Biodiesel Production via the Esterification of Free Fatty Acids Catalyzed by Novel Recyclable and Reusable Thiadiazole-Based Solid Organic Acid Catalyst

Novel thiadiazole-based solid organic acids were designed and synthesized, and their catalytic performance was evaluated in the esterification of free fatty acids (FFAs) and methanol to produce biodiesel. Characterization results suggested that high-purity solid organic acids (DSTD and MSTD) were successfully synthesized via a one-step method. Among them, the DSTD with a high acid site strength and acid density exhibited excellent catalytic activity, and the conversion of FFAs (C10–C18) was greater than 96% at 70 °C for 1 h. Notably, DSTD could be easily recovered by filtration after the removal of methanol and generated water from the reaction mixture under vacuum, and the conversion of lauric acid remained above 97% after 10 cycles of experiments, demonstrating outstanding recyclability and reusability. Kinetic studies for the lauric acid esterification revealed that DSTD has a lower activation energy (Ea) of 29.67 kJ·mol–1, and the thermodynamic parameters were determined as ΔH = 26.95 kJ·mol–1, ΔS = −0.197 kJ·mol–1·K–1, and ΔG = 88.61–94.51 kJ·mo–1. In general, the simple preparation procedure, excellent catalytic performance, and favorable recyclability and reusability make DSTD a promising industrial catalyst for biodiesel production via esterification.