Exploring Cobalt, Nickel, and Palladium Complexes for the Hydrogenation Reactions: A Review

Catalyst design and synthesis are among the most important aspects of catalysis, especially for hydrogenation reactions. This work, through a systematic review of relevant literature, seeks to identify key parameters for the application of homogeneous cobalt, nickel, and palladium catalysts in hydrogenation reactions, particularly in the partial hydrogenation of biodiesel. The hydrogenation of biodiesel using the Co, Ni, and Pd catalysts stabilized by different ligands is dependent on the activity of the catalyst and its interaction with a ligand that can stabilize it at low oxidation states during the catalytic cycle. A Pd catalyst coordinated to a monodentate nitrogen ligand shows the highest activity, giving 100% conversion of methyl linoleate (C18:2) to methyl stearate (C18:0). The Pd (II) catalyst with a bidentate ligand, however, shows high activity but low stability as it reduces to Pd(0) species. The product selectivity depends on the time; hence, the desired product or composition mixture of the biodiesel is achieved by kinetic control of the reaction process. The optimized composition of the partially hydrogenated biodiesel sample oil sample was found to be 18.2% C18:0, 49.2% C18:1, 8.1% C18:2, and 23.4% C16:0, which showed improvement of the oxidative stability by 3 h. The use of homogeneous catalysts in biodiesel studies remains a challenge due to the difficulty in separating the catalysts from the hydrogenated material. Highly active catalysts are unsuitable for partial biodiesel hydrogenation as they produce excessive saturated substrates, which are undesirable.