Alcohol Stabilization of Low Water Content Pyrolysis Oil during High

Nov 7, 2017 - The addition of alcohols is a promising method to pretreat and stabilize pyrolysis oil by converting carboxylic acids and reactive carbo...
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Alcohol Stabilization of Low Water Content Pyrolysis Oil during High Temperature Treatment Laibao Zhang,† Yan Luo,‡ Rangana Wijayapala,§ and Keisha B. Walters*,∥ †

Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Sustainable Bioproducts, Mississippi State University, Mississippi State, Mississippi 39762, United States § Swalm School of Chemical Engineering, Mississippi State University, Mississippi State, Mississippi 39762, United States ∥ School of Chemical, Biological and Materials Engineering, The University of Oklahoma, Norman, Oklahoma 73019, United States ‡

S Supporting Information *

ABSTRACT: The addition of alcohols is a promising method to pretreat and stabilize pyrolysis oil by converting carboxylic acids and reactive carbonyl compounds into esters, ethers, and acetals. In this study a series of alcoholsmethanol, ethanol, 1propanol, 2-propanol, or 1-octanolwas added to crude pyrolysis oil, and these mixtures were stored at 200 °C for different periods from 6 to 50 h to investigate the impact of heat treatment on the oils’ physicochemical properties. All oil/alcohol mixtures were characterized by Karl Fisher titration, viscometer/rheometer, differential scanning calorimetry (DSC), gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, and gas chromatography−mass (GC-MS) spectrometry. Phase separation is observed for all aged oil/alcohol mixtures. The time-dependent rheologies of heat treated oil/ 1-propanol, oil/2-propanol, and oil/1-octanol mixtures are found to be well fitted by the Herschel−Bulkley model. Isothermal DSC traces directly confirm that low molecular mass (LMM) alcohols (methanol, ethanol, 1-propanol, and 2-propanol) improve the stability of pyrolysis oil. Although 1-octanol is less efficient in slowing the aging reactions, it significantly reduces the increase rate of viscosity and molecular weight of pyrolysis oil compared with LMM alcohols. FTIR spectra suggest reactive carbonyl and aldehyde groups are captured by the added monofunctional alcohols. GC-MS results indicate esterifications contribute significantly to mitigate aging reactions. The introduction of LMM alcohols or a combination of LMM alcohols and HMM alcohols is a promising pretreatment method before the further catalytic upgrading procedure on the crude pyrolysis oil.

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INTRODUCTION Pyrolysis oil, also known as bio-oil, is produced from fast pyrolysis of biomass using elevated temperature and oxygenfree conditions.1 Pyrolysis oil is neither chemical nor thermally stable; resultant increases in water content, average molecular weight, and viscosity, termed as aging,2 obstruct its further upgrading using conventional petroleum processes. The acidity originates primarily from high concentrations of formic and acetic acid, while the chemical instability of pyrolysis oil is believed to be due to the presence of compounds containing reactive carbonyl, hydroxyl, and aldehyde groups that can readily undergo esterification, acetalization, oxidization, dimerization, and condensation/polymerization reactions to form compounds with higher molecular weights.3 These drawbacks make the reduction of oxygen content necessary to transform pyrolysis oil into a biorefinery feedstock, heating fuel, and/or transportation fuel. Deoxygenation reactions via both catalytic and noncatalytic means have been proved to be very effective to improve the quality of pyrolysis oil. Noncatalytic methods have generally been carried out in the presence of supercritical alcohols.4−7 To reduce cost in the upgrading process, some research has focused on processing pyrolysis oils in existing hydrotreatment and fluid catalytic cracking (FCC) facilities which are routinely used in the fossil oil industry. Catalytic cracking occurring at high temperatures causes rejection of oxygen through simultaneous dehydration and decarboxylation processes. Low © XXXX American Chemical Society

