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Comparative investigation on bio-oil production from eucalyptus via liquefaction in subcritical water and supercritical ethanol
Industrial Crops & Products 140 (2019) 111695
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Comparative investigation on bio-oil production from eucalyptus via liquefaction in subcritical water and supercritical ethanol
T
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Xiao-Fei Wu, Jing-Jing Zhang, Yan-Hui Huang, Ming-Fei Li , Jing Bian, Feng Peng Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Liquefaction Subcritical water Supercritical ethanol Eucalyptus Bio-oil
The present work addresses the liquefaction of eucalyptus for the production of bio-oil, examining the effects of solvent on the conversion process. The liquefaction was conducted at 260–320 °C in subcritical water and supercritical ethanol, and the bio-oil produced was analyzed by elemental components, higher heating value, GC–MS, and FTIR. Results showed that reaction medium and temperature exhibited notable influence on the process. The optimum condition for the preparation of bio-oil from eucalyptus should be conducted under water at 300 °C for 30 min, which can produce a high bio-oil yield of 30.1%. As the hydrothermal liquefaction temperature increased, the contents of aldehydes, phenols, and alkanes in the heavy oil decreased, while those of esters and ketones increased. After liquefaction in ethanol, the main substances in the heavy oil were esters, phenols, ketones, alcohols, alkanes and olefins. In light oil, esters, alcohols, phenols and ketones were the main substances. The properties of the bio-oil produced made it suitable to be applied as a promising precursor of biobased aromatics and/or phenolic-rich antioxidant.
1. Introduction
for liquefaction of lignocellulose. Different solvents result in diverse products. Water shows unique properties under sub- and supercritical conditions. It has been utilized as solvent for dissolve lignocellulose components, such as hemicelluloses (Gao et al., 2016; Peng and She, 2014). In addition, it has also been utilized as a medium for the production of bio-char via carbonization, bio-crude oil via liquefaction, and hydrogen through gasification. Water is considered to be a natural choice for liquefaction since the use of water as a medium can omit the cost drying step (Liu et al., 2017; Xu et al., 2015; Zou et al., 2010). As compared to pyrolysis, liquefaction in water produces bio-oil with lower yield but less oxygen and higher heating value. However, it should be noted that the process had some weaknesses including corrosiveness problems due to water, hardness for product separation, as well as low yield of bio-oil (Kritzer, 2004). The supercritical point for water is 374 °C and 22.1 MPa. Many investigations have been conducted for liquefaction of lignocellulose under super and subcritical water. Hydrothermal liquefaction of softwood indicated that sugars and furans achieved the max yield at 170–190 °C and 210–230 °C whereas the highest yields of organic acids, phenolics, and bio-oil of 10.4%, 3.5%, and 7.4% were obtained at 230–270 °C (Sipponen et al., 2016). During the liquefaction of lignin, with the increase of temperature and prolongation of retention time, catechol yield got lower, and no catechol was detected at temperatures above 350 °C (Schuler et al., 2019).
Liquid fuels provide ease for long-distance transportation. Thermochemical conversion of solid lignocellulose into liquid fuels mainly includes liquefaction and pyrolysis. Pyrolysis can produce biooil but it exhibits poor stability since the bio-oil contains a high content of acid and oxygenated components (Oliveira et al., 2017). As compared to pyrolysis, liquefaction is conducted in a suitable solvent other than in an inert gas atmosphere. The solvent applied in liquefaction made the conversion performed at lower temperatures but high pressures, producing bio-oil with higher yield and fewer char. Generally, the liquefaction process is operated under 250–400 °C and 5–40 MPa. During the liquefaction, the macromolecules of lignocellulose were decomposed into intermediates, and the unstable components polymerize to form bio-oils. The major function of solvent is solubilizing products, dispersing the reaction intermediates with high molecular weight, and impeding the coke formation. The degradation of lignocellulose in solvents yields bio-oil with better physicochemical features than that in an inert atmosphere. The major functions of solvent are stabilizing intermediates via hydrogen donation, improving the stability of the products and updating the characteristics of the products (Akhtar and Amin, 2011). Water, alcohols, and acetones are examples of the solvents applied
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Corresponding author. E-mail address: [email protected] (M.-F. Li).
https://doi.org/10.1016/j.indcrop.2019.111695 Received 19 July 2019; Received in revised form 16 August 2019; Accepted 16 August 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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products. The liquid phase was concentrated under reduced pressure at 30 °C (for ethanol) or 70 °C (for water) using a rotary evaporator to obtain a residue by removing water or ethanol, noted as light oil, LO. The solid fraction was then extracted in a Soxhlet apparatus with acetone until the extract liquor was colorless. The solid was dried in an oven at 80 °C for 24 h and the solid fraction was noted as solid residue, SR. The acetone phase was concentrated under reduced pressure at 30 °C using a rotary evaporator to remove acetone to obtain a liquid residue (noted as heavy oil, HO).
In addition, the liquefaction of lignin with water can be conducted under various catalysts, aiming at obtaining a high yield of bio-oil, which has been reviewed by Kozliak et al. (2016). Supercritical ethanol (Tc > 243 °C, Pc > 6.3 MPa) has been applied to liquefaction of lignocellulose. Under supercritical conditions, ethanol has the advantages of excellent solubility of degraded components, hydrogen donor capacity, as well as the easiness of recovery for its low boiling point. Ethanol serves as a reaction medium for solubilization of degraded products, as well as a stabilizer to impede the formation of char from reaction intermediates. In addition, it also promotes the deoxygenation to enhance the heating value of the bio-oil. Supercritical ethanol has been reported to show excellent properties to reduce the combination of reaction intermediates, producing bio-oil with high yield (Oh et al., 2016). Ethanol also showed the capacity of removal of oxygen in the form of CO, CO2, H2O, producing energy-intensified biooil (Brand et al., 2013). In a report on liquefaction of cellulose and xylose, it was found that the bio-oil produced from supercritical ethanol had notably different properties from those from pyrolysis (Brand and Kim, 2015). The liquefaction of palm oil fiber at 300–500 °C showed that sugar derivatives were produced at low temperature but alcohols and phenolic were the major products at high temperature (Oliveira et al., 2017). In a study on liquefaction of hazelnut shell in ethanol, the highest yield of bio-oil of 64.2% was obtained at 300 °C (Demirkaya et al., 2019). Although many studies have demonstrated the potential of water and ethanol as media for liquefaction of lignocellulose, the comparative investigation of the two solvents has rarely reported except an investigation on subcritical water and supercritical ethanol of red pine sawdust, while the focus was to explore the heating rate on liquefaction reaction mechanism and product distribution (Brand et al., 2014). In view of this, in the present study, subcritical water and supercritical ethanol were applied for liquefaction of lignocellulose to explore the role of water and ethanol. Since temperature contributes to the reaction with respect to the product formation, the liquefaction was conducted under various temperatures. The present study explored the fundamental principles and product distribution of liquefaction of eucalyptus in water and ethanol, which is meaningful for the development of feasible process for lignocellulose liquefaction. This study provides an in-depth analysis of the bio-oil obtained and the insights on the reactions involved in the liquefaction process.
