Acidic ZnCl 2 /ZnBr 2 Molten Salt Hydrates for Cellulose Deconstruction and Selective Conversion to 5- Hydroxymethylfurfural

Abstract The single-vessel conversion of cellulose into 5-hydroxymethylfurfural (HMF) provides an efficient route for valorizing lignocellulosic biomass. In this study, an acidic ZnCl 2 /ZnBr 2 molten salt hydrate system was developed for the two-stage conversion of microcrystalline cellulose into HMF. Low-temperature pretreatment effectively disrupted the intra- and intermolecular hydrogen-bonding network of cellulose, reducing its crystalline order. The combined Brønsted and Lewis acidity of the mixed molten salt system facilitated cellulose depolymerization and subsequent HMF formation. Systematically tuning the ZnBr 2 fraction concurrently optimized zinc coordination, medium acidity, and the solvation environment; moderate ZnBr₂ fractions enhanced cellulose deconstruction and HMF selectivity, whereas excessive ZnBr 2 fractions promoted HMF rehydration to levulinic acid. The response surface model predicted a maximum HMF yield of 60.69% at 163°C, 150 min, 2.63 wt% HCl, and 10.35 mol% ZnBr 2 . Three independent validation experiments under these conditions gave an average HMF yield of 59.73%, in close agreement with the model prediction. Density functional theory calculations indicated that local mixed-halide Zn coordination reduced the calculated electronic energy barrier of the representative 1,2-hydride-transfer step by 40.6 kJ·mol -1 . These results delineate clear composition–structure–performance relationships for halide-regulated cellulose deconstruction and selective conversion in zinc-based molten salt hydrates.