Recoverable and reusable hydrochloric acid used as a homogeneous catalyst for biodiesel production

Several homogeneous acid catalysts (nitric, sulfuric, and hydrochloric acids), were selected to investigate their recoverability and reusability for esterifying enzyme-hydrolyzed FFAs and methanol to produce biodiesel. Although all of the three catalysts drove the reaction at high yield, hydrochloric acid is the only recoverable and reusable catalyst, as indicated by partitioning data. Hence, esterifying FFAs and methanol was catalyzed using hydrochloric acid; and the reaction conversion, which was affected by the reaction conditions, was optimized using response surface methodology. A maximal reaction conversion of 98.19% was obtained at 76.67 °C, at a methanol/FFAs molar ratio of 7.92, a catalyst concentration of 0.54 M, and after a reaction time of 103.57 min. The catalyst could be reused at least five times to drive the reaction to a conversion of 97%. This study demonstrated that recoverable and reusable hydrochloric acid is promising for potential applications, including biodiesel production.