Sulfonated-silica/carbon composites from rice husk as heterogeneous catalysts in fructose conversion: The effect of controlling carbonization temperature of rice husk on its physicochemical properties and catalytic activities

Abstract

A simple preparation of sulfonated-silica/carbon (SiO2/C–SO3H) composites from rice husk at different carbonization temperatures have been demonstrated. These composites have been employed as heterogeneous Brønsted acid catalysts in converting fructose into methyl levulinate (ML) in a one-pot reaction that involves dehydration, etherification, and rehydration processes. As the carbonization temperature increased, the results from N2 adsorption-desorption isotherm showed porosity and the surface area of the composites were increased. The acid strength was also increased, as confirmed by the Temperature-Programmed Desorption of Ammonia (NH3-TPD) profile. Energy-Dispersive X-ray (EDX) analysis revealed the SiO2/C weight ratios of the composite were increased, but S/C weight ratios were decreased. 90% yield of ML was synthesized at 165 °C for 7 h in methanol solvent. Pearson's correlation test showed that hydrophilicity is the main factor in obtaining high turnover number (TON). The yield was dropped in recycling experiments due to carbonaceous deposition, which caused deactivation, as corroborated by the Thermogravimetric Analysis (TGA) and N2 sorption analysis. These composites were also able to convert fructose into 56–74% yield of 5-hydroxymethylfurfural (HMF) in dimethylsulfoxide (DMSO) solvent at 130 °C for 5 h.