Chromium ions within zeolites. Part 2.—A quantum-chemical study of the properties of chromium ions in faujasites

Abstract

The CNDO (complete neglect of differential overlap) method is used for calculation of the physico-chemical characteristics of faujasites with Cr^II, Cr^III, Cr^V and Cr^VI ions in the S_II cation position, modelled by Si₃Al₃O₆(OH)₁₂Cr clusters. It is demonstrated that the Cr ions are bonded to the zeolite skeleton by electron donor–acceptor bonds whose formation leads to electron donation from the skeleton to the cation, thus stabilizing the valence states of the Cr ions. The most probable valence state of the Cr ions in the zeolites was found to be Cr^III; however, it can readily be oxidized to Cr^V. In contrast, Cr^VI and Cr^V ions were found to exhibit high electron-acceptor ability and thus are readily reduced.