OXIDATIVE DEPOLYMERIZATION OF LIGNIN TO LOW MOLECULAR WEIGHT AROMATICS

To date, most lignocellulosic biorefinery strategies have focused on optimizing conversion of cellulose to ethanol, leaving lignin as an underutilized biomass constituent. Lignin is engineered by nature with the intent to protect plants from chemical and biological attack; this leaves lignin with high structural irregularity and recalcitrance, rendering conversion of the lignin macromolecule to valuable products particularly challenging. Nevertheless, given that the economics of cellulosic ethanol production are strongly dependent on the value that can be obtained for the lignin co-product, the successful valorization of lignin is a crucial step in the transition towards a bio-based economy. This thesis focuses on lignin depolymerization using oxidative methods, specifically, the oxidation and cleavage of the β-O-4 linkage. Heterogeneous catalysis in this case is more desirable than homogenous catalysis as the catalyst easily recovered, and it is better suited for industrial applications. Initially, layered double hydroxide (LDH) supported gold nanoparticles were characterized and screened in the oxidation of various lignin model compounds using molecular oxygen, leading to the discovery of an Au/Li-Al LDH heterogeneous catalyst active for oxidative cleavage of the β-O-4 linkage. The Au/Li-Al LDH catalyst was then applied to oxidatively depolymerize Indulin AT kraft lignin and γ-valerolactone (GVL) extracted lignin, high yields of monomers being observed when the oxidized lignins underwent subsequent base-catalyzed hydrolysis. Thereafter, different literature oxidative lignin depolymerization methods were tested on kraft lignin and GVL lignin, and the results compared to the Au/Li-Al LDH catalyst (coupled with hydrolysis) system to determine the most effective oxidative depolymerization method