hydrocarbon yields, catalyst deactivation, formation of undesirable products (e.g., coke, tar, and char), and reactor plugging represent significant barriers to commercialization.8−12 Catalytic hydrotreatment with precious metal catalysts and conventional catalysts developed for petroleum hydroprocessing has been identified as a promising method to improve the properties of pyrolysis liquids and make them suitable as a refinery feed. Hydrotreating is performed at high temperatures (typically 250−450 °C), and oxygen is eliminated in the form of water, CO, CO2, etc.13,14 While this method consumes a considerable amount of hydrogen, adjustments are needed to the conventional hydroprocessing applied to petroleum feed stocks. Both hydrotreatment and catalytic cracking involve high temperatures, and it is well established that heat treatment can accelerate the aging reactions in pyrolysis oil. During catalytic upgrading processes, severe aging reactions occur when pyrolysis oil is heated to above 200 °C.15 An efficient method to suppress aging reactions is via the addition of commercially available chemicals. A variety of small molecules, including methanol,16−22 ethanol,17,20,23−26 isopropanol,20 acetone,17 ethyl acetate,17,27 methyl isobutyl ketone,17 methyl isobutyl ketone−methanol mixture,17 acetone−methanol mixture,17 and Received: August 4, 2017 Revised: November 1, 2017

A

DOI: 10.1021/acs.energyfuels.7b02276 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels methanol−ethyl acetate−acetone mixture28 have been identified as effective stabilizers. Considering the cost and beneficial effect in decreasing the rate of aging reactions, alcohols, especially methanol is the most promising additive. However, this alcohol additive strategy has mainly been investigated on aqueous solutions of pyrolysis oil at low temperatures (≤90 °C). To the authors’ knowledge, no work has been done on the stabilization of pyrolysis oil at high temperatures suitable for catalytic upgrading. In the present study, stabilization effects on pyrolysis oil with the addition of different alcohols were investigated during high temperature treatment. Alcohols with different structures and carbon contentmethanol, ethanol, 1propanol, 2-propanol, and 1-octanolwere introduced into the pyrolysis oil at a 1:6 ratio (v/v) while at room temperature, and then the mixtures were stored/aged at 200 °C for a specified time period. The effects of alcohol addition on water content, high molecular mass (HMM) lignin content, viscosity, molecular weight, and chemical composition of control and heat treated (aged) pyrolysis oil/alcohol mixtures are discussed.

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Gel Permeation Chromatography. Gel permeation chromatography (GPC) characterizations were carried out on a custom-built GPC with a Waters 600 controller, Waters 610 fluid pump, and Varian Star 9040 refractive index detector. Two columns (Waters Styragel HR 5E and Agilent mesopore 300 × 7.5 mm) were connected in series for the separation of pyrolysis oil components. Tetrahydrofuran (THF) was used as the mobile phase flowing at 2.0 mL/min. The pyrolysis oil/alcohol sample was dissolved into THF at approximately 2 wt % and then filtered through a 0.45 μm PVDF syringe filter. Polystyrene standards (PSS Polymer Stands) with molecular weights 126, 266, 486, 582, 891, 2780, 6480, 10 261, and 18 200 Da were used to generate the calibration curve. ATR-FTIR Spectroscopy. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected using a Nicolet 6700 spectrometer (MCT-A* detector, 4 cm−1 resolution) with a Pike VeeMAX III accessory and ZnSe ATR crystal. GC-MS Spectroscopy. Gas chromatography−mass spectroscopy (GC-MS) was performed on a Hewlett-Packard 5890 II gas chromatograph equipped with a Hewlett-Packard 5971 mass spectrometer. A column (30 m × 0.32 mm × 0.25 μm) coated with 5% phenyl 95% dimethylpolysiloxane was used with the temperature program that ramped from 40 to 280 °C at 5 °C/min followed by an isothermal hold at 280 °C for 15 min. The injector temperature was 270 °C. The mass spectrometer employed a 70 eV electron impact ionization mode. The source and interface temperatures were 250 and 270 °C, respectively. The oil/alcohol mixtures were dissolved into CH2Cl2 and then filtered with a 0.45 μm PVDF syringe filter to remove solids and the HMM lignin. Chlorobenzene (anhydrous, 99.8%) was used as an internal standard for the semiquantitative analysis.

