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 bee n prepared via a direct hydrothermal synthesis. A change of structure of me soporous molecular sieves and coordination of chromium was investigated by X-ray diffraction (XRD), nitrogen adsorption and X-ray absorption measureme nts after various redox gas treatments at high temperature. It was determin ed from the XRD patterns that the Cr containing material has a hexagonal MC M-41 and a cubic MCM-48 structure, respectively, and the structure is not a ffected significantly by redox gas treatments. The pore structure and surfa ce area, analyzed from nitrogen isotherms, of Cr-substituted MCM-41 and MCM -48 are maintained after reduction and re-oxidation, which were performed s equentially after calcination. It is suggested from X-ray absorption near e dge structure that the local environment of Cr in the MCM-41 and MCM-48 str uctures was transformed from tetrahedral to octahedral coordination by redu ction, and vice versa by re-oxidation. (C) 2001 Elsevier Science B.V. All r ights reserved.