Catalytic hydrolysis of cellulose by phosphotungstic acid-supported functionalized metal-organic frameworks with different electronegative groups

It is found that strong electronegative groups can selectively adsorb cellulose by hydrogen bonds. Grafting strong negatively charged groups onto catalysts to achieve the functionalization of the catalyst can give it the ability to selectively adsorb cellulose without affecting its catalysis, which is of great significance for the hydrolysis of cellulose. In this study, PTA@MIL-101-X (X = -Br, -NH2, -Cl, -NO2) materials were synthesized to investigate the effect of grafting different electronegative groups on carriers to the directional hydrolysis of cellulose. The synthesized catalysts used phosphotungstic acid as the catalytic center while treated MIL-101 structure as the carrier. The grafting of different electronegative groups changed the crystal structure of the metal organic framework without affecting its stability during the reaction. The strong negative functional groups can selectively adsorb cellulose by forming hydrogen bonds with cellulose hydroxyl groups and weaken the hydrogen bonds within cellulose molecules. This hydrogen bond can reduce the side reaction of glucose, lighten the difficulty of cellulose hydrolysis, and improve the efficiency of cellulose conversion at the same time. The hydrolysis rate of cellulose increased with the electronegativity enhancement of the grafted functional groups, and the grafted -NO2 catalyst PTA@MIL-101-NO2 obtained the highest glucose yield of 16.2% in the cellulose-directed hydrolysis. The -NH2 can form a chemical linkage with PTA through electrostatic interaction to get the highest immobilization stability and exhibit excellent stability in the recycling of catalysts.