Base-catalyzed depolymerization of residual lignin-enriched biomass from biochemical conversion processes and its bacterial treatment

Lignin is a heterogeneous, alkyl-aromatic polymer and could eventually serve as a feedstock for the production of hydrocarbon fuels and chemicals. However, lignin depolymerization to upgradable intermediates remains challenging in a process context. In previous study, we have demonstrated that base-catalyzed depolymerization (BCD) of residual, lignin-rich biomass after a conventional biochemical conversion process (the remaining substrates after disk refining pretreatment of deacetylated corn stover followed by enzymatic hydrolysis) could be depolymerized in sodium hydroxide at 270-330°C to produce significant yields of small molecular weight aromatics. In this study, we investigated BCD effects on the residual lignin-rich biomass under milder conditions with 10 wt% solid loading in 1-2 wt% NaOH at 60-240°C for 40 min. Accurate mass balances of obtained products (aqueous soluble fraction, insoluble solid residue and gas) were measured, and characterization of the soluble fraction was performed using chemical methods (GPC and GC/LC-MS), which suggest that a 37-63% of the starting lignin-rich material was depolymerized to small molecular weight, soluble species. Bacterial treatments were also conducted in the different soluble fractions (which were used as a sole carbon source) to identify BCD conditions that maximize microbial carbon conversion. Pseudomonas putida KT2440 was able to grow satisfactorily in all those liquors excluding the ones from BCD treatments at 220 and 240ºC (1-2 wt% NaOH) and 200ºC with 1 wt% NaOH. Results demonstrate how mild BCD of lignin is a promising tool for lignin solubillization and further biological lignin upgrading.