Zeolite chiral catalysis

Zeolite-anchored complexes may exhibit a greater activity and selectivity in chiral catalysis. This has been demonstrated by Corma and colleagues.181-184 They prepared a chiral ligand from L-proline (1 in Fig. 7.3). 

Chiral catalysis using zeolites has been considered, however, intercalation of chiral metal chelates or the use of asymmetric organic pillaring agents may be a new way of introducing such selectivity. Iwai and coworkers have used asymmetric organic pillars, (R)- or (S)- phenylethylammonium cations in several layered phases and have observed some molecidar recognition Certainly further work is needed in this rather different definition of selective catalysis. 

Zeolites have also transformed the field of chiral catalysis, because of the possibility of introducing extra-framework active sites in the pores of these materials. There is, however, one subject yet to be investigated-the synthesis of chiral zeolite structures and/or chiral framework sites. 

Although the main applications of zeohtic sohds in catalysis will continue to be as solid acids in the synthesis and transformations of petrochemicals and commodity chemicals they continue to be considered as catalysts and catalyst supports for a range of reactions of synthetic and industrial relevance. The most important of these are of titanium- and tin-containing solids in selective oxidations. Other well-studied reactions over zeohtes include light hydrocar-bons-to-aromatics (Ga-zeolites) selective catalytic reduction of NO (transition metal exchanged zeolites) C C bond formation (Pd zeohtes) selective alkane oxyfunctionalisation with air (MAPOs, M Mn, Fe, Co) and chiral catalysis over encapsulated chiral complexes. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

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. 

Catalysis by zeolites is a rapidly expanding field. Beside their use in acid catalyzed conversions, several additional areas can be identified today which give rise to new catalytic applications of zeolites. Pertinent examples are oxidation and base catalysis on zeolites and related molecular sieves, the use of zeolites for the immobilization of catalytically active guests (i.e., ship-in-the-bottle complexes, chiral guests, enzymes), applications in environmental protection and the development of catalytic zeolite membranes. Selected examples to illustrate these interesting developments are presented and discussed in the paper. 

Stereoselective catalysis in zeolites is still one of the ultimate goals in zeolite science. Earlier work in this field was summarized recently [4]. More recently, Mahrwald et al. [95] reported that the addition of aluminophosphate molecular sieves in the liquid phase alkylation of a-chiral benzaldehydes by butyllithium results in an increased proportion of the so-called Cram product in the diastereomeric mixture. It is argued that in this Grignard type reaction the adsorption of the reactants on the molecular sieves favors the attack at the sterically less hindered position of the molecule. This shape selectivity effect is even observed when the reactant is adsorbed at the outer crystal surface, as demonstrated for the case of the small-pore AIPO4-I7. 

There is a continuous flow of new zeolite structures which might have a considerable potential in catalysis. Recent examples are NCL-1, NU-86, NU-87, SSZ-26 and MCM-22. However, zeolites with intrinsically chiral channels are not yet in sight, therefore stereoselective catalysis in zeolites still relies on the presence of chiral guests. Important reactions studied over acid zeolite catalysts include skeletal isomerization of n-butenes and of long-chain... 

The individual chemical species with chiral catalytic properties, such as complex, organometallic compounds, organic ligands or molecules, anchored or grafted into the channels of microporous and mesoporous materials, and some microporous compounds possessing chiral channels or their pore structures composed of the chiral motifs, all promise further development and potential application in microporous chiral (asymmetric) catalysis and separations. It is an important frontier direction in the zeolite catalytic field at present. Therefore, the synthesis and assembly of chiral microporous compounds and materials are of particular interest for researchers engaged in porous materials. This is a research field in rapid development. 

Among the many types of catalytic reactions, asymmetric catalysis is of great importance in industrial production of enantiomerically pure products. During the past few decades, much research effort has been devoted to the development of chiral zeolites and some other chiral porous materials having asymmetric catalytic sites. However, the traditional preparation procedures of zeolites require the removal of surfactant templates at the high temperatures of 400-550°C. Under such harsh conditions, the chirality of the preintroduced chiral surfactants, which are used to integrate silicate-surfactant assemblies into chiral conformations, is irreversibly destroyed. Therefore, an enantiomerically pure form of zeolite is not available to date. Compared to the syntheses of zeolites, homochiral MOFs can be... 

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. 

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