STRUCTURE OF MOLYBDENUM SUPPORTED ON ALPHA-ALUMINA, GAMMA-ALUMINA, AND CHI-ALUMINA IN RELATION TO ITS EPOXIDATION ACTIVITY

The structures of Mo supported on alpha-, gamma-, and chi-aluminas wer e analyzed by ESCA, XRD, IR, and XAFS techniques, and the relationship between the structure of Mo and its catalytic activity for the epoxid ation of allyl alcohol was discussed. The ranges of the Mo-monolayer f ormation as estimated by the ESCA technique were less than 18, 15, and 1 wt% of Mo-loading for gamma-, chi-, and alpha-aluminas, respectivel y. However, MoO3 was detected by an XRD analysis at 10 wt% of Mo-loadi ng for gamma- and chi-aluminas; the ESCA technique overestimated the r ange of the monolayer. The monolayer on gamma- and chi-aluminas as app arently determined by the ESCA analysis consisted of isolated tetrahed ral Mo, octahedral polymolybdate, and a small amount of MoO3. The mono layer on a-alumina contained only tetrahedral Mo-species. MoO3 was pre dominantly formed after the completion of the monolayer on all alumina s. The epoxidation activity of Mo was low in the monolayer region; esp ecially, isolated tetrahedral Mo was completely inactive. MoO3 was the most active species for the epoxidation, and peroxo-type oxygen bound to Mo was deduced to be the active species. (C) 1998 Academic Press.