ON THE NATURE OF THE ACTIVE-SITES FOR ETHYLENE HYDROGENATION IN METAL-FREE ZEOLITES

The hydrogenation of ethylene has been studied over acid faujasites, mordenite, and ZSM-5 zeolites at reaction temperatures varying between 623 and 723 K and a hydrogen pressure of 1 MPa. The active sites responsible for the hydrogenation of ethylene change from sodium cations to Brønsted acid sites when the reaction temperature is increased from 623 K to 723 K. The change of the active sites is reflected in the temperature dependence of the ethane yield. It is suggested that the hydrogenation of ethylene on Brønsted acid sites requires only a single site and occurs via a pentacoordinated carbonium ion mechanism. Different behavior of acid sites in various zeolites is attributed to topological differences. © 1992.