Unraveling the Mechanisms of the Selective Oxidation of Methanol to Formaldehyde in Vanadia Supported on Titania Catalyst

A computational study based on B3LYP calculations was carried out to investigate the kinetic and mechanistic aspects of the selective oxidation of methanol to formaldehyde using titania-supported vanadate as a catalyst model. A complete picture of the possible mechanisms to obtain formaldehyde is given. Statistical mechanics as well as transition state theory (TST) were utilized to determine the rate coefficients and equilibrium constants of the most plausible mechanism. A tetrahedral vanadia containing a methoxy species is found to be the most stable intermediate. The rate-limiting step in the most commonly accepted mechanism is the hydrogen transfer from the tetrahedral methoxy intermediate to the catalyst sites V-O-Ti (46.4 kcal/mol) or V=O (41.0 kcal/mol) via a spin-crossing process. The transition states associated to these steps are biradicaloid. The simultaneous formation of H-2 and formaldehyde can be discarded because it proceeds with a higher energetic barrier of 57.0 kcal/mol. The plausibility of a more reactive site involving fivefold coordinated vanadium species along a H-transfer process with a energetic barrier of 20.1 kcal/mol is discussed. Finally, the dependence of the calculated values of energy barriers for the rate-limiting step on the functional used is analyzed.