Electrothermal Conversion of Methane to Methanol at Room Temperature with Phosphotungstic Acid

Traditional methods for the aerobic oxidation of methane to methanol frequently require the use of noble metal catalysts or flammable H₂-O₂ mixtures. While electrochemical methods enhance safety and may avoid the use of noble metals, these processes suffer from low yields due to limited current density and/or low selectivity. Here, we design an electrothermal process to conduct aerobic oxidation of methane to methanol at room temperature using phosphotungstic acid (PTA) as a redox mediator. When electrochemically reduced, PTA activates methane with O₂ to produce methanol selectively. The optimum productivity reaches 29.45 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>μ</mml:mi> <mml:mi>m</mml:mi> <mml:mi>o</mml:mi> <mml:mi>l</mml:mi> <mml:mspace></mml:mspace> <mml:msubsup><mml:mi>g</mml:mi> <mml:mrow><mml:mi>P</mml:mi> <mml:mi>T</mml:mi> <mml:mi>A</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msubsup> <mml:msup><mml:mrow><mml:mi>h</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> </mml:mrow> <mml:annotation>${\mu mol\ {g}_{PTA}^{-1}{h}^{-1}}$</mml:annotation> </mml:semantics> </mml:math> with approximately 20.3 % overall electron yield. Under continuous operation, we achieved 19.90 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>μ</mml:mi> <mml:mi>m</mml:mi> <mml:mi>o</mml:mi> <mml:mi>l</mml:mi> <mml:mspace></mml:mspace> <mml:msubsup><mml:mi>g</mml:mi> <mml:mrow><mml:mi>P</mml:mi> <mml:mi>T</mml:mi> <mml:mi>A</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msubsup> <mml:msup><mml:mrow><mml:mi>h</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> </mml:mrow> <mml:annotation>${\mu mol\ {g}_{PTA}^{-1}{h}^{-1}}$</mml:annotation> </mml:semantics> </mml:math> catalytic activity, over 74.3 % methanol selectivity, and 10 hours durability. This approach leverages reduced PTA to initiate thermal catalysis in solution phase, addressing slow methane oxidation kinetics and preventing overoxidations on electrode surfaces. The current density towards methanol production increased over 40 times compared with direct electrochemical processes. The in situ generated hydroxyl radical, from the reaction of reduced PTA and oxygen, plays an important role in the methane conversion. This study demonstrates reduced polyoxotungstate as a viable platform to integrate thermo- and electrochemical methane oxidation at ambient conditions.