Comparisons of alkali, organosolv and deep eutectic solvent pre-treatments on the physiochemical changes and lignin recovery of oak and pine wood

Pre-treatment is a crucial and the most energy-intensive step for successful valorisation of lignocellulosic biomass in a biorefinery. This study provides systematic comparisons of the biomass fractionation and physiochemical changes of oak and pine wood waste using three pre-treatment methods: 1). an alkali process (2.5 M NaOH and 0.4 M Na2S), 2). an Organosolv process (70 % ethanol in water with 1 % w/w H2SO4), and 3). an emerging Deep Eutectic Solvent (DES) process (p-Toluenesulfonic acid: Choline Chloride: Glycerol in 2:1:1 molar ratio). Among the three pre-treatments, DES exhibits the highest process efficiency, achieving a solid residue weight loss of 43.5 % (oak) and 35.3 % (pine) corresponding to the highest delignification yield of 66.1 % (oak) and 79.2 % (pine) at a much shorter time (30 min vs 180 min) and less energy consumption (1/5 to 1/8) than alkali and Organosolv. DES-treated wood shows higher sensitivity to thermal degradation at lower temperatures, indicating better potential as a solid fuel. It also shows a lower degree of crystallinity of 28.6 % (oak) and 26.7 % (pine) and higher phenolic content, suggesting potentially better enzymatic susceptibility during bioconversion. On the contrary, Organosolv-treated wood presents preserved crystallinity of 41.6 % (oak) and 35.5 % (pine) and macro-/nano-structures, indicating potentially better mechanical properties for functional materials. It is hoped that this study will provide insights to informing sustainable solvent selections in biorefinery and biomass product development for value-added applications.