Photocatalytic upgrading of 5-hydroxymethylfurfural - aerobic or anaerobic?

Cellulose-derived 5-hydroxymethylfurfural (HMF) is a key platform in biomass valorisation, with a market value exceeding USD 60 million. Its redox reformed products are highly desirable, serving as the precursors of various sustainable polymers, fuels and fine chemicals. By decoupling the photocatalytic redox species, HMF can be upgraded under different routes, via hydroxymethyl and/or aldehyde groups, to diverse value-added products. Here, we focus on the latest heterogeneous photocatalysis for converting HMF to 2,5-diformylfuran (DFF), 2,5-furandicarboxylic acid (FDCA) and 2,5-bis(hydroxymethyl)furan (BHMF). Particular attention is paid to the effect of aerobic/anaerobic conditions on active species, redox pathways and derived products. Under aerobic conditions, reactive oxygen species (O-center dot(2)-, O-1(2) and (OOH)-O-center dot) generated from photogenerated holes/electrons and molecular oxygen act as the active species, driving the oxidation of HMF to DFF and FDCA via oxidative dehydrogenation or insertion reactions, along with H2O2 generation as the side product. In comparison, photogenerated holes can directly oxidise HMF to DFF coupled with electron-involved H2 production. In aerobic conditions, HMF can be hydrogenated to BHMF using solvent protons or in situ generated H2 enabled by decoupling and activation of charge carriers and hydrogenation sites. Challenges and fundamental insights associated with anerobic and anaerobic approaches for selective redox transformations of HMF are discussed, which pave the way for the design of highly efficient and net-zero solar biorefineries.