Compositional Changes to Low Water Content Bio-oils during Aging: An NMR, GC/MS, and LC/MS Study

Abstract

Bio-oil generated by the fast pyrolysis of biomass is an unstable material, undergoing chemical and physical transformations as the oil ages at room temperature. In this study, electrostatic precipitator (ESP) pine wood-derived bio-oil, which contains less water and does not undergo phase-separation upon aging, was characterized following accelerated aging. Bulk oil properties (percent water and viscosity) were found to increase in ways similar to conventional bio-oils. The unaged and aged bio-oil samples were characterized by gel permeation chromatography (GPC), solvent fractionation, solution 13C NMR, gas chromatography/mass spectrometry (GC/MS), and chip-based nanoelectrospray ionization, liquid chromatography, quadrupole time-of-flight (nanoESI-LC-Q-TOF) MS/MS. Using the formation of the silyated derivatives to extend the range of detectable compounds, GC/MS analysis was used to identify specific compounds that showed elevated reactivity, extending the understanding of reactivity characteristics beyond the known reactivity of aldehydes and some aromatics to distinguishing the reactivity of ring-conjugated aromatics and certain polyhydroxylated benzenes, specifically the 1,3-di-, 1,2,3-tri-, and 1,2,4-trihydroxy substituted compounds. To explain the enhanced reactivity of these compounds, we propose acid-catalyzed formation of quinone methides as important intermediates. Additionally, we find significant changes to the composition of mono- and disaccharides, where specific monosaccharides (arabinose, xylose, and glucose) increased in concentration with aging and high reactivity was observed for certain sugars with furano-ring mass spectral characteristics. In contrast, we also found that three anhydrosugars (levoglucosan, mannosan, and galactosan) were largely stable with respect to aging. High mass resolution nanoESI-LC/MS/MS analyses of peracetylated samples permitted the analysis and chromatographic separation of both lignin and carbohydrate-derived oil components and were used for the identification of a putative formaldehyde–trihydroxybenzene dimer. This work provides further insights into chemically specific entities and the processes responsible for bio-oil aging.

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

“…This is supported by Joseph et al [1], who found catechols to have a reduced reactivity in model studies at 80°C for a period time of 40 h. A reduced activity of catechol rings can be ascribed to the lack of ring activation due to counteracting effects when two hydroxyl groups are substituted in a 1,2-pattern. Also, retene (or 7-isopropyl-1-methyl phenanthrene) was found to be relatively stable.…”

“…The yield of insolubles in the aged LO (aLO_EIS_80) and the aged HO (aHO_TIS_80) after 1 week of ageing is 0.5 and 3.1%, respectively, showing that the ageing rate of HO is faster than that of LO at 80°C for 1 week. The formation rate of insolubles (aLO_EIS_80 and aHO_TIS_80) in this study is relatively low compared to the biomass-pyrolysis bio-oil [1,7,[13][14][15]. The LO and HO produced by the HTL of lignin have, thus, a comparably high degree of stability at 80°C.…”

“…Although relatively few studies can be found in the literature regarding the stability of bio-oil during storage, and particularly at elevated temperatures, there are some regarding bio-oil produced from biomass-pyrolysis [1]. However, the biomasspyrolysis bio-oil has high contents of water (20-30%), oxygen (45-60%) and acid (pH 2.0-4.0), which make it thermally and chemically unstable [2].…”

“…The amount of insolubles in biomasspyrolysis bio-oil was found to increase considerably during accelerating ageing at elevated temperature (i.e. 0.5-26%) in various studies [1,7,[13][14][15]. Further important properties to study are the variations in the chemical structural changes (gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR)) and the molecular weight (Mw) during ageing.…”

Accelerated aging of fast pyrolysis bio-oil: a new method based on carbonyl titration

“…17,18 Furthermore, the phase stability of FP bio-oils has recently been reviewed. 19 The chemical composition of aged biooils has been reported, [20][21][22][23][24][25] and other advanced characterizations have been performed. 26 It has also been shown that bio-oil aging is an exothermic process.…”