2.3. Analysis of the products The product yield of bio-oil was calculated by the equations below: Xm=Massm/Massfeedstock×100%, in which m represents the heavy oil, light oil, and solid residue. The yield of gas and others was calculated by subtraction. Elemental analysis of bio-oil samples was conducted using an elemental analyzer (Vario ELIII, Germany). Carbon, hydrogen, nitrogen contents of the sample were detected simultaneously by quantitatively examination of the combustion gas products. The content of oxygen was calculated by subtraction. The higher heating value was evaluated by Dulong’s formula as shown below: HHV (MJ/kg) = 0.338 C + 1.428 (H–O/8), where C, H and O are the content of carbon, hydrogen and oxygen, respectively. FTIR spectra were recorded on a Bruker Tensor II spectrometer. The sample was prepared by mixing the oils ample with KBr at a content of 2%. The scanning range was 4000–400 cm−1 and the resolution was 0.1 cm−1. Bio-oil sample was first diluted with acetone to a final concentration of 1 mg/mL, and then 1 μL of the solution was analyzed by Agilent 7890A-5975C equipped with a capillary column CP9205 VF-WAXms (30 m ×0.25 mm id ×0.25 μm film thickness) in splitless model. Elution was carried out by using the protocol below: an initial temperature of 50 °C was held for 3 min, and a ramble to 250 °C at a rate of 5 °C/min for 3 min. Mass spectra were recorded in EI model with a mass range of 50–550 m/z after 5 min of the solvent delay. Compounds of bio-oil were identified using NIST mass spectral library, and normalization of the areas of the compounds were conducted for comparison. 3. Results and discussion
2. Materials and methods
3.1. Effect of reaction condition on production distribution
2.1. Materials
The product distribution of eucalyptus liquefaction under subcritical water and supercritical ethanol is illustrated in Fig. 1. For liquefaction in water (Fig. 1a), the highest HO yield of 20.9% was obtained at 280 °C. As temperature increased from 260 °C to 320 °C, the yield of bio-oil (heavy oil and light oil) reached a maximum of 30.1% at 300 °C and then slightly decreased, and the yield of SR decreased constantly. This was because when liquefaction at a high temperature, more bio-oil produced was converted into solid residue/gases, especially gases, resulting in a low yield of bio-oil. When liquefaction at high temperatures over 300 °C, due to the formation of more solid residue, the yield of bio-oil decreased with increasing temperature (Gai et al., 2015). The increase in solid yield and the decrease in bio-oil yield are associated with increased carbonation of lignin and condensation of phenolic compounds into solids (Lucian et al., 2018). The variation of product yield was consistent with the results of the sub/supercritical liquefaction studies of straw reported by Yang et al. (2018), in which a high temperature resulted in a high yield of gas product, i.e., regarding liquefaction for 30 min, the gas yield increased from 22.80% for liquefaction at 270 °C to 43.46% for liquefaction at 345 °C. With respect to liquefaction in ethanol (Fig. 1b), the yield of bio-oil decreased from 27.6% to 14.6%, and SR yield decreased from 55.1% to 42.5%, and the yield of gas and other volatile components increased from 17.3% to 43.0%. The decrease of bio-oil was mainly due to the reaction in
The feedstock used in the present study was eucalyptus sawdust obtained from Hainan Province, China. The sample was sieved to obtain a fraction with sizes smaller than 20 mesh and then dried before experiment. Chemicals of analytical grade were used as received. 2.2. Liquefaction experiment and production separation Liquefaction of lignocellulose was conducted in a batch reactor (150 mL) equipped with a mechanical stirrer. The reactor was composed of a steel cylinder and the maximum operation temperature and pressure were up to 350 °C and 20 MPa, respectively. For each run, 4 g of the powder and 40 mL of solvent (water or ethanol) were added into the reactor. Next, the reactor was sealed and nitrogen was put in to the reactor to discharge the air in the reactor. It was heated to the desired temperature (260, 280, 300, and 320 °C) using a furnace under a stirring rate of 300 rpm. The system temperature lasted for 30 min and then it was cooled to 50 °C by air. The maximum pressures ranged from 4.0 to 9.5 MPa in subcritical water and from 6.1 to 8.2 MPa in supercritical ethanol for liquefaction at 260–320 °C. Subsequently, the reactor was opened and the products were collected. The mixed products were filtered by vacuum filtration to obtain a solid fraction and liquid 2
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Table 1 The element analysis and HHV of heavy and light bio-oils. Sample
Element (%) C
O
Liquefaction in water 260-HO 66.51 26.06 280-HO 64.80 27.65 300-HO 66.40 28.74 320-HO 67.42 29.80 260-LO 51.95 42.70 280-LO 51.40 43.25 300-LO 51.01 44.55 320-LO 52.01 42.54 Liquefaction in ethanol 260-HO 65.88 27.42 280-HO 65.76 26.93 300-HO 66.32 26.08 320-HO 67.50 25.95 260-LO 62.51 31.12 280-LO 63.12 30.19 300-LO 68.03 23.79 320-LO 65.49 27.57
H/C
O/C
HHV(MJ/kg)
H
N
5.74 5.35 5.61 6.31 5.08 5.35 4.22 5.22
0.09 0.18 0.16 0.26 0.15 0.11 0.05 0.09
1.03 0.98 1.01 1.11 1.16 1.25 0.99 1.20
0.29 0.32 0.32 0.33 0.62 0.63 0.66 0.61
26.03 24.62 25.33 26.48 17.21 17.22 15.34 17.46
6.50 7.08 7.30 6.41 6.30 6.65 8.16 6.88
0.21 0.23 0.30 0.15 0.07 0.04 0.02 0.05
1.18 1.28 1.31 1.13 1.20 1.25 1.43 1.25
0.31 0.31 0.30 0.29 0.37 0.36 0.26 0.32
26.65 27.52 28.18 27.33 24.57 25.44 30.38 27.04
Fig. 1. Product distribution of eucalyptus liquefaction in subcritical water (a) and supercritical ethanol (b).
ethanol liquefaction. Liquefaction can be roughly divided into two process (Huang et al., 2011), i.e., the first one was cyclization, condensation, and repolymerization of liquid products to form solids and decomposition of liquid to produce gases; the other process was decomposition of solid and gas agglomeration/condensation of gas to generate liquid. Based on the results of this experiment, elevation of temperature facilitated to the former process. Comparatively speaking, the optimum condition for the preparation of bio-oil from eucalyptus should be conducted under water at 300 °C for 30 min, which can produce a high bio-oil yield.
3.2. Characterization of bio-oil The elemental components of the oils are illustrated in Table 1. For the liquefaction in water, the oxygen content of the heavy oil component increased from 26.06% in 260-HO to 29.80% in 320-HO, while the nitrogen content was less than 0.26% for all the heavy oil samples. The contents of nitrogen and oxygen in the light oil sample were generally higher than those in the heavy oil component, and the carbon content was lower than that in the heavy oil. The nitrogen content of light oil was less than 0.15%, and the oxygen element reached the maximum value (44.55%) at 300 °C. The formation of heavy oil was mainly related to the decarboxylation reaction, while the degree of dehydration and decarboxylation was slight, but it undergoes more oxidation, resulting in higher oxygen content (Fig. 2). The HHVs of the heavy oils were 43.0%–51.7% higher than the corresponding light oils from the same liquefaction temperature. Therefore, in order to use bio-oil as a fuel, the water-soluble light oil should be further upgraded to remove nitrogen and oxygen. With respect to the liquefaction in ethanol,
Fig. 2. Van Krevelen diagram of heavy and light oils from liquefaction of eucalyptus in subcritical water (a) and supercritical ethanol (b).
lignocellulose underwent decarboxylation in the process of forming heavy oil and light oil after degradation of macromolecules. The C, H and HHV values of the two bio-oils were similar, but the O content in the light oil was 2.78%–4.19% higher than those in the oil obtained at 260 °C and 280 °C. This result indicated that bio-oil need a further deoxygenation. FTIR spectra were used to characterize the functional groups of the bio-oils (Figs. 3 and 4). According to literatures (Liu et al., 2018; Wu et al., 2019; Xi et al., 2018), the signals of the spectra can be assigned as −1), vibration of −OH (3200–3600 cm −1), vibration of C–OH (1000–1050 cm 3
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Fig. 3. FTIR spectra of heavy oils from liquefaction of eucalyptus in subcritical water (a) and supercritical ethanol (b). C
–H and = CH stretching vibration (characteristic peak of alkanes and olefin, 2900–3000 cm −1), and carbonyl group, ether bond, and aromatic ring (1800–800 cm −1). Specifically, the peak at 1750–1650 cm −1 (CO ] stretching vibration) was related to acid and ester substances. Vibration at 1350–1500 cm −1 (aromatic CCe stretching) indicated the presence of aromatic compounds. The peaks between 900 and 1350 cm −1 (C–H and C –O stretching vibration) were related to the presence of alcohols and esters. The C–H out-of-plane bending vibration at 900–700 cm −1 corresponded to aromatic compounds and their derivatives. For heavy oil and light oil from hydrothermal liquefaction, the −OH characteristic peak at 3200–3600 cm −1 was weaker than that at 2800–3000 cm −1 (CH2 and CH3). The heavy oil and light oil had obvious characteristic peaks at 1210–1100 cm −1 (stretching vibration of C–O–C). The signal peak at 1358–1600 cm -1 corresponded to the
aromatic C]C absorption, and the characteristic peak of this part in the heavy oil was stronger than that in the light oil. The intensity of the C–H out-of-plane bending characteristic peak of aromatic hydrocarbons and their derivatives at 700–900 cm −1 was weak because of the ring opening or hydrogenation reaction during subcritical hydrothermal liquefaction. Different to the case in hydrothermal liquefaction, the change of the signals at 2950-2800 cm -1 in heavy oil and light oil showed a diverse trend. That in heavy oil was intensified from 260-HO to 300-HO and then waken to 320-HO, while that for light oil in this region was weaken overall. This means that heavy oil contains more C–H. The peak signal at 1510 and 1462 cm −1 indicated the enrichment of benzene derivatives (Yip et al., 2009). The signal at 1710 cm −1 (ketones) showed a strong absorption in bio-oil, indicating that the degradation of cellulose and hemicellulose. The intensity at 650600 cm-1 in the light oil component increased with the elevation of 4
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Fig. 4. FTIR spectra of light oils from liquefaction of eucalyptus in subcritical water (a) and supercritical ethanol (b).
