Article pubs.acs.org/EF
Upgrading of Bio-Oil Using Supercritical 1‑Butanol over a Ru/C Heterogeneous Catalyst: Role of the Solvent Xingmin Xu,†,‡ Changsen Zhang,†,‡ Yunpu Zhai,†,‡ Yonggang Liu,†,‡ Ruiqin Zhang,*,†,‡ and Xiaoyan Tang‡,§ †
College of Chemistry and Molecular Engineering, and ‡Research Institute of Environmental Science, Zhengzhou University, Zhengzhou 450001, China § College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China S Supporting Information *
ABSTRACT: Bio-oil was upgraded using supercritical 1-butanol over a Ru/C heterogeneous catalyst. The results clearly demonstrated that the use of a supercritical solvent resulted in an upgraded product with improved properties compared to upgraded bio-oil using no solvent or subcritical solvent conditions. These improvements included a decrease in oxygen content from 24.9 or 21.4 to 14.5% and an increase in the high heating value (HHV) from 27.9 or 30.5 to 32.0 MJ kg−1. The most important improvement is that carbon deposition was limited to only 0.2% through the use of a supercritical solvent compared to 9.9% without a solvent. Thus, coking was overcome effectively during the upgrading process. The solvent played several roles: in addition to being the reaction medium and reactant, the solvent facilitated hydrogen dissolution, protected the catalyst, and enhanced the product properties. The reaction pathways in supercritical bio-oil upgrading primarily include esterification, etherification, acetalization, hydrogenation, and hydrodeoxygenation. In this study, the properties of upgraded bio-oil purified via vacuum distillation to remove the solvent were quantified.
1. INTRODUCTION Alternative energy sources are urgently needed because of increasing energy demand and the decreasing supply of fossil fuels worldwide, in addition to environmental concerns.1,2 One promising method for fuel production in the future is the conversion of biomass into bio-oil, followed by upgrading of the bio-oil to biofuel.3 Bio-oil can be produced from lignocellulosic biomass by fast pyrolysis and is highly regarded as a substitute for petroleum fuels.4 However, bio-oil has numerous inherent limitations, including high oxygen content, high acidity, high moisture, low heating value, and thermal instability.5 These undesirable properties render bio-oil unsuitable as a transportation fuel. A process for upgrading bio-oil is required to enable the practical application of this alternative energy source. The hydrodeoxygenation (HDO) process is the most important route for upgrading bio-oils to generate liquid transportation fuels. The HDO process has been extensively investigated.3,6,7 However, the critical disadvantage of the HDO process is the thermal instability of bio-oil, which results in coke formation, thus leading to catalyst deactivation.3,8 The use of solvent to reduce coke formation and enhance the HDO of biooil is a promising approach. In fact, the use of solvents in bio-oil upgrade processes have been previously evaluated.9−11 In these previous studies, the solvent enhanced the quality of bio-oil through physical or chemical methods. For example, decalin and tetraline have been used as hydrogen-donor solvents to reduce coke formation during the upgrading of bio-oil.12,13 Subsequently, hydrocarbons were used as solvents for bio-oil upgrading.14,15 However, hydrocarbons are not easily blended with bio-oil because of their hydrophobicity, which leads to only a partial upgrade of bio-oil components. © XXXX American Chemical Society
Recently, supercritical solvents have been employed for bio-oil upgrading. Supercritical fluid conditions can enhance mass and heat transfer; supercritical fluids can also behave similarly to a liquid, with extremely high dissolving power, and similarly to a gas, with extremely high diffusivity, resulting in sufficient dissolution of the reactant and in the formation of a homogeneous reaction environment.16 The supercritical solvents used in upgrading bio-oil mainly include methanol and ethanol.17−22 Previous supercritical solvent studies have most commonly evaluated factors that affect the upgrading process18,19,22 or the performance of different catalysts.17,20 Unfortunately, the role of solvent has not been elucidated in these studies. In particular, solvent-mediated reactions have not been specifically studied. In addition, both methanol and ethanol exhibit high polarity (polarity index of approximately 5.1)23 and are highly soluble in water (100 wt %), resulting in a solvent miscible with the water in upgraded bio-oil. Consequently, the upgraded bio-oil contains a high water content, e.g., 16− 30%.21,22 Finally, the reported properties of upgraded bio-oil are masked by the presence of large amounts of solvent, resulting in misleading information, e.g., overestimations of the high heating value (HHV) of 25−29 MJ kg−1.17 In addressing the aforementioned concerns, we undertook this study to illustrate the role of the solvent in the bio-oil upgrading process using supercritical 1-butanol with a Ru/C catalyst. The selection of 1-butanol (supercritical conditions: T = 287 °C, P = 4.9 MPa) is based on its rather weak polarity and its previous use Received: April 29, 2014 Revised: June 11, 2014
A
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Table 1. Experimental Results for Bio-Oil Upgrading with/without Solvent over a Ru/C Catalyst under Different Conditionsa upgraded bio-oil without solvent cases properties (organic phase) density (kg L−1) viscosity (40 °C, cSt) moisture (wt %) HHV (MJ kg−1) pH elemental analysis (wt %) C H O N DOD (%) organic phase yield (wt %)b solid yield (coke, wt %)
bio-oil
250 °C
with solvent 300 °C
250 °C
280 °C
300 °C
1.2 22.3 21.2 17.9 2.6
1.2 254.4 14.1 25.4 3.0
1.2 156.1 11.5 27.9 3.2
0.9 7.4 10.7 29.5 4.2
0.9 6.6 8.4 30.5 4.3
0.9 5.7 6.7 32.0 4.4
44.6 6.9 48.0 0.5
61.7 7.3 30.5 0.5 36 52.0 (H); 0 (L) 2.5
67.0 7.6 24.9 0.5 48 40.3 (H); 2.0 (L) 9.9
64.3 10.5 24.7 0.2 46 0 (H); 84.6 (L) 0.3
67.5 10.6 21.4 0.3 53 0 (H); 76.5 (L) 0.3
72.4 11.3 14.5 0.2 68 0 (H); 69.4 (L) 0.2
Note: All experimental runs were 3 h, and the initial H2 pressure was 2 MPa; solid yield (wt %) = (msolid − mcat.)/mfeed × 100%. (H) - heavy oil, (L) - light oil; 1-butanol was used as the solvent in the process of upgrading bio-oil in the presence of solvent. bWithout the solvent process, this refers to heavy oil (light oil not analyzed because of its negligible amount). With the solvent process, this refers to light oil (no heavy oil phase is present). a
