Effects of Mild Alkali Pretreatment and Hydrogen-Donating Solvent on

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Effects of mild alkali pretreatment and hydrogen-donating solvent on hydrothermal liquefaction of eucalyptus woodchips Zhixia Li, Yaming Hong, Jiangfei Cao, Zhentao Huang, Kai Huang, Hao Gong, Lingyun Huang, Song Shi, Masakazu Kawashita , and Yue Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01625 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Effects of mild alkali pretreatment and hydrogendonating solvent on hydrothermal liquefaction of eucalyptus woodchips Zhixia Li,*,† Yaming Hong,† Jiangfei Cao,‡ Zhentao Huang,† Kai Huang,‡ Hao Gong,† Lingyun Huang,† Song Shi,† Masakazu Kawashita,§ Yue Li† †

School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi

530004, China ‡

State Key Laboratory of Non-food Biorefinery Enzymolysis, National Engineering Research

Center for Non-food Biorefinery, Guangxi Academy of Sciences, Nanning 530007, China §

Department of Biomedical Engineering, Graduate School of Biomedical Engineering, Tohoku

University, Aoba-ku, Sendai 980-8579, Japan

ACS Paragon Plus Environment

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Abstract: Eucalyptus woodchips (EWC) were pretreated with mild aqueous NaOH solution, followed by liquefaction in water-hydrogen donor (tetralin) mixture at a temperature from 220 to 330 °C to produce bio-oil. As a reference, pure tetralin and water were also used as liquefaction media. Compositional analysis and SEM observation were performed to study the effect of the pretreatment on EWC structural change. The influences of temperature, and tetralin content in mixed solvent on conversion and heavy oil (HO) yield were determined. SEM image showed that pretreatment made the fiber structure loose and destroy biomass through hemicellulose and lignin dissolution. Compared with water or tetralin as solvent, water-tetralin mixed solvent (WTMS) shows better effect on the conversion and HO yield in the tested temperature range. The highest conversions for liquefaction of untreated EWC were 66 wt%, 88.1 wt%, and 88.3 wt%, and the highest HO yields were 19.7 wt%, 26.4 wt%, and 43.2 wt%, with water, tetralin, and WTMS, respectively. The optimized conditions to achieve both high conversion (97.3 wt%) and HO yield (57.3 wt%) is liquefaction in WTMS at 300 °C using 1.0 wt% NaOH-pretreated EWC. GC-MS analysis showed that phenols, ketones, aromatics, and alkenes are the main components in HOs.

Keywords: NaOH pretreatment; eucalyptus woodchips; liquefaction; hydrogen donor; watertetralin mixed solvent


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1. INTRODUCTION Development of renewable biofuels from biomass to substitute fossil fuels is increasingly important to keep sustainable energy development and cope with the global climate change.1 The hot and moist climate conditions of south china have encouraged local government to develop various non-food biomass resources such as fast-growing eucalyptus and sugarcane as raw materials of industry. Now eucalyptus plantation area in china has reached 3.6 million hm2, and the plantation will increase year by year. 2‒4 Eucalyptus is mainly used for pulp production and furniture making, and in these processes, substantial wood waste is produced and is not utilized effectively. Wood wastes, such as eucalyptus sawdust, woodchips, and bark, have great potential applications in the preparation of bio-oil, a prospective renewable biofuel, because of their availability and low cost. Bio-oil can be produced via thermochemical conversion approaches e.g. pyrolysis and direct liquefaction. Compared with pyrolysis, direct liquefaction has following advantages: the presence of solvent prevents the cross linked reaction by diluting the liquefaction products; reaction occurs at a relatively low temperature and counsumes less energy. As water is used as solvent, the biomass materials don’t need to be dried.5-7 Although the hydrothermal liquefaction of biomass has been investigated extensively in recent decades, the commercial application of hydrothermal liquefaction still faces many challenges, such as low bio-oil yield and biomass conversion. Liquefaction efficiency is also considerably affected by the type of biomass. Wood biomass is one of the most difficult biomass types to liquefy in water without catalysis due to its high lignin content. Extensive pretreatments by physical, chemical, or biological means were suggested to increase the bio-oil yield and decrease the reaction temperature. Shi at al.8,

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performed ultrasonic pretreatment of cellulose and cornstalk, demonstrated that, compared with


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untreated samples, pretreatment increased the bio-oil yields by 22.1 wt% for cellulose and 10 wt% for cornstalk. Performing acid-chlorite pretreatment of cornstalk10, 11 prior to hydrothermal liquefaction at 200–260 °C was reported to effectively increase the water-soluble bio-oil yield. Additionally, the bio-oil yield could be enhanced by combining the hydrothermal liquefaction process with alkaline pretreatment of biomass resources.12, 13 Hamieh et al. demonstrated that the biological pretreatment and direct liquefaction of green wastes (a mixture of straw, wood, and grass) promoted the efficiency of the liquefaction process in terms of the yield and quality of the resulting bio-oil (oxygen content and energy value).14 These results indicate that appropriate pretreatment is necessary to destroy the rigid structure of lignocelluloses and thereby enhance bio-oil production. In addition, the uses of organic solvents show remarkable effects on liquefaction process. The often used solvents are acetone, ethanol, methanol, ethylene glycol, phenol etc.15-19 These solvents showed better effects on enhancing biomass conversion than water. It is believed that organic solvent can dissolve and dilute liquefaction products, and prevent their repolymerization reaction. Co-solvent systems, such as methanol-water, ethanol-water, perform better than a single solvent, probably due to the synergistic effects between two solvents, which increase dissolution of complex compounds exceeding with a single solvent. Our recent study reported13 liquefaction of bagasse at 300 °C with different solvents, conversion increased from 67 wt% to 86 wt% when water as solvent was replaced with water-tetralin mixture (tetralin content: 50 wt%) (WTMS), and further increased from 86 wt% to 99 wt% when mildly alkali-pretreated bagasse was used. These results demonstrated that both the use of WTMS and mild alkali pretreatment are effective techniques to promote biomass liquefaction process. Tetralin is a typical hydrogendonating solvent (HDS), and thus WTMS is expected to have hydrothermal decomposing and


