Impacts of Thermal Processing on the Physical and Chemical

Article pubs.acs.org/EF

Impacts of Thermal Processing on the Physical and Chemical Properties of Pyrolysis Oil Produced by a Modified Fluid Catalytic Cracking Pyrolysis Process Laibao Zhang,† Andres Chaparro Sosa,‡ and Keisha B. Walters*,§ †

Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, Colorado 80309, United States § School of Chemical, Biological and Materials Engineering, The University of Oklahoma, Norman, Oklahoma 73019, United States ‡

ABSTRACT: Accelerated aging was examined over a broad temperature range (40−290 °C) to determine the stability and different aging mechanisms for pyrolysis oil produced using fluid catalytic cracking (FCC) of solid particulate biomass material. Oil properties, including water content, high-molecular-mass (HMM) lignin content, viscosity, molecular weight, and chemical composition, were measured both before and after the accelerated aging study. Notably, at all aging times and temperatures explored, all samples remained as a single phase. Chemical composition of the pyrolysis oils was monitored using infrared and mass spectroscopies. Oils aged at temperatures above 60 °C showed marked increases in HMM lignin content and molecular weight with increasing aging time. Calorimetry results indicated that, at and above 200 °C, there were alternate aging mechanisms that occur, including sugar decomposition. A rapid increase in the water content and shift in the HMM fraction molecular weight corroborated the decomposition of sugars, which led to the formation of water and reactive monomers. Fourier transform infrared (FTIR) results showed that similar aging reactions occurred for oils aged between 40 and 80 °C and between 150 and 290 °C. Gas chromatography−mass spectrometry (GC−MS) results suggested the reactions among furfural, 2-cyclopenten-1-one, 2-hydroxy-3-methyl-2-cyclopenten-1-one, hydroxybenzenes with available ortho/para sites, and the active sites on HMM lignin contributed to property changes as a result of heat treatment.

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INTRODUCTION Fossil fuels continue to play a dominant role in worldwide energy consumption. Rapid consumption over the past few decades has highlighted some challenges, including nonrenewability, as well as environmental concerns. Therefore, the development of renewable energy has increasingly become a focus of research and industrial development. One type of renewable energy, pyrolysis oil (also known as bio-oil), is typically produced from fast pyrolysis, when a biomass feedstock is rapidly heated to temperatures around 500 °C in a nonoxidizing environment during a short residence time.1 Pyrolysis oil is a promising alternative energy source because it does not generate greenhouse gas emissions and is considered sustainable, if appropriate feedstock sources are available. There are challenges associated with pyrolysis oil production, including feedstock type and processing, reactor design and maintenance, condensation, oil filtration, and upgrading.2−5 Crude pyrolysis oil is a complex mixture of a large number of compounds that can be broadly classified into volatile organic compounds (e.g., hydroxyacetaldehyde, formic acid, and acetic acid), furanic compounds, monosubstituted phenols, sugars (fermentable and cross-linked), lignin oligomers, and water.6−8 Crude pyrolysis oil suffers many deleterious properties, such as high water content, high oxygen content, high viscosity, high ash/solids content, and immiscibility with hydrocarbon liquids. The major issue with pyrolysis oil is that the components will continue to react until equilibrium is reached because it is not in thermodynamic equilibrium as a result of the rapid quench.9 All of these drawbacks necessitate that substantial physical and © 2016 American Chemical Society

chemical improvements in the quality are necessary before pyrolysis oil can be used as a biorefinery feedstock, heating fuel, and/or transportation fuel. A number of physical and chemical pyrolysis oil upgrading technologies, including filtration,10−13 catalytic cracking,14−16 hydrotreating,17,18 and steam reforming,19−22 have shown that, in general, chemically upgraded pyrolysis oils are more stable. Catalytic hydrotreatment with heterogeneous catalysts at high temperature and pressure (e.g., 500 °C and 300 bar) has been identified as a promising method to improve the properties of pyrolysis liquids and make them suitable as a refinery feed.17,23 However, at temperatures above 200 °C, the decarboxylation and repolymerization processes occur more quickly than the hydrodeoxygenation processes.17,24 This can lead to reactor blockages and/or deactivation of the upgrading catalysts. Understanding how and why these reactions, including polymerization, occur in pyrolysis oil is important to design treatments to stabilize or transform pyrolysis oil before further upgrading. Thermal treatments have previously been used to examine the continuing “aging” chemical reactions as a result of thermodynamic inequilibrium and determine if a standard procedure can be used to mitigate or even prevent aging reactions. Heat-induced physical and chemical changes in pyrolysis oils are usually investigated using carefully sealed samples to simulate the aging process during storage and to prevent the Received: May 21, 2016 Revised: August 5, 2016 Published: September 6, 2016 7367

