidants.123

Since the pioneering work of ENI (Italian chemical company)researchers with TS-1 (Ti-silicalite),124–130 other Ti-containingzeolites, layered zeolites and structured mesoporous molecularsieves with wider pores that could be used as catalysts using tert-butylhydroperoxide as oxidizing agent have been reported.123,131,132

In the case of zeolites and, in general, in Ti-silicates, the presenceof silanols should be avoided either by passivating them bysilylation or by synthesizing zeolites without internal defects(silanol groups).133 The latter was achieved by preparing highlycrystalline Ti-Beta zeolites using fluoride rather than hydroxide asmineralizer in the synthesis.123,134 The remarkable crystallinity ofthe Ti-Beta zeolites synthesised by the fluoride method encounter,however, the limitation of the relatively narrow pore size ofthis zeolite that, particularly for liquid-phase epoxidation usingtert-butylhydroperoxide was inefficient when large alkenes wereemployed as substrates.123,135 The pore size limitation was solvedby developing Ti containing MCM-41 materials in which theresidual silanol groups were passivated by silylation.123,136 It hasto be remarked that solid catalyst based on a Ti-silicate with anarrow distribution of pore size in the mesoporous range hasbeen industrially successful for carrying out the epoxidation ofpropylene. Moreover, TS-1 has allowed the development of indus-trial process for propylene epoxidation, phenol hydroxylation andcyclohexanone oxidation.137–140

In reference to oxidation reactions, it is worth pointing out thatFe-ZSM-5 has been successfully used for hydroxylation of phenolwith N2O141 and Sn-Beta142 has given excellent results for Baeyer–Villiger oxidations with hydrogen peroxide143,144 as well as for manyother Lewis acid catalysed oxidation reactions.145

Two metal organic frameworks, [Cu(2-pymo)2] (2-pymo = 2-hydroxypyrimidinolate) and [Co(PhIM)2] (PhIM = phenylimi-dazolate) with Cu2+ and Co2+ ions and anionic diazahetero-cyclic ligands (pyrimidinolate and phenylimidazolate) as organiclinkers, have been used for the aerobic oxidation of 1,2,3,4-tetrahydronaphthalene (tetralin), yielding a-tetralone as the mainproduct.146 [Cu(2-pymo)2] was highly active for the activationof tetralin to produce tetralin hydroperoxide with negligibleinduction period, but less efficient in decomposing the perox-ide. In remarkable contrast, the cobalt based MOF showedas catalyst a long induction period for the reaction, but oncetetralin hydroperoxide is formed, Co2+ rapidly transforms thisinto tetralone with high tetralone-to-tetralol ratio of 7. Therefore,it was concluded that a combination of both copper and cobaltMOFs in the adequate proportion is the most active system sinceit combines short induction period and high tetralin oxidationactivity (copper) with a high selectivity towards the wanted ol/oneproducts (cobalt).146

In the case of MOFs, MIL-101, a mesoporous chromium metalorganic framework, was found to be an efficient heterogeneouscatalyst for selective oxidation of tetralin to 1-tetralone usingtert-butyl hydroperoxide as an oxidant.147 It has also beendemonstrated that the choice of solvent plays a vital role inachieving high percentage conversion of tetralin. Acetonitrile,chlorobenzene and benzene were found to be better choice overtetrahydrofuran as solvent. In any case, the use of ethers isnever advisable for oxidation reactions due to the possibility ofgeneration of explosive a-hydroperoxide that can cause hazardand risk during the reaction workup. Tetrahydrofuran was a poor

solvent with tert-butylhydroperoxide as oxidant. In the best case,oxidation of tetralin resulted in the formation of tetralone with85.5% selectivity with 1 : 2 mole ratio of tetralin with oxidant. 1-Tetralone was also accompanied with 1-tetralol (2.5%), naphthol(9.9%) and naphthalene (2.1%).147 Fig. 9 shows the influence withthe temperature of the conversion and the selectivity.

