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Ind. Eng. Chem. Res. 2009, 48, 10152–10155
Hydrolysis and Decomposition of Cellulose in Bro¨nsted Acidic Ionic Liquids Under Mild Conditions Ananda S. Amarasekara* and Onome S. Owereh Department of Chemistry, Prairie View A&M UniVersity, Prairie View, Texas 77446
Cellulose dissolves in Bro¨nsted acidic ionic liquids 1-(1-propylsulfonic)-3-methylimidazolium chloride and 1-(1-butylsulfonic)-3-methylimidazolium chloride up to 20 g/100 g ionic liquid by gentle mixing at room temperature. Hydrolysis of cellulose can be carried out in these ionic liquid solutions by the addition of 2.0 equiv of water per glucose unit of cellulose and heating the solution at 70 °C and at atmospheric pressure with or without preheating to give glucose along with other reducing sugars. The hydrolysis of Sigmacell cellulose (DP ∼ 450) in 1-(1-propylsulfonic)-3-methylimidazolium chloride produced the highest total reducing sugar (62%) and glucose (14%) yields and was attained in 1 h of preheating at 70 °C and 30 min of heating at 70 °C, after adding water. 1. Introduction Cellulose, the most abundant form of biomass, has been the target of passionate research efforts for over a century,1-3 and these efforts have been intensified recently due to the search for nonfood sources for bioethanol production.4-8 Hydrolysis of cellulose to fermentable sugars is an essential step in any practical cellulosic-ethanol process before the microbial action to produce ethanol. Two methods, i.e. acid hydrolysis and enzymatic hydrolysis are currently known for cellulosic biomass hydrolysis and both methods have their deficiencies, such as requiring a drastic pretreatment at high pressure and temperature to disrupt the strong hydrogen bonding network before the hydrolysis. Furthermore, commercialization of the enzymatic process is hindered by the prohibitive cost of the currently available enzyme preparations as well.9 Therefore, there is an urgent demand for more efficient and simple green technologies for the degradation of cellulose to produce cellulosic ethanol as an alternative to corn ethanol. In 2002, Rogers et al. reported10 the use of 1-n-butyl-3-methylimidazolium ([C4mim]+) salts, which are room temperature ionic liquids for the dissolution of cellulose. They showed that high molecular weight pulp cellulose (DP ∼ 1000) slowly dissolves (5-10 g/100 g ionic liquid) in [C4mim]+ ionic liquids with Cl-, Br-, and SCNanions, when heated to 100 °C, yielding viscous solutions. Since this first report on the dissolution of cellulose, considerable effort has been devoted to improve the solubility and to build up on this initial discovery.11-13 For instance, the ionic liquids can also be applied as reaction media for the synthesis of cellulose derivatives like carboxymethyl cellulose and cellulose acetate.14-17 Furthermore, the regeneration of cellulose from ionic liquid solutions has been applied to uses like fabrication into films, gels,18 and composite materials.19 Even though the cellulose ionic liquid system has attracted attention as a breakthrough, ionic liquid medium has been studied only once for the hydrolysis or degradation of cellulose. In that study, Zhao et al. reported20,21 that cellulose could be hydrolyzed by adding catalytic amounts of sulfuric acid to the cellulose-ionic liquid solution. In this important and recent (2007) study, they found that cellulose-[C4mim]+Cl- solution with H2SO4/cellulose mass ratio of 0.92 produces total reducing * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +1 936 261 3107. Fax: +1 936 261 3117.
sugars (TRS) and glucose in 59% and 36% yields respectively, within 3 min. Further reducing the acid/cellulose mass ratio to 0.46 produced higher yields after 42 min, and when the mass ratio was dropped to 0.11, the yields of TRS and glucose reached 77% and 43%, respectively, in 9 h. This is an exciting finding because this reaction system was operated under mild conditions using essentially a catalytic amount of H2SO4 and no pretreatment was required. As any fermentation of glucose and reducing sugars by microbial fermentation requires the separations of both the acid catalyst and the ionic liquid, we have envisioned that incorporation of the acidic function into the ionic liquid would yield a more efficient process and we have investigated the use Bro¨nsted acidic ionic liquids for the dissolution and hydrolysis of cellulose. These Bro¨nsted acidic ionic liquids can behave as both the solvent and catalyst as well; additionally, no neutralization and separation of the acid catalyst is required, and there is no waste in acid, as the acid is in the solvent itself. Furthermore, a higher concentration of -SO3H active sites is expected to accelerate the reaction and lower the operating temperature, thus saving energy. In this communication, we report the first application of Bro¨nsted acidic ionic liquids for the dissolution and hydrolysis of cellulose under moderate reaction temperatures and atmospheric pressure. Three different types of Bro¨nsted acidic ionic liquids (Figure 1) based on methyl imidazolium (1a,b),22 pyridinium (2),22 and triethanolammonium (3)23 were studied for their ability to dissolve and hydrolyze cellulose under mild reaction temperatures. 2. Experimental Section 2.1. Materials and Instrumentation. R-Cellulose (DP ∼ 100), microcrystalline (MC)-cellulose (DP ∼ 240), Sigmacell cellulose (DP ∼ 450), 1-methylimidazole, 1,3-propanesultone,
Figure 1. Bro¨nsted acidic ionic liquids.
