Catalytic Oxidation of Methane to Oxygenated Products: Recent Advancements and Prospects for Electrocatalytic and Photocatalytic Conversion at Low Temperatures

Abstract

Methane is an important fossil fuel and widely available on the earth's crust. It is a greenhouse gas that has more severe warming effect than CO 2. Unfortunately, the emission of methane into the atmosphere has long been ignored and considered as a trivial matter. Therefore, emphatic effort must be put into decreasing the concentration of methane in the atmosphere of the earth. At the same time, the conversion of less valuable methane into value-added chemicals is of significant importance in the chemical and pharmaceutical industries. Although, the transformation of methane to valuable chemicals and fuels is considered the "holy grail," the low intrinsic reactivity of its C-H bonds is still a major challenge. This review discusses the advancements in the electrocatalytic and photocatalytic oxidation of methane at low temperatures with products containing oxygen atom(s). Additionally, the future research direction is noted that may be adopted for methane oxidation via electrocatalysis and photocatalysis at low temperatures.

Methane Conversion Under Mild Conditions Using Semiconductors and Metal-Semiconductors as Heterogeneous Photocatalysts: State of the Art and Challenges

“…This involves some strategies and methods to control the composition, sizes and morphologies of the metals and the semiconductors, the doping of the semiconductors and the interactions between them, for instance, depositing metals on specific facets of the semiconductor, joining semiconductors to facilitate the separation of photogenerated charge carriers, etc. The metal organic framework (MOF) has the property of adsorbing large amounts of methane and, therefore, can be an interesting for developing hybrid photocatalysts for methane conversion ( Shah et al, 2020 ). However, although the knowledge in this area has advanced, the possibilities of combinations are endless and there is a plenty of room to be investigated and improved.…”

Photoelectrochemical Conversion of Methane into Value-Added Products

“…The CH 4 molecule is very stable and symmetrical, with low polarizability (2.84 × 10 −40 C 2 •m 2 •J −1 ) owing to the low electronegativity difference between hydrogen and carbon atoms [4]. Thus, a relatively strong local electric field is required to cause the polarization of CH 4 to initiate electrophilic or nucleophilic chemical reactions.…”

“…Methyl cations (CH 3 + ) are unstable among carbocations, making methane the least reactive alkane in terms of hydride ion abstraction. It also shows low reactivity towards acids and bases due to its weak acidity (pK a ≈ 48) and low proton affinity (543.9 kJ•mol −1 ) [4]. Therefore, the chemical transformation of methane generally requires harsh reaction conditions.…”

“…Usually, this energy is provided by increasing the temperature of the system. Practically, the oxidation of CH 4 should always be conducted at high temperatures (>700 • C), which often results in reactions being mediated by a free radical pathway with inherently low selectivity [4]. High-temperature conditions, which are often accompanied by high pressures, cause high costs and safety issues on a commercial scale.…”

Development of a Combined Plasma‐Matrix Reformer for Solid Oxide Fuel Cell Application

“…The technology that converts the greenhouse gas methane into useful energy is a thermal conversion using conventional catalysts. Other alternative technologies include electrochemical, photochemical, and biochemical conversion 1–4. Among them, some thermal and electrochemical conversions have been put into practical use and applied to the field.…”