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Characterization of Woody Biomass Pyrolysis Oils’ Water Soluble Fraction Filip Stankovikj, Armando G. McDonald, Gregory L. Helms, Mariefel Valenzuela Olarte, and Manuel Garcia-Perez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02950 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017
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Characterization of Woody Biomass Pyrolysis Oils’ Water Soluble Fraction Filip Stankovikj1, Armando G. McDonald3, Gregory L. Helms 2, Mariefel V. Olarte4, Manuel Garcia-Perez1* 1
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120, USA
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 Science, University of Idaho, Moscow, ID, 83844, USA 4
Pacific Northwest National Laboratory, Richland, WA 99354, USA
(Paper submitted to Energy and Fuels)
Abstract: 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, hydrolysable sugars and total carbohydrates), it is possible to quantify only between 45 and 50 wt.% of it. Our results confirm that most of the total carbohydrates (hydrolysable sugars and non-hydrolysable) are soluble in water. The ion chromatography hydrolysis method showed that between 11.6 and 17.3 wt.% of these oils were hydrolysable 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 were quantified by titration, Folin-Ciocalteu,
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P-NMR and 1H-NMR. The WS fraction contains
between 5.5 and 6.2 mmol/g of carbonyl groups, between 0.4 and 1.0 mmol/g of carboxylic acid groups, between 1.2 and 1.8 mmol/g phenolic -OH, and between 6.0 and 7.9 mmol/g of aliphatic
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alcohol groups. Translation into weight fractions of the WS was done by supposing surrogate structures for the water soluble phenols, carbonyl and carboxyl groups and we estimated the content of WS phenols (21-27 wt.%), carbonyl (5-14 wt.%), and carboxyl (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 hydrolysable sugars is the missing furanic fraction. Further refinement of the suggested methods and development of separation schemes to obtain and quantify sub-fractions with homogeneous composition (e.g. carbohydrates, high molecular weight WS phenols, furans, and dehydrated sugars) warrant further investigation. Keywords: fast pyrolysis, characterization, fractionation, FTIR,
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pyrolytic humins, furanics, dehydrated sugars, water soluble phenols.
*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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P-NMR, carbonyl groups,
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1.
Introduction
The world’s energy demand is closely correlated to the population and living standard growth, and predictions point towards an energy demand increase of about 37% in the next thirty years.1 Agricultural, forest and waste biomass is readily and widely available to satisfy a portion of the energy needs and through fast pyrolysis to produce transportation fuels and chemicals.2–4 Fast pyrolysis is a technology the converts lignocellulosic materials into 60 to 70% crude liquid oil through rapid thermal decomposition in absence of oxygen at temperatures between 350 to 550ºC.5,6 Though important for building sustainable future, the complexity of the oil has been imposing challenges on its handling and upgrading7–10 These oils are composed of hundreds of highly reactive oxygenated compounds with a wide range of molecular weights.9-12 In addition, most of the bio-oil compounds are multi-functional and poorly described as a mixture of acids, alcohols, aldehydes, esters, ketones, sugars, phenols, guaiacols, syringols, furans and others.9–15 Precipitation in cold water is the most common method used for bio-oil fractionation followed by subsequent characterization.16–20 By this method the bio-oil is divided into a water soluble (WS) and a water insoluble (WIS) fraction (also known as pyrolytic lignin).16,19,20 Greater details on the analytical characterization techniques that follow, their advantages and weaknesses, and their application on the whole pyrolysis oil may be found in excellent reviews elsewhere.21–23
The biggest problems for production of transportation fuels and chemicals from these oils are polymerization and coke formation during bio-oil hydrotreatment.24,25 One of the culprits for biooil rapid polymerization and catalyst coking has been narrowed down to the WS fraction and its unknown chemical makeup.7,26,27 While the heavy pyrolytic lignin WIS fraction still remains a processing challenge,26,28 much less has been known about the WS oligomeric fraction which is present in higher quantities in the pyrolysis oils, and because its reactivity induces coke formation during hydrodeoxygenation.29
Although an effort in understanding the pyrolytic lignin structure by using comprehensive set of analytical tools has been done in the recent past by Meier et al,16,30–32 no such comprehensive study has been found in literature on the characterization of the WS fraction. Bio-oil characterization studies typically result in the quantification of between 70 and 90 wt.% of the oil.14,15,19,33 There seems to be consensus in the literature that the unknown bio-oil material is the 3 ACS Paragon Plus Environment
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high molecular weight (HMW) WS molecules.19,33 These highly reactive O-laden structures, result of dehydration reactions,15 lead to initial “gunking” of the catalyst, and later coke formation leading to deactivation of upgrading catalysts. Formation of star like carbonyl reach reactive HMW compounds through aldol condensation of light volatile molecules has been hypothesized as a “gunking” culprit,34 however the existence of these molecules in the fresh biooils has not been recognized. Recombination of the phenolic and the carbohydrate elements to form this gunking material has only been recently studied in pyrolysis oils.35,36 Growth of these compounds during catalytic hydroprocessing must be avoided, or their content in the pyrolysis oil minimized through new separation strategies and better control of the pyrolysis oil production conditions in order to improve the economy and viability for industrial scale-up of the envisioned refining strategies.
