Glycerol-Derived Solvents for Tractable Organosolv Extraction of Softwood Lignins

Abstract

In the present study, glycerol-derived ethers (GDEs) are evaluated as green solvents to facilitate organosolv extraction of lignins from softwood biomass. GDEs assessed in the present investigation consist of glycerol skeletons etherified with alkyl constituents (e.g., methyl, ethyl), including 1,3-glycerol diethers and 1,2,3-glycerol triethers. GDEs are derived from glycerol, a high-volume byproduct produced by the biodiesel industry. In comparison to glycerol, GDEs are ∼3 orders of magnitude less viscous while maintaining high boiling points (>160 °C), facilitating improved processability and extraction of lignins under mild temperatures and pressures. The influence of organosolv conditions, specifically temperature (120–180 °C), batch holding time (10–240 min), Brønsted acid concentration, and solvent composition on the extraction of lignins from softwood feedstocks are evaluated. Under optimal conditions (150 °C, 60 min), over 70 wt % of lignins are extracted from softwood biomass. More severe conditions (e.g., 160 °C, 60 min; 180 °C, 60 min) promoted higher delignification (80–90 wt %) but at the expense of lower pulp yields (<40 wt %). The influence of process conditions and solvent speciation on the molecular structure of residual pulps and isolated lignins is further interrogated by IR and NMR spectroscopy, electron microscopy, and mass spectrometry. The “greenness” of GDE organosolv was quantitatively benchmarked against other biobased, high-boiling point solvents (glycerol, Cyrene, γ-valerolactone (GVL)) applied for organosolv in the literature by evaluating two mass-based green chemistry metrics (e.g., mass intensity, solvent intensity). Green chemistry benchmarking demonstrated that GDE organosolv is highly competitive with previous reports. Overall, the present study advances insights into the application of eco-friendly solvents to support organosolv extraction of lignins under mild conditions and supports sustainable biorefining of lignocellulosic biomass.