Figure 1. Schematic of the bio-oil upgrading experiment. from Shanxi Rock New Materials (Baoji, China). The Ru content of the catalyst was 5 wt % (dry basis), with a total surface area of 800 m2 g−1 and an average catalyst particle size of 74 μm. 1-Butanol (analytical grade, obtained from Sinopharm Chemical Reagent) was used without further purification. Hydrogen, argon/helium (carrier gas), and calibration gases were all purchased from Beijing Ruizhx Technology. 2.2. Experimental Procedures. Bio-oil hydrogenation experiments were performed in a batch-scale autoclave (500 mL) equipped with a magnetic stirrer (Weihai Automatic Control). First, control experiments (without solvent) were performed to investigate coke formation at different temperatures (250 and 300 °C). Then, bio-oil was mixed with solvent and used as the feedstock to determine the role of solvent and the possible reaction pathways. A total of 50 g of bio-oil was mixed with 50 g of 1-butanol and 5 g of catalyst (Ru/C). After the autoclave was flushed with N2 and purged with 2 MPa H2 (to replace nitrogen) three times, the reactor was pressurized with 2 MPa H2 at room temperature. The reactor was then heated to the intended reaction temperature (250−300 °C, with a corresponding pressure of 8.8−11.5 MPa) and run for 3 h. In order to keep the same reaction pressure at the desired temperature, the system pressure was adjusted by a regulating valve installed in the reaction system. After 3 h, the reactor was allowed to cool to ambient temperature by cooling water. The volume of off-gas was measured using a wet-type gas meter, and the off-gas was collected with a gas bag to detect the gaseous composition by gas chromatography-thermal conductivity detector (GC-TCD). The reactor was then opened, and the suspension was centrifuged to separate the liquid phases from the solids, which include the spent catalyst. Separation of the aqueous, light oil, and heavy oil phases was performed using a separatory funnel. The weight of all phases
as a solvent for the conversion of biomass to bio-oil and for biooil upgrading.24−26 The objectives of this study are (1) to outline the role of the solvent in the supercritical upgrading of bio-oil; (2) to clarify the solvent-mediated reactions that occur between bio-oil and upgraded bio-oil by monitoring chemical compounds to determine the different reaction pathways; (3) to quantify the properties of actual upgraded bio-oil by employing vacuum distillation to separate the solvent from upgraded bio-oil; (4) to conduct control experiments without solvent addition to determine the extent of enhancement with solvent; and (5) to perform experiments (280 °C) close to supercritical conditions (300 °C) with solvent addition to determine the effects of using supercritical 1-butanol. We hope that the results of the present study further expand the current understanding of supercritical fluids for bio-oil upgrading, with the expectation that this process will generate high-quality upgraded bio-oil for subsequent applications or further upgrading.
2. EXPERIMENTAL SECTION 2.1. Materials. Bio-oil obtained from fast pyrolysis of pine sawdust was provided by the Key Laboratory of Environmental Chemistry and Low Carbon Technologies of Henan Province, Zhengzhou University (Zhengzhou, China). The typical properties of the bio-oil are presented in Table 1. The commercial noble-metal catalyst Ru/C was obtained B
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⎛ wt% Oproduct ⎞ DOD = ⎜1 − ⎟100% wt% Ofeed ⎠ ⎝
was determined for mass balance calculations. The organic phase was analyzed using various techniques, as described in section 2.3. The solid was thoroughly washed with alcohol and dried (overnight at room temperature, followed by 3 h at 80 °C) before further characterization, described in section 2.4. Because the excess solvent present in the upgraded product can mask the properties of the upgraded bio-oil, distillation was performed to separate the solvent from the upgraded bio-oil. This separation was performed using reduced-pressure distillation at 60 °C and −0.1 MPa for 40 min in a RE-52AA rotary evaporator (Shanghai Yarong Biochemical Instrument). A schematic of the upgrading experiment is presented in Figure 1. 2.3. Product Analysis. Gas chromatography−mass spectrometry (GC−MS) analyses were performed using an Agilent 7890A-5975C equipped with a VF-1701ms capillary column (30 m × 0.25 mm × 0.25 μm) to determine the compositions of the bio-oil and the organic-phase products. The GC split was 1:100, and the injector temperature was set to 250 °C; the injection volume was 1 μL (bio-oil and upgraded products were diluted 10-fold with CH2Cl2). The oven temperature was maintained at 50 °C for 3 min, increased to 200 °C at a rate of 3 °C min−1, and then maintained at 200 °C for 50 min. Helium was used as the carrier gas, with a constant flow rate of 1 mL min−1. The mass spectrometer was equipped with an electron-impact ionization source with an electron energy of 70 eV. The compositions of the bio-oil and the organic-phase products were identified using the NIST08 mass spectral library. Relative contents were determined by area normalization. Gas analysis was performed as described elsewhere.27 The elemental compositions of the bio-oil and upgraded products were determined using a Thermo Electron Flash EA 1112 analyzer (Delft, The Netherlands). The heating values of the samples were measured with a ZDHW-6000 automatic calorimeter (Hebi Instrument, Henan). The water content in the samples was determined using a Karl Fischer KF-1A automatic titrator (Shanghai Baoshan Fine Working Electronic Instrument). Sample viscosities were determined at 40 °C using a kinematic viscosity instrument (TimePower Measure and Control Equipment, Beijing). The pH values of the bio-oil and upgraded products were determined using a PHS-25 analyzer (LeiCi Analysis Instrument Factory, Hangzhou). 2.4. Catalyst Characterization. X-ray diffraction (XRD) analyses were performed on an X’Pert PRO (PANalytical, The Netherlands) Xray diffractometer equipped with a Cu Kα radiation source. Diffraction patterns were recorded by scanning at angles from 10 to 90° in 0.05° step increments and with an acquisition period of 10 s per step. Thermogravimetric (TG) analyses of fresh and spent catalysts were performed on a STA-449C TG analyzer (NETZSCH, Germany). Catalyst samples (ca. 0.01 g) were placed in corundum crucibles and subsequently heated at a constant heating rate of 10 °C min−1 from ambient temperature to 1000 °C. All measurements were conducted in air (0.1 MPa). The carbon residue deposited on the catalyst was determined from the difference in weight loss (after TG) of the fresh and spent catalysts. Nitrogen adsorption−desorption isotherms were measured with a Micromeritics Nova 1000e system (Quantachrome). The surface area of the catalyst was calculated according to the Brunauer−Emmett−Teller (BET) equation for relative pressures (P/P0) between 0.0 and 0.2. The total pore volume was determined from the adsorption and desorption branches of the nitrogen isotherms at P/P0 = 0.99. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 20 STWIN transmission electron microscope (FEI, The Netherlands) operated at an accelerating voltage of 200 kV. 2.5. Performance Evaluation. For experiments where bio-oil was mixed with solvent, the conversion of each individual component in the bio-oil is impossible to evaluate. Thus, several important gross parameters were used to evaluate the oil yield (Yobs) and the degree of deoxygenation (DOD): Yobs =