DOI: 10.1021/acs.energyfuels.6b01220 Energy Fuels 2016, 30, 7367−7378

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Figure 1. (a) FTIR, (b) GPC, (c) DSC, and (d) TG plots of separated HMM lignin and LMM fraction of control pyrolysis oil.

Figure 2. Water content as a function of the heat treatment time for pyrolysis oil aged at 40, 60, 80, 150, 200, and 290 °C.

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removal of volatile components.25−30 Czernik et al.28 concluded that chemical reactions were quite similar over the temperature range of 37−90 °C, because equivalent viscosities were obtained in oak pyrolysis oil after aging for 3 months at 37 °C, 4 days at 60 °C, or 6 h at 90 °C. It has since been advised that a sealed sample of pyrolysis oil held at 80 °C29,30 should be used as an accelerated “aging” test. While prior studies conducted at low temperature (at or below 90 °C) significantly improve our understanding about the polymerization or stabilization of pyrolysis oils, even the lowest temperature hydrotreatment processes are carried out at ∼200 °C. To the authors’ knowledge, no report has investigated the applicable temperature range for this accelerated aging method. In the present study, the physical and chemical changes were investigated for pyrolysis oil aged at discrete temperatures and times over a broad temperature range, from 40 to 290 °C, in sealed, high-pressure reactors. An understanding of “aging” reactions that occur in pyrolysis oil is critical to propose potential measures to stabilize or transform pyrolysis oil before further upgrading.

EXPERIMENTAL SECTION

Heat Treatment. The pyrolysis oils used in this study were obtained directly from KiOR, LLC. The oils were produced with a phosphorus-promoted ZSM-5/silica-containing binder catalyst31 using a modified pyrolysis process32 based on the fluid catalytic cracking (FCC) technology currently used in the fossil oil industry. Heat treatments were performed by heating samples (∼10 mL) in sealed vessels (1/2 in. 316 stainless-steel tubes with caps) for different times at 40, 60, 80, 150, 200, and 290 °C. A void space of ∼10% was left in each sample vessel to allow for gas expansion. After heating for a specified period of time, the pyrolysis oil samples were allowed to cool to room temperature, transferred to glass vials, and stored at 2.5 °C prior to testing. Solvent Extraction. Using CH2Cl2, the high-molecular-mass (HMM) lignin (CH2Cl2-insoluble portion) was separated from the pyrolysis liquid oil. The mixture was shaken for 10 min at room temperature and left to equilibrate for 3 h, forming distinct bottom and top phases, thereby the low-molecular-mass (LMM) fraction was preferentially extracted into the top CH2Cl2 phase and separated by decantation from the bottom phase (HMM). Excess CH2Cl2 was removed by evaporation at room temperature. Water Content Analysis. The water content was measured by Karl Fisher titration following ASTM E203-01 using a Hydranal 2E 7368

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Figure 3. HMM lignin content as a function of the heat treatment time for pyrolysis oil aged at 40, 60, 80, 150, 200, and 290 °C.

Figure 4. TG plots of pyrolysis oil aged at 40, 60, 80, 150, 200, and 290 °C. Each plot contains traces for samples aged at a common temperature but for different aging times. ∼2 mg. Pyrolysis oils were placed in sealed aluminum pans and heated at 5 °C/min from −60 to 290 °C under a N2 atmosphere. Untreated (control) pyrolysis oil samples were analyzed using isothermal runs at 80, 150, and 200 °C with a N2 flow rate of 50 mL/min. Attenuated Total Reflectance Fourier Transform Infrared (ATR−FTIR) Spectroscopy. ATR−FTIR spectra were collected using a Nicolet 6700 spectrometer with a ZnSe 60° ATR crystal (MCT-A* detector, 4 cm−1 resolution). Gel Permeation Chromatography (GPC). GPC characterization of the pyrolysis oil was carried out on custom-built GPC with a Waters 610 fluid pump, Waters 600 controller, Varian Star 9040 refractive