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hydrogen-donating abilities. An example on the use of HDS liquefaction of sawdust20 showed that HDS has better effects on hydrogenation ability than reducing gases e.g. H2 or syngas. An extensive research on HDS applied on biomass liquefaction is desired to expand its application for production of liquid fuels. However, so far as I know, a few of works have been carried out to use tetralin or WTMS as solvent in the liquefaction of wood biomass. In this study, eucalyptus woodchips (EWC) were used as a wood biomass sample. Liquefaction behaviors of wood biomass in water, tetralin and WTMS were comparatively studied. Mild alkali pretreatment was performed, and the effects of pretreatment on EWC structure and liquefaction efficiency were determined. The influences of liquefaction temperature, tetralin content in solvent on liquefaction efficiency, product distribution, and characteristics of solid residue (SR) were explored. 2. EXPERIMENTAL SECTION 2.1. Materials and Pretreatment EWC was obtained from Guangxi Jingui Pulp co., LTD (Nanning, China). The air-dried woodchips were firstly crushed using a crusher, and sieved through 20 mesh. Only the portion with particle sizes able to pass through 20-mesh was used. The crushed EWC was dried and pretreated according to the method as reported in our previous study. 13 2.2. Liquefaction process All liquefaction experiments were carried out in a high pressure autoclave (volume: 1000 ml) at a raw material/solvent mass ratio of 1:8. The untreated or pretreated EWC samples were liquefied in water, tetralin, and WTMS (with 50 wt% of tetralin content), respectively. All


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liquefactions were conducted at the chosen temperature (from 220 to 330 °C) with magnetically stirring at 300 rpm and held for 60 min. The working pressure was between 1.5 and 9.8 MPa, depending on the reaction temperature and solvent. The details about the liquefaction procedure had been revealed in our last study.13 Additionally, the tetralin content in WTMS was adjusted to be 0, 10 wt%, 50 wt%, 90 wt%, and 100 wt%. The effects of the tetralin content change on conversion and HO yield were also investigated. The experiments were performed with 30 g of untreated EWC at 300 °C for 60 min. The conversion and HO yield were calculated using the following two formulas: Conversion (%) = 100 × [1 ‒ (weight of SR)]/(weight of starting EWC) HO yield (%) = 100 × (weight of HO)/(weight of starting EWC) 2.3. Analytical methods Cellulose, lignin and hemicellulose contents of raw and pretreated EWC samples were analyzed according to the HNO3/ethanol method 21 and Chinese standard methods.13 Surface morphologies of EWC samples before and after pretreatment were observed using scanning electron microscope (SEM, VE–8800, Keyence, Japan). To elucidate the effect of the liquefaction solvent on the decomposition of main components of EWC, the SRs of runs were analyzed using a Thermogravimetric Analyzer (TGA, Q50). Thermogravimetry (TG) and derivative thermogravimetry (DTG) profiles of raw EWC and the SRs of runs were obtained by heating under nitrogen gas with a heating rate of 20 °C/min.


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The chemical compositions of the HO samples were analyzed using Fourier transform infrared spectrometry (FT-IR, FTIR-8400S, Shimadzu, Japan) by recording the transmission spectra within the range of 4000‒400 cm‒1 using the KBr disk technique, in which one drop of HO was spread on the surface of a 150-mg KBr disk. The compositions of HO samples were also identified by a gas chromatograph equipped with a mass spectrometer (GC-MS, Shimadzu 17A/QP5000, Japan). 13 A HP–5MS column (30 m×0.25 mm×0.25 µm) was used. The carrier gas (He) velocity was kept at 1.2 mL/min. The oven program was set as follows: 5 min isothermal at 60 °C, and heated to 300 °C at 10 °C/min, and then held the temperature for 25 min. 3. RESULTS AND DISCUSSION 3.1. Effects of pretreatment on the composition and structure of EWC Raw EWC were pretreated with different dosages of NaOH (0‒4.0 wt% of EWC), and then rinsed and dried. After that, the chemical compositions were analyzed; the results are shown in Figure 1. Compared with the untreated EWC (dosage of NaOH: 0), the cellulose content in the pretreated EWC increased from 42.38 wt% to 44.50 wt% as the NaOH dosage increased from 0 to 4.0 wt%, whereas the hemicellulose and lignin contents decreased from 36.29 wt% to 32.52 wt% and from 25.00 wt% to 21.88 wt%, respectively. Thus, NaOH pretreatment increases the likelihood of hemicellulose and lignin degradation as well as structural damage to lignincarbohydrate complexes (LCC) .22 A SEM was used to observe changes in the physical structure of EWC during NaOH pretreatment. The microstructures of untreated and pretreated EWC are shown in Figure 2.


