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Pores of uniform dimensions characterize zeolite catalysts. If the pores are small, the fate of reactants and the probability of forming products are determined by molecular dimensions and configurations as well as by the types of catalytically active sites present. Reactant shape selectivity occurs when some of the molecules in a reactant mixture are too large to diffuse into the zeolite pores. Product selectivity occurs when, among all the product molecules formed within the pores, only those with the proper dimensions can diffuse out and appear as products. In restricted transition state type selectivity, certain reactions are prevented because the corresponding transition state requires more space than is available. Most commercial applications of shape selectivity involve either (1) cracking of undesirable molecules to smaller, easily removable fragments, or (2) avoiding undesirable competing reactions such as coking and transalkylation. Applications discussed are distillate and lube oil dewaxing, the production of pxylene, ethylbenzene, and pax-ethyltoluene, and the methanolto-gasoline, methanol-to-olefins, and olefins-to-gasoline-anddistillates processes. Most of these processes use the pentasil type ZSM-5 zeolite.

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