High Yield, Low Temperature Oxidation of Methane to Methanol

Abstract

Developing methods for the direct oxidation of alkanes to fuel and chemicals will lead to a paradigm shift in the manufacture of chemicals and fuels in the 21^st century. We wish to report the development of novel catalysts that allow the direct, low temperature, oxidative conversion of methane to a methanol equivalent product in 70% one-pass yield. To our knowledge, this is the highest one-pass yield ever reported for methane oxidation to a methyl product. The keys to achieving this high yield are: A) the development of novel catalysts that are stable and active for the oxidation of the CH bonds of methane at temperatures as low as 100°C and B) the chemical “protection” of the methanol product from over-oxidation by esterification. The catalysts utilized are novel ligated Pt complexes based on the bidiazine ligand family. A particularly effective oxidation system is based on 20mM solutions of (bipyrimidine)PtC1₂ in concentrated sulfuric acid. Reaction of methane at 500 psig at 250°C with this solution results in 90% conversion of methane to methyl bisulfate in 80% selectivity (70% one-pass yield) based on added methane.

Mechanistic studies show that Pt(II) is the most active oxidation state of the platinum species for reaction with methane. The reaction proceeds via CH activation of methane to generate a platinum-methyl species that is oxidized to generate the product, methyl bisulfate. Isotopic labeling studies suggest that a platinum-methane complex may be involved as an intermediate before CH activation. The bipyrimidine ligand plays a key role in minimizing platinum black formation. The generation of platinum black has been shown to lead to unselective combustion of methane and catalyst death. Control reactions show that the high selectivity and yield is possible because the methyl bisulfate is at least 10 – 100 times less reactive than methane. This suggests that the reaction of the ligated Pt(II) with a CH bond Proceeds via an electrophilic process and is retarded by the presence of electron withdrawing groups such as bisulfate.