Magnetic solid catalysts for sustainable and cleaner biodiesel production: A comprehensive review

Biodiesel has attracted widespread attention as a potential alternative energy source to fossil-derived fuels. The application of heterogeneous catalysts for biodiesel production can offer an environmentally-friendly more attractive process for sustainable and cleaner production requirement. Small-sized heterogeneous catalysts generally show better activities due to their higher surface area and less mass transfer limitation. However, the separation of theses solid catalysts is particularly difficult; and the serious mass loss during the separation process together with more time-and energy-consumption precludes their practical utilization especially for high vis-cosity reaction mixtures. Magnetic solid catalysts have recently become the research hotpot for the trans-esterification and esterifications for biodiesel production, because of their easy separation with minimal mass loss by an external magnetic field. In this review, various magnetic solid catalysts, including magnetic solid acid, alkali, enzyme and acidic-basic bi-functional catalysts were introduced. The catalyst preparation, catalytic performances, reaction mechanism, and recyclability of the catalysts and their influential parameters with some justifications are discussed from previous literatures to the field of interest, mainly focusing on the influence of active species, surface properties and their synergisms on the catalytic performances. For achieving the goal of good progress in efficient biodiesel production, the evaluation of the present research state, the challenge faced by the magnetic catalysts and future development trend are put forward. Possible reaction mechanisms for different types of the catalysts are also presented, giving some critical guidance in the design of magnetic solid catalysts. Finally, the key influential parameters on the catalytic performances as well as current challenges and future perspectives are provided for further improvement.