Structure of Molybdenum Supported on α-, γ-, and χ-Aluminas in Relation to Its Epoxidation Activity

Abstract

The structures of Mo supported on α-, γ-, and χ-aluminas were analyzed by ESCA, XRD, IR, and XAFS techniques, and the relationship between the structure of Mo and its catalytic activity for the epoxidation of allyl alcohol was discussed. The ranges of the Mo-monolayer formation as estimated by the ESCA technique were less than 18, 15, and 1 wt% of Mo-loading for γ-, χ-, and α-aluminas, respectively. However, MoO3was detected by an XRD analysis at 10 wt% of Mo-loading for γ- and χ-aluminas; the ESCA technique overestimated the range of the monolayer. The monolayer on γ- and χ-aluminas as apparently determined by the ESCA analysis consisted of isolated tetrahedral Mo, octahedral polymolybdate, and a small amount of MoO3. The monolayer on α-alumina contained only tetrahedral Mo-species. MoO3was predominantly formed after the completion of the monolayer on all aluminas. The epoxidation activity of Mo was low in the monolayer region; especially, isolated tetrahedral Mo was completely inactive. MoO3was the most active species for the epoxidation, and peroxo-type oxygen bound to Mo was deduced to be the active species.