Accelerated aging of bio-oil from lignin conversion in subcritical water

Abstract

Accelerated aging of bio-oil derived from lignin was investigated at different aging temperatures (50°C and 80°C) and times (1 hour, 1 day, 1 week, and 1 month). The bio-oil used was produced by the hydrothermal liquefaction of kraft lignin, using phenol as the capping agent, and base (potassium carbonate and potassium hydroxide) and zirconium dioxide as the catalytic system in subcritical water. Elemental composition, molecular weight (by using gel permeation chromatography), and chemical composition (by using gas chromatography–mass spectrometry and 2D nuclear magnetic resonance [18.8 T, DMSO-d6]) of the bio-oil were measured to gain better understanding of the changes that occurred after being subjected to an accelerated aging process. The ligninderived hydrothermal liquefaction bio-oil was quite stable compared with biomass-pyrolysis bio-oil. The yield of the low molecular weight fraction (light oil) decreased from 64.1% to 58.1% and that of tetrahydrofuran insoluble fraction increased from 16.5% to 22.2% after aging at 80°C for 1 month. Phenol and phenolic dimers (Ar–CH2–Ar) had high reactivity compared with other aromatic substituents (i.e., methoxyl and aldehyde groups); these may participate in the polymerization/condensation reactions in the hydrothermal liquefaction bio-oil during accelerated aging. Moreover, the 2D heteronuclear single quantum coherence nuclear magnetic resonance spectra of the high molecular weight fraction (heavy oil) in the aged raw oil in the aromatic region showed that the structure of this fraction was a combination of phenol-alkyl patterns, and the guaiacol cross-peaks of Ar2, Ar5, and Ar6 after aging indicate that a new polymer was formed during the aging process.