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Organosolv liquefaction of sugarcane bagasse catalyzed by acidic ionic liquids
Bioresource Technology 214 (2016) 16–23
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Organosolv liquefaction of sugarcane bagasse catalyzed by acidic ionic liquids Zhengjian Chen a, Jinxing Long a,b,⇑ a b
Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou Education University, Guiyang 550018, PR China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
h i g h l i g h t s An intensified process for bagasse all-components liquefaction is provided. The efficient delignification substantially promotes the carbohydrate conversion. 97.5% of bagasse is liquefied with 66.46% of volatile product yield at 200 °C. No obvious activity loss is shown for the acid IL catalyst even after five runs. A signification decrease of the reaction temperature is demonstrated.
a r t i c l e
i n f o
Article history: Received 11 March 2016 Received in revised form 15 April 2016 Accepted 17 April 2016 Available online 20 April 2016 Keywords: Bagasse Ionic liquids Liquefaction Aromatic products Promotion effect
a b s t r a c t An efficient and eco-friendly process is proposed for sugarcane bagasse liquefaction under mild condition using IL catalyst and environmental friendly solvent of ethanol/H2O. The relationship between IL acidic strength and its catalytic performance is investigated. The effects of reaction condition parameters such as catalyst dosage, temperature, time and solvent are also intensively studied. The results show that ethanol/H2O has a significant promotion effect on the simultaneous liquefaction of sugarcane bagasse carbohydrate and lignin. 97.5% of the bagasse can be liquefied with 66.46% of volatile product yield at 200 °C for 30 min. Furthermore, the IL catalyst shows good recyclability where no significant loss of the catalytic activity is exhibited even after five runs. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Catalytic valorization of renewable biomass to biochemical and biofuel attracts steady attention with the depletion of fossil fuel and the increasing concern on the sustainable chemistry (Li et al., 2015; Long et al., 2012a; Zhang et al., 2013b). Sugarcane is a typical economic crop in the tropical and subtropical areas such as Brazil, Cuba and China. As a byproduct from the cane sugar industry, bagasse is produced in large quantities annually, for example, more than ten million tons in China (Yi et al., 2015). Generally, sugarcane bagasse contains 80% of carbohydrate and no more than 20% of lignin. Comparing with the wood, this herbaceous plant has looser inner structure, suggesting that it has a great potential to be an excellent starting material for the renewable biofuel and biochemical production. However, it is so pity that less than 10% of ⇑ Corresponding author at: Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou Education University, Guiyang 550018, PR China. E-mail address: [email protected] (J. Long). http://dx.doi.org/10.1016/j.biortech.2016.04.089 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
sugarcane bagasse is currently industrially utilized. Instead, most of it is directly combusted as low value energy, causing serious resource waste and environmental problems. And thus, novel and efficient strategies for the valorization of this renewable resource are highly impressive and desired. Conventional thermal chemical techniques for biomass utilization, such as gasification, pyrolysis and liquefaction, are efficient and robust for bagasse depolymerization (Galadima and Muraza, 2015; Ma et al., 2012). However, the intrinsically rigid operation condition (high temperature and pressure), wide product distribution and serious char formation during the gasification and pyrolysis process are inevitable (Lange, 2015). Therefore, a series of post-treatment processes, such as gas-synthesis for gasification products and upgradation for the pyrolysis bio-oil, are necessary (Zhang et al., 2013a,c). In contrast, hydrothermal liquefaction is more popular because of its mild condition and high product selectivity (Chumpoo and Prasassarakich, 2010; Cunha et al., 2014; Hassan and Shukry, 2008; Prado et al., 2014; Zhang et al., 2007).
Z. Chen, J. Long / Bioresource Technology 214 (2016) 16–23
For example, Prado and the coauthors reported that bagasse can be efficiently liquefied in the subcritical water, where 95% degree of the liquefaction could be obtained under optimized condition (Prado et al., 2014). The bagasse liquefaction in other solvents, such as methanol, ethanol, phenol and polyol, was also investigated with H2SO4 and HCl catalyst (Chumpoo and Prasassarakich, 2010; Cunha et al., 2014; Hassan and Shukry, 2008; Zhang et al., 2007), and good to excellent results could be achieved. However, the application of the strong proton acid and sometime the toxic solvent generally brings serious corrosion and pollution, in discordance with the requirement of the green chemistry and sustainable development. Previously, an efficient and environmental benign process for sugarcane bagasse liquefaction was proposed using ionic liquid (IL) catalyst in the green solvent of subcritical water. With the acidic IL, 96.1% of the feedstock could be liquefied with 48.6% volatile product yield at 270 °C for 30 min (Long et al., 2011). However, it should be noticed that the reaction temperature is high, resulting in large energy consumption, which can be ascribed to the poor dissolution of the carbohydrate and lignin in the water. Therefore, more energy-efficient strategies for bagasse valorization are important and necessary. Here, an intensified process for bagasse liquefaction is provided using acidic IL as catalyst and the mixture of ethanol and water as solvent. In this process, lignin, which composes of the aromatic structure and is regarded as the main barrier for the efficient biomass valorization, is dissolved by ethanol/water with the degradation of the carbohydrate (cellulose and hemicellulose). Simultaneously, the dissolved lignin is depolymerized under the catalysis of acid IL. The efficient delignification substantially enlarges the contact between carbohydrate and IL catalyst. Meanwhile, the eco-friendly solvent of ethanol/H2O is also an excellent medium for the dispersion of the phenolic chemicals and excellent inhibitor of char formation (Long et al., 2015). Therefore, with the efficient solvent effect and the synergistic effect between the delignification and lignin depolymerization, a significant decrease of the reaction temperature for the bagasse all-components conversion is achieved, which results in substantially reduced energy consumption. The reduced energy consumption process can also be a good reference for the high valorization the renewable biomass which is considered to be excellent feedstock for sustainable biofuel or biochemical production.
2. Experimental section 2.1. Materials and methods N-methylimidazolium, 1, 4-butane sultone, ethyl chloropropionate, 1-chlorobutane and tetrahydrofuran (THF, HPLC grade) were purchased from Acros (Belgium) and used without further purification. Other reagents were supplied by Tianjin Sci-Tech Co., Ltd. (Tianjin, China) and redistilled prior to use. The sugarcane was first milled and sieved to get powder with particles size among 40–60 meshes. Then it was dried under vacuum at 80 °C for 24 h. Composition analysis shows that it contains 46.47% of cellulose, 33.35% of hemicellulose, 18.96% of lignin, and 1.06% of inorganic salts. ILs 1-(4-sulfobutyl)-3-methyl imidazolium hydrosulfate ([C4H8SO3Hmim]HSO4), N-methyl imidazolium hydrosulfate ([mim] HSO4), 1-butyl-3-methyl imidazolium hydrosulfate ([bmim] HSO4), and 1-(2-carboxyethyl)-3-methyl imidazolium chloride ([C2H4COOHmim]Cl) were synthesized according to the reported procedures (Cole et al., 2002; Fei et al., 2004; Singh et al., 2005) and characterized by 1H-NMR, 13C-NMR, electrospray ionizationmass spectrometry (ESI-MS) and thermogravimetric analysis (TG). Ion chromatography and element analysis demonstrate that their purities are all greater than 97%. Their detailed characteriza-
17
tion data are listed as follows. [C4H8SO3Hmim]HSO4: 1H-NMR (D2O, 600 MHz): d = 1.42 (m, 2H,); 1.69 (m, 2H); 2.62 (t, 2H, J = 7.2); 3.57 (s, 3H); 3.92 (t, 2H, J = 7.2); 7.13 (s, 1H); 7.17 (s, 1H); 8.4 (s, 1H). 13C-NMR (D2O, 100 MHz): d = 21.3; 28.2; 35.9; 49.1; 50.3; 122.3; 123.9; 136.0. ESI-MS: m/z (+): 219.4; m/z (): 97.5. Onset decomposition temperature: 325 °C; [mim]HSO4: 1HNMR (d6-DMSO, 600 MHz): d = 3.92 (s, 3H); 7.75 (d, 2H, J = 7.2); 9.24 (s, 1H). 13C-NMR (d6-DMSO, 100 MHz): d = 35.6; 28.2; 119.5; 122.9; 135.1. ESI-MS: m/z (+): 83.1; m/z (): 97.2. Onset decomposition temperature: 335 °C; [bmim]HSO4: 1H-NMR (d6-DMSO, 600 MHz): d = 0.90 (t, 3H, J = 7.2); 1.19 (m, 2H); 1.71 (m, 2H); 3.83 (s, 3H); 4.14 (t, 2H, J = 7.2); 7.67 (d, 1H, J = 3.6); 7.75 (d, 1H, J = 3.6); 9.16 (s, 1H).13C-NMR (100 MHz, d6-DMSO): 13.0, 18.9, 31.7, 35.9, 49.3, 121.8, 123.3, 135.8. ESI-MS: m/z (+): 138.8; m/z (): 97.1. Onset decomposition temperature: 345 °C; [C2H4COOHmim]Cl: 1H-NMR (D2O, 600 MHz): d = 2.15 (s, 1H,); 5.04 (s, 2H); 3.87 (s, 3H); 7.42 (d, 1H, J = 3.6); 7.42 (d, 2H, J = 7.2); 8.72 (s, 1H). 