Elucidating the effect of the physicochemical properties of organosolv lignins on its solubility and reductive catalytic depolymerization

Abstract

Unsatisfactory solubility and recalcitrant condensed structures of isolated lignins (technical lignins) severely limit their processability into renewable materials, fuels, and chemicals. Therefore, a comprehensive understanding of the structural and physicochemical properties of isolated lignins is of great value in explaining their performance in the solubilization and further depolymerization to high-value applications. In this work, we investigated the solubilization and depolymerization performance of organosolv lignins from three different sources (beech (L01), birch (L02), and wheat straw (L03)). We applied sequential solvent fractionation as a tool to subdivide parent lignins into four more homogeneous subfractions (SF1, SF2, SF3, and IF), with a gradual increase in molecular weight and interunit linkages and a decrease in phenolic –OH group from SF1 to IF. The solubility tests of the parent lignins and their subfractions at ambient conditions were carried out in six organic solvents (n-hexane, diisopropyl ether, ethyl acetate, ethanol, methanol, and acetone) with large differences in Hansen solubility parameters. Among the solvents studied, the solvents with δp > 8 MPa1/2 demonstrated a high solubilization yield of organosolv parent lignins and their subfractions. The solubilization behavior of parent lignins and subfractions was further described based on their inherent structural properties, such as molecular weight and functional group content (phenolic –OH and –COOH). The structure–reactivity characteristics of parent organosolv lignin and their subfractions were also investigated under different conditions in the presence of catalyst-containing transition (NiHTC-500) and noble (Pd/C) metals as active centers. Strong structural correlations between parent lignin/subfractions and depolymerized lignin oils were derived using GC-FID/MS, GPC, 1H–13C HSQC NMR, and 31P NMR.