Liquid–Liquid Extraction of Biomass Pyrolysis Bio-oil - Energy & Fuels

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Liquid−Liquid Extraction of Biomass Pyrolysis Bio-oil Yi Wei, Hanwu Lei,* Lu Wang, Lei Zhu, Xuesong Zhang, Yupeng Liu, Shulin Chen, and Birgitte Ahring Bioproducts, Sciences and Engineering Laboratory, Department of Biological Systems Engineering, Washington State University, Richland, Washington 99354-1671, United States ABSTRACT: Various organics with quite different oxygen-containing functional groups mixing together in bio-oil lead to the high instability of bio-oils. A liquid−liquid extraction method was developed to separate the bio-oil into different chemical groups by their polarities to stabilize bio-oils and improve the quality. The separated bio-oil had similar oxygen-containing functional groups in different phases. Biomass pyrolysis using Douglas fir pellet and characterization of the bio-oil chemical compounds were conducted, followed by bio-oil liquid−liquid extractions with several solvents (e.g., hexane, petroleum ether, and chloroform). In comparison to the raw bio-oil, the solvent phase had high concentrations (85%) of phenols and guaiacols, while no sugar and very low acid and alcohol contents were detected, which were left in the water phases.

1. INTRODUCTION Because petroleum plays a major role in the world economy, rapid growing consumption of fossil fuel and depletion of total crude oil reservation lead to a global energy crisis. Meanwhile, fossil fuel causes a negative impact on the environment and public health because of its pollutant gas emissions. The increasing emissions of greenhouse gas introduced by burning fossil fuel lead to global warming, which has become one of the biggest environmental issues in human history.1,2 As a result, various research focuses on biomass as the most significant renewable resource to produce alternative liquid fuels. Bio-oil produced from biomass pyrolysis has been considered as a promising renewable liquid fuel source. Pyrolysis is a thermochemical conversion process, which runs at 350−650 °C in the absence of oxygen to produce solid, liquid, and syngas.3 The bio-oil produced from biomass pyrolysis is a complex compound mixture containing alkenes, aromatics, phenolics, guaiacols, furans, esters, aldehydes, ketones, alcohols, sugars, and acids, with 35−40% oxygen content.4 The complex organic compounds with different functional groups are mixed in bio-oils.5 Because of its low heating value and high oxygen content, bio-oil has to be upgraded before being used as a liquid fuel or chemical product.6−8 However, upgrading of bio-oil is rather difficult currently, because of the catalyst coking.9 Principally, coke is formed through polymerization and polycondensation reactions.10 Among all of the chemicals in bio-oils, oxygen-containing function groups, predominantly sugars, guaiacols, and furanic rings, are most likely to form coke with aldehydes because of their instability and deficiency in molar balance.9 These precursors react on the catalytic surface and fill up the pores, which contribute to inactivation of catalysts during the upgrading and hydrodeoxygenation processes of biooils.5,9,11,12 Small aldehyde molecules are easily condensed together with aromatics to form polymers.5,13−16 Although increasing the hydrogen pressure and reaction temperature while reducing the acidity of the catalyst can drive down coking on the catalyst surface, it is still a significant challenge to minimize the hydrogen consumption and coking at mild reaction conditions.17 © 2014 American Chemical Society

In comparison to strictly controlling the catalysis reaction conditions, a new method broadens the horizon of minimizing polymerization and polycondensation reactions by extracting and separating the oxygen-containing function groups. The chemical compounds, such as aromatics, phenols, guaiacols, sugars, and acids, have distinct polarities. There is few research using organic solvents for bio-oil extraction. Hu et al. reported that the bio-oil can be separated into two phases, an upper water-like phase and a lower oil-like phase, by adding water but 20% (w/w) of weak polarity organic compounds are still left in the water-like phase.5,18 Extraction of bio-oils with water followed by chloroform could effectively separate sugars from aromatics.5 However, Hu and co-authors only conducted the extraction experiments with a single chemical solvent for model compound extraction without extracting the mixture of real crude bio-oils. Other research by Wang et al. was conducted on separation of phenols and pyrolytic lignins from the waterinsoluble phase of bio-oils, while it required multi-steps and did not concern sugar derivatives and aldehyde compounds.19 Using organic solvent liquid−liquid extraction to separate the raw bio-oil into different chemical groups by their polarities has not been found in the literature. How to selectively extract precursors of polymerization and polycondensation reactions from bio-oil and effectively separate them are two main challenges because of the complexity of biooils. Because bio-oil has a relatively high water content and high viscosity, the liquid−liquid extraction method has great potential to improve the quality, stabilize bio-oils, and separate the bio-oil into different chemical groups compared to leaching and solid-phase extractions. The separated bio-oil should have similar oxygen-containing functional groups. In the present study, liquid−liquid extraction was used to purify the water-like phase with a strategy of separating the guaiacol and sugar derivatives and aldehyde compounds. The bio-oil was produced from microwave pyrolysis of Douglas fir sawdust pellets. Received: December 17, 2013 Revised: January 29, 2014 Published: January 29, 2014 1207

