11-40: Lignin Structural Changes Associated with Oxidative Pretreatment Catalyzed by Cu-diimine Complexes

A wide range of alkaline-oxidative treatments of lignocellulose are known from the literature and practice of chemical pulping and bleaching and from more recent cellulosic biofuels applications.  Alkaline hydrogen peroxide (AHP) has been developed as a pretreatment or post-treatment for increasing biomass digestibility by decreasing the recalcitrance of high-lignin biomass. Traditional AHP pretreatment uses the high pH of 11.5 for deprotonation of hydrogen peroxide which leads to the formation of oxidative radicals (∙OH and ∙O₂^-), whereas the low selectivity of the radical reactions and the non-productive decomposition of hydrogen peroxide to oxygen at alkaline pH limit the economic competitiveness of AHP pretreatment. Significantly, oxidative delignification catalyzed by coordinated transition metal ligands within laccases and peroxidases is the route taken for lignin degradation in nature.  Catalyzed oxidative pretreatments offer the promise of potentially mild temperatures, high reaction specificities, and high oxidizing efficiency that can simultaneously generate a material that is susceptible to enzymatic carbohydrate depolymerization, be performed at decreased pH with correspondingly less inorganics load, and generate modified low MW aromatics that may be potentially upgraded to petroleum-displacing fuels and chemicals. In this work, the effect of Cu-diimine complexes during catalyzed oxidative pretreatment on model lignins is characterized.  Model lignins include a Kraft lignin and dioxane milled-wood and enzymatic lignins from select hardwoods and grasses, while characterization is performed using ¹³C NMR and SEC which show distinct structural changes during pretreatment including, among others, clear evidence of lignin depolymerization.  Additionally, improved cellulolytic enzyme digestibility is found for select catalyzed pretreatments.