(136c) Transition Metal Catalyzed Oxidation of Lignin and Lignin Model Compounds in Room Temperature Ionic Liquids

Lignocellulosic biomass is a renewable resource that has the potential to serve as a feedstock for the production of fuels, chemicals, and energy. Dissolution of wood and other lignocellulosic materials is a critical step for the valorization of biomass. Once dissolved, lignin is susceptible to a wide range of transformations to yield high value aromatic products. In this work, lignin was dissolved in 1-ethyl-3-methyl imidazolium diethyl phosphate (EMIM DEP) and catalytically oxidized using a CoCl2?6H2O catalyst precursor in the presence of NaOH and molecular oxygen. The system dynamics, including changes that occurred in the lignin as well as the formation of the active catalyst species, were monitored using in-situ ATR-IR, Raman, and UV-Visible spectroscopy. This spectroscopic analysis revealed increases in alcohol and aldehyde functionality in lignin during the course of the reaction and also provided information regarding the characteristics of the catalyst. In addition to lignin, reactions of several lignin model compounds that contain the salient features of lignin were conducted. These model compounds included those with phenolic, methoxy, and benzylic functionality and beta-O-4, 5-5', and phenylcoumaran linkages, which represent the most abundant linkages in the lignin polymer. Analysis of these compounds revealed that the catalyst system readily oxidized alcohol functionality to form aldehydes. For example, veratryl alcohol, which is a simple non-phenolic lignin model compound, was oxidized to form veratraldehyde and veratrylic acid with a maximum turnover frequency of 1440/h relative to 10-15/h for previously described systems. In addition to the oxidation of the propyl side-chain contained in cinnamyl alcohol to form cinnamaldehyde, the catalyst system was also capable of disrupting the carbon-carbon double bond to form benzoic acid. The role played by the components of the catalytic system, including the interaction of the diethylphosphate anion, hydroxide, substrate, and molecular oxygen with the cobalt to yield this high oxidative catalytic activity, will be discussed. We conclude with a discussion of the potential application of this catalyst system as a lignin pretreatment method in a biorefinery or, alternatively, as a step to functionalize the compounds formed after other lignin monomeric-compound forming pretreatment steps.