Transition-metal catalysts for the conversion of Lignin and Lignin Models to High Value Chemicals.

Lignin is considered the second most abundant biopolymer and the first source of aromatics on earth. There is great interest in the development of processes that can transform lignin to fuels and high value chemicals. However, current technology is insufficient for efficiently transforming high volumes of biorefinery lignin into marketable products at low cost and high yield. In this dissertation we evaluated different homogeneous transition metal catalyst systems for the oxidative depolymerization and conversion of lignin models into value-added chemicals. First, we examined the direct carbon-hydrogen functionalization of monomeric, dimeric and polymeric lignin models using a ruthenium catalysts. High yields of alkylated product were obtained when using monomeric and dimeric lignin models. It was found that the yield of alkylated product in monomeric ketone lignin models is a function of their degree of substitution. Also, the methoxy groups create steric hindrance that reduce the rate of alkylation. Increasing the reaction time and the amount of olefin lead to the formation of dialkylated products. Second, we focused in the evaluation of cobalt-Schiff base catalysts for the oxidative depolymerization of lignin. Some quinones produced in the oxidative depolymerization of lignin deactivated the Co-Schiff base catalysts used in the reaction. Even catalysts with sterically bulky ligands were susceptible to deactivation by quinones when a coordinating base is added. We found that the formation of complexes between Co-Schiff bases catalyst and quinones and quenching of the superoxo radical can explain the deactivation of the catalyst. Finally, we examined the promotion of lignin models oxidation by using Co-Schiff bases catalyst mediated by aminoxyl radicals. Our attempts to increase the formation of phenoxy radical were challenged by the preference of the aminoxyl radicals to attack the benzylic hydrogens instead of the phenolic hydrogens.