titrant and a Hydranal solvent. To reduce the error caused by slow response of the electrolyte, the pyrolysis oil samples were first dissolved into Hydranal solvent prior to titration. Thermogravimetric Analysis (TGA). TGA was performed using a TA Instruments Q600 SDT under a nitrogen atmosphere or an air atmosphere with a 50 mL/min flow rate. Dynamic ramps were performed on both heat-treated and untreated samples at a rate of 5 °C/min, from room temperature to 800 °C in N2 and from room temperature to 550 °C in air. Differential Scanning Calorimetry (DSC). DSC data were collected with a TA Q2000 DSC instrument using a sample size of 7369

DOI: 10.1021/acs.energyfuels.6b01220 Energy Fuels 2016, 30, 7367−7378

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Figure 5. DTG plots of pyrolysis oil aged at 40, 60, 80, 150, 200, and 290 °C. The curves in each plot are staggered for clarity. index detector, Varian Mesopore guard column (50 × 4.6 mm internal diameter), and Varian Mesopore column (250 × 4.6 mm internal diameter). Polystyrene standards (PSS Polymer Stands) with molecular weights of 126, 266, 486, 582, 891, 2780, 6480, 10 261, and 18 200 Da were used to generate the calibration curve. The operation flow rate of tetrahydrofuran was 0.2 mL/min with 50 μL injections. Prior to injection, pyrolysis oil samples were diluted to 1−2 mg/mL using Optima tetrahydrofuran and filtered with 0.45 μm polyvinylidene fluoride (PVDF) membrane filters. Gas Chromatography−Mass Spectroscopy (GC−MS). GC− MS analyses were performed using Hewlett-Packard (HP) 5890 II GC equipped with HP 5971 MS. A silica capillary column (30 m length, 0.32 mm internal diameter, and 0.25 μm film thickness) coated with 5% phenyl methylpolysiloxane was used. The injector temperature was 270 °C. The column temperature was initially 40 °C, increased at 5 °C/min to 280 °C, and then held at 280 °C for 15 min. The mass spectrometer employed a 70 eV electron impact ionization mode with source and interface temperatures of 250 and 270 °C, respectively. Prior to injection, the pyrolysis oil/alcohol mixtures were dissolved into CH2Cl2 and then filtrated with 0.45 μm PVDF membrane filters. Chlorobenzene (anhydrous, 99.8%) was used as an internal standard.

oil was mixed with water, methanol, ethanol, and isopropanol. A mixture of methanol and dichloromethane (50:50, v/v)26 was used as the solvent, and the solids content was measured to be 3.3 ± 0.19 wt %. The pyrolysis oil can be divided into a CH2Cl2-soluble portion (which contains LMM compounds, such as LMM lignin and other extractives) and a CH2Cl2insoluble portion (which contains HMM compounds, mainly HMM lignin).37 The HMM lignin content was measured to be 33.0 ± 1.1 wt %. Figure 1 displays property data of the separated HMM and LMM fractions. As a result of the aldehyde, ketone, and ester compounds present, the LMM fraction had a much higher carbonyl (CO) content and a lower molecular weight, in comparison to the HMM fraction. As expected, GPC data indicated that HMM lignin had a significantly higher molecular weight than the LMM fraction. The thermal behaviors were investigated by DSC and TGA. DSC traces exhibited two negative peaks that can be assigned to the dehydration38,39 and decomposition of sugar derivatives40 for both the HMM and LMM fractions. The higher relative composition of volatile compounds in the LMM fraction resulted in rapid weight loss, as shown in the thermogravimetric (TG) plots; the LMM fraction lost 60 wt % versus 20 wt % for the HMM fraction by 300 °C. Water Content. The water content changes as a function of aging time at different temperatures are shown in Figure 2. In general, the water content in the pyrolysis oil increased with the duration of storage, but the magnitude of this increase was strongly affected by the storage temperature. Despite the increases in the water content, all aged samples remained

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RESULTS AND DISCUSSION Characterization of Untreated Pyrolysis Oil. Pyrolysis oil properties are determined by many factors, such as pyrolysis reactor type, operating conditions, and feedstock type, and consistency. While the water content was 4.77 ± 0.27 wt %, significantly lower than reported values in the literature,33−36 the FCC pyrolysis oil examined in this study was found to be immiscible with petroleum ether as a result of its high oxygen content. Phase separation was observed when the pyrolysis 7370

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Figure 6. DSC plots of pyrolysis oil aged at 40, 60, 80, 150, 200, and 290 °C. The curves in each plot are staggered for clarity.

