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Selective conversion of cellulose to levulinic acid via microwave-assisted synthesis in ionic liquids
Bioresource Technology 129 (2013) 616–619
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Selective conversion of cellulose to levulinic acid via microwave-assisted synthesis in ionic liquids Huifang Ren, Yonggui Zhou, Li Liu ⇑ Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" SO3H-functionalized ILs selectively
"
" " "
catalyzed cellulose conversion to levulinic acid. Microwave irradiation improved production of levulinic acid from cellulose. The highest yield of levulinic acid was 55.0%. Catalytic activities of SFILs depend on anions. Further esterification enabled product separation and SFIL recovery.
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 18 December 2012 Accepted 19 December 2012 Available online 26 December 2012 Keywords: Biomass Cellulose Ionic liquids Levulinic acid Microwave
a b s t r a c t A highly selective approach to produce levulinic acid from cellulose was developed via microwaveassisted synthesis in SO3H-functionalized ionic liquids (SFILs). The effects of reaction conditions and ionic liquid structures on the yield of levulinic acid have been investigated, where the highest yield of 55.0% was obtained. The catalytic activities of SFILs depend on the anions and decrease in the order: HSO4 > CH3SO3 > H2PO4 , which is in good agreement with their acidity order. The SFILs are efficient catalysts for cellulose conversion into levulinic acid and the subsequent esterification, which facilitates the separation of product and reuse of ionic liquids. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Diminishing fossil resources and increasing concern about sustainable development have prompted the research on production of liquid hydrocarbon fuels and chemicals from biomass, which is the only renewable resource of fixed carbon (Ragauskas et al., 2006). As the most abundant biomass and sustainable raw materials, cellulose (Klemm et al., 2005) can be utilized to produce platform chemicals such as levulinic acid (LA) (Corma et al., 2007; Rackemann and Doherty, 2011), a versatile building block for fuel additives, polymer precursors, herbicides, pharmaceuticals, flavor ⇑ Corresponding author. Tel.: +86 411 84379903; fax: +86 411 84795945. E-mail address: [email protected] (L. Liu). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.12.132
substances and chemical intermediates. Recent work demonstrated that LA can serve as initial feedstock for existing petrochemical processing operations (Bozell, 2010), thus production of LA from cellulose has attracted considerable attention and become one of the key steps for the biomass refining. However, cellulose is generally regarded as a difficult material to work with due to its densely packed structure and insolubility in water (Jarvis, 2003). Various catalytic systems have been attempted to convert cellulose into LA directly, including mineral acids (Girisuta et al., 2007), metal chlorides (Seri et al., 2002), solid acid catalysts (Zakzeski et al., 2012) and acidic polymers (Vyver et al., 2011), whereas the limiting issues were focused on improving the selectivity of LA and developing efficient strategies for product separation and catalyst recovery.
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H. Ren et al. / Bioresource Technology 129 (2013) 616–619
Ionic liquids have been widely recognized as green reaction media due to their negligible volatility and excellent thermal stability (Welton, 1999). Vast combinations of cations and anions could be used to design tailor-made ionic liquids for many different performances. SO3H-functionalized ionic liquids (SFILs) with strong Brønsted acidity were firstly reported by Davis and coworkers (Cole et al., 2002), which have large potentials in replacing conventional acidic catalysts for they are flexible, recyclable, and could be used as dual solvents and catalysts. Tao et al. (2011) found that MnCl2 in SO3H-functionalized ionic liquids were effective catalysts for the production of 5-hydroxymethylfurfural (HMF) and furfural from cellulose, whereas levulinic acid was obtained as a side product. Herein for the first time, SO3H-functionalized ionic liquids such as 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ([C3SO3Hmim]HSO4) were employed in the selective conversion of cellulose to levulinic acid under microwave (MW) irradiation, whereafter the reaction yield was quantitatively determined by 1H NMR with ionic liquid as internal standard. The effects of reaction conditions and SFILs structures on the yield of levulinic acid were studied in detail.
