Heterogeneous Catalysis in Liquid-Phase Oxidation of Olefin. II. Dependence of the Structure of Vanadium–Chromium Binary Oxide Catalyst for Oxidation of Cyclohexene on the Method of Preparation

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Abstract
The influence of preparative conditions on the structure and catalytic activity of V–Cr oxide catalyst for the liquid-phase oxidation of cyclohexene has been investigated. Three series of binary oxide, V–Cr-D, V–Cr-E, and V–Cr-F, were prepared from acidic, neutral, and alkaline media, respectively. The specific activity of the catalyst was found to be in the order: V–Cr-D>V–Cr-E>V–Cr-F, the catalyst prepared from acidic medium showing the highest activity. The results of structure analysis showed the formation of several chromium isopolyvanadates, i.e., chromium pyrovanadate, chromium metavanadate, and unknown phase (probably chromium polyvanadates), together with V2O5, Cr2O3, and solid solution of Cr6+ in V2O5 lattice. The formation of chromium isopolyvanadate having a higher degree of condensation of the vanadate anion was promoted by use of a more acidic medium in the catalyst preparation. The higher activity for autoxidation was obtained with the catalyst containing the more condensed isopolyvanadate. Epoxidation seems to proceed by the hydroperoxide intermediate mechanism on the active site differing from that for autoxidation.

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