Energy Fuels 2010, 24, 737–745 Published on Web 01/07/2010
: DOI:10.1021/ef901215m
Solubility of Carbohydrates in Ionic Liquids Mazgorzata Ewa Zakrzewska,† Ewa Bogel-yukasik,† and Rafaz Bogel-yukasik*,†,‡ †
REQUIMTE, Departamento de Quı´mica, Faculdade de Ci^ encias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal and ‡Laborat orio Nacional de Energia e Geologia, I.P., Unit of Bioenergy, Estrada do Pac-o do Lumiar 22, 1649-038, Lisboa, Portugal Received October 26, 2009. Revised Manuscript Received December 17, 2009
While carbohydrates serve as an abundant, diverse, and reusable source of carbon, their derivatization for industrial applications is still a challenging task because of the low solubility in solvents other than water. Ionic liquids are recognized as green solvents for carbohydrate processing. However, only a limited number of studies have been carried out to investigate their ability to dissolve and modify saccharides. Most of the works focus on cellulose. The aim of this Review is to assess the current state of knowledge regarding the solubility of carbohydrates in ionic liquids but not on modifications of carbohydrates in ionic liquids. We herein collect all of the available literature data about the solubility of various carbohydrates in ionic liquids and highlight their interactions with carbohydrates. The subject of functionalization of carbohydrates in ionic liquids is not discussed in this Review.
classical volatile solvents. Nevertheless, it is important to underline that basic properties in the environmental risk assessment of ILs are scarce. Because of this fact, they need to be treated with the same caution as any other chemical with a limited data about toxicity and biodegradability.4 It is notable that toxicity of the ILs is mainly ascribed to the alkyl chain and that the toxicity of imidazolium and pyridinium ILs increases with their cation chain length.5 In addition, the measurements by Garcı´ a et al.5c using the closed bottle test showed that the [bmim]([BF4], [PF6], [NTf2], and [N(CN)2]) ILs are poorly biodegradable (less than 5% biodegradation after 28 days). Nevertheless, ILs are still considered as more environmentally friendly than their volatile, toxic, and organic counterparts. ILs are compounds composed solely of ions with immeasurable combinations of anions and cations. They possess widely tunable properties, such as hydrophobicity,6 polarity, and miscibility with other solvents.7 Using ILs as solvents or catalysts8 allows for the modification of the properties of
Introduction Carbohydrates are the most abundant organic compounds on the earth. They are composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio. Carbohydrates interact with the aqueous environment through numerous hydroxyl groups and build hydrogen bonds. Dependent upon their molecular weight, simple, low-molecular-weight carbohydrates, such as mono- (arabinose, glucose, fructose, mannose, and xylose) and di- (sucrose, lactose, and maltose) saccharides or more complex, high-molecular-weight oligo- (dextrins) and poly(cellulose, chitin, chitosan, starch, amylose, amylopectin, agarose, inulin, and xylan) saccharides are distinguished. Structures of the carbohydrates reported in this work are shown in Schemes 1 and 2 in the Supporting Information. The carbohydrates are relatively inexpensive and a renewable feedstock, and because of that, they find many industrial applications in such diverse areas as chemistry, fermentation, petroleum production, and food, paper, and pharmaceutical industries.1 Unfortunately, carbohydrates are poorly soluble in almost all solvents except water. This is a major hurdle that significantly hinders their use. Ionic liquids (ILs) are solvents that facilitate more green applications in reactions and separations because of their unique properties, such as negligible vapor pressure2 and high thermal stability.3 Their very low vapor pressure reduces the risk of exposure that is a clear advantage over the use of the
(4) http://www.il-eco.uft.uni-bremen.de/. (5) (a) Swatloski, R. P.; Holbrey, J. D.; Memon, S. B.; Caldwell, G. A.; Cladwell, K. A.; Rogers, R. D. Chem. Commun. 2004, 6, 668–669. (b) Ranke, J.; Molter, K.; Stock, F.; Bottin-Weber, U.; Poczobutt, J.; Hoffman, J.; Ondruschka, B.; Filser, J.; Jastorff, B. Ecotoxicol. Environ. Saf. 2004, 58, 396–404. (c) Garcia, M. T.; Gathergood, N.; Scammells, P. J. Green Chem. 2005, 7, 9–14. (d) Pernak, J.; Kalewska, J.; Ksycinska, H.; Cybulski, J. Eur. J. Med. Chem. 2001, 36, 899–907. (e) Bernot, R. J.; Brueseke, M. A.; EvansWhite, M. A.; Lamberti, G. A. Environ. Toxicol. Chem. 2005, 24, 87–92. (f) Romero, A.; Santos, A.; Tojo, J.; Rodriguez, A. J. Hazard. Mater. 2008, 151, 268–273. (g) Rebros, M.; Gunaratne, H. Q. N.; Ferguson, J.; Seddon, K. R.; Stephens, G. Green Chem. 2009, 11, 402–408. (h) Pretti, C.; Chiappe, C.; Baldetti, I.; Brunini, S.; Monni, G.; Intorre, L. Ecotoxicol. Environ. Saf. 2009, 72, 1170–1176. (6) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156–164. (7) (a) Chiappe, C.; Pieraccini, D. J. Phys. Org. Chem. 2005, 18, 275– 297. (b) Domanska, U.; Bogel-yukasik, E. Ind. Eng. Chem. Res. 2003, 42, 6986–6992. (c) Vasiltsova, T. V.; Verevkin, S. P.; Bich, E.; Heintz, A.; Bogelyukasik, R.; Domanska, U. J. Chem. Eng. Data 2005, 50, 142–148. (8) (a) Forsyth, S. A.; MacFarlane, D. R.; Thomson, R. J.; von Itzstein, M. Chem. Commun. 2002, 714–715. (b) Bogel-yukasik, R.; Lourenco, N. M. T.; Vidinha, P.; Gomes da Silva, M. D. R.; Afonso, C. A. M.; Nunes da Ponte, M.; Barreiros, S. Green Chem. 2008, 10, 243–248.