hydroxymethylfuran was present in the heavy oil. Cellulose and hemicelluloses were mainly degraded to form aldehydes and ketones, while phenolic substances such as guaiacol were originated from the degradation of lignin. As the hydrothermal liquefaction temperature increased, the contents of aldehydes, phenols, and alkanes in the heavy oil component decreased, while those of esters and ketones increased. In 260-HO, the main aldehydes were furfural and 5-hydroxymethylfurfural, and the main phenols were phenol and guaiacol. Dehydration of glucose and xylose during hydrothermal liquefaction produced unstable intermediates (furfural and 5-hydroxymethylfurfural), which can be degraded to carboxylic acids by further oxidation (Yue et al., 2018). The content of phenols decreased as the liquefaction temperature increased. Light oil was mainly composed of alcohols, phenols and ketones. The
liquefaction temperature, indicating that the light oil in high temperature contained acetylene. The above findings suggested that biooils mainly contained aromatic hydrocarbons, alkanes, alkenes, alcohols, phenols, ketones, and aldehydes. The main components of bio-oil were qualitatively and quantitatively analyzed by GC–MS, and their main substances and peak areas were classified and summarized as shown in Tables 2 and 3. The formation of aldehydes, ketones, and phenols in light oils and phenols and esters in heavy oils was related to dehydrogenation, dehydration, decarboxylation, deoxygenation and aromatic isomerization in hydrothermal liquefaction. The production of aldehydes and ketones was related to the degradation of cellulose, such as 5-hydroxymethylfurfural, 2-methyl-2-cyclopenten-1-ketone. In the present study, after hydrothermal liquefaction of eucalyptus at 260 °C, 55
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Table 2 Compounds of heavy oil identified by GC–MS analysisa. Compounds
Acids Aldehydes Furans, pyrroles and pyridines Esters Phenols Ketones Alcohols Ethers Amides Olefins and alkanes Others a
Liquefaction in water
Liquefaction in etahnol
260-HO
280-HO
300-HO
320-HO
260-HO
280-HO
300-HO
320-HO
2.45 12.69 0.46 5.03 41.69 0.69 1.07 – – 22.07 13.85
– 5.27 – 11.75 18.32 41.77 – – 0.65 7.23 15.01
3.73 – – 20.71 17.92 44.07 1.47 – – 7.37 4.73
1.08 – 3.04 29.07 10.15 20.51 0.67 0.83 – 3.43 31.22
0.07 1.26 0.08 8.93 5.96 0.98 1.63 – – 27.42 53.67
– 1.5 3.93 19.33 1.69 1.35 – – – 19.29 52.91
2.28 – 1.65 21.67 0.31 8.59 0.38 – – 21.71 43.41
0.52 – – 8.21 1.85 73.17 – – – 10.96 5.29
Indicating it was not detected.
(2011) found that lignin was decomposed into low-molecular phenolic substances in the liquefaction under supercritical methanol, and the phenolic products obtained by depolymerization of lignin had high antioxidant capacity (Moussa et al., 2019). It can be inferred that the phenolic compounds of bio-oil during the liquefaction in supercritical ethanol had the potential as a liquid antioxidant.
contents of alcohols and phenols increased with the increase of liquefaction temperature, but aldehydes had the opposite trend. After the liquefaction in ethanol, the main substances in the heavy oil were esters, phenols, ketones, alcohols, alkanes and olefins. In light oil, esters, alcohols, phenols and ketones were the main substances. The result was consistent with those of FT-IR. As the temperature increased, the contents of aldehydes, phenols, alkanes and olefins in heavy oil decreased, while that of ketones increased. The contents of ketones and alkanes in light oils decreased with increasing temperature, and phenols continued to increase. Light oil contained a large amount of esters, attributed to the esterification reaction between the acid formed from cellulose and hemicelluloses. The decrease of the content of furfural under a higher temperature indicated that furfural was converted into other compounds. This finding was consistent with the report of (Brand and Kim, 2015) who found a similar change of the content of furfural in bio-oil during the liquefaction in supercritical ethanol. They also found that after the liquefaction in supercritical ethanol, low molecular weight acids and C1-C3 aldehydes (such as formic acid, acetic acid, acetaldehyde, glycer aldehyde) were not observed in the bio-oil. These substances were mainly found in bio-oil obtained by hydrothermal liquefaction. D-xylose produced C4-C5 chain acids during ethanol liquefaction, such as 3-methyl-4-(2,3-dihydroxyphenyl)-4-oxobutyric acid and 2, 3-dihydroxy-2-methylglutaric acid, in light oil, but they were not detected in heavy oil. The compounds with benzene rings accounted for a large proportion, especially esters and phenolic compounds with benzene ring, which were mainly derived from degradation of lignin during ethanol liquefaction. The lignin in eucalyptus is mainly composed of guaiacyl and syringyl units. 4-Methylguaiacol, 4-ethylguaiacol, 2,6-dimethoxyphenol and ortho-benzene were detected in the oils. As the liquefaction temperature increased, the contents of alkanes and olefins decreased whereas those of ketones and esters increased. Kang et al.
4. Conclusion The compositions of bio-oils obtained from subcritical water liquefaction and supercritical ethanol liquefaction were complicated. Heavy oil mainly contained esters, phenols and ketones during hydrothermal liquefaction, while it was mainly composed of esters, phenols, ketones, alcohols, alkanes and olefins during ethanol liquefaction. Light oil was rich in alcohols, phenols, ketones, in which the contents of alcohols and phenols increased with temperature for liquefaction in water whereas the main components of light oil were esters, alcohols, phenols and ketones for ethanol liquefaction. The formation of aldehydes, ketones, phenols and esters was related to dehydrogenation, dehydration, decarboxylation, deoxygenation and aromatic ring isomerization reactions during liquefaction. The formation of aldehydes and ketones was mainly related to the degradation of hemicelluloses and cellulose, and phenolic substances correlated to lignin. During supercritical ethanol liquefaction, as the temperature increased, cellulose, hemicelluloses, and lignin broke down into volatile materials, and the yield of bio-oil decreased. During the liquefaction process, the liquid products underwent continuous cyclization, condensation and repolymerization to form a solid, and decomposed to form gases. Acknowledgments This work was financially supported by the Fundamental Research
Table 3 Compounds of light oils identified by GC–MS analysisa. Compounds
Carboxylic acids and acid anhydrides Aldehydes Furans and pyrans Esters Alcohols and phenols Ketones Amides Alkane and olefins Others a
Liquefaction in water
Liquefaction in ethanol
260-LO
280-LO
300-LO
320-LO
260-LO
280-LO
300-LO
320-LO
6.06 3.82 3.24 7.87 19.05 5.52 – – 54.44
5.2 0.69 0.81 7.22 30.42 11.91 4.87 – 38.88
– 0.71 – 16.2 36.57 0.94 – – 45.58
10.33 – 3.7 2.82 41.28 15.12 – – 26.75
2.92 5.87 0.15 37.58 17.18 14.41 – 3.91 17.98
6.44 – 3.12 48.99 16.73 9.27 – – 15.45
– 2.24 6.8 36.18 22.15 6.52 – 2.30 24.00
– – 5.95 40.90 31.81 6.15 – 1.00 14.00
Indicating it was not detected. 6
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Funds for the Central Universities (2019ZY06) and the National Natural Science Foundation of China (21706014).
