Lignin valorization through integrated biological funneling and chemical catalysis

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.

Keywords: aromatic degradation; biofuels; biorefinery; lignocellulose.

Fig. 1.

Integrated production of fuels, chemicals, and materials from biomass-derived lignin via natural aromatic catabolic pathways and chemical catalysis. Biomass fractionation can yield streams enriched in lignin and polysaccharides, which can be converted along parallel processes. The challenges associated with lignin’s heterogeneity are overcome in a “biological funneling” process through upper pathways that produce central intermediates (e.g., protocatechuic acid). Dioxygenases cleave the aromatic rings of these intermediates, which are metabolized through the β-ketoadipate (β-KA) pathway to acetyl-CoA. As shown, residual glucose and acetate present will also be metabolized to acetyl-CoA, the primary entry point to mcl-PHA production via fatty acid synthesis. We demonstrate mcl-PHA production, which are biodegradable polymers. mcl-PHAs are converted to alkenoic acids, and further depolymerized and deoxygenated (“depoly-deoxy”) into hydrocarbons, thus demonstrating the production of fuels, chemicals, and materials from lignin and other biomass-derived substrates.

Fig. 2.

Biological conversion of lignin-derived aromatic molecules and carbohydrate-derived products in APL to mcl-PHAs in P. putida. (A) Conversion and mcl-PHA production from representative model compounds present in APL, each at 2 g/L. (B) Conversion and mcl-PHA production of a mixture of four representative model compounds from APL, each loaded at 0.5 g/L. (C) Flow cytometry of Nile Red-stained cells for mcl-PHA accumulation. Cell counts are plotted as a function of fluorescence intensity in the initial inoculum (t = 0) and cultures at t = 48 h for a model substrate, p-coumaric acid, and APL grown in 250-mL shake flasks, with the corresponding total mcl-PHA production from APL shown in Inset. (D) Fluorescence imaging of cells at 0, 12, and 48 h stained with Nile Red demonstrates mcl-PHA production from APL (Upper). Fluorescence quantitation of P. putida cells from the APL conversion as a function of time adjusted to an equivalent cell density (Lower). Biological APL conversion by P. putida in a 14-L fermenter (Right).

Fig. 3.

APL-derived mcl-PHA physicochemical properties and catalytic upgrading to chemical precursors and fuels. (A) Example of thermal-catalytic upgrading pathway for mcl-PHAs to chemical precursors and hydrocarbon fuels. (B) APL-derived mcl-PHAs and physicochemical properties including weight-average molecular weight (MW_w), polydispersity index (PDI), glass transition temperature (T_g), melting point (T_m), and 5% decomposition temperature (T_d). (C) Initial mcl-PHA hydroxyacid composition (Left) and alkane distribution (Right) after thermal depolymerization and catalytic deoxygenation.