Electrochemical Oxidation of Methane in Superacid
Authors: Li Guangfang , Fang Yuxin , Arges Christopher George , Flake John 
Published: February 2020

Methane is abundant in nature including approximately ~6,000,000 Tg located undersea or underground and 5000 Tg (~1%) in the atmosphere. In the last 200 years, atmospheric methane concentrations have doubled from 0.8 ppm to 1.6 ppm. The fifth report of the Intergovernmental Panel on Climate Change (IPCC) claims that its global warming potential is 25~30 times greater than CO2, which contributes more than one-third of today’s anthropogenic greenhouse global warming. Capturing and reusing methane offers a unique opportunity to reduce its anthropogenic and natural emissions and simultaneously bring economic benefit. Unfortunately, conventional Fischer-Tropsch route via synthesis gas is operated at extremely high temperature and pressure (700 ~1000 oC, 15~40 atm) in expensive reactors with heat transfer and coke management challenge. A more efficient pathway is therefore highly desired to directly convert methane to valued products.

A prototypical system of particular interest has been superacid-catalytic model proposed by Olah using “magic acid” (HSO3F-Sb5F) as reaction media, which could directly convert methane into higher hydrocarbons under mild conditions.1-2 In addition, electrocatalysis has recently emerged as an alternative pathway for C-H activation with a lower cost and more energy efficient process.3 In this work, we explore a viable electro-oxidation process for coupling of methane in superacid in a simple Teflon made two-electrode cell, with Pt wire as the working electrode and graphite rod as both counter electrode and reference electrode. The acidity of superacid also plays a critical role during methane conversion process and only strong acid with hammett acid function (H0
) in excess of approximately -12 is capable of methane conversion. Through employing appropriate superacid system as electrolyte, methane could be directly converted into ethane accompanied with certain amount of ethylene at room temperature. Methane oxidation could be modulated through adjusting potential in ternary acid (HF/H2SO4/HSO3F), and the Faradaic efficiency (FE) reaches to 17 % for ethane generation with a high yield of 194 mmol m-2 h-1 at 1.0 V (vs. RHE). Moreover, the cationic Pt is proposed to work as active sites for methane conversion based on anodic potential scan and X-ray photoelectron spectroscopy (XPS) analysis. This study provides a new insight for methane electro-oxidation process and has a great impact on mitigating methane emissions as well.

References

1.Olah, G. A.; Schlosberg, R. H., Chemistry in super acids. I. Hydrogen exchange and polycondensation of methane and alkanes in FSO3H-SbF5 ("magic acid") solution. Protonation of alkanes and the intermediacy of CH5
+ and related hydrocarbon ions. The high chemical reactivity of "paraffins" in ionic solution reactions. J. Am. Chem. Soc.
1968, 90 (10), 2726-2727.

2.Olah, G. A.; Klopman, G.; Schlosberg, R. H., Super acids. III. Protonation of alkanes and intermediacy of alkanonium ions, pentacoordinated carbon cations of CH5
+ type. Hydrogen exchange, protolytic cleavage, hydrogen abstraction; polycondensation of methane, ethane, 2,2-dimethylpropane and 2,2,3,3-tetramethylbutane in FSO3H-SbF5. J. Am. Chem. Soc.
1969, 91(12), 3261-3268.

3.Sauermann, N.; Meyer, T. H.; Qiu, Y.; Ackermann, L., Electrocatalytic C–H Activation. ACS Catalysis
2018, 8 (8), 7086-7103.