https://doi.org/10.1016/j.indcrop.2017.06.026. Lucian, M., Volpe, M., Gao, L., Piro, G., Goldfarb, J.L., Fiori, L., 2018. Impact of hydrothermal carbonization conditions on the formation of hydrochars and secondary chars from the organic fraction of municipal solid waste. Fuel 233, 257–268. https:// doi.org/10.1016/j.fuel.2018.06.060. Moussa, I., Khiari, R., Moussa, A., Mortha, G., Mhenni, M.F., 2019. Structural characterization and antioxidant activity of lignin extracted from Ficus Carica L. J. Renew. Mater. 7 (4), 345–354. https://doi.org/10.32604/jrm.2019.04011. Oh, S., Kim, U.J., Choi, I.G., Choi, J.W., 2016. Solvent effects on improvement of fuel properties during hydrodeoxygenation process of bio-oil in the presence of Pt/C. Energy 113, 116–123. https://doi.org/10.1016/j.energy.2016.07.027. Oliveira, A.L.P.C., Almeida, P.S., Campos, M.C.V., Franceschi, E., Dariva, C., Borges, G.R., 2017. Thermoliquefaction of palm oil fiber (Elaeis sp.) using supercritical ethanol. Bioresour. Technol. 230, 1–7. https://doi.org/10.1016/j.biortech.2017.01.029. Peng, P., She, D., 2014. Isolation, structural characterization, and potential applications of hemicelluloses from bamboo: a review. Carbohyd. Polym. 112, 701–720. https:// doi.org/10.1016/j.carbpol.2014.06.068. Schuler, J., Hornung, U., Dahmen, N., Sauer, J., 2019. Lignin from bark as a resource for aromatics production by hydrothermal liquefaction. GCB Bioenergy 11, 218–229. https://doi.org/10.1111/gcbb.12562. Sipponen, M.H., Ozdenkci, K., Muddassar, H.R., Melin, K., Golam, S., Oinas, P., 2016. Hydrothermal liquefaction of softwood: selective chemical production under oxidative conditions. ACS Sustain. Chem. Eng. 4, 3978–3984. https://doi.org/10.1021/ acssuschemeng.6b00846. Wu, X.-F., Yin, S.-S., Zhou, Q., Li, M.-F., Peng, F., Xiao, X., 2019. Subcritical liquefaction of lignocellulose for the production of bio-oils in ethanol/water system. Renew. Energy. https://doi.org/10.1016/j.renene.2019.01.041. Xi, D.K., Jiang, C.C., Zhou, R.W., Fang, Z., Zhang, X.H., Liu, Y., Luan, B.Y., Feng, Z., Chen, G.L., Chen, Z., Liu, Q.H., Yang, S.Z., 2018. The universality of lignocellulosic biomass liquefaction by plasma electrolysis under acidic conditions. Bioresour. Technol. 268, 531–538. https://doi.org/10.1016/j.biortech.2018.08.025. Xu, Y.P., Duan, P.G., Wang, F., 2015. Hydrothermal processing of macroalgae for producing crude bio-oil. Fuel Process. Technol. 130, 268–274. https://doi.org/10.1016/ j.fuproc.2014.10.028. Yang, T.H., Wang, J., Li, B.S., Kai, X.P., Xing, W.L., Li, R.D., 2018. Behaviors of rice straw two-step liquefaction with sub/supercritical ethanol in carbon dioxide atmosphere. Bioresour. Technol. 258, 287–294. https://doi.org/10.1016/j.biortech.2018.02.099. Yip, J., Chen, M.J., Szeto, Y.S., Yan, S.C., 2009. Comparative study of liquefaction process and liquefied products from bamboo using different organic solvents. Bioresour. Technol. 100, 6674–6678. https://doi.org/10.1016/j.biortech.2009.07.045. Yue, Y., Kastner, J.R., Mani, S., 2018. Two-stage hydrothermal liquefaction of sweet sorghum biomass-part 1: production of sugar mixtures. Energy Fuel. 32, 7611–7619. https://doi.org/10.1021/acs.energyfuels.8b00668. Zou, S.P., Wu, Y.L., Yang, M.D., Kaleem, I., Chun, L., Tong, J.M., 2010. Production and characterization of bio-oil from hydrothermal liquefaction of microalgae Dunaliella tertiolecta cake. Energy 35, 5406–5411. https://doi.org/10.1016/j.energy.2010.07. 013.
References Akhtar, J., Amin, N.A.S., 2011. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renew. Sustain. Energy Rev. 15, 1615–1624. https://doi.org/10.1016/j.rser.2010.11.054. Brand, S., Hardi, F., Kim, J., Suh, D.J., 2014. Effect of heating rate on biomass liquefaction: differences between subcritical water and supercritical ethanol. Energy 68, 420–427. https://doi.org/10.1016/j.energy.2014.02.086. Brand, S., Kim, J., 2015. Liquefaction of major lignocellulosic biomass constituents in supercritical ethanol. Energy 80, 64–74. https://doi.org/10.1016/j.energy.2014.11. 043. Brand, S., Susanti, R.F., Kim, S.K., Lee, H.S., Kim, J., Sang, B.I., 2013. Supercritical ethanol as an enhanced medium for lignocellulosic biomass liquefaction: influence of physical process parameters. Energy 59, 173–182. https://doi.org/10.1016/j.energy. 2013.06.049. Demirkaya, E., Dal, O., Yüksel, A., 2019. Liquefaction of waste hazelnut shell by using sub- and supercritical solvents as a reaction medium. J. Supercrit. Fluids 150, 11–20. https://doi.org/10.1016/j.supflu.2019.03.019. Gai, C., Li, Y., Peng, N.N., Fan, A.N., Liu, Z.G., 2015. Co-liquefaction of microalgae and lignocellulosic biomass in subcritical water. Bioresour. Technol. 185, 240–245. https://doi.org/10.1016/j.biortech.2015.03.015. Gao, Y.F., Wang, H.T., Guo, J.H., Peng, P., Zhai, M.Z., She, D., 2016. Hydrothermal degradation of hemicelluloses from triploid poplar in hot compressed water at 180-340 °C. Carbohyd. Polym. 126, 179–187. https://doi.org/10.1016/j.polymdegradstab. 2016.02.003. Huang, H.J., Yuan, X.Z., Zeng, G.M., Wang, J.Y., Li, H., Zhou, C.F., Pei, X.K., You, Q.A., Chen, L.A., 2011. Thermochemical liquefaction characteristics of microalgae in suband supercritical ethanol. Fuel Process. Technol. 92, 147–153. https://doi.org/10. 1016/j.fuproc.2010.09.018. Kang, S., Li, X., Li, B., Fan, J., Chang, J., 2011. Effects of lignins on antioxidant biodiesel production in supercritical methanol. Energy Fuel. 25, 2746–2748. https://doi.org/ 10.1021/ef2004249. Kozliak, E.I., Kubatova, A., Artemyeva, A.A., Nagel, E., Zhang, C., Rajappagowda, R.B., Srnirnova, A.L., 2016. Thermal liquefaction of lignin to aromatics: efficiency, selectivity, and product analysis. ACS Sustain. Chem. Eng. 4, 5106–5122. https://doi. org/10.1021/acssuschemeng.6b01046. Kritzer, P., 2004. Corrosion in high-temperature and supercritical water and aqueous solutions: a review. J. Supercrit. Fluid. 29, 1–29. https://doi.org/10.1016/S08968446(03)00031-7. Liu, C.Z., Wufuer, A., Kong, L.P., Wang, Y.Y., Dai, L.Y., 2018. Organic solvent extractionassisted catalytic hydrothermal liquefaction of algae to bio-oil. RSC Adv. 8, 31717–31724. https://doi.org/10.1039/c8ra04668a. Liu, H., Ma, M., Xie, Xa., 2017. New materials from solid residues for investigation the mechanism of biomass hydrothermal liquefaction. Ind. Crop. Prod. 108, 63–71.
