Mechanistic Origin of Selective Methane to Methanol Oxidation on Vanadium-Doped Mesoporous Amorphous Silica Photocatalyst

Abstract

Selective direct oxidation of methane to methanol is an important reaction for efficient utilization of natural gas. Photocatalytic routes for methane oxidation processes have advantages over traditional thermal catalysis as they can be more easily deployed at small-scale and remote off-grid sites, potentially reducing the rate of methane release and flaring. Hu and co-workers identified single-site vanadium-doped mesoporous amorphous silica photocatalysis which demonstrated methane conversion with high methanol selectivity (J. Photochem. Photobiol., A20132644855). Photocatalysts using amorphous SiO2 frameworks are significantly less studied than other heterogeneous photocatalysts. As a result, computational studies are important for elucidating mechanisms in order to gain fundamental understanding as to how amorphous SiO2-based photocatalysts can be used for partial oxidation of methane. This work uses density functional theory to establish the reaction barriers associated with the photocatalytic methane to methanol reaction combined with microkinetic modeling. The work is able to elucidate the role of terminal versus bridging oxygens at the photocatalytic center in enhancing photocatalytic efficiency. In addition, avoiding non-Mars–van Krevelen reaction pathways, in which the oxygen vacancy is filled before methanol is formed, is found to be important, owing to overstabilization of reaction intermediates leading to subsequent high energy reaction barriers. Finally, the amorphous SiO2 surface is found to modify reaction barriers by stabilizing intermediates through dispersion and preventing the photocatalytic center from forming the optimum coordination geometry.

Reactivity of Group 5 and 6 Single-Site Photocatalysts for Partial Oxidation of Methane: Comparison of Chromium, Niobium, and Tungsten-Doped Mesoporous Amorphous Silica

“…Additionally, Int2 lies lower in energy than Int1, while Int3 lies higher in energy than Int1. The observed behavior for this step in Nb is significantly different from that in V where Int3 is favored, which causes the reaction to be channeled into a low energy intermediate that reduces the rate of methanol formation . If Int3 is formed, the energy barrier to cross Int3 → Int2 is 8.06 kcal mol –1 with Int2 lying 21.51 kcal mol –1 lower in energy than Int3.…”

“…However, in addition to the higher reaction barrier, Int5 is higher in energy than Int2, although Int3 → Int5 is exothermic, suggesting that the path to Int2 will be favored instead of forming Int5. If Int5 is formed, the subsequent reaction to form methanol is 3.94 kcal mol –1 , and so, Int5 is not a low-energy intermediate that inhibits the formation of methanol as in the V catalyst …”

“…Therefore, further studies are necessary to understand the POM mechanism to develop more effective and economically viable photocatalytic technologies. Previous work has focused exclusively on vanadium-doped mesoporous materials. ,, However, a remaining question is whether vanadium is the most efficient dopant or whether other transition metals may have superior performance.…”