(517b) Upgrading Chemically Depolymerized Lignin into Value-Added Products: Maximizing Feedstock Utilization

Lignocellulosic biomass is a bountiful source of renewable carbon for the sustainable production of fuels and chemicals.

Lignocellulosic biomass is composed of primarily three polymers: cellulose, hemicellulose, and lignin. Hemicellulose is a heteropolymer of C₅ and C₆ carbohydrates, whereas cellulose is a homopolymer of β(1→4) linked glucose. These polysaccharides comprise 65-80 wt% of the plant biomass and considered to be easier to upgrade to value-added products (e.g., paper, clothing, and liquid fuels). Lignin, on the other hand, is a heteropolymer of aromatic compounds and is recalcitrant to chemical and/or biological upgrading. As lignin constitutes 20-35 wt% of biomass, and it is the largest source of renewable aromatics on the planet, it is desirable to obtain value-added products from lignin for maximum utilization of the feedstock.

In this work, we have used chemical and biological upgrading in tandem to extract maximum value from lignin. We will show that relatively native lignin can be isolated from lignocellulosic biomass using γ-valerolactone (GVL) and water as the solvent system with sulfuric acid as the catalyst under mild reaction conditions. We demonstrate that this GVL-lignin is successfully depolymerized by hydrogenolysis over a Pd/C catalyst into a mixture of aromatic compounds. This mixture contains molecules with similar chemical structures and presents a challenging separation problem. Indeed, this has been a bottleneck for obtaining value-added products from lignin. We solve this problem by upgrading the mixture of aromatic compounds to a single compound via biological funneling. Specifically, we use engineered Novosphingobium aromaticivorans (DSM12444) to transform the mixture of aromatic compounds containing syringyl, guaiacyl, and p-hydroxyphenyl substructures to a single product, 2-pyrone-4,6-dicarboxylic acid (PDC). Thus, we show that a complex mixture of lignin hydrogenolysis products can be reduced to a single product leading to easier separation. Additionally, we show that this strategy is agnostic to the biomass type by successfully applying this strategy to lignin extracted from hardwoods (poplar and maple) and grasses (switchgrass and sorghum).