The objective of this paper is, for the first time, to use a comprehensive set of available analytical tools, both spectroscopic and wet chemistry quantification techniques, to advance our understanding on the composition and structural motifs of the WS fraction in wood pyrolysis oils.
2.
Material and Methods
2.1. Pyrolysis oils and water soluble fraction Two biomass pyrolysis oils produced by two distinct technologies were used for our studies. BTG-BTL bio-oil 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 < 2s, and condensation temperature: 40°C (one step condensation). Amaron Energy bio-oil (http://www.amaronenergy.com/) was produced from Arbor Pellets (arborpellet.com) in rotating drum reactor (6”/15.2 cm pipe, heated length 48”/122 cm, temperature 450°C, 18 rpm). The particle size used was 6.4 mm (1/4”), approximate reactor residence times for the solid particles was 5 min, and for the gas/vapor products 2 s. A two stage condenser was used to trap the condensable gases, where the first stage temperature was held at 88°C (this fraction used for analysis), and the second stage temperature at 37°C.
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The pyrolysis oils were partitioned into a WS and WIS fractions following the procedure described elsewhere.19,20 Briefly, about 3 g of bio-oil in methanol (1:1 wt.) was injected into ice cold water (100 mL) that was stirred vigorously with a vortex mixer and a precipitation was observed (WIS). The mixture was vacuum filtered through a Whatman N°42 filter paper, and the WS filtrate collected and concentrated to dryness at 40°C in a rotary evaporator. The concentrated WS fraction was stored in a freezer prior to analysis. 2.2. Overall Characterization Table 1 lists the analytical techniques used to analyze the water soluble fraction of the bio-oils and the information that these tools provide. Table 1. Summary of analytical tools used to analyze the water soluble fraction. Analytical Technique Objective 1 2 3
Water content Elemental Analysis* Thermogravimetric analysis*
4
GC-MS
5
Ion Chromatograph (IC)
6
Sulfuric acid–UV method
7 8 9
Non-aqueous titration by Faix Folin – Ciocalteu Method ESI-MS
10
FT-ICR-MS
11
UV Florescence
12
UV Absorption
13
ATR-FTIR
14
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15
1
16
13
17
HSQC-NMR*
P-NMR
H-NMR C-NMR
Quantification of water in the WS (ASTM E203–08) Quantification of C, H, N and O mass fraction (ASTM D5373 – 08) Quantification of volatiles, and fixed carbon.19 Quantification and identification of individual volatile compounds;37 data used to further quantify functional groups (carboxylic, carbonyl, phenol, aliphatic -OH).15 Quantification of hydrolysable sugars based on calibration for arabinose, galactose, glucose, xylose and levoglucosan.38 Quantification of total carbohydrates that include hydrolysable and non-hydrolysable sugars, and already present furnanics.39 Quantification of carbonyl groups.40 Quantification of total phenols.41, 42 Molecular weight distribution.43 Compositional analysis through assignment of elemental compositions to each of the peaks: van-Krevelen diagrams, DBE vs. C, grouping compounds by number of C and O, molecular weight distribution of the non-volatile fraction.44 Information on the extent of conjugation. Possible quantification of phenolic/aromatic functionalities.15 Quantification of furanic compounds. Fast and simple method for analyzing pyrolysis oils;45 spectral region 1850-1490 cm-1 is deconvoluted before analysis of the carbonyls, carboxylic acids, and aromatic compounds.46 Quantification of -OH functional groups (carboxylic, phenol and aliphatic).47 Quantification of aldehyde, aromatic, aliphatic and other moeieties.48 Quantification of carbonyl, aromatic, aliphatic and methoxy moieties.49 Allows for more precise assignment of 1H-13C and structure elucidation.50–52