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Figure 2(a) shows that the untreated EWC exhibits fibrous particle aggregates and that its surface is rough and compact, whereas NaOH-pretreated EWC has an orderly and loose surface structure (Figures 2(b) and 2(c)). One possible reason is that the partial dissolution of hemicellulose and lignin by NaOH pretreatment exposes more of the skeleton structure of cellulose. Koo et al.23 conducted acidic organosolv pretreatment (using ethanol) of wood plant, showed that lignin was isolated and migrated onto the biomass surface, forming lignin droplets. However, in our experiment, the SEM images did not reveal any lignin droplets on the surface, likely because lignin is soluble in NaOH. This suggests that NaOH pretreatment altered the physical structure of EWC, increasing the surface exposure and roughness of the fine fibrils and thereby creating favorable conditions for the subsequent liquefaction reaction. 3.2. Liquefaction of untreated EWC The liquefaction of untreated EWC with water, tetralin, and WTMS was studied under identical experiment conditions at various temperatures from 220 to 330 °C for 60 min. The water/tetralin mass ratio in WTMS was fixed at 1:1. The average conversions and HO yields from three replicate tests are shown in Figure 3. Among the tested solvents, WTMS has the best effect on the conversion and HO yield in the temperature range from 220 °C to 320 °C. The highest conversions were 66%, 88.1%, and 88.3%, and the highest HO yields were 19.7 wt%, 26.4%, and 43.2 wt% with water, tetralin, and WTMS, respectively. However, when the temperature exceeded 300 °C, the HO yield decreased, likely because of char formation through repolymerization and cyclization reaction, and/or secondary decompositions of HO into the light fraction or gas products.24 Additionally, when the temperature was relatively low (< 270 °C), both water and WTMS resulted in higher conversions, likely because of water’s ability to decompose cellulose and hemicelluloses at low temperature.25 Meanwhile, the conversion


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achieved using tetralin as the solvent rapidly increased from 250 to 300 °C, indicating that TL mainly follows solvothermal and thermolysis mechanisms which require a higher temperature. By contrast, WTMS liquefaction included the several reaction mechanisms as discussed above (hydrothermal, solvothermal and thermolysis), and thus performed more effectively. Furthermore, in WTMS, the presence of HDS helps to stabilize decomposition fragments resulting from hydrolysis and solvolysis reactions. This prevents the decomposition intermediates from reacting to produce SRs and thereby results in high conversion.23 The liquefaction of untreated EWC in WTMS with different tetralin contents was also studied. Based on the above analysis, a high conversion could be obtained when the temperature exceeded 300 °C when WTMS was used. Therefore, EWC liquefaction in WTMS with different tetralin contents (0, 10 wt%, 50 wt%, 90 wt% and 100 wt%) was performed at 300 °C for 60 min with a EWC/WTMS mass ratio of 1/8. The content of tetralin was changed by setting water/tetralin mass ratios at 1:0, 9:1, 1:1, 1:9 and 0:1. As shown in Figure 4, the conversion and HO yield increase firstly, and then decrease with the ratio changed from 1:0 to 0:1, and get the maximum values at a tetralin content of approximately 50 wt%. This is in agreement with the results described in our previous study13: tetralin content at 50 wt% contributed a higher bagasse conversion than other tetralin contents. These results are likely attributed to the synergic effect between two solvents.13 In WTMS liquefaction, the presence of water contributes to increasing the reaction pressure, thereby shifting the gas-liquid state equilibrium of tetralin towards the liquid state in the reactor; Liquid tetralin has a higher density, and better dissolving ability and hydrogen shuttling abilty.24 In other words, the hydrogen-donating ability of tetralin has been enhanced by the presence of water. These are responsible for the high solid conversion and biooil yield in the case of WTMS.


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3.3. Liquefaction of pretreated EWC The influences of NaOH amount on EWC liquefaction in three solvents are shown in Figure 5. These experiments were performed at 300 °C and 60 min of residence time with 30 g of EWC. Clearly, using an appropriate amount of NaOH exerts positive effects on the conversion and HO yield from WL and WTMS liquefaction, while a higher amount of NaOH, e.g., 4 wt%, decreased the HO yields of WTMS liquefaction and TL. NaOH pretreatment could damage the LCC structure and promote hydrolysis reactions involving lignocellulosic materials, thus enhancing the efficiency of WL and WTMS liquefaction. A higher amount of NaOH probably facilitates the formation of gas products, thereby decreasing the HO yield. This was confirmed by our experiment, which showed that a higher amount of NaOH led to a higher final reaction pressure (the pressure in the reactor after the liquefaction reaction is complete and the reactor has been cooled to room temperature), indicating formation of more gas products. In the tested range of NaOH amounts, the highest conversions for WTMS liquefaction and WL were 97.9 wt% and 89.1 wt%, respectively. The near-complete conversion by WTMS liquefaction indicates that both NaOH pretreatment and WTMS have synergetic effects on preventing both the repolymerization of the intermediate products of the liquefaction process and residue (char) formation. The low solubility of NaOH in tetralin and the different decomposing mechanisms of TL (mainly solvolysis and thermal degradation reaction mechanisms) are likely responsible for the low effectiveness of NaOH pretreatment on TL.26 Recently, several studies have investigated the effect of alkali catalysis on biomass liquefaction efficiency. Mazaheri et al.27 studied the effects of NaOH, Na2CO3 and K2CO3 as catalysts on the hydrothermal liquefaction of oil palm shells (OPS) and concluded that the addition of 10 wt% NaOH resulted in the highest solid conversion and liquid product yield: 84


