The synthesis of functionalized-silica/carbon catalysts from rice husk for the conversion of hexose sugar into methyl levulinate and 5-hydroxymethylfurfural.

Recently, silica/carbon composite material has received much attention in heterogeneous catalysis due to its unique hybrid properties. However, the difficulties in designing green synthetic methods remain challenging. In this study, varied proportion of silica/carbon heterogeneous catalyst was prepared directly from rice husk by varying the carbonization temperatures (250, 300, 350, 400, and 450 °C). The prepared composites were functionalized separately with sulfonic acid and aluminium oxide giving the following catalysts: sulfonated-silica/carbon (SO3H-SiO2/C-250, SO3H-SiO2/C-300, SO3H-SiO2/C-350, SO3H-SiO2/C-400, SO3H-SiO2/C-450) and silica/carbon-supported aluminium oxide (Al-SiO2/C-350, Al-SiO2/C-400, and Al- SiO2/C-450). For the SO3H-SiO2/C catalysts, the XRD data confirms these composites were amorphous. Higher carbonization temperature showed an increase in porosity (pore size increased from 40 to 53 Å), total pore volume (increased from 0.031 to 0.288 cm3 /g), and surface area (increased from 39 to 260 m2/g) of the composites. EDX analysis showed the silicon-to-carbon weight ratios of the composite increased from 0.114to 1.12 with increase in carbonization temperature. However, the sulfur-to- carbon weight ratio showed a decrease from 0.040 to 0.0055. In the case of Al-SiO2/C catalyst, XPS data showed the presence of aluminium in the form of Al2O3 (1.03 wt%). The XRD data confirms this catalyst was amorphous. N2 adsorption-desorption analysis shows no major impact on textural properties after Al2O3 grafting, as the surface area just decreased from 179 to 141 m2/g. The total pore volume decreased from 0.155 to 0.124 cm3/g, and the pore width increased from 44 to 48 Å. The SO3H- SiO2/C catalysts were used as heterogeneous Brønsted acid catalyst in the conversion of fructose into methyl levulinate (ML) and 5-hydroxymethylfurfural (HMF). A product yield of 90% of ML was obtained at 165 °C in 7 hours in methanol solvent. The yield decreased in the recycling experiments due to continued carbonaceous deposition resulting in deactivation. These composites were also able to convert fructose to HMF in 56 – 74% yield in dimethyl sulfoxide (DMSO) solvent at 130 °C in 5 hours. For the Al-SiO2/C composite, it was used as a bifunctional heterogeneous Lewis-Brønsted acid catalyst to convert glucose into HMF. The use of the catalyst resulted in 52% yield of HMF at 170 °C in 5 hours in N-methylpyrrolidone (NMP) solvent. This catalyst was stable without significant loss of yield after several recycling experiments.