Upgrading the Water-Soluble Fraction of Bio-oil by Simultaneous

Mar 7, 2014 - Esterification and Acetalation with Online Extraction. Fei Qin,. †. Hongyou Cui,* ... University of Technology, Zibo 255049, People,s ...
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Upgrading the Water-Soluble Fraction of Bio-oil by Simultaneous Esterification and Acetalation with Online Extraction Fei Qin,† Hongyou Cui,*,† Weiming Yi,*,‡ and Chuanbo Wang† †

School of Chemical Engineering, and ‡Shandong Research Center of Engineering and Technology for Clean Energy, Shandong University of Technology, Zibo 255049, People’s Republic of China S Supporting Information *

ABSTRACT: Upgrading the water-soluble fraction, which was obtained by water extraction of rice husk fast pyrolysis oil, was investigated with simultaneous esterification and acetalation with online solvent extraction (SEAWOSE) in butanol. It was found that, in comparison to direct esterification and acetalation without extraction, almost all of the acids and aldehydes in the watersoluble fraction can be converted to the corresponding esters, hemiacetals, and acetals by SEAWOSE. With the aid of online extraction, the saccharides could be transformed into the upgraded oil gradually via first hydrolysis into aldehyde derivatives and then acetalation. As a result, the char formation was significantly suppressed. The effect of oxidation and reduction of the watersoluble fraction as pretreatment before SEAWOSE was also investigated. By hydrogen peroxide oxidation, the aldehydes could be first converted into acids and subsequently esterified to esters, consequently without char formation. The upgraded oil was with high oil quality, less than 3% in moisture, higher than 30 MJ/kg in high heating value, and less than 2 mg of KOH/g in acidity. distillation,5 solvent extraction,6 and supercritical CO2 extraction.7 The chemical upgrading way includes catalytic reforming,8 hydrotreating9,10 catalytic esterification,9 etc. Catalytic reforming is expected to deoxygenate bio-oil in the form of carbon dioxide and water to produce high-quality fuel. Various catalysts have been attempted thus far, including HZSM-5, 12−15 ZSM-5,16,17 Al-SBA-15,10 and alumina.11 Although catalytic reforming can promote the quality of biooils under appropriate catalysts, it suffers from catalyst deactivation, reactor clogging, and low energy efficiency because of the formation of char, tar, and coke.12 For example, catalytic reforming of maple wood bio-oil in a fixed reactor with a HZSM-5 catalyst afforded 12.8% char yield, 10.2% coke yield at 370 °C, and a weight hourly space velocity of 1.8 h−1, while the refined oil yield was only 29.5%.13 Another effective mean to refine the bio-oils is hydrotreating, which can be further categorized into hydrogenation and hydrodeoxygenation. Hydrogenation usually aims to improve oil stability and minimize the aldehyde and other unsaturated compounds,14 but the promotion on the heating value is very limited. Hydrodeoxygenation expects the oxygen contained in bio-oil to be removed in the form of water. Because hydrogen is a precious energy resource, this technology encounters an economic issue. Besides, coking and rapid catalyst deactivation might occur in the bio-oil hydrodeoxygenation.15 One-step hydrogenation−esterification of aldehyde and acid to ester over bifunctional Pt catalysts was also examined.16 The major objective of catalytic esterification of bio-oil is to minimize its acidity and boost the heating value of bio-oils at relatively mild conditions. Inorganic acids and their acidic salts,24,25 ionic liquids,17 ion-exchange resins,9,18 solid acids,19