2. Methods Microcrystalline cellulose with an average particle size of 50 lm was purchased from Acros Organics (USA) and used as received. Ethyl levulinate (EL) was obtained from TCI Chemical Co. (Japan). 1,3-Propane sultone, 1,4-butane sultone and 1-methylimidazole were freshly distilled prior to use. The ionic liquids including 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ([C3SO3 Hmim]HSO4), 1-methyl-3-(3-sulfopropyl)imidazolium dihydrogen phosphate ([C3SO3Hmim]H2PO4), 1-methyl-3-(3-sulfopropyl)imidazolium methanesulfonate ([C3SO3Hmim]CH3SO3), 1-methyl-3(4-sulfobutyl)imidazolium hydrogen sulfate ([C4SO3Hmim]HSO4), N-(3-sulfopropyl)pyridinium hydrogen sulfate ([C3SO3HPy]HSO4) and N,N,N-trimethyl-N-(3-sulfopropyl)ammonium hydrogen sulfate ([C3SO3HN111]HSO4) were synthesized by acidification of the respective zwitterions according to literature (Cole et al., 2002), and characterized by NMR before use (Supplementary data). 1H NMR and 13C NMR spectra were recorded on Bruker DRX-400 and Avance III 500 MHz spectrometers. Microwave experiments were realized in the MicroSYNTH plus microwave system (Milestone, Italy). UV–vis spectra were recorded on UV-2550 spectrophotometer (Shimadzu, Japan). The gas chromatography analysis was carried on Agilent 7890A with a FID detector and a HP-5 capillary column (30.0 m 320 lm 0.25 lm). In a typical reaction, cellulose (250 mg), ionic liquid (3.3 mmol) and de-ionized water (2.000 g) were mixed in a 50 mL quartz tube and the mixture was heated in MicroSYNTH plus at 800 W, 160 °C for 30 min. After filtration of insolubles and removal of water, the crude products were analyzed by 1H NMR using ionic liquid as internal standard. The yields of products were calculated from the equation: yield (%) = (mol of the product)/(mol of glucose unit in cellulose) 100%. The ionic liquids were reused as following. In the first run, the reaction of cellulose (0.400 g), [C3SO3Hmim]HSO4 (1.000 g, 3.3 mmol) and de-ionized water (2.000 g) was heated in MicroSYNTH plus at 800 W, 160 °C for 30 min. After filtration and removal of water, ethanol (1 mL, 17 mmol) was added to the mixture and the esterification reaction was carried out in an oil bath of 100 °C for 1 h. Afterwards, ethyl levulinate (EL) was extracted by n-hexane 5 mL 4, and then subjected for GC analysis. The lower layer of ionic liquid layer was separated, dried under high vacuum and reused in the second and third run as above. The yields of EL were calculated from the equation: yield (%) = (mol of EL)/(mol of glucose unit in cellulose) 100%.
Table 1 The effects of reaction time and temperature.a Entry
Time (min)
Temperature (°C)
1 2 3 4 5 6 7 8 9 10 11
5 10 20 30 40 30 30 30 30 30 30
160 160 160 160 160 80 100 120 140 150 170
Yield (%) LA
Glucose
15.7 21.1 35.1 44.5 34.8 0 0 0.3 9.7 38.2 43.2
20.9 20.5 0 0 0 0 4.7 15.6 22.9 14.8 0
a Reaction condition: 250 mg cellulose, 1.000 g [C3SO3Hmim]HSO4, 2.000 g H2O, MW.