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Comparative investigation on bio-oil production from eucalyptus via liquefaction in subcritical water and supercritical ethanol
T
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Xiao-Fei Wu, Jing-Jing Zhang, Yan-Hui Huang, Ming-Fei Li , Jing Bian, Feng Peng Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Liquefaction Subcritical water Supercritical ethanol Eucalyptus Bio-oil
The present work addresses the liquefaction of eucalyptus for the production of bio-oil, examining the effects of solvent on the conversion process. The liquefaction was conducted at 260–320 °C in subcritical water and supercritical ethanol, and the bio-oil produced was analyzed by elemental components, higher heating value, GC–MS, and FTIR. Results showed that reaction medium and temperature exhibited notable influence on the process. The optimum condition for the preparation of bio-oil from eucalyptus should be conducted under water at 300 °C for 30 min, which can produce a high bio-oil yield of 30.1%. As the hydrothermal liquefaction temperature increased, the contents of aldehydes, phenols, and alkanes in the heavy oil decreased, while those of esters and ketones increased. After liquefaction in ethanol, the main substances in the heavy oil were esters, phenols, ketones, alcohols, alkanes and olefins. In light oil, esters, alcohols, phenols and ketones were the main substances. The properties of the bio-oil produced made it suitable to be applied as a promising precursor of biobased aromatics and/or phenolic-rich antioxidant.
1. Introduction
for liquefaction of lignocellulose. Different solvents result in diverse products. Water shows unique properties under sub- and supercritical conditions. It has been utilized as solvent for dissolve lignocellulose components, such as hemicelluloses (Gao et al., 2016; Peng and She, 2014). In addition, it has also been utilized as a medium for the production of bio-char via carbonization, bio-crude oil via liquefaction, and hydrogen through gasification. Water is considered to be a natural choice for liquefaction since the use of water as a medium can omit the cost drying step (Liu et al., 2017; Xu et al., 2015; Zou et al., 2010). As compared to pyrolysis, liquefaction in water produces bio-oil with lower yield but less oxygen and higher heating value. However, it should be noted that the process had some weaknesses including corrosiveness problems due to water, hardness for product separation, as well as low yield of bio-oil (Kritzer, 2004). The supercritical point for water is 374 °C and 22.1 MPa. Many investigations have been conducted for liquefaction of lignocellulose under super and subcritical water. Hydrothermal liquefaction of softwood indicated that sugars and furans achieved the max yield at 170–190 °C and 210–230 °C whereas the highest yields of organic acids, phenolics, and bio-oil of 10.4%, 3.5%, and 7.4% were obtained at 230–270 °C (Sipponen et al., 2016). During the liquefaction of lignin, with the increase of temperature and prolongation of retention time, catechol yield got lower, and no catechol was detected at temperatures above 350 °C (Schuler et al., 2019).
Liquid fuels provide ease for long-distance transportation. Thermochemical conversion of solid lignocellulose into liquid fuels mainly includes liquefaction and pyrolysis. Pyrolysis can produce biooil but it exhibits poor stability since the bio-oil contains a high content of acid and oxygenated components (Oliveira et al., 2017). As compared to pyrolysis, liquefaction is conducted in a suitable solvent other than in an inert gas atmosphere. The solvent applied in liquefaction made the conversion performed at lower temperatures but high pressures, producing bio-oil with higher yield and fewer char. Generally, the liquefaction process is operated under 250–400 °C and 5–40 MPa. During the liquefaction, the macromolecules of lignocellulose were decomposed into intermediates, and the unstable components polymerize to form bio-oils. The major function of solvent is solubilizing products, dispersing the reaction intermediates with high molecular weight, and impeding the coke formation. The degradation of lignocellulose in solvents yields bio-oil with better physicochemical features than that in an inert atmosphere. The major functions of solvent are stabilizing intermediates via hydrogen donation, improving the stability of the products and updating the characteristics of the products (Akhtar and Amin, 2011). Water, alcohols, and acetones are examples of the solvents applied
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Corresponding author. E-mail address: [email protected] (M.-F. Li).
https://doi.org/10.1016/j.indcrop.2019.111695 Received 19 July 2019; Received in revised form 16 August 2019; Accepted 16 August 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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products. The liquid phase was concentrated under reduced pressure at 30 °C (for ethanol) or 70 °C (for water) using a rotary evaporator to obtain a residue by removing water or ethanol, noted as light oil, LO. The solid fraction was then extracted in a Soxhlet apparatus with acetone until the extract liquor was colorless. The solid was dried in an oven at 80 °C for 24 h and the solid fraction was noted as solid residue, SR. The acetone phase was concentrated under reduced pressure at 30 °C using a rotary evaporator to remove acetone to obtain a liquid residue (noted as heavy oil, HO).
In addition, the liquefaction of lignin with water can be conducted under various catalysts, aiming at obtaining a high yield of bio-oil, which has been reviewed by Kozliak et al. (2016). Supercritical ethanol (Tc > 243 °C, Pc > 6.3 MPa) has been applied to liquefaction of lignocellulose. Under supercritical conditions, ethanol has the advantages of excellent solubility of degraded components, hydrogen donor capacity, as well as the easiness of recovery for its low boiling point. Ethanol serves as a reaction medium for solubilization of degraded products, as well as a stabilizer to impede the formation of char from reaction intermediates. In addition, it also promotes the deoxygenation to enhance the heating value of the bio-oil. Supercritical ethanol has been reported to show excellent properties to reduce the combination of reaction intermediates, producing bio-oil with high yield (Oh et al., 2016). Ethanol also showed the capacity of removal of oxygen in the form of CO, CO2, H2O, producing energy-intensified biooil (Brand et al., 2013). In a report on liquefaction of cellulose and xylose, it was found that the bio-oil produced from supercritical ethanol had notably different properties from those from pyrolysis (Brand and Kim, 2015). The liquefaction of palm oil fiber at 300–500 °C showed that sugar derivatives were produced at low temperature but alcohols and phenolic were the major products at high temperature (Oliveira et al., 2017). In a study on liquefaction of hazelnut shell in ethanol, the highest yield of bio-oil of 64.2% was obtained at 300 °C (Demirkaya et al., 2019). Although many studies have demonstrated the potential of water and ethanol as media for liquefaction of lignocellulose, the comparative investigation of the two solvents has rarely reported except an investigation on subcritical water and supercritical ethanol of red pine sawdust, while the focus was to explore the heating rate on liquefaction reaction mechanism and product distribution (Brand et al., 2014). In view of this, in the present study, subcritical water and supercritical ethanol were applied for liquefaction of lignocellulose to explore the role of water and ethanol. Since temperature contributes to the reaction with respect to the product formation, the liquefaction was conducted under various temperatures. The present study explored the fundamental principles and product distribution of liquefaction of eucalyptus in water and ethanol, which is meaningful for the development of feasible process for lignocellulose liquefaction. This study provides an in-depth analysis of the bio-oil obtained and the insights on the reactions involved in the liquefaction process.
