A Mercury-Catalyzed, High-Yield System for the Oxidation of Methane to Methanol

A homogeneous system for the selective, catalytic oxidation of methane to methanol via methyl bisulfate is reported. The net reaction catalyzed by mercuric ions, Hg(II), is the oxidation of methane by concentrated sulfuric acid to produce methyl bisulfate, water, and sulfur dioxide. The reaction is efficient. At a methane conversion of 50 percent, 85 percent selectivity to methyl bisulfate (∼43 percent yield; the major side product is carbon dioxide) was achieved at a molar productivity of 10 –7 mole per cubic centimeter per second and Hg(II) turnover frequency of 10 –3 per second. Separate hydrolysis of methyl bisulfate and reoxidation of the sulfur dioxide with air provides a potentially practical scheme for the oxidation of methane to methanol with molecular oxygen. The primary steps of the Hg(II)-catalyzed reaction were individually examined and the essential elements of the mechanism were identified. The Hg(II) ion reacts with methane by an electrophilic displacement mechanism to produce an observable species, CH 3 HgOSO 3 H, 1. Under the reaction conditions, 1 readily decomposes to CH 3 OSO 3 H and the reduced mercurous species, Hg 2 2+ . The catalytic cycle is completed by the reoxidation of Hg 2 2+ with H 2 SO 4 to regenerate Hg(II) and byproducts SO 2 and H 2 O. Thallium(III), palladium(II), and the cations of platinum and gold also oxidize methane to methyl bisulfate in sulfuric acid.