(723i) Advancing Delignification of Ponderosa Pine with High-Boiling Point Solvents to Produce Lignin for Downstream Valorization

Lignin, the most abundant aromatic polymer, can be catalytically depolymerized to produce low molecular weight aromatic compounds such as vanillin, acetovanillone, and vanillic acid. These compounds can serve as precursors for bioconversion into bioplastic monomers such as pyrone-2,4-dicarboxylic acid and muconic acid (a precursor for nylon-6,6). Compared to traditional methods, producing nylon-6,6 from lignin-derived aromatic compounds can result in a 70% reduction in greenhouse gas emissions.

The Four Forest Restoration Initiative aims to mitigate forest fires by mechanically thinning 250,000 acres of Ponderosa pine (Pinus ponderosa) forests. This approach is expected to yield approximately 25 green tons of residual biomass per acre, with about 30% lignin. This lignin could be utilized for various applications, alongside cellulose, a traditional product of the pulp and paper industry and a focus of the growing biorefineries.

Mechanically refined Ponderosa pine (P. ponderosa) pulp was subjected to organosolv pulping using two high-boiling point solvents—1,4-butanediol (BDO; bp = 235 °C, protic) and γ-valerolactone (GVL; bp = 205 °C, aprotic)—under acidolytic conditions at low temperatures and autocatalytic conditions at high temperatures. Both solvents are biorefinery-derived and renewable, and are considered safe for operation due to their low vapor pressures. GVL, which has a strong affinity for lignin, as predicted based on Hansen solubility theory, is compared with BDO, which has a lower affinity but can stabilize lignin through benzyl alkoxylation, neutralizing the reactive benzyl carbocations.

The isolated lignins were characterized for their chemical composition (wet analysis), molecular weight distribution (GPC), free phenolic hydroxyl groups (31P NMR), β-O-4 bonds (2D HSQC), glass transition temperature, and thermal properties (thermogravimetric analysis). Softwood kraft lignin and P. ponderosa milled wood lignin were used as references.

Overall, the GVL process produces lignin of higher purity (up to 96%) than the BDO process. The higher recovery yields and lower purity of BDO lignins compared to GVL lignins are attributed to BDO alkoxylation of lignin at the Cα position, as confirmed by 2D HSQC. Additionally, BDO lignins exhibit significantly higher molecular weights than GVL lignins. Especially under acidolytic conditions, more β-O-4 bonds are retained, resulting in lower free phenolic hydroxyl groups (PhOH), making them strong candidates for depolymerization strategies in biofunneling applications.