Catalysis by MOFs

Horike, S., Dinca, M., Tamaki, K. and Long, J. R., Size-selective lewis acid catalysis in a microporous metal-organic framework with exposed Mn coordination sites , J. Am. Chem. Soc. 2008, 130, 5854-5855. 

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Most popular examples for catalytic apphcations belong to framework catalysis by metal ions in the MOFs even though the metal ion and the ligand are usually selected as the building blocks rather than as catalysts. After the pioneer works of Clearfield on phosphonates [136-138], framework catalysis by MOFs now includes cyanosilylation, the Diels-Alder reaction, hydrogenation, esterification, CO etc. [139-143]. 

Quite a few excellent reviews on catalysis by MOFs can be recommended to the reader [32,67]. One of the earliest reports on MOF-based catalysis by Fujita et al. [55] concerned the cyanosilylation of aldehydes in the presence of a 2D MOF based on 4,4 -bipyridine and cadmium nitrate Cd(4,4 -bpy)2(N03)2, which does not possess permanent porosity. The substrate size and shape selectivity was observed in the presence of this catalyst. The active... 

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

Studies of the catalytic activity of MOFs are in their infancy with some encouraging results emerging in enantioselective catalysis. By contrast, meso-porous solids have already been studied extensively as catalytic supports, particularly of complexes too large to be encapsulated in zeolites. One of the most significant developments in this area is the observation that the constrained encapsulation of chiral catalysts in mesopores can raise the enantioselectivities of reactions well above those observed when the reaction is performed homogeneously. 

Scheme 17.8 Heterogeneous catalysis by Cd(ii) MOF 22, behind oxidation reactions of alkylbenzenes to corresponding ketones.

Most catalytic systems based on MOFs exhibit catalytic activity due to metal ions (or atoms) as the active sites. However, there are reported examples of catalysis by organic ligands that contain no metal. 

From the temperature variation of the equilibrium constant, thermodynamic parameters for the reaction were also obtained. The extent of formation of [Mo(CO)5l]" was found to be cation-dependent, and while equilibrium constants of 39 and 21 atm L moF were obtained for Bu4P and pyH+, none of the anionic iodide complex was observed for Na. Despite this variation, there seemed to be no correlation between the concentration of [Mo(CO)5l]" and the rate of the catalytic carbonylation reaction. It was proposed that [Mo(CO)5] and [Mo(CO)5l] are spectator species, with the catalysis being initiated by [Mo(CO)5]. Based on the in situ spectroscopic results and kinetic data, a catalytic mechanism was suggested, involving radicals formed by inner sphere electron transfer between EtI and [Mo(CO)5]. 

In the absence of added nucleophiles, nitrosation occurs virtually irreversibly by an acid-catalysed pathway, presumably by attack by HjNO or NO". The third order rate constant from the rate equation equivalent to (46) has a value of 840 dm moF s- at 31°C (c/. 456 and 6960 dm mol- s for cysteine and thiourea respectively at 25°C) which suggests that for this neutral substrate the reaction rate is somewhat less than that expected for an encounter-controlled process. There is a major difference between the nitrosation of alcohols and that of thiols in that, whilst the former reactions are reversible (with equilibrium constants around 1), the reactions of thiols are virtually irreversible. It is possible to effect denitrosation of thionitrites but only at high acidity and in the presence of a nitrous acid trap to ensure reversibility (Al-Kaabi et al., 1982). Direct comparisons are not possible, but it is likely that nitrosation at sulphur is much more favoured than reaction at oxygen (by comparison of the reactions of N-acetylpenicillamine and t-butyl alcohol). This is in line with the greater nucleophilicity expected of the sulphur atom in the thiol. For the reverse reaction of denitrosation [(52) and (53)], the acid catalysis observed suggests the intermediacy of the protonated forms... 

The relevant properties of MOFs have prompted their use for catalysis, gas storage, and separation (Janiak, 2003 Chun et al., 2005 Kaye and Long, 2005 Rowsell and Yaghi, 2006), as well as for fuel cells (Mueller et al., 2006), Li-based batteries (Li et al., 2006 Ferey et al., 2007), and electrocatalysis (Wang et al., 2008). Conversely, MOFs can be synthesized electrochemically, as described by Mueller et al. (2006). MOFs are indirectly related with other electrochemical applications acting as a template for the synthesis of porous carbon to be applied as double-layer electrochemical capacitor (Liu et al., 2008). 

Chi the basis of E. values we can make a conclusion about the difhision factors which are some of the most conqilicated points concerning catalysis with immobilized enzymes. The value for the activation energy on peroxidase oxidation of phenol with catalase immobilized on "NORIT" soot is E, =10.95 kJ.mof which is an indication that the process takes place under diSusion regime. The latter means that the enzymatic reaction rate is determined by the mass tranfer of substrate to the surfoce of the carrier particles and its diffiision into the carrier. 

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