A long-chain Brønsted acid ionic liquid (IL), 3-(N,N-dimethyldodecylammonium)propanesulfonic acid p-toluenesulfonate ([DDPA][Tos]), was prepared and characterized by FT-IR, 1H NMR, 13C NMR, UV/vis and TGA. The H0 (Hammett function) value of the IL was also determined. The IL as catalyst was applied to the catalytic synthesis of biodiesel from free fatty acids (FFAs). The influencing factors, such as the type and amount of catalyst, reaction time, molar ratio of fatty acid to methanol and reaction temperature, were investigated. The results indicated that the long-chain Brønsted acid IL showed high catalytic activity and fair reusability. Esterification was accomplished under the following optimized conditions: molar ratio of alcohols to FFA at 1.5 : 1, mole fraction of ionic liquid at 10%, 60 °C, and 3 h. The products could be separated from the catalyst system by liquid–liquid biphase separation at room temperature with good yields of 92.5% to 96.5%. The catalyst could be reused nine times after the removal of water and alcohol. Therefore, the long-chain Brønsted acid IL has good potential for the synthesis of biodiesel from low-cost feedstocks such as waste oil and woody plant oils.