1. INTRODUCTION Concerns about the depletion of fossil fuels and the environment pollution because of the use of fossil fuel have stimulated the incentive to explore and exploit sustainable energy resources. For this purpose, a great deal of effort has been devoted in the past few decades around the world. Because biomass originated from the photosynthesis radically, they are viewed as a renewable and abundant energy resource on the earth. Fast pyrolysis of biomass into bio-oil has been commercialized exemplarily in several plants. In comparison to fossil fuels, bio-oil is with very low sulfur and nitrogen contents.1 Therefore, it is expected to play an important part in the future energy consumption. Because the residence time of biomass in the fast pyrolysis reactor is extremely short (about 1−2 s), the compounds in the bio-oil have not reached their chemical reaction equilibria. For this reason, bio-oil is an unstable liquid mixture. When stored for a long time, especially at high temperatures, both its physical properties and chemical composition might change. Moreover, bio-oil is such a complicated mixture in composition that hundreds of compounds have been recognized, involving almost all species of oxygenated organic compounds (alcohols, aldehydes, ketones, carboxylic acids, furanoids, pyranoilds, phenols, saccharides, etc). In addition, bio-oil has many undesired properties in comparison to petroleum oils: (1) higher water content, (2) high acidity, leading to high corrosiveness, (3) high viscosity, (4) high ash content, and (5) high oxygen content, leading to a low heating value.2 These undesired properties present many obstacles to use bio-oil as a substitute for petroleum-based oils. In the past few decades, the researchers around the world have performed considerable work on ameliorating bio-oil quality. There are two ways with respect to the bio-oil quality promotion: the physical route and the chemical route. The physical route involves emulsification,3 distillation,4 molecular © 2014 American Chemical Society

Received: November 19, 2013 Revised: March 6, 2014 Published: March 7, 2014 2544

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and solid bases20 have been examined as the catalysts. The reactor coupling technology was also examined for bio-oil esterification.21 To enhance the acid conversion, an adsorber was designed to connect with the esterification reactor, by which the water removal could shift the esterification equilibria toward ester formation. It was testified that 99.9% ester yield was obtained when esterifying oleic acid with ethanol at 110 °C under continuous water removal conditions. Despite the relatively mild conditions in esterification of biooil, the formation of char and coke is still a problem, unless a large amount of alcohol is used. In this paper, our research objective is to examine whether the extractive reaction process can suppress or eliminate the char and coke formation during bio-oil upgrading. We present a new process for upgrading the water-soluble fraction of bio-oil, simultaneous esterification and acetalation with online solvent extraction (SEAWOSE). In comparison to the direct conversion without solvent extraction, this technique could significantly suppress the char and coke formation. The effects of oxidation and reduction of the water-soluble fraction on the subsequent upgrading were also investigated. Almost all of the acids and aldehydes were transformed to esters, hemiacetals, and acetals. Consequently, the upgraded oil demonstrated high oil quality in view of acidity, water content, and heating value.

on a setup illustrated in Figure 1. It was composed of a four-necked bottom flask with a mechanical agitator, a condenser, a water knockout

2. MATERIALS AND METHODS

Figure 1. Schematic drawing of the experimental setup for upgrading the WSF of bio-oil by SEAWOSE.

2.1. Materials. The bio-oil used was obtained by fast pyrolysis of rice husk at 500 °C with a heating rate of about 1500 °C/s in a fluidized bed with a residence time of less than 2 s. The apparatus for the fast pyrolysis of rice husk was similar to that in ref 22. The rice husk was dried and pulverized to powders with a particle diameter of 200−350 μm before pyrolysis. Table 1 summarizes the primary properties of the bio-oil. All of the other chemicals used were of analytical purity.

trap, and an electric jacket. That is very similar to the esterification experiment with a knockout trap. The major difference is that the WSF of bio-oil was added to the water knockout trap, while butanol and catalyst (sulfuric acid) were added to the round-bottom flask. When an experiment is performed, about 30.0 g of aqueous fraction (WSF, OWSF, or RWSF) was added into the water knockout trap, while 80.0 g of butanol and 1.0 g of sulfuric acid (98 wt %) were added to the four-necked bottom flask. When the butanol started boiling, its vapor was condensed and flowed into the water knockout trap, where the extractable compounds, such as acids, aldehydes, and ketones, in the WSF were extracted by butanol. The temperature in the water knockout trap was about 110 °C. After extraction, the extractant (oil phase) flowed back to the bottom flask, where the extracted acids and aldehydes reacted with butanol to form esters, hemiacetals, and acetals. The formed water by esterfication and acetalation were continuously removed by the refluxing butanol. In this way, butanol formed a cycle between the flask and the water knockout trap, extracted the aldehydes and acids continuously, and converted them into esters and acetals. When the setup is installed, the upper liquid surface in the water knockout trap should be slightly higher than the neck inlet of the flask, so that butanol can flow back into the flask by gravity. The temperature in the flask was about 140 °C. By such a recycling mode, the acids and aldehydes in the WSF were extracted and converted gradually, while the water formed during the esterification and acetalation reaction was removed away from the reactor. As a result, the acids and aldehydes could be completely converted into esters and acetals. The reaction time was about 10−12 h until there was no water formed. 2.3. Analysis. Qualitative analysis of the bio-oil samples was carried on gas chromatography−mass spectrometry (GC−MS) (GC6890/ MS5973N, Agilent), with the following conditions: column, Innowax 19091N-136 (60 m × 0.25 mm × 0.25 μm); carrier gas, 1 mL/min He; split ratio, 60; injection sample, 0. 2 μL; inlet temperature, 280 °C; interface temperature between GC and MS, 250 °C; and oven temperature, from 60 to 120 °C at a ramp rate of 5 °C/min and then to 240 °C at a ramp rate of 5 °C/min. The build-in mass spectra library with Agilent MS5973N, NIST 98 was used to identify the compounds.

