Thermal stability of low and high Mw fractions of bio-oil derived from lignin conversion in subcritical water

摘要

The thermal stability of bio-oil influences its application in industry and is, therefore, a very important factor that must be taken into consideration. In this study, the stability of low and high molecular weight (Mw) fractions of bio-oil obtained from the hydrothermal liquefaction (HTL) of lignin in subcritical water was studied at an elevated temperature (80 A degrees C) for a period of 1 h, 1 day and 1 week. The changes in molecular weight (gel permeation chromatography (GPC)) and chemical composition (gas chromatography-mass spectrometry (GC-MS) and 2D heteronuclear single quantum correlation (HSQC) NMR (18.8 T, DMSO-d(6))) of low and high Mw fractions of the HTL bio-oil (i.e. light oil (LO) and heavy oil (HO)) were evaluated before and after ageing. It was found that only a slight formation of high Mw insoluble structures was obtained during ageing at elevated temperature for 1 week: 0.5% for the LO and 3.1% for the HO. These higher Mw moieties might be formed from different polymerisation/condensation reactions of the reactive compounds (i.e. anisoles, guaiacols, phenols, methylene (-CH2-) groups in phenolic dimers and xanthene). The high Mw insolubles in both the LO and the HO were analysed for structural composition using 2D HSQC NMR to obtain a better understanding of the changes in the composition of bio-oil fractions during the accelerated ageing process. In addition, a chemical shift database in DMSO-d(6) was analysed for a subset of phenolic model compounds to simplify the interpretation of the 2D HSQC NMR spectra.