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2.2.1. Water content Water content of the bio-oil samples was determined by Karl Fischer titration according to ASTM E203–08 (Standard Test Method for Water Using Volumetric Karl Fischer Titration) using a Schott Instruments TitroLine Karl Fischer Volumetric Titrator. Deionized water (>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 temperature 45 °C (10 min) ramped at 3 °C/min to 250 °C (5 min). More details on sample preparation and quantification can be found in NREL’s laboratory analytical procedure by Christensen et al.37 2.2.3. Hydrolysable Sugars Approximately 1 g of the diluted WS fraction (0.05 g/mL) was further diluted in water (10 g) to which approximately 0.3 mL H2SO4 was added and the mixture hydrolyzed at 0.5M H2SO4 and at 130°C for 4 h. An aliquot of the hydrolysate (0.1 mL) was diluted with water (5.0 mL) and neutralized with 1 M NaOH. The sugars were then analyzed by anion exchange chromatography using a Carbopac PA20 column coupled to a Dionex ICS-3000 system equipped with an AS 50 auto-sampler, GP50 gradient pump, and ED50 electrochemical detector at a flow rate of 0.50 mL/min. The mobile phase was a mixture of 80% water and 20% 50 mM NaOH. A 10 mM NaOH storage solution was added post-column to maintain a pH of 10.4 in the detector. The injection volume of the sample was 10 µL, and the column temperature was constant (35 °C). The sugar standards used were levoglucosan, arabinose, galactose, glucose, and xylose (Sigma–
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Aldrich). The calibration curves for all the sugars analyzed were linear in the range of concentrations studied. 2.2.4. Total carbohydrates (Sulfuric acid–UV method) The sulfuric acid assay was employed to determine total carbohydrates based on the method from Albalasmeh et al. was used on triplicate samples.39 The diluted aqueous fraction (1 mL, about 0.05 g/mL, see section 2.2.3) was mixed with cH2SO4 (3 mL) for 30 s. The temperature of the mixture rises rapidly, and the reaction proceeds to completion i.e. dehydration of carbohydrates and formation of furfural derivatives. Next, the mixture was cooled in an ice bath for 2 min, transferred to a polystyrene cuvette, and the absorbance measured at 315 nm using a Shimadzu UV-2550PC spectrophotometer with water as a blank. The calibration curve was obtained by using levoglucosan solutions prepared in concentrations between 3 and 100 mg/L. 2.2.5. ESI-MS The WS fraction and the bio-oils were diluted to 1 mg/mL−1 in methanol and water containing 1% acetic acid. The sample was introduced to a Finnigan LCQ-Deca instrument (ThermoQuest) at a flow rate of 10 µL/min. Both negative and positive ion ESI-MS scans in the range m/z 1002000 were performed for the same sample. The ion source and capillary voltages were 4.48 kV and 47 V, respectively, and the temperature was 275 °C. Both the number average molar mass (Mn = ΣMiNi/ΣNi) and the weight average molar mass (Mw = ΣMi2Ni/ΣMiNi) were calculated from the ESI-MS spectra, where the Mi is the m/z and Ni is the intensity of the ith ion.43 2.2.6. FT-ICR-MS The pyrolysis oils and WS fractions (0.1 mg/mL in methanol) were analyzed using ESI in negative-ion mode on a Bruker Solarix 9.4T instrument and was tuned to provide an adequate signal in the mass range of interest and to minimize possible aggregations and/or fragmentations. Various ion flight times were tested for FT-ICR, and 0.6 ms was used in the final data acquisition. The data acquisition size for FT-ICR was 8 M with a transient length of approximately 0.9 s. The peak list was produced by DataAnalysis (Bruker) imported to Composer (Sierra Analytics, Modesto, CA) for petroleomic analysis. The mass accuracy of the Composer was limited to 3 ppm in the chemical composition analysis, and the relative ion abundance was limited to 0.1%. 7 ACS Paragon Plus Environment
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2.2.7. UV Florescence The oils were diluted in HPLC grade methanol at 20 ppm and analyzed on Shimadzu RF 5301 pc (software: Panorama Fluorescence 2.1) spectrometer. Synchronous fluorescence spectra at constant wavelength difference were set up. Excitation wavelength was scanned from 250 to 500 nm and emission wavelengths were recorded with a 10 nm differences (from 260 to 510 nm). The excitation slit width and emission slit width were set up at 3 nm. Data was collected every 1 nm. Additional concentrations of 40 ppm, 30 ppm, and 5 ppm were run in order to confirm the linear response of the fluorescent spectra. 2.2.8. UV Absorption WS fractions were diluted to 250 mg/L in 18.2 MΩ-cm E-pure water and analyzed on Shimadzu UV-2550PC UV/Vis Spectrophotometer. The samples were scanned from 190 nm to 400 nm, with a fast scan speed setting, and the slit was set to 0.2 nm. Three of the furanic compounds known to be present in the WS (furfural, 5-HMF, 2(5H)-Furanone) were used for calibration at 20, 10, 5, and 1.5 mg/L, with R2>0.995. 