Table 1. Physical Properties of the Bio-oil Used in the Experiments element analysis (%) C H O N

46.18 6.08 45.02 2.72

properties relative density (g/cm3) viscosity (mPa s) high heating value (MJ/kg) moisture (wt %)

1.163 20.50 14.00 32.97

2.2. Methods. 2.2.1. Water Extraction. A total of 200.0 g of bio-oil and 120.0 g of distilled water were sufficiently mixed in a 500 mL Erlenmeyer flask with a magnetic stirrer at room temperature.23 After stratification, about 242.48 g of water-soluble fraction (upper layer) and 77.52 g of oil-soluble fraction (bottom layer) were obtained. For convenience, the water- and oil-soluble fractions are designated as WSF and OSF, respectively. Table 3 gives the major properties of these two fractions. 2.2.2. Oxidation of the WSF. A mixture of 50.0 g of WSF and 15.0 g of hydrogen peroxide (30 wt %) in a 100 mL Erlenmeyer flask was subjected to oxidation for 2.0 h at room temperature and atmosphere pressure under stirring conditions. The oxidized water-soluble fraction is called OWSF in the following. 2.2.3. Reduction of the WSF. To 50.0 g WSF in a 100 mL Erlenmeyer flask, 4.0 g of hydrochloric acid (37 wt %) was added and stirred mechanically. Then, 3.0 g of iron powder was slowly added in batches to reduce the WSF sample at room temperature and atmosphere pressure at least for 2.0 h, followed by filtration to remove the unreacted iron powder and collection of the filtrate (RWSF). 2.2.4. Chemical Conversion of the WSF Coupling with Solvent Extraction. The upgrading of the WSF by SEAWOSE was performed 2545

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The moisture content in the samples was determined by Karl Fischer titration (870KF Trino plus, Metrohm, Switzerland). The high heating value (HHV) was measured by calorimetry (C200 calorimetric, IKA, Germany). The acidity was determined by potentionmetric acid−base titration, while the aldehyde and saccharide contents were measured by an anthrone colorimetric method with UV757CRT spectrometry (Shanghai Precise Scientific Instrument, Limited). Thermogravimetric analysis (WCT-1C, Beijing Optical Apparatus Factory) was used to characterize the quality of the upgraded oil.

It can be seen that the composition of the bio-oil was very complicated. A total of 41 compounds that had been identified occupied only 68.34% of the total bio-oil. There were still a lot of compounds that were not identified, even though they were detected by GC−MS, which occupied about 31.66%. Nevertheless, it should be noted that only the evaporable compounds in the bio-oil were detected by GC−MS, while the compounds with a high boiling point, such as saccharides and inorganic salt, were excluded in the GC−MS analysis results. From Table 2, the acids in bio-oil were predominantly formic, acetic, propanoic, and 3-(2-hydroxyphenyl)acrylic acids, about 9.56% of the bio-oil; aldehydes involved 2-hydroxylacetaldehyde, furfural, 5-hydroxymethyl furfural, 5-methylfurfural, and 3-hydroxyl-4-methoxyphenyl aldehydes, occupying 9.27%; ketones were composed of hydroxyl acetone, 1-hydroxyl-2butanone, 2,3-butanedione, 5-hydrofuran-2-one, and other heterocyclic ketones, accounting for 10.63%; and 12 phenols had been identified, accounting for 14.36%. It has been reported that levoglucosan is one of the sugars that can be detected by GC−MS, with a content as high as 10%.24 In our bio-oil, its content was 9.35%, in good agreement with the literature result. Although the moisture measured by GC−MS was merely 4.84%, it was as high as 32.97% by Karl Fischer titration. The latter is more reliable than the former because the GC−MS result was not calibrated. 3.2. Water Extraction. When 120.0 g of distilled water was added to 200.0 g of bio-oil, phase splitting occurred in the biooil spontaneously. This process was called water extraction by Vitaseri et al.23 The upper layer was a WSF, and the bottom layer was an OSF. On the mass basis of the original bio-oil, about 61.24% was in the WSF, while 38.76% was left in the OSF. Determination of their acidity, moisture, and HHV showed that, after water extraction, the acidity of OSF decreased from 90.88 to 50.24 mg of KOH/g and moisture decreased from 32.97 to 14.36%, while HHV increased from 14.00 to 22.75 MJ/kg, in comparison to the original bio-oil (Table 3). GC−MS in Figure 2a clearly shows that polar