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min. Finally, the flask was kept in a tightly closed container away from sunlight for 24 h before analysis. After separating the solvent and water phase II, organics were analyzed by GC−MS. In water phase II, the water content was tested by the KF compact titrator to calculate the organics. In the solvent phase, the organics were calculated by the mass gain of the solvent phase from a reference mass weight in the control group. The pH values of the solvent phase were measured on both after vigorous shaking and after 24 h of storage (pH meter, pH 510 Benchtop Meter, Oakton). The experiments were conducted under room temperature, which was constant around 25 ± 1 °C in a fuming hood during the entire separation and extraction processes. 2.6. GC−MS Analysis. The chemical composition of bio-oil was determined by Agilent GC−MS (GC, Agilent 7890A; MS, Agilent 5975C) with a DB-5 capillary column. GC was programmed by maintaining at 45 °C for 3 min, followed by heating to 300 °C at a heating rate of 10 °C/min. The injection took place at 300 °C, with a sample injection size of 1 μL. The flow rate of the carrier gas (helium) was 0.6 mL/min. The ion source temperature was 230 °C for the mass selective detector.21 The compounds were identified by comparing the spectral data to the National Institute of Standards and Technology (NIST) mass spectral library.27−29 2.7. Solvent (Chloroform) Distillation and Recycling. Among all of the three organic solvents, only chloroform has a constant boiling point. Petroleum ether is a mixture with boiling points ranging from 40 to 60 °C, while hexane is easier to form azeotrope with water, alcohols, and acids, which makes it difficult to control and set the distillation temperature. Only chloroform was recycled in this study. The chloroform phase was collected and put in a rotary evaporator (Heidolph Glassware G3 Vertical). The flask was rotated at 150 rpm. The vacuum pump was set at a pressure of 20 kPa, with the heating bath set at a temperature of 30 °C. The chloroform phase was continuously evaporated until no more liquids came out from the condenser. Then, both of the liquids stayed in the flask, and the liquids that evaporated out were weighed and stored separately.

Douglas fir is a soft wood in the coniferous family, which contains 44% cellulose, 21% hemicelluloses, and 32% lignin, and its pyrolysis has a high bio-oil yield.20 In comparison to conventional heating mechanisms used in biomass pyrolysis, microwave pyrolysis induces heat at the molecular level, which converts the electromagnetic energy into heat directly.21 Ruan et al. studied catalytic microwave pyrolysis to produce valueadded chemicals, such as furfural.22,23 Recent research conducted on microwave pyrolysis showed significant phenol and guaiacol yields.24−27 It is necessary to separate the bio-oil into chemical groups with similar functional groups before biooil upgrading or use as chemical feedstock. The objective of this study was to investigate the liquid− liquid extraction of bio-oils and determine the effects of extraction solvents and volume ratios on chemical separation with similar functional groups. The organic yield was determined, and compounds were characterized by gas chromatography−mass spectrometry (GC−MS). Solvent distillation and recycling were also investigated.

