When Electrochemistry Met Methane: Rapid Catalyst Oxidation Fuels Hydrocarbon Functionalization

Modern energy challenges have motivated the development of electrochemical transformations of abundant feedstocks into valuable fuels and commodity chemicals. Now, reporting in ACS Central Science, Surendranath and co-workers introduce an electrochemical strategy that enables an organic transformation long-considered a grand challenge: selective methane activation. Methane, the primary component of natural gas, is globally abundant and inexpensive, making it an increasingly important part of the global energy portfolio. However, the dearth of efficient methods for converting this gaseous carbon feedstock into liquid fuels and commodity chemicals has limited our ability to take full advantage of this hydrocarbon, fueling decades of research into ways to overcome methane’s chemical inertness. This endeavor is made even more challenging by methane’s propensity for overoxidation and low selectivity. While the majority of industrial processes in the petrochemical industry utilize reactor beds with metal oxide or supported metal catalysts, the heterogeneous approach commercialized for the conversion of methane to methanol entails an indirect two-step route that operates at high temperatures, rendering it cost-prohibitive for more widespread use. Molecular catalysts are an attractive alternative to heterogeneous systems for the selective C−H functionalization of methane because they promise to operate at low temperatures and with high selectivity. To get past the chemical inertness of methane, these catalysts must be electrophilic, and most exploit high-valent transition metals, such as Pt(IV). C−H bond activation and functionalization by these electrophilic intermediates reduce the catalyst to a low-valent species that must be reoxidized in a two-electron process to regenerate the reactive, high-valent complex and turn over the catalyst. Building on the seminal work of Shilov, who demonstrated in the early 1970s that K2PtCl4 catalyzes the oxidation of methane to methanol and chloromethane with stoichiometric K2PtCl6, molecular catalysts have been developed for methane oxidation using stoichiometric oxidants. These include the system reported by Periana and co-workers in which (2,2′-bipyrimidyl)platinum(II) dichloride catalyzes the selective oxidation of methane to methyl bisulfate in concentrated sulfuric acidwith sulfuric acid acting as both solvent and oxidizing reagent. While this system boasts high conversion and selectivity, it still crawls along at rates of around 10 h−1. Recent work by Schüth and co-workers demonstrated that both (2,2′-bipyrimidyl)platinum(II) dichloride and K2PtCl4 can carry out similar chemistry in 20% oleum (a H2SO4/SO3 mixture) with turnover frequencies that are orders of magnitude higher than the original Periana system. The ideal terminal oxidant for a large scale commercial processes is O2, but the sluggish kinetics associated with direct oxidation by O2 renders it too impractical to be used directly. Many reported systems thus attempt to exploit intermediate chemical oxidants that can be directly regenerated by O2, such