(570d) Preserving the Structure of Softwood Lignin Using Green High-Boiling POINT Solvents

As a method to mitigate forest fires, the Four Forest Restoration Initiative aims to mechanically thin 250,000 acres of Ponderosa pine (Pinus Ponderosa; Ppine) forests, yielding ~25 green tons of residual biomass per acre.

Mechanically refined Ppine pulp was subjected to organosolv pulping via two high-boiling point (bp) solvents—1,4, butanediol (BDO; bp = 235 °C; protic) and γ-valerolactone (GVL; bp = 205 °C; aprotic)—to isolate lignin with a relatively preserved native structure for high-value applications. These solvents exhibit a good affinity toward lignin, are biorefinery products, renewable, and safe to operate owing to their low vapor pressures.

The pulping was performed under acid-catalyzed conditions at a low temperature (90 °C, 1.5 h, 100 mM H₂SO₄) and autocatalytic conditions at a higher temperature (180 °C, 4 h) with GVL or BDO (solvent: water = 7:3), yielding four lignin fractions: BDO_180, BDO_90, GVL_180, and GVL_90. 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), and glass transition temperature and thermogravimetric analysis. Softwood kraft lignin and pine milled wood lignin were used as references.

Overall, the GVL process produces lignin of higher purity (up to 96% for GVL_180) than the BDO process. The higher recovery yields and lower purity of the BDO lignins compared to those of the GVL lignins are attributed to the BDO alkoxylation of lignin at the Cα position as is confirmed by 2D HSQC. Further, the BDO lignins also exhibit significantly higher molecular weights than the GVL lignins. Under acidolytic conditions, more β-O-4 bonds are retained, yielding lower free phenolic hydroxyl groups (PhOH) and may be more suitable for depolymerization strategies for biofunneling applications. Additionally, contact angle measurements of the lignins revealed that the acidolytic lignins (BDO_90 and GVL_90) are more hydrophobic than their autocatalytic counterparts (BDO_180 and GVL_180). These observed differences might be attributed to the higher PhOH groups in the latter. Therefore, the autocatalytic lignins may be more suitable for polymeric applications such as lignin-based thermoplastics, resins, and carbon fibers. Further, SEM revealed the particles of BDO_90 and GVL_90 to be at the nanoscale, indicative of their potential for use as nanoparticles.