Figure 7. Isothermal DSC aging plots of pyrolysis oil tested at 80, 150, and 200 °C.

temperatures (≥150 °C), even for short times, resulted in the net formation of water. HMM Lignin Content. HMM lignin has been reported to be a major reason for the instability of pyrolysis oil.42 Most HMM lignin is not distillable, which makes it unsuitable for combustion in engines. The high molecular weight also hinders its usage in spray combustors. The unsaturated structure of HMM lignin in comparison to native and industrial lignin leads to differences in properties.42,43 Polymerizations of lowmolecular-weight components44 via reactive functional groups, such as phenols, aliphatic alcohols, and double bonds, can occur during the heat treatment aging process. As shown in Figure 3, significant increases in the HMM lignin content were observed

homogeneous. Only slight increases in the water content were observed for oils stored below 80 °C, even after long-term storage. Higher storage temperatures (≥150 °C) resulted in significantly larger increases in the water content. The water content reached 11.8 ± 0.3 wt % when the oil was stored for 150 min at 290 °C, which was almost twice the value of 6.6 ± 0.3 wt % measured in samples aged for 56 days at 80 °C. During the aging process, several kinds of reactions occur; etherification, esterification, hemiacetal/acetal formation, and condensation yield water, while hydration consumes water.41 Therefore, the average water content during aging is a result of competing water-producing and water-consuming reactions. The overall water content changes observed indicated that high 7371

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Figure 8. GPC traces of pyrolysis oil aged at 40, 60, 80, 150, 200, and 290 °C.

Table 1. GPC Determined Mn, Mw, and PDI Values of Control and Heat-Treated Pyrolysis Oil Samples temperature (°C)

time (day)

Mn

Mw

PDI

290a

control 20 50 70 100 120 150 1 2 5 9 1 3.5 7 8 9 6 12 24 35 56 7 14 21 35

506 600 721 688 718 678 715 845 882 889 1100 817 787 878 814 908 578 702 780 679 711 601 544 586 608

1330 1480 1710 1700 1740 1660 1710 2303 2248 2532 2678 2033 1954 2085 2063 2153 1475 2100 2020 1550 1780 1430 1459 1480 1460

2.63 2.46 2.37 2.47 2.42 2.45 2.39 2.72 2.55 2.85 2.44 2.49 2.48 2.37 2.53 2.37 2.55 2.99 2.59 2.28 2.51 2.38 2.68 2.52 2.40

200

150

80

60

Table 1. continued temperature (°C) 40

a

time (day)

Mn

Mw

PDI

49 33 45 52

627 597 641 675

1870 1510 1670 1760

2.99 2.52 2.61 2.60

Time in minutes.

for oils aged at and above 60 °C, while only slight changes were observed for oils aged at 40 °C. High temperatures resulted in significant increases in the HMM lignin content. Indeed, the HMM lignin content was over 90 wt % for oils aged at 200 °C for 9 days, and the oil sample solidified. TGA. TGA was used to examine more closely the thermal stability of oil and measure the fraction of volatile components by monitoring the weight change as a specimen is heated. Figures 4 and 5 show the TG and differential thermogravimetric (DTG) traces of the control oil sample and samples heat-treated at different temperatures. The peak temperature for maximum weight loss shifted to higher temperatures for longer aging times and higher aging temperatures. Mass loss in the control oil sample began at low temperatures with a mass loss maxima at ∼65 °C, corresponding to removal of volatile compounds. In thermal characterization, the previously heattreated samples lost less weight than the control samples, with removal of light compounds and water occurring below 120 °C. Above 120 °C, the dehydration of sugar compounds had been reported.38,39,42 The decreasing intensity of peaks in the 7372