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wt% and 53.4 wt%, respectively. Wang et al.28 demonstrated that K2CO3 and KOH as catalysts on the liquefaction of pinewood sawdust in water or supercritical carbon dioxide increased the bio-oil yield and decreased SR. The roles of alkali catalysts might be reduced to: swelling polymer chains by breaking the intermolecular hydrogen bonds in lignocellulosic structure; Causing the cleavage and condensation of the products. In this study, pretreatment of EWC with a low dosage of NaOH (0.5‒4.0 g/100 g of EWC) resulted in high conversions—97.9 wt% and 89.1 wt%—for WTMS liquefaction and WL, respectively. This might be attributed to the damage of LCC structure caused by dissolution of partial hemicellulose and lignin by pretreatment, thereby increasing the likelihood of decomposition of cellulose and lignin.29 Besides, NaOH also catalyzes the liquefaction reaction. Thermal degradation of raw EWC and liquefaction SR from the untreated and pretreated EWC samples are shown in Figures 6(a) and 6(b), respectively. As Figure 6 shows, raw EWC decomposes through three steps: main weight loss at 200‒320 °C, is attributable to the decomposition of hemicelluloses and cellulose; the gradual weight loss at 320‒415 °C, is mainly caused by the decomposition of cellulose of higher crystalline quality; the weight loss from 415 °C to 470 °C reflects the decomposition of lignin.26 Two obvious peaks can be seen in the DTG curve of untreated EWC: one relatively large peak at approximately 280 °C, and a moderate peak at approximately 430 °C. Unlike untreated EWC, the peak at 280 °C disappears and a stronger peak appears between 400 °C and 500 °C in all DTA curves of the EWC SRs (Figure 6 (a)). Thus, all hemicelluloses and most of the cellulose in the raw EWC decomposed during the liquefaction processes with three different solvents, and a considerable number of lignin molecules remained in the SRs. It can be seen from Figure 6 (a) that peaks in DTG curves differ in height and position. The temperature corresponding to the


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peak height (Tp) is inversely proportional to the reactivity.27 Comparing the Tp between 400 °C and 500 °C reveals the following: Tp of SR from WL > from WTMS liquefaction > from TL; Tp of SR from TL is similar to the pattern exhibited by untreated EWC. This indicates that some thermally stable macromolecules easily formed in WL process but not in TL process. The higher burnout temperature (Tb, approximately 505 °C) of the SR from WL confirms that some thermally stable macromolecules indeed exist in this material. The thermal degradation of SRs from liquefaction of the pretreated EWC is shown in Figure 6(b). Compared with the SRs derived from untreated EWC (Figure 6(a)), the SRs from pretreated EWC show lower Tp and Tb values in their DTG curves when the same solvent is used, indicating SRs from pretreated EWC are more reactive. These results demonstrate that NaOH pretreatment can prevent repolymerization of the intermediate products of liquefaction process and, thus the formation of thermally stable compounds. 3.4. Compositional analysis of the HO The compositions of HO derived using different solvents were studied using FT-IR and GCMS. The liquefaction operations were performed at 300 °C for 60 min in 240 mL of water, tetralin, or WTMS (tetralin content: 50 wt%) using untreated and pretreated EWC samples. All of the HO samples shared very similar curve profiles and band positions in their FT-IR spectra (Figure 7). The band at 3350 cm‒1 can be assigned to the O–H stretching vibrations of phenolic and alcohol moieties. The weak absorption at about 3000 cm‒1 is caused by C‒H stretching vibration in aromatic rings. The absorption band between 3000 and 2800 cm‒1 are attributed to symmertrical and asymmetrical C‒H stretching vibrations of methyl and methylene groups. Among them, the weak absorption at 2960 cm‒1 in curves a, b and c is caused by C‒H


