Eric P. Weeda

Biomass pretreatment methods are commonly used to isolate carbohydrates from biomass, but they often lead to modification, degradation, and/or low yields of lignin. Catalytic fractionation approaches provide a possible solution to these challenges by separating the polymeric sugar and lignin fractions in the presence of a catalyst that promotes cleavage of the lignin into aromatic monomers. Here, we demonstrate an oxidative fractionation method conducted in the presence of a heterogeneous non-precious-metal Co-N-C catalyst and O2 in acetone as the solvent. The process affords a 15 wt% yield of phenolic products bearing aldehydes (vanillin, syringaldehyde) and carboxylic acids (p-hydroxybenzoic acid, vanillic acid, syringic acid), complementing the alkylated phenols obtained from existing reductive catalytic fractionation methods. The oxygenated aromatics derived from this process have appealing features for use in polymer synthesis and/or biological funneling to value-added products, and the non-alkaline conditions associated with this process support preservation of the cellulose, which remains insoluble at reaction conditions and is recovered as a solid.

Depolymerization of lignin into aromatic monomers is one of the highest priority targets for valorization of lignin obtained from biomass pretreatment. Oxidative lignin depolymerization proceeds rapidly under alkaline conditions at elevated temperature with O2; however, the aromatic products are susceptible to degradation under the same conditions, complicating practical application of these conditions. Here, we report a continuous-flow aerobic alkaline lignin depolymerization method using an O2-permeable membrane reactor. The flow reactor allows for continuous oxygen delivery to the alkaline lignin solution and precise control of the temperature and reaction time. Reaction time-course analysis provides direct insights into the rates of lignin depolymerization and monomer decomposition, enabling process optimization. Aromatic yields up to 43 wt % are observed with a residence time of less than 4 min. This process is applied to the depolymerization of multiple lignin materials derived from different biomass pretreatment methods and from both softwood and hardwood sources.

Methods for catalytic fractionation of biomass provide a means to convert lignin directly into monomers while generating a high-quality cellulosic stream, contrasting conventional biomass pretreatment strategies that prioritize the cellulosic fraction. Here, a nonprecious-metal Co-N-P-C catalyst is identified for aerobic oxidative catalytic fractionation (OCF) of poplar feedstock in dimethyl carbonate that achieves significantly higher yields of aromatic monomers (24 wt %) relative to those obtained with a recently reported Co-N-C catalyst in acetone solvent (15 wt %). Mechanistic studies indicate that the acidic properties of the catalyst contribute to its improved performance by promoting extraction of lignin from insoluble polysaccharides. This OCF process is complemented by the development of a new centrifugal partition chromatography (CPC) method that supports isolation of all five major aromatic monomers (syringic acid, syringaldehyde, vanillic acid, vanillin, and para-hydroxybenzoic acid) in a single liquid–liquid extraction purification step. This OCF/CPC sequence has important implications for future lignin valorization efforts.

Carbon-carbon σ bond activation is difficult due to the strength of the σ bond and the steric hindrance around the bond. Using a quinolinyl ketone system, activation can be achieved with a rhodium catalyst, allowing not only carbon-carbon σ bond activation, but also functionalization using boronic acids. The exchange of aryl substituents between quinolinyl ketones and boronic acids, proceeding through carbon-carbon σ bond activation, is the subject of a kinetic study. NMR and kinetic studies are used to determine the rate law of the reaction using ortho-fluoro quinolinyl ketone and 4-trifluoromethyl boronic acid. In addition, NMR studies are used to compare the rates of various aryl substituents of both boronic acids and quinolinyl ketones to ortho-fluoro quinolinyl ketone and 4-trifluoromethyl boronic acid exchange.