EXPERIMENTAL SECTION

Pyrolysis Oil/Alcohol Mixture Preparation. The crude pyrolysis oil used in this research was obtained from KiOR, LLC. It is produced from pine wood chips using a modified fluid catalytic cracking (FCC) process29 similar to what is used in the fossil oil industry. Pyrolysis oil was mixed with one of five different alcohols: methanol (Fisher Chemical, 99.9%), ethanol (Sigma-Aldrich, >99.5%), 1-propanol (Sigma-Aldrich, 99.7%), 2-propanol (Sigma-Aldrich, 99.5%), or 1octanol (Sigma-Aldrich, 99.9%). A total of 10 mL of alcohol was added to 60 mL of pyrolysis oil with mechanical stirring at room temperature. These pyrolysis oil/alcohol mixtures were then held at room temperature for 1 week. Pyrolysis oil/alcohol mixtures without heat treatment were used as controls for the samples aged at 200 °C. The crude pyrolysis oil was also shown in plots and tables as a comparison of the property change. Pyrolysis Oil/Alcohol Aging. Approximately 10 mL of pyrolysis oil/alcohol mixtures were sealed in N2 purged 1/2 in. SS316 Swagelok tubes and stored at 200 °C in an oven for specific time intervals. The tubes were then cooled to room temperature, and weight losses after heat treatment were measured. In all cases, the sample mass losses were below 0.1 wt %. The samples were then transferred to 50 mL vials and stored at 2.5 °C before the physicochemical properties were measured. Solvent Extraction. CH2Cl2 was used to separate the HMM lignin (CH2Cl2-insoluble part). The oil/alcohol/CH2Cl2 mixtures were shaken for about 10 min and then left to equilibrate for 3 h at room temperature. The CH2Cl2-soluble fraction was separated by decantation from the bottom phase. HMM lignin was weighted after the removal of CH2Cl2 by evaporation at room temperature. Water Content. Water content was measured by Karl Fisher titration following ASTM method E-203-01 using Hydranal titrant and Hydranal solvent. Phase separations were observed for all heat treated pyrolysis oil/alcohol mixtures. The formed water phase was decanted, and Karl Fisher titrations were performed on the remaining homogeneous fraction. All samples were first dissolved into Hydranal solvent before the measurement. Rheology. Rheology test was performed on a TA Instruments AR 1500ex rheometer with a 60 mm aluminum parallel plate accessory. Step-flow shear data were collected at 40 °C over the 0.1−100 s−1 shear rate range with 10 points per decade. The maximum limit for the rheometer is 15 000 cP. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) data were collected with a TA Q2000 DSC instrument using a sample size of ∼2 mg. Untreated (control) pyrolysis oil and oil/alcohol samples were analyzed using isothermal runs at 200 °C with a N2 flow rate of 50 mL/min.

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RESULTS AND DISCUSSION The presence of large amounts of oxygenated compounds is the primary reason for the physical and chemical property differences between fossil fuels and pyrolysis oils resulting in an incompatibility for pyrolysis oil to be integrated into existing fossil fuel refinery processes. The drawbacks such as thermal instability, acidity/corrosiveness, and low heating value imparted by these oxygen compounds can be mitigated by introducing new chemicals to reduce the content of reactive chemical species, notably aldehydes, ketones, carboxylic acids, alkenes, and guaiacol-type molecules.3 Thermodynamically favored etherification and esterification reactions effectively resulted in reduced acid content, increased pH value, and improved stability of pyrolysis oil.17 Using ASPEN PLUS simulations, Diebold and Czernik17 found that at 25 °C both the reaction of methanol with formaldehyde to make dimethoxymethane and the reaction of methanol with acetic acid to make methyl acetate went nearly to completion, while these reactions were not as highly favored at higher temperatures. Based on the above considerations, the pyrolysis oil/alcohol mixtures were mixed and left on a shaker at room temperature for 1 week in order to allow the reactions to reach equilibrium before further analysis. Water Content. The water content of the crude pyrolysis oil was 4.77 ± 0.27 wt %. Phase separation was not observed for any of the heat treated crude pyrolysis oils. In contrast, 2−5 vol % water appeared on the surface of pyrolysis oil/alcohol mixtures after 6 h of storage at 200 °C. The separation was irreversible, and therefore, the formed water was decanted and Karl Fisher titrations were performed on the remaining homogeneous fraction. Figure 1 shows the water content as a function of aging time for the crude pyrolysis oil and oil/ alcohol mixtures. All samples showed similar trends with aging time at 200 °C; water content initially increased, peaked after about 10 h heat treatment, and then decreased and leveled off. B