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stretching vibration of methyl group, and the absorptions at 2930 cm‒1 and 2850 cm‒1 are attributed to C‒H stretching vibration of methylene groups in cyclanes. 28 The band at 1366 cm‒1 is caused by C‒H bending vibrations. The absorption at 1745 cm‒1 in curves a, b, c, and g is caused by C=O stretching vibrations in non-conjugated ketone, carbonyl, or ester groups.30 The absorption at 1710 cm‒1 in curves d, e, and f is probably attributable to the presence of a conjugated C=O group. The absorption bands at 1610 cm‒1, 1505 cm‒1, and 1450 cm‒1 correspond to aromatic C=C bending vibrations and indicate the presence of compounds with aromatic ring structures.31 The bands at 1210, 1100, and 1050 cm‒1 correspond to C‒O stretching vibrations in phenolic hydroxyl group and secondary and primary alcohols, respectively.18 Compared with untreated EWC (curve g), the adsorptions at 3350 cm‒1 and 1100 cm‒1 in all the HO samples are obviously weaker, indicating that HO samples contain fewer O–H groups and primary alcohols. This could be caused by dehydration and condensation reactions involving the intermediate products derived from EWC degradation. Meanwhile, the adsorption at 1210 cm‒1 in HOs from WL and TL (curves a, b, c, and d) is increased, indicating that the WL- and TL-derived HOs contain increased numbers of phenolic hydroxyl groups. In addition, the stronger adsorption at 1100 cm‒1 in HOs from WTMS liquefaction (curves e and f) suggests that these HOs contain a relatively high amount of secondary alcohol groups. The chemical components of these HOs analyzed by GC-MS are summarized in Table 1. HO component classifications are shown in Table 2. Calculations were performed based on the peak areas of the GC-MS chromatograms. As seen in Table 1, the main difference between liquefaction in the three tested solvents was the resulting chemical composition. The HOs produced by the liquefaction of untreated EWC


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with the three tested solvents all contained 2-methyl-1,5-hexadiene, 2-methyl-2-cyclopenten-1one, 2-methoxyphenol, 2-methoxy-4-methylphenol, and 2,6-dimethoxyphenol at significantly different relative abundances (e.g., 2-methyl-1,5-hexadiene was observed to make up 5.82%, 5.96%, and 14.09% of the total area for WL, TL, and WTMS liquefaction, respectively). The major phenolic compounds were 2-methoxyphenol and 2,6-dimethoxyphenol for WL and TL but 2-methoxy-4-methylphenol (12.72%) for WTMS liquefaction. Greater quantities of 2-methyl-2cyclopenten-1-one and 2-methyl-1,5-hexadiene were produced by WTMS liquefaction. The differences in the chemical compositions of the HOs produced by WTMS liquefaction could be attributed to the synergistic reaction between water and tetralin: hot-pressed water accelerates EWC decomposition by hydrolysis, while tetralin likely acts as a substrate, further reacting with the decomposition fragments by providing active hydrogen and increasing the formation of substituted phenolic compounds, ketones, and alkenes. The EWC HOs were very complex mixtures. Tables 1 and 2 show the following: (1) All HO samples contained high contents of oxygenated compounds (78.53‒88.55 wt%), mainly consisting of phenolic compounds, ketones, aromatic compounds, and alkenes. (2) The solvent type substantially affected the product distribution. In contrast to WL and TL, WTMS liquefaction products contained more ketones (36.8 wt%), aromatic compounds (23.27 wt%), and alkenes (18.01 wt%). (3) The formation of ketones is significantly promoted and the formation of phenolic compounds limited by NaOH pretreatment, regardless of the solvent used. The presence of large amount of phenols in the HOs could be related to the stable structure of these compounds.32 Phenols are also a degradation product of furans and furfurals, which are derived from the hydrolysis and pyrolysis of carbohydrates (cellulose and hemicelluloses). The mechanisms of phenol formation via furan or furfural degradation have been reported by Luijkx


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et al.33 Phenols can also be derived from lignin by thermal cracking of the phenyl-propane units of the macromolecular lattice by cleavage of β-aryl ether linkages.17 Although the degradation of cellulose and lignin are enhanced under WL conditions through a combination of hydrolysis and pyrolysis reaction mechanisms, thereby resulting in the formation of abundant phenols during WL, the high phenol content may also accelerate the formation of SR via polymerization reactions and thus decrease the conversion.17 The reason for the remarkably decreased phenol contents in HO obtained via WTMS liquefaction may be the hydrogenation of phenols to 2-methyl-2-cyclopenten-1-one and other hydrogenation products because of the hydrogen-donating properties of tetralin in WTMS. However, no decrease in the phenol content of HO from pure TL is observed, suggesting that water likely play a key role to improve tetralin’s hydrogen-donating ability. As discussed in previous section, the better hydrogen-donating ability of WTMS likely results from the higher reaction pressure. Table 2 shows that the ketone contents in HOs obtained via liquefaction in the three tested solvents significantly increased when NaOH-pretreated EWC was used. This is likely because NaOH stabilizes the intermediate products derived from the liquefaction process. As a result, some relatively reactive ketone products are formed. This can be confirmed by TG analyses of EWC SR: SR from NaOH-pretreated EWC show a lower Tp than SR from untreated EWC (see Figure 6(a) and (b)), indicating that SR from pretreated EWC is more reactive. This result is in good agreement with that reported in a previous study25: the use of NaOH as a catalyst in the liquefaction of OPS in subcritical WL favored the formation of relatively reactive OPS SRs compared the SRs produced using K2CO3 or Na2CO3 as a catalyst or uncatalyzed liquefaction.


