Oxidation of Methanol to Formaldehyde on Silica-Supported Molybdena: Density Functional Theory Study on Models of Mononuclear Sites

Isolated molybdenum centers bearing either one (oxomolybdenum system) or two (dioxomolybdenum system) terminal oxo ligands are considered, which are modeled by appropriate mononuclear oxomolybdenum methoxides and oxomolybdasilsesquioxanes. Although the oxidation process in both systems is characterized by the same fundamental steps, that is, dissociative addition of methanol followed by rate-determining hydrogen abstraction from the methoxy group, the mechanism of the oxidation reaction differs in each case. In the oxomolybdenum system, the first step leads to cleavage of a bond in a Mo−O−Si sequence and the formation of a surface molybdenum methoxide species. Hydrogen is then abstracted from the methoxide ligand by a terminal oxo ligand in a process entailing a closed-shell transition structure. In contrast, the preferred mechanism in the dioxomolybdenum system involves a hydroxomolybdenum methoxide intermediate formed without cleavage of a bond in a Mo−O−Si sequence. Furthermore, the hydrogen abstractio...