2. MATERIALS AND METHODS 2.1. Materials. Douglas fir pellets (7 mm in diameter and 15 mm in length) were purchased from Bear Mountain Forest Products, Inc., Cascade Locks, OR. Chloroform (ACS reagent grade, 99.8%) and petroleum ether (extra pure, boiling range of 40−60 °C, Acros Organics) were purchased from Fisher Scientific. Hexane [highperformance liquid chromatography (HPLC) grade, 98.5%] was purchased from VWR International. 2.2. Experimental Design. A full factorial design (two factors with 3 × 3 factorial treatment structure with duplicates) was used to optimize the liquid−liquid extraction. Three different solvents (chloroform, hexane, and petroleum ether) and three volume ratios (solvent/water phase of 1:0.5, 1:1, and 1:2) were employed as independent variables in the design. Solvent was a categorical factor, while the volume ratio was a numeric factor. 2.3. Microwave Pyrolysis. A Sineo MAS-II batch microwave oven (Shanghai, China) with a rated power of 1000 W was used to produce bio-oil. The reaction conditions were set at 723 K, 25 min, and a fixed microwave power input of 700 W according to the report by Bu et al.2 During this pyrolysis reaction, a bio-oil with a high yield of guaiacols and furans, which were regarded as the precursors in catalytic coking, was obtained. A total of 200 g of Douglas fir sawdust pellets was put in the pyrolysis reaction vessel. After reaching the desired reaction temperature, the microwave reactor automatically controlled the temperature and maintained the power and temperature for 25 min. Condensers were used to cool the hot heavier volatiles into liquids as bio-oils. The light volatile that escaped from the end of the condenser was pyrolysis gas. After the pyrolysis, char was left in the flask. The bio-oil and char were weighed, and gas was determined using the following equation according to a previous report:21

3. RESULTS AND DISCUSSION 3.1. Product and Organic Yield Distribution. During the pyrolysis reaction, the 700 W power setting resulted in a heating rate of about 70 °C min−1 before reaching the reaction temperature. After the pyrolysis, bio-oil and char were weighed separately and gas was determined according to previous a report and calculated by mass balance.21 The bio-oil yields were from 37.80 to 38.25 wt % of the feedstock, while char yield was about 30 wt %. The gas yield was about 32 wt % of the feedstock. A total of 300 mL of water was added to the bio-oil according to the 1:1 (v/v) ratio of water/bio-oil. The water yield is about 19% based on original biomass because of the original moisture in biomass pellets and water produced during the microwave pyrolysis. The organic yield distribution after bio-oil separation and water contents are shown in Table 1. Table 1. Organic Yield Distribution and Water Contents

weight of gas = weight of biomass − bio‐oil mass − char mass 2.4. Bio-oil Separation. The bio-oil obtained from microwave pyrolysis was collected, and the same volume of water was added to separate the bio-oil into two phases. After that, the mixture was standing for 30 min, followed by centrifuges under 10 000 rpm for 15 min. Then, both the upper water-like phase and the lower pasty oil-like phase were separated and weighed. The moisture contents were determined by a Karl Fischer (KF) compact titrator (V20 Compact Volumetric KF Titrator, Mettler-Toledo). The organics were calculated by mass balance. 2.5. Liquid−Liquid Extraction. The water-like phase (water phase I) was used for liquid−liquid extraction. In this study, different extraction solvents and volume ratios were investigated. A fixed volume (20 mL) of the water phase and relative volume of extraction solvents were weighed and added in a 100 mL Erlenmeyer flask. The flask was shaken vigorously for 30 min and magnetically stirred for 30

original oil phase water phase I

organic yield (%)

organic (g/200 g of biomass)

100.00 38.18 61.82

38.18 23.60 14.58

Although there were more organics in water phase I (61.82 wt % of total organics) than in the oil phase (38.18 wt % of total organics), the oil phase had a much higher organic concentration (89.99 wt %) and lower water content. According to the result, water separation can concentrate the organics from original bio-oils. To obtain a higher organic yield from original bio-oils, further solvent extraction is necessary to gather more weak-polarity organics from water phases. 1208

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Table 2. Full Factor Design of Liquid−Liquid Extraction Based on 20 mL of Water Phase I run

solvent

ratio (v/v)

organics in the solvent phase (g)

percentage of total organics (%)a

organics in water phase I (g)

percentage of total organics (%)a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

petroleum ether hexane hexane hexane chloroform petroleum ether chloroform chloroform petroleum ether chloroform hexane petroleum ether hexane chloroform petroleum ether petroleum ether chloroform hexane

1:2 0.5:1 1:2 1:1 1:1 1:2 1:2 0.5:1 1:1 0.5:1 1:2 0.5:1 1:1 1:2 0.5:1 1:1 1:1 0.5:1

0.70 0.39 0.45 0.43 1.38 0.63 1.50 1.03 0.73 1.03 0.47 0.27 0.46 1.51 0.25 0.73 1.39 0.40

34.07 19.06 21.48 20.39 66.73 30.67 73.54 50.04 34.28 49.96 22.50 13.64 21.90 74.14 12.19 34.04 66.61 19.29

1.35 1.67 1.64 1.70 0.69 1.42 0.54 1.03 1.40 1.03 1.61 1.74 1.66 0.53 1.77 1.41 0.70 1.66

65.93 80.94 78.52 79.61 33.27 69.33 26.46 49.96 65.72 50.04 77.50 86.36 78.10 25.86 87.81 65.96 33.39 80.71

a

The total organics stands for the organics in water phase I.

