The efficient fractionation and production of individual biomass components is pivotal process in biorefinery concept. However, the recalcitrant nature of lignocellulose biomass is one of the main obstacles in wider application of biomass-based chemicals and materials. In this study, thiourea was used as nucleophilic fractionation chemical in aqueous acidic condition. At relatively mild conditions (100°C for 30 – 90 min), around 90% of lignin could be removed softwood and the obtained solid carbohydrate fraction exhibited mild yellowish color, being contradict to many delignification processes where severe darkening of the fiber fraction has been observed. Cellulosic fraction had high degree of polymerization and was easily bleached to bright white using halogen-free bleaching, and both bleached and non-bleached cellulose fibers could be easily disintegrated into cellulose nanofibers by mechanical treatment. Dissolved lignin could be precipitated from washing liquor with high yield (around 90%). Precipitated lignin was light in color and after alkaline treatment, it could be dissolved in organic solvents. In addition, small amount of water-soluble lignin was obtained and both alkaline treated and water-soluble lignin had high molecular weight (around 40 000 g/mol). Tentative reaction mechanism of fractionation is via formation of isothiouronium-modified lignin during the reaction, allowing the dissolution of lignin in reaction solution in relatively mild condition. The formation of isothiouronium minimize the degradation and condensation of lignin during the fractionation, which in turn results in production of high molecular weight lignin and prevents the darkening carbohydrate and lignin fractions.

Acidic thiourea fractionation of softwood lignocellulose

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