13C-NMR (100 MHz, D2O): 27.1, 35.9, 49.8, 125.3, 126.7, 179.8. ESI-MS: m/z (+): 155.8; m/z (): 35.7. Onset decomposition temperature: 278 °C. 2.2. Hydrothermal liquefaction of the sugarcane bagasse The hydrothermal liquefaction of the sugarcane bagasse was carried out in a 100 mL stainless autoclave equipped with a mechanical stirring (Tongda Co. Ltd. Dalian, China). In a typical process, 2.0 g bagasse, 21 mL ethanol, 9 mL H2O and 1.5 mmol acidic IL were charged into the autoclave reactor in sequence. After the careful displacement of the air using nitrogen for three times, the reactor was heated to 200 °C for 30 min. When the designated time was elapsed, the autoclave reactor was cooled down to room temperature using flowing water during 30 min. 2.3. Procedure for product separation Fig. 1 shows the procedure for liquefaction product separation. The gaseous product from this process was collected using an airbag. The mixture of liquid product and the residual solid was first separated by filtration. And then, the solid fraction was thoroughly washed using deionized water and THF, respectively. After that, it was dried at 80 °C under vacuum until constant weight. The filtrate was first submitted for ethanol removal, and then it was extracted by ethyl acetate for two times (10 mL 2). The organic phase was defined as the ethyl acetate soluble fractions (EtOAc soluble product). The raffinate was further submitted to rotary evaporation for water removal, then, the obtained slurry was washed using THF. The THF insoluble product was designated as the water soluble fraction. 2.4. Product analysis The gaseous product was analyzed on an Agilent 7890B gas chromatography (Agilent, America) with a thermal conductivity cell detector (TCD). The volatile chemicals in the liquid fraction were identified by gas chromatography-mass spectrometer (GC– MS) according to Agilent 5977 MS library (Agilent, America). The chemicals were separated in a HP-5MS column (30 m 0.25 mm 0.25 lm) with high purity helium as carrier gas. The initial oven temperature was 60 °C (hold for 3 min), and then it was ramped to 250 °C (hold for another 10 min) with the speed of 8 °C min1. The qualitative analysis of the water soluble product was conducted on an Agilent 1260 high performance liquid chromatography (HPLC) with a refractive index detector (RID), where commercial chemicals (purity >99%, purchased from Acros, Belgium) were used as the standard compound. A SH 1011 column was used for chemical separation. 0.5 wt% H2SO4 aqueous
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Fig. 1. Procedure for bagasse liquefaction and product separation. Notes: r.t. is room temperature. The mass balance is measured according to the results from the optimized condition. The yield of low boiling product includes that from step (a) and (b), which accounts for 29.38% and 6.98%, respectively. The THF soluble fraction composes of that from step (b) and (c), which accounts for 21.2% and 3.3%, respectively.
was used as the eluent with the flow rate of 0.6 mL min1. The molecular weight distribution of the THF soluble fraction was achieved by gel permeation chromatography (GPC) on an Agilent 1260 HPLC with a RID as well. 1.0 mL min1 THF was used as the eluent. The average molecular weight was measured using polystyrene as the standard compound. The measurements of the carbon content of feedstock, products and residual solid were carried out on a vario EL III element analyzer (Germany). The Fourier transform infrared spectroscopy (FT-IR) spectra of the raw bagasse and the residual solids were recorded on a Nicolet is 50 FT-IR spectrometer (Thermo scientific, America) by KBr pelleting method. 2.5. Degree of liquefaction and product yield measurement The degree of the bagasse liquefaction was calculated by the carbon content comparison of residual solid and feedstock which measured by elemental analysis (Eq. (1)). The gaseous fraction, which is collected by airbag, composes of CO2, CO, CH4 and other hydrocarbon with two to four carbons as detected by GC–TCD. However, this fraction is less than 0.02 g each run, namely, the weight of the gaseous product is less than 1% of the original bagasse. Therefore, the gaseous products generated from this process are negligible and were not calculated in the following mass balance. The yield of EtOAc product was measured by gas chromatography with a flame ionized detector (GC–FID) where the same chromatography column and temperature program as the GC–MS analysis were used (Eq. (2)). The yields of water-soluble and THF-soluble fraction were determined by the comparison of the carbon content as well after careful removal of the solvent under reduced pressure until constant weight (Eqs. (3) and (4)). The yield of low-boiling product was measured by the mass balance according to Eq. (5). Triplicate experiments were conducted to reduce the relative error.
W R CR 100% Degree of liquefaction ðDL Þ ¼ 1 W F CF Pi Yield of EtOAc fraction ðY E Þ ¼
C Ei 100% W F CF
i¼1 W Ei
ð1Þ
ð2Þ
Yield of Water soluble fraction ðY W Þ ¼ Yield of THF soluble fraction ðY T Þ ¼
W W CW 100% W F CF
W T CT 100% W F CF
ð3Þ ð4Þ
Yield of Low boiling product ðY L Þ ¼ ðDL Y E Y W W T Þ 100% ð5Þ WR: weight of the residual solid, CR: carbon content of the residual solid, WF: weight of the feedstock (bagasse), CF: carbon content of the feedstock, WE: weight of the EtOAc soluble fraction, CE: carbon content of the EtOAc soluble fraction, WW: weight of the water soluble fraction, CW: carbon content of the water soluble fraction, WT: weight of the THF soluble fraction, CT: carbon content of the THF soluble fraction. 3. Results and discussion 3.1. Bagasse catalytic liquefaction with various IL catalysts Generally, an intensive competition process of liquefaction and hydrolysis exists in the biomass hydrothermal conversion (Long et al., 2011). And the main products from the latter are monosaccharides and polysaccharides. These nonvolatile sugars are insoluble in the organic solvents such as EtOAc and THF, so it generally exists as the water-soluble fraction during this process. It can be seen from Fig. 2 that acidic IL catalyst is crucial for bagasse liquefaction. For example, 26% of bagasse was liquefied in the absence of acid IL. Further analysis demonstrates that water soluble fraction is the most abundant products (Fig. 2a), suggesting that hydrolysis promoted by the thermal effect and the self-catalysis of the H+ from subcritical water (Savage, 1999) is the dominant without catalyst. However, when an acidic catalyst is presented, both the bagasse liquefaction and the product yield are significantly improved. For example, 54% degree of bagasse liquefaction can be obtained with the carboxyl functionalized IL [C2H4COOHmim]Cl and 95.7% for that with the SO3H functionalized IL [C4H8SO3Hmim]HSO4 (Fig. 2a). Meanwhile, the product distribution is also changed substantially where an
Z. Chen, J. Long / Bioresource Technology 214 (2016) 16–23
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Fig. 2. The relationship between (a) IL catalytic performance and (b) its acidic strength IL A: [C2H4COOHmim]Cl; IL B: [bmim]HSO4; IL C: [mim]HSO4; IL D: [C4H8SO3Hmim] HSO4 Conditions: bagasse 2.0 g, 80% (v/v) ethanol aqueous 30 mL, catalyst dosage 2 mmol (H2SO4 concentration: 5 wt%), reaction temperature 200 °C, reaction time 30 min.
obvious yield increase of the EtOAc-soluble and THF-soluble fraction is demonstrated (for example, 3.28 and 3.8% of EtOAc-soluble and THF-soluble fraction are shown without catalyst, however, when [C4H8SO3Hmim]HSO4 is used, they sharply increase to 31% and 23.5%, respectively), implying the efficient liquefaction over the acid IL catalyst. The relationship between the IL structure and the catalytic activity on the sugarcane bagasse liquefaction is further investigated. As shown in Fig. 2(b), the acidic strength of the IL is closely related to its structure. [C2H4COOHmim]Cl is found to be the weakest acid, while dual-functionalized IL [C4H8SO3Hmim]HSO4 is the strongest, in accordance with the weaker acidity of the carboxyl group than the SO3H. Surely, the acidity of the HSO 4 anion is also a main reason for the IL acidic strength. Comparing with IL [bmim] HSO4, [mim]HSO4 exhibits higher acidity because of the less sidechain length (Greaves and Drummond, 2008). Fig. 2(a) shows that the catalytic performance of the IL catalyst is linearly proportional to its acidic strength. For example, with the weakest catalyst [C2H4COOHmim]Cl, 54.0% of the bagasse is liquefied at 200 °C for 30 min, whereas more than 95.7% of the degree of liquefaction is obtained over the strongest IL acid [C4H8SO3Hmim]HSO4 at the same condition. Furthermore, the product distribution depended on the acidic strength of the IL catalyst as well, where increasing EtOAc-soluble and THF-soluble fractions are demonstrated with the increasing IL acidity (Fig. 2a). However, it should be noticed that the yield of the water-soluble products suffers a various change of first increase and then decrease, in which, the most abundant of 30.52% of that is achieved when IL [bmim]HSO4 is used. In this process, both liquefaction and hydrolysis are existed and competed under the catalysis of the IL acid (Long et al., 2011). At lower acid density, hydrolysis is the primary which generally results in the water-soluble product, while liquefaction becomes more remarkable in the presence of stronger acid. The catalytic activities of the IL catalyst and the conventional proton acid such as H2SO4 are also examined. The results show that the SO3H functionalized IL catalyst of [C4H8SO3Hmim]HSO4 is far more efficient than the proton acid (Fig. 2a). For example, with 5 wt% H2SO4, merely 83% of the bagasse is liquefied. The special chemical bond connecting in IL (He et al., 2012) is responsible for the high catalytic activity. Furthermore, the IL catalyst shows obvious advantage of less corrosion (Uerdingen et al., 2005), so the present process using IL catalyst has a great potential to be a promising alternative for the conventional proton acid technologies in biomass utilization.
