Chiral zeolites

Starting from the Pt-cinchona modified system, more recently an interesting concept has been developed by Feast and coworkers [144], A chiral acidic zeolite was created by loading one molecule of iM,3-dithianc-l-oxide per supercage of zeolite Y, either during or after the zeolite synthesis. Other chiral zeolites were formed by adsorbing ephedrine as a modifier on zeolites X and Y for the Norrish-Yang reaction [145],... 

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. 

An ideal approach to achieving chiral induction in a constrained medium such as zeolite would be to make use of a chiral medium. No zeolite that can accommodate organic molecules, currently exists in a stable chiral form. Though zeolite beta and titanosilicate ETS-10 have unstable chiral polymorphs, no pure enantiomorphous forms have been isolated. Although many other zeolites can, theoretically, exist in chiral forms (e.g., ZSM-5 and ZSM-11) none has been isolated in such a state. In the absence of readily available chiral zeolites, one is left with the choice of creating an asymmetric environment within zeolites by the adsorption of chiral organic molecules. 

Synthesis of Microporous Compounds with Chiral Channels or Chiral Structural Features In theory, there are 66 chiral space groups among 230 space groups, such as PA, P43, P6i, P62, P3i, P3>2, P65, P64, etc. The framework structures crystallized in these space groups do not contain any symmetric element, and this results in the chiral structures. However, the occurrence of chiral frameworks is rare, and an optically pure chiral zeolite material has never been found. Among the large variety of zeolites and related compounds, only a few are known to have chiral channels or chiral structural features. Therefore, it is difficult to summarize the rules for the synthesis of such compounds. Herein, we will present a rough overview of the synthesis of chiral microporous structures or microporous compounds with chiral structural features based on their framework compositions. 

It is therefore believed that the approach employing a rigid and optically active metal complex or organic amine as a template will become an efficient method for the synthesis of chiral microporous compounds. However, developing a route to the preparation of chiral zeolites, in particular the design of appropriate chiral SDAs, remains a challenging task. 

Y. Li, J. Yu, Z. Wang, J. Zhang, M. Guo, and R. Xu, Design of Chiral Zeolite Frameworks with Specified Channels through Constrained Assembly of Atoms. Chem. Mater, 2005,17,4399-4405. 

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. 

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... 

Zeolites are widely used in heterogeneous catalysis. In principal, their highly controllable porous structures have great potential for use as enantioselective catalysts. A considerable research effort has been devoted to the development of chiral zeolites [32]. Only zeolite beta and titanosiUcate ETSIO exist in chiral form [33, 34], although it is very difficult to obtain zeolite in enantiopure form [32]. Zeolites are typically synthesized in the presence of surfactant templates, which are removed by high-temperature calcination, a process that invariably destroys the chiral conformation of such assemblies [32]. Low enantioselectivities attributable to the chiral zeolite framework structure have been observed by Davis and Lobo [35] for the ring opening of trans-stilbene oxide with water. 

Sun J, Bonneau C, Cantfn A, Corma A, Dfaz-Cabanas Ml, Moliner M, et al. The lTQ-37 mesoporous chiral zeolite. Nature 2009 458 1154-7. 

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