3. RESULTS AND DISCUSSION 3.1. Original Bio-oil. Table 2 summarizes the residence time and relative peak area of the compounds that have been detected and identified by GC−MS. Table 2. Various Compounds and Relative Content in the Bio-oil by GC−MS

number

residence time

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

4.187 4.279 4.63 5.503 5.711 5.913 6.77 7.468 8.612 8.673 9.763 10.18 5 10.24 11.397 11.716 13.95 14.895 15.994 16.716 17.42 18.675 19.11 19.648 20.303 20.54 21.558 22.592 23.517 23.89 24.104 25.047 25.157 25.475 26.16 27.104 27.918 28.077 30.02 30.46 33.439 36.79

compound

relative peak area (%)

water methanol methyl acetate formic acid 2-hydroxyacetaldehyde acetic acid 1-hydroxy-2-acetone propanoic acid methyl acetate 1-hydroxy-2-butanone 2,3-butanedione furfural 2-methylfuran 2-oxopropyl acetate 2-methylallyl acetate 5-methyl furfural 5-hydrofuran-2-one 2-hydroxy-3-methylcyclopent-2-enone phenol 2-methoxyphenol 3-hydroxy-2-methyl-4-pyrone 4-methylphenol 4-hydroxymethyl lactone 3-methyl-4-methoxyphenol cyclopentanol 4-ethylphenol 2-methoxy-4-ethylphenol 1,4:3,6-dianhydro-D-glucopyranose 3-(2-hydroxyphenyl)acrylic acid 4-methyl-2-acetylphenol 5-hydroxymethyl fufural o-dihydroxyl benzene 2,4-dimethoxyphenol 2-methoxy-4-allylphenol 3-methyl-1,2-dihydroxybenzene 1,4-dihydroxyl benzene 3-hydroxy-4-methoxyphenyl aldehyde 4-tert-butyl-1,2-dihydroxyl benzene 4-butoxyphthalaldehyde levoglucosan 4-hydroxy-2-methoxycinnamic aldehyde unidentified

4.84 1.57 0.80 1.07 4.73 6.67 4.75 0.79 1.22 0.80 1.60 1.64 0.62 0.62 0.61 0.56 1.11 1.46 0.91 2.55 0.91 0.81 1.25 1.52 1.30 0.87 0.90 0.73 1.03 1.12 1.08 1.67 1.47 0.86 0.82 0.81 1.26 0.65 0.89 8.62 0.85 31.66

Table 3. Moisture, Acidity, and HHV of the Bio-oil and Various Fractions sample

moisture (%)

acidity (mg of KOH/g)

HHV (MJ/kg)

original bio-oil OSF WSF OWSF RWSF UO OUO RUO AR ORA RAR

32.97 14.36 72.14 74.40 72.14 3.80 2.59 2.52 94.60 96.10 94.77

90.88 50.24 59.14 101.60 64.20 1.23 1.60 1.40 1.20 2.90 2.20

14.00 22.75 4.28a 3.10a 4.28a 32.76 33.00 33.32 2.48a 0.77a 1.59a

a

The HHV was calculated by energy balance because the HHV of the aqueous fraction was too low to be measured by the calorimetric method.

compounds with a low molecular weight, such as acids, alcohols, adehydes, ketones, and saccharides, were inclined to dissolving in WSF, while the weakly polar and nonpolar compounds were enriched in OSF (Figure 2b), including phenols, esters, and heterocyclic and aromatic compounds. The increase in the HHV of OSF was mainly attributed to the 2546