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HMM lignin content were observed for all the oil/alcohol mixturesexcept oil/methanol. The aging process could be slowed by the formation of more stable compounds,17 for example acetals generated through reactions of alcohols with aldehydes, ketones, and/or anhydrosugars, or polyesters produced by reversible reactions of acidic oligomers with monofunctional alcohols and esters by transesterification reactions. Additionally, the added alcohols serve to physically dilute the concentration of reactive monomers/oligomers which may slow the aging rate, unless the reaction is zero order. HMM lignin content generally decreased with increasing carbon chain length of the added alcohols. The lower HMM lignin content in oil/1-octanol samples might be explained by the better solubility of 1-octanol in the oil. Indeed, 1-octanol has been used as an emulsifier37 to produce a stable pyrolysis oil/biodiesel emulsion without precipitating lignin. Rheology. Viscosity is an important criterion for the practical usage of pyrolysis oil. The viscosity of pyrolysis oil was immediately reduced by the introduction of alcohols. As displayed in Figure 3, without heat treatment the control

Figure 1. Water content of the crude pyrolysis oil and pyrolysis oil/ alcohol mixtures as a function of aging time at 200 °C.

While some research groups2,3,30−32 reported that water content in pyrolysis oil increased with the length of storage, Boucher et al.18 found no clear tendency. During the aging process, etherification, esterification, hemiacetal/acetal formation, dehydration, adol condensation, and other condensation reactions of aldehyde yield water, while hydration consumes water.3 The change in water content reflected different reactions dominated at specific periods during heat treatment. Moreover, the irreversible phase separation of pyrolysis oil/ alcohol mixtures after heat treatment also contributes the variation in water content measurement. HMM Lignin Content. The unsaturated structures33,34 of HMM lignin, as compared to native and industrial lignin, lead to instability issues for pyrolysis oil during storage. Molecular weight discrepancies between control and heat treated pyrolysis oils suggest that linking reactions between two lignin macromolecules rarely occur.2,35,36 The reactive sites in lignin, e.g., double bond, phenolic −OH, −OCH3, likely react with low molecular weight compounds present in pyrolysis oil or added alcohols resulting in an increase in HMM lignin content and molecular weight during aging. As shown in Figure 2, the HMM lignin content of pyrolysis oil increased rapidly from 33 to 69 wt % after 50 h. In contrast, only slight changes in the

Figure 3. log−log plot of the apparent viscosities for the nonheat treated crude pyrolysis oil and pyrolysis oil/alcohol mixtures (controls, aging time = 0) as a function of shear rate.

pyrolysis oil and all of the oil/alcohol samples exhibited shear thinning behavior. Methanol and ethanol displayed limited impact on reducing the viscosity of pyrolysis oil. 1-Propanol and 2-propanol showed similar effects with an initial 17% decrease in the viscosity. In terms of reducing the pyrolysis oil viscosity, 1-octanol was the most effective among the five alcohols tested. This is due to the higher hydrocarbon content enhancing its solubility in the pyrolysis oil. Pyrolysis oil typically demonstrates an increase in viscosity with aging time and temperature, and this viscosity increase is undesirable as it makes the properties dependent on storage time, increases the pumping duty, and adds difficulty and cost for atomization when the oil is used for combustion. The previously reported superiority of methanol17,20,23 in preventing aging-related viscosity increases of pyrolysis oil was not observed. Notable is that after heat treatment, all pyrolysis oils without added alcohols, oil/methanol and oil/ethanol samples had become tacky and their viscosities were outside of the rheometer test limit of 15 000 cP. Viscosities of oil/1propanol and oil/2-propanol samples aged for 50 h at 200 °C also could not be tested for the same reason. Figure 4 shows viscosity measurements as a function of shear rate at 40 °C of

Figure 2. HMM lignin contents of the crude pyrolysis oil and pyrolysis oil/alcohol mixtures as a function of aging time at 200 °C. C

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Figure 4. log−log plots of apparent viscosities as a function of shear rate for pyrolysis oil/alcohol mixtures aged at 200 °C for up to 50 h. Data for the nonheat treated crude pyrolysis oil is also shown.

