Mild thermolytic solvolysis of technical lignins in polar organic solvents to a crude lignin oil

A mild thermal solvolysis process using alcohols for the valorization of technical lignin into crude lignin oil (CLO) is presented. The solubilization process results in lower molecular weight lignin fragments (1250-1550 g/mol cf. 2500 g/mol of parent lignin), while rejecting heavy compounds and other solid impurities. The quantitative and qualitative influence of reaction temperature (100-350 °C), residence time (0.5-4 h), lignin:solvent ratio (1:15-1:2 w/v) and alcohol solvent is investigated. At high lignin loading (ratio < 1:5 w/v) and optimum conditions for lignin solubilization (T = 200 °C, t = 0.5 h), the condensation reactions and solvent consumption is minimized. Methanol exhibits the highest solvolytic efficacy solubilizing also some heavier lignin fractions originating from condensation reactions of reactive intermediates. Hansen solubility parameters are employed to discuss the effect of solvent on the solubilization process. Gel permeation chromatography and heteronuclear single quantum coherence NMR of solubilized fractions revealed cleavage of β-Ο-4 bonds during thermal solvolysis, explaining the molecular weight reduction. Methanol is the most favourable solvent and is utilized in solubilization of 5 different biorefinery lignins. In all cases, this led to CLO with a lower molecular weight of the lignin fragments, a lower polydispersity and an increased hydroxyl group content.