*To whom correspondence should be addressed. Telephone: þ351210924714. Fax: þ351217163636. E-mail:
[email protected]. (1) Lichtenthaler, F. W. In Methods and Reagents for Green Chemistry: An Introduction; Tundo, P., Perosa, A., Zecchini, F., Eds.; John Wiley and Sons, Inc.: New York, 2007; pp 23-63. (2) (a) Paulechka, Y. U.; Kabo, G. J.; Blokhin, A. V.; Vydrov, O. A.; Magee, J. W.; Frenkel, M. J. Chem. Eng. Data 2003, 48, 457–462. (b) Kabo, G. J.; Blokhin, A. V.; Paulechka, Y. U.; Kabo, A. G.; Shymanovich, M. P.; Magee, J. W. J. Chem. Eng. Data 2004, 49, 453–461. (c) Earle, M. J.; Esperanc-a, J. M. S. S.; Gilea, M. A.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831–834. (3) (a) Doma nska, U.; Bogel-yukasik, R. J. Phys. Chem. B 2005, 109, 12124–12132. (b) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermochim. Acta 2000, 357, 97–102. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 737–745
: DOI:10.1021/ef901215m
carbohydrates dissolved in ILs in a chemical, physical, or enzymatic way.9-11 The comprehensive use of environmentally preferable solvents and renewable feedstock enrolls in principles of green chemistry proposed by Anastas et al.12 ILs, being designed as green solvents, might be applied in various chemical reactions, including carbohydrate processing. The solvation ability of ILs is an important feature because of industrial applications as replacements for VOCs. The list of cations and anions constituting ILs considered in this Review is presented in Tables 1 and 2. Until now, the literature mainly concerned the application of ILs in the modification of cellulose trapped in the lignocellulosic biomass.13 In the case of the dissolution of carbohydrates in ILs other than cellulose, only limited studies were presented.14,15 Lignocellulosic Biomass Conversion: Dissolution of Cellulose and Mechanism. Cellulose is a linear polymer stabilized by a large number of intra- and intermolecular hydrogen bonds forming a highly ordered, crystalline structure. The lignocellulosic material is constituted by hemicellulose and lignin. This results in obtaining a three-dimensional network that is recalcitrant to any transformations of the noncovalent interactions.16 The first reports on the solubility of cellulose in ILs were published in the 1930s.17 Graenacher investigated the usage of molten N-ethylpyridinum chloride in the presence of nitrogencontaining base. His finding has little practical use because of the relatively high melting point of the salt (120 °C) and the volatility of co-solvents. Husemann and Siefert lowered the temperature of the system to 77 °C by the addition of 50% N,N-dimethylformamide (DMF) or dimethylsulfoxide (DMSO).18 In the 2002, the group of Rogers demonstrated that some imidazolium-based ILs are capable of dissolving considerable amounts (up to 25 wt %) of cellulose, forming highly viscous solutions.19 It was shown that the assistance of microwave irradiation (or sonification20) enhances the efficiency of dissolution compared to thermal heating. However, because of possible excessive, localized overheating, polymer decomposition and incomplete dissolution might occur.21 Rogers and co-workers suggested that a high chloride concentration and activity of 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) IL is responsible for breaking the extensive hydrogen-bonding network in the polysaccharide and promoting the dissolution.19 The proposed mechanism