Fig. 9 Effect of temperature on the catalytic activity of MIL-101 usingt-BuOOH in chlorobenzene. Squares indicates the conversion of tetralin(taken from ref. 147).

Different MOFs with Sc and Y were synthesised with 1,5-and 2,6-naphthalenedisulfonates as a ligand and these MOFswas tested as active and selective bifunctional heterogeneouscatalysts in the epoxidation of 3,7-dimethylocta-1,6-dien-3-ol(linalool) and oxidation of sulfides (Scheme 13).148 Linalool wasoxidised to pyranoid and furanoid ethers, promoted by Sc-basedbifunctional redox-acid catalysts, in 24 h with a conversion ofmore than 60%. In the case of Y as central metal atom with 1,5-naphthalenedisulfonates as ligand, the conversion of linalool was100%.148 For both catalytic reactions, it must be noted that thesmall size of the pores in these structures prevent the access ofthe substrate to the metallic centres and thus the catalytic reactiontakes place only on the external surface of the particles.148 Thisis clearly an unsatisfactory situation since the main advantage ofusing microporous solids is lost when only the external surfaceis catalytically active. Also, considering the large particle size ofmost MOFs generally in the micron length scale, the exclusive useof the external surface will make MOFs notably disadvantageouswith respect to conventional metal oxides that can be preparedconveniently in very large surface areas.

Scheme 13 Oxidation of linalool and methylphenyl sulfide using Sc andY based MOF catalysts.

It must be pointed out that Ti-Al-MCM-41 is already usedfor oxidation of linalool to pyranoid and furanoid ethers withexcellent activity and selectivity. Remarkably the ratio of the


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furanoid to the pyranoid ether was very similar to that obtained bythe natural molybdase enzyme (0.7). In the case of Sc and Y MOFsthat ratio were 0.45 and 1 for the two Sc MOFs having 1,5 and2,6-napthalenedisulfonates, respectively, and 0.3 for Y MOF. Ti-Beta and Ti-MCM-41 materials also showed an excellent activityto convert sulfide organic molecules into sulfoxides and sulfonesusing either hydrogen peroxide or organic peroxides as oxidants.149

In this case, the catalyst can be regenerated by thoroughwashing.

Baiker and co-workers have developed a new route for thesynthesis of highly porous mixed-linker [Zn4O(BDC)3-x(ABDC)x]MOFs where 5–10% of the BDC linkers have been substituted by afunctionalized linker, namely 2-aminobenzene-1,4-dicarboxylate(ABDC),34 as shown in Fig. 10. The thermal stability of theresulting MOF decreased when increasing the degree of NH2

substitution. Nevertheless, all materials showed thermal stabilityup to at least 350 ◦C in oxidizing atmosphere that is sufficientlyhigh for most catalytic applications. The amino group at thefunctionalized linker proved to be beneficial for the introductionof Pd species.34 The Pd loading could be controlled depending onthe percentage of amino groups in the material. Pd-loaded, mixed-linker MOFs was successfully applied as catalysts in the oxidationof CO at elevated temperatures.

Fig. 10 Structure of Zn-based mixed-linker metal–organic frameworks(MIXMOFs) containing BDC and ABDC linkers. The two linkermolecules are randomly distributed in the structure (taken from ref. 34).

Ytterbium-based MOFs have been synthesised by solvothermalsynthesis with the formula Yb(OH)(2,6-AQDS)(H2O) (AQDS =anthraquinone-2,6-disulfonate) as a porous solid.150 This materialwas also tested as bifunctional redox-acid catalyst in the transfor-mation of (linalool) to hydroxy ethers, in acetonitrile, at 353 K,with an excess of H2O2. The material is very active as redox catalystbut acts poorly as acid catalyst, with 36% of conversion after28 h, with a ratio 1 : 100 Yb : substrate.150 In the later case, a totalconversion is achieved after 28 h, with the same Yb : substrateratio, and with a high selectivity to the furanoid vs. pyranoidproducts (9 : 1 in the first run; 7 : 3 in the second run).