3. Results and discussion The selectivity and yield of the products vary during the reaction course (Table 1). The yield of levulinic acid rose up with the reaction time increasing from 5 to 30 min and decreased afterwards. On the other hand, glucose was formed in the yield of 20.9% at 5 min and converted totally after 20 min. Therefore, the optimum reaction time was determined to be 30 min with 44.5% yield of levulinic acid. The yield of levulinic acid increased at higher temperature. As shown in Table 1, levulinic acid was not formed in the temperature range of 80 to 120 °C, and reached the maximum yield at 160 °C. On the contrary, glucose was formed exclusively below the temperature of 120 °C, but could not be detected above 160 °C. Apparently higher temperature is in favor of levulinic acid rather than glucose. Fig. 1a shows the effect of ionic liquid dosage on the yield of levulinic acid. Neither levulinic acid nor glucose could be detected in the absence of SFIL, indicating that SFIL acts as essential catalyst in the conversion of cellulose. The yield of levulinic acid increased and finally levelled off when more than 1 g of ionic liquid was added. While glucose reached maximum yield of 34.2% with the addition of 100 mg of ionic liquid and was transformed completely with the addition of more than 1 g of ionic liquid. Fig. 1b shows the effect of water amount on the yield of levulinic acid. In the presence of less than 2 g of water, the yield of levulinic acid increased to 44.5% and no glucose detected. In the presence of more than 2 g of water, the yield of levulinic acid decreased, whereas the yield of glucose rose up. Fig. 1c shows the effect of initial cellulose intake on the yield of levulinic acid. Levulinic acid reached the highest yield of 55.0% at an initial cellulose intake of 50 mg. Only trace amount of glucose could be detected at initial cellulose intake over 400 mg. In order to establish the relationship between SFILs structures and their catalytic activities, the Brønsted acidities of SFILs were determined by Hammett method (Gu et al., 2005) using UV–vis spectrophotometer with 4-nitroaniline as indicator. The maximal absorbance of the unprotonated form of the indicator was observed at 381 nm in H2O, which decreased after adding acidic ionic liquid. Under the same concentration of 4-nitroaniline (4 mg/L) and ionic liquids (50 mmol/L) in H2O, Hammett acidity functions (H0) of SFILs with different cations and anions were calculated and compared thereafter. As shown in Table 2, for SFILs with the same cationic structure, the acidities of the SFILs depend on the anions and decrease in the order: HSO4 > CH3SO3 > H2PO4 . Different types of cations (imidazolium, pyridinium and ammonium) have negligible effect on the acidity of SFILs, and no obvious variation could be either observed when the alkyl chain near the sulfonic group elongates from propyl to butyl. The activity order of SFILs to catalyze the conversion of cellulose into LA is in good
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In the catalytic process towards the target product of levulinic acid, glucose and HMF have been observed under incomplete conversion of cellulose, implying the reaction pathway is very likely that cellulose depolymerizes to glucose first, which transforms to HMF and further dehydrates to form levulinic acid and formic acid (Girisuta et al., 2007; Ragauskas et al., 2006). Efficient catalytic systems for selective formation of levulinic acid are highly desirable. Recent catalytic system in the presence of AlCl3 (Yang et al., 2012) showed high activity for the conversion of glucose into furans (HMF and 5-ethoxymethylfurfural) but low activity for the subsequent formation of LAs. In order to catalyze multistep process towards levulinic acid, Ya’aini et al. (2012) have developed a strategy of modifying HY zeolite by introducing CrCl3 to form a hybrid catalyst. In the current catalytic system of SFILs, as only trace amount of HMF could be detected (<0.1%), suggesting the dehydration of HMF to levulinic acid is faster than the transformation of glucose to HMF. Combination of SFILs and microwave irradiation (Liu et al., 2010; Polshettiwar and Varma, 2008) provides an efficient approach to produce levulinic acid from cellulose with high selectivity, whereabout both intermediates of glucose and HMF (Amarasekara and Owereh, 2009; Tao et al., 2011) could be completely converted. In contrast, 1.2% of