Adipic acid production from lignin
Lignin is an alkyl-aromatic polymer present in plant cell walls for defense, structure, and water transport. Despite exhibiting a high-energy content, lignin is typically slated for combustion in modern biorefineries due to its inherent heterogeneity and recalcitrance, whereas cellulose and hemicellulose are converted to renewable fuels and chemicals. However, it is critical for the viability of third-generation biorefineries to valorize lignin alongside polysaccharides. To that end, we employ metabolic engineering, separations, and catalysis to convert lignin-derived species into cis,cis-muconic acid, for subsequent hydrogenation to adipic acid, the latter being the most widely produced dicarboxylic acid. First, Pseudomonas putida KT2440 was metabolically engineered to funnel lignin-derived aromatics to cis,cis-muconate, which is an atom-efficient biochemical transformation. This engineered strain was employed in fed-batch biological cultivation to demonstrate a cis,cis-muconate titer of 13.5 g L−1 in 78.5 h from a model lignin-derived compound. cis,cis-Muconic acid was recovered in high purity (>97%) and yield (74%) by activated carbon treatment and crystallization (5 °C, pH 2). Pd/C was identified as a highly active catalyst for cis,cis-muconic acid hydrogenation to adipic acid with high conversion (>97%) and selectivity (>97%). Under surface reaction controlling conditions (24 °C, 24 bar, ethanol solvent), purified cis,cis-muconic acid exhibits a turnover frequency of 23–30 s−1 over Pd/C, with an apparent activation energy of 70 kJ mol−1. Lastly, cis,cis-muconate was produced with engineered P. putida grown on a biomass-derived, lignin-enriched stream, demonstrating an integrated strategy towards lignin valorization to an important commodity chemical.