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In fact, some polar and non-polar light compounds probably formed through hydrothermal and/or solvothermal liquefaction reaction of biomass, were likely dissolved in liquefaction solvent and filtered out during the separation of the insoluble material from the reaction mixture. As a result, they would not have been identified in HO. Because tetralin is a high-boiling point solvent (209 °C), it is difficult to collect the tetralin-soluble fraction by distillation (solvent removal in a rotary evaporator). The complete characterization of liquid products and investigation of the application of liquid products as a resource for fuel or chemical production are currently being pursued. The obtained HOs are unsuitable for use as an energy resource for boilers or engines because of their high oxygen contents. Some refining techniques, such as catalytic hydrogenation or catalytic cracking, are required to improve the quality of such HO by removing the oxygen. The high phenol contents in HOs from WL indicate that WL-derived HO probably represents a promising resource for preparing new composite materials and environment friendly adhesives. Additionally, ketones are considered to be important platform chemicals, and can be widely used in the chemical industry. 4. CONCLUSIONS NaOH pretreatment altered the main components and physical structures of EWC, and effectively enhanced the conversion and HO yield of WL and WTMS liquefaction. TG analysis showed all hemicelluloses and most of cellulose decomposed during liquefaction process with three solvents. It was more difficult to decompose lignin in TL process. Some thermally stable macromolecules were formed by WL process. The optimized conditions to achieve both high conversion (97.3 wt%) and HO yield (57.3 wt%) was liquefaction in WTMS at 300 °C using 1.0


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wt% NaOH-pretreated EWC. GC-MS analysis showed that phenols, ketones, aromatics, and alkenes were the main components of the produced HO. The formation of ketones was promoted and the formation of phenols limited by NaOH pretreatment.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 771 3274209. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by the National Natural Science Foundation of China (21266002, 21566004), Scientific Research Foundation of Guangxi University (XGZ120081, XTZ140787), and Guangxi Natural Science Foundation (2013GXNSFFA019005, 2012GXNSFBA053028). ABBREVIATIONS DTG, derivative thermogravimetry; EWC, eucalyptus woodchips; FT-IR, Fourier transform infrared spectrometry; GC-MS, gas chromatograph-mass spectrometer; HDS, hydrogen-donating solvent; HO, heavy oil; OPS, oil palm shells; SEM, scanning electron microscope; SR, solid residue; TGA, thermogravimetric analyzer; Tb, burnout temperature; TG, thermogravimetry; TL, tetralin liquefaction; Tp, temperature corresponding to peak height; WL, water liquefaction; WTMS, water-tetralin mixed solvents. REFERENCES


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hot-compressed water for bio-oil production. J. Agric. Food Chem. 2011, 59, 10524–10531. (11) Liu, H.M.; Feng, B.; Sun, R.C. Enhanced bio-oil yield from liquefaction of cornstalk in suband supercritical ethanol by acid−chlorite pretreatment. Ind. Eng. Chem. Res. 2011, 50, 10928–10935. (12) Liu, H. M.; Wang, F. Y.; Liu, Y. L. Alkaline pretreatment and hydrothermal liquefaction of cypress for high yield bio-oil production. J. Anal. Appl. Pyrol. 2014, 108, 136–142. (13) Li, Z.; Cao, J.; Huang, K.; Hong, Y.; Li, C.; Zhou, X.; Xie, N.; Lai, F., Shen, F.; Chen, C. Alkaline pretreatment and the synergic effect of water and tetralin enhances the liquefaction efficiency of bagasse. Bioresour. Technol. 2015, 177, 159–168. (14) Hamieh, S.; Beauchet, R.; Lemee, L.; Toufaily, J.; Koubaissy, B.; Hamieh, T.; Pouilloux, Y.; Pinard, L. Bio oil synthesis by coupling biological biomass pretreatment and catalytic hydroliquefaction process. Bioresour. Technol. 2014, 156, 389–394. (15) Liu, Z.; Zhang, F. S. Effects of various solvents on the liquefaction of biomass to produce fuels and chemical feedstocks. Energy Convers. Manage. 2008, 49, 3498– 3504. (16) Zhao, Y. P.; Zhu, W. W.; Wei, X. Y.; Fan, X.; Cao, J. P.; Dou, Y. Q.; Zong, Z. M.; Zhao, W. Synergic effect of methanol and water on pine liquefaction. Bioresour. Technol. 2013, 142, 504–509. (17) Demirbas, A. Mechanisms of liquefaction and pyrolysis reactions of biomass. Energy Convers. Manage. 2000, 41, 633–646. (18) Fan, S.P.; Zakaria,S.; Chia, C.H.; Jamaluddin, F.; Nabihah, S.; Liew, T.K.; Pua, F.L. Comparative studies of products obtained from solvolysis liquefaction of oil palm empty fruit bunch fibres using different solvents. Bioresour. Technol. 2011, 102, 3521–3526. (19) Yip, J.; Chen, M.; Szeto, Y. S.; Yan, S. Comparative study of liquefaction process and


