A team led by Prof. XU Jun and DENG Feng from the Innovation Academy for Precision Measurement Science and Technology of the Chinese Academy of Sciences (CAS), in cooperation with Prof. Graham Hutchings' group from Cardiff University, has recently realized partial oxidation of methane using molecular oxygen on zeolite catalyst.

They designed a gold (Au) modified ZSM-5 (Au/ZSM-5) catalyst and achieved the catalytic oxidation of methane to methanol and acetic acid with O₂ under mild conditions in aqueous solution. The reaction mechanism was revealed. Results have been published in Nature Catalysis.

Methane is the most abundant and cleanest natural carbon resource. Since most of methane is stored in remote areas, transforming methane to transportable oxygenates (methanol, formic acid, acetic acid, etc.) on mining site could improve the efficiency of its utilization. However, its strong C-H bond needs harsh condition (high temperature and pressure) to split hydrogen atom from methane, which is the first step for converting methane to new chemicals. 

It just gets trickier by the fact that the desired oxygenates products are often more easily over-oxidized into byproducts (e.g., CO₂) than methane. Therefore, the selective methane oxidation is one of the "holy grail" reactions in catalysis. Seeking a route of direct methane oxidation at mild condition with the potential of industrialization has attracted great attention from both academia and industry.

To address this issue, the researchers prepared Au supported ZSM-5 zeolite (Au/ZSM-5) catalyst. Interestingly, this catalyst can achieve methane oxidation using O₂ at 120-240 °C in aqueous solution, showing high selectivity to methanol and acetic acid.

Quantitative Nuclear Magnetic Resonance (NMR) analysis showed that a maximum oxygenate productivity of 7.3 mol/molAu/h can be obtained at short reaction times. Compared with the Cu-zeolite catalysts on which only C1 products can be produced, the formation of C2 oxygenates as major products on the Au-ZSM-5 catalyst demonstrates a different reaction route.

Two dimensional ¹H-¹³C correlation NMR method combined with ¹³C and ¹²C isotopes tracing technique were employed to study the detailed reaction mechanism. Au nanoparticles were found to promote the activation of molecular oxygen to surface active oxygen species, which readily reacted with methane. The reaction involves different surface bound intermediates (such as methyl-, methylperoxy- and acetyl-) rather than the species in fluid phase.

Importantly, only O₂ was employed to produce C2 oxygenates. This is different from the reported noble metals modified zeolite catalysts, such as Rh and Ir, on which necessities CO co-reactant.

This work presents a proof-of-concept study on catalytic methane conversions on heterogeneous catalyst using oxygen oxidant and it was supported by the National Natural Science Foundation of China and the Hubei Provincial Department of Science and Technology, etc.

Schematic diagram of the catalytic oxidation of methane to methanol and acetic acid on Au/ZSM-5. (Image by QI, G., Davies, T.E., Nasrallah, A. et al.)