mproduct mfeed
100%
(2)
where mproduct and mfeed are the mass of the product and feedstock, respectively, and wt % Oproduct and wt % Ofeed are the mass percent of oxygen in the product and feedstock, respectively.3
3. RESULTS AND DISCUSSION 3.1. Upgrading Bio-Oil in the Absence of Solvent over a Ru/C Catalyst. The thermal instability of bio-oil plays an important role in the catalytic upgrading of bio-oil. Therefore, experiments were first conducted to investigate the effect of temperature on polymerization for coke formation of bio-oil in the absence of a solvent. As shown in Table 1, two temperatures (250 and 300 °C) were investigated using the same initial H2 pressure (2 MPa) and reaction time (3 h) over a Ru/C catalyst. The results indicate that the solid yield (excluding the mass of catalyst) was only 2.5% at 250 °C but increased substantially to approximately 10% at 300 °C (Table 1). The solid likely originated from the polymerization of oxygenated compounds, which are abundant in bio-oil.7,28 The organic phase fractions (including heavy oil and light oil) decreased from 52 to 42% with increasing temperature, resulting in larger aqueous and gaseousphase fractions (Figure S1 of the Supporting Information). However, the properties of the organic phases (mainly heavy oil) improved with increasing temperature. For example, as the temperature was increased from 250 to 300 °C, the HHV increased from 25.4 to 27.9 MJ kg−1, the moisture decreased from 14.1 to 11.5%, and the oxygen content decreased from 30.5 to 24.9% (Table 1). Nevertheless, the viscosity increased by an order of magnitude compared to that of bio-oil (22.3 cSt) because of polymerization (Table 1). Moreover, the mass of the gas-phase products increased with increasing temperature. The gas phase consisted primarily of CO, CO2, CH4, C2H4, and C2H6 (data not shown). 3.2. Upgrading Bio-Oil Using 1-Butanol over a Ru/C Catalyst. 3.2.1. Effect of Solvent. Previous experiments have demonstrated that coke formation is a serious problem in the upgrading process. To investigate the effect of solvent on the upgrading process, we used 1-butanol as an organic solvent to promote bio-oil upgrading and to overcome coke formation. The results of the experiments with 1-butanol are presented in Table 1 with the results from the control experiments (without solvent). As evident from the results in the table, solvent addition significantly enhanced all of the measured parameters. The viscosity decreased by 2 orders of magnitude (from 254 to 7 cSt at 250 °C). In addition, elemental analysis showed that the carbon and hydrogen contents of the upgraded bio-oils with solvent addition increased and that the oxygen content decreased substantially. For example, at 250 °C, the carbon content increased from 62% (without solvent) to 64%, hydrogen content increased from 7.3% to 10.5%, and oxygen content decreased from 30.5% to 24.7%. More significantly, the amount of solid product decreased from 2.5 to 0.3%, indicating much less coke formation in the presence of the solvent. Under solvent-addition conditions, the quality of the upgraded bio-oil was further improved with increasing temperature; for example, when the temperature was increased from 250 to 280 °C, the viscosity decreased from 7.4 to 6.6 cSt, the water content decreased from 10.7% to 8.4%, the HHV increased from 29.5 to 30.5 MJ kg−1, and the DOD increased from 46% to 53%. 3.2.2. Effect of Supercritical Solvent. The upgrading of bio-oil in supercritical (300 °C) 1-butanol was also performed over a
(1) C
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Figure 2. GC-MS chromatograms of the bio-oil and upgraded bio-oil in supercritical 1-butanol over a Ru/C catalyst (at 300 °C, 11.5 MPa, 3 h).
analyzed by GC/MS; GC/MS chromatograms are presented in Figure 2. The peaks in the chromatograms are identified according to their respective retention time. Almost all of the peaks in the chromatogram of upgraded bio-oil are newly formed (labeled N1−N46), with only a few compounds from the original bio-oil remaining (peaks 18, 19, 25, 30, 35, and 41). Detailed component analyses of the bio-oil and upgraded bio-oil are presented in Table 2. Acetic acid (peak 1), the most abundant compound in the biooil, completely disappeared from the upgraded bio-oil. Other carboxylic acids present in bio-oil, such as propanoic acid (peak 3), 4-oxo-pentanoic acid (peak 23), and 4-hydroxy-3-methoxybenzeneacetic acid (peak 41), were also not detected in the upgraded bio-oil. However, 4-hydroxy-3-methoxy-benzoic acid (peak 33) was still present in the upgraded bio-oil. Additionally, a small amount of butanoic acid (peak N7) was generated in the upgraded bio-oil. Overall, the content of carboxylic acids decreased substantially (18.4% in bio-oil compared to 2.8% in upgraded bio-oil); these carboxylic acids were converted into their corresponding esters via esterification with 1-butanol. Dozens of these esters were formed, accounting for more than 40% of the total corresponding peak area (specific reaction pathways will be discussed later). Another notable change is the complete disappearance of 1-hydroxy-2-propanone (peak 2), the second-most abundant compound in bio-oil. This compound can be transformed into esters, alcohols, or other ketones.19 Because
Ru/C catalyst; the results are summarized in Table 1. Under supercritical conditions, the quality of the upgraded bio-oil was further improved compared to the quality of the bio-oil upgraded under subcritical conditions (280 °C) (Table 1): the viscosity decreased from 6.6 to 5.7 cSt, the oxygen content decreased from 21.4% to 14.5%, and the DOD significantly increased from 53% to 68%. Although the increased temperature may play a role in this enhancement, in our opinion, the supercritical conditions may be responsible for this subtle yet noticeable improvement in quality. To the best of our knowledge, this work represents the first attempt to quantify the advantages of using a supercritical solvent on the basis of the properties of upgraded bio-oil. Because supercritical fluids exhibit better dissolving power,16 these fluids are better able to dissolve pyrolytic lignin and microcarbon, resulting in reduced polymerization reactions. In addition, supercritical fluids exhibit gas-like flow properties (low viscosity and high diffusivity) as well as fast rates of mass and heat transfer.17,29 These features of supercritical fluids certainly contribute to the enhanced quality of upgraded bio-oil. In short, this study clearly demonstrates that, in general, the use of a solvent improves the quality of bio-oil. In addition, the use of a supercritical solvent, in particular, further enhances the quality of bio-oil. 3.2.3. Changes in Organic Components Induced by the Supercritical Upgrading Process. The organic components of bio-oil and upgraded bio-oil (using supercritical 1-butanol) were D
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Table 2. Main Components of Bio-Oil and Upgraded Bio-Oil Using Supercritical 1-Butanol bio-oil
upgraded bio-oil
peak no.
