Shape-selective catalyst design

N. Y. Chen, T. F. Degman, Jr., C. M. Smith, Molecular Transport and Reaction in Zeolites - Design and Application of Shape Selective Catalysts, VCH, New York, 1994. 

The design of shape selective catalysts may be described as the science of coupling chemical reactions with sorption and diffusion characteristics of the zeolite to alter the reaction pathway and the product selectivity of known chemical reactions. 

One may wish to take advantage of the difference in the diffusivity of the molecules in the zeolite channels to achieve high selectivity. In this case, besides the selection of a specific zeolite structure and chemical composition, the successful design of a shape selective catalyst often depends on the ability to alter and control the morphology of the crystals and the means to adjust the concentration of active sites within each crystal. 

If it can be shown that the photooxidation of hydrocarbons in zeolites is a general method, then the shape and size-selective properties of zeolites may potentially be used to control the selectivity of specific oxidation reactions (2,3). For example, ZSM-5 is an important shape-selective catalyst in many reactions, such as the disproportionation of toluene (4). Para-xylene is the dominant product because the transport of the other isomers, ortho- and meta-xylene, is restricted due to the pore size of ZSM-5. Thus, stereochemical aspects of selective photooxidation reactions may also be influenced by the zeolite and may be used to design environmentally benign processes for the synthesis of industrially useful molecules. 

Chen NY, Degnan Jr TF, Smith CM. Molecular transport and reaction in zeolites, design and application of shape selective catalysts. New York VCH Pubhshers 1994. 

As was shown here in some examples, the field of catalysis over zeolites, although marnre, is still very much alive. The chemists who work with the synthesis zeolites continue to be very creative, the focus now being placed on the synthesis of materials that can catalyze reactions other than the acidic ones and/or reactions of bulkier molecules, that is, synthesis of zeolites with larger micropores or with a very large external surface, such as nanosize and delaminated zeolites. New concepts related to the mode of action of zeolite catalysts continue to emerge, as shown here with the shape selectivity of the external surface. These concepts are particularly useful to scientifically design selective and stable catalysts. 

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 strategy used to design active and selective catalysts was based on the following five factors for regulation, (i) conformation of ligands coordinated to Rh atom (ii) orientation of a vacant site on Rh (iii) cavity with the template molecular shape for reaction space produced behind template removal (iv) architecture of the cavity wall and (v) micropore in inorganic polymer-matrix overlayers stabilizing the active species at the surface [46, 47, 71]. 

J. Haggin, Shape selectivity key to designed catalysts. Chem. Eng. News, December 13, 9-15 (1982). 

It is appropriate and very instructive to briefly discuss a relatively new and very successful approach, namely, the development of catalysts with designed and atomically engineered active centers. Thomas and coworkers used micro- and meso-porous solids and carried out delicate structural and compositional variations to prepare specific catalysts capable of promoting regioselective, shape-selective, and enantioselective conversions.183-185 This strategy resulted in the development of framework-substituted CoALPO-18 and MnALPO-18 molecular sieves for the selective aerobic oxidation of linear alkanes to the corresponding monocarboxylic acids,186 and that of hexane to adipic acid.187 Framework-substituted MALPO-36... 

Another approach to designing shape-selective heterogeneous oxidation catalysts was to use redox metal oxides as the pillaring agents in the preparation of pillared clays. These redox pillared clays have been used for a number of selective oxidations. Chromium pillared montmorillonite (Cr-PILC) is an effective catalyst for the selective oxidation of alcohols with tert-butyl hydroperoxide. 7 Primary aliphatic and aromatic alcohols are oxidized to the aldehydes in very good yields. Secondary alcohols are selectively oxidized in the presence of a primary hydroxy group of a diol to give keto alcohols in excellent yields (Eqn. 21.12). 2... 

The examples have shown that it is possible to explain promotion by poisoning of metal catalysts by ensemble control. It nay give ideas for the design of more selective catalysts exposing almost two-dimensional shape selectivity. 

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