oil/1-propanol, oil/2-propanol, and oil/1-octanol before and after heat treatment at 200 °C. Heat treated samples of oil/1propanol and oil/2-propanol showed very similar behaviors; viscosity increased rapidly in the first 6 h and then leveled off over the next 17 h. These trends are consistent with those observed in aging below 90 °C2,3,38 with the most rapid property changes occurring within the first days with significantly slower changes afterward. The viscosity of oil/1octanol samples continued to increase with aging time over the duration of 50 h. This extended time may be due to the pyrolysis oil/1-octanol sample not having reached chemical equilibrium even after 1 week of storage at room temperature. Non-Newtonian fluids can generally be described by some common viscoelastic models. Three models were examined for these pyrolysis oil/alcohol mixtures. These include the Power Law (eq 1), Bingham Plastic (eq 2), and Herschel−Bulkley (eq 3) models τ = kγ ṁ

(1)

τ = τB + ηBγ ṅ

(2)

τ = τH + Kγ ṅ

(3)

Table 1. Herschel−Bulkley Model Rheology Results for the Crude Pyrolysis Oil and Oil/Alcohol Samples Aged at 200 °Ca variable sample

time (h)

τH

K

n

R2

pyrolysis oil oil/methanol oil/ethanol oil/ 1-propanol

0 0 (control) 0 (control) 0 (control) 6 18 23 0 (control) 6 18 23 0 (control) 6 18 23 50

3.99545 3.20645 2.37029 2.61257 8.15583 10.3197 11.6716 2.53266 10.0105 11.7225 12.1647 0.10872 0.02896 0.03854 0.05494 0.28645

1.57772 1.44216 2.54001 0.38005 3.08682 2.20653 1.45529 0.51232 2.18978 1.47118 1.60345 0.30869 0.95077 0.84833 1.31906 1.72650

1.07311 1.00389 0.85430 1.03815 0.83220 0.92099 1.00060 0.98587 1.00916 1.01019 1.05445 0.99811 1.03475 1.02106 1.06851 1.02995

0.99967 0.99963 0.99867 0.99980 0.99658 0.99520 0.99900 0.99989 0.99990 0.99895 0.99907 0.99999 1.00000 1.00000 0.99980 0.99999

oil/ 2-propanol

oil/1-octanol

a

All pyrolysis oil/alcohol mixtures were kept at room temperature for 1 week before further heat treatment. The nonheat treated mixtures were used as controls for the samples aged at 200 °C.

where τ is the shear stress, ηB is the apparent viscosity, γ̇ is the shear rate, m and n are the Power Law and Herschel−Bulkley flow indices, respectively, and indicate the degree of deviation from Newtonian flow character, k and K are consistency indices which reflect the viscous nature of the sample, and τB and τH are the yield stresses. For the oil/alcohol mixtures, the Power Law model showed the poorest correlation (R2 < 0.98), and the Bingham Plastic model gave negative values for τB. The rheological behavior of all test samples, before and after heat treatment (aging), was best described by the Herschel−Bulkley model, which has also successfully been used to model the rheological behavior of pyrolysis oil/coal slurries.39 Table 1 summarizes the fitted rheological parameters for the pyrolysis oil and oil/alcohol mixtures before and after heat treatment. The flow index was close to 1 for all of the unaged samples, with the exception of oil/ethanol. An examination of the low yield stress values shows that 1-octanol had a higher compatibility with the pyrolysis oil than the other alcohols tested. Both the flow index, n, and yield stress, τB, generally increased with aging time. The increase of n indicates a change from pseudoplastic flow behavior to dilatant behavior. Since the dilatancy effect is generally associated with highly concentrated suspensions,40 it is probable that the observed dilatant flow behavior could be connected to the slightly higher content of HMM lignin in aged samples relative to the controls.

DSC. Chemical reactions that occur during aging can be classified as exothermic and endothermic according to whether thermal energy is released or absorbed. The DSC profile for the fast pyrolysis oil was published in our former work.41 In this work, an isothermal DSC technique has been used to monitor the net heat flows in and out of pyrolysis oil/alcohol mixtures with an empty pan as the inert reference. Figure 5 displays the isothermal DSC plots for pyrolysis oil/alcohol mixtures at 200 °C. In our prior work,41 multiple endothermic peaks caused by decomposition of sugar derivatives were observed for pyrolysis oil aged at 200 °C. In contrast, except for oil/1-octanol, no heat flow signals were observed for oil/alcohol mixtures indicating the addition of low molecular weight alcohols improved the thermal stability of pyrolysis oil. The poor performance of 1octanol can be explained by the large sterically hindered alkane group in 1-octanol which reduced its reaction rate with the active compounds in pyrolysis oil. Doerr et al.42 found that the concentration of methyl formate reached 95% of its maximum value after 72 h while methyl acetate reached 40% of the maximum value attained after 26 days. The slower formation rate of methyl acetate as compared to methyl formate suggests D