levulinic acid and 11.9% of glucose were obtained under the same reaction condition except conventional heating source of oil bath. Practically, it is necessary to separate the product and reuse ionic liquids. As catalyzed by the inherent SFIL, the reaction mixture was subjected to esterification upon addition of ethanol, resulting in an upper phase containing ethyl levulinate (EL) and lower phase containing ionic liquid. Through facile phase separation, the recyclability of ionic liquid has been demonstrated over three cycles with the EL yields of 27.8%, 26.1% and 27.7%, respectively. Another two batches also verified the recyclability of ionic liquids without significant loss of catalytic activity. The overall yield of EL is in the range of 26–34% based on glucose unit in cellulose, which is comparable to that of ethanolysis approach (Mascal and Nikitin, 2010). 4. Conclusions
Fig. 1. The effects of (a) [C3SO3Hmim]HSO4 dosage (250 mg cellulose, 2.000 g H2O, MW, 160 °C, 30 min), (b) H2O amount (250 mg cellulose, 1.000 g [C3SO3Hmim]HSO4, MW, 160 °C, 30 min) and (c) cellulose intake (1.000 g [C3SO3Hmim]HSO4, 2.000 g H2O, MW, 160 °C, 30 min). Table 2 Hammet acidity (H0) and catalytic activity of SFILs.a SFILs
– [C3SO3Hmim]H2PO4 [C3SO3Hmim]CH3SO3 [C3SO3Hmim]HSO4 [C4SO3Hmim]HSO4 [C3SO3HPy]HSO4 [C3SO3HN111]HSO4
Amax
0.38 0.31 0.26 0.24 0.23 0.23 0.23
[I] (%)
100 82 68 63 61 61 61
[IH+] (%)
0 18 32 37 39 39 39
In summary, for the first time, a highly selective approach to direct conversion of cellulose into LA was developed via microwaveassisted synthesis in SO3H-functionalized ionic liquids, where the highest yield of 55.0% was obtained. SO3H-functionalized ionic liquids are efficient catalysts for cellulose conversion into LA and the subsequent esterification, which facilitates the separation of ethyl levulinate and reuse of ionic liquids. This approach offers significant improvements for production of LA from cellulose, regarding high selectivity, product separation and catalyst recovery, thereby provides an environmentally friendly route to biomass utilization. Acknowledgements
H0b
– 1.65 1.32 1.22 1.18 1.18 1.18
Yield (%) LA
Glucose
0 3.5 36.3 44.5 41.4 40.5 43.2
0 10.5 0 0 0 0 0
a Reaction condition: 250 mg cellulose, 3.3 mmol SFIL, 2.000 g H2O, MW, 160 °C, 30 min. b H0 = pK(I)aq + log ([I]/[IH+]). Indicator: 4-nitroaniline (pK(I)aq = 0.99).
agreement with their acidity order, indicating the determining role of the catalytic acidity.
We thank DICP-100-Talents Program and financial support. 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.2012.12. 132. References Amarasekara, A.S., Owereh, O.S., 2009. Hydrolysis and decomposition of cellulose in brönsted acidic ionic liquids under mild conditions. Ind. Eng. Chem. Res. 48, 10152–10155. Bozell, J.J., 2010. Connecting biomass and petroleum processing with a chemical bridge. Science 329, 522–523.
H. Ren et al. / Bioresource Technology 129 (2013) 616–619 Cole, A.C., Jensen, J.L., Ntai, I., Tran, K.L.T., Weaver, K.J., Forbes, D.C., Davis Jr., J.H., 2002. Novel brønsted acidic ionic liquids and their use as dual solvent-catalysts. J. Am. Chem. Soc. 124, 5962–5963. Corma, A., Iborra, S., Velty, A., 2007. Chemical routes for the transformation of biomass into chemicals. Chem. Rev. 107, 2411–2502. Girisuta, B., Janssen, L.P.B.M., Heeres, H.J., 2007. Kinetic study on the acid-catalyzed hydrolysis of cellulose to levulinic acid. Ind. Eng. Chem. Res. 46, 1696–1708. Gu, Y., Zhang, J., Duan, Z., Deng, Y., 2005. Pechmann reaction in nonchloroaluminate acidic ionic liquids under solvent-free conditions. Adv. Synth. Catal. 347, 512–516. Jarvis, M., 2003. Chemistry: cellulose stacks up. Nature 426, 611–612. Klemm, D., Heublein, B., Fink, H.-P., Bohn, A., 2005. Cellulose: fascinating biopolymer and sustainable raw material. Angew. Chem. Int. Ed. 44, 3358– 3393. Liu, L., Jiang, X., Zhang, J., 2010. Anion-linked cucurbit[6]uril frameworks formed by microwave-assisted synthesis in ionic liquids. CrystEngComm 12, 3445–3447. Mascal, M., Nikitin, E.B., 2010. Comment on processes for the direct conversion of cellulose or cellulosic biomass into levulinate esters. ChemSusChem 3, 1349– 1351. Polshettiwar, V., Varma, R.S., 2008. Microwave-assisted organic synthesis and transformations using benign reaction media. Acc. Chem. Res. 41, 629–639. Rackemann, D.W., Doherty, W.O.S., 2011. The conversion of lignocellulosics to levulinic acid. Biofuels, Bioprod. Biorefin. 5, 198–214.