3.2. Effect of catalyst dosage Fig. 3(a) shows the effect of IL catalyst [C4H8SO3Hmim]HSO4 dosage on the bagasse liquefaction and the product distribution. For a homogeneous catalyst, larger catalyst dosage generally indicates more catalytic active center, so the degree of sugarcane bagasse liquefaction increases from 86.8% to 95.7% when the IL [C4H8SO3Hmim]HSO4 dosage increases from 0.5 to 1.5 mmol. Simultaneously, the yield of water soluble product decreases from 34.28% to 7.9%. It further confirms that bagasse hydrolysis is the primary at lower acid density, whereas liquefaction becomes dominant with stronger acid or higher acid IL dosage (Fig. 2a). However, it should be noticed that the bagasse depolymerization is generally concomitant with the repolymerization of the unsaturated products such as polysaccharide and phenolic oligomer during the liquefaction (Long et al., 2011, 2012b). Both of these processes could be substantially promoted by the acid IL as well. Furthermore, the repolymerization product has a more complex and recalcitrant chemical structure than the original bagasse and it usually coats on the surface of the raw material (Long et al., 2014) hampering further degradation of the bagasse. Therefore, with the continuous increase of the IL amount, a stepwise declined degree of liquefaction is shown because of the more remarkable repolymerization of unsaturated products at high acid concentration (Fig. 3a). 3.3. Effect of the solvent Comparing with the effect of catalyst dosage, that of solvent is more significant (Fig. 3b). 81.1% of the degree of bagasse liquefaction is shown in the single solvent of ethanol, whereas, 97.5% of bagasse is liquefied when the ethanol concentration (aqueous) decreases to 70%. It was reported that a biomass swelling process is necessary before depolymerization (Xu et al., 2014). The fact that water substantially favors the bagasse wetting and swelling than the ethanol is the main reason for the enhanced bagasse liquefaction. However, too high water content hinders the delignification (Long et al., 2013). And lignin is more recalcitrant for degradation and generally coats on the surface of the carbohydrate. Therefore, the inefficient delignification is responsible for the decrease of the bagasse liquefaction at low ethanol concentration. Fig. 3(b) also illustrates that the product distribution changes substantially with the decrease of the ethanol concentration. For example, a significant increase of the water-soluble fraction with decrease yield of the EtOAc-soluble fraction and low-boiling products is shown
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Fig. 3. Effect of IL catalyst dosages (a) and ethanol concentration (b) on the bagasse liquefaction conditions: (a) bagasse 2.0 g, 80% (v/v) ethanol aqueous 30 mL, catalyst [C4H8SO3Hmim]HSO4, reaction temperature 200 °C, reaction time 30 min; (b) bagasse 2.0 g, ethanol aqueous solution 30 mL, catalyst [C4H8SO3Hmim]HSO4 1.5 mmol, reaction temperature 200 °C, reaction time 30 min.
when the ethanol concentration is lower than 60%. It indicates the efficient hydrolysis rather than liquefaction at lower ethanol content.
3.4. Effect of reaction temperature and time Generally, thermal effect is a crucial factor for biomass liquefaction (Alhassan et al., 2016; Elliott et al., 2015). Hence, the effect of the reaction temperature on the sugarcane bagasse liquefaction is further investigated with the IL catalyst of [C4H8SO3Hmim]HSO4. As shown in Fig. 4(a), with the increase of the reaction temperature from 160 to 200 °C, the degree of bagasse sharply increases from 91.5% to 97.5% with a gradual increase of the low boiling and THF soluble fraction. Previous studies showed that both the delignification and lignin depolymerization are highly temperature dependent (Long et al., 2013, 2015), which is the main reason for these increases of the degree of bagasse liquefaction and the yields of THF soluble and low boiling product. However, with the continuously increasing of the reaction temperature, the condensation of the unsaturated product such as phenolic oligomer is enhanced. The condensation product is more recalcitrant for degradation
and coats on the surface of the carbohydrate. Therefore, a declination of the bagasse liquefaction is displayed when the reaction temperature is higher than 200 °C. The same change tendency is exhibited for the effect of the reaction time on the bagasse liquefaction. Fig. 4(b) illustrates that the degree of liquefaction increases sharply from 89.6% to 97.5% when the reaction time increases from 5 to 30 min. And then, it decreases to 91.5% with the prolonged time of 60 min because of the repolymerization of the unsaturated products. Summarily, with the optimized acidic IL [C4H8SO3Hmim]HSO4 of 1.5 mmol, ethanol concentration of 70% (v/v), reaction temperature of 200 °C and time of 30 min, 97.5% of the bagasse can be liquefied with 66.46% of volatile product yield. These small molecule volatile chemicals are promising starting materials for high quality bio-oil production.
3.5. Recyclability of the IL catalyst Comparing with the conventional homogeneous acidic catalyst, the most prominent advantage of the IL is the recyclability. Fig. S1 demonstrates that the efficient IL catalyst [C4H8SO3Hmim]HSO4 is
Fig. 4. Effects of temperature (a) and time (b) on the bagasse hydrothermal liquefaction conditions: (a) bagasse 2.0 g, 70% (v/v) ethanol aqueous solution 30 mL, catalyst [C4H8SO3Hmim]HSO4 1.5 mmol, reaction time 30 min; and (b) bagasse 2.0 g, 70% (v/v) ethanol aqueous solution 30 mL, catalyst [C4H8SO3Hmim]HSO4 1.5 mmol, temperature 200 °C.
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Z. Chen, J. Long / Bioresource Technology 214 (2016) 16–23 Table 1 The most abundant 25 volatile products detected by GC–MS analysis.a
a b
Nos.
RT (min)
Compounds
Formula
Percentage (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
4.29 4.78 6.22 6.43 6.56 6.85 7.25 7.49 8.24 9.19 9.38 9.55 9.85 10.14 11.56 12.73 13.61 14.22 15.71 16.59 18.51 18.69 19.54 21.69 22.05
Propanoic acid, 2-hydroxy-, ethyl ester Furfural Butanoic acid, 2-hydroxy-, ethyl ester Ethanone, 1-(2-furanyl)4-Pentenoic acid ethyl ester Butanoic acid, 3-hydroxy-, ethyl ester Butane, 1,1-diethoxy-3- methyl2-Furancarboxaldehyde, 5-methylPentanoic acid, 2-hydroxy-, ethyl ester Benzeneacetaldehyde 2-Furancarboxylic acid, ethyl ester Pentanoic acid, 4-oxo-, ethyl ester 2-Furaldehyde diethyl acetal Phenol, 2-methoxyPhenol, 4-ethyl6-Hydroxyl guaiacol Phenol, 4-ethyl-2-methoxy2-Methoxy-4-vinylphenol Vanillin Phenol, 2-methoxy-4-propyl2,40 -Dihydroxy-30 -methoxyacetophenone Phenol, 2,6-dimethoxy- 4-(2-propenyl)Benzaldehyde, 4-hydroxy-3,5-dimethoxy p-Hydroxycinnamic acid, ethyl ester Phenol, 4-(1-methylpropyl)-
C5H10O3 C5H4O2 C6H12O3 C6H6O2 C7H12O2 C6H12O3 C9H20O2 C6H6O2 C7H14O3 C8H8O C7H8O3 C7H12O3 C9H14O3 C7H8O2 C8H10O C7H8O3 C9H12O2 C9H10O2 C8H8O3 C10H14O2 C9H10O4 C11H14O3 C9H10O4 C11H12O3 C10H14O
1.40 43.37 0.47 0.20 0.21 0.26 0.09 1.03 0.44 0.25 0.25 11.24 0.73 0.20 5.21 20.73 1.71 0.70 0.65 1.41 0.68 0.36 0.81 6.50 0.68
The initial oven temperature was 60 °C (hold for 3 min), and then it was ramped to 250 °C (hold for another 10 min) with the speed of 8 °C min1; Based on the peak area.
