Chemical Diversity of Oligomers in Biomass Fast Pyrolysis Oils, Part 1: Dimers, Extractives, Humin-Like Compounds, and Hybrid Oligomers from Dichloromethane-Soluble Pyrolytic Lignin

Lignocellulosic biomass-derived products are of increasing interest as renewable and nonfood-competitive carbon sources. Fast pyrolysis can efficiently convert biomass into bio-oil (BO), offering potential advantages such as easy storage and transport. However, BO’s high oxygen content and instability can result in low calorific values and rapid aging, requiring further upgrading. This work is the first in a three-part series aimed at thoroughly analyzing BO through advanced separation and characterization techniques. This series introduces an expanded method involving bio-oil fractionation by cold-water precipitation, followed by dichloromethane (DCM) extraction and silica gel column chromatography using solvents of increasing polarity. This specific paper focuses on characterizing the water-insoluble DCM-soluble fraction of pine BO, also known as DCM-soluble pyrolytic lignin (PL). This fraction constituted 15.5 wt % of the BO and was further separated using a novel open-column chromatography method into three distinct subfractions: ethyl ether (56 wt %), ethyl acetate (23 wt %), and methanol (20 wt %). Comprehensive characterization using UV fluorescence, FT-IR, 1H, 13C, and 2D HSQC NMR, and FT-ICR MS revealed significant compositional differences among the fractions. This fraction exhibited varying elemental ratios (H/C: 1.20:1.26; O/C: 0.26:0.38; double bond equivalent (DBE): 9.15:9.46), positioning them between the water-soluble fraction (part 3) (H/C: 1.41:1.73; O/C: 0.69:0.81; DBE: 4:8) and the water-DCM insoluble fraction (part 2) (H/C: 0.82:1.02; O/C: 0.26:0.36; DBE: 14:20). Phenolic dimers dominated all DCM-soluble PL, but differed in associated compound types: the ethyl ether fraction was enriched in terpenoids and lipids, the ethyl acetate fraction featured hybrid carbohydrate–lignin and larger lignin oligomers and moderately dehydrated sugars (humin-like compounds), and the methanol fraction contained hybrid carbohydrate–lignin oligomers and moderately dehydrated sugars. Representative structures proposed for each fraction illustrate their diverse chemical compositions. Thus, this work provides a detailed compositional analysis of PL and introduces a successful fractionation strategy to concentrate and elucidate its constituent compounds.