3.2. Full Factorial Design Analysis. The full factor design and mass balance of liquid−liquid extraction using three different solvents (polarity: chloroform, 4.1; hexane, 0.0; and petroleum ether, 0.1)30 with three levels of volume ratios are shown in Table 2. After solvent extraction, the solvent phase had a weight increase from 0.25 to 1.51 g, as the organics were extracted from water phases to solvent phases. According to the organic content shown in Table 1, the solvent had extracted 12.19−74.14 wt % of total organics from the water phase. It is obvious that chloroform had a better extraction result in weight increases than hexane and petroleum ether solvents. According to the results of the experiment, the regression equations showing the weight of solvent extraction were obtained as a function of the solvent (A1, chloroform; A2, hexane) and ratio of solvent/water phase I (B).

describe the organic yield in the water phase after solvent extraction. The effect of interactions on extraction effects can be visualized in Figure 1. In Figure 1A, there was a strong interaction between the solvent and its ratio to water phase I on the organic yield in solvent phases. The ANOVA test gave a p value of 0.011 < α = 0.05, indicating a strong interaction. A similar result appears in Figure 1B for the yield of water phase II organics. A strong interaction was found between the solvent and its ratio to water phase I, which significantly affected the yield of water phase II organics with a p value of 0.0029 < α = 0.05, indicating a significant interaction. 3.3. Effect of Extraction Conditions on Organic Distribution. 3.3.1. Effect of the Solvent on the Extraction Result. As shown in Figure 2, chloroform had the strongest affinity for the organics in the water phase among all three solvents. The highest amount of organics was extracted to the chloroform solvent phase, followed by petroleum ether and hexane, as shown in Figure 2. Petroleum ether had a better extraction result than hexane when the ratio of solvent/water phase I was 1:1 (v/v), because the organic mass in the petroleum ether phase were more than that in the hexane phase. It was found that 66.67 wt % of organics in water phase I were extracted to the chloroform phase, while only 21.14 and 34.16 wt % were extracted to the hexane phase and petroleum ether phase, respectively, when the ratio of solvent/water phase I was 1:1 (v/v) (Figure 2). The variation in extraction results were mainly caused by different solvent polarities. Although three solvents were nonpolar, chloroform had a stronger polarity than hexane and petroleum ether, which was more affinity to the organic compounds in water phase I. Consequently, chloroform was selected as the optimum solvent in the extraction process. 3.3.2. Effect of the Solvent/Water Phase I Ratio on the Extraction Result. An increase of the organic mass in the solvent phase was observed when more solvent was added. A significant increase on the organic mass of the solvent phase in chloroform and petroleum ether resulted from an increase of the solvent/bio-oil ratio (Figure 3). A slight increase of

Ysolvent = 0.93 + 0.55A1 − 0.34A 2 + 0.16B + 0.079A1B − 0.11A 2 B − 0.21B2

(1)

Ywater = 1.19 − 0.57A1 + 0.36A 2 − 0.15B − 0.092A1B + 0.11A 2 B + 0.14B2

(2)

According to the analysis of variation (ANOVA) test, the p value of eq 1 was 0.0001 < α = 0.05, which is significant and can describe the weight of organics obtained by solvent extraction. The p values of both factors were less than 0.0001, which indicated that they are significant to affect organic yields. The coefficient of determination for eq 1 was 0.98, which meant that the regression model was significant to describe the organic yield from solvent extraction with its significant factors. In reference to the extraction method introduced above, the organics in water phase II stand for the organics that were not extracted by the solvents and the yield was described by eq 2. The p value of eq 2 was 0.0001 < α = 0.05, which meant that the equation was significant to describe the weight of nonextractable chemicals. The p values of both factors were less than 0.0001, and the coefficient of determination for eq 2 was 0.99, indicating that the regression model was significant to 1209

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Figure 3. (A and B) Effect of ratios on extraction: (A) effect of the ratio on the extracted organic mass in the solvent phase and (B) effect of the ratio on the organic distribution based on the chloroform solvent.