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time. This may be related to differences in the degree of branching for lignin caused by condensation reactions but warrants further examination. DSC. Figure 6 shows the DSC plots of oil samples aged at different temperatures. For all samples, two endothermic peaks assigned to the dehydration38,39 and decomposition40 of sugar derivatives were observed in the first heating process. No peaks were detected during the cooling process or in the second heating−cooling cycle. No obvious trends were observed for the dehydration and decomposition temperatures with aging temperature and aging time. Isothermal DSC can be used to monitor the net heat flows adsorbed or released from the sample. Figure 7 illustrates isothermal DSC plots at 200, 150, and 80 °C. At 80 °C, no obvious changes in heat flow were observed within 500 min. It was reasonable to postulate that positive heat flows would be detected using a large amount of samples because the aging of pyrolysis oil was an exothermic48 process. At 150 °C, positive heat flows were observed, indicating that the heat-release reactions (e.g., polymerization/condensation) were dominant. However, negative heat flows were observed in isothermal processing at 200 °C, indicating the decomposition of sugar derivatives.45,49 Hydrolysis of levoglucosan, the most common anhydrate sugar,50 in pyrolysis oils was observed above 90 °C, and the rate of that process had been shown to accelerate at higher temperatures, especially in the presence of acids and phenolics.49 The primary resulting product, glucose, can undergo dehydration, retro-aldol condensation, and decomposition

Figure 9. FTIR spectra of control and aged pyrolysis oils.

180−220 °C range with heat treatment time and temperature can be attributed to the decomposition of sugar derivatives.45 Pyrolytic lignin degradation occurred in the 350−425 °C temperature range and involved breakage of the interunit bond,27,46 which released monomeric compounds, such as phenols. The slight weight loss observed above 500 °C was likely related to lignin degradation and decomposition of released aromatic ring structures.47 There was an increase in the non-volatile residue remaining at 800 °C as a function of aging

Figure 10. FTIR PHRs as a function of the heat treatment time for pyrolysis oil aged at 40, 60, 80, 150, 200, and 290 °C. 7373

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Figure 11. GC−MS chromatogram of the CH2Cl2-soluble faction of control pyrolysis oil.

Table 2. Composition of the CH2Cl2-Soluble Faction of Control Pyrolysis Oil as Determined by GC−MS

into various reactive compounds, including levulinic acid, formic acid, acetic acid, etc., which, in turn, further facilitate these reactions. Indeed, the increase in the acetic acid content was confirmed by comparing the composition of pyrolysis oil/ 1-octanol mixtures before and after heat treatment; the abundance of octyl acetate measured by GC−MS increased by 29 times as a result of aging at 200 °C for 50 h. GPC. The control and heat-treated pyrolysis oils were investigated using GPC to examine the average molecular weight and molecular weight distributions. The retention time determined by GPC depends upon not only the absolute molecular size but also the detector type, solvent quality, and functional groups present in the samples. For accurate results, it is crucial to use GPC calibration standards, which possess the same functional groups as the compounds to be tested. However, because pyrolysis oil is a mixture of many compounds containing many different functional groups, all known standards for GPC calibration are non-ideal. Therefore, in this study, traditional polystyrene standards were used, knowing that absolute molecular weights would not be obtained. Figure 8 displays the GPC curves of the control and heattreated oil samples. All oil samples exhibited multiple peaks. With aging, the relative proportion of the high-molecularweight fraction increased. This effect was more pronounced for the oils aged at higher temperatures and longer times. It is known that heating can result in cross-linking and polymerization reactions of lignin oligomers that lead to an increase in average molar mass.26,28 The increase in the high-molecularweight fraction resulted in an increase of the average molecular weight. Table 1 lists the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of the control and heat-treated oil samples. While both Mn and Mw generally increased with aging time, some deviations from this trend were observed. Czernik et al.28 also observed a poor correlation between Mw and aging conversion, where the conversion was defined as the ratio of the actual

number

time (min)

compound

area (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

3.74 6.19 9.99 10.58 13.15 16.58 18.48 19.56 20.49 21.75 22.8 23.32 23.39 23.97 25.18 26.63 27.43 28.14 28.57 29.28 30.72 31.73 32.91 33.66 34.67 37.58 39.28 54.43

