Reversible coordination change of chromium in Cr-MCM-41 and Cr-MCM-48 studied by X-ray absorption near edge structure

Chromium-substituted MCM-41 and MCM-48 mesoporous molecular sieves have been prepared via a direct hydrothermal synthesis. A change of structure of mesoporous molecular sieves and coordination of chromium was investigated by X-ray diffraction (XRD), nitrogen adsorption and X-ray absorption measurements after various redox gas treatments at high temperature. It was determined from the XRD patterns that the Cr containing material has a hexagonal MCM-41 and a cubic MCM-48 structure, respectively, and the structure is not affected significantly by redox gas treatments. The pore structure and surface area, analyzed from nitrogen isotherms, of Cr-substituted MCM-41 and MCM-48 are maintained after reduction and re-oxidation, which were performed sequentially after calcination. It is suggested from X-ray absorption near edge structure that the local environment of Cr in the MCM-41 and MCM-48 structures was transformed from tetrahedral to octahedral coordination by reduction, and vice versa by re-oxidation. (C) 2001 Elsevier Science B.V. All rights reserved.