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Quantification of Bio-oil Functional Groups and Evidences of the Presence of Pyrolytic Humins Filip Stankovikj, Armando G. McDonald, Gregory L. Helms, and Manuel Garcia-Perez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01242 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016
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Energy & Fuels
Quantification of Bio-oil Functional Groups and Evidences of the Presence of Pyrolytic Humins
Filip Stankovikj1, Armando G. McDonald3, Gregory L. Helms 2, Manuel Garcia-Perez1* 1
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120
2
Center for NMR Spectroscopy, PO Box 644630, Washington State University, Pullman, WA 99164–4630, USA 3
Renewable Materials Program, Department of Forest, Rangeland, and Fire Sciences, University of Idaho, Moscow, ID, 83844-1132, USA
Abstract: 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 1 ACS Paragon Plus Environment
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acids varies between 1.1 and 2.1 mmol/g. Between 52 and 66 % of these acids were detectable by GC/MS, the rest is in the oligomeric form. These results confirm that the GC/MS detectable fraction although 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 with oligomeric materials with up to 29 carbon atoms and 17 oxygen atoms. The Van Krevelen plots of the non-volatile 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.
Keywords: Fast pyrolysis, bio-oil, characterization, fractionation, FTIR, 1H-NMR, non-aqueous
titration,
FT-ICR-MS,
Van-Krevelen
diagrams,
carbohydrates, sulfuric acid assay, UV-Fluorescence 2 ACS Paragon Plus Environment
Folin
31
P-NMR,
Ciocalteu,
total
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*Corresponding author: Manuel Garcia-Perez, Associate Professor, Biological Systems Engineering Department, Washington State University e-mail:
[email protected] Phone number: 509-335-7758
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1.
Introduction
Over the last century the energy demand has been steadily growing, and the main driving factors for this trend, the growing population and increasing standards of living, show no signs of slowing down. The International Energy Agency projects 37% increase in energy demand by 2040.1 Biomass is a widely and readily available renewable resource and has the potential to satisfy a significant portion of this energy need as an important part of the future energy mix.2 Therefore, it is a realistic source of green carbon based fuels and chemicals. Developing advanced biofuels may not only offset the forthcoming environmental crisis by mitigating the effects of the global warming, but it may provide an incentive for spurring rural development, and stabilizing political systems by removing the dependence on foreign fossil fuel imports and developing domestic sources of energy.3,4 There are multiple biological and thermochemical technologies and strategies for the utilization of biomass for production of liquid fuels; fast pyrolysis followed by mild and deep hydrotreatment has been found as one of the most viable technologies for drop in biofuels production.5–9 In fast pyrolysis, the thermal decomposition of organic material, in absence of oxygen, is characterized by short residence times of the vapors, and moderate temperatures from 450 to 550°C in order to produce high, up to 75% on biomass dry basis, yield of liquid products.10
The main hurdle preventing large-scale utilization of fast pyrolysis oils, and hence more extensive development of biomass based economies are the problems associated with the refining of these highly oxygenated oils. The performance of bio-oils during storage, handling and upgrading depends not only of the oxygen content, but on the type and reactivity of these oxygen carrying functional groups.11,12 Our limited understanding of the content and distribution 4 ACS Paragon Plus Environment
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of bio-oil functional groups is a major hurdle for the development of rational strategies to overcome the refining problems currently encountered.
The bio-oils are comprised of compounds that can be classified as acids, alcohols, aldehydes, esters, ketones, phenols, guaiacols, syringols, sugars, furans, alkenes, aromatics, few nitrogen compounds, and miscellaneous oxygenates that are found in low concentrations. In the last decade several groups have investigated pyrolysis oil in terms of its composition, its chemical and physical properties.13–15 A review of thermal, chromatographic, mass spectroscopic, infrared and NMR techniques was published by Bahng et al,16 and an overview of various fractionation schemes was given by Mohan et al.17 Bio-oil complexity is such that several analytical techniques need to be used for their chemical characterization. However, instead of qualifying individual compounds, one may be able to follow the basic chemistry of bio-oil aging and upgrading reactions by following the changes in the content of functional groups. The limited information on pyrolysis oil functional groups available is mostly based on titration methods.18 A brief summary of the methods reported in the literature for the quantification of carbonyl, acids, phenols, and carbohydrates follows.
