From homogeneous to heterogeneous catalysts in biodiesel production

The production of biodiesel as a fuel in diesel engines greatly increased in recent years and is expected to grow more and more in the near future. Increasing biodiesel consumption requires optimized production processes allowing high production capacities, simplified operations, high yields, and the use of more economic feedstocks such as waste oils and fats. However, the latter often contain large amounts of free fatty acids and cannot be processed with the commonly practiced technology based on the use of alkaline catalysts in the homogeneous phase that requires the use of highly refined oil as raw materials. Therefore, the development of processes for low-cost biodiesel production requires the individuation of heterogeneous catalysts that are very efficient in promoting the transesterification reaction also in the presence of free fatty acids and water, allowing the prompt separation of pure glycerol and not requiring expensive purification of this byproduct. In the present contribution, the performances of different heterogeneous catalysts are compared both in the absence and in the presence of free fatty acids. In some cases, the resistance of the catalysts to the presence of water and the eventual deactivating effects after the first use have also been tested. The catalysts considered are both basic and acidic in nature, such as hydrotalcite, MgO, TiO2 grafted on silica, vanadyl phosphate, and different metals-substituted vanadyl phosphate of the type Me(H2O),VO1-x.,PO4 center dot 2H(2)O, where Me is a trivalent cation such as Al, Ga, Fe, and Cr and where x = 0.18-0.20. Finally, the understanding of the kinetic behavior of the most stable catalyst TiO2/SiO2 has been deepened.