highly recyclable, in which, no obvious loss of the catalytic activity is exhibited even after five runs at the operation condition. And thus, it is considered that the present bagasse liquefaction technique with IL catalyst has a great potential in the industrialization of biomass valorization and IL application due to the high efficiency, mild condition and simple technology. 3.6. GC–MS analysis of the volatile products Fig. S2(a) illustrates the main volatile products including lowboiling and EtOAc soluble fraction from this process. As shown in this figure and Table 1, aliphatic compounds such as furfural and its derivate, 5-hydroxylmethyl furfural (HMF), ethyl levulinate and other ester, which are generally originated from the degradation of the carbohydrate, are detected by GC–MS. Simultaneously, phenolic monomers and other aromatic chemicals, which are the typical products from lignin depolymerization, are also achieved. It suggests that both the carbohydrate and the lignin are concurrently cracked in the solvent of ethanol/water with the IL catalyst. Furthermore, this process is selective for biochemical product. Furfural, ethyl levulinate and 2-hydroxyl guaiacol, which are the typical depolymerization products from hemicellulose, cellulose and lignin, are the dominated, representing for 43.47%, 11.24% and 20.73%, respectively (Table 1). All this three compounds are versatile platform molecule and promising sustainable starting materials for bio-material and biofuel. For example, a high quality jetfuel can be obtained from these molecules through the consecutive processes of Aldol condensation and hydrodeoxygenation (Huber et al., 2005). Furthermore, the GC–MS analysis of the EtOAc soluble fraction also shows that most of the aromatic chemicals could be extracted by this solvent (Table S1), which can also be perceived as the excellent feedstock for biogasoline product (Zhang et al., 2013b). 3.7. HPLC analysis of the water soluble fractions The identification of the water soluble fraction from the bagasse liquefaction was carried out by HPLC. It can be seen from Fig. S2(b)
that this fraction mainly composes of the monosaccharide including both pentose and hexose, such as xylose, glucose and arabinose. Polysaccharide such as cellobiose is also detected, but its content is far lower than the monosaccharide. These sugars are insoluble in THF, so it exists as the water-soluble products in this process. However, it is noteworthy that both the monosaccharide and the polysaccharide can be further converted to the volatile chemicals as listed in the Table 1 in the presence of an acidic catalyst. For example, furfural, the most abundant component in the volatile product, is generally originated from the xylose dehydrolysis. Ethyl levulinate, another dominant volatile product, is obtained from the pentose from a consecutive process of glucose isomerization, fructose dehydration for HMF, HMF rehydrolysis for levulinic acid, and the esterification of levulinic acid with ethanol. It can be verified by the existence of HMF and levulinic acid in the HPLC spectrum (Fig. S2b). The fact of sugar decomposition over IL acid also explains well with the change tendency of the water soluble product over stronger IL acid (Fig. 2a) and larger IL dosage (Fig. 3a). It should be noticed that an undesired reaction for humin generation from all of above sugar and HMF (Weingarten et al., 2012), a main reason for the efficiency decrease in current carbohydrate conversion, is enhanced as well with higher acidic strength and IL dosage, resulting in lower degree of liquefaction (Fig. 3a). 3.8. GPC analysis of the THF soluble fraction As described above, the bagasse is readily depolymerized in the green solvent of ethanol/water with the IL catalyst, in which the nonvolatile products is dissolved by THF. Here, the molecular weight distribution of this fraction is further investigated. The results listed in Table 2 show that the weight average molecular weight of this fraction is in the range of 690–903 g mol1, implying that the THF soluble product mainly composes of the phenolic oligomer with four to six structural units. Solvent effect has a significant influence on the bagasse liquefaction and the products distribution as exhibited in Fig. 3(b), where the bagasse liquefaction is enhanced with the increase of the ethanol content when it is lower than 70%. Fig. S3 shows that the molecular weight distri-
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Table 2 The average molecular weight of the THF soluble fraction. Ethanol (%)
Mn
Mw
Mz
D
40 50 60 70 80
258 285 293 307 245
764 784 803 913 690
1893 1684 1802 2184 1636
2.97 2.75 2.74 2.98 2.81
Table 3 the main FT-IR absorption and its assignment. Band (cm1)
Assignment
3438 2924 2843 1730 1706 1649 1604 1517 1462 1440 1379 1321 1255 1168 1051 1017 910 839 765 667
OH stretching vibration CH2 stretching vibration OCH3 stretching vibration C@O stretching vibration Conjugated C@O stretching vibration HAOAH bending vibration Benzene ring skeleton vibration Benzene ring skeleton vibration CH2 bending vibration Benzene ring skeleton vibration Benzene ring skeleton vibration CAH bending vibration CACAH deformation vibration CAO bending vibration CAO bending vibration OAH bending vibration CAOAH bending vibration of cellulose Out-of-plane bending vibration of benzene ring –CH2 rocking vibration CAH bending vibration
bution of the THF soluble fraction also changes obviously with the various ethanol concentrations. It can be seen that the average molecular weight of the THF soluble fraction slightly increases with the increase of the ethanol content when it is less than 70%. According to the previous study (Long et al., 2011), the bagasse fragment with low degree of polymerization is easier to be degraded with the acid catalyst. Therefore, the slight increase of the molecular weight of the THF soluble fraction can be ascribed to the increased degree liquefaction. Furthermore, the lignin with large size and molecular weight is also more flexible for separation with the increase of the ethanol concentration (Long et al., 2013). This organosolv lignin is THF dissoluble but is more recalcitrant for degradation than the carbohydrate. And thus, this lignin and partly degraded lignin fragment are also contributed to the molecular weight increase of the THF soluble product. However, it should be noticed that, the molecular weight of the THF soluble fraction is substantially declined with further increase of the ethanol concentration to 80%. The efficient depolymerization of the lignin (Long et al., 2015) is the main reason for this phenomenon. 3.9. FT-IR analysis of the original bagasse and residue The structure change of the bagasse during the liquefaction is also investigated using FT-IR spectra (Fig. S4 and Table 3). According to the reported studies (Mayo et al., 2004; Silverstein et al., 2005), the peaks at 3438 and 2924 cm1 are designated as the characteristic absorption of the OH and CH2, respectively (Mayo et al., 2004). The infrared absorption at 2843 cm1 is ascribed to the methoxyl group in the lignin molecule (Sharma et al., 2004). 1730 and 1706 cm1 are considered to be the stretching vibration of C@O. The benzene ring skeleton vibration shows the characteristic infrared absorption at 1604, 1517, 1440 and 1379 cm1. The CAOAH bending vibration of cellulose shows the characteristic infrared absorption at 910 cm1. The band at 839 cm1 represents the out-of-plane bending vibration of benzene ring. It can be seen
from Fig. S4 that the original bagasse shows a typical FT-IR spectrum of herbaceous biomass, where the characteristic absorption of carbohydrate and lignin can be clearly observed. However, after the liquefaction in ethanol/water, the FT-IR absorptions of the residues are changed obviously. For example, the strong absorption of the unconjugated C@O stretching vibration, which generally peaks at 1730 cm1 and is considered to be the characteristic absorption of carbonyl group in cellulose and hemicellulose, disappears in the residues. It indicates that cellulose and hemicellulose are more feasible for degradation in the acidic environment. Instead, an infrared absorption peaked at 1706 cm1, which is assigned as the stretching vibration of the C@O conjugated with the electrophilic benzene ring (Sun et al., 2015), becomes more remarkable with the increase of the ethanol concentration, indicating that aromatic compounds from the lignin is the main constitute of the residues. More evidences can also be found as follows: (1) the peak at 839 cm1, which represents the aromatic fragment of lignin, weakens when the ethanol content is less than 60%, and then it sharply enhances with the continuous increase of the ethanol, and (2) the infrared absorption peaks of the carbohydrate (cellulose and hemicellulose) disappear thoroughly in the FT-IR spectra of the residues from bagasse liquefaction with 70% and 80% ethanol contents (Fig. S4, curves e and f). As shown in Fig. 3(b), the degree of bagasse liquefaction increases significantly with the increase of the ethanol concentration, and 97.5% degree of liquefaction is exhibited at the optimized solvent composition of 70% ethanol/water. FT-IR spectra (Fig. S4) demonstrate that the most recalcitrant fragment of the bagasse mainly composes of aromatic compound. Generally, lignin depolymerization to phenolics and the recondensation of the unsaturated phenolic oligomer occur simultaneously (Long et al., 2014). And the products from the latter are insoluble in the solvent and are more rigid for further degradation (Long et al., 2014). Therefore, it is considered that the non-degradable instinct ingredient of bagasse and the product from phenolic oligomer condensation are the main constituents of the liquefied residues at higher ethanol concentration. 4. Conclusion All-components of sugarcane bagasse are efficiently degraded in the eco-friendly solvent of ethanol/H2O using acidic IL catalyst. Under mild condition of 200 °C for 30 min, 97.5% bagasse is converted with 66.46% of volatile product yield. The results of product analyses using GC–MS, GPC and FT-IR demonstrate that both carbohydrate and lignin are simultaneously degraded. Furthermore, the catalyst shows excellent recyclability, where no significant activity loss is exhibited after five runs. However, it should be noticed that the wide product distribution results in a significant difficulty in separation. Therefore, novel strategies for selective depolymerization of biomass all-components are highly desired. Acknowledgments The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (No. 51306191 and 21503050), the Construction Project of Key Laboratories from the Education Department of Guizhou Province (No. QJHKY[2015] 329) and the start-up fund from Guizhou Education University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2016.04. 089.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Organosolv liquefaction of sugarcane bagasse catalyzed by acidic ionic liquids Zhengjian Chen a, Jinxing Long a,b,⇑ a b
Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou Education University, Guiyang 550018, PR China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
h i g h l i g h t s An intensified process for bagasse all-components liquefaction is provided. The efficient delignification substantially promotes the carbohydrate conversion. 97.5% of bagasse is liquefied with 66.46% of volatile product yield at 200 °C. No obvious activity loss is shown for the acid IL catalyst even after five runs. A signification decrease of the reaction temperature is demonstrated.
