The nature of molybdenum oxide species mounted on alumina: An oligomer model

Abstract

A model for the structure of the molybdenum species mounted on alumina is developed on the basis of the following considerations. (1) The alumina surface hydroxyl groups remaining after thermal depopulation are found largely in rows. (2) Molybdate species react with the support hydroxyl groups. (3) Molybdate species undergo oligomerization on the alumina support. The structure developed using these criteria has the molybdenum species attached in rows to the support with μ-oxo linkages between adjacent molybdenum atoms along these rows. This requires each alumina surface oxygen atom in these rows to carry one molybdenum atom. The average interatomic molybdenum to molybdenum separation along these surface-attached polymer chains is therefore equal to the ionic diameter of the alumina oxygen atoms (viz., approx. 2.8 Å). On reduction of the molybdenum atoms in these rows, metal-metal bond formation is proposed to occur. On the (111)-face of the alumina steric and charge considerations suggest that in the formation of the polymer chain, one oxygen atom per two molybdenum(VI) atoms is placed in the support in a vacancy formed during partial dehydration of the surface alumina layer. This oxygen is difficult to remove either by reduction or sulfidation. “Fixed” hydrogen, formed on reduction of the catalyst with hydrogen, is associated with isolated relatively immobile μ-OH groups. Catalytic hydrodesulfurization activity is ascribed to dimer species and the utility of alumina as a catalyst support is thought to reside in its ability to promote both the oligomerization and the dispersion of the molybdenum species over the support.