the ratio changed from 1 to 2, while the extracted organics were still significantly increased in the chloroform phase. The organic matter was increased from 50.00 to 66.67 wt % in the chloroform phase when the ratio was increased from 0.5 to 1. The highest amount of organics (73.84%) was extracted in the chloroform phase when a ratio of 2 was used. A total of 7.17% more organics extracted by chloroform may not meet the expense on scaling up the liquid−liquid extraction equipment and processes by doubling the amount of solvents, which require a larger extractor. Many liquid−liquid extraction technology companies have studied the relationship between scale and efficiency of extraction, showing that a larger diameter of the extraction reactor and lower extraction speed lead to a lower efficiency of extraction.31 Consequently, a solvent/water phase I ratio of 1 was regarded as the optimum extraction condition. 3.4. GC−MS Characterization. 3.4.1. Chemical Characterization on Water Extraction. GC−MS analysis was carried out to determine the composition of bio-oil and extracted organics (Figure 4). The original bio-oil was mainly composed of 52.25% phenols and guaiacols, 3.08% acids, 3.9% ketones, 1.25% alcohols, 0.39% hydrocarbons, 15.57% furans, 2.41% esters, and 0.65% sugars. In comparison to the original bio-oil, the oil phase had higher concentrations of hydrocarbons, phenols, and guaiacols, along with very low acid and alcohol contents and without sugars. Water phase I had large amounts of esters, and nearly 82.7 wt % of total furans stayed in water phase I. Although water phase I had only 37.56% phenols and guaiacols, it still retained the equal mass of phenols and guaiacols in the oil phase. According to the result, water

Figure 1. Effect of the interaction between the solvent and ratio on extraction results: (A) solvent phase and (B) water phase II. Solid lines represent the standard mean and trend of the data, and dash lines show the 95% confidence. The red, green, and blue spots stand for chloroform, hexane, and petroleum ether, respectively.

Figure 2. Effect of the solvent on organic distribution based on a ratio of 1:1 (v/v).

extracted organics occurred in hexane solvent when the ratio was increased. The increase of extracted organic mass was not significant (p value > 0.5) for hexane and petroleum ether when 1210

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Figure 5. Chemical characterization by GC−MS on different solvents and ratios.

solvent among the three referring to the GC−MS results. Chloroform extracted 82.99 wt % guaiacols, 42.39 wt % phenols, and 70.63 wt % furans from water phase I (Table 2), while only 0.93 wt % acids, 6 wt % alcohols, and 0.65 wt % sugars were extracted out at the same time. Petroleum ether was observed as poor performance in organic extraction by mass because only 35.77 wt % guaiacols were extracted. Similar extraction results occurred on the hexane solvent with low organic affinity. Therefore, chloroform was considered the best solvent in the liquid−liquid extraction process. 3.4.4. Chemical Characterization by the Ratio Effect. Figure 5 shows the influence of the solvent/water phase I volume ratio levels on liquid−liquid extraction. As the ratio of solvent/water phase I increased, the extracted ketones increased from 1.62 to 2.72% for all three solvents. Furan contents were increased with the increase of the solvent ratios. Guaiacols had a better affinity to solvents than other compounds because guaiacols were first extracted with high concentrations. About 50 wt % organics was extracted, which contained about 55% guaiacols when a 0.5 ratio was used. A total of 31.9 wt % more guaiacols was extracted in the chloroform phase when the solvent ratio was increased from 0.5 to 1. By adding more solvent with ratios from 1 to 2, more organics were extracted in the solvent phase, while most of the extracted organics did not belong to the guaiacol category because the guaiacol content was slightly different (56.16 versus 55.55%). Decreases were observed for the guaiacol content using hexane and petroleum ether when ratios were increased from 1 to 2. In contrast, the content of furans significantly increased with more solvents added. This indicated that certain organics (e.g., furans) required more solvents to extract at a ratio more than 1:1 (v/v). Other compounds, such as alcohols and esters, were not responsive to the change of the solvent ratios. In summary, the low ratio of 0.5 could not extract organics sufficiently, the ratio of 1 could be sufficient to extract guaiacol and phenol organics, and the high ratio of 2 extracted more ketones, furans, and other organic compounds to solvent phases. A 1:1 (v/v) ratio was considered the optimum solvent level for the liquid−liquid extraction. 3.5. Chloroform Distillation and Recycling. Chloroform had been proven to be the optimum solvent for liquid−liquid extraction among all three solvents. Because chloroform can transform to chlorine hydride through the photodecomposition process, the pH value of the chloroform phase was measured by a pH meter before distillation. The pH value of the original chloroform phase was 3.11. The pH value changed to 3.10