a r t i c l e
i n f o
Article history: Received 11 March 2016 Received in revised form 15 April 2016 Accepted 17 April 2016 Available online 20 April 2016 Keywords: Bagasse Ionic liquids Liquefaction Aromatic products Promotion effect
a b s t r a c t An efficient and eco-friendly process is proposed for sugarcane bagasse liquefaction under mild condition using IL catalyst and environmental friendly solvent of ethanol/H2O. The relationship between IL acidic strength and its catalytic performance is investigated. The effects of reaction condition parameters such as catalyst dosage, temperature, time and solvent are also intensively studied. The results show that ethanol/H2O has a significant promotion effect on the simultaneous liquefaction of sugarcane bagasse carbohydrate and lignin. 97.5% of the bagasse can be liquefied with 66.46% of volatile product yield at 200 °C for 30 min. Furthermore, the IL catalyst shows good recyclability where no significant loss of the catalytic activity is exhibited even after five runs. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Catalytic valorization of renewable biomass to biochemical and biofuel attracts steady attention with the depletion of fossil fuel and the increasing concern on the sustainable chemistry (Li et al., 2015; Long et al., 2012a; Zhang et al., 2013b). Sugarcane is a typical economic crop in the tropical and subtropical areas such as Brazil, Cuba and China. As a byproduct from the cane sugar industry, bagasse is produced in large quantities annually, for example, more than ten million tons in China (Yi et al., 2015). Generally, sugarcane bagasse contains 80% of carbohydrate and no more than 20% of lignin. Comparing with the wood, this herbaceous plant has looser inner structure, suggesting that it has a great potential to be an excellent starting material for the renewable biofuel and biochemical production. However, it is so pity that less than 10% of ⇑ Corresponding author at: Guizhou Provincial Key Laboratory of Computational Nano-material Science, Guizhou Education University, Guiyang 550018, PR China. E-mail address: [email protected] (J. Long). http://dx.doi.org/10.1016/j.biortech.2016.04.089 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
sugarcane bagasse is currently industrially utilized. Instead, most of it is directly combusted as low value energy, causing serious resource waste and environmental problems. And thus, novel and efficient strategies for the valorization of this renewable resource are highly impressive and desired. Conventional thermal chemical techniques for biomass utilization, such as gasification, pyrolysis and liquefaction, are efficient and robust for bagasse depolymerization (Galadima and Muraza, 2015; Ma et al., 2012). However, the intrinsically rigid operation condition (high temperature and pressure), wide product distribution and serious char formation during the gasification and pyrolysis process are inevitable (Lange, 2015). Therefore, a series of post-treatment processes, such as gas-synthesis for gasification products and upgradation for the pyrolysis bio-oil, are necessary (Zhang et al., 2013a,c). In contrast, hydrothermal liquefaction is more popular because of its mild condition and high product selectivity (Chumpoo and Prasassarakich, 2010; Cunha et al., 2014; Hassan and Shukry, 2008; Prado et al., 2014; Zhang et al., 2007).
Z. Chen, J. Long / Bioresource Technology 214 (2016) 16–23
For example, Prado and the coauthors reported that bagasse can be efficiently liquefied in the subcritical water, where 95% degree of the liquefaction could be obtained under optimized condition (Prado et al., 2014). The bagasse liquefaction in other solvents, such as methanol, ethanol, phenol and polyol, was also investigated with H2SO4 and HCl catalyst (Chumpoo and Prasassarakich, 2010; Cunha et al., 2014; Hassan and Shukry, 2008; Zhang et al., 2007), and good to excellent results could be achieved. However, the application of the strong proton acid and sometime the toxic solvent generally brings serious corrosion and pollution, in discordance with the requirement of the green chemistry and sustainable development. Previously, an efficient and environmental benign process for sugarcane bagasse liquefaction was proposed using ionic liquid (IL) catalyst in the green solvent of subcritical water. With the acidic IL, 96.1% of the feedstock could be liquefied with 48.6% volatile product yield at 270 °C for 30 min (Long et al., 2011). However, it should be noticed that the reaction temperature is high, resulting in large energy consumption, which can be ascribed to the poor dissolution of the carbohydrate and lignin in the water. Therefore, more energy-efficient strategies for bagasse valorization are important and necessary. Here, an intensified process for bagasse liquefaction is provided using acidic IL as catalyst and the mixture of ethanol and water as solvent. In this process, lignin, which composes of the aromatic structure and is regarded as the main barrier for the efficient biomass valorization, is dissolved by ethanol/water with the degradation of the carbohydrate (cellulose and hemicellulose). Simultaneously, the dissolved lignin is depolymerized under the catalysis of acid IL. The efficient delignification substantially enlarges the contact between carbohydrate and IL catalyst. Meanwhile, the eco-friendly solvent of ethanol/H2O is also an excellent medium for the dispersion of the phenolic chemicals and excellent inhibitor of char formation (Long et al., 2015). Therefore, with the efficient solvent effect and the synergistic effect between the delignification and lignin depolymerization, a significant decrease of the reaction temperature for the bagasse all-components conversion is achieved, which results in substantially reduced energy consumption. The reduced energy consumption process can also be a good reference for the high valorization the renewable biomass which is considered to be excellent feedstock for sustainable biofuel or biochemical production.
2. Experimental section 2.1. Materials and methods N-methylimidazolium, 1, 4-butane sultone, ethyl chloropropionate, 1-chlorobutane and tetrahydrofuran (THF, HPLC grade) were purchased from Acros (Belgium) and used without further purification. Other reagents were supplied by Tianjin Sci-Tech Co., Ltd. (Tianjin, China) and redistilled prior to use. The sugarcane was first milled and sieved to get powder with particles size among 40–60 meshes. Then it was dried under vacuum at 80 °C for 24 h. Composition analysis shows that it contains 46.47% of cellulose, 33.35% of hemicellulose, 18.96% of lignin, and 1.06% of inorganic salts. ILs 1-(4-sulfobutyl)-3-methyl imidazolium hydrosulfate ([C4H8SO3Hmim]HSO4), N-methyl imidazolium hydrosulfate ([mim] HSO4), 1-butyl-3-methyl imidazolium hydrosulfate ([bmim] HSO4), and 1-(2-carboxyethyl)-3-methyl imidazolium chloride ([C2H4COOHmim]Cl) were synthesized according to the reported procedures (Cole et al., 2002; Fei et al., 2004; Singh et al., 2005) and characterized by 1H-NMR, 13C-NMR, electrospray ionizationmass spectrometry (ESI-MS) and thermogravimetric analysis (TG). Ion chromatography and element analysis demonstrate that their purities are all greater than 97%. Their detailed characteriza-
17
tion data are listed as follows. [C4H8SO3Hmim]HSO4: 1H-NMR (D2O, 600 MHz): d = 1.42 (m, 2H,); 1.69 (m, 2H); 2.62 (t, 2H, J = 7.2); 3.57 (s, 3H); 3.92 (t, 2H, J = 7.2); 7.13 (s, 1H); 7.17 (s, 1H); 8.4 (s, 1H). 13C-NMR (D2O, 100 MHz): d = 21.3; 28.2; 35.9; 49.1; 50.3; 122.3; 123.9; 136.0. ESI-MS: m/z (+): 219.4; m/z (): 97.5. Onset decomposition temperature: 325 °C; [mim]HSO4: 1HNMR (d6-DMSO, 600 MHz): d = 3.92 (s, 3H); 7.75 (d, 2H, J = 7.2); 9.24 (s, 1H). 13C-NMR (d6-DMSO, 100 MHz): d = 35.6; 28.2; 119.5; 122.9; 135.1. ESI-MS: m/z (+): 83.1; m/z (): 97.2. Onset decomposition temperature: 335 °C; [bmim]HSO4: 1H-NMR (d6-DMSO, 600 MHz): d = 0.90 (t, 3H, J = 7.2); 1.19 (m, 2H); 1.71 (m, 2H); 3.83 (s, 3H); 4.14 (t, 2H, J = 7.2); 7.67 (d, 1H, J = 3.6); 7.75 (d, 1H, J = 3.6); 9.16 (s, 1H).13C-NMR (100 MHz, d6-DMSO): 13.0, 18.9, 31.7, 35.9, 49.3, 121.8, 123.3, 135.8. ESI-MS: m/z (+): 138.8; m/z (): 97.1. Onset decomposition temperature: 345 °C; [C2H4COOHmim]Cl: 1H-NMR (D2O, 600 MHz): d = 2.15 (s, 1H,); 5.04 (s, 2H); 3.87 (s, 3H); 7.42 (d, 1H, J = 3.6); 7.42 (d, 2H, J = 7.2); 8.72 (s, 1H). 13C-NMR (100 MHz, D2O): 27.1, 35.9, 49.8, 125.3, 126.7, 179.8. ESI-MS: m/z (+): 155.8; m/z (): 35.7. Onset decomposition temperature: 278 °C. 2.2. Hydrothermal liquefaction of the sugarcane bagasse The hydrothermal liquefaction of the sugarcane bagasse was carried out in a 100 mL stainless autoclave equipped with a mechanical stirring (Tongda Co. Ltd. Dalian, China). In a typical process, 2.0 g bagasse, 21 mL ethanol, 9 mL H2O and 1.5 mmol acidic IL were charged into the autoclave reactor in sequence. After the careful displacement of the air using nitrogen for three times, the reactor was heated to 200 °C for 30 min. When the designated time was elapsed, the autoclave reactor was cooled down to room temperature using flowing water during 30 min. 2.3. Procedure for product separation Fig. 1 shows the procedure for liquefaction product separation. The gaseous product from this process was collected using an airbag. The mixture of liquid product and the residual solid was first separated by filtration. And then, the solid fraction was thoroughly washed using deionized water and THF, respectively. After that, it was dried at 80 °C under vacuum until constant weight. The filtrate was first submitted for ethanol removal, and then it was extracted by ethyl acetate for two times (10 mL 2). The organic phase was defined as the ethyl acetate soluble fractions (EtOAc soluble product). The raffinate was further submitted to rotary evaporation for water removal, then, the obtained slurry was washed using THF. The THF insoluble product was designated as the water soluble fraction. 