2.2.9. Quantification of carbonyl groups by non-aqueous titration Determination of carbonyl groups developed by Faix et al. based on modified oximation reaction was used.53 Briefly, 100 mg of oil WS fraction was added to a reaction vial, followed by 1 mL DMSO, 2 mL hydroxylamine hydrochloride, and 2 mL triethanolamine solutions and the vessel sealed and. heated to 80 °C for 2 h with frequent shaking and stirring. The reaction contents were washed off with aqueous 80 % ethanol and transferred quantitatively into titration beakers. An automatic titrator Mettler Toledo T50 was used to titrate standardized 0.1N HCl against the portion of triethanolamine that was not consumed by the oximation reaction. Each of the samples was run in triplicate and the results are presented on mmol/g bio-oil basis. The method was validated using known concentration of 4-(benzyloxy)benzaldehyde (4-BBA), and the preparation of the blank followed the same procedure as above, just avoided the addition of biooil. The concentration of carbonyl groups expressed in mmol/g of water soluble fraction was calculated using the equation below:
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∗
Where EPBA and EP are the endpoints of titration for the blank, and for the sample respectively, expressed in mL; the weight of the bio-oil is given in grams, and the molarity of titrant is expressed in mmol/L. Solution preparation: Hydroxylamine hydrochloride solution – Add 7.7g of hydroxylamine hydrochloride and 50 mL of deionized water to a 250 mL volumetric flask. Swirl till all solids are dissolved, then dilute up to the mark with ethanol; Triethanolamine solution – Add 17.4 mL of triethanolamine to a 250 mL volumetric flask, then dilute up to the mark with ethanol. More details on sample preparation and quantification can be found in NREL’s laboratory analytical procedure by Black et al.40 2.2.10. Quantification of total phenols by Folin - Ciocalteu Method The total phenol content was determined by the Folin–Ciocalteu (FC) method with 5 replicates.54,55 Briefly, bio-oil (0.3 g) is weighed into a 15 mL centrifuge tube to which ethanol (5 mL), and water (5 mL) are added, vigorously mixed, and diluted to about 500 mg/L. A 40 µL aliquot is added to deionized water (3.16 mL), followed by 200 µL of FC reagent (Sigma Aldrich F9252) and mixed for 8 min after which 600 µL of aq 20% sodium carbonate solution was added. The mixture was incubated to develop color for 2 h at room temperature and absorbance was measured at 765 nm on a Spectronic 20 Genesys instrument. Gallic acid was used as a calibration standard, six points were used to make the calibration curve between concentrations of 50 and 500 mg/L and the average of five measurements is reported in wt.% gallic acid equivalents (GAE). 2.3. Functional Groups by Spectroscopic Techniques 2.3.1. Quantification of OH functional- groups using 31P-NMR Oil WS fraction (18 mg) samples (in duplicate) were dissolved in anhydrous pyridine (0.9 mL), to which 0.6 mL internal standard solution (ISTD, 21 mg of TPPO, 3 mg of chromium(III) acetylacetonate (Cr(acac)3), and 1.15 mL of CDCl3) and 0.16 mL of the phosphitylating reagent 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) were added in a vial, under a N2 atmosphere.49,56,57 Reagent concentrations used are: pyridine: CDCl3=1.6:1, Cr(acac)3 9 ACS Paragon Plus Environment
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concentration of 0.003 mmol/mL solvent, TPPO concentration of 0.025 mmol/mL solvent, and the minimum pyridine:water ratio of 185. 31P-NMR spectra were acquired using an inverse gated decoupling pulse sequence, 90o pulse width, 25 s pulse delay, acquisition time of 1.2 s, and 96 scans on Varian VNMRS 600 with 5 mm broadband probe. The spectra were phase and baseline (Bernstein polynomial, fit parameter = 6) corrected, the TMDP peak was assigned at 175.514 ppm, and the following regions were assigned for integration: aliphatic OH (145 – 152 ppm), phenolic OH (138 – 145 ppm), and carboxylic acid OH (134.6 – 138 ppm), TPPO (27 – 29 ppm). The ratio of spectral regions was compared to the TPPO region and the amount of functional groups calculated in terms of mmol OH/ g bio-oil. More details on sample preparation and quantification can be found elsewhere.58 2.3.2. 