Carbonyl group: The carbonyl group might be the single most important group to be followed in order to observe changes in pyrolysis oil composition during hydrotreatment or aging, as well as a relevant parameter to compare pyrolysis oils from different sources.11,12,19 The content of aldehydes and ketones in the volatile fraction of pyrolysis oils can be estimated from the GC/MS results.20 The total content of carbonyl can be obtained by an oximation reaction with hydroxylamine hydrochloride followed by titration of the products with base sodium
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hydroxide,18,21 and recently by derivatization with 4-(trifuoromethyl)phenylhydrazine followed by
19
F-NMR spectroscopy.22 Nicolaides in 1984 was the first to measure the carbonyl group
content of pyrolysis oils based on the oximation reaction method developed by Bryant et al.23 Faix introduced few modifications to this method, which now increased the throughput by lowering the reaction time, made it safer by replacing pyridine with triethanolamine, reduced the sample size and the dependence of the measurement from the amount of reagent left at reaction end.21 The Faix method has been found to be more reliable by accounting for the carbonyl content derived from carbohydrate fraction of the biomass and by demonstrating lower standard deviation in the interlaboratry tests.24,25There are very few studies on the use of IR spectroscopic techniques for semi-quantitative estimations of carbonyl group content in pyrolysis oils.21,26,27
Acids: The acidic species in pyrolysis oils contribute to its corrosive characteristics, impeding its use in standard petroleum refineries; furthermore, they have been considered to catalyze polymerization reactions. The content of volatile acids in pyrolysis oils is typically quantified by gas or liquid chromatography, and more recently by 31P-NMR.19,28,29 The standard titration test ASTM D664 has been developed for determination of acidity in mineral oils, however in many cases of pyrolysis oils the inflection points are weak and indistinguishable, and this method is almost not applicable for measuring acidic compounds with dissociation constants lower than 9. To remedy this problem and obtain better resolution of the titration curve many researchers tried different compounds as solvents, titrants and electrode electrolytes.18,30–33 Although semiquantitative estimation of acid groups has been reported for liquefied coal products,34–36 there are very few studies on the quantification of bio-oil acids by spectroscopic techniques.27
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Phenols: Estimation of phenols in natural products has always been of interest, and hence many methods based on titration, colorimetry, and spectroscopy have been developed.37 In the case of bio-oils the content of volatile phenols can be typically measured by GC/MS. The content of heavy oligomeric phenols (also known as pyrolytic lignin) has been typically measured by the cold water precipitation method.14,38–41 Quantification of phenols by UV spectroscopy using Folin-Ciocalteu (FC) reagent is sufficiently reliable and forgiving that it is a standard method for measuring phenol content of wine.42,43 This method reports the content of phenols in gallic acid equivalents. The chemistry behind this process is complex.37,44,45 Nevertheless, it has been proven that the response factors for various phenolic species are similar, their molar absorptivity is higher than that of the interfering species present in typical pyrolysis oils, and the interfering species such as levoglucosan and cellobiosan together with formic, acetic, and propionic acids and others have a low response factor.45 FC results are typically close to the quantification of phenols by tedious liquid-liquid extraction.46 It was not possible to find any reference on the use of spectroscopic techniques for direct quantification of total phenols in bio-oils.
Carbohydrates: There are many methods reported in the literature for total carbohydrate quantification (chromatography, electrophoresis, IR, light scattering, NMR) as summarized by Albalasmeh.47 Some of these techniques have also been applied to quantify carbohydrates in biooils.38,48–50 The content of carbohydrates in bio-oils has been reported as: free anhydrosugars, hydrolysable sugars and total carbohydrates.38 Simple, quick, and direct methods for estimation of carbohydrate content are in demand.38,51 The presence of carbohydrates in the pyrolysis oil is linked to their instability during upgrading, and tendency for catalyst coking due to formation of
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humin type polymers under certain conditions.52 It was not possible to find any report on the use of spectroscopic techniques for the direct quantification of carbohydrates in bio-oils.
Our literature review confirms the limited information on the use of spectroscopic techniques for the quantification of functional groups in bio-oils. Spectroscopic techniques are widely used tools for the quantification of functional groups. In this paper we made thorough characterization of pyrolysis oils and we established correlations between the functional groups obtained by titration, chromatography and mass spectrometry, and the information obtained by spectroscopic techniques such as FTIR, 1H-NMR and UV-fluorescence. The goal is to contribute to the use of these spectroscopy techniques for the quantification of bio-oil functional groups and to advance the understanding of bio-oil complex chemistry.
2.
Material and Methods
2.1. Pyrolysis oils Based on availability we used a selection of oils that covers as wide spectrum of pyrolysis oil types as possible. Ten biomass Pyrolysis oils produced by three distinct technologies from three different feedstocks and at wide range of temperatures were used for our studies. BTG-BTL biooil from the Biomass Technology Group was produced from pine wood using a rotating cone reactor (http://www.btg-btl.com/). Briefly, the average particle size was 3 mm, the average reactor temperature 510°C, gas residence time 18.18 MΩ·cm) was used for calibration.
2.2.2. Gas Chromatography/Mass Spectroscopy (GC-MS) GC-MS analysis was performed on an Agilent Technologies 7890A GC with Restek Rtx-1701 column: 60m x 250µm x 0.25µm, Agilent 5975C MS with NIST 2.0f Mass Spectral Search Program. Acetonitrile was used as solvent to prepare 10 wt% concentration of pyrolysis oil samples. Three internal standards, isoamyl ether, 1-octanol, and methyl laurate were used together with 34 standard compounds (see Table 1) to create eight-point calibration curves to quantify the major components of these pyrolysis oils. Each sample was filtered through a 0.2 µm PTFE syringe filter before injection. The method used is as following: He flow rate of 1 mL/min, injection volume 1µl, injection port temperature 250 °C, split ratio 30:1, initial oven 9 ACS Paragon Plus Environment
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temperature 45°C (10 min) ramped at 3°C/min to 250°C (5min). More details on sample preparation and quantification can be found in NREL’s laboratory analytical procedure by Christensen et al.54
2.2.3. Hydrolysable Sugars The content of hydrolysable sugars in the bio-oil was quantified by ion exchange chromatography. Briefly, 1 g of bio-oil was dispersed in water (10 mL) in a centrifuge tube and sonicated for 30 min while the temperature was kept
