PNNL

Abstract

This paper, the second in a series, investigates the water-insoluble CH2Cl2-insoluble fraction known as high molecular weight pyrolytic lignin (HMW-PL) from BTG biomass pyrolysis oil. HMW-PL accounts for 9.2 wt.% of the original oil, increases upon aging, and is associated with coke formation during bio-oil upgrading. Understanding its composition is essential for controlling pyrolysis oil behavior in processing. HMW-PL was fractionated using a silica gel column with solvents of increasing polarity (ethyl acetate (EA), acetone (AC), isopropanol (ISO), and methanol (MeOH)). The majority (49.2 wt.%) eluted with EA, followed by AC (19.9 wt.%), ISO (8.2 wt.%), and MeOH (8.1 wt.%), while 14.1 wt.% remained in the column. The resulting subfractions were characterized by UV-fluorescence, FTIR, NMR (13C, 1H), and (-)APCI-FT-Orbitrap MS. Subfractions showed decreasing molecular weights and DBE (double bond equivalence) values (20 to 14) with increasing solvent polarity and had average H/C ratios (0.82-1.02) lower than the water–insoluble–CH2Cl2 soluble fraction (1.20-1.26), but comparable O/C ratios (0.26-0.36). Compared to the water-soluble fraction (on average, H/C: 1.41-1.73, O/C: 0.69-0.81), the water-insoluble fractions were lower in oxygen and hydrogen, indicating the H/C ratio's usefulness in predicting bio-oil oligomer solubility. UV-fluorescence revealed conjugated ring systems in all fractions. The EA fraction contained predominantly lignin-derived dimers and trimers, while the AC, ISO, and MeOH fractions had fewer aromatics but more humins and hybrid oligomers. FTIR supported significant compositional differences among different fractions, and NMR confirmed methyl-aromatic structures consistent with lignin derivatives. (-)APCI-FT-Orbitrap MS detected dimers and trimers (C#: 13~20, O#: 5), mostly with 180-400 Da molecular weights. This fractionation approach enabled the definition of four distinct subfractions and the proposal of surrogate molecular structures for each, enhancing the understanding of HMW-PL composition.