2.3. Analysis of the products The product yield of bio-oil was calculated by the equations below: Xm=Massm/Massfeedstock×100%, in which m represents the heavy oil, light oil, and solid residue. The yield of gas and others was calculated by subtraction. Elemental analysis of bio-oil samples was conducted using an elemental analyzer (Vario ELIII, Germany). Carbon, hydrogen, nitrogen contents of the sample were detected simultaneously by quantitatively examination of the combustion gas products. The content of oxygen was calculated by subtraction. The higher heating value was evaluated by Dulong’s formula as shown below: HHV (MJ/kg) = 0.338 C + 1.428 (H–O/8), where C, H and O are the content of carbon, hydrogen and oxygen, respectively. FTIR spectra were recorded on a Bruker Tensor II spectrometer. The sample was prepared by mixing the oils ample with KBr at a content of 2%. The scanning range was 4000–400 cm−1 and the resolution was 0.1 cm−1. Bio-oil sample was first diluted with acetone to a final concentration of 1 mg/mL, and then 1 μL of the solution was analyzed by Agilent 7890A-5975C equipped with a capillary column CP9205 VF-WAXms (30 m ×0.25 mm id ×0.25 μm film thickness) in splitless model. Elution was carried out by using the protocol below: an initial temperature of 50 °C was held for 3 min, and a ramble to 250 °C at a rate of 5 °C/min for 3 min. Mass spectra were recorded in EI model with a mass range of 50–550 m/z after 5 min of the solvent delay. Compounds of bio-oil were identified using NIST mass spectral library, and normalization of the areas of the compounds were conducted for comparison. 3. Results and discussion
2. Materials and methods
3.1. Effect of reaction condition on production distribution
2.1. Materials
The product distribution of eucalyptus liquefaction under subcritical water and supercritical ethanol is illustrated in Fig. 1. For liquefaction in water (Fig. 1a), the highest HO yield of 20.9% was obtained at 280 °C. As temperature increased from 260 °C to 320 °C, the yield of bio-oil (heavy oil and light oil) reached a maximum of 30.1% at 300 °C and then slightly decreased, and the yield of SR decreased constantly. This was because when liquefaction at a high temperature, more bio-oil produced was converted into solid residue/gases, especially gases, resulting in a low yield of bio-oil. When liquefaction at high temperatures over 300 °C, due to the formation of more solid residue, the yield of bio-oil decreased with increasing temperature (Gai et al., 2015). The increase in solid yield and the decrease in bio-oil yield are associated with increased carbonation of lignin and condensation of phenolic compounds into solids (Lucian et al., 2018). The variation of product yield was consistent with the results of the sub/supercritical liquefaction studies of straw reported by Yang et al. (2018), in which a high temperature resulted in a high yield of gas product, i.e., regarding liquefaction for 30 min, the gas yield increased from 22.80% for liquefaction at 270 °C to 43.46% for liquefaction at 345 °C. With respect to liquefaction in ethanol (Fig. 1b), the yield of bio-oil decreased from 27.6% to 14.6%, and SR yield decreased from 55.1% to 42.5%, and the yield of gas and other volatile components increased from 17.3% to 43.0%. The decrease of bio-oil was mainly due to the reaction in
The feedstock used in the present study was eucalyptus sawdust obtained from Hainan Province, China. The sample was sieved to obtain a fraction with sizes smaller than 20 mesh and then dried before experiment. Chemicals of analytical grade were used as received. 2.2. Liquefaction experiment and production separation Liquefaction of lignocellulose was conducted in a batch reactor (150 mL) equipped with a mechanical stirrer. The reactor was composed of a steel cylinder and the maximum operation temperature and pressure were up to 350 °C and 20 MPa, respectively. For each run, 4 g of the powder and 40 mL of solvent (water or ethanol) were added into the reactor. Next, the reactor was sealed and nitrogen was put in to the reactor to discharge the air in the reactor. It was heated to the desired temperature (260, 280, 300, and 320 °C) using a furnace under a stirring rate of 300 rpm. The system temperature lasted for 30 min and then it was cooled to 50 °C by air. The maximum pressures ranged from 4.0 to 9.5 MPa in subcritical water and from 6.1 to 8.2 MPa in supercritical ethanol for liquefaction at 260–320 °C. Subsequently, the reactor was opened and the products were collected. The mixed products were filtered by vacuum filtration to obtain a solid fraction and liquid 2
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Table 1 The element analysis and HHV of heavy and light bio-oils. Sample
Element (%) C
O
Liquefaction in water 260-HO 66.51 26.06 280-HO 64.80 27.65 300-HO 66.40 28.74 320-HO 67.42 29.80 260-LO 51.95 42.70 280-LO 51.40 43.25 300-LO 51.01 44.55 320-LO 52.01 42.54 Liquefaction in ethanol 260-HO 65.88 27.42 280-HO 65.76 26.93 300-HO 66.32 26.08 320-HO 67.50 25.95 260-LO 62.51 31.12 280-LO 63.12 30.19 300-LO 68.03 23.79 320-LO 65.49 27.57
H/C
O/C
HHV(MJ/kg)
H
N
5.74 5.35 5.61 6.31 5.08 5.35 4.22 5.22
0.09 0.18 0.16 0.26 0.15 0.11 0.05 0.09
1.03 0.98 1.01 1.11 1.16 1.25 0.99 1.20
0.29 0.32 0.32 0.33 0.62 0.63 0.66 0.61
26.03 24.62 25.33 26.48 17.21 17.22 15.34 17.46
6.50 7.08 7.30 6.41 6.30 6.65 8.16 6.88
0.21 0.23 0.30 0.15 0.07 0.04 0.02 0.05
1.18 1.28 1.31 1.13 1.20 1.25 1.43 1.25
0.31 0.31 0.30 0.29 0.37 0.36 0.26 0.32
26.65 27.52 28.18 27.33 24.57 25.44 30.38 27.04
Fig. 1. Product distribution of eucalyptus liquefaction in subcritical water (a) and supercritical ethanol (b).
ethanol liquefaction. Liquefaction can be roughly divided into two process (Huang et al., 2011), i.e., the first one was cyclization, condensation, and repolymerization of liquid products to form solids and decomposition of liquid to produce gases; the other process was decomposition of solid and gas agglomeration/condensation of gas to generate liquid. Based on the results of this experiment, elevation of temperature facilitated to the former process. Comparatively speaking, the optimum condition for the preparation of bio-oil from eucalyptus should be conducted under water at 300 °C for 30 min, which can produce a high bio-oil yield.
3.2. Characterization of bio-oil The elemental components of the oils are illustrated in Table 1. For the liquefaction in water, the oxygen content of the heavy oil component increased from 26.06% in 260-HO to 29.80% in 320-HO, while the nitrogen content was less than 0.26% for all the heavy oil samples. The contents of nitrogen and oxygen in the light oil sample were generally higher than those in the heavy oil component, and the carbon content was lower than that in the heavy oil. The nitrogen content of light oil was less than 0.15%, and the oxygen element reached the maximum value (44.55%) at 300 °C. The formation of heavy oil was mainly related to the decarboxylation reaction, while the degree of dehydration and decarboxylation was slight, but it undergoes more oxidation, resulting in higher oxygen content (Fig. 2). The HHVs of the heavy oils were 43.0%–51.7% higher than the corresponding light oils from the same liquefaction temperature. Therefore, in order to use bio-oil as a fuel, the water-soluble light oil should be further upgraded to remove nitrogen and oxygen. With respect to the liquefaction in ethanol,
Fig. 2. Van Krevelen diagram of heavy and light oils from liquefaction of eucalyptus in subcritical water (a) and supercritical ethanol (b).
lignocellulose underwent decarboxylation in the process of forming heavy oil and light oil after degradation of macromolecules. The C, H and HHV values of the two bio-oils were similar, but the O content in the light oil was 2.78%–4.19% higher than those in the oil obtained at 260 °C and 280 °C. This result indicated that bio-oil need a further deoxygenation. FTIR spectra were used to characterize the functional groups of the bio-oils (Figs. 3 and 4). According to literatures (Liu et al., 2018; Wu et al., 2019; Xi et al., 2018), the signals of the spectra can be assigned as −1), vibration of −OH (3200–3600 cm −1), vibration of C–OH (1000–1050 cm 3
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Fig. 3. FTIR spectra of heavy oils from liquefaction of eucalyptus in subcritical water (a) and supercritical ethanol (b). C
–H and = CH stretching vibration (characteristic peak of alkanes and olefin, 2900–3000 cm −1), and carbonyl group, ether bond, and aromatic ring (1800–800 cm −1). Specifically, the peak at 1750–1650 cm −1 (CO ] stretching vibration) was related to acid and ester substances. Vibration at 1350–1500 cm −1 (aromatic CCe stretching) indicated the presence of aromatic compounds. The peaks between 900 and 1350 cm −1 (C–H and C –O stretching vibration) were related to the presence of alcohols and esters. The C–H out-of-plane bending vibration at 900–700 cm −1 corresponded to aromatic compounds and their derivatives. For heavy oil and light oil from hydrothermal liquefaction, the −OH characteristic peak at 3200–3600 cm −1 was weaker than that at 2800–3000 cm −1 (CH2 and CH3). The heavy oil and light oil had obvious characteristic peaks at 1210–1100 cm −1 (stretching vibration of C–O–C). The signal peak at 1358–1600 cm -1 corresponded to the
aromatic C]C absorption, and the characteristic peak of this part in the heavy oil was stronger than that in the light oil. The intensity of the C–H out-of-plane bending characteristic peak of aromatic hydrocarbons and their derivatives at 700–900 cm −1 was weak because of the ring opening or hydrogenation reaction during subcritical hydrothermal liquefaction. Different to the case in hydrothermal liquefaction, the change of the signals at 2950-2800 cm -1 in heavy oil and light oil showed a diverse trend. That in heavy oil was intensified from 260-HO to 300-HO and then waken to 320-HO, while that for light oil in this region was weaken overall. This means that heavy oil contains more C–H. The peak signal at 1510 and 1462 cm −1 indicated the enrichment of benzene derivatives (Yip et al., 2009). The signal at 1710 cm −1 (ketones) showed a strong absorption in bio-oil, indicating that the degradation of cellulose and hemicellulose. The intensity at 650600 cm-1 in the light oil component increased with the elevation of 4
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Fig. 4. FTIR spectra of light oils from liquefaction of eucalyptus in subcritical water (a) and supercritical ethanol (b).