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liquefied products from bamboo using different organic solvents. Bioresour. Technol. 2009,100, 6674–6678. (20) Wang, G.; Li, W.; Li, B.; Chen, H. Direct liquefaction of sawdust under syngas. Fuel 2007, 86, 1587–1593. (21) Wang, L. F.; Cheng, Y. C. Determination the content of cellulose by nitric acid ethanol method. Chem. Res. 2011, 22, 52–55. (22) Henriksson, G. Lignin, in: Ek, M.; Gellerstedt, G.; Henriksson, G. (eds.) Pulp and Paper Chemistry and Technology; DE Gruyter: Berlin, 2009; pp.132–133. (23) Koo, B. W.; Kim, H. Y.; Park, N.; Lee, S. M.; Yeo, H.; Choi, I. G. Organosolv pretreatment of Liriodendron tulipifera and simultaneous saccharification and fermentation for bioethanol production. Biomass Bioenergy 2011, 35, 1833–1840. (24) Akhtar, J.; Amin, N.A.S. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renewable Sustainable Energy Rev. 2011, 15, 1615– 1624. (25) Jakab, E.; Liu, K.; Meuzelaar, H. L. C. Thermal decomposition of wood and cellulose in the presence of solvent vapors. Ind. Eng. Chem. Res. 1997, 36, 2087–2095. (26) Beauchet, R.; Pinard, L.; Kpogbemabou, D.; Laduranty, J.; Lemee, L.; Lemberton, J. L.; Bataille, F.; Magnoux, P.; Ambles, A.; Barbier, J. Hydroliquefaction of green wastes to produce fuels. Bioresour. Technol. 2011, 102, 6200–6207. (27) Mazaheri, H.; Lee, K. T.; Mohamed, A. R. Influence of temperature on liquid products yield of oil palm shell via subcritical water liquefaction in the presence of alkali catalyst. Fuel Process. Technol. 2013, 110, 197–205. (28) Wang, Y.; Wang, H.; Lin, H.; Zheng, Y.; Zhao, J.; Pelletier, A., Li, K. Effects of solvents


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and catalysts in liquefaction of pinewood sawdust for the production of bio-oils. Biomass Bioenergy 2013, 59, 158–167. (29) Cağlar, A.; Demirbas, A. Conversion of cotton cocoon shell to liquid products by supercritical fluid extraction and low pressure pyrolysis in the present of alkalis. Energy Convers. Manage. 2001, 42, 1095–1104. (30) Chen, Y.; Tshabalala M.A.; Gao, J.; Stark, N.M.; Fan, Y. Color and surface chemistry change of pine wood flour after extraction and delignification. Bioresources 2014, 9, 2937−2948. (31) Agrawalla, A.; Kumar, S.; Singh, R.K. Pyrolysis of groundnut de-oiled cake and characterization of the liquid product. Bioresour. Technol. 2011,102, 10711–10716. (32) Sinağ, A.; Gülbay, S.; Uskan B.; Güllü, M. Comparative studies of intermediates produced from hydrothermal treatment of sawdust and cellulos. J. of Supercritical Fluids 2009, 50, 121–127. (33) Luijkx, G.C.A.; Rantwijk, F.; Bekkum, H. Hydrothermal formation of 1,2,4-benzenetriol from 5-hydroxymethyl-2-furaldehyde and D-fructose. Carbohyd. Res.1993, 242, 131–139.


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Tables Table Caption Table 1 Main compounds in HO obtained from liquefaction of untreated- and pretreated EWC in water, tetralin and WTMS, respectively. Table 2 Composition classification of HO derived from liquefaction of untreated- and pretreated EWC at 300 °C for 60 min in different solvents.


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Table 1. Main compounds in HO obtained from liquefaction of untreated- and pretreated EWC in water, tetralin and WTMS, respectively. RT

Compound

Formula

(min)

In water (area%)

In tetralin (area%)

In WTMS (area%)

