Reduced surface sulphonic acid concentration Alleviates carbon-based solid acid catalysts deactivation in biodiesel production

In this study, esterification of oleic acid with methanol using solid acid catalysts synthesised from different waste biomasses (bamboo, wheat straw, and peanut shells) using the carbonisation–sulphonation method was investigated. The results showed that biomass with higher lignin content had a lower optimum carbonisation temperature and the corresponding catalyst contained more acidic groups. All three catalysts catalyzed esterification reactions under optimum reaction conditions (10 wt% catalyst loading, 8:1 methanol/oleic acid, at 75 °C for 8 h) with >96% conversion. The oleic acid conversion decreased with catalyst cycles, reaching approximately 80% after four cycles. In addition to the leaching of active groups during the reaction, the reaction of the sulphonic acid on the catalyst surface with excess methanol (i.e., sulphonic ester formation) is an important cause of catalyst deactivation. The high concentration of sulphonic acid groups on the catalyst surface was the major cause of sulphonic ester formation during esterification. Therefore, reducing the concentration of sulphonic acid groups by increasing the catalyst volume and specific surface area is an effective way to avoid reduction in the catalyst activity while maintaining a constant total number of sulphonic acid groups.