Figure 4. (A and B) Chemical characterization of different extraction phases by GC−MS: (A) chemical composition of bio-oil, oil phase, and water phase I and (B) chemical composition of both solvent phase and water phase II after extraction.

extraction cannot efficiently extract phenols and guaiacols, indicating that further liquid−liquid extraction is a necessary step to achieve similar oxygen-containing functional groups in the same phase. 3.4.2. Chemical Characterization on Liquid−Liquid Solvent Extraction. Water phase I was further extracted using solvents. Figure 4B illustrates the chemical composition of both solvent phase and water phase II after extraction by GC−MS. A total of 67.84 wt % of organics in water phase I was extracted in the solvent phase, which contained over 60% phenols and guaiacols, about 20% furans, less than 5% ketones and esters, and trace sugars, acids, and alcohols. The extracted guaiacols were made up of phenol, 2-methoxy- (15.06%), phenol, 2methoxy-4-methyl- (26.38%), phenol, 4-ethyl-2-methoxy(10.41%), and phenol, 2-methoxy-4-(1-propenyl)- (4.32%). The chemical compounds in the furan category are furfural (11.48%) and 2-furancarboxaldehyde, 5-methyl- (6.46%). Phenols and guaiacols had a better solubility in these solvents than in water. Acids, alcohols, and sugars were hardly soluble in extraction solvents, in which they stayed in water phase II. 3.4.3. Chemical Characterization by the Solvent Effect. The GC−MS analysis indicated that chloroform represented a better affinity to ketones and phenols, in comparison to two other solvents (Figure 5). Hexane and petroleum ether had quite similar organic concentrations and characteristics, while the latter had a higher content of guaiacols but lower content of furans and ketones. All three solvents had poor solubility with acids, sugars, and alcohols. Chloroform would be the best 1211

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when the chloroform phase was distillated in the rotary vessel. It was convinced that there was trace of chlorine hydride generated in the extraction process, indicating that photodecomposition of chloroform could be neglectable. A total of 20 mL of the chloroform phase (30.3 g, 1:1 ratio of solvent/water phase I) with 1.39 g of organics was distilled in the rotary evaporator. After 0.5 h of distillation, 28.00 g of liquid was distilled out, while 1.31 g of organics was left in the rotary vessel. The liquid in the rotary vessel with a pH value of 3.58 was chemicals extracted in the solvent phase. The distillate was analyzed by HPLC. Trace phenolics or guaiacols and no furans were found, indicating an eligible purity of chloroform. A total of 96.78 wt % of chloroform was distilled and could be reused. The mass loss of chloroform in the process was mainly due to the open environment in lab-scaled equipment and a small lab scale of the extraction process. The mass loss of chloroform would be significantly reduced when extraction is performed in a closed process environment with large industrial-scale reactors.

4. CONCLUSION Liquid−liquid extraction was used to purify the water-like phase of bio-oils. According to the result, the extraction solvent and volume ratio had a strong influence on liquid−liquid extraction, producing phenols and guaiacols. A total of 66.67 wt % of organics in water phase I was extracted while acids, sugars, and alcohols stayed with the water phases. The optimum extraction condition was found using a chloroform solvent at a solvent/ water phase I ratio of 1, during which an acceptable level of mass and concentrations of phenols and guaiacols were obtained. The phenols and guaiacols were at 85% recovery efficiency in the extracted phase by liquid−liquid extraction. The extracted fractions with high phenol and guaiacol contents were mixed together with the pasty oil phase. The mixture could be upgraded by catalytic cracking25 or hydrodeoxygenation (HDO)2,26 processes to produce aromatics and phenols. The water phase II contains acids, alcohols, and furfurals, which can be converted to more stable esters using the esterification process with ethanol, followed by a similar process to produce hydrocarbons.32 The chloroform solvent can be distilled and reused at a recovery rate of 96.78 wt % with eligible purity. Future works will be focused on seeking organic solvents with a higher extraction yield and higher phenolic selectivity at different extraction temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 509-372-7628. Fax: 509-372-7690. E-mail: hlei@ wsu.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported in part by the Joint Center for Aerospace and Technology Innovation (JCATI) and Chinese Scholarship Council.

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REFERENCES

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dx.doi.org/10.1021/ef402490s | Energy Fuels 2014, 28, 1207−1212