Untreated Pretreated



4.11

4-Methyl-3-penten-2-one

C6H10O

1.10

1.70

0.90

3.03

1.64

1.91

5.16

2-Cyclopenten-1-one

C5H6O

1.54

1.92

0.85

2.44

1.91

1.89

6.22

2-Methyl-1,5-hexadiene

C7H12

5.82

7.78

5.96

8.53

14.09

12.75

6.72

1,3,5,7-Cyclooctatetraene

C8H8

1.67

1.99

3.87

2.43

3.91

3.99

6.86

Cyclohexanone

C6H10O

3.30

4.09

1.81

4.92

2.93

5.08

7.25

2-Methyl-2-cyclopenten-1-one

C6H8O

15.87

19.47

8.79

23.82

27.93

30.60

7.62

1-Methylethylbenzene

C9H12

1.27

3.06

1.93

1.89

1.55

1.34

7.91

2,5-Hexanedione

C6H10O2

0.98

2.52

0.54

−

1.16

−

8.91

3-Methyl-2-cyclopenten-1-one

C6H8O

2.95

2.55

3.82

6.78

1.23

1.16

10.24

(E)-2-Octen-1-ol

C8H16O

2.90

0.76

0.85

−

−

−

10.48 3,4,4-Trimethyl-2-cyclopenten-1-one

C8H12O

1.20

2.03

2.93

6.68

−

−

11.01 2-Methylphenol

C7H8O

1.27

1.44

1.74

0.84

−

−

11.57 2-Methoxyphenol

C7H8O2

6.08

7.37

7.54

0.41

1.10

1.54

12.15 4-Propyl-1,6-heptadien-4-ol

C10H18O

−

−

1.03

−

−

−

13.27 Naphthalene

C10H8

2.05

1.98

3.50

−

5.02

4.07

13.43

2-Methoxy-4-methylphenol

C8H10O2

2.49

1.69

4.79

11.68

12.72

7.82

14.60 4-Methyl-3-(2-methyl-2-propenyl)2(5H)-furanone

C10H16O

2.10

1.27

1.58

2.97

−

0.73

14.83 4-Ethyl-2-methoxyphenol

C9H12O2

4.74

2.83

4.64

2.93

−

1.29

14.99 2,3-Dihydro-1H-Inden-1-one

C9H8O

1.10

2.05

2.61

−

−

−

15.80 1-Methoxy-1,4-cyclohexadiene

C7H10O

−

−

0.98

2.27

−

−

8

16.09 2,6-Dimethoxyphenol

C H10O3

27.88

20.17

21.03

3.58

5.27

6.71

16.19

C10H14O2

−

−

2.92

1.83

−

−

16.96 1-(3-Hydroxyphenyl)ethanone

C8H8O2

−

−

0.88

−

−

−

17.38 1,2,4-Trimethoxybenzene

C9H12O3

6.37

4.56

7.49

2.65

11.70

9.77

18.41 1,2,3-Trimethoxy-5-methylbenzene

C10H14O3

2.93

3.82

3.64

2.36

4.28

5.20

20.27 2,6-Dimethoxy-4-(2-propenyl)phenol

C11H14O3

1.56

2.82

2.02

3.09

−

1.25

21.52 (E,Z)-2,4-Hexadienedioic acid-3,4diethyl-dimethyl ester

C12H18O4

2.18

1.16

0.71

2.86

0.93

0.95

2-Methoxy-4-propylphenol

−, Not detected.


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Table 2. Composition classification of HO derived from liquefaction of untreated- and pretreated EWC at 300 °C for 60 min in different solvents.

Composition classification

In water

In tetralin

In WTMS

untreated

pretreated

untreated pretreated

untreated

pretreated

88.55

84.23

84.12

85.15

78.53

79.97

Non-oxygenated compounds 10.81

14.82

10.26

12.85

19.56

18.09

Phenols

44.02

36.33

44.69

24.37

19.08

18.61

Aromatic compounds

12.61

13.43

16.57

16.90

23.27

20.38

Ketones

30.15

37.59

24.71

40.65

36.80

41.37

Alcohols

2.90

0.76

1.88

−

−

−

Alkenes

7.49

9.77

10.81

13.23

18.01

16.74

Esters

2.18

1.16

0.71

2.86

0.93

0.95

Others

0.64

0.96

0.62

2.00

1.91

1.95

Oxygenated compounds


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Figures Figure

Caption

Figure 1 Chemical compositions of unpretreated and pretreated EWC using different NaOH amount (0.5 ∼ 4.0 wt% of EWC). Figure 2 SEM observation of EWC before and after NaOH pretreatment, NaOH amount: (a) 0.0 wt%, (b) 1.0 wt%, (c) 4.0 wt%. Figure 3 Effects of temperature on the conversion (a) and HO yield (b) of EWC liquefied in water, tetralin and WTMS for 60 min. Figure 4 Effects of water/tetralin mass ratio in WTMS on EWC liquefaction. Figure 5 Effects of NaOH amount on conversion and HO yield in the liquefaction of the pretreated EWC in different solvents: (a) conversion at different NaOH amount (0, 0.5, 1.0 and 4.0 wt%); (b) HO yield at different NaOH amount (0, 0.5, 1.0 and 4.0 wt%). Figure 6 TG/DTG curves of raw EWC and EWC SR from liquefaction of untreated EWC (a) and pretreated EWC (b) in different solvents (water, tetralin, and WTMS). Figure 7 FTIR spectra of raw EWC and HO from liquefaction of untreated- and pretreated EWC in different solvents (water, tetralin, and WTMS). Full line indicates HO from the untreated EWC; broken line indicates HO from pretreated EWC.


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Figure 1. Chemical compositions of unpretreated and pretreated EWC using different NaOH amount (0.5 ∼ 4.0 wt% of EWC).


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(a)

(b)

(c)

Figure 2. SEM observation of EWC before and after NaOH pretreatment, NaOH amount: (a) 0.0 wt%, (b) 1.0 wt%, (c) 4.0 wt%.


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Figure 3. Effects of temperature on the conversion (a) and HO yield (b) of EWC liquefied in water, tetralin and WTMS for 60 min.


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Figure 4. Effects of water/tetralin mass ratio in WTMS on EWC liquefaction.


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Figure 5. Effects of NaOH amount on conversion and HO yield in the liquefaction of the pretreated EWC in different solvents: (a) conversion at different NaOH amount (0, 0.5, 1.0 and 4.0 wt%); (b) HO yield at different NaOH amount (0, 0.5, 1.0 and 4.0 wt%).


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(a)

(b)

Figure 6. TG/DTG curves of raw EWC and EWC SR from liquefaction of untreated EWC (a) and NaOH-pretreated EWC (b) in different solvents (water, tetralin, and WTMS).


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Figure 7. FTIR spectra of raw EWC and HO from liquefaction of untreated- and NaOHpretreated EWC in different solvents (water, tetralin, and WTMS). Full line indicates HO from the untreated EWC; broken line indicates HO from pretreated EWC.


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Effects of Mild Alkali Pretreatment and Hydrogen-Donating Solvent on

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