Recently, the ligand 1,4-bis[(3,5-dimethyl)-pyrazol-4-yl]benzene(bdpb) has been reacted with suitable cobalt(II) salts un-

der solvothermal conditions leading to formation of a seriesof novel cobalt(II)-based MOF compounds one of which is,[CoII

4O(bdpb)3] (MFU-1, where MFU is an acronym for MOFsmade at the University of Ulm). MFU-1 has been used asheterogeneous catalyst in the liquid-phase oxidation of cyclo-hexene using tert-butyl hydroperoxide as oxidant to form 2-cyclohexenyl-1-tert-butylperoxide accompanied by 2-cyclohexen-1-one and cyclohexene oxide (Scheme 14).151 The maximumsubstrate conversion achieved after 22 h was 27.5%. It is worthnoting that although cyclohexene conversion was not high, theselectivity toward allylic substitution leading to the formation of2-cyclohexenyl-1-tert-butylperoxide (ca. 66%) exhibited by MFU-1 is remarkable.

Scheme 14 Oxidation of cyclohexene with MFU-1 as heterogeneouscatalyst using t-butylhydroperoxide as terminal oxidant.

During the catalytic study with tert-butylhydroperoxide, it wasnoticed that the microcrystals of MFU-1 undergo a slow colourchange from blue to green. However, the MFU-1 crystal structureis retained, as evidenced by powder XRD data of the recoveredcatalyst which gave no indication of structural decomposition.The BET surface area of MFU-1 decreases from 1485 to 1018 m2

g-1 after the first run. The partial loss of activity and thereduction in surface area was ascribed to the formation of polarreaction products during catalysis that might block active sitesand adsorption sites in the crystal lattice. It should be mentionedthat cobalt compounds and complexes are used as homogeneouscatalysts for aerobic oxidation employing molecular oxygen asterminal oxidant. Thus, it would be of interest to know if MFU-1has some activity as a heterogeneous catalyst when oxygen is usedas oxidant.

Recently, Cu-MOF Cu(bpy)(H2O)2(BF4)2(bpy) has also beenfound to exhibit a promising catalytic activity and high selectivityfor the allylic oxidation of cyclohexene with molecular oxygen asthe sole oxidant.152 Cu(bpy)(H2O)2(BF4)2(bpy) belongs to the typeof so-called latent porous MOFs that do not exhibit porosity inthe resting state, but in the presence of certain solvents and due tothe lattice flexibility then expand the pore size. Using this latentCu-MOF, the selectivity for cyclohexene hydroperoxide reachedto 90% working at reaction conditions close to ambient and inthe absence of solvent. However, the combination of catalyticand spectroscopic studies indicated that the reaction occurs at thesurface of the latent porous framework and both bpy and waterare involved in the active complex. Thus, in this case, the materialdoes not really act as a MOF since there is not porosity in thesematerials.

The original structure of as-prepared Cu(bpy)(H2O)2-(BF4)2(bpy) is poorly active for allylic oxidation of cyclohexenebut the crystal-to-crystal restructuring during the complete dehy-dration under high vacuum is sufficient to provide a catalyst that issuperior to the reference Cu-MOF catalyst prepared according toliterature. The small difference in the activities of Cu-MOF uponconsecutive cycles of dehydration and rehydration compared tothe as-synthesised sample is probably related to the differencesin their particle size and thus differences in the external surface


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area of the particles. A reliable determination of the BET surfaceareas is, however, not possible due to restructuring (dehydration)of the framework during the necessary pretreatment at elevatedtemperature. The catalyst stability was reduced by dehydration ofwater molecules but the activity and stability of this Cu-MOF isregained again by rehydration that is accompanied by structuralchanges (Fig. 11).

Fig. 11 XRD patterns of Cu(bpy)(H2O)2(BF4)2(bpy) before (a) and after(b) reaction at 50 ◦C: (x) marks diffraction peaks originating from theinternal standard Cu (taken from ref. 152).