compounds
area (%)
peak no.a
compounds
areab (%)
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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
acetic acid 1-hydroxy-2-propanone propanoic acid 1-hydroxy-2-butanone acetic acid methyl ester butanedial 2-cyclopenten-1-one furfural 2-methyl-2-cyclopenten-1-one 1,2-ethanediol monoacetate 3-methyl-2-cyclopenten-1-one 2(5H)-furanone 2-hydroxy-3-methyl-2-cyclopenten-1-one 3-methyl-2(5H)-furanone 2-(3-chloropropyl)-1,3-dioxolane phenol 2-methoxy-phenol 3-ethyl-2-hydroxy-2-cyclopenten-1-one 2-methyl-phenol 4-methyl-5H-furan-2-one 4-methyl-phenol 2-methoxy-4-methyl-phenol 4-oxo-pentanoic acid O-(3-methylbutyl)-hydroxylamine 4-ethyl-2-methoxy-phenol 1,4:3,6-dianhydro-α-D-glucopyranose thymol 2-methoxy-3-(2-propenyl)-phenol 5-(hydroxymethyl)-2-furancarboxaldehyde 2,6-dimethoxy-phenol 2-methoxy-4-(1-propenyl)-(E)-phenol 2-methoxy-4-(2-propenyl)-(E)-phenol 4-hydroxy-3-methoxy-benzoic acid vanillin 1,2,3-trimethoxy-5-methyl-benzene 1-(3-hydroxy-4-methoxyphenyl)-ethanone homovanillyl alcohol 2,6-dimethoxy-4-(2-propenyl)-phenol N-(4-methoxyphenyl)-2-hydroxyimino-acetamide 2,6-dimethoxy-4-(3-propenyl)-phenol 4-hydroxy-3-methoxy-benzeneacetic acid 4-hydroxy-3,5-dimethoxy-benzaldehyde 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone 4-hydroxy-2-methoxycinnamaldehyde 1-(2,4,6-trihydroxy-3-methylphenyl)-1-butanone
13.3 8.7 0.5 0.9 1.5 1.5 1.1 2.3 0.9 1.2 1.1 2.8 3.2 1.0 0.9 0.8 5.5 0.5 0.6 0.7 0.5 4.4 0.7 0.8 1.7 0.7 0.5 2.8 0.9 5.9 1.6 2.8 2.9 3.6 1.5 2.5 1.4 2.7 1.2 2.3 0.9 3.5 2.2 1.7 1.2
N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 N12 N13 N14 N15 N16 N17 18 19 N20 N21 N22 N23 N24 25 N26 N27 N28 N29 30 N31 N32 N33 N34 35 N36 N37 N38 N39 N40 41 N42 N43 N44 N45 N46
1-butanol butyl 2-methylpropanoate n-butyl acetate n-butyl ether p-xylene 2-methyl-cyclopentanone butanoic acid butyl propanoate 2-methyl-propanoic acid butyl ester 2-ethyl-hexanal butanoic acid butyl ester 3-ethyl-4-methyl-3-heptanol 2-ethyl-hexenal butyrolactone 2-ethyl-1-hexanol pentanoic acid butyl ester 2,3-dimethyl-2-cyclopenten-1-one phenol 2-methoxy-phenol 3,6,6-trimethyl-cyclohex-2-enol hexanoic acid butyl ester octahydro-7a-hydroxy-1H-inden-1-one 1,1-dibutoxy-butane 5-methyl-2-(1-methylethyl)-2-cyclohexen-1-one 2-methoxy-4-methyl-phenol 3,5-dimethyl-1H-pyrazole 2-hydroxy-3-methyl-2-cyclopentan-1-one butyl glycolate butanoic acid (tetrahydro-2-furanyl)methyl ester 4-ethyl-2-methoxy-phenol 2-[2-[2-[2-(2-acetyloxyethoxy)ethoxy]ethoxy]ethoxy]ethyl acetate 2-isopropyl-5,5-dimethylcyclohex-2-enone 2-methoxy-4-propyl-phenol 2-butyl-phenol 2,6-dimethoxy-phenol 5-butyldihydro-2(3H)-furanone 2′,4′-dihydroxy-3′-methylpropiophenone 1-(4-hydroxy-3-methoxyphenyl)-2-propanone 2-(1-methylpropyl)-cyclopentanone 2,7-dimethyl-(E,Z)-3,5-octadiene 4-hydroxy-3-methoxy-benzoic acid 3-methoxy-4-propoxybenzaldehyde 5-tert-butylpyrogallol flopropione 1,1′-[[(4-methylphenyl)thio]methylene] bis-benzene bis-benzenea phlorobutyrophenone
0.5 26.5 1.5 0.7 1.0 0.6 3.4 0.9 0.6 4.7 1.8 0.9 0.5 0.7 1.7 0.5 0.5 2.8 0.7 0.6 1.2 13.9 0.9 3.5 0.7 0.5 0.7 0.7 2.9 1.2 0.5 5.2 0.5 1.8 0.8 0.6 0.5 0.7 1.7 2.1 0.6 1.4 0.5 2.9 1.9
a
Peak no. with prefix N refers to newly formed compound. bContent of 1-butanol (50%) is excluded.