2-propanone, 1-hydroxymethyl isobutyl ketone furfural benzene, chloro2-cyclopenten-1-one phenol 2-cyclopenten-1-one, 2-hydroxy-3-methylphenol, 2-methylphenol, 4-methylphenol, 2,6-dimethylphenol, 2-ethylphenol, 3,5-dimethylphenol, 2,3-dimethylphenol, 4-ethyl1,2-benzenediol phenol, 2-ethyl-5-methyl1,2-benzenediol, 3-methylethanone, 1-(2,5-dihydroxyphenyl)1,2-benzenediol, 4-methyl2-methoxybenzyl alcohol 1,3-benzenediol, 4,5-dimethyl1,3-benzenediol, 4-ethyl2-allyl-4-methylphenol phenol, 4-ethyl-2-methoxy1,3-benzenediol, 4-propylbenzamide, 2-(methylamino)1-naphthalenol, 2-methylphenanthrene, 1-methyl-7-(1-methylethyl)-

0.50 1.28 1.65 3.34 1.04 5.83 1.15 3.66 6.72 1.16 0.51 3.21 1.03 2.51 9.33 1.58 4.06 1.05 8.64 0.69 1.16 6.54 1.05 1.24 1.09 1.13 0.60 1.16

increase in Mw to the increase that would theoretically occur after infinite storage time. This deviation was thought to be due to the complexity of the oil composition and reactions that 7374

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Figure 12. Relative GC−MS peak area change of identified compounds in CH2Cl2-soluble fractions during accelerated aging.

occur during aging. The increase in Mw was less than 1000 Da, indicating that linking reactions between separate lignin chains rarely occurred. It could therefore be inferred that CH2Cl2-soluble low-molecular-weight components, such as acids, phenols, and alcohols, could be reacting with pyrolytic lignin reactive sites, such as phenolic −OH, aliphatic −OH, −OMe, and double bonds, to give rise to increasing yields of pyrolytic lignins as well as increased molecular weights of existing lignins. ATR−FTIR. Changes in the specific functional groups present in the control and heat-treated samples were evaluated by ATR−FTIR spectroscopy. Figure 9 shows the FTIR spectra of the control and typical heat-treated pyrolysis oils. All samples showed large broad peaks at 3300 cm−1 for the O−H stretch, indicating the presence of OH and COOH functional groups.24 The weak peak at 3040 cm−1 was assigned to the C−H stretch of aromatics.44 The peaks at 1510 and 1465 cm−1 were related to the C−C stretch for aromatics and substituted aromatic ring skeletal vibrations.42 The peak for the protonated carboxyl acid groups51,52 at 1740 cm−1 overlapped with the strong band at 1700 cm−1 assigned to the carbonyl group in aldehydes, ketones, carboxylic acids, and esters.42,53 The bands at 1600, 1510, and 1440 cm−1 corresponding to aromatic ring vibrations were partly overlapped with the conjugated ketone CO stretch at 1650 cm−1.53 The strongest band at 1200 cm−1 was assigned to the C−O stretch, C−C stretch, and in-plane C−H bending.53 Below 1500 cm−1, the absorption bands from different components were heavily superimposed, which made peak interpretation difficult. The transmittance intensity percentages were normalized to the highest peak and used to represent the relative concen-

trations of selected functional groups. Figure 10 displays the evolution of the peak height ratios (PHRs) with aging time. Obvious changes were observed for all aged oils, except those aged at the lowest temperature in the study, 40 °C. Higher aging temperatures led to higher rates of change in PHRs. The increase in O−H stretching PHR was in agreement with higher water contents, as measured by Karl Fisher titration for the aged oils. For all heat-treated samples, aromatic skeletal vibration PHRs did not show significant change, indicating that ring formation reactions were not prevalent. The intensities of the bands assigned to −CH2− and −CH3 increased with aging time and can be attributed to the formation of new ester and/or ether groups during aging. For oils aged below 150 °C, the intensities of carbonyl bands centered around 1700 cm−1 slightly increased with aging time, which can be explained by the formation of carbonyl groups through oxidation reactions of primary and secondary alcohols.11,44,54 Decreases in carbonyl peak intensities with time for oils aged at and above 150 °C had also been observed by other researchers.48,55,56 This was due to aldehydes and ketones alone reaction in aldol condensations or reaction with some lignin monomers, resulting in an increase in the HMM lignin fraction.55 Zero-valent metals, in this case Fe, could accelerate the conversion of carbonyl groups assigned to ketones and aldehydes into methylene groups at high temperatures.57 While the decomposition of sugar compounds and oxidation reactions of primary and secondary alcohols led to an increase in the carbonyl content, the net effect of all aging reactions resulted in a decrease in the carbonyl content. GC−MS. The compositions of the control and aged pyrolysis oil samples were investigated by GC−MS. CH2Cl2 was used to extract low-molecular-weight compounds prior to 7375