Iron and copper metal organic frameworks [Fe(BTC) andCu3(BTC)2] have been used as heterogeneous catalysts for theoxidation of benzylic compounds153 with tert-butyl hydroperoxideas oxidant in acetonitrile with moderate to good yields (20–98%)(Scheme 15). The synthetic interest of this reaction derives fromthe importance of aromatic ketones and their transformation intoa wide range of derivatives.

Scheme 15 Oxidation of xanthene to xanthone catalysed by Fe(BTC)with tert-butyl hydroperoxide.

Oxidation of hydroquinone to p-benzoquinone has been re-ported with microporous MOFs namely M3(BTC)2(H2O)x (M =Cu(II), Co(II) or Ni(II); BTC = benzene-1,3,5-tricarboxylate; x = 3for MOF Cu(II) and 12 for Co(II) and Ni(II)) using molecularoxygen as oxidant.154 The catalyst stability during the courseof the oxidation reaction was, however, not determined. Theheterogeneity of the reaction still has to be demonstrated.

As commented earlier, zeolites, silicates and aluminophosphateswith transition metals such as Ti, Sn, Fe, Co, Mn, Cu, V in thelattice are active and selective for oxidation reactions. Attentionhas to be made during the synthesis to avoid the presence ofextraframework metal. Moreover one has to be aware about thepossibility that metal leaching may occur during the reaction.In this respect, it is of much interest to extend the possibilitiesof MOFs as selective oxidation catalysts, while studying theirstability and leaching properties. Formation of encapsulated metal

complexes that exhibit catalytic activity has also been extensivelyreported,155,156 and it may also be a possibility to be prepare thesewithin the pores of MOFs.

10. Hydrogenation

Hydrogenation of carbon–carbon multiple bonds is a generalreaction in organic chemistry that is normally effected by hydrogengas and heterogeneous noble or transition-metal catalysts.

Kaskel et al. have shown that MOF-5 can be convenientlyused as a support for the deposition of Pd which was usedfor the hydrogenation of olefins.157 Pd/MOF-5, showed a higheractivity towards the hydrogenation of styrene than palladium onactivated carbon with similar palladium loadings. Unfortunately,one serious disadvantage of the Pd/MOF-5 catalyst was itsinstability in contact with water or humid air. Hence it is moreconvenient to use as palladium matrix some material which canovercome these difficulties. Instead of MOF-5, MIL-101 hasbeen an attractive material which can act as support due to itslong-term stability in the presence of moisture, very high porevolume and a relatively high thermal stability. Hence, palladiumwas deposited on MIL-101 by a wet impregnation method andtested for catalytic activity and stability in liquid-phase and gas-phase hydrogenation of styrene and ethylene.158 In contrast topreviously reported Pd/MOF-5, all Pd/MIL-101 materials couldbe synthesised and handled in air due to their superior stability.Under the same reaction conditions, hydrogenation of styrenewith Pd/MOF-5 yields 80% hydrogenation products after thesame reaction time whereas Pd/Norit A (Pd supported on acommercial activated carbon) affords 65%, and Pd/C less than50% ethylbenzene. In this case, only supported catalysts with apalladium content of 1 wt% were used. The results prove the higheractivity of Pd/MIL-101 towards the hydrogenation of styrene(Scheme 16). In addition, Pd/MIL-101 clearly shows the highestactivity in the hydrogenation of cis-cyclooctene compared to allother palladium supported catalysts tested. The higher activity ofPd/MIL-101 compared to Pd/MOF-5 may be caused by the largerpore size of the former. Besides the high initial activity, Pd/MIL-101 also showed significant activities after 145 h time-on-stream,indicating a slow deactivation of the catalyst. The observed slowand continuous degradation in performance with time on-streamcould possibly be explained by blocking of the pores with residualhydrocarbon.

Scheme 16 Hydrogenation of styrene to ethylbenzene catalysed byPd/MIL-101.