However, identification of every ketone translation route is not possible because of the complexity of the reaction system. Aldehydes present in bio-oil, such as vanillin (peak 34), butanedial (peak 6) and 4-hydroxy-3,5-dimethoxy-benzaldehyde (peak 42), and furans, such as furfural (peak 8), 2(5H)-furanone (peak 12), and 3-methyl-2(5H)-furanone (peak 14), were not detected in the upgraded bio-oil. However, newly produced 2ethyl-hexanal (peak N10), 2-ethyl-hexenal (peak N13), 3methoxy-4-propoxybenzaldehyde (peak N42), and 5-butyldihydro-2(3H)- furanone (peak N36) were detected in the upgraded bio-oil, although in very low quantities.
hydrogen and 1-butanol exist in this system, 1-hydroxy-2propanone may have undergone C−O bond formation and C−C bond cleavage to directly form n-butyl acetate (peak N3), similar to the results in a related report.19 Similarly, 1-hydroxy-2butanone (peak 4) may have been transformed into butyl propanoate (peak N8). Other transformations also occurred during the upgrading process: 2-methyl-2-cyclopenten-1-one (peak 9) to 2-methyl-cyclopentanone (peak N6) via hydrogenation and 2-hydroxy-3-methyl-2-cyclopenten-1-one (peak 13) to 2-hydroxy-3-methyl-2-cyclopentan-1-one (peak N27). E
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Table 3. Compounds Formed during the Upgrading Process and the Possible Formation Pathwaysa
a
Note: Only formed compounds with area >1% are listed * Formation pathways not known.
Phenolic compounds present in bio-oil, such as phenol (peak 16), 2-methoxy-phenol (peak 17), 2-methoxy-4-methyl-phenol (peak 22), and 2,6-dimethoxy-phenol (peak 30), were detected in diminished amounts and were identified as peaks 18, 19, 25, and 35, respectively, in the upgraded bio-oil. The phenolic derivatives of 2-methoxy-3-(2-propenyl)-phenol (peak 28) and
2-methoxy-4-(1-propenyl)-(E)-phenol (peak 31) present in biooil were not detected in the upgraded bio-oil; however, these compounds can be transformed into 2-methoxy-4-propyl-phenol (peak N33) or other phenols via hydrogenation and isomerization. Overall, the content of phenols decreased substantially (32% in bio-oil compared to 19% in the upgraded bio-oil). F
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Figure 3. Role of 1-butanol as a solvent. A: Serving as a reaction medium, B: Serving as a reactant, C: Enhancing hydrogen dissolution, D: Protecting the catalyst, E: Improving the product properties.
cyclopentanone (N6) and 2-methoxy-4-propyl-phenol (N33) were formed by hydrogenation of their corresponding unsaturated double-bond compounds, which were also present in the upgraded bio-oil without solvent (Table S1, Supporting Information). Octahydro-7a-hydroxy-1H-inden-1-one (N22) may have formed through both hydrogenation and an accompanying coupling reaction. Compounds N43 (5-tertbutylpyrogallol) and N46 (phlorobutyrophenone) were formed through phenolic conversion, which includes isomerization and demethylation. However, the functional phenolic hydroxyl group was still present in the upgraded products. This result indicates that the transformation of phenolic compounds is relatively difficult, consistent with the results of previous studies.17,20,27 Compounds N12 (3-ethyl-4-methyl-3-heptanol) and N40 (2,7dimethyl-(E,Z)-3,5-octadiene) might have been formed by the aldol condensation of 1-hydroxy-2-propanone, accompanied by hydrogenation or dehydration. Such conversion pathways have not been previously reported. These compounds are likely produced through transformations of several intermediates. Moreover, some components may also undergo polymerization to form oligomers, such as compound N45. Esterification and etherification were clearly the dominant reactions, and along with hydrogenation and hydrodeoxygenation, resulted in a certain extent of deoxygenation of upgraded bio-oil. Other reactions that occurred during the upgrading process include acetalization, aldol condensation, ring-opening, isomerization, and demethylation (shown in Table 3). The significant decrease in the contents of carboxylic acids, aldehydes, ketones, phenols, and furans and the increase in the contents of alcohols, esters, ethers, and hydrocarbons (Figure 2) clearly demonstrate the increased stability and facile hydrocarbon blending capability of the upgraded bio-oil.
Obviously, the greatest change in the upgraded bio-oil was that esters and ethers were newly formed via the upgrading process, including n-butyl acetate (up to 27%, peak N3), followed by 1,1dibutoxy-butane (14%, peak N23), propanoic acid butyl ester (3.4%, peak N8), butanoic acid butyl ester (4.7%, peak N11), pentanoic acid butyl ester (1.7%, peak N16) and n-butyl ether (1.5%, peak N4). Because bio-oil is a highly complex mixture, a detailed reaction pathway for the generation of every newly formed component cannot easily be determined. Nevertheless, we attempted to formulate the pathways for the main products according to basic rules for chemical reactions, as shown in Table 3. Of course, the new 1-butanol peak was mainly due to the added solvent. n-Butyl acetate (N3) was formed via esterification of acetic acid and 1butanol. Other esters (N8, N11, N16, and N31) were derived from esterification of the corresponding acids (e.g., acetic acid and propanoic acid) and alcohols. Some acids are present in insignificant amounts in bio-oil (e.g., butanoic acid and pentanoic acid); however, these acids can be formed via the conversion of certain oxygenated compounds. For example, 2(5H)-furanone (peak 12) can be transformed into butanoic acid through ringopening and hydrogenation, and 4-oxo-pentanoic acid (peak 23) can be transformed into propanoic acid by HDO (Table 3). Multiethoxy-ester (N31) is formed by esterification and etherification, whereas ethers are mainly formed through etherification and acetalization.20 For example, n-butyl ether (N4) likely originated from etherification of 1-butanol, and 1,1dibutoxy-butane (N23) likely originated from the acetalization of n-butanal and 1-butanol. We noted that n-butanal was not present in the original bio-oil but can be generated as an intermediate compound from certain oxygenated compounds, including through the HDO of carboxylic acid.30 2-MethylG
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role of the solvent under supercritical conditions for the upgrading of bio-oil. 3.2.5. Properties of Actual Upgraded Bio-Oil. A significant fraction of 1-butanol (25.6%, Table 4) was present in the
