Single site Mo oxide catalyst supported on substrates such as silica and alumina are important olefin metathesis catalysts. While there is a general consensus that metathesis process occurs through a metal carbene intermediate, the structure of the single site oxide precursor and the formation of the carbene intermediate are not well elucidated. We report characterization of the single site Mo oxide species using several different structural characterization techniques. Mo on silica and alumina were made through a sol-gel process where Mo precursors molecules (NH4MoO4 and (NH4)6Mo7O24) were co-condensed with silica and aluminum alkoxies. For comparison, catalyst were also made through wet impregnation of silica gel. Structural characterization of the Mo site across the range of different catalyst preparations were carried out by EXAFS. The structure refined from models on silica indicate that the Mo has a dioxo functional group, similar to what it observed in supported Cr(VI) catalysts. Unlike the Cr catalyst the total coordination number is 6 consisting of the two terminal Mo=O bonds, two bridges to Si and two dative bonds arising from the silica substrate. To further support and elucidate the structure, detailed Raman studies were carried out on transparent sol-gel derived Mo/SiO2 monoliths. The Raman bands showed characteristic stretches for the symmetric and antisymmetric Mo(=O)2 group. Further bands were identified and the normal modes calculated. Vibrational assignments were confirmed by 18O isotopic labeling.

Spectroscopy and structrual characterization of single site Mo metathesis catalyst

2019 Spring National Meeting

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