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Ragauskas, A.J., Williams, C.K., Davison, B.H., Britovsek, G., Cairney, J., Eckert, C.A., Frederick Jr., W.J., Hallett, J.P., Leak, D.J., Liotta, C.L., Mielenz, J.R., Murphy, R., Templer, R., Tschaplinski, T., 2006. The path forward for biofuels and biomaterials. Science 311, 484–489. Seri, K., Sakaki, T., Shibata, M., Inoue, Y., Ishida, H., 2002. Lanthanum(III)-catalyzed degradation of cellulose at 250 °C. Bioresour. Technol. 81, 257–260. Tao, F., Song, H., Chou, L., 2011. Hydrolysis of cellulose in SO3H-functionalized ionic liquids. Bioresour. Technol. 102, 9000–9006. Vyver, S.V., Thomas, J., Geboers, J., Keyzer, S., Smet, M., Dehaen, W., Jacobs, P.A., Sels, B.F., 2011. Catalytic production of levulinic acid from cellulose and other biomass-derived carbohydrates with sulfonated hyperbranched poly(arylene oxindole)s. Energy Environ. Sci. 4, 3601–3610. Welton, T., 1999. Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev. 99, 2071–2083. Ya’aini, N., Amin, N.A.S., Asmadi, M., 2012. Optimization of levulinic acid from lignocellulosic biomass using a new hybrid catalyst. Bioresour. Technol. 116, 58–65. Yang, Y., Hu, C., Abu-Omar, M.M., 2012. Conversion of glucose into furans in the presence of AlCl3 in an ethanol–water solvent system. Bioresour. Technol. 116, 190–194. Zakzeski, J., Grisel, R.J.H., Smit, A.T., Weckhuysen, B.M., 2012. Solid acid-catalyzed cellulose hydrolysis monitored by in situ ATR-IR spectroscopy. ChemSusChem 5, 430–437.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Selective conversion of cellulose to levulinic acid via microwave-assisted synthesis in ionic liquids Huifang Ren, Yonggui Zhou, Li Liu ⇑ Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" SO3H-functionalized ILs selectively
"
" " "
catalyzed cellulose conversion to levulinic acid. Microwave irradiation improved production of levulinic acid from cellulose. The highest yield of levulinic acid was 55.0%. Catalytic activities of SFILs depend on anions. Further esterification enabled product separation and SFIL recovery.
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 18 December 2012 Accepted 19 December 2012 Available online 26 December 2012 Keywords: Biomass Cellulose Ionic liquids Levulinic acid Microwave
a b s t r a c t A highly selective approach to produce levulinic acid from cellulose was developed via microwaveassisted synthesis in SO3H-functionalized ionic liquids (SFILs). The effects of reaction conditions and ionic liquid structures on the yield of levulinic acid have been investigated, where the highest yield of 55.0% was obtained. The catalytic activities of SFILs depend on the anions and decrease in the order: HSO4 > CH3SO3 > H2PO4 , which is in good agreement with their acidity order. The SFILs are efficient catalysts for cellulose conversion into levulinic acid and the subsequent esterification, which facilitates the separation of product and reuse of ionic liquids. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Diminishing fossil resources and increasing concern about sustainable development have prompted the research on production of liquid hydrocarbon fuels and chemicals from biomass, which is the only renewable resource of fixed carbon (Ragauskas et al., 2006). As the most abundant biomass and sustainable raw materials, cellulose (Klemm et al., 2005) can be utilized to produce platform chemicals such as levulinic acid (LA) (Corma et al., 2007; Rackemann and Doherty, 2011), a versatile building block for fuel additives, polymer precursors, herbicides, pharmaceuticals, flavor ⇑ Corresponding author. Tel.: +86 411 84379903; fax: +86 411 84795945. E-mail address: [email protected] (L. Liu). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.12.132
substances and chemical intermediates. Recent work demonstrated that LA can serve as initial feedstock for existing petrochemical processing operations (Bozell, 2010), thus production of LA from cellulose has attracted considerable attention and become one of the key steps for the biomass refining. However, cellulose is generally regarded as a difficult material to work with due to its densely packed structure and insolubility in water (Jarvis, 2003). Various catalytic systems have been attempted to convert cellulose into LA directly, including mineral acids (Girisuta et al., 2007), metal chlorides (Seri et al., 2002), solid acid catalysts (Zakzeski et al., 2012) and acidic polymers (Vyver et al., 2011), whereas the limiting issues were focused on improving the selectivity of LA and developing efficient strategies for product separation and catalyst recovery.
