Abstract:

Methane is a promising energy source with vast reserves, and is considered one of the promising alternatives to nonrenewable petroleum resources because it can be converted into valuable hydrocarbon feedstocks and hydrogen through appropriate reactions. Recently, the conversion of CH₄ into other high-value-added products has received increasing attention because of their sustainability for energy and the environment. However, methane has a tetrahedral geometry with four equivalent C―H bonds due to the sp³ hybridization of the central carbon atom, with a C―H bond length of 0.1087 nm and an H-C―H bond angle of 109.5°. The absence of a dipole moment and the small polarizability (2.84 × 10⁻⁴⁰ C²·m²·J⁻¹) imply that methane requires a high local electric field for polarization and for nucleophilic or electrophilic attack. Nevertheless, it is believed that an effective method to activate CH₄ would be available, so that not only methanol, formaldehyde, and ethylene but also other industrially valuable raw materials can be obtained. On the other hand, the conversion of this combustible gas into the corresponding liquid fossil fuel proceeds via secondary chemical conversion, and it can greatly reduce transportation costs. From the economic viewpoint, this can still provide considerable benefits. Homogeneous catalysts have been reported to catalyze methane, but most of them operate at high pressures (2–7 MPa), or in strongly acidic media and at high temperatures (up to 500 K). Heterogeneous catalysts reported in the literature are also active only at high temperatures. Therefore, finding an efficient method to active methane has become a hot research topic. Photocatalysis technology is recognized as the optimal solution for the conversion of CH₄ since solar energy is by far the largest exploitable resource of energy. In the past years, much effort has been undertaken for the conversion of CH₄ under light at low temperature. In this regard, several photocatalysts, including silica-alumina-titania, silica-supported oxides, and ceria- and zeolite-based materials, have been developed. In photocatalytic methane conversion, the C―H bond can be selectively activated by adjusting the wavelength and intensity of the incident light and the oxidation capacity of the photocatalysts, thereby avoiding the formation of byproducts. This review summarizes a series of photocatalytic direct methane conversion systems developed in recent years, including methane oxidation and coupling processes. The effects of the catalyst composition and structure, oxidant, and electron transfer on the activation of the C―H bond of methane are detailed. Finally, future perspectives and challenges for the photocatalytic conversion of methane are discussed.

Key words: Photocatalysis, Methane conversion, Oxidation to methanol, Anaerobic coupling, Oxidative coupling