A solar energy-driven sustainable process for synthesis of ethylene glycol from methanol

Direct photocatalytic coupling of methanol to ethylene glycol (EG) is highly attractive. The first metal oxide photocatalyst, a tantalum-based semiconductor, is reported for preferential activation of C-H bonds within methanol to form hydroxymethyl radical (* CH₂OH) and subsequent C-C coupling to EG. The nitrogen doped tantalum oxide (N-Ta₂O₅) photocatalyst is an environmentally friendly and highly stable candidate for photocatalytic coupling of methanol to EG.

The photochemistry of the future will support human industry without smoke, and usher in a brighter civilization based on the utilization of solar energy instead of fossil energy. Photochemistry has been used in controlling many reaction processes, especially for the challenging reactions containing selective C-H activation and C-C coupling in chemical synthesis. It is of great interests that a 'dream catalytic reaction' of direct coupling of methanol to ethylene glycol (2CH₃OH—HOCH₂CH₂OH + H₂, denoted as MTEG) could be achieved through the solar energy-driven C-H activation and C-C coupling processes, and this MTEG reaction has not been achieved through thermocatalysis yet.

Ethylene glycol (EG) is an important monomer for the manufacture of polymers (e.g., poly(ethylene terephthalate), PET), and can also be used as antifreeze and fuel additive. The annual production of EG is more than 25 million tons, which is primarily produced from petroleum-derived ethylene industrially. Methanol is a clean platform chemical, which can not only traditionally produced from natural gas and coal, but also has been directly synthesized from biomass and CO₂. Thus, the solar energy-driven MTEG route provides an alternative process for sustainable synthesis of EG and H₂ from methanol directly with great attractions.

Although direct photocatalytic coupling of methanol to EG is highly attractive, the reported photocatalysts for this reaction are all metal sulfide semiconductors, which may suffer from photocorrosion and have low stability. Thus, the development of non-sul?de photocatalysts for efficient photocatalytic coupling of methanol to EG and H2 with high stability is urgent but extremely challenging.

Recently, a research team led by Prof. Ye Wang from Xiamen University and Yanshan University, China reported the first metal oxide photocatalyst, tantalum-based semiconductor, for preferential activation of C-H bond within methanol to form hydroxymethyl radical (* CH₂OH) and subsequent C-C coupling to EG. Compared with other metal oxide photocatalysts, such as TNO₂, ZnO, NO₃, Nb₂O₅, tantalum oxide (Ta₂O₅) is unique in that it can realize the selective photocatalytic coupling of methanol to EG. The co-catalyst free nitrogen doped tantalum oxide (2%N-Ta₂O₅) shows an EG formation rate as high as 4.0 mmol/g/h, about 9 times higher than that of Ta₂O₅, with a selectivity higher than 70%. The high charge separation ability of nitrogen doped tantalum oxide plays a key role in its high activity for EG production. This catalyst also shows excellent stability longer than 160 h, which has not been achieved over the reported metal sulfide photocatalysts. Tantalum-based photocatalyst is an environmentally friendly and highly stable candidate for photocatalytic coupling of methanol to EG. The results were published in Chinese Journal of Catalysis.