Mechanistic insights into synergistic effect between homogeneous Brønsted acid and solvents on synthesis of 5-hydroxymethylfurfural from cellulose

Solvent and catalyst are important factors in the synthesis of 5-hydroxymethylfurfural (HMF). Herein, different solvents and catalysts were selected to explore the synergistic mechanism to boost the conversion of cellulose to HMF. It was found that low-polarity aprotic (molecular polarizability index <15) and medium basicity (800 kJ/mol < protonation affinity <840 kJ/mol) solvents, such as 1,4-dioxane, tetrahydrofuran (THF) and methyl isobutyl ketone (MIBK) were optimal for homogeneous Brønsted acid-catalyzed HMF production from cellulose. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy gap of Brønsted acid-catalyzed cellulose conversion were significantly reduced by adding these solvents. Molecular dynamics (MD) simulation revealed that in 1,4-dioxane/H2O system, H2SO4 was aggregated to the ring oxygen of cellobiose, as well as the C1-OH and C2-OH groups of glucose. This aggregation process effectively promoted the protonation of cellobiose at the O1 position. The reaction conditions were optimized via response surface methodology leading to a maximum HMF yield of 56.5 % in 1,4-dioxane/H2O (water content 5.3 %),under the conditions of 193 °C and 67 min. The synergistic mechanism will be beneficial for the rational selection of system in the production of high-valued chemicals from biomass.