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Characterization of Lignin Dissolved During Alkaline Pretreatment of Softwood and Hardwood

Various alkaline pretreatments were applied to Scots pine (Pinus sylvestris) and silver/white birch (Betula pendula/pubescens) wood chips and the characterization of sulfur-free lignin dissolved was performed. The behavior of lignin during these pretreatments (alkali charge 1–8% NaOH, time 30–120 minutes, and temperature 130–150°C) was studied mainly in terms of lignin removal efficiency and molar mass distribution of dissolved lignin. The amount of lignin in pretreatment liquors increased along with an increase in the alkali charge of 0.8–4.4% and 0.6–3.4% of o.d. pine and birch wood, respectively. The most significant parameter affecting the molar mass of the dissolved lignin was shown to be the alkali charge, since an increase in this parameter clearly increased the -values of dissolved lignin, varying from 2,260 g/mol to 7,050 g/mol and from 2,200 g/mol to 5,550 g/mol for pine and birch lignin, respectively.