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H. Ren et al. / Bioresource Technology 129 (2013) 616–619
Ionic liquids have been widely recognized as green reaction media due to their negligible volatility and excellent thermal stability (Welton, 1999). Vast combinations of cations and anions could be used to design tailor-made ionic liquids for many different performances. SO3H-functionalized ionic liquids (SFILs) with strong Brønsted acidity were firstly reported by Davis and coworkers (Cole et al., 2002), which have large potentials in replacing conventional acidic catalysts for they are flexible, recyclable, and could be used as dual solvents and catalysts. Tao et al. (2011) found that MnCl2 in SO3H-functionalized ionic liquids were effective catalysts for the production of 5-hydroxymethylfurfural (HMF) and furfural from cellulose, whereas levulinic acid was obtained as a side product. Herein for the first time, SO3H-functionalized ionic liquids such as 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ([C3SO3Hmim]HSO4) were employed in the selective conversion of cellulose to levulinic acid under microwave (MW) irradiation, whereafter the reaction yield was quantitatively determined by 1H NMR with ionic liquid as internal standard. The effects of reaction conditions and SFILs structures on the yield of levulinic acid were studied in detail.
2. Methods Microcrystalline cellulose with an average particle size of 50 lm was purchased from Acros Organics (USA) and used as received. Ethyl levulinate (EL) was obtained from TCI Chemical Co. (Japan). 1,3-Propane sultone, 1,4-butane sultone and 1-methylimidazole were freshly distilled prior to use. The ionic liquids including 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ([C3SO3 Hmim]HSO4), 1-methyl-3-(3-sulfopropyl)imidazolium dihydrogen phosphate ([C3SO3Hmim]H2PO4), 1-methyl-3-(3-sulfopropyl)imidazolium methanesulfonate ([C3SO3Hmim]CH3SO3), 1-methyl-3(4-sulfobutyl)imidazolium hydrogen sulfate ([C4SO3Hmim]HSO4), N-(3-sulfopropyl)pyridinium hydrogen sulfate ([C3SO3HPy]HSO4) and N,N,N-trimethyl-N-(3-sulfopropyl)ammonium hydrogen sulfate ([C3SO3HN111]HSO4) were synthesized by acidification of the respective zwitterions according to literature (Cole et al., 2002), and characterized by NMR before use (Supplementary data). 1H NMR and 13C NMR spectra were recorded on Bruker DRX-400 and Avance III 500 MHz spectrometers. Microwave experiments were realized in the MicroSYNTH plus microwave system (Milestone, Italy). UV–vis spectra were recorded on UV-2550 spectrophotometer (Shimadzu, Japan). The gas chromatography analysis was carried on Agilent 7890A with a FID detector and a HP-5 capillary column (30.0 m 320 lm 0.25 lm). In a typical reaction, cellulose (250 mg), ionic liquid (3.3 mmol) and de-ionized water (2.000 g) were mixed in a 50 mL quartz tube and the mixture was heated in MicroSYNTH plus at 800 W, 160 °C for 30 min. After filtration of insolubles and removal of water, the crude products were analyzed by 1H NMR using ionic liquid as internal standard. The yields of products were calculated from the equation: yield (%) = (mol of the product)/(mol of glucose unit in cellulose) 100%. The ionic liquids were reused as following. In the first run, the reaction of cellulose (0.400 g), [C3SO3Hmim]HSO4 (1.000 g, 3.3 mmol) and de-ionized water (2.000 g) was heated in MicroSYNTH plus at 800 W, 160 °C for 30 min. After filtration and removal of water, ethanol (1 mL, 17 mmol) was added to the mixture and the esterification reaction was carried out in an oil bath of 100 °C for 1 h. Afterwards, ethyl levulinate (EL) was extracted by n-hexane 5 mL 4, and then subjected for GC analysis. The lower layer of ionic liquid layer was separated, dried under high vacuum and reused in the second and third run as above. The yields of EL were calculated from the equation: yield (%) = (mol of EL)/(mol of glucose unit in cellulose) 100%.
