Preparation and Catalytic Properties of Graphene Oxide/ Phosphotungstic Acid Composites

Background: Cellulose structures are in stable crystalline form. The hydrolysis of cellulose to small reducing sugars is difficult, but essential for its utilization. Objective: To investigate the effect of graphene oxide (GO) loading on the catalytic performance of phosphotungstic acid (HPW) for the catalyzed hydrolysis of cellulose, with the purpose to get high yield of total reducing sugar (TRS). Methods: Graphene oxide/phosphotungstic acid (GO/HPW) composites were prepared using a liquid-phase composite method. The materials were applied to catalyze hydrolysis of microcrystalline cellulose in 1-butyl-3-methylimidazole chloride ionic liquid ([Bmim]Cl). The samples were characterized by powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), field emission scanning electron micrographs (FE-SEM), pyridine IR and acid-base chemical titration. Results: The Brønsted acidic sites were the main source of acidity in the composites and its concentration was determined to be 0.96 mmol/g. With the use of the GO/HPW composite as catalysts for cellulose hydrolysis, high TRS yield of 90.5% was obtained. Conclusion: GO/HPW composites retained the functional groups of both materials. It was the Brønsted acidic sites in the materials that effectively promoted the cellulose hydrolysis reaction. The structures of GO/HPW with the agglomeration of HPW scattered on GO had high accessibility of acidic sites and fast mass transfer of the reducing sugars to the outside of the catalysts in time to prevent their further conversion into by-products. TRS yield of 90.5% was obtained from the hydrolysis of cellulose catalyzed by the GO/HPW (1:1.5) composites at 115°C for 4 h using catalysts to cellulose 1:1 ratio.