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pounds occurred at and above 200 °C, resulting in a significant increase in the water content and reactive monomer species. FTIR results showed that the aging reactions were similar over the temperature range from 40 to 80 °C and from 150 to 290 °C. Therefore, the commonly used accelerated aging method should be carried out below 150 °C. GC−MS results suggested that furfural, 2-cyclopenten-1-one, 2-hydroxy-3methyl-2-cyclopenten-1-one, hydroxybenzenes having available ortho/para sites, and active sites of HMM lignin contributed to increases in the water content, HMM lignin content, and molecular weight. Because this reaction was more severe at higher temperatures, additives with low-molecular-weight and monofunctional groups should be added to slow or stop this reaction before further high-temperature hydrotreatments or other upgrading processes.

testing. As a result of an injector temperature of 270 °C, the GC analysis could only detect 25−38 wt % of the oil depending upon the aging conditions. Figure 11 shows the total ion chromatogram (TIC) GC−MS of the CH2Cl2-soluble fraction, and peak identification is presented in Table 2. Although trends in the carbonyl content changing, as shown by FTIR, were opposite for oils treated below and above 150 °C, qualitatively, the major GC−MS peaks present after aging matched those of unaged pyrolysis oil, regardless of the aging temperature range. By normalization of the spectra based on the area of the internal standard, chlorobenzene, comparisons could be made regarding the relative concentrations of compounds before and after aging. Figure 12 shows a comparison of the normalized areas of the major peaks present in the GC spectra for the control and heat-treated pyrolysis oils at 80 and 290 °C. The concentrations of compounds 3, 5, 7, 8, 9, 10, 11, 14, 15, 16, 18, 19, 20, 22, 23, 24, and 27 decreased with aging time, and rates of change increased at higher temperatures. In contrast, 1-methyl-7(1-methylethyl)phenanthrene (28) content significantly increased with aging time. The abundance for 1-hydroxy-2-propanone (1) initially increased during 24 days at 80 °C and then decreased as aging progressed. The fact that the maximum yield of 1-hydroxy-2-propanone was attained by torrefaction of biomass feedstock at 280 °C58 helped to explain the higher content of 1-hydroxy-2-propanone and its increase in content upon heat treatment at 290 °C. Furfural (3) is very reactive and can react with phenolic compounds via condensation reactions in the presence of acids.59 The carbon double bond and hydroxyl group can activate 2-cyclopenten-1-one (5) and 2-hydroxy-3-methyl-2-cyclopenten-1-one (7) toward polymerization.49 Phenols can react with aldehydes in the presence of acids. While phenol (6) showed minor decreases in the concentration after heat treatment, it was reasonable to conclude that 4-ethylphenol (14), 4-methyl-1,2-benzenediol (19), 4-ethyl-2methoxyphenol (24), and 2-methyl-1-naphthalenol (27) would be mostly consumed given enough reaction time. Generally, the contents for all hydroxylbenzenes having available ortho/para sites decreased with aging time, and this trend was more distinctive when the ortho/para sites were activated by the augmenting effect of alkyl, hydroxyl, or methoxy groups. 2-Allyl-4methylphenol (23) was the only detected lignin product with double bond side chains, which can easily react with itself or with other functional groups to produce larger molecular weight compounds.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 405-325-0465. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This effort was funded by KiOR, LLC, and the experimental efforts were conducted while the authors were affiliated with the Swalm School of Chemical Engineering at Mississippi State University (MSU). The authors acknowledge the efforts of Dr. Yan Luo at MSU for carrying out GC−MS measurements.

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CONCLUSION Thermal processing under sealed conditions of pyrolysis oil formed via catalytic cracking was carried out over a broad temperature range, from 40 to 290 °C. Control and heattreated oil samples were characterized by a combination of techniques to measure water content, HMM lignin content, thermal behaviors, molecular weight, and chemical compositions before and after thermal processing. All heat-treated samples showed no phase separation as water contents remained relatively low (