Table 1 The effects of reaction time and temperature.a Entry
Time (min)
Temperature (°C)
1 2 3 4 5 6 7 8 9 10 11
5 10 20 30 40 30 30 30 30 30 30
160 160 160 160 160 80 100 120 140 150 170
Yield (%) LA
Glucose
15.7 21.1 35.1 44.5 34.8 0 0 0.3 9.7 38.2 43.2
20.9 20.5 0 0 0 0 4.7 15.6 22.9 14.8 0
a Reaction condition: 250 mg cellulose, 1.000 g [C3SO3Hmim]HSO4, 2.000 g H2O, MW.
3. Results and discussion The selectivity and yield of the products vary during the reaction course (Table 1). The yield of levulinic acid rose up with the reaction time increasing from 5 to 30 min and decreased afterwards. On the other hand, glucose was formed in the yield of 20.9% at 5 min and converted totally after 20 min. Therefore, the optimum reaction time was determined to be 30 min with 44.5% yield of levulinic acid. The yield of levulinic acid increased at higher temperature. As shown in Table 1, levulinic acid was not formed in the temperature range of 80 to 120 °C, and reached the maximum yield at 160 °C. On the contrary, glucose was formed exclusively below the temperature of 120 °C, but could not be detected above 160 °C. Apparently higher temperature is in favor of levulinic acid rather than glucose. Fig. 1a shows the effect of ionic liquid dosage on the yield of levulinic acid. Neither levulinic acid nor glucose could be detected in the absence of SFIL, indicating that SFIL acts as essential catalyst in the conversion of cellulose. The yield of levulinic acid increased and finally levelled off when more than 1 g of ionic liquid was added. While glucose reached maximum yield of 34.2% with the addition of 100 mg of ionic liquid and was transformed completely with the addition of more than 1 g of ionic liquid. Fig. 1b shows the effect of water amount on the yield of levulinic acid. In the presence of less than 2 g of water, the yield of levulinic acid increased to 44.5% and no glucose detected. In the presence of more than 2 g of water, the yield of levulinic acid decreased, whereas the yield of glucose rose up. Fig. 1c shows the effect of initial cellulose intake on the yield of levulinic acid. Levulinic acid reached the highest yield of 55.0% at an initial cellulose intake of 50 mg. Only trace amount of glucose could be detected at initial cellulose intake over 400 mg. In order to establish the relationship between SFILs structures and their catalytic activities, the Brønsted acidities of SFILs were determined by Hammett method (Gu et al., 2005) using UV–vis spectrophotometer with 4-nitroaniline as indicator. The maximal absorbance of the unprotonated form of the indicator was observed at 381 nm in H2O, which decreased after adding acidic ionic liquid. Under the same concentration of 4-nitroaniline (4 mg/L) and ionic liquids (50 mmol/L) in H2O, Hammett acidity functions (H0) of SFILs with different cations and anions were calculated and compared thereafter. As shown in Table 2, for SFILs with the same cationic structure, the acidities of the SFILs depend on the anions and decrease in the order: HSO4 > CH3SO3 > H2PO4 . Different types of cations (imidazolium, pyridinium and ammonium) have negligible effect on the acidity of SFILs, and no obvious variation could be either observed when the alkyl chain near the sulfonic group elongates from propyl to butyl. The activity order of SFILs to catalyze the conversion of cellulose into LA is in good