1H-NMR studies Oils were diluted in DMSO-d6 (10 wt. %) and 1H NMR spectra were acquired on a Varian Inova 500 spectrometer (11.75T), using a Nalorac HF dual tuned probe with Z-axis gradients, with 16 scans, 90° pulse, a 10 s relaxation delay, and with and without water suppression. Assignment of chemical shifts was done following the procedure described by Mullen et. al.48 Suppression of the broad water peak centered around 3.4 ppm was performed with spin-echo attenuation using a CPMG pulse sequence.59 An 8 s relaxation delay was followed by a 90º pulse and a CPMG spin echo train consisting of 12 cycles of τ-180º pulse-τ where the τ delay was 965 µs in duration. A total of 24 ms of transverse relaxation time was sufficient for complete elimination of the broad water signal. A 2-second acquisition time was used, 16 scans were accumulated for each sample and the data were processed by zero filling once followed by Fourier Transformation. 2.3.3. 13C-NMR studies Quantitative 13C-NMR samples were prepared by mixing 300 µL stock solvent (8.33 mg/mL of Cr(acac)3 in DMSO-d6) and ~200 mg of WS or oil. An Agilent DD2 600 with 5 mm OneNMR probe was used to collect 2048 scans, with 90º pulse angle, 1.2s acquisition time, relaxation delay d1 of 3 s and inverse-gated 1H decoupling. The FID was apodized with 5 Hz exponential line broadening and the data was Fourier transformed. After phasing the spectrum, the reference for the DMSO-d6 was set to 39.52 ppm, and Bernstein polynomial fit (parameter >6) was used to flatten the baseline. The sum of the following integration regions was normalized to 100%:
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Carbonyl (215-166.5), Aromatic (166.5-95.8), Aliphatic C-O (95.8-60.8), Methoxyl (60.8-55.2), and Aliphatic (55.2-0).49 2.3.4. ATR-FTIR FTIR spectra (in duplicate) were obtained using a Shimadzu IRPrestige 21 spectrometer equipped with MIRacle™ single reflection ATR Ge probe. A drop of oil was applied to cover the crystal window, and the spectra acquired with (32 scans, 600 to 4000 cm-1, resolution of 4 cm-1). The spectra were baseline corrected and band fitted between 1490 cm-1 and 1850 cm-1 using 9 Gaussian bands.46 The position and the width of the bands were kept fixed, and only the amplitude was allowed to change during the fitting process. Absorbance area was calculated in arbitrary units.
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Results and Discussion
The WS parts of the BTG and Amaron oils after fractionation were 51.3 and 50.3%, respectively. The section that follows focuses on the overall analysis of these fractions. 3.1. Quantification of independent compounds and typical compositional analysis 3.1.1. Water content and elemental analysis Table 2 shows the water content of the pyrolysis and the water soluble fractions obtained after a couple of hours in the rotary evaporator. For comparison data for the oils from our previous study were presented here.15 Typical elemental compositions of the organic phases of the WS fractions are given in Table S1 (C ~49 %, O ~44 %, H ~7%, N ~0.1%). Table 2. Water content of the oils studied. Sample Water [wt.%] BTG* 26.2 Amaron* 18.6 BTG WS 3.2 Amaron WS 4.0 15 *Data from reference.
STD [-] 0.2 0.2 0.1 0.2
3.1.2. Analysis of the volatile fractions by Gas Chromatography/Mass Spectroscopy (GC/MS) The compounds in the oil and WS fractions were identified and quantified by GC/MS analyses and are given in Table 3 and Table 4. A match quality of >70 % was accepted for the identification of the indirectly quantified compounds; additionally, their MS signatures were visually compared with the database entries for best match. The compounds in Table 3 were directly quantified using external standards, while the compounds in Table 4 were quantified based on the assumption that each of those compounds has a similar response factor to at least one of the directly quantified compounds in Table 3 based on similarity in structure and functional groups.
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Table 3. Compounds identified in the pyrolysis oils and WS fractions by GC/MS (wt.%); ND stands for non-detected; 0.0 stands for compounds that were detected with quantities