hydroxymethylfuran was present in the heavy oil. Cellulose and hemicelluloses were mainly degraded to form aldehydes and ketones, while phenolic substances such as guaiacol were originated from the degradation of lignin. As the hydrothermal liquefaction temperature increased, the contents of aldehydes, phenols, and alkanes in the heavy oil component decreased, while those of esters and ketones increased. In 260-HO, the main aldehydes were furfural and 5-hydroxymethylfurfural, and the main phenols were phenol and guaiacol. Dehydration of glucose and xylose during hydrothermal liquefaction produced unstable intermediates (furfural and 5-hydroxymethylfurfural), which can be degraded to carboxylic acids by further oxidation (Yue et al., 2018). The content of phenols decreased as the liquefaction temperature increased. Light oil was mainly composed of alcohols, phenols and ketones. The
liquefaction temperature, indicating that the light oil in high temperature contained acetylene. The above findings suggested that biooils mainly contained aromatic hydrocarbons, alkanes, alkenes, alcohols, phenols, ketones, and aldehydes. The main components of bio-oil were qualitatively and quantitatively analyzed by GC–MS, and their main substances and peak areas were classified and summarized as shown in Tables 2 and 3. The formation of aldehydes, ketones, and phenols in light oils and phenols and esters in heavy oils was related to dehydrogenation, dehydration, decarboxylation, deoxygenation and aromatic isomerization in hydrothermal liquefaction. The production of aldehydes and ketones was related to the degradation of cellulose, such as 5-hydroxymethylfurfural, 2-methyl-2-cyclopenten-1-ketone. In the present study, after hydrothermal liquefaction of eucalyptus at 260 °C, 55
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Table 2 Compounds of heavy oil identified by GC–MS analysisa. Compounds
Acids Aldehydes Furans, pyrroles and pyridines Esters Phenols Ketones Alcohols Ethers Amides Olefins and alkanes Others a
Liquefaction in water
Liquefaction in etahnol
260-HO
280-HO
300-HO
320-HO
260-HO
280-HO
300-HO
320-HO
2.45 12.69 0.46 5.03 41.69 0.69 1.07 – – 22.07 13.85
– 5.27 – 11.75 18.32 41.77 – – 0.65 7.23 15.01
3.73 – – 20.71 17.92 44.07 1.47 – – 7.37 4.73
1.08 – 3.04 29.07 10.15 20.51 0.67 0.83 – 3.43 31.22
0.07 1.26 0.08 8.93 5.96 0.98 1.63 – – 27.42 53.67
– 1.5 3.93 19.33 1.69 1.35 – – – 19.29 52.91
2.28 – 1.65 21.67 0.31 8.59 0.38 – – 21.71 43.41
0.52 – – 8.21 1.85 73.17 – – – 10.96 5.29
Indicating it was not detected.
(2011) found that lignin was decomposed into low-molecular phenolic substances in the liquefaction under supercritical methanol, and the phenolic products obtained by depolymerization of lignin had high antioxidant capacity (Moussa et al., 2019). It can be inferred that the phenolic compounds of bio-oil during the liquefaction in supercritical ethanol had the potential as a liquid antioxidant.
contents of alcohols and phenols increased with the increase of liquefaction temperature, but aldehydes had the opposite trend. After the liquefaction in ethanol, the main substances in the heavy oil were esters, phenols, ketones, alcohols, alkanes and olefins. In light oil, esters, alcohols, phenols and ketones were the main substances. The result was consistent with those of FT-IR. As the temperature increased, the contents of aldehydes, phenols, alkanes and olefins in heavy oil decreased, while that of ketones increased. The contents of ketones and alkanes in light oils decreased with increasing temperature, and phenols continued to increase. Light oil contained a large amount of esters, attributed to the esterification reaction between the acid formed from cellulose and hemicelluloses. The decrease of the content of furfural under a higher temperature indicated that furfural was converted into other compounds. This finding was consistent with the report of (Brand and Kim, 2015) who found a similar change of the content of furfural in bio-oil during the liquefaction in supercritical ethanol. They also found that after the liquefaction in supercritical ethanol, low molecular weight acids and C1-C3 aldehydes (such as formic acid, acetic acid, acetaldehyde, glycer aldehyde) were not observed in the bio-oil. These substances were mainly found in bio-oil obtained by hydrothermal liquefaction. D-xylose produced C4-C5 chain acids during ethanol liquefaction, such as 3-methyl-4-(2,3-dihydroxyphenyl)-4-oxobutyric acid and 2, 3-dihydroxy-2-methylglutaric acid, in light oil, but they were not detected in heavy oil. The compounds with benzene rings accounted for a large proportion, especially esters and phenolic compounds with benzene ring, which were mainly derived from degradation of lignin during ethanol liquefaction. The lignin in eucalyptus is mainly composed of guaiacyl and syringyl units. 4-Methylguaiacol, 4-ethylguaiacol, 2,6-dimethoxyphenol and ortho-benzene were detected in the oils. As the liquefaction temperature increased, the contents of alkanes and olefins decreased whereas those of ketones and esters increased. Kang et al.
4. Conclusion The compositions of bio-oils obtained from subcritical water liquefaction and supercritical ethanol liquefaction were complicated. Heavy oil mainly contained esters, phenols and ketones during hydrothermal liquefaction, while it was mainly composed of esters, phenols, ketones, alcohols, alkanes and olefins during ethanol liquefaction. Light oil was rich in alcohols, phenols, ketones, in which the contents of alcohols and phenols increased with temperature for liquefaction in water whereas the main components of light oil were esters, alcohols, phenols and ketones for ethanol liquefaction. The formation of aldehydes, ketones, phenols and esters was related to dehydrogenation, dehydration, decarboxylation, deoxygenation and aromatic ring isomerization reactions during liquefaction. The formation of aldehydes and ketones was mainly related to the degradation of hemicelluloses and cellulose, and phenolic substances correlated to lignin. During supercritical ethanol liquefaction, as the temperature increased, cellulose, hemicelluloses, and lignin broke down into volatile materials, and the yield of bio-oil decreased. During the liquefaction process, the liquid products underwent continuous cyclization, condensation and repolymerization to form a solid, and decomposed to form gases. Acknowledgments This work was financially supported by the Fundamental Research
Table 3 Compounds of light oils identified by GC–MS analysisa. Compounds
Carboxylic acids and acid anhydrides Aldehydes Furans and pyrans Esters Alcohols and phenols Ketones Amides Alkane and olefins Others a
Liquefaction in water
Liquefaction in ethanol
260-LO
280-LO
300-LO
320-LO
260-LO
280-LO
300-LO
320-LO
6.06 3.82 3.24 7.87 19.05 5.52 – – 54.44
5.2 0.69 0.81 7.22 30.42 11.91 4.87 – 38.88
– 0.71 – 16.2 36.57 0.94 – – 45.58
10.33 – 3.7 2.82 41.28 15.12 – – 26.75
2.92 5.87 0.15 37.58 17.18 14.41 – 3.91 17.98
6.44 – 3.12 48.99 16.73 9.27 – – 15.45
– 2.24 6.8 36.18 22.15 6.52 – 2.30 24.00
– – 5.95 40.90 31.81 6.15 – 1.00 14.00
Indicating it was not detected. 6
Industrial Crops & Products 140 (2019) 111695
X.-F. Wu, et al.
