Structure of intrazeolite molybdenum oxide clusters and their catalysis of the oxidation of ethyl alcohol

Abstract

Molybdenum oxide clusters confined in zeolite supercages were synthesized using a precursor Mo(CO)₆ adsorbed in the pores of zeolite and characterized by XAFS, HREM, LRS and XPS. The structure of the intrazeolite Mo oxide clusters and their catalysis of the oxidation of ethyl alcohol to acetaldehyde were investigated. It was found by Mo K-edge XAFS that intrazeolite Mo(VI) oxide dimer species with an octahedral symmetry of Mo are formed by thermal oxidation of Mo(CO)₆ entrapped in an NaY zeolite irrespective of the Mo loading up to two Mo atoms per supercage. HREM and XRD detected no degradation of the crystallinity of the host zeolite. The dimer catalyst showed a specific activity for the oxidation of ethyl alcohol 10 times higher than an impregnation catalyst, in which isolated tetrahedral Mo oxide species are formed. The Mo oxide dimer species were transformed to monomeric species by a heat treatment at 673 K. This led to a considerable decrease in catalytic activity. The Mo oxide clusters prepared by thermal oxidation of Mo(CO)₆ encaged in the cavity of a high silica FAU zeolite (Si/Al = 630) showed a significantly higher specific activity than the Mo oxide clusters in NaY. From the Mo K-edge EXAFS analysis, it was found that Mo oxide clusters containing several Mo atoms are constructed in the pores of the high silica zeolite. It is concluded that the structure and size of Mo oxide clusters encaged in zeolite are controlled by the zeolite composition and heat treatment and that larger Mo oxide clusters exhibit a higher specific activity for the partial oxidation of ethyl alcohol.