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addition, all samples showed an initial molecular weight decrease followed by a significant increase in Mw and Mn with aging time at 200 °C. Both the control and heat treated oil/alcohol mixtures exhibited polydisperse behavior, and the PDI of the oil/alcohol samples decreased with aging (Table 2). To better understand the change in molecular weight during heat treatment, the GPC traces for the crude pyrolysis oil and oil/alcohol samples before and after discrete aging times were deconvoluted into primary fractions using Gaussian distributions (Figures S1−S6). The fitted Mw peak area ratio represents the mass fraction of the five primary groups of species. Peaks 1 and 2 are grouped together as the fractions of the higher molecular mass compounds. Mw of the fractions decreases with increasing peak index number. The peak areas of these five primary Mw fractions are plotted in Figure 6. The high molecular mass fractions (peaks 1 and 2) for pyrolysis oil increased with aging time. In comparison, for all of the oil/ alcohol mixtures, the areas for peaks 1 and 2 initially increased and then tended to level off. The decreasing content of lower molecular mass fraction (peak 4, mainly benzenediols and alkylsubstituted benzenediols, Mp = 170 by GPC) suggests benzenediols and alkyl-substituted benzenediols were consumed in the aging reactions. These rapid transformations mainly occurred during the initial six hours, which is consistent with the observations in the viscosity measurements. The mass fractions of peaks 1 and 2 in pyrolysis oil and oil/methanol rapidly reached ∼70% after 6 h heat treatment at 200 °C, and the values then remained higher than the other oil/alcohol mixtures. This trend agreed well with the observations on the HMM lignin content as measured by solvent extraction. ATR-FTIR Spectroscopy. Chemical composition changes due to the addition of the alcohols were monitored by examining the functional group content of the samples using FTIR spectroscopy. The changes can be semiquantitated using a peak area ratio technique.30 All absorption bands were normalized to the strong (and relatively constant) band centered at 1200 cm−1 that can be ascribed to C−O stretch, C−C stretch, and in-plane C−H bending.43 Figure 7 shows the FTIR spectra of the crude pyrolysis oil and pyrolysis oil/alcohol mixtures after 1 week of storage at room temperature. The increase in intensity of the broad peak at 3300 cm−1 attributed to −OH and −C(O)OH functional groups43 with the introduction of alcohols can include contributions due to esterification, hemiacetal and acetal formations.20 Significant increases in the three peaks between 2960 and 2866 cm−1, assigned to the C−H stretch of methyl and methylene,43 were observed upon the introduction of alcohols. The 1700 cm−1 peak area, which corresponds to unconjugated CO stretch in aldehydes, ketones, carboxylic acids, and esters,34 changed but was within experimental error (±2%). The bands at 1600, 1510, and 1440 cm−1 for aromatic ring vibrations were partly overlapped with the conjugated CO stretch at 1650 cm−1. The obvious decrease in intensities of the 1650 and 1600 cm−1 peaks can be explained by the reduced content of conjugated carboxyl groups upon the introduction of alcohols. No further interpretations were made on bands below 1500 cm−1 because of the heavy superimposition of multiple absorption bands. Figure 8 compares FTIR spectra of pyrolysis oil and oil/ alcohol mixtures before and after heat treatment (200 °C, 50 h). All samples showed increases in the 3300 cm−1 peak which indicates the formation of water during heat treatment. The concurrent decrease of peaks at 1700, 1650, and 1600 cm−1 were more severe in the presence of alcohols. Carboxylic acids

Figure 5. Isothermal DSC curves of pyrolysis oil/alcohol samples tested at 200 °C. (Plots are staggered for clarity.)

