Enzymes, zeolites mimicking

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

More than MePc complexes, MePOR complexes have offered a great contribution to the understanding of monooxygenase and peroxidase enzymes.[75,76] The similarity in behaviour in selective oxidations with synthetic and natural systems has been the impetus for the search of mimicking the protein cavity of natural enzymes. In this context zeolites have always had a privileged role. Next to the in situ synthesis, zeolite adsorption of pre-synthesized FemPOR has been attempted as well. 

To date, no chiral zeolite or molecular sieve has been obtained. However, Newsam et al. (48) have shown that zeolite beta is an intergrowth of two distinct structures polymorph A and B. Polymorph A forms an enantiomorphic pair. Thus, synthesis of one of the enantiomorphs of polymorph A would yield the first chiral zeolite and initiate the possibility of performing intrazeolitic asymmetric catalysis. Shape selective asymmetric catalysis would be the ultimate achievement in shape selective catalysis, and would certainly be a step closer toward truly mimicking enzyme catalysis. 

Schulz-Ekloff G, Lipski RJ, Jaeger NI, Hulstede P, Kubelkova L (1995) Catal Lett 30 65 Parton R, den Vos D, Jacobs PA (1992) Enzyme mimicking with zeolites. In Derouane et al. (eds) Zeolites zeolite microporous solids synthesis, structure, and reactivity. Kluwer, Dordrecht, p 555 Herron N (1988) J Coord Chem 19 25... 

Additional reviews on biomimetic catalytic systems are available[20,21]. Shilov l reviews transition-metal complex systems that have related activities to biocatalytic systems. The review by de Vos et al.P l compares the reactivities of zeolite and layered hydroxide-based enzyme-mimicking systems. 

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