2.4. Product analysis The gaseous product was analyzed on an Agilent 7890B gas chromatography (Agilent, America) with a thermal conductivity cell detector (TCD). The volatile chemicals in the liquid fraction were identified by gas chromatography-mass spectrometer (GC– MS) according to Agilent 5977 MS library (Agilent, America). The chemicals were separated in a HP-5MS column (30 m 0.25 mm 0.25 lm) with high purity helium as carrier gas. The initial oven temperature was 60 °C (hold for 3 min), and then it was ramped to 250 °C (hold for another 10 min) with the speed of 8 °C min1. The qualitative analysis of the water soluble product was conducted on an Agilent 1260 high performance liquid chromatography (HPLC) with a refractive index detector (RID), where commercial chemicals (purity >99%, purchased from Acros, Belgium) were used as the standard compound. A SH 1011 column was used for chemical separation. 0.5 wt% H2SO4 aqueous
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Fig. 1. Procedure for bagasse liquefaction and product separation. Notes: r.t. is room temperature. The mass balance is measured according to the results from the optimized condition. The yield of low boiling product includes that from step (a) and (b), which accounts for 29.38% and 6.98%, respectively. The THF soluble fraction composes of that from step (b) and (c), which accounts for 21.2% and 3.3%, respectively.
was used as the eluent with the flow rate of 0.6 mL min1. The molecular weight distribution of the THF soluble fraction was achieved by gel permeation chromatography (GPC) on an Agilent 1260 HPLC with a RID as well. 1.0 mL min1 THF was used as the eluent. The average molecular weight was measured using polystyrene as the standard compound. The measurements of the carbon content of feedstock, products and residual solid were carried out on a vario EL III element analyzer (Germany). The Fourier transform infrared spectroscopy (FT-IR) spectra of the raw bagasse and the residual solids were recorded on a Nicolet is 50 FT-IR spectrometer (Thermo scientific, America) by KBr pelleting method. 2.5. Degree of liquefaction and product yield measurement The degree of the bagasse liquefaction was calculated by the carbon content comparison of residual solid and feedstock which measured by elemental analysis (Eq. (1)). The gaseous fraction, which is collected by airbag, composes of CO2, CO, CH4 and other hydrocarbon with two to four carbons as detected by GC–TCD. However, this fraction is less than 0.02 g each run, namely, the weight of the gaseous product is less than 1% of the original bagasse. Therefore, the gaseous products generated from this process are negligible and were not calculated in the following mass balance. The yield of EtOAc product was measured by gas chromatography with a flame ionized detector (GC–FID) where the same chromatography column and temperature program as the GC–MS analysis were used (Eq. (2)). The yields of water-soluble and THF-soluble fraction were determined by the comparison of the carbon content as well after careful removal of the solvent under reduced pressure until constant weight (Eqs. (3) and (4)). The yield of low-boiling product was measured by the mass balance according to Eq. (5). Triplicate experiments were conducted to reduce the relative error.
W R CR 100% Degree of liquefaction ðDL Þ ¼ 1 W F CF Pi Yield of EtOAc fraction ðY E Þ ¼
C Ei 100% W F CF
i¼1 W Ei
ð1Þ
ð2Þ
Yield of Water soluble fraction ðY W Þ ¼ Yield of THF soluble fraction ðY T Þ ¼
W W CW 100% W F CF
W T CT 100% W F CF
ð3Þ ð4Þ
Yield of Low boiling product ðY L Þ ¼ ðDL Y E Y W W T Þ 100% ð5Þ WR: weight of the residual solid, CR: carbon content of the residual solid, WF: weight of the feedstock (bagasse), CF: carbon content of the feedstock, WE: weight of the EtOAc soluble fraction, CE: carbon content of the EtOAc soluble fraction, WW: weight of the water soluble fraction, CW: carbon content of the water soluble fraction, WT: weight of the THF soluble fraction, CT: carbon content of the THF soluble fraction. 3. Results and discussion 3.1. Bagasse catalytic liquefaction with various IL catalysts Generally, an intensive competition process of liquefaction and hydrolysis exists in the biomass hydrothermal conversion (Long et al., 2011). And the main products from the latter are monosaccharides and polysaccharides. These nonvolatile sugars are insoluble in the organic solvents such as EtOAc and THF, so it generally exists as the water-soluble fraction during this process. It can be seen from Fig. 2 that acidic IL catalyst is crucial for bagasse liquefaction. For example, 26% of bagasse was liquefied in the absence of acid IL. Further analysis demonstrates that water soluble fraction is the most abundant products (Fig. 2a), suggesting that hydrolysis promoted by the thermal effect and the self-catalysis of the H+ from subcritical water (Savage, 1999) is the dominant without catalyst. However, when an acidic catalyst is presented, both the bagasse liquefaction and the product yield are significantly improved. For example, 54% degree of bagasse liquefaction can be obtained with the carboxyl functionalized IL [C2H4COOHmim]Cl and 95.7% for that with the SO3H functionalized IL [C4H8SO3Hmim]HSO4 (Fig. 2a). Meanwhile, the product distribution is also changed substantially where an
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Fig. 2. The relationship between (a) IL catalytic performance and (b) its acidic strength IL A: [C2H4COOHmim]Cl; IL B: [bmim]HSO4; IL C: [mim]HSO4; IL D: [C4H8SO3Hmim] HSO4 Conditions: bagasse 2.0 g, 80% (v/v) ethanol aqueous 30 mL, catalyst dosage 2 mmol (H2SO4 concentration: 5 wt%), reaction temperature 200 °C, reaction time 30 min.
obvious yield increase of the EtOAc-soluble and THF-soluble fraction is demonstrated (for example, 3.28 and 3.8% of EtOAc-soluble and THF-soluble fraction are shown without catalyst, however, when [C4H8SO3Hmim]HSO4 is used, they sharply increase to 31% and 23.5%, respectively), implying the efficient liquefaction over the acid IL catalyst. The relationship between the IL structure and the catalytic activity on the sugarcane bagasse liquefaction is further investigated. As shown in Fig. 2(b), the acidic strength of the IL is closely related to its structure. [C2H4COOHmim]Cl is found to be the weakest acid, while dual-functionalized IL [C4H8SO3Hmim]HSO4 is the strongest, in accordance with the weaker acidity of the carboxyl group than the SO3H. Surely, the acidity of the HSO 4 anion is also a main reason for the IL acidic strength. Comparing with IL [bmim] HSO4, [mim]HSO4 exhibits higher acidity because of the less sidechain length (Greaves and Drummond, 2008). Fig. 2(a) shows that the catalytic performance of the IL catalyst is linearly proportional to its acidic strength. For example, with the weakest catalyst [C2H4COOHmim]Cl, 54.0% of the bagasse is liquefied at 200 °C for 30 min, whereas more than 95.7% of the degree of liquefaction is obtained over the strongest IL acid [C4H8SO3Hmim]HSO4 at the same condition. Furthermore, the product distribution depended on the acidic strength of the IL catalyst as well, where increasing EtOAc-soluble and THF-soluble fractions are demonstrated with the increasing IL acidity (Fig. 2a). However, it should be noticed that the yield of the water-soluble products suffers a various change of first increase and then decrease, in which, the most abundant of 30.52% of that is achieved when IL [bmim]HSO4 is used. In this process, both liquefaction and hydrolysis are existed and competed under the catalysis of the IL acid (Long et al., 2011). At lower acid density, hydrolysis is the primary which generally results in the water-soluble product, while liquefaction becomes more remarkable in the presence of stronger acid. The catalytic activities of the IL catalyst and the conventional proton acid such as H2SO4 are also examined. The results show that the SO3H functionalized IL catalyst of [C4H8SO3Hmim]HSO4 is far more efficient than the proton acid (Fig. 2a). For example, with 5 wt% H2SO4, merely 83% of the bagasse is liquefied. The special chemical bond connecting in IL (He et al., 2012) is responsible for the high catalytic activity. Furthermore, the IL catalyst shows obvious advantage of less corrosion (Uerdingen et al., 2005), so the present process using IL catalyst has a great potential to be a promising alternative for the conventional proton acid technologies in biomass utilization.
