Electrochemical oxidation of biomass derived 5-hydroxymethylfurfural (HMF): pathway, mechanism, catalysts and coupling reactions

Electrochemical conversion is emerging as a powerful and promising method to produce a wide range of high-value chemicals on account of mild operation conditions, controllable selectivity, and scalability. 5-Hydroxymethylfurfural (HMF), with simple molecular structures including furan rings, -C = O, and -OH groups, is considered one of the most versatile platform molecules. The electrooxidation of bio-based HMF to furandicarboxylic acid (FDCA), a crucial bio-based precursor of polyethylene furanoate (PEF), which would probably replace petroleum-based polyethylene terephthalate (PET), has recently attracted increasing attention. Here, we review the HMF electrochemical oxidation, from reaction pathway/mechanism to catalysts and coupling reactions. First, a pH-dependent reaction pathway is proposed, and the reaction mechanism (direct oxidation and indirect oxidation) is summarized systematically, which is also suitable for electrochemical oxidation of other small organic molecules containing aldehyde/alcohol groups (e.g., methanol, ethanol, glycerol, and glucose) to some extent. Then, the progress, advantages and disadvantages of HMF electrooxidation catalysts of noble metals, non-noble metals, and non-metals are reviewed, particularly on non-noble metal catalysts. Furthermore, for more efficient energy utilization, HMF electrooxidation coupled with H-2 evolution, CO2 reduction, N-2 reduction, and organic reduction are discussed. Finally, a few unique insights into reaction mechanism, an assessment about catalyst performance and an outlook for further development of this topic are provided. This review can offer a guideline for the in-depth understanding of electrochemical oxidation of small organic molecules as well as the design of advanced anodic electrocatalysts towards the utilization and production of renewable resources.