Conversion of biomass to hydroxymethylfurfural: A review of catalytic systems and underlying mechanisms.

Conversion of biomass waste to hydroxymethylfurfural (HMF), a value-added platform chemical, has captured great research interests driven by the economic and environmental incentives. This review evaluates the recent development of biomass conversion systems for high HMF yield and selectivity, with a focus on the performance of emerging catalysts and solvents from a mechanistic view. We highlight that the ratio and strength of Brønsted and Lewis acid in bifunctional catalyst are critical for maximizing HMF production by selective improvement in the kinetics of desirable reactions (hydrolysis, isomerization, and dehydration) over undesirable reactions (rehydration, polymerization). The characteristics of solvent mixture such as functional groups and speciation govern the reactivity of substrate towards desirable reactions and stability of HMF and intermediates against side reactions. Research efforts to unravel the interactions among co-catalysts/co-solvents and between catalysts and solvents are encouraged, thereby engineering a synergistic conversion system for biomass valorization.