Quantification of Bio-Oil Functional Groups and Evidences of the Presence of Pyrolytic Humins

Quantification of functional groups (carbonyl, carboxyl, hydroxyl, phenolics) in biomass-derived pyrolysis oils is crucial to advance our understanding of bio-oil compositional changes during production, storage, aging, and upgrading. Traditionally, most of the methods reported in the literature on this subject are based on titration. There are very few studies on the use of spectroscopic techniques for the quantification of functional groups in bio-oils. The distribution of functional groups between the volatile and the heavy fraction is also very poorly understood. The content of functional groups in the volatile fraction estimated by GC/MS was compared with their content in the total oil determined by titration and P-31 NMR. The carbonyl groups are almost equally distributed between the volatile and the oligomeric fractions. The content of total phenols varies between 1.6 and 3.1 mmol/g. It is important to note that between 85 and 95% of the phenols in bio-oil are in the form of oligomers. The content of carboxylic acids varies between 1.1 and 2.1 mmol/g. Between 52 and 66% of these acids were detectable by GC/MS, and the rest is in the oligomeric form. These results confirm that the GC/MS-detectable fraction-although it only represents around 30 wt % of the whole oil-contains more than half of the very reactive carbonyl and carboxyl functional groups of the oil. Our results suggest that as an average 56% of all the oxygen derived from the carbohydrate fraction that is collected in the oil is in the form of water. Around 20% is in the form of carbonyl groups, close to 12% is in the form of carboxylic groups, and only 17% is in the form of OH in aliphatic chains. This result clearly shows the importance of dehydration reactions (close to 70% of the oxygen in the oil is in the form carbonyl or water). The oil was studied by FT-ICR-MS. The heavy fraction is composed of oligomeric materials with up to 29 carbon atoms and 17 oxygen atoms. The Van Krevelen plots of the nonvolatile fraction show for the first time the existence of heavy unknown water-soluble oligomers produced by the gradual dehydration of cellulose primary depolymerization products. This unknown fraction is herein called pyrolytic humin. The oils were also analyzed by H-1 NMR, FTIR, and UV fluorescence spectroscopies. H-1 NMR results confirm that, with appropriate calibrations, this technique could be used to quantify the content of phenols and water. The correlations observed between FTIR spectra and titration results confirm that, with appropriate calibrations, this technique can be used for the quantification of water, carboxylic acids, and phenolics in bio-oils. A good correlation was obtained between the total content of phenols measured by Folin-Ciocalteu and the area of the UV fluorescence peaks.