In the absence of solvent, the reaction mechanism for bio-oil upgrading appears to differ. In a conventional HDO process without solvent, the following reactions can occur: deoxygenation (decarbonylation, decarboxylation form CO and CO2); demethoxylation, e.g., 4-ethyl-phenol present in upgraded bio-oil (Table S1) may originate from demethoxylation of 4-ethyl-2methoxy-phenol; hydrodeoxygenation or dehydroxylation of OH groups (from phenols to benzene derivatives), e.g., ethylbenzene and o-xylene generation (Table S1); and hydrogenation, e.g., the approximately 9% of 2-methoxy-4-propylphenol that is generated results from hydrogenation of 2methoxy-4-(1-propenyl)-(E)-phenol. Finally, large molecules polymerized, resulting in coke formation. 3.2.4. Role of 1-Butanol as a Solvent. 1-Butanol exhibits moderate polarity and is miscible with both bio-oil and organic liquids. Additionally, 1-butanol exhibits a certain degree of hydrophobicity: the solubility of 1-butanol in the aqueous phase is 7%,31 which should not affect the separation of organic products from the aqueous phase. On the basis of these properties, we selected 1-butanol as the organic solvent for biooil upgrading. The role of 1-butanol as a solvent is illustrated in Figure 3 and includes five functions: (1) serving as the reaction medium; (2) acting as a reactant; (3) enhancing hydrogen dissolution; (4) protecting the catalyst; and (5) improving the product properties. First, 1-butanol as a reaction medium provides a homogeneous reaction environment. In fact, bio-oil is composed of pyrolytic lignins wrapped with microcarbon, oxy-organics, and water to form a network matrix.32 In the presence of 1-butanol, the hydroxyl group, which is the main factor that determines the polarity of an alcohol,33 likely interacts with the small micelles in bio-oil, breaking the microscopic multiphase structures and forming a micro homogeneous system. Simultaneously, the components of bio-oil are dispersed and diluted by the 1-butanol solvent, which leads to a low reactant concentration. Consequently, the chance of interactions between reactive oxygenic groups is reduced, polymerization reactions are limited, and coke formation is significantly diminished (Table 1). Furthermore, a solvent that completely dissolves the organic bio-oil components into a single liquid phase should improve reaction rates by reducing resistance to mass transfer. Second, 1-butanol serves as a reactant in the upgrading process, as it was involved in esterification reactions with the carboxylic acids present in the bio-oil, thereby reducing the acidity and corrosiveness of the upgraded bio-oil. In addition, 1butanol also participates in the condensation reaction with aldehydes present in the bio-oil to form ethers. The stability of the upgraded bio-oil was improved because of the increased levels of esters and ethers, which was accompanied by decreased levels of acids and aldehydes. Third, in the presence of 1-butanol, H2 solubility can reach 0.1 mol mol−1 at 250 °C and 8.8 MPa,34 and this solubility should be further enhanced at higher temperatures and pressures. The higher dissolved hydrogen concentration led to increase hydrogenation and hydrodeoxygenation reactions of oxygenated groups in the bio-oil during the upgrading process. Fourth, 1-butanol present in the reaction system can protect the catalyst and prevent deactivation because of its ability to inhibit coke formation and reduce the change of catalyst texture (these will be discussed later). Fifth, the properties of upgraded bio-oil were significantly improved, as previously discussed. To the best of our best knowledge, this work represents the first attempt to elucidate the
Table 4. Comparison of the Properties of Upgraded Bio-Oil, Actual Upgraded Bio-Oil (Excluding the Solvent), the Distillation Liquid, and the 1-Butanol Solventa properties density (kg L−1) viscosity (40 °C, cSt) moisture (wt %) HHV (MJ kg−1) pH elemental analysis (wt %) C H O DOD (%) specific components 1butanol (wt %) n-butyl acetate (wt %) distillation fraction (wt %)
upgraded bio-oil
actual upgraded bio-oil distillation residue
distillation liquid
1-butanolb
0.9
1.0
0.8
0.8
5.7
19.2
2.4
3.6
6.7
0.4
10.2
0.1
32.0
35.2
31.5
36.1
4.4
4.0
4.6
6.8
72.4 11.3 14.5 68
71.4 11.1 16.5 64
61.7 11.2 26.8 41
64.9 13.5 21.6
25.6
5.4
56.3
99.5
10.0
0.5
20.0
0.2
54
45
Note: Upgrade conditions: Ru/C, 300 °C, 11.5 MPa, 3 h. Distillation conditions: 60 °C, −0.1 MPa, 40 min. bProperties of 1-butanol from reagent specifications. a
upgraded bio-oil, which clearly masked the properties of the upgraded bio-oil. Hence, we distilled the upgraded bio-oil to separate it into two fractions: actual upgraded bio-oil (distillation residue) and distillation liquid (solvent and other light components); the properties of these fractions are reported in Table 4. A comparison of the properties of the upgraded oil (Table 1) to the properties of the individual fractions highlights the extent of masking. Unfortunately, complete exclusion of 1butanol under such distillation conditions is difficult because some light volatile compounds are also collected in the distillation liquid, which means that the distilled bio-oil is missing those volatile compounds. Nonetheless, this work demonstrates some differences between the properties of biooils upgraded with and without a solvent. A comparison between 1-butanol and the distillation liquid reveals similar values between the two liquids, with the exception of a significantly increased moisture content in the distillation liquid (0.1 compared to 10.2%) due to water enrichment in the distillation process and decreased pH (6.8 to 4.6) due to the presence of acids. Differences in their other parameters are expected because the distillation liquid includes other lightweight compounds. In fact, the distillation liquid includes 56% 1butanol, 20% n-butyl acetate, 10% water (Table 4), and other light components (data not shown). H
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With respect to the actual upgraded bio-oil (distillation residue), some properties were improved compared to those of upgraded bio-oil using solvent, with the exception of viscosity, which increased. Notably, the moisture content decreased from 6.7 to 0.4%, and the HHV of the distillation residue was enhanced from 32 to 35 MJ kg−1 compared to that of upgraded bio-oil using solvent. However, the oxygen content increased slightly (from 14.5 to 16.5%), resulting in a lower DOD. This work represents the first attempt to quantify the difference in quality between upgraded bio-oil and distilled upgraded bio-oil. Moreover, all properties were substantially improved compared to those of upgraded bio-oil without solvent: the viscosity decreased by 87%, the moisture decreased by 96%, and the HHV increased by 26%. This improvement is obvious from the elemental analysis results, especially because the oxygen content decreased from 24.9 to 16.5%. The advantage of 1-butanol solvent addition is clearly apparent in terms of the improved properties of the actual upgraded bio-oil. 3.3. Catalyst Characterization. To study the solvent’s ability to protect the catalyst and prevent deactivation, the catalysts before and after reaction were characterized by XRD, TEM, TG, and N2 adsorption−desorption. XRD was performed on three catalysts: (a) fresh Ru/C catalyst, (b) Ru/C catalyst used in the presence of solvent, and (c) Ru/C catalyst used in the absence of solvent. As shown in Figure 4, a weak Ru crystallite peak was detected at 2θ = 44° in
Table 5. Size of Ru Crystallites at Different Crystal Faces under Different Conditionsa size of Ru crystallites (nm) catalysts fresh Ru/C used Ru/C (with solvent) used Ru/C (without solvent) a
2θ = 38.7° (100)
2θ = 42.4° (002)
2θ = 44.3° (101)
8.4
11.0
2.8 16.6
11.0
14.3
20.0
Note: at 300 °C, 11.5 MPa, 3 h; with solvent 1-butanol (50 wt %).
