(482b) HPLC/Qtof-MS for Analysis of Oligomeric Compounds (pyrolytic lignin) from Crude Bio-Oils and Products from Pyrolysis of Model Compounds

Fast pyrolysis of biomass has become a popular way to obtain liquid products with complex mixtures, including medium-polar and polar monomers, water, and oligomeric compounds (or pyrolytic lignin, PL). Among these, the oligomeric fraction, accounted for about 20 wt.%, have negative effects on bio-oil and are difficult for further utilization. Therefore, it is of vital importance to understand the structure and formation of pyrolysis lignin when attempting to make full use of bio-oil.

In this work, pyrolysis bio-oil derived from three kinds of lignocellulosic biomass (Rice Straw, RS; Cotton Stem, CS; Walnut Shell, WS) were used to obtain PL samples (simplified as RS-PL, CS-PL, WS-PL) as fine homogeneous powder by precipitation method. Besides, aromatic monomers (guaiacol, eugenol, vanillin) and acetic acid were mixed together as model compounds of bio-oil vapor for fast pyrolysis at 550℃ to simulate the actual pyrolysis condition to study the formation of pyrolytic lignin. The products were collected as model compounds pyrolysis sample (M-PL). All the samples mentioned above were dissolved in methane for further analysis.

Since high-performance liquid chromatography coupled with UV detector and quadrupole-time-of-flight mass spectrometry (HPLC/QTOF-MS) is a powerful tool for separation and structural analysis of big unknown molecules in complex mixtures, it is used here to obtain molecular weight and structure information on the major components of the three PL samples and model compounds pyrolysis sample.

The results show that the UV chromatograms and total ion chromatograms (TIC) of the three PL samples have high similarity on distribution and intensity of chromatographic peaks, indicating a lot of compounds in PL samples are similar. Combined with TOF MS spectra and TOF MS^2 spectra, molecular formula and possible structure of the common components are confirmed, such as C₁₉H₁₈O₄, C₁₈H₁₈O₄, C₁₇H₁₆O₄, C₁₆H₁₄O₄, C₁₃H₁₂O₂, C₁₂H₁₀O₂, C₉H₁₂O₂, C₈H₁₀O₂. They are mostly aromatic dimers and aromatic monomers. RC-PL sample was then taken as a representative to compare with M-PL sample and some similarities in UV and TIC chromatograms were found as well. According to extra ion chromatogram (XIC), aromatic monomers (100≤Mm≤180, 4≤DBE<8) of the two samples match really well, and a certain number of aromatic dimers or trimers (180<Mm≤400, DBE≥8) with high intensity are also found to be the same. The results indicate that the oligomeric fraction of crude bio-oil may form from aromatic monomers through free-radical polymerization during the pyrolysis process. Furthermore, seven compounds are proposed to be considered as typical components in oligomers fraction. The molecular formulas (m/z, DBP) are C₁₆H₁₄O₂ (237.0921, 10.0), C₁₄H₁₂O₂ (211.0765, 9.0), C₁₄H₁₀O₃ (225.0557, 10.0), C₁₃H₁₂O₂ (199.0765, 8.0), C₁₃H₁₀O₂ (197.0608, 9.0), C₁₂H₁₀O₂ (185.0608, 8.0), C₁₂H₈O₂ (183.0452, 9.0).