Selective and quantitative methanol production from methane under mild conditions is the first step towards the so-called future “methanol economy” alternative. The large amounts of methane extracted from fossil fuels (natural gas, fracking) can be further exploited via a simple on-site conversion technique of methane to methanol. The industrial approach involves harsh conditions mediated by syn gas production. The laboratory probed techniques, including metal oxide surfaces, zeolites, molecular catalysts, suffer from low yields or low selectivity. The fundamental reason is that the C-H bonds of methanol are weaker and are oxidized easier than methane. Therefore, the cessation of the methane oxidation at the first stage is challenging and seemingly impossible. Suggested remedies in the literature include the rapid removal of methanol from the metal center or the discovery of new catalysts which exhibit higher activation barriers for methanol activation. The combination of our theoretical studies and the photoelectron spectroscopic studies of Prof. Kit Bowen (Johns Hoppkins University) reveal that anionic metal centers can provide this fast removal due to the 3-4 times weaker metal-methanol interaction compared to cationic metal centers. In this work we present complete catalytic cycles for M− + N2O +CH4 → M− + N2 + CH3OH and compare the catalytic efficiency for M = Ni, Pd, Pt, and Fe. Insights and lessons for future studies are discussed and the superior performance of metal anions is demonstrated. In addition, we show that dicationic metal methyl complexes (MCH32+) can be used as catalysts and that surprisingly the activation/oxidation of methanol has similar or lower reaction rate than methane.

Selective methane to methanol catalytic partial oxidation achieved by anionic oxides and dicationic methyl molecular complexes of transition metals

ACS Spring 2022

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