the formation of even larger esters, e.g., octyl ester, would require a much longer reaction time. GPC. The pyrolysis oil control showed a number average molecular weight, Mn, of 620, a weight average molecular weight, Mw, of 2196, and polydispersity index, PDI, of 3.04. Table 2 lists Mn, Mw, and PDI for the control and heat treated pyrolysis oil and oil/alcohol mixtures. Upon the alcohol Table 2. Molecular Weights and Molecular Weight Distributions of the Crude Pyrolysis Oil and Pyrolysis Oil/ Alcohol Mixtures Aged at 200 °C sample pyrolysis oil

oil/methanol

oil/ethanol

oil/1-propanol

oil/2-propanol

oil/1-octanol

time (h)

Mn

Mw

PDI

0 2 6 11 23 50 0 (control) 6 18 23 50 0 (control) 6 18 23 50 0 (control) 6 18 23 50 0 (control) 6 18 23 50 0 (control) 6 18 23 50

620 712 741 758 877 919 591 852 746 834 841 589 792 732 824 833 552 792 885 765 752 515 651 739 688 819 492 652 673 793 853

2196 2103 2213 2179 2432 2649 2130 2922 2020 2767 2629 1907 2510 1877 2463 2552 1723 2238 2650 2343 2428 1737 1924 1934 2010 2359 1632 2054 2277 2394 2563

3.04 2.96 2.99 2.88 2.77 2.88 3.60 3.43 2.71 3.32 3.13 3.24 3.17 2.56 2.99 3.06 3.12 2.83 2.99 3.06 3.23 3.37 2.96 2.62 2.92 2.88 3.32 3.15 3.38 3.02 3.00 E

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Figure 6. Mass fractions of fitted GPC peaks as a function of aging time for the crude pyrolysis oil and pyrolysis oil/alcohol mixtures. Peaks 1 → 5 correspond to higher → lower mass fractions, respectively.

bands at 1700, 1650, and 1600 cm−1 corresponded to decreases in carbon content relative to oxygen content. Based on these FTIR results, it is reasonable to conclude that heat treatment of pyrolysis oil in the presence of alcohols increased the C/O ratio, which is favorable for combustion. GC-MS. The chemical composition of control and aged samples was analyzed by GC-MS. CH2Cl2 was used as the solvent in sample preparation because prior work41 suggests the CH2Cl2 soluble fraction of pyrolysis oil contributes significantly to the aging reactions. A disadvantage of this method is that the low molecular weight alcohols and some volatile compounds cannot be determined because of the relatively long solvent delay time. The introduction of alcohols to pyrolysis oil results in the formation of new compounds. Indeed, acetic acid octyl ester was observed in the control (nonheat treated) oil/1octanol sample. However, esters with short alkyl chains were not detected because of long delay time. By normalizing the spectra using chlorobenzene as an internal standard, comparisons could be made regarding the relative concentration of compounds before and after heat treatment using peak area ratios. Figure 9 illustrates the normalized peak area ratio change for the major peaks in the GC-MS spectra of control and aged (200 °C, 50 h) pyrolysis oil/alcohol mixtures. For all oil/alcohol mixtures, 1-hydroxy-2propanone and furfural were consumed after heat treatment. 1Hydroxy-2-propanone can react with aldehydes by aldol reactions. Furfural has a strong tendency to polymerize44 due to the high reactivity of carbonyl group. The significant decrease in 2-hydroxy-3-methyl-2-cyclopenten-1-one content can be explained by its high reactivity toward polymerization due to the existence of a carbon double bond and a hydroxyl group.44 There was a slight change in concentration of phenols

Figure 7. FTIR spectra of the nonheat treated crude pyrolysis oil and pyrolysis oil/alcohol mixtures (control, aging time = 0).

can react with alcohols to form esters. Aldehydes are very active functional groups in the aging reactions. Aldehydes can react with each other to form polyacetal oligomers and polymers which have limited solubility in water. Additionally, aldehydes can also react with the hydroxyl groups in phenols, sugars, alcohols, and lignins to form hemiacetals or acetals.32 For all of the oil/alcohol samples, the highest peaks attributed to C−O stretch, C−C stretch, and in-plane C−H bending became narrower and shifted to higher wavenumbers due to the formation of C−O bond after heat treatment. Scholze and Meier34 studied the composition of eight types of pyrolytic lignin obtained from pyrolysis oils produced by different fast pyrolysis processes. They found that increases in the absorption F