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Figure 2. GC−MS chromatograms of the fractions after water extraction as well as after pretreatment: (a) WSF, (b) OSF, (c) OWSF, and (d) RWSF.

facile to be dehydrated and carbonized. Therefore, a high content of saccharides in the bio-oil would result in severe charring and coking issues and deactivate the catalysts quickly. Xu et al.25 had surveyed the esterification of bio-oil pretreated with ozone. The acidity of the rice husk pyrolysis oil increased from 45.4 to 118.4 mg of KOH/g after ozone oxidation. After subsequent esterification, the moisture in the esterified oil was only 1−2%, while the original bio-oil was with 45% water content. Xiong et al.17 investigated esterifying a rice husk bio-oil with in situ reduction of formic acid in methanol solution. They found that the pH value of the hydrogenated bio-oil increased from 2.17 to 4.5 and the HHV approached 22 MJ/kg. With reduction, no coking was observed during treatment. Because

decrease in the moisture and oxygen contents after water extraction. The decrease in acidity of OSF is expected because the acids in bio-oil were mainly composed of acetic, formic, and propanoic acids, which are readily water-soluble. It should be noted that the decrease in acidity of WSF was due to the dilution effect. 3.3. Effect of Oxidation/Reduction on the WSF. Given the fact that aldehydes are generally chemically active compounds, they take to a great extent responsibility for the thermal and chemical instability of bio-oil.16 For example, aldehydes can react with phenols to form phenolic resins at acidic conditions and change the viscosity of the bio-oil. Saccharides at high-temperature and hydropenic conditions are 2547

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markedly. The hydrolysis of levoglucosan during the treatment might be a good explanation. It should be mentioned that only aldoses can be determined by anthrone spectrophotometry. Therefore, the determined saccharides might be lower than the actual content. After treatment, the saccharide contents in OWSF and RWSF were decreased to 3.62 and 3.04%, respectively, with converions of 34.61 and 45.00%, respectively. 3.4. Upgrading of the WSFs by SEAWOSE. The upgrading of the three aqueous fractions, WSF, OWSF, and RWSF, were separately carried out in butanol at refluxing temperatures. After SEAWOSE upgrading, an oil fraction and an aqueous residue fraction were collected from the roundbottom flask and the water knockout trap, respectively, in each experiment. UO, OUO, and RUO were designated to the oil fractions obtained by upgrading of WSF, OWSF, and RWSF, respectively, and AR, OAR, and RAR denoted the aqueous residues, respectively. From Table 3, the moisture contents were very low, 3.80% in UO, 2.59% in OUO, and 2.52% in RUO, suggesting that SEAWOSE can efficiently remove the water from the reactor. Esterification and acetaltion are both reversible reactions and accompanied with water formation. The effective segregation of water can accelerate the reactions and enhance the conversions of acids and aldehydes. For this reason, the acidity values in UO, OUO, and RUO were merely 1.23, 1.60, and 1.40 mg of KOH/g, respectively, and those in AR, OAR, and RAR declined to 1.20, 2.90, and 2.20 mg of KOH/g, correspondingly. On the basis of the acidity in WSF, OWSF, and RWSF before upgrading, the calculated acidity conversions (α) reached up to 98.69, 97.96, and 97.63%, respectively. From the viewpoint of energy efficiency, the HHVs were boosted up to 32.76, 33.00, and 33.32 MJ/kg, although they were the HHVs of the mixtures containing the unreacted butanol. If assuming the esterification and acelation, both have no reaction enthalpy changes; then, the HHVs for the aqueous residues (AR, OAR, and RAR) were calculated to be 2.48, 0.77, and 1.59 MJ/kg, respectively. The calculated energy efficiencies were 62.53, 82.24, and 74.33%. The formulas for calculating acidity and energy efficiency are as follows: m a − m2a 2 α= 1 1 × 100 m1a1