3.2. Effect of catalyst dosage Fig. 3(a) shows the effect of IL catalyst [C4H8SO3Hmim]HSO4 dosage on the bagasse liquefaction and the product distribution. For a homogeneous catalyst, larger catalyst dosage generally indicates more catalytic active center, so the degree of sugarcane bagasse liquefaction increases from 86.8% to 95.7% when the IL [C4H8SO3Hmim]HSO4 dosage increases from 0.5 to 1.5 mmol. Simultaneously, the yield of water soluble product decreases from 34.28% to 7.9%. It further confirms that bagasse hydrolysis is the primary at lower acid density, whereas liquefaction becomes dominant with stronger acid or higher acid IL dosage (Fig. 2a). However, it should be noticed that the bagasse depolymerization is generally concomitant with the repolymerization of the unsaturated products such as polysaccharide and phenolic oligomer during the liquefaction (Long et al., 2011, 2012b). Both of these processes could be substantially promoted by the acid IL as well. Furthermore, the repolymerization product has a more complex and recalcitrant chemical structure than the original bagasse and it usually coats on the surface of the raw material (Long et al., 2014) hampering further degradation of the bagasse. Therefore, with the continuous increase of the IL amount, a stepwise declined degree of liquefaction is shown because of the more remarkable repolymerization of unsaturated products at high acid concentration (Fig. 3a). 3.3. Effect of the solvent Comparing with the effect of catalyst dosage, that of solvent is more significant (Fig. 3b). 81.1% of the degree of bagasse liquefaction is shown in the single solvent of ethanol, whereas, 97.5% of bagasse is liquefied when the ethanol concentration (aqueous) decreases to 70%. It was reported that a biomass swelling process is necessary before depolymerization (Xu et al., 2014). The fact that water substantially favors the bagasse wetting and swelling than the ethanol is the main reason for the enhanced bagasse liquefaction. However, too high water content hinders the delignification (Long et al., 2013). And lignin is more recalcitrant for degradation and generally coats on the surface of the carbohydrate. Therefore, the inefficient delignification is responsible for the decrease of the bagasse liquefaction at low ethanol concentration. Fig. 3(b) also illustrates that the product distribution changes substantially with the decrease of the ethanol concentration. For example, a significant increase of the water-soluble fraction with decrease yield of the EtOAc-soluble fraction and low-boiling products is shown
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Fig. 3. Effect of IL catalyst dosages (a) and ethanol concentration (b) on the bagasse liquefaction conditions: (a) bagasse 2.0 g, 80% (v/v) ethanol aqueous 30 mL, catalyst [C4H8SO3Hmim]HSO4, reaction temperature 200 °C, reaction time 30 min; (b) bagasse 2.0 g, ethanol aqueous solution 30 mL, catalyst [C4H8SO3Hmim]HSO4 1.5 mmol, reaction temperature 200 °C, reaction time 30 min.
when the ethanol concentration is lower than 60%. It indicates the efficient hydrolysis rather than liquefaction at lower ethanol content.
3.4. Effect of reaction temperature and time Generally, thermal effect is a crucial factor for biomass liquefaction (Alhassan et al., 2016; Elliott et al., 2015). Hence, the effect of the reaction temperature on the sugarcane bagasse liquefaction is further investigated with the IL catalyst of [C4H8SO3Hmim]HSO4. As shown in Fig. 4(a), with the increase of the reaction temperature from 160 to 200 °C, the degree of bagasse sharply increases from 91.5% to 97.5% with a gradual increase of the low boiling and THF soluble fraction. Previous studies showed that both the delignification and lignin depolymerization are highly temperature dependent (Long et al., 2013, 2015), which is the main reason for these increases of the degree of bagasse liquefaction and the yields of THF soluble and low boiling product. However, with the continuously increasing of the reaction temperature, the condensation of the unsaturated product such as phenolic oligomer is enhanced. The condensation product is more recalcitrant for degradation
and coats on the surface of the carbohydrate. Therefore, a declination of the bagasse liquefaction is displayed when the reaction temperature is higher than 200 °C. The same change tendency is exhibited for the effect of the reaction time on the bagasse liquefaction. Fig. 4(b) illustrates that the degree of liquefaction increases sharply from 89.6% to 97.5% when the reaction time increases from 5 to 30 min. And then, it decreases to 91.5% with the prolonged time of 60 min because of the repolymerization of the unsaturated products. Summarily, with the optimized acidic IL [C4H8SO3Hmim]HSO4 of 1.5 mmol, ethanol concentration of 70% (v/v), reaction temperature of 200 °C and time of 30 min, 97.5% of the bagasse can be liquefied with 66.46% of volatile product yield. These small molecule volatile chemicals are promising starting materials for high quality bio-oil production.
3.5. Recyclability of the IL catalyst Comparing with the conventional homogeneous acidic catalyst, the most prominent advantage of the IL is the recyclability. Fig. S1 demonstrates that the efficient IL catalyst [C4H8SO3Hmim]HSO4 is
Fig. 4. Effects of temperature (a) and time (b) on the bagasse hydrothermal liquefaction conditions: (a) bagasse 2.0 g, 70% (v/v) ethanol aqueous solution 30 mL, catalyst [C4H8SO3Hmim]HSO4 1.5 mmol, reaction time 30 min; and (b) bagasse 2.0 g, 70% (v/v) ethanol aqueous solution 30 mL, catalyst [C4H8SO3Hmim]HSO4 1.5 mmol, temperature 200 °C.
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Z. Chen, J. Long / Bioresource Technology 214 (2016) 16–23 Table 1 The most abundant 25 volatile products detected by GC–MS analysis.a
a b
Nos.
RT (min)
Compounds
Formula
Percentage (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
4.29 4.78 6.22 6.43 6.56 6.85 7.25 7.49 8.24 9.19 9.38 9.55 9.85 10.14 11.56 12.73 13.61 14.22 15.71 16.59 18.51 18.69 19.54 21.69 22.05
Propanoic acid, 2-hydroxy-, ethyl ester Furfural Butanoic acid, 2-hydroxy-, ethyl ester Ethanone, 1-(2-furanyl)4-Pentenoic acid ethyl ester Butanoic acid, 3-hydroxy-, ethyl ester Butane, 1,1-diethoxy-3- methyl2-Furancarboxaldehyde, 5-methylPentanoic acid, 2-hydroxy-, ethyl ester Benzeneacetaldehyde 2-Furancarboxylic acid, ethyl ester Pentanoic acid, 4-oxo-, ethyl ester 2-Furaldehyde diethyl acetal Phenol, 2-methoxyPhenol, 4-ethyl6-Hydroxyl guaiacol Phenol, 4-ethyl-2-methoxy2-Methoxy-4-vinylphenol Vanillin Phenol, 2-methoxy-4-propyl2,40 -Dihydroxy-30 -methoxyacetophenone Phenol, 2,6-dimethoxy- 4-(2-propenyl)Benzaldehyde, 4-hydroxy-3,5-dimethoxy p-Hydroxycinnamic acid, ethyl ester Phenol, 4-(1-methylpropyl)-
C5H10O3 C5H4O2 C6H12O3 C6H6O2 C7H12O2 C6H12O3 C9H20O2 C6H6O2 C7H14O3 C8H8O C7H8O3 C7H12O3 C9H14O3 C7H8O2 C8H10O C7H8O3 C9H12O2 C9H10O2 C8H8O3 C10H14O2 C9H10O4 C11H14O3 C9H10O4 C11H12O3 C10H14O
1.40 43.37 0.47 0.20 0.21 0.26 0.09 1.03 0.44 0.25 0.25 11.24 0.73 0.20 5.21 20.73 1.71 0.70 0.65 1.41 0.68 0.36 0.81 6.50 0.68
The initial oven temperature was 60 °C (hold for 3 min), and then it was ramped to 250 °C (hold for another 10 min) with the speed of 8 °C min1; Based on the peak area.