10.1021/ie901047u CCC: $40.75 2009 American Chemical Society Published on Web 10/26/2009
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Table 1. Average Percent Yields of TRS and Glucose Produced in Duplicate Experimentsa temp (°C)/time (min) entry
IL/cellulose
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1a/R-cellulose 1a/ΜC-cellulose 1a/Sigmacell 1a/Sigmacell 1a/Sigmacell 1a/Sigmacell 1a/Sigmacell 1a/Sigmacell 1a/Sigmacell 1a/Sigmacell 1a/Sigmacell 1b/R-cellulose 1b/ΜC-cellulose 1b/Sigmacell 2/R-cellulose 2/ΜC-cellulose 2/Sigmacell 3/R-cellulose 3/ΜC-cellulose 3/Sigmacell
yield (%)
before adding H2O
after adding H2O
TRS
glucose
70/60 70/60 70/60
70/30 70/30 70/30 70/30 70/30 70/60 70/240 50/960 90/30 90/30 90/240 70/30 70/30 70/30 70/30 70/30 70/30 70/30 70/30 70/30
59 12 62 39 56 42 29 32 34 26 15 32 7 12 14 8 16 5 2 10
15 4 14 12 12 7 4 3 3 2 2
70/40 70/60 70/60 90/30 70/60 70/60 70/60 70/60 70/60 70/60 70/60 70/60 70/60
a 10% w/w cellulose in Bro¨nsted acidic ionic liquid solutions and 2.0 equiv of H2O per glucose unit of cellulose were added in all hydrolysis experiments.
1,4-butanesultone, pyridine, and triethanolamine were purchased from Aldrich Chemical Co. Bro¨nsted acidic ionic liquids 1a,b, 2, and 3 were prepared22,23 by condensation of the corresponding nitrogen bases with sultones, acidification of the resulting salts with conc. HCl, and removal of water under vacuum. Ionic liquids with chloride ions as the anions were chosen because Cl- ion containing ionic liquids have a good cellulose dissolution capacity10 without derivatization. Meanwhile, other cellulose dissolving ionic liquids like alkylimidazolium acetates are known24 to acetylate cellulose under acidic conditions. Total reducing sugars (TRS) and glucose concentrations in aqueous solutions were determined using a Carey 50 UV-vis spectrophotometer and 1 cm quartz cells. 2.2. General Experimental Procedure for Hydrolysis of Cellulose Samples Using Bro¨nsted Acidic Ionic Liquid Medium and Determination of TRS and Glucose. Cellulose (0.030 g, 0.185 mmol of glucose unit of cellulose) was dissolved in 0.300 g of ionic liquid by mixing with a glass rod at room temperature to give a clear solution. Then, the sample was treated with deionized water (6.7 µL, 2.0 equiv/glucose unit of cellulose) with or without preheating and was warmed in a thermostatted water bath for a predetermined time for the hydrolysis of cellulose. The reaction was quenched by diluting with 10.0 mL of deionized water and immediately neutralized by dropwise addition of 0.5 M aq NaOH. The resulting lightly turbid solution was transferred to a centrifuge tube and centrifuged at 3500 rpm for 6 min to precipitate the solids before total reducing sugar determination using the 3,4-dinitrosalicylic acid (DNS) method.25 The glucose formed was measured using glucose oxidase/peroxidase enzymatic assay26,27 using the Sigma-Aldrich Glu 20 kit. The percent yields of total reducing sugars and glucose formed during the hydrolysis are shown in Table 1. TRS Assay. A 2.50 mL portion of the clear solution from the centrifuge tube was transferred to a vial, and 0.50 mL of DNS reagent25 was added. The resulting solution was heated at 90 °C for 5.0 min to develop the red-orange color. Then the absorbance was measured at 540 nm using 1 cm quartz cells and Carey 50 UV-vis spectrophotometer. The TRS concentra-
Figure 2. Optical microscope images (×400) of dissolution of Sigmacell cellulose (DP ∼ 450) in 1-(1-propylsulfonic)-3-methylimidazolium chloride (1a) at room temperature (23 °C) and atmospheric pressure, after 0, 60, and 150 s.
