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

Abstract

A mild thermal solvolysis process using alcohols for the valorization of technical Protobind soda lignin into crude lignin oil (CLO) is presented. The solubilization process results in lower molecular weight lignin fragments (1250–1550 g mol⁻¹cf. 2500 g mol⁻¹ of parent lignin), while rejecting heavy compounds and other solid impurities. The influence of the reaction temperature (100–350 °C), residence time (0.5–4 h), lignin : solvent ratio (1 : 15–1 : 2 w/v) and alcohol solvent (methanol, ethanol, 1-propanol, 1-butanol, and 1-octanol) on the amount and type of products is investigated. At a high lignin loading (ratio < 1 : 5 w/v) and under optimum conditions for lignin solubilization (T = 200 °C, t = 0.5 h), the condensation reactions and solvent consumption are minimized. Methanol exhibits the highest solvolytic efficacy resulting in an overall lignin solubilization degree of 61 wt%, which includes some heavier lignin fractions originating from condensation reactions. The other alcohols resulted in a lignin solubilization degree of 57 wt% for ethanol, 53 wt% for 1-propanol, 51 wt% for 1-butanol and 38 wt% for 1-octanol. The solvent losses based on GC-MS analysis of the obtained CLOs were 1.1 wt% for methanol, 1.4 wt% for ethanol and 2.2 wt% for 1-butanol. 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-O-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.