Reusable chemical catalysts for sustainable biodiesel production: The role of metallic elements

In recent decades, biodiesel has emerged as a renewable and environmentally benign fuel compared with its fossil counterpart. From an industrial perspective, homogeneously-catalyzed transesterification has been established as the principal method for biodiesel synthesis owing to the moderate reaction conditions. However, homogeneous catalysts cannot be reused, and large amounts of wastewater accompany their separation from the products, making the production process detrimental to the environment and contrary to the sustainable development objectives. This grim reality confronting green fuel can be avoided by using heterogeneous catalysts that can be recycled and reused several times. Metal elements have played a crucial role in the development of such catalysts. These species are readily available in the environment and provide solid catalysts with high activity. Due to their significant contribution to achieving a sustainable production method for biodiesel, this paper reviews the role of metallic elements in fabricating functional materials, including metal oxides, mixed metal oxides, and metal-doped porous frameworks. The optimized reaction conditions focused on reusability were reported and analyzed for each class of catalysts. Challenges and future requirements for boosting the catalysts' activity and reusability in the production process were also discussed. Leaching of active sites and pore blockage were the primary factors detrimental to reusability. These issues could be minimized by supported metal atoms on porous materials, providing a stronger bond of the metal sites and the support, and utilizing membrane reactors to continuously remove the products from a reaction mixture.