Predictive modelling and optimization of lignin extraction efficiency and quality in birch-wood mild ethanosolv fractionation in a semi-continuous flow-through reactor

Lignin, a complex and abundant biopolymer found in plants, holds immense potential for sustainable materials and chemicals. However, conventional extraction methods often lead to structural deterioration of the native-like aryl ether structure <i>via</i> condensation and other chemical alterations, limiting lignin utility. High delignification in conjunction with preservation of the versatility and functionality of lignin structure for high-value applications can be achieved using advanced mild extraction techniques. In this study, an integrated modeling-experimental approach is used to attain a scalable framework for lignin-first biorefining. Temperature and flow rate were optimized in a flow-through mild ethanosolv system utilizing crude birch-wood chips (without extractive-removal) to balance solvent use, delignification, and structure preservation. Delignification and lignin yield were monitored separately as was its quality in terms of preservation of its native aryl-ether structure, as determined <i>via</i> 2D HSQC NMR and GPC. Extraction kinetics were monitored using UV-Vis spectroscopy to allow for maximizing efficient solvent utilization. Response surface methodology identified optimal conditions (145-151 °C, 8 g_solvent min^-1 flow rate), revealing temperature as the primary driver for extraction, exhibiting synergistic effects with the flow rate. Notably, higher flow rates at elevated temperatures (≥140 °C) mitigated β-O-4 linkage degradation without compromising delignification efficiency. Experimental validation of the optimized model at 150 °C and 8 g_solvent min^-1 achieved 82 wt% delignification and yielded lignin with high β-O-4 linkage content (59.4 per 100 aromatic units (ArU)), aligning closely with model predictions (81-87 wt%, ≥52 β-O-4 per 100 ArU). Solvent consumption was optimized from the model (13.1 mL g^-1, solvent : biomass) and realized a reduction of over 40% of solvent consumption when compared with solvent consumption from typical batch organosolv systems (22.9 mL g^-1, solvent : biomass). Finally, the optimization reduced the extraction time significantly from typically 2 hours to 30 min when compared with previous standard extraction conditions (120 °C, 2 g_solvent min^-1 flow rate), without compromising on extraction efficiency and lignin quality. This study shows the potential of mild organosolv extraction with alcohol with optimized conditions.