aldehydes and some unsaturated compounds that have carbon− carbon double bonds are very active compounds, they might be responsible for the char, coking, and/or tar formation during bio-oil upgrading. Therefore, we surveyed the effect of decreasing aldehydes and unsaturated compounds on the side reactions, which lead to the char, coke, and/or tar formation, by oxidation and reduction of the WSF samples before upgrading by SEAWOSE. Hydrogen peroxide and iron powder were separately used to oxidize/reduce the WSF. The preferably oxidized or reduced compounds are aldehydes and the unsaturated compounds. By hydrogen peroxide oxidation, the aldehydes are expected to be first converted into acids and subsequently esterified to esters, while the unsaturated compounds might be decomposed by the breakage of the double bonds. By reduction, the aldehydes are expected to be converted into alcohols, at least partially, while the unsaturated compounds with double bonds could be saturated. Table 3 summarizes changes in the acidity, moisture, and HHV after treatment. The acidity of OWSF increased from 59.14 to 101.60 mg of KOH/g after oxidation. The determination of the aldehydes and saccharides in WSF by the anthrone colorimetric method showed that there were 0.71 g of aldehydes and 5.53 g of saccharides per 100 g of WSF (both on the basis of glucose). After oxidation and reduction, the aldehyde content decreased to 0.14 and 0.23 g/100 g, respectively, with conversion of 80.45 and 67.96% (Figure 3). From the GC−MS chromatogram for

β= Figure 3. Effect of the oxidation/reduction on the content of aldehydes and saccharides in WSF and aqueous residue after upgrading.

m1h1 − m2h2 × 100 m1h1

where m1 and m2 are the mass of the aqueous fractions before and after SEAWOSE upgrading (g), a1 and a2 are their acidities in mg of KOH/g, and h1 and h2 are their HHVs in MJ/kg. After SEAWOSE upgrading, aldehydes in the aqueous residues (AR, OAR, and RAR) were very low, approaching 100% conversion (Figure 3). For OWSF upgrading, the aldehydes in the WSF was first oxidized to acids and subsequently esterified to butyl esters. The unoxidized aldehydes and the aldehydes derived by the hydrolysis of saccharides were extracted and transformed into acetals and hemiacetals. Panels a, b, and c of Figure 4 are the GC−MS chromatograms of AR, OAR, and RAR after SEAWOSE, respectively. Only n-butanol (nBu), 2-hydroxyl acetone (HA), butyl acetate (BA), and levoglucosan (LG) were detected in AR and RAR. These evidence suggest that most of the compounds in the WSFs can be extracted and finally transformed into the upgraded oils, except the very strongly polar compounds, such

OWSF (Figure 2c), it can be seen that the peaks of hydroxyl acetaldehyde (HAA), propanal (PA1), furfural (Fur), 5hydroxymethyl furfural (HMF), 3-hydroxyl-4-methoxyphenylaldehyde (HMoPh), and 4-hydroxyl-2-methoxyl cinnamaldehyde (HMCA) became weak or disappeared, while the peaks of acids were intensified. These are evidence that the acidity increase of OWSF was attributed to the oxidation of aldehydes into acids, at least partially. The marked increase of formic acid (FA) might be due to oxidation of the aldehyde group on the aromatic ring and the carbon−carbon double bonds at the end of carbon chains. After reduction with iron powder, the acidity change was unnoticeable because the reduction of aldehydes formed alcohols or others rather than acids (Figure 2d). In addition, both oxidation and reduction had effect on levoglucosan 2548

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Figure 4. continued

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Figure 4. GC−MS chromatograms of the aqueous residues after upgrading by SEAWOSE, (a) AR, (b) OAR, and (c) RAR, and GC−MS chromatograms of the oil fractions after upgrading by SEAWOSE, (d) UO, (e) OUO, and (f) RUO.

as HA and LG. Interestingly, the amount of HAA in OAR was even higher than that in OWSF before SEAWOSE upgrading (Figure 4b). This fact provides strong evidence that HAA could derive from hydrolysis of saccharides during SEAWOSE upgrading. It had been expected the saccharides could be left in the water knockout trap because they are extremely difficult to be extracted by butanol. However, all of WSF, OWSF, and RWSF samples afforded very high saccharide conversions, 92−98%. High saccharide conversions might be due to the high temperature in the water knockout trap that enables hydrolysis of saccharides into their derivatives, such as 5-hydroxymethyl furfural, levulic acid, hydroxyacetaldehyde, etc. To verify this inference, esterification of WSF was also attempted in ethanol using benzene as an extraction solvent, which allowed for extraction at a lower temperature (about 60 °C). It was found that the saccharide conversion was much lower, with about 70% conversion. This result indicates that the saccharides in the WSF are readily hydrolyzed, accounting for the decrease of saccharides during pretreatment of WSF by oxidation and reduction. It can be seen from the GC−MS chromatograms in panels d−f of Figure 4 that new peaks of butyl esters appeared, such as butyl formate (BF), butyl acetate (BA), butyl propionate (BP), butyl butyrate (BB), butyl 2-hydroxyacetate (BHA), butyl levulinate (BL), dibutyl succinate (DBS), and dibutyl 2hydroxysuccinate (DHS) (Scheme 1). The fact that the BHA peak in Figure 4e is stronger than the counterparts in panels d and f of Figure 4 might be related to the oxidation of 2hydroxyacetaldehyde into 2-hydroxylacetic acid and subsequent esterification with butanol. The formation of butoxy(5(hydroxymethyl)furan-2-yl)methanol (BHMFM), butoxyl ace-

