Valorization of fructose: Catalytic pathways from 5-Hydroxymethylfurfural (HMF) to 2,5-Diformylfuran (DFF)

The sustainable conversion of carbohydrate feedstocks into high-value platform chemicals is central to contemporary biorefinery research. This review compiles and critically evaluates recent advances (2020–2025) in catalytic methods for converting fructose to 5-hydroxymethylfurfural (HMF) and subsequently oxidizing HMF to 2,5-diformylfuran (DFF). Emphasis is placed on catalyst design (heterogeneous and homogeneous), solvent engineering (DMSO, DES/NADES, biphasic systems, ionic liquids), reactor formats (batch, microreactors, continuous μPBR), and enabling technologies (microwave, photocatalysis, bio- and chemoenzymatic systems). Key findings include: (i) sulfonated carbon and mesoporous solid acids, MOF-based composites, and tailored deep eutectic solvent systems achieve HMF yields often exceeding 90% under optimized conditions; (ii) photocatalytic, transition-metal, vanadium-based, MnO 2 polymorphs and metal-free P/N co-doped carbons provide efficient and selective routes to DFF with high conversions and minimal over-oxidation; (iii) process intensification strategies such as in-situ extraction, substrate protection (acetalization), and continuous flow microreactors suppress humin formation and raise productivity; and (iv) theoretical studies (DFT, mechanistic probes) and enzyme/whole-cell approaches are expanding mechanistic understanding and enabling greener oxidation routes. Remaining challenges include catalyst stability under realistic, high-concentration feeds, humin suppression at scale, oxygen-efficient oxidation pathways without precious metals, and integrated one-pot processes from sugars to DFF. The review concludes with recommendations for catalyst robustness testing, techno-economic and life-cycle assessments, and combined experimental–computational efforts to accelerate scale-up of sustainable furan chemistry.