Factors affecting shape selectivity of zeolites in sorption

Abstract

Factors affecting the shape selectivity of high silica zeolites ( viz. ZSM-5, ZSM-8, ZSM-11 and mordenite) in sorption have been investigated using GC pulse techniques at temperatures close to those used in catalytic processes. The shape selectivity of zeolites is found to be strongly influenced by its channel structure (when channel diameter is nearly the same), nature of the cation and its degree of exchange and also by the presence of poison molecules in the zeolite channels. It is increased when H + cations are replaced by Na + cations and also when the zeolite is poisoned by pyridine, because of an increase in the diffusional resistance and/or hindrance to diffusion in the zeolite channels. The shape selectivity of ZSM-5, ZSM-8 and ZSM-11 zeolites in the sorption of p- and m- xylenes is found to be in the following order: ZSM-8> ZSM-5> ZSM-11.