tion in the solution was calculated by employing a standard curve prepared using glucose. Glucose Assay. A 0.10 mL portion of the clear solution from the centrifuge tube was transferred to a vial, 0.90 mL deionized water, and 2.0 mL of glucose assay reagent reagent26,27 (SigmaAldrich Glu 20 kit) were added. The resulting solution was heated at 37 °C for 30 min, the reaction was quenched by adding 2.0 mL of 12 N H2SO4 to give a pink solution. Then, the absorbance was measured at 540 nm using 1 cm quartz cells and Carey 50 UV-vis spectrophotometer. Glucose concentration in the solution was calculated by employing a standard curve prepared using glucose. 2.3. Measurement of Average Degree of Polymerization (DP) of the Precipitated Cellulose from Experiment 2.2, Entry 4 (Table 1). Precipitated cellulose in experiment 2.2, entry 4 (Table 1), was collected after centrifugation, repeatedly washed (dispersion in deionized water followed by centrifugationssix cycles) to remove traces of ionic liquid, and dried at 50 °C under vacuum for 24 h. The DP of this cellulose residue was calculated by using intrinsic viscosity of the cupriethylenediamine solution, employing the Mark-Houwink equation (1).28,29 The intrinsic viscosity of cellulose in 0.5 M cupriethylenediamine solution was measured according to ASTM standard method D1795-96. [η] ) K(DP)R
(1)
in which [η] is the intrinsic viscosity of cellulose, K is a constant 1.7, and R is a constant 0.8. The DP of the Sigmacell cellulose used for experiment 2.2 was also measured by the same method as the reference. 3. Results and Discussion Three different types of cellulose samples, R-cellulose (DP ∼ 100), MC-cellulose (DP ∼ 240), and Sigmacell cellulose (DP ∼ 450), were used in hydrolysis and dissolution studies. All three types of cellulose samples are found to dissolve in imidazolium type Bro¨nsted acidic ionic liquids (1a,b) at room temperature and atmospheric pressure. For example, we have observed that Sigmacell cellulose (DP ∼ 450, cat. no. S6790 from Sigma-Aldrich) completely dissolves in up to about 20 g of cellulose per 100 g of ionic liquid 1-(1-propylsulfonic)-3methylimidazolium chloride (1a) giving a clear solution at room temperature (23 °C) in 2.5 min, when subjected to gentle mixing with a glass rod. This dissolution capability is significantly higher than the reported dissolution of cellulose in the neutral ionic liquid, 1-n-butyl-3-methylimidazolium chloride, which is typically10 5-10 g/100 g, with prolonged heating at 100 °C. Optical microscopic images during the dissolution of Sigmacell cellulose in 1-(1-propylsulfonic)-3-methylimidazolium chloride (1a) are shown in the Figure 2. These cellulose samples were found to dissolve in pyridinium (2) and triethanolammonium
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based ionic liquids (3) when allowed to stand at room temperature for 24 h. Initial cellulose hydrolysis experiments shown in entries 1-3 (Table 1) were carried out using acidic ionic liquid 1a, under identical conditions to compare the hydrolysis of three types of cellulose samples. In these experiments, cellulose-ionic liquid solutions were heated for 60 min at 70 °C before adding 2.0 equiv of water and, then, heated for 30 min at 70 °C after adding water. In the next set of experiments (entries 4-11), the hydrolysis of Sigmacell cellulose was studied by varying the heating time and temperature conditions. Furthermore, entries 4, 8, 9, and 11 (Table 1) were carried out without preheating, i.e., 2.0 equiv of water were added immediately after 2 min mixing with a glass rod. Experiments in entries 12-20 were carried out under identical conditions to compare the catalytic activities of the ionic liquids 1b, 2, and 3, using three types of cellulose samples. Imidazolium acidic ionic liquid mediums (1a,b) gave better yields of TRS and glucose than pyridinium (2) ionic liquids as shown in Table 1. Triethanolammonium acidic ionic liquid (3) gave poor yields with all cellulose samples tested (entries 18-20). 