Microbial Upgrading of Lignin Depolymerization: Enhancing Efficiency with Lignin-First Catalysis

Chemical depolymerization of lignin is a non-selective process that often generates a wide distribution of product compounds, denoted herein as lignin breakdown products (LBPs). To address this limitation, we developed a hybrid lignin conversion process that employs a lignin-first catalytic approach on biomass and subsequent microbial upgrading. A Pd/C catalyst was used for reductive catalytic fractionation (RCF) of poplar biomass, and Rhodococcus opacus PD630 (R. opacus PD630) was then cultivated on the resulting LBPs. This RCF approach increases the total biomass utilization by R. opacus PD630 over base-catalyzed depolymerization (BCD) reactions that were performed in the absence of Pd/C and molecular hydrogen (H₂)_. LBPs generated using RCF resulted in higher cell growth per gram of biomass. Cellulose in the residual biomass after RCF treatment also showed enhanced enzymatic digestibility due to saccharification yields over 40%. Techno-economic analysis (TEA) and life cycle analysis (LCA) of this hybrid lignin conversion scheme, integrated into a cellulosic bioethanol plant, decreased the minimum ethanol selling price from $4.07/gallon (base case) to $3.94/gallon. Global warming potentials ranged from 29 and 30.5 CO_2,eq/MJ. These results highlight the potential for an industrial hybrid conversion-based biorefinery scheme that utilizes lignin-first catalytic deconstruction and R. opacus PD630 upgrading.