the peaks in the XRD pattern of catalyst c at 2θ = 26.6°, 50.4°, and 77.5° are stronger than those in the pattern of catalyst b, which is attributed to graphite carbon according to reference code 00-001-0646. This result indicates carbon deposition occurred because of the lack of solvent protection. To determine the differences in morphology of catalysts before and after the reaction, we performed TEM analysis on catalysts a, b, and c. As shown in Figure 5, the texture of the fresh
Figure 5. TEM images of fresh Ru/C catalyst and used Ru/C catalyst (at 300 °C, 11.5 MPa, 3 h). (a) Fresh catalyst, (b) used catalyst with solvent, (c, d) used catalyst without solvent (under different resolution).
Figure 4. XRD patterns of fresh Ru/C catalyst and used Ru/C catalyst (at 300 °C, 11.5 MPa, 3 h). (a) Fresh catalyst, (b) used catalyst with solvent, (c) used catalyst without solvent.
catalyst is homogeneous, and Ru crystallites are difficult to observe due to their small size and good dispersion, which is consistent with the XRD results. In contrast, the Ru particle size in catalyst b increased slightly; however, the dispersion of Ru particles is still homogeneous (with the exception of a partial region). However, the Ru particle size of catalyst c is significantly larger compared to that of catalyst b, which is likely due to a layer of carbon deposition on the Ru crystallite. Figure 5d shows a high-resolution TEM image of the catalyst used without solvent; two distinguishable patterns are observed (Types (I) and (II)). Type (I) shows a clear crystal structure (stripe texture), indicating the presence of Ru crystallites. In contrast, Type (II) indicates the deposition of a layer of amorphous carbon (i.e., no crystal structure). On the basis of these observations, we concluded that the Ru particle sizes after the reaction both with and without solvent increased to a certain degree because of
the XRD pattern of the fresh Ru/C catalyst, indicating a fine dispersion of Ru on the carbon support. The XRD patterns of the used catalysts (b and c) are similar: the peaks at 2θ = 38.7°, 42.4°, 44.3°, and 58.7° are attributed to Ru crystallites.35 In fact, the peaks at 2θ = 69.6°, 79.1°, 85.2° can also be attributed to Ru crystallites (reference code: 00-001-1253). However, the size of the Ru crystallites in catalysts a, b, and c differ based on different Ru crystal face calculations performed using the Scherrer equation (Table 5). The results indicate that the Ru crystallites grow after the reaction and that the Ru crystallite size of catalyst c is slightly larger than that of catalyst b because of the absence of solvent protection. Unfortunately, carbon deposition and the carbon support cannot be distinguished by XRD because the amorphous carbon peaks are all observed at 2θ = 24°.36 However, I
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4. CONCLUSIONS Bio-oil was upgraded using supercritical 1-butanol over a Ru/C heterogeneous catalyst. The results indicated that the properties of the upgraded bio-oil were significantly improved with the HHV increasing from 17.9 to 32 MJ kg−1, viscosity decreasing from 22.3 to 5.7 cSt, moisture decreasing from 21.2 to 6.7%, and oxygen content decreasing from 48.0 to 14.5%; moreover, with insignificant coke formation (0.2%). The acids, aldehydes, ketones, phenols, and furans contents of the upgraded bio-oil were decreased significantly, whereas the esters and ethers contents were increased. The reaction pathways primarily included esterification, etherification, acetalization, hydrogenation, and hydrodeoxygenation. The solvent played numerous roles: serving as a reaction medium and as a reactant, enhancing H2 dissolution, protecting the catalyst, and improving the product properties.
metal particle migration and sintering under high temperatures. However, in the process without solvent, a significant amount of coke was deposited on the catalyst surface. Figure 6 shows the weight-loss analysis results for the Ru/C catalysts. The weight loss of the catalyst used without solvent was
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ASSOCIATED CONTENT
* Supporting Information S
Mass balance closure for the hydrodeoxygenation of bio-oil using Ru/C catalyst under different conditions (Figure S1), and main components of upgraded bio-oil without solvent (Table S1). This material is available free of charge via the Internet at http:// pubs.acs.org/.
Figure 6. Weight losses determined by TG analysis of fresh Ru/C catalyst and used Ru/C catalysts with and without solvent.
■
the largest (96%), compared with 93% for the catalyst used with solvent and 90% for the fresh catalyst. The significant increase in weight loss for the catalyst used without solvent may be due to combustion of the additional carbon deposited on the catalyst. The TG data are consistent with the TEM images, which indicated that coke deposits were present on the catalyst after the reaction. The N2 physisorption results indicate a significant reduction in the BET specific surface area for the catalyst used without solvent (82%) compared to those of the fresh catalyst (808 m2 g−1) and the catalyst used with solvent (42%) (Table 6). The total pore
Corresponding Author
*Tel.: +86 371 67781284. Fax: +86 371 67781163. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors sincerely acknowledge financial support from the Key Programs of Science and Technology of Henan Province and Zhengzhou City (Project 10ZDGG121 and Project 111PCXTD165) and from the National Science Foundation of China (Project No. 21201153). The authors also thank Miss Qingxia Meng and Mr. Kang Zhai for their assistance in performing the laboratory analyses.