Scheme 1. Esterification Reaction

taldehyde (BoA), 4-butoxyphthalaldehyde (BPDA), 2-butoxytetrahydrofuran (BTHF), dibutyl ether (DBE), dibutoxyl methane (DBM), and 1,1,3,3-tetrabutoxy acetone (TBA) evidenced that the acetalation and etherification happened (Schemes 2 and 3). Transesterification reactions were also accompanied with the esterification and acelation simultaneously because the peaks of methyl formate (MA), glycerol diacetate (GDA), 2-oxopropyl acetate (OPA), and 4-hydroxymethyl butyrolactone (HMBL) disappeared or were weakened after upgrading. For comparison purposes, the control test was also performed. A total of 30.0 g of WSF and 80.0 g of butanol were directly added to the four-necked round-bottom flask and subjected to esterification and acetalation in the presence of 1.0 g of sulfuric acid with refluxing and water segregation. After reaction, the mixture left in the bottom flask was filtrated and the filter cake was dried at 101 °C. A total of 3.50 g of char was obtained, corresponding to 11.67% char yield on the basis of the WSF mass. In contrast, the char yields for upgrading WSF, OWSF, and RWSF by SEAWOSE were 3.67, 0.00, and 4.33%, respectively. These results strongly suggest that aldehydes and saccharides were responsible for the char formation. 3.5. Thermogravimetric Analyses of the Upgraded Oils. Volatility is one of the important indexes for evaluating 2550

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Scheme 2. Acetalation Reaction

as saccharides. OWSF showed higher weight loss because hydrogen peroxide is an aqueous solution and brought in water during the oxidation. Although oxidation can convert aldehydes into acids and break the double bond in the unsaturated compounds, its effect on the volatility of the upgraded oil was unnoticeable. Less weight loss for RWSF was attributed to FeCl2 formed during reduction with iron powder and hydrochloric acid. AR and OAR displayed a similar weight loss, about 92%, while RAR showed 77.87% weight loss, when the temperature rose to 350 °C. Such a low weight loss for RAR is due to the concentration effect of FeCl2 because the amount of RAR was only about half of RWSF. If the concentration effect is taken into account, the weight losses for AR, OAR, and RAR were all much lower than those of WSF, OWSF, and RWSF, indicating that the organic compounds had been effectively extracted and transformed into the upgraded oils.

Scheme 3. Etherification Reaction

the quality of a fuel for an internal combustion engine. Figure 5 shows the thermogravimetric curves of the oil and aqueous fractions before and after upgrading with and without pretreatment.

4. CONCLUSION Water extraction of bio-oil can enrich the polar compounds, such as acids, adehydes, ketones, alcohols, and saccharides, with low molecular weight in the WSF, while the weakly polar and nonpolar compounds (such as phenols, esters, and ligin-derived compounds) were left in the OSF. The aldehydes and saccharides in the bio-oil take great responsibility for the chemical and thermal instability of bio-oils. Simultaneous esterification and acetalation with online solvent extraction can effectively eliminate the occurrence of side reactions that finally lead to the char formation during upgrading of the WSF. Almost all of the acids and aldehydes were converted to esters, hemiacetals, and acetals by esterification and acetalation. The upgraded oils had very low acidity and moisture content but high HHV and good thermal stability. Moreover, SEAWOSE could markedly suppress char formation and enhance the energy efficiency compared to the direct esterification and acetalation without solvent extraction, with more than 97% acid conversion and 100% aldehyde conversion. Moreover, more than 92% saccharides was converted into the upgraded oils simultaneously. Oxidation of the WSF as a pretreatment afforded more esters and less acetals, with no char formation.