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In the catalytic process towards the target product of levulinic acid, glucose and HMF have been observed under incomplete conversion of cellulose, implying the reaction pathway is very likely that cellulose depolymerizes to glucose first, which transforms to HMF and further dehydrates to form levulinic acid and formic acid (Girisuta et al., 2007; Ragauskas et al., 2006). Efficient catalytic systems for selective formation of levulinic acid are highly desirable. Recent catalytic system in the presence of AlCl3 (Yang et al., 2012) showed high activity for the conversion of glucose into furans (HMF and 5-ethoxymethylfurfural) but low activity for the subsequent formation of LAs. In order to catalyze multistep process towards levulinic acid, Ya’aini et al. (2012) have developed a strategy of modifying HY zeolite by introducing CrCl3 to form a hybrid catalyst. In the current catalytic system of SFILs, as only trace amount of HMF could be detected (<0.1%), suggesting the dehydration of HMF to levulinic acid is faster than the transformation of glucose to HMF. Combination of SFILs and microwave irradiation (Liu et al., 2010; Polshettiwar and Varma, 2008) provides an efficient approach to produce levulinic acid from cellulose with high selectivity, whereabout both intermediates of glucose and HMF (Amarasekara and Owereh, 2009; Tao et al., 2011) could be completely converted. In contrast, 1.2% of levulinic acid and 11.9% of glucose were obtained under the same reaction condition except conventional heating source of oil bath. Practically, it is necessary to separate the product and reuse ionic liquids. As catalyzed by the inherent SFIL, the reaction mixture was subjected to esterification upon addition of ethanol, resulting in an upper phase containing ethyl levulinate (EL) and lower phase containing ionic liquid. Through facile phase separation, the recyclability of ionic liquid has been demonstrated over three cycles with the EL yields of 27.8%, 26.1% and 27.7%, respectively. Another two batches also verified the recyclability of ionic liquids without significant loss of catalytic activity. The overall yield of EL is in the range of 26–34% based on glucose unit in cellulose, which is comparable to that of ethanolysis approach (Mascal and Nikitin, 2010). 4. Conclusions
Fig. 1. The effects of (a) [C3SO3Hmim]HSO4 dosage (250 mg cellulose, 2.000 g H2O, MW, 160 °C, 30 min), (b) H2O amount (250 mg cellulose, 1.000 g [C3SO3Hmim]HSO4, MW, 160 °C, 30 min) and (c) cellulose intake (1.000 g [C3SO3Hmim]HSO4, 2.000 g H2O, MW, 160 °C, 30 min). Table 2 Hammet acidity (H0) and catalytic activity of SFILs.a SFILs
– [C3SO3Hmim]H2PO4 [C3SO3Hmim]CH3SO3 [C3SO3Hmim]HSO4 [C4SO3Hmim]HSO4 [C3SO3HPy]HSO4 [C3SO3HN111]HSO4
Amax
0.38 0.31 0.26 0.24 0.23 0.23 0.23
[I] (%)
100 82 68 63 61 61 61
[IH+] (%)
0 18 32 37 39 39 39
In summary, for the first time, a highly selective approach to direct conversion of cellulose into LA was developed via microwaveassisted synthesis in SO3H-functionalized ionic liquids, where the highest yield of 55.0% was obtained. SO3H-functionalized ionic liquids are efficient catalysts for cellulose conversion into LA and the subsequent esterification, which facilitates the separation of ethyl levulinate and reuse of ionic liquids. This approach offers significant improvements for production of LA from cellulose, regarding high selectivity, product separation and catalyst recovery, thereby provides an environmentally friendly route to biomass utilization. Acknowledgements
H0b
– 1.65 1.32 1.22 1.18 1.18 1.18
Yield (%) LA
Glucose
0 3.5 36.3 44.5 41.4 40.5 43.2
0 10.5 0 0 0 0 0
a Reaction condition: 250 mg cellulose, 3.3 mmol SFIL, 2.000 g H2O, MW, 160 °C, 30 min. b H0 = pK(I)aq + log ([I]/[IH+]). Indicator: 4-nitroaniline (pK(I)aq = 0.99).
agreement with their acidity order, indicating the determining role of the catalytic acidity.
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