1-(1-Propylsulfonic)-3-methylimidazolium chloride (1a) medium produced the highest yields of TRS and glucose with most of the cellulose samples studied. Both R-cellulose and Sigmacell cellulose produced moderate TRS yields in these experiments, whereas MC-cellulose generally produced lower yields in all of the hydrolysis experiments. The experiment with a 1a/Sigmacell cellulose solution showed the highest TRS (62%) and glucose (14%) yields, which were attained with 1 h preheating at 70 °C and 30 min heating at 70 °C, after adding water, as shown in entry 3. Heating the sample at a lower temperature for a longer time (entry 8) or shorter time at a higher temperature (entry 9) without preheating failed to give better yields, indicating the importance of preheating. Furthermore, longer heating times (entry 7) and higher temperatures (entries 10, 11) produced excessive charring of the sample, giving black residues, and thus lowering the TRS and glucose yields. Out of the four Bro¨nsted acidic ionic liquids studied, two imidazolium ionic liquids showed better catalytic activity, and among these, 1-(1-propylsulfonic)-3-methylimidazolium chloride (1a) medium produced the highest activity. This may probably due to three carbon separation from imidazole nucleus to sulfonic acid group gives a better approach of the sulfonic acid group to glycosidic oxygen for the protonation, as compared to the four carbon arm in 1b. The DP of the original Sigmacell cellulose and precipitated solids in entry 4 of experiment 2.2 (Table 1) were measured as described in experiment 2.3. The original Sigmacell cellulose showed a DP value of 457, whereas the partially degraded cellulose precipitated in entry 4 of experiment 2.2 was calculated as 143 from the average of triplicate experiments. This result also supports the observation that cellulose undergoes a facile degradation under mild conditions when exposed to Bro¨nsted acidic ionic liquids. 4. Conclusion In conclusion, for the first time, we have shown that imidazolium Bro¨nsted acidic ionic liquids are effective in dissolution and hydrolysis of cellulose under mild reaction temperatures and at atmospheric pressure, in a single operation without any pretreatment. This process could be developed into an efficient, green, and economical cellulose hydrolysis method for the cellulosic-ethanol production with the recycling of the Bro¨nsted acidic ionic liquid medium.
Acknowledgment The authors would like to thank the Center for Environmentally Beneficial Catalysis (CEBC), University of Kansas, and the American Chemical Society, PRF grant UR1-49436, for financial support. Literature Cited (1) Kamide, K. Cellulose and Cellulose DeriVatiVes; Elsevier B.V: Amsterdam, 2005. (2) Teeri, T. T.; Brumer, H., III.; Daniel, G.; Gatenholm, P. Biomimetic engineering of Cellulose-based materials. Trends Biotechnol. 2007, 25, 299– 306. (3) Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A. Cellulose: Fascinating Biopolymer and Sustainable Raw Material. Angew. Chem., Int. Ed. 2005, 44, 3358–3393. (4) Goettemoeller, J.; Goettemoeller, A. Sustainable Ethanol: Biofuels, Biorefineries, Cellulosic Biomass, Flex-fuel Vehicles, and Sustainable Farming for Energy Independence; Prairie Oak: Maryville, MO, 2007. (5) Katzen, R.; Schell, D. J. In Biorefineries - Industrial processes and Products; Kamm, B., Gruber, P. R., Kamm, M., Eds.; Wiley-VCH: Weinheim, 2006; Vol. 1, pp 129-138. (6) Wyman, C. Handbook on Bioethanol: Production and Utilization; Taylor and Francis: Washington, D.C., 1996. (7) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. ReV. 2006, 106, 4044–4098. (8) Balat, M.; Balat, H. Recent trends in global production and utilization of bio-ethanol fuel. Appl. Energy 2009, 86, 2273–2282. (9) Sukumaran, R. K.; Singhania, R. R.; Mathew, G. M.; Pandey, A. Cellulase production using biomass feed stock and its application in lignocellulose saccharification for