Synergistic activation of hydroxyl groups by hierarchical acid sites and deep eutectic solvents for the dehydration of fructose to 5-hydroxymethylfurfural under mild temperature

The activation of C–OH bond shows great importance for fructose dehydration to produce 5-hydroxymethylfurfural (HMF). In the deep eutectic solvent (DES), the synergistic improvement of surface sulfonic acid groups on hierarchical porous carbon solid acids (HPCSA) and choline chloride (ChCl) on activation of fructose hydroxyl groups was studied to achieve high catalytic performance under mild conditions. The results show a high yield of 90.7 % for HMF at relatively mild temperature of 70 °C. The polarizing effect of ChCl on fructose hydroxyl groups enhances the interaction energy between fructose and sulfonic acid groups on carbon surface, resulting in elongated C–OH bond of fructose from 1.412 Å to 1.456 Å. The hierarchical pore structure can improve the accessibility to acid active sites and increase the quantity of surface sulfonic acid groups by enlarging specific surface areas. A reaction mechanism considering the synergistic effect of various components in DES is proposed to explain the promotion of fructose dehydration under mild conditions. This work provides an effective solution for utilizing biomass to produce renewable energy.