Chromium ions within zeolites. Part 3.—The influence of the valence state of chromium on the catalytic activity of Cr zeolites in ethylene polymerization

Abstract

The rate of ethylene polymerization on CrHY and Cr-stabilized Y zeolites has been measured in a static apparatus by the increase in the weight of zeolite with time. The ethylene polymerization depends particularly on the valence state of the Cr ions localized in the cation position of the zeolite. A further necessary condition for ethylene polymerization is a vacant coordination sphere in the isolated Cr ions and their localization in accessible sites. The zeolite catalytic activity increases with increasing electron-acceptor ability of the Cr ions, i.e. from Cr^II to Cr^III and higher valence states. However, the highest catalytic activity of Cr zeolites is not connected with the presence of Cr^VI but with Cr^V ions.