bio-ethanol production. Renewable Energy 2009, 34, 421–424. (10) Swatloski, R. P.; Spear, S. K.; Holbery, J. D.; Rogers, R. D. Dissolution of Cellulose with Ionic Liquids. J. Am. Chem. Soc. 2002, 124, 4974–4975. (11) Rogers, R. D.; Voth, G. A. Ionic Liquids. Acc. Chem. Res. 2007, 40, 1077–1078. (12) Zhang, H.; Wu, J.; Zhang, J.; He, J. 1-Allyl-3-methylimidazolium Chloride Room Temperature Ionic Liquid: A New and Powerful Nonderivatizing Solvent for Cellulose. Macromolecules 2005, 38, 8272–8277. (13) Zavrel, M.; Bross, D.; Funke, M.; Bu¨chs, J.; Spiess, A. C. Highthroughput screening for ionic liquids dissolving (ligno-)cellulose. Bioresour. Technol. 2009, 100, 2580–2587. (14) Liu, C.-F.; Sun, R.-C.; Zhang, A.-P.; Qin, M.-H.; Ren, J.-L.; Wang, X.-A. Preparation and Characterization of Phthalated Cellulose Derivatives in Room-Temperature Ionic Liquid without Catalysts. J. Agric. Food Chem. 2007, 55, 2399–2406. (15) Cao, Y.; Wu, J.; Meng, T.; Zhang, J.; He, J.; Li, H.; Zhang, Y. Acetone-soluble cellulose acetates prepared by one-step homogeneous acetylation of cornhusk cellulose in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl). Carbohyd. Polym. 2007, 69, 665–672. (16) Liu, C. F.; Sun, R. C.; Zhang, A. P.; Ren, J. L.; Wang, X. A.; Qin, M. H.; Chao, Z. N.; Luo, W. Homogeneous modification of sugarcane bagasse cellulose with succinic anhydride using a ionic liquid as reaction medium. Carbohyd. Res 2007, 342, 919–926. (17) Liu, C. F.; Sun, R. C.; Zhang, A. P.; Ren, J. L. Preparation of sugarcane bagasse cellulosic phthalate using an ionic liquid as reaction medium. Carbohyd. Polym. 2007, 68, 17–25. (18) Kadokawa, J.; Murakami, M.; Kaneko, Y. A facile preparation of gel materials from a solution of cellulose in ionic liquid. Carbohyd. Res. 2008, 43, 769–772. (19) Kadokawa, J.; Murakami, M.; Kaneko, Y. A facile method for preparation of composites composed of cellulose and a polystyrene-type polymeric ionic liquid using a polymerizable ionic liquid. Compos. Sci. Technol. 2008, 68, 493–498. (20) Li, C.; Zhao, Z. K. Efficient Acid-Catalyzed Hydrolysis of Cellulose in Ionic Liquid. AdV. Synth. Catal. 2007, 349, 1847–1850. (21) Li, C.; Wang, Q.; Zhao, Z. K. Acid in ionic liquid: An efficient system for hydrolysis of lignocellulose. Green Chem. 2008, 2, 177–182. (22) Yang, Q.; Wei, Z.; Xing, H.; Ren, Q. Bronsted acidic ionic liquids as novel catalysts for the hydrolyzation of soybean isoflavone glycosides. Catal. Commun. 2008, 9, 1307–1311. (23) Zhu, G. Y.; Wang, R.; Liu, G. H.; Xu, L. Q.; Zhang, B.; Wu, X. Q. Synthesis of multi-hydroxyl and sulfonyl dual-functionalized room temperature ionic liquids. Chin. Chem. Lett. 2007, 18, 633–635.
Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 (24) Kohler, S.; Liebert, T.; Schobitz, M.; Schaller, J.; Meister, F.; Gunther, W.; Heinze, T. Interactions of Ionic Liquids with Polysaccharides. Unexpected Acetylation of Cellulose with 1-Ethyl-3-methylimidazolium Acetate. Macromol. Rapid Commun. 2007, 28, 2311–2317. (25) Breuil, C.; Saddler, J. N. Comparison of the 3,5-dinitrosalicylic acid and Nelson-Somogyi methods of assaying for reducing sugars and determining cellulase activity. Enzyme Microb. Tech. 1985, 7, 327–332. (26) Bergmeyer, H. U.; Bernt, E. In Methods of Enzymatic Analysis; Bergmeyer, H. U., Eds.; Academic Press: New York, 1974; pp 12051212. (27) Southgate, D. A. T. Determination of Food Carbohydrates; Applied Science Publishers, Ltd.: London, 1961.
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(28) Xu, W.; Reddy, N.; Yang, Y. Extraction, characterization and potential applications of cellulose in corn kernels and Distillers’ dried grains with solubles (DDGS). Carbohyd. Polym. 2009, 76, 521–527. (29) Halidan, M. N.; Wumanjiang, E. Determination of polymerization degree of plant cellulose by viscosimeter. Hecheng Xianwei Gongye. 2006, 29, 40–42.
ReceiVed for reView June 30, 2009 ReVised manuscript receiVed September 28, 2009 Accepted October 16, 2009 IE901047U