Funds for the Central Universities (2019ZY06) and the National Natural Science Foundation of China (21706014).
https://doi.org/10.1016/j.indcrop.2017.06.026. Lucian, M., Volpe, M., Gao, L., Piro, G., Goldfarb, J.L., Fiori, L., 2018. Impact of hydrothermal carbonization conditions on the formation of hydrochars and secondary chars from the organic fraction of municipal solid waste. Fuel 233, 257–268. https:// doi.org/10.1016/j.fuel.2018.06.060. Moussa, I., Khiari, R., Moussa, A., Mortha, G., Mhenni, M.F., 2019. Structural characterization and antioxidant activity of lignin extracted from Ficus Carica L. J. Renew. Mater. 7 (4), 345–354. https://doi.org/10.32604/jrm.2019.04011. Oh, S., Kim, U.J., Choi, I.G., Choi, J.W., 2016. Solvent effects on improvement of fuel properties during hydrodeoxygenation process of bio-oil in the presence of Pt/C. Energy 113, 116–123. https://doi.org/10.1016/j.energy.2016.07.027. Oliveira, A.L.P.C., Almeida, P.S., Campos, M.C.V., Franceschi, E., Dariva, C., Borges, G.R., 2017. Thermoliquefaction of palm oil fiber (Elaeis sp.) using supercritical ethanol. Bioresour. Technol. 230, 1–7. https://doi.org/10.1016/j.biortech.2017.01.029. Peng, P., She, D., 2014. Isolation, structural characterization, and potential applications of hemicelluloses from bamboo: a review. Carbohyd. Polym. 112, 701–720. https:// doi.org/10.1016/j.carbpol.2014.06.068. Schuler, J., Hornung, U., Dahmen, N., Sauer, J., 2019. Lignin from bark as a resource for aromatics production by hydrothermal liquefaction. GCB Bioenergy 11, 218–229. https://doi.org/10.1111/gcbb.12562. Sipponen, M.H., Ozdenkci, K., Muddassar, H.R., Melin, K., Golam, S., Oinas, P., 2016. Hydrothermal liquefaction of softwood: selective chemical production under oxidative conditions. ACS Sustain. Chem. Eng. 4, 3978–3984. https://doi.org/10.1021/ acssuschemeng.6b00846. Wu, X.-F., Yin, S.-S., Zhou, Q., Li, M.-F., Peng, F., Xiao, X., 2019. Subcritical liquefaction of lignocellulose for the production of bio-oils in ethanol/water system. Renew. Energy. https://doi.org/10.1016/j.renene.2019.01.041. Xi, D.K., Jiang, C.C., Zhou, R.W., Fang, Z., Zhang, X.H., Liu, Y., Luan, B.Y., Feng, Z., Chen, G.L., Chen, Z., Liu, Q.H., Yang, S.Z., 2018. The universality of lignocellulosic biomass liquefaction by plasma electrolysis under acidic conditions. Bioresour. Technol. 268, 531–538. https://doi.org/10.1016/j.biortech.2018.08.025. Xu, Y.P., Duan, P.G., Wang, F., 2015. Hydrothermal processing of macroalgae for producing crude bio-oil. Fuel Process. Technol. 130, 268–274. https://doi.org/10.1016/ j.fuproc.2014.10.028. Yang, T.H., Wang, J., Li, B.S., Kai, X.P., Xing, W.L., Li, R.D., 2018. Behaviors of rice straw two-step liquefaction with sub/supercritical ethanol in carbon dioxide atmosphere. Bioresour. Technol. 258, 287–294. https://doi.org/10.1016/j.biortech.2018.02.099. Yip, J., Chen, M.J., Szeto, Y.S., Yan, S.C., 2009. Comparative study of liquefaction process and liquefied products from bamboo using different organic solvents. Bioresour. Technol. 100, 6674–6678. https://doi.org/10.1016/j.biortech.2009.07.045. Yue, Y., Kastner, J.R., Mani, S., 2018. Two-stage hydrothermal liquefaction of sweet sorghum biomass-part 1: production of sugar mixtures. Energy Fuel. 32, 7611–7619. https://doi.org/10.1021/acs.energyfuels.8b00668. Zou, S.P., Wu, Y.L., Yang, M.D., Kaleem, I., Chun, L., Tong, J.M., 2010. Production and characterization of bio-oil from hydrothermal liquefaction of microalgae Dunaliella tertiolecta cake. Energy 35, 5406–5411. https://doi.org/10.1016/j.energy.2010.07. 013.
References Akhtar, J., Amin, N.A.S., 2011. A review on process conditions for optimum bio-oil yield in hydrothermal liquefaction of biomass. Renew. Sustain. Energy Rev. 15, 1615–1624. https://doi.org/10.1016/j.rser.2010.11.054. Brand, S., Hardi, F., Kim, J., Suh, D.J., 2014. Effect of heating rate on biomass liquefaction: differences between subcritical water and supercritical ethanol. Energy 68, 420–427. https://doi.org/10.1016/j.energy.2014.02.086. Brand, S., Kim, J., 2015. Liquefaction of major lignocellulosic biomass constituents in supercritical ethanol. Energy 80, 64–74. https://doi.org/10.1016/j.energy.2014.11. 043. Brand, S., Susanti, R.F., Kim, S.K., Lee, H.S., Kim, J., Sang, B.I., 2013. Supercritical ethanol as an enhanced medium for lignocellulosic biomass liquefaction: influence of physical process parameters. Energy 59, 173–182. https://doi.org/10.1016/j.energy. 2013.06.049. Demirkaya, E., Dal, O., Yüksel, A., 2019. Liquefaction of waste hazelnut shell by using sub- and supercritical solvents as a reaction medium. J. Supercrit. Fluids 150, 11–20. https://doi.org/10.1016/j.supflu.2019.03.019. Gai, C., Li, Y., Peng, N.N., Fan, A.N., Liu, Z.G., 2015. Co-liquefaction of microalgae and lignocellulosic biomass in subcritical water. Bioresour. Technol. 185, 240–245. https://doi.org/10.1016/j.biortech.2015.03.015. Gao, Y.F., Wang, H.T., Guo, J.H., Peng, P., Zhai, M.Z., She, D., 2016. Hydrothermal degradation of hemicelluloses from triploid poplar in hot compressed water at 180-340 °C. Carbohyd. Polym. 126, 179–187. https://doi.org/10.1016/j.polymdegradstab. 2016.02.003. Huang, H.J., Yuan, X.Z., Zeng, G.M., Wang, J.Y., Li, H., Zhou, C.F., Pei, X.K., You, Q.A., Chen, L.A., 2011. Thermochemical liquefaction characteristics of microalgae in suband supercritical ethanol. Fuel Process. Technol. 92, 147–153. https://doi.org/10. 1016/j.fuproc.2010.09.018. Kang, S., Li, X., Li, B., Fan, J., Chang, J., 2011. Effects of lignins on antioxidant biodiesel production in supercritical methanol. Energy Fuel. 25, 2746–2748. https://doi.org/ 10.1021/ef2004249. Kozliak, E.I., Kubatova, A., Artemyeva, A.A., Nagel, E., Zhang, C., Rajappagowda, R.B., Srnirnova, A.L., 2016. Thermal liquefaction of lignin to aromatics: efficiency, selectivity, and product analysis. ACS Sustain. Chem. Eng. 4, 5106–5122. https://doi. org/10.1021/acssuschemeng.6b01046. Kritzer, P., 2004. Corrosion in high-temperature and supercritical water and aqueous solutions: a review. J. Supercrit. Fluid. 29, 1–29. https://doi.org/10.1016/S08968446(03)00031-7. Liu, C.Z., Wufuer, A., Kong, L.P., Wang, Y.Y., Dai, L.Y., 2018. Organic solvent extractionassisted catalytic hydrothermal liquefaction of algae to bio-oil. RSC Adv. 8, 31717–31724. https://doi.org/10.1039/c8ra04668a. Liu, H., Ma, M., Xie, Xa., 2017. New materials from solid residues for investigation the mechanism of biomass hydrothermal liquefaction. Ind. Crop. Prod. 108, 63–71.
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