of dissolution of carbohydrates in [bmim][Cl] was further confirmed using nuclear magnetic resonance (NMR) spectroscopy. Additionally, Heinze et al.22 compared the 13C NMR spectra of cellulose dissolved in a studied IL and spectra of a known, nonderivatizing DMSO/tetrabutylammonium fluoride trihydrate (TBAF) solvent system. Their results proved that [bmim][Cl] is a nonderivatizing solvent. Moulthrop et al. demonstrated that [bmim][Cl] is able to disrupt the structure of cellulose oligomers (cellobiose, cellotriose, and cellohexaose).23 Remsing et al.24 performed 13C and 35/37Cl NMR relaxation measurements to examine the interactions between cellulose and IL at a microscopic level. This investigation demonstrated that the interaction between the carbohydrate and the anion of an IL is predominant compared to the interactions of the carbohydrate with the cation. The chloride ion was reported to have the strongest hydrogen-bonding basicity among bromide ([Br]), thiocyanate ([SCN]), hexafluorophosphate ([PF6]) and tetrafluoroborate ([BF4]) anions of ILs.19 However, the effect of the anion cannot be considered in isolation. The fact that the chemical structure of cations affects the carbohydrate dissolution was already proven. Rogers and co-workers found that the solubility of cellulose decreases with an increase of the alkyl chain length in the imidazolium cation.19 A diminishing of the solubility of the carbohydrate might be explained by the effective reduction of the concentration of chloride anion in the solution. Zhang et al.25 demonstrated that a short alkyl chain in the cation (i.e., [amim]) enhances the solubility compared to, e.g., the [bmim] cation. It is caused by the positive entropic effect of the smaller cation and increased relaxation of the carbohydrate structure because of the formation of two hydrogen bonds (one between the oxygen atom of the hydroxyl group of the carbohydrate and the cation of ILs and the second between the chloride anion and the hydrogen atom of the hydroxyl group of sugar). Various effects were observed for cations containing the oxygenated side chains;26,27 however, usually the oxygen atom present in the molecule serves as a hydrogen-bond acceptor and interacts with carbohydrates to enhance the solubility of the carbohydrate in the IL. Inspection of Tables 3-7 reveals that the oxygen atoms present in the molecule may exert their influences on the solubility of carbohydrates by increasing the flexibility of the cation chains. In fact, it appears from Table 6 that the effect of the interactions between sucrose and the MeOEt group of the cation is negligibly small. In contrast, a comparison of the solubility of lactose in [bmim][N(CN)2], [Bt14][N(CN)2], and [Bt1Bn][N(CN)2] indicates that the flexibility of the cation tail group plays an important role. Nevertheless, additional experimental and theoretical studies are still needed to fully elucidate the effect of the nature of specific anions and cations on the solvation ability. The gas-liquid chromatography with ILs as stationary phases is a promising and useful tool for this task.28
(9) Murugesana, S.; Linhardt, R. J. Curr. Org. Synth. 2005, 2, 437– 452. (10) Laus, G.; Bentivoglio, G.; Schottenberger, H.; Kahlenberg, V.; Kopacka, H.; R€ oder, T.; Sixta, H. Lenzinger Ber. 2005, 84, 71–85. (11) Liebert, T.; Heinze, T. BioResources 2008, 3, 576–601. (12) Green Chemistry: Theory and Practice; Anastas, P., Warner, J., Eds.; Oxford University Press: New York, 1998. (13) Pinkert, A.; Marsh, K. N.; Pang, S.; Staiger, M. P. Chem. Rev. 2009, 109, 6712–6728. (14) El Seoud, O. A.; Koschella, A.; Fidale, L. C.; Dorn, S.; Heinze, T. Biomacromolecules 2007, 8, 2629–2647. (15) Rosatella, A. A.; Branco, L C.; Afonso, C. A. M. Green Chem. 2009, 11, 1406–1413. (16) Lynd, L. R.; Weimer, P. J.; van Zyl, W. H.; Pretorius, I. S. Microbiol. Mol. Biol. Rev. 2002, 66, 506–577. (17) Graenacher, C. U.S. Patent 1,943,176, 1934. (18) Husemann, E.; Siefert, E. Makromol. Chem. 1969, 128, 288–291. (19) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2002, 124, 4974–4975. (20) Mikkola, J.-P.; Kirilin, A.; Tuuf, J.-C.; Pranovich, A.; Holmbom, B.; Kustov, L. M.; Murzin, Y. D.; Salmi, T. Green Chem. 2007, 9, 1229–1237. (21) Egorov, V. M.; Smirnova, S. V.; Formanovsky, A. A.; Pletnev, I. V.; Zolotov, Y. A. Anal. Bioanal. Chem. 2007, 387, 2263–2269.
