ABSTRACT
Small, uniform intracrystalline cavities and pores characterize zeolite catalysts. If the overwhelming majority of the catalytic sites are confined within this pore structure and if the pores are small, the fate of reactant molecules and the probability of forming product molecules are determined mostly by molecular dimension and configuration. Only molecules whose dimensions are less than a critical size can enter the pores, have access to internal catalytic sites, and react there. Shape selective catalysis was first reported almost 30 years ago by Weisz and Frilette,1 and subsequently many applications have appeared in the literature. Excellent reviews on this aspect of catalysis have been reported. Shape-selective effects in catalysis by zeolites are usually predominant when configuration diffusion is essential, i.e., a diffusion regime in which there must exist a continuous matching of size, shape, and configuration between the diffusing species and the available free space in the host catalyst, i.e., its pores, cages, or channels.
