Oxidative dehydrogenation of methanol to formaldehyde

Thermochemical results from ab initio quantum chemical calculations (generalized valence bond) are used to examine the reactions mechanism for H₃COH + 1 2O₂→H₂CO + H₂O as catalyzed by MoO₃. We find that surface dioxo sites {A figure is presented} are critical to activating the methanol, but we find that the single-site single-step proposal {A figure is presented} is not favorable (ΔH ≈ + 31.5 kcal). Our conclusion is that an important catalytic site involves two adjacent surface dioxo units (the dual dioxo site), with each dioxo site extracting one H in a sequence of steps. The required dual dioxo site exists on the (010) surface of MoO₃ but does not exist on the other low index surfaces. This mechanism is supported by atmospheric pressure experimental studies which indicate that MoO₃(010) is selective for CH₂O products. A detailed sequence of reaction steps and the associated thermochemistry is proposed.