Lignin valorization through integrated biological funneling and chemical catalysis

TLDR

Lignin, the second most abundant biopolymer, is energy‑dense but underused because its heterogeneity and recalcitrance hinder selective valorization, although some organisms possess metabolic pathways that funnel lignin‑derived aromatics into central intermediates. The study aims to use *Pseudomonas putida* KT2440 to convert lignin‑enriched streams into medium‑chain‑length polyhydroxyalkanoates. By exploiting the organism’s aromatic‑catabolic pathways, the authors converted both model aromatic compounds and heterogeneous lignin‑derived substrates into mcl‑PHAs. The produced mcl‑PHAs matched conventional carbohydrate‑derived polymers in physicochemical properties and could be catalytically transformed into chemical precursors and fuel‑range hydrocarbons, demonstrating lignin valorization into fuels, chemicals, and materials.

Abstract

Lignin is an energy-dense, heterogeneous polymer comprised of phenylpropanoid monomers used by plants for structure, water transport, and defense, and it is the second most abundant biopolymer on Earth after cellulose. In production of fuels and chemicals from biomass, lignin is typically underused as a feedstock and burned for process heat because its inherent heterogeneity and recalcitrance make it difficult to selectively valorize. In nature, however, some organisms have evolved metabolic pathways that enable the utilization of lignin-derived aromatic molecules as carbon sources. Aromatic catabolism typically occurs via upper pathways that act as a "biological funnel" to convert heterogeneous substrates to central intermediates, such as protocatechuate or catechol. These intermediates undergo ring cleavage and are further converted via the β-ketoadipate pathway to central carbon metabolism. Here, we use a natural aromatic-catabolizing organism, Pseudomonas putida KT2440, to demonstrate that these aromatic metabolic pathways can be used to convert both aromatic model compounds and heterogeneous, lignin-enriched streams derived from pilot-scale biomass pretreatment into medium chain-length polyhydroxyalkanoates (mcl-PHAs). mcl-PHAs were then isolated from the cells and demonstrated to be similar in physicochemical properties to conventional carbohydrate-derived mcl-PHAs, which have applications as bioplastics. In a further demonstration of their utility, mcl-PHAs were catalytically converted to both chemical precursors and fuel-range hydrocarbons. Overall, this work demonstrates that the use of aromatic catabolic pathways enables an approach to valorize lignin by overcoming its inherent heterogeneity to produce fuels, chemicals, and materials.