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Figure 8. FTIR spectra of the crude pyrolysis oil and pyrolysis oil/alcohol samples before (control) and after heat treatment at 200 °C for 50 h.

effectively mitigated the aging reactions. Although hemiacetals/ acetals were not detected in the GC-MS chromatograms of the CH2Cl2 soluble fractions, it is plausible to conclude the quick reaction of aldehydes with alcohol to form hemiacetals/acetals would contribute to mitigating polymerizations and/or crosslinking reactions of aldehydes.

and methyl substituted phenols. In contrast, hydroxybenzenes having available ortho/para sites activated by the augmenting effect of alkyl groups were active during heat treatment; 1,2benzenediol, 3-methyl-1,2-benzenediol, 4-methyl-1,2-benzenediol, and 4-ethyl-1,3-benzenediol all decreased significantly during aging and this corroborates the GPC results. Esterifications were observed in all of the oil/alcohol mixtures. The content of n-propyl acetate increased significantly in aged oil/1-propanol. Unreacted 1-octanol in the control oil/ 1-octanol sample completely disappeared after heat treatment at 200 °C while the content of acetic acid octyl ester increased substantially. Newly formed esters were also detected in the heat treated oil/alcohol mixtures: propanoic acid ethyl ester and butanoic acid ethyl ester in aged oil/ethanol; butanoic acid pentyl ester and propanoic acid propyl ester in aged oil/1propanol; acetic acid 1-methylethyl ester in aged oil/2propanol; and propanoic acid octyl ester in aged oil/1-octanol. The production of new esters suggests the formation of acids by degradation of susceptible components.45 For example, sugars can decompose to form anhydrosugars. Levoglucosan could participate in polymerization reactions or undergo hydrolysis to glucose. Glucose could undergo dehydration or elimination reactions to form reactive intermediates such as furans, acetic and formic acids, and hydroxypropanone, which in turn further facilitate these reactions.44 As confirmed by GCMS, the active acids formed during high temperature treatment were captured by the added monofunctional alcohols which

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CONCLUSIONS Pyrolysis oils subjected to elevated temperatures characteristically show increased viscosity and molecular weight over time. The introduction of alcohols, methanol, ethanol, 1-propanol, 2propanol, and 1-octanol, to pyrolysis oil prior to aging at 200 °C mitigated these viscosity and molecular weight changes. Time-dependent viscoelastic behavior of pyrolysis oil and oil/ alcohol mixtures were found to be fitted well by the Herschel− Bulkley model. The rheological characteristics of aged oil/1propanol, oil/2-propanol, and oil/1-octanol samples changed from pseudoplastic to dilatant with increased aging times. 1Octanol was the most effective among the five alcohols in reducing the viscosity of pyrolysis oil and rate of viscosity increase. Isothermal DSC results suggested that low molecular weight alcohols (methanol, ethanol, 1-propanol, and 2propanol) can effectively increase the thermal stability of pyrolysis oil. FTIR confirmed the introduction of alcohols accelerated the decarboxylation during high temperature treatment and reduced the oxygen content. Benzenediol and alkyl-substituted benzenediol played an important role in G

DOI: 10.1021/acs.energyfuels.7b02276 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 9. GC-MS peak area ratios of major compounds evident in pyrolysis oil/alcohol samples before (control) and after heat treatment at 200 °C for 50 h.

increasing the molecular weight and viscosity of pyrolysis oil/ alcohol mixtures. GC-MS data also supported that aging reactions were mitigated by reactions of added alcohols with active components formed during high temperature treatment. The introduction of low molecular weight alcohols or combinations of low and high molecular weight alcohols is a promising strategy in terms of both improving the stability and reducing the viscosity and molecular weight of pyrolysis oils before catalytic upgrading processes at high temperatures.

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Keisha B. Walters: 0000-0002-0875-2659 Funding

This effort was funded in part by KiOR, LLC. Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02276. Detailed information regarding deconvolution of GPC curves of the crude pyrolysis oil and pyrolysis oil/alcohol mixtures before and after heat treatment at 200 °C. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 405-325-0465. ORCID

Laibao Zhang: 0000-0002-2325-5124 H

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DOI: 10.1021/acs.energyfuels.7b02276 Energy Fuels XXXX, XXX, XXX−XXX