Sequential solvent fractionation of lignin for selective production of monoaromatics by Ru catalyzed ethanolysis

Abstract

The purpose of this study was to investigate the impact of lignin structure, especially its average molecular weight (M_w) on the distribution of catalytically depolymerized products. Soda lignin (SL) was sequentially fractionated by organic solvents (ethyl acetate: F1, methanol: F2, acetone: F3, dioxane/water: F4, and insoluble fraction: F5). The M_w of SL, F1, F2, F3, and F4 were 2800, 1120, 2860, 5850, and 7200 Da, while that of F5 could not be determined due to its low solubility in GPC solvent. 2D-HSQC-NMR analysis revealed that the relative abundance of the β-O-4 linkage increased upon increasing the M_w of the lignin fraction. Each fraction was efficiently depolymerized into lignin-oil under the combination of supercritical ethanol (350 °C) and Ru/C (5 wt%). The yield of lignin-oil, mixture of monoaromatic compounds, as well as high molecular phenolics, ranged from 62.5 to 81.4 wt%. The yield of total monoaromatic compounds in lignin-oil varied from ca. 3.4 to 10 wt% depending on the lignin fraction. The selectivity of monoaromatic compounds produced from the lignin depolymerization process was clearly affected by the M_w value of lignin. Specifically, F1 and F2 yielded higher amounts of non-alkylated phenols, methylated-, and ethylated phenols compared to the other lignin fractions.