Many precedents have been reported for this reaction withvarious heterogeneous catalysts just to avoid the use of noblemetals and its cost, as well as the use of reagents alternative tohydrogen for the hydrogenation of olefins. The list includes formateand hydrazine or its derivatives and even alcohols. MOFs withaluminium coordinated with benzenedicarboxylic acid has beenreported as an efficient heterogeneous catalyst for the reduction


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of carbon–carbon multiple bonds in alkenes, alkynes and a,b-unsaturated esters with hydrazine hydrate in acetonitrile undermild conditions.159 Isomerization of alkenes was not observed withthe mentioned protocol. Also, the potential utility of this catalystwas tested with many substrates in moderate to good yield.

Recently, a calcium-based MOF has been synthesised, charac-terized and its catalytic activity was studied in the hydrogenationof styrene160 with hydrogen at elevated temperature. While noble-metal catalysts are active for this reaction it is interesting to observethat a Ca-based MOF can be used as hydrogenation catalyst. Themaximum conversion (100%) was achieved in 2 h at 100 ◦C withhigh selectivity without other detectable products. The catalystdeactivation kinetics was not reported in detail although thestability of the fresh catalyst was studied.

The combination of noble metals and zeolites is a very large fieldthat has applications in large-scale chemical and petrochemicalindustrial process. There is abundant information about how toprepare highly dispersed noble metals supported in zeolites todevelop monofunctional hydrogenation catalysts with excellentactivities and reactivities, as well as bifunctional (acid + hydro-genation) catalysts that perform C–C bond cleavage (acid sites)followed by the hydrogenation of the resultant olefins. It is clearthat the efficiency and the productivity of zeolites for this gas-phase process will not be achieved with MOFs, since the stability ofMOFs for this process is very poor. Therefore, the opportunity touse MOFs in hydrogenations will only be found most probably infine chemistry, working in liquid-phase under mild conditions forlow vapour pressure or multifunctional compounds. Nevertheless,they will have to compete in those cases with well optimised metalsupported catalysts with excellent activities and selectivities.

11. Concluding remarks

The purpose of this review has been to compare the performanceand potential of MOFs with respect to zeolites for some reactions.As we have seen there are still many issues in MOFs that haveto be solved to achieve their maximum potential as catalysts.The preparation of single site, structurally stable catalysts withlarger pore dimensions and void volumes, with controlled particlesize and morphology, should drive towards active and selectivecatalysts. In the case of zeolites reactants accessibility for largermolecules, especially in liquid-phase should be improved. This canbe achieved by employing small crystallite zeolites, samples withhierarchical systems of meso- and micropores, layered zeolitesand zeolites with larger pores. It is clear that in zeolites, such as inMOFs, one can achieve well defined single and even multiple activesites. It is of interest to remember the possibility of modifying thepolarity of zeolites and its capacity to work in the presence ofwater. Finally, when the catalyst becomes poisoned, zeolites canbe easily regenerated. This advantage of zeolites over MOFs canmake both materials complementary rather than competitors sinceboth can be useful for different reactions, or even for the samereaction under different conditions. We just have to make use ofthe most adequate catalyst for a particular reaction regardless if itis homogeneous or heterogeneous, micro- or mesoporous MOFsor zeolitic. Therefore, both materials zeolites and MOFs shouldbe considered independently in order to determine which is themost appropriate for each application.

Finally, we have commented that asymmetric heterogeneouscatalysis is a promising area in which MOFs can easily makesignificant contributions. In principle, chiral MOFs should be easyto prepare in large quantities and stability should not be a majorproblem considering the mild conditions in which these reactionsare carried out. This can be a clear advantage of the MOFs withrespect to inorganic molecular sieves.

Acknowledgements

Financial support by the Spanish MICINN (project CTQ2009-11568 and CTQ2010-18671) is gratefully acknowledged. Fund-ing of European Commission through an integrated project(MACADEMIA) is also acknowledged.

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delineating similarities and dissimilarities in the use of metal organic frameworks and zeolites as...

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