Abstract
This study investigates the possibility of silica produced from rice husks as a precursor for the development of solid acid catalysts, indicating mostly heterogeneous behavior under the conditions examined to produce biodiesel. Rice husk silica (RHS) was prepared via controlled thermal treatment and subsequently functionalized with sulfonic acid groups via a post-grafting-oxidation method using 3-mercaptopropyltrimethoxysilane as a precursor. The influence of calcination temperature (700–900 °C) on the structural, textural, and acidic properties of the resulting catalysts was systematically examined. It was found that lower calcination temperatures favor the preservation of surface silanol groups, which are essential for effective grafting and high acid site density. Among the synthesized materials, RHS-700-SO3H exhibited the highest acidity (0.55 mmol H+ g−1) and demonstrated the highest catalytic activity among the synthesized catalysts. Under investigated conditions, a maximum free fatty acid conversion of 92.7% was achieved in the esterification of palm fatty acid distillate (PFAD). The observed catalytic activity is correlated with the physicochemical properties of the synthesized catalysts, including surface acidity and pore structure, although further characterization is required to fully elucidate the nature of the active sites. The RHS-700-SO3H catalyst showed reasonable reusability. However, a gradual decline in activity was observed due to partial leaching of sulfonic acid groups. These findings demonstrate that rice husk-derived silica is a promising platform for the development of biomass-derived solid acid catalysts, although further evaluation of economic feasibility and process efficiency is required.