(22) Heinze, T.; Schwikal, K.; Barthel, S. Macromol. Biosci. 2005, 5, 520–525. (23) Moulthrop, J. S.; Swatloski, R. P.; Moyna, G.; Rogers, R. D. Chem. Commun. 2005, 1557–1559. (24) Remsing, R. C.; Swatloski, R. P.; Rogers, R. D.; Moyna, G. Chem. Commun. 2006, 1271–1273. (25) Zhang, H.; Wu, J.; Zhang, J.; He, J. Macromolecules 2005, 38, 8272–8277. (26) Liu, Q.; Janssen, M. H. A.; van Rantwijk, F.; Sheldon, R. A. Green. Chem. 2005, 7, 39–42. (27) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759–6761. (28) Armstrong, D. W.; He, L.; Liu, Y.-S. Anal. Chem. 1999, 71, 3873– 3876.
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Energy Fuels 2010, 24, 737–745
: DOI:10.1021/ef901215m Table 1. List of Cations of ILs Presented in This Work
Water is found to decrease the solubility of carbohydrates considerably. Water links units of sugar causing aggregation and decreases carbohydrates accessibility and
reactivity. In addition, water hydrolyses reagents used for functionalization and leads to side reactions, e.g., a chain degradation. On the other hand, water can be used for a 739
Energy Fuels 2010, 24, 737–745
: DOI:10.1021/ef901215m precipitation of carbohydrates19 that reduces the amount of the produced wastes. Swatloski et al. discovered that even 1 wt % of water dramatically decreases the solubility of cellulose in ILs and causes the precipitation of cellulose.19 Water is one of the major impurities of IL and has a strong impact on the solubility of carbohydrates in ILs. Water significantly modifies the solvation ability of ILs. Therefore, it is very important to report that the real ability of IL to dissolve carbohydrates as water efficiently masks the solubility behavior and is one of the major hindrances in the dissolution of carbohydrates in ILs. As already mentioned, the chloride anion as a strong proton acceptor plays a key role in the dissolution process. However, because of a high melting point and a relatively high viscosity, the processing of carbohydrates with chloride ILs is expensive and inefficient. This demands an employment of a newly designed IL that exhibits a low melting temperature, relatively low viscosity, and sufficient polarity. ILs containing carboxylate anions show low viscosity
facile regeneration of carbohydrates already dissolved in an IL. The addition of water, alcohol, or acetone results in a Table 2. List of Anions of ILs Presented in This Work anions abbreviation
name
[Cl] [Br] [I] [SCN] [N(CN)2] [N(CF3SO2)2] [CF3SO3] [CF3COO] [CH3COO] [HCOO] [PF6] [BF4] [CH3SO3] [CH3OSO3] [CH2CH3OSO3] [(CH3O)2PO2]
chloride bromide iodide thiocyanate dicyanamide bis(trifluoromethanesulfonyl)amide trifluoromethanesulfonate trifluoroacetate acetate formate hexafluorophosphate tetrafluoroborate methanesulfonate methylsulfate ethylsulfate dimethylphosphate
Table 3. Solubility of Cellulose in ILs carbohydrate cellulose
IL [amim][Cl] [amim][Cl] [amim][Cl] [ammim][Br] [amim][HCOO] [bmim][Br] [bmim][Cl] [bmim][Cl] [bmim][Cl] [bmim][I] [bmim][HCOO] [bmim][N(CN)2] [bmim][N(CF3SO2)2] [bmpy][Cl] [emim][Cl] [emim][CH3COO] [emim][CH3COO] [emim][CH3COO] [emim][(CH3O)2PO2] [emim][(CH3O)2PO2] [hmim][Cl] [H(OEt)2-mim][CH3COO] [H(OEt)3-mim][CH3COO] [H(OEt)2-mim][Cl] [mmim][(CH3O)2PO2] [MeOMemim][Br] [Me(OEt)2eim][Cl] [Me(OEt)2eim][CH3COO] [Me(OEt)3eim][CH3COO] [Me(OEt)4eim][CH3COO] [Me(OEt)7eim][CH3COO] [Me(OPr)3-eim][CH3COO] [Me(OEt)3-bim][CH3COO] [omim][CH3COO] [P66614][N(CN)2] [pmpy][Cl] [tbpm][HCOO] [Bu4N][HCOO] [Me(OEt)2-Et3N][CH3COO] [Me(OEt)3-Et3N][CH3COO] [MeMe(EtOH)NH][CH3COO] [(MeOEt)2NH2][CH3COO] [MeMe(MeOEt)NH][CH3COO] [Me(MeOEt)2NH][CH3COO] [Amm110][Cl] [Amm110][N(CN)2] [Amm110][HCOO] [Amm110][CH3COO]
water content nd ∼0.7%