highly recyclable, in which, no obvious loss of the catalytic activity is exhibited even after five runs at the operation condition. And thus, it is considered that the present bagasse liquefaction technique with IL catalyst has a great potential in the industrialization of biomass valorization and IL application due to the high efficiency, mild condition and simple technology. 3.6. GC–MS analysis of the volatile products Fig. S2(a) illustrates the main volatile products including lowboiling and EtOAc soluble fraction from this process. As shown in this figure and Table 1, aliphatic compounds such as furfural and its derivate, 5-hydroxylmethyl furfural (HMF), ethyl levulinate and other ester, which are generally originated from the degradation of the carbohydrate, are detected by GC–MS. Simultaneously, phenolic monomers and other aromatic chemicals, which are the typical products from lignin depolymerization, are also achieved. It suggests that both the carbohydrate and the lignin are concurrently cracked in the solvent of ethanol/water with the IL catalyst. Furthermore, this process is selective for biochemical product. Furfural, ethyl levulinate and 2-hydroxyl guaiacol, which are the typical depolymerization products from hemicellulose, cellulose and lignin, are the dominated, representing for 43.47%, 11.24% and 20.73%, respectively (Table 1). All this three compounds are versatile platform molecule and promising sustainable starting materials for bio-material and biofuel. For example, a high quality jetfuel can be obtained from these molecules through the consecutive processes of Aldol condensation and hydrodeoxygenation (Huber et al., 2005). Furthermore, the GC–MS analysis of the EtOAc soluble fraction also shows that most of the aromatic chemicals could be extracted by this solvent (Table S1), which can also be perceived as the excellent feedstock for biogasoline product (Zhang et al., 2013b). 3.7. HPLC analysis of the water soluble fractions The identification of the water soluble fraction from the bagasse liquefaction was carried out by HPLC. It can be seen from Fig. S2(b)
that this fraction mainly composes of the monosaccharide including both pentose and hexose, such as xylose, glucose and arabinose. Polysaccharide such as cellobiose is also detected, but its content is far lower than the monosaccharide. These sugars are insoluble in THF, so it exists as the water-soluble products in this process. However, it is noteworthy that both the monosaccharide and the polysaccharide can be further converted to the volatile chemicals as listed in the Table 1 in the presence of an acidic catalyst. For example, furfural, the most abundant component in the volatile product, is generally originated from the xylose dehydrolysis. Ethyl levulinate, another dominant volatile product, is obtained from the pentose from a consecutive process of glucose isomerization, fructose dehydration for HMF, HMF rehydrolysis for levulinic acid, and the esterification of levulinic acid with ethanol. It can be verified by the existence of HMF and levulinic acid in the HPLC spectrum (Fig. S2b). The fact of sugar decomposition over IL acid also explains well with the change tendency of the water soluble product over stronger IL acid (Fig. 2a) and larger IL dosage (Fig. 3a). It should be noticed that an undesired reaction for humin generation from all of above sugar and HMF (Weingarten et al., 2012), a main reason for the efficiency decrease in current carbohydrate conversion, is enhanced as well with higher acidic strength and IL dosage, resulting in lower degree of liquefaction (Fig. 3a). 3.8. GPC analysis of the THF soluble fraction As described above, the bagasse is readily depolymerized in the green solvent of ethanol/water with the IL catalyst, in which the nonvolatile products is dissolved by THF. Here, the molecular weight distribution of this fraction is further investigated. The results listed in Table 2 show that the weight average molecular weight of this fraction is in the range of 690–903 g mol1, implying that the THF soluble product mainly composes of the phenolic oligomer with four to six structural units. Solvent effect has a significant influence on the bagasse liquefaction and the products distribution as exhibited in Fig. 3(b), where the bagasse liquefaction is enhanced with the increase of the ethanol content when it is lower than 70%. Fig. S3 shows that the molecular weight distri-
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Table 2 The average molecular weight of the THF soluble fraction. Ethanol (%)
Mn
Mw
Mz
D
40 50 60 70 80
258 285 293 307 245
764 784 803 913 690
1893 1684 1802 2184 1636
2.97 2.75 2.74 2.98 2.81
Table 3 the main FT-IR absorption and its assignment. Band (cm1)
Assignment
3438 2924 2843 1730 1706 1649 1604 1517 1462 1440 1379 1321 1255 1168 1051 1017 910 839 765 667
OH stretching vibration CH2 stretching vibration OCH3 stretching vibration C@O stretching vibration Conjugated C@O stretching vibration HAOAH bending vibration Benzene ring skeleton vibration Benzene ring skeleton vibration CH2 bending vibration Benzene ring skeleton vibration Benzene ring skeleton vibration CAH bending vibration CACAH deformation vibration CAO bending vibration CAO bending vibration OAH bending vibration CAOAH bending vibration of cellulose Out-of-plane bending vibration of benzene ring –CH2 rocking vibration CAH bending vibration
bution of the THF soluble fraction also changes obviously with the various ethanol concentrations. It can be seen that the average molecular weight of the THF soluble fraction slightly increases with the increase of the ethanol content when it is less than 70%. According to the previous study (Long et al., 2011), the bagasse fragment with low degree of polymerization is easier to be degraded with the acid catalyst. Therefore, the slight increase of the molecular weight of the THF soluble fraction can be ascribed to the increased degree liquefaction. Furthermore, the lignin with large size and molecular weight is also more flexible for separation with the increase of the ethanol concentration (Long et al., 2013). This organosolv lignin is THF dissoluble but is more recalcitrant for degradation than the carbohydrate. And thus, this lignin and partly degraded lignin fragment are also contributed to the molecular weight increase of the THF soluble product. However, it should be noticed that, the molecular weight of the THF soluble fraction is substantially declined with further increase of the ethanol concentration to 80%. The efficient depolymerization of the lignin (Long et al., 2015) is the main reason for this phenomenon. 3.9. FT-IR analysis of the original bagasse and residue The structure change of the bagasse during the liquefaction is also investigated using FT-IR spectra (Fig. S4 and Table 3). According to the reported studies (Mayo et al., 2004; Silverstein et al., 2005), the peaks at 3438 and 2924 cm1 are designated as the characteristic absorption of the OH and CH2, respectively (Mayo et al., 2004). The infrared absorption at 2843 cm1 is ascribed to the methoxyl group in the lignin molecule (Sharma et al., 2004). 1730 and 1706 cm1 are considered to be the stretching vibration of C@O. The benzene ring skeleton vibration shows the characteristic infrared absorption at 1604, 1517, 1440 and 1379 cm1. The CAOAH bending vibration of cellulose shows the characteristic infrared absorption at 910 cm1. The band at 839 cm1 represents the out-of-plane bending vibration of benzene ring. It can be seen
from Fig. S4 that the original bagasse shows a typical FT-IR spectrum of herbaceous biomass, where the characteristic absorption of carbohydrate and lignin can be clearly observed. However, after the liquefaction in ethanol/water, the FT-IR absorptions of the residues are changed obviously. For example, the strong absorption of the unconjugated C@O stretching vibration, which generally peaks at 1730 cm1 and is considered to be the characteristic absorption of carbonyl group in cellulose and hemicellulose, disappears in the residues. It indicates that cellulose and hemicellulose are more feasible for degradation in the acidic environment. Instead, an infrared absorption peaked at 1706 cm1, which is assigned as the stretching vibration of the C@O conjugated with the electrophilic benzene ring (Sun et al., 2015), becomes more remarkable with the increase of the ethanol concentration, indicating that aromatic compounds from the lignin is the main constitute of the residues. More evidences can also be found as follows: (1) the peak at 839 cm1, which represents the aromatic fragment of lignin, weakens when the ethanol content is less than 60%, and then it sharply enhances with the continuous increase of the ethanol, and (2) the infrared absorption peaks of the carbohydrate (cellulose and hemicellulose) disappear thoroughly in the FT-IR spectra of the residues from bagasse liquefaction with 70% and 80% ethanol contents (Fig. S4, curves e and f). As shown in Fig. 3(b), the degree of bagasse liquefaction increases significantly with the increase of the ethanol concentration, and 97.5% degree of liquefaction is exhibited at the optimized solvent composition of 70% ethanol/water. FT-IR spectra (Fig. S4) demonstrate that the most recalcitrant fragment of the bagasse mainly composes of aromatic compound. Generally, lignin depolymerization to phenolics and the recondensation of the unsaturated phenolic oligomer occur simultaneously (Long et al., 2014). And the products from the latter are insoluble in the solvent and are more rigid for further degradation (Long et al., 2014). Therefore, it is considered that the non-degradable instinct ingredient of bagasse and the product from phenolic oligomer condensation are the main constituents of the liquefied residues at higher ethanol concentration. 4. Conclusion All-components of sugarcane bagasse are efficiently degraded in the eco-friendly solvent of ethanol/H2O using acidic IL catalyst. Under mild condition of 200 °C for 30 min, 97.5% bagasse is converted with 66.46% of volatile product yield. The results of product analyses using GC–MS, GPC and FT-IR demonstrate that both carbohydrate and lignin are simultaneously degraded. Furthermore, the catalyst shows excellent recyclability, where no significant activity loss is exhibited after five runs. However, it should be noticed that the wide product distribution results in a significant difficulty in separation. Therefore, novel strategies for selective depolymerization of biomass all-components are highly desired. Acknowledgments The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (No. 51306191 and 21503050), the Construction Project of Key Laboratories from the Education Department of Guizhou Province (No. QJHKY[2015] 329) and the start-up fund from Guizhou Education University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2016.04. 089.
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