Table 6. N2 Physisorption Data for the Ru/C Catalystsa used catalysts
a
catalysts
fresh
with solvent
without solvent
BET surface area (m2 g−1) total pore volume (cm3 g−1)
808 0.6
467 0.4
148 0.2
AUTHOR INFORMATION
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Note: at 300 °C, 11.5 MPa, 3 h; with solvent 1-butanol (50 wt %).
REFERENCES
(1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106 (9), 4044−4098. (2) Huber, G. W.; Corma, A. Angew. Chem., Int. Ed. 2007, 46, 7184− 7201. (3) Mortensen, P. M.; Grunwaldt, J. D.; Jensen, P. A.; Knudsen, K. G.; Jensen, A. D. Appl. Catal., A 2011, 407, 1−19. (4) Bridgwater, A. V. Biomass Bioenerg. 2012, 38, 68−94. (5) Lu, Q.; Li, W. Z.; Zhu, X. F. Energy Convers. Manage. 2009, 50, 1376−1383. (6) Elliott, D. C. Energy Fuels 2007, 21 (3), 1792−1815. (7) Wildschut, J.; Melián-Cabrera, I.; Heeres, H. J. Appl. Catal., B 2010, 99, 298−306. (8) Choudhary, T. V.; Phillips, C. B. Appl. Catal., A 2011, 397, 1−12. (9) Oasmaa, A.; Kuoppala, E.; Selin, J. F.; Gust, S.; Solantausta, Y. Energy Fuels 2004, 18, 1578−1583. (10) Mahfud, F.; Meli_an-Cabrera, I.; Manurung, R.; Heeres, H. Process Saf. Environ. 2007, 85, 466−472. (11) Hilten, R. N.; Bibens, B. P.; Kastner, J. R.; Das, K. C. Energy Fuels 2010, 24, 673−682. (12) Sheu, Y. H. E.; Anthony, R. G.; Soltes, E. J. Fuel. Process. Technol. 1988, 19, 31−50.
volume for fresh Ru/C was 0.6 cm3 g−1; the pore volume decreased to 0.4 and 0.2 cm3 g−1 in the cases of the catalysts used with solvent and without solvent, respectively. These results suggest that the solvent played a role in protecting the integrity of the catalyst. In summary, the fact that the catalyst was protected against deactivation by the supercritical 1-butanol system is demonstrated by several observations: the Ru crystallite size increased less than that of the catalyst used without solvent, as determined by XRD analysis; no clear coke formation on the active site of the catalyst used with solvent was observed by TEM or by TG analysis; and the change in catalyst texture, as determined by N2 physisorption, was smaller in the case of the catalyst used with solvent than in the case of the catalyst used without solvent under the same reaction conditions. J
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Energy & Fuels
Article
(13) Zhang, S.; Yan, Y.; Li, T.; Ren, Z. Bioresour. Technol. 2005, 96, 545−550. (14) Agblevor, F. A.; Mante, O.; Mcclung, R.; Oyama, S. T. Biomass Bioenerg. 2012, 45, 130−137. (15) Zhao, C.; Lercher, J. A. Angew. Chem., Int. Ed. 2012, 51, 1−7. (16) Baiker, A. Chem. Rev. 1999, 99, 453−473. (17) Zhang, J.; Luo, Z.; Dang, Q.; Wang, J.; Chen, W. Energy Fuels 2012, 26, 2990−2995. (18) Li, W.; Pan, C.; Sheng, L.; Liu, Z.; Chen, P.; Lou, H.; Zheng, X. Bioresour. Technol. 2011, 102, 9223−9228. (19) Li, W.; Pan, C.; Zhang, Q.; Liu, Z.; Peng, J.; Chen, P.; Lou, H.; Zheng, X. Bioresour. Technol. 2011, 102, 4884−4889. (20) Peng, J.; Chen, P.; Lou, H.; Zheng, X. Bioresour. Technol. 2009, 100, 3415−3418. (21) Tang, Z.; Lu, Q.; Zhang, Y.; Zhu, X.; Guo, Q. Ind. Eng. Chem. Res. 2009, 48, 6923−6929. (22) Dang, Q.; Luo, Z.; Zhang, J.; Wang, J.; Chen, W.; Yang, Y. Fuel 2013, 103, 683−692. (23) http://www.erowid.org/archive/rhodium/pdf/solvent. miscibility.pdf. (24) Shin, J. D.; Hong, S. G.; Choi, W. S.; Park, S. K. Biotechnol. Bioproc. E 2013, 18, 956−964. (25) Zhang, Z. J.; Wang, Q. W.; Tripathi, P.; Pittman, J. C. U. Green Chem. 2011, 13, 940−949. (26) Alcala, A.; Bridgwater, A. V. Fuel 2013, 109, 417−426. (27) Xu, X.; Zhang, C.; Liu, Y.; Zhai, Y.; Zhang, R. Chemosphere 2013, 93, 652−660. (28) Venderbosch, R. H.; Ardiyanti, A. R.; Wildschut, J.; Oasmaac, A.; Heeres, H. J. J. Chem. Technol. Biotechnol. 2010, 85, 674−686. (29) Wen, D.; Jiang, H.; Zhang, K. Prog. Nat. Sci. 2009, 19, 273−284. (30) Chen, L.; Zhu, Y.; Zheng, H.; Zhang, C.; Zhang, B.; Li, Y. J. Mol. Catal. A: Chem. 2011, 351, 217−227. (31) http://www.solvents.basf.com/portal/load/fid228768/n_ Butanol_e_03_08.pdf. (32) Pérez, M. G.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Rodrigue, D.; Roy, C. Energy Fuels 2006, 20, 364−375. (33) Bentley, T. W.; Koo, I. S. Org. Biomol. Chem. 2004, 2, 2376−2380. (34) Safamirzaei, M.; Modarress, H.; Mohsen-Nia, M. Fluid Phase Equilib. 2010, 289, 32−39. (35) Hong, U. G.; Kim, J. K.; Lee, J.; Lee, J. K.; Song, J. H.; Yi, J.; Song, I. K. Appl. Catal., A 2014, 469, 466−471. (36) Bindwal, A. B.; Vaidya, P. D. Ind. Eng. Chem. 2013, 52, 17781− 17789.
K
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