Chemical Diversity of Oligomers in Biomass Fast Pyrolysis Oils. Part 3: Water-Soluble Fraction

Bio-oils from the fast pyrolysis of lignocellulosic materials are complex mixtures with numerous compounds. This study, the third in a series, characterizes the BTG bio-oil water-soluble fractions commonly known as pyrolytic sugars. The bio-oil was initially fractionated via cold water precipitation, yielding a water-soluble fraction representing 51.5 wt % of the original oil. Subsequent chromatography using Sepabeads resin separated this fraction into sugars (55.6 wt %) and phenols (44.4 wt %). The sugars were concentrated through rotary evaporation and further fractionated using silica gel column chromatography with solvents of increasing polarity (dichloromethane (DCM), tetrahydrofuran (THF), ethyl acetate (EA), methanol (MeOH), and water (H2O)). Each fraction underwent comprehensive analysis via FTIR, UV-Fluorescence, Py-GC/MS, NMR, ESI, and HESI-FT Orbitrap MS. Generated chemical formula representations indicated that fractions had average O/C ratios between 0.69 and 0.84 and H/C ratios between 1.41 and 1.76, comparable to cellulose and hemicellulose pyrolysis derivatives (O/C = 0.833, H/C = 1.66). Increased solvent polarity correlated with higher O/C and H/C values suggesting higher content of phenols in the DCM and THF soluble fractions. EA, MeOH, and H2O fractions were rich in sugars with a low degree of dehydration and fragmentation. Sugars eluted with H2O had higher molecular weights than those eluted with EA and MeOH. Molecular representations for each fraction were proposed to enhance our understanding of the chemistry of pyrolytic sugars. Summarizing this series, an expanded analytical method was proposed, categorizing bio-oil oligomers into nine subgroups based on solubility and solvent polarity. These groups ranged from sugars with varying dehydration levels to lignin-derived oligomers, terpenoids, lipids, monophenols, and volatile compounds. This comprehensive separation scheme significantly advances the characterization and understanding of pyrolysis oil fractions.