Optimization of delignification from Douglas fir sawdust by alkaline pretreatment with sodium hydroxide and its effect on structural and chemical properties of lignin and pyrolysis products

The extraction of softwood lignin from Douglas fir (Pseudotsuga menziesii) through alkaline pretreatment was optimized at moderate treatment conditions. A central composition experimental design (CCD) was used to investigate the effect of three extraction factors: extraction temperature (63.2–96.8 °C), sodium hydroxide loading (0.10–0.60 g/g feedstock) and heating time (2.6–9.4 h). Response surface methodology indicates the interactive effects of sodium hydroxide loading and extraction temperature on enhancing the recovery of solid alkaline lignin at intermediate conditions. Optimization analysis of alkaline lignin extraction was performed with the emphasis on product yield and process economy, respectively. Extracted alkaline lignins were characterized by FT-IR spectroscopy, UV–vis spectrometry, elemental analysis, and pyrolysis product analysis. The diversity of functional groups was retained and removal of methoxyl groups was significantly impacted by alkali loading. Extraction condition of alkaline lignin affected product yield and chemical compound distribution in bio-oil from lignin pyrolysis.