Copper-Substituted Polyoxotungstates as Catalysts for the Electrocatalytic Oxygenation of Light Alkanes

The low-temperature oxidation of alkanes and arenes using molecular oxygen under ambient conditions is still one of the grand challenges of catalysis. Inspired by the alkane hydroxylation activity of the copper-based metalloenzyme, particulate methane monooxygenase, a tetra-copper polyoxometalate, [Cu₄(H₂O)₂(PW₉O₃₄)₂]^10-, was investigated as an electrocatalyst for the cathodic (reductive) oxidation of hydrocarbons with emphasis on oxidation of ethane. Controlled potential electrolysis (CPE) in water at -0.45 V versus NHE showed the formation of ethanol, acetaldehyde, and then acetic acid. Divided cell CPE confirmed that oxidation of ethane to ethanol occurs only at the cathode, while further oxidation can occur at the cathode and anode. Experiments using a chemical reductant ¹⁸O₂ and halides confirmed the formation of an active species at the cathode. Different combinations of redox-active copper and redox-inactive zinc within the polyoxometalate framework demonstrate that the O₂ activation probably requires three copper atoms. The use of kinetic isotope effects and probe molecules indicates a rebound mechanism for aliphatic substrates and an electrophilic species for oxidation of arenes. The transformation of ethane to either ethanol or acetic acid is an important transformation, especially in scenarios where ethane is not recovered from natural gas.