Figure 5. Thermogravimetric curves of the upgraded oils and the aqueous residues.

For conventional light diesel fuel and heavy diesel fuel, their boiling point ranges are about 150−370 and 350−410 °C, respectively. From Figure 5, it can be seen that only 70% of the original oil was volatile below 350 °C. This is because there is a great amount of high-molecular-weight compounds in the biooil resulting from the insufficient pyrolysis of hemicellulose, cellulose, and lignin. For example, saccharides and polyphenols are non-volatile compounds. On the other hand, side reactions, such as phenol-aldehyde polymerization might occur at high temperatures and lead to char and coke formation. Noticeable char was indeed observed in the crucible after thermogravimetric analysis. The volatility of OSF was even worse, less than 60% below 350 °C, because most of the phenols and other high-boilingpoint compounds were enriched in the OSF. This is consistent with GC−MS results. The upgraded oils (UO, OUO, and RUO) showed more than 90% weight loss, indicating their high volatility. In addition, both oxidation and reduction seemed to have little effect on the volatility of the upgraded oil. WSF was with 70% weight loss before 125 °C, consistent with its high water content. When the temperature rose to 350 °C, there was still about 10% in weight left. This suggests that high-boiling-point compounds are contained in the WSF, such

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ASSOCIATED CONTENT

* Supporting Information S

GC−MS chromatogram of the raw bio-oil (Figure S1). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2551

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ACKNOWLEDGMENTS This work is financial supported by the National Natural Science Foundation of China (Grant 51276103) and the National High-Tech Program (2012AA101808) granted by the Ministry of Science and Technology of the People’s Republic of China.

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NOMENCLATURE 2M3APh = 2-methoxy-3-allylphenol 2M4APh = 2-methoxy-4-allylphenol 2MoPh = 2-methoxyphenol 4MoPh = 4-methoxyphenol 5MF = 5-methylfurfural AC = acetic acid AF = 2-acetyl furan BA = butyl acetate BB = butyl butyrate BDPh = 4-tert-butyl-1,2-dihydroxyl benzene BF = butyl formate BHA = butyl hydroxylacetate BHMFM = butoxy(5-(hydroxymethyl)furan-2-yl)methanol BL = butyl levulinate BoA = butoxyl acetaldehyde BoPy = 2-butoxy-2-hydropyrane BP = butyl propionate BPDA = 4-butoxyphthalaldehyde BPh = 4-tert-butylphenol BTHF = 2-butoxytetrahydrofuran CH = cyclohexane-1,2,3,4,5,6-hexaol CP = cyclopentanol DBE = dibutyl ether DBM = dibutoxyl methane DBS = dibutyl succinate DHS = dibutyl 2-hydroxysuccinate DHT = 3,5-dihydroxyl toluene DiMoPh = 2,4-dimethoxyphenol DMAPh = 2,6-dimethoxy-4-allylphenol Eph = 4-ethylphenol FA = formic acid Fur = furfural GDA = glycol diacetate HA = hydroxyl acetone HAA = 2-hydroxylacetaldehyde HBA = 3-hydroxybutan-2-one HF = 5-hydrofuran-2-one HFK = 4-hydroxylfuran-2-(3H)one HMBL = 4-hydroxymethyl butyrolactone HMCA = 4-hydroxy-2-methoxycinnamic aldehyde HMCP = 2-hydroxy-3-methyl-2-cyclopentenone HMoPh = 3-hydroxy-4-methoxyphenyl aldehyde HPAA = 3-(2-hydrophenyl)acrylic acid LG = levoglucosan MA = methyl acetate MB = 2-methyl butanol MeOH = methanol MMMPh = 2-methoxy-3-(methoxymethyl)phenol MMoPh = 3-methyl-4-methoxyphenol MoEPh = 2-methoxy-4-ethylphenol MPh = 2-methylphenol MPK = 3-methyl-2-heptanone MPPh = 2-methoxy-4-(1-propenyl)phenol nBu = n-butanol

o-DPh = o-dihydroxybenzene OPA = 2-oxopropyl acetate PA = propanoic acid Pal = propionic aldehyde PDO = 2-propyl-1,3-dioxane Ph = phenol PO = propylene oxide TBA = 1,1,3,3-tetrabutoxy acetone

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