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 31P 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 fractionalthough it only represents around 30 wt % of the whole oilcontains 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 1H NMR, FTIR, and UV fluorescence spectroscopies. 1H 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.
This paper reports a study of the chemical composition of the water-soluble (WS) fraction obtained by cold water precipitation of two commercial wood pyrolysis oils (BTG and Amaron). The fraction studied accounts for between 50.3 and 51.3 wt % of the oils. With the most common analytical techniques used today for the characterization of this fraction (KF titration, GC−MS, hydrolyzable sugars, and total carbohydrates), it is possible to quantify only between 45 and 50 wt % of the fraction. Our results confirm that most of the total carbohydrates (hydrolyzable sugars and nonhydrolyzable) are soluble in water. The ion chromatography hydrolysis method showed that between 11.6 and 17.3 wt % of these oils were hydrolyzable sugars. A small quantity of phenols detectable by GC−MS (between 2.5 and 3.9 wt %) were identified. It is postulated that the unknown high molecular weight fraction (30−55 wt %) is formed by highly dehydrated sugars rich in carbonyl groups and WS phenols. The overall content of carbonyl, carboxyl, hydroxyl, and phenolic compounds in the WS fraction was quantified by titration, the Folin−Ciocalteu method, 31 P NMR, and 1 H NMR. The WS fraction contains between 5.5 and 6.2 mmol/g carbonyl groups, between 0.4 and 1.0 mmol/g carboxylic acid groups, between 1.2 and 1.8 mmol/g phenolic −OH, and between 6.0 and 7.9 mmol/g of aliphatic alcohol groups. Translation into weight fractions of the WS was done by supposing surrogate structures for the water-soluble phenols, carbonyl groups, and carboxyl groups, and we estimated the content of WS phenols (21−27 wt %), carbonyls (5−14 wt %), and carboxyls (0−4 wt %). Together with the total carbohydrates (23−27 wt %), this approach leads to >90 wt % of the WS material in the bio-oils being quantified. We speculate the larger portion of the difference between the total carbohydrates and hydrolyzable sugars is the missing furanic fraction. Further refinement of the suggested methods and development of separation schemes to obtain and quantify subfractions with homogeneous composition (e.g., carbohydrates, high molecular weight WS phenols, furans, and dehydrated sugars) warrant further investigation.
Thin films (∼115 μm thick) of milled wood lignin from hybrid poplar and acid-washed hybrid poplar were pyrolyzed at 500 °C and ∼55 °C/s at five pressures (4, 250, 500, 750, and 1000 mbar) to determine the impact of secondary liquid intermediate reactions on the product distribution. For both milled wood lignin extracted from poplar and acid-washed hybrid poplar wood, pressure had a significant effect on the product distribution for thin film pyrolysis between 4 and 1000 mbar. For lignin, lowering the pressure from 1000 mbar to 4 mbar reduced the char yield from 36 to 23% and enhanced production of large cluster pyrolytic lignin. However, the pressure did not dramatically impact the gas yield (CO 2 , CO, methane, H 2 , ethane, propane, and butane), nor did it significantly impact the release of monomeric phenolic compounds. ICR-MS shows limited changes in the composition of lignin oligomers. The increase in the production of large lignin oligomers observed by UV fluorescence and the reduction of char yield with vacuum confirm the importance of oligomeric combination reactions to form large polyaromatic structures in the liquid intermediate. For hybrid poplar, lowering the pressure from 1000 mbar to 4 mbar decreased the char yield from 19 to 7% and enhanced production of heavy sugars (cellobiosan and cellotriosan). ICR-MS results clearly show the importance of dehydration reactions in the liquid intermediate. Lowering the pressure also enhanced production of CO, CO 2 , and methane due to heterogeneous catalysis by residual alkali and alkaline earth metals in the solid wood matrix. However, it also decreased production of levoglucosan from 10 to 6.1 wt %. The yields of levoglucosan and cellobiosan obtained for hybrid poplar were higher and lower, respectively, compared with those expected if the pyrolysis products were the result of the additive contribution of hybrid poplar constituents. This result could be explained by the tendency of lignin liquid intermediate to bubble vigorously, contributing in this way to the removal of cellulose oligomers from the liquid intermediate.
In this work, we examine the evolution of functional groups (carbonyl, carboxyl, phenol, and hydroxyl) during hydrotreatment at 100–200 °C of two typical wood derived pyrolysis oils from BTG and Amaron in a batch reactor over Ru/C catalyst for reaction time of 4 h. An aqueous and an oily phase were obtained. The contents of the functional groups in both phases were analyzed by GC/MS, 31P NMR, 1H NMR, CHN, KF titration, UV fluorescence, carbonyl groups by Faix and phenols by Folin−Ciocalteu method. The consumption of hydrogen was between 0.007 and 0.016 g/(g of oil), and 0.001–0.020 g of CH4/(g of oil), 0.005–0.016 g of CO2/(g of oil), and 0.03–0.10 g of H2O/(g of oil) were formed. The contents of carbonyl, hydroxyl, and carboxyl groups in the volatile GC-MS detectable fraction decreased (80, 65, and ∼70%, respectively), while their behavior in the total oil and hence in the nonvolatile fraction was more complex. The carbonyl groups initially decreased having a minimum at ∼125–150 °C and then increased, while the hydroxyl groups had a reversed trend. This might be explained by the initial hydrogenation of the carbonyl groups to form hydroxyls, followed by continued dehydration reactions at higher temperatures that may have increased their content. The 31P NMR analysis was on the limit of its sensitivity for the carboxylic groups to precisely detect changes in the upgraded nonvolatile fraction; however, the more precise titration method showed that the concentration of carboxylic groups in the nonvolatile fraction remains constant with increased hydrotreatment temperature. The UV fluorescence results show that repolymerization increases with temperature, starting as low as 125 °C. ATR-FTIR method coupled with deconvolution of the region between 1490 and 1850 cm–1 was shown to be a good tool for following the changes in carbonyl groups and phenols of the stabilized pyrolysis oils. The deconvolution of the IR bands around 1050 and 1260 cm–1 correlated very well with the changes in the 31P NMR silent O groups (likely ethers). Most of the H2O formation could be explained from the significant reduction of these silent O groups (from 12% in the fresh oils, to 6 to 2% in the stabilized oils) most probably belonging to ethers.
In this perspective, we discuss the standardization of analytical techniques for pyrolysis biooils, including the current status of methods, and our opinions on future directions. First, the history of past standardization efforts is summarized, and both successful and unsuccessful validation of analytical techniques highlighted. The majority of analytical standardization studies to-date has tested only physical characterization techniques. Here, we present results from an international round robin on the validation of chemical characterization techniques for bio-oils. Techniques tested included acid number, carbonyl titrations using two different methods (one at room temperature and one at 80 °C), 31 P NMR for determination of hydroxyl groups, and a quantitative gas chromatography-mass spectrometry (GC-MS) method. Both carbonyl titration and acid number methods have yielded acceptable inter-laboratory variabilities. 31 P NMR produced acceptable results for aliphatic and phenolic hydroxyl groups, but not for carboxylic hydroxyl groups. As shown in previous round robins, GC-MS results were more variable. Reliable chemical characterization of bio-oils will enable upgrading research and allow for detailed comparisons of bio-oils produced at different facilities. Reliable analytics are also needed to enable an emerging bioenergy industry, as processing facilities often have different analytical needs and capabilities than research facilities. We feel that correlations in reliable characterizations of bio-oils will help strike a balance between research and industry, and will ultimately help to determine metrics Perspective: Bio-oil Analytical Standardization JR Ferrell III et al.
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