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Biomacromolecules 2004, 5, 266-268
Homogeneous Acetylation of Cellulose in a New Ionic Liquid Jin Wu,† Jun Zhang,*,† Hao Zhang,† Jiasong He,† Qiang Ren,‡ and Meili Guo‡ State Key Laboratory of Engineering Plastics, Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing, 100080, China, and School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing,100083, China Received October 8, 2003; Revised Manuscript Received January 7, 2004
Acetylation of cellulose has been accomplished in a new room-temperature ionic liquid, 1-allyl-3methylimidazolium chloride, in the absence of any catalysts, and cellulose acetates with a wide range of degree of substitution have been obtained directly under homogeneous reaction conditions. Homogeneous functionalization of cellulose has been one focus of cellulose research for a long time, although heterogeneous methods are the actually applied ones in production of most commercial cellulose derivatives.1 Advantages of the homogeneous reaction include: creating more options to induce novel functional groups, opening new avenues for the design of products, and opening up the opportunity to control the total degree of substitution (DS) value. For the homogeneous reaction, eagerly needed are suitable cellulose solvents that can dissolve cellulose and provide a feasible reaction environment. However, due to the stiff molecule and close chain packing via numerous intermolecular and intramolecular hydrogen bonds, cellulose is extremely difficult to dissolve. To date, only a limited number of solvent systems have been found, for example, DMAc/LiCl, DMF/N2O4, NMNO, and DMSO/TBAF and some molten salt hydrates, such as LiClO4*3H2O, and LiSCN*2H2O. Homogeneous cellulose derivatizations, such as esterification, etherification, and other reactions, for producing novel products in these solvents have also been reported.1-8 However, there remain limitations such as toxicity, cost, difficulty for solvent recovery, or instability in above processing. Room-temperature ionic liquids (ILs), being considered as desirable green solvents for a wide range of separation and as reaction media for processes including catalysis, have recently received significant attention.9,10 Particularly, used as reaction media, ILs have several advantages such as enhancement of reaction rates, improvement of selectivity and yields, or ease of recycling catalysts.11,12 For example, acetylation of alcohols and saccharides was conducted in a dicyanamide based ionic liquid.13 This IL was not only an excellent solvent but also an active base catalyst for O-acetylation. Additionally, the physicochemical properties of ILs may be easily adjusted through the changing structure of cations or anions, which will broaden their application fields. In previous literatures, dissolution and reactions of cellulose in benzylpyridium chloride, ethylpyridium chloride, * To whom correspondence should be addressed. † The Chinese Academy of Sciences. ‡ Beijing University of Aeronautics and Astronautics.
Figure 1. Chemical structure of 1-allyl-3-methylimidazolium chloride.
or their mixtures with pyridine were reported.14,15 These solvents are closely similar to current frequently used ILs, except for relatively high melt points of the former. Recently, dissolution of cellulose with ILs was first reported by Swatloski et al.16 It is considered that the high chloride concentration and activity in IL play an important role in cellulose dissolution. More recently, a novel IL, 1-allyl-3-methylimidazolium chloride (AMIMCl), was synthesized in our lab and was found to have outstanding capability for dissolving cellulose.17 The chemical structure of AMIMCl is shown in Figure 1. It is worth noticing that one substituent on nitrogen is alkenyl instead of saturated alkyl. This substituent makes this IL have a relatively low melting point (ca. 17 °C) and keep high thermal stability (decomposition temperature, 273 °C) compared to other ILs substituted by saturated alkyl containing the same number of carbon atoms, e.g., PMIMCl18 (1-methyl-3-propylimidazolium chloride, mp, 60 °C, decomposition temperature, 282 °C). Just like 1-butyl-3-methylimidazolium chloride (BMIMCl),16 AMIMCl is a nonderivatizing solvent for cellulose. Interestingly, at 100 °C, AMIMCl can dissolve 5%(wt) cellulose without any pretreatment or activation within only 15 min, which prevents substantial degradation of cellulose upon dissolution.17 A solution of 10%(wt) cellulose in AMIMCl was also obtained and kept clear and transparent after cooling to room temperature, except that it was highly viscous. Therefore, this IL is expected to be a promising direct solvent for cellulose. In the present paper, we are going to report a homogeneous acetylation of cellulose in AMIMCl. The cellulose used was dissolving pulp with a degree of polymerization (DP) of ca. 650. The acetylating reagent was acetic anhydride. In a typical reaction procedure, a certain amount of acetic anhydride was added with a pipet into about 10 g cellulose/ AMIMCl solutions (containing 4.0% or 2.9% of cellulose by weight) in flask. The mixture was heated in oil bath for a certain time under N2 atmosphere with or without stirring. The products were isolated by precipitation into excess
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Communications Table 1. Conditions and Results of the Acylation of Cellulose Dissolved in AMIMCl entrya
wt% molar T cellulose ratiob (°C)
A1 A2 A3 A4 A5 A6 A7 A8 B1 B2 B3 C1 C2 C3
4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 2.9 2.9 2.9
5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 4:1 6.5:1 8:1 3:1 4:1 5:1
time (h)
80 0.25 80 0.5 80 1.0 80 2.0 80 3.0 80 4.0 80 8.0 80 23.0 80 4.0 80 4.0 80 4.0 100 3.0 100 3.0 100 3.0
solubilityc DS 0.94 1.39 1.61 1.80 1.86 2.21 2.49 2.74 2.15 2.43 2.38 1.99 2.09 2.30
acetone chloroform + + + + + + + + + +
+ + + + +
a Group C was stirred during reaction group A and B were not. b Molar ratio: acetic anhydride/anhydroglucose unit (AGU). c “+” stands for soluble (able to dissolve no less than 1wt %), and “-” stands for insoluble. All of the samples listed were soluble in DMSO.
deionized water or isopropyl alcohol, filtered and washed several times, and then dried in a vacuum at 50 °C. The DS (degree of substitution) of all products was analyzed by titration method and NMR method in DMSO-d6. The structure of products was characterized by FTIR and NMR spectroscopy. Under the conditions for the acetylation, the system remained completely homogeneous as the reaction proceeded. Cellulose was also homogeneously acetylated after a certain time in this system even at 20 °C. To obtain a reasonable reaction speed, a relatively high temperature was adopted. Experimental results are given in Table 1. In group A and B, reactions were carried out without agitation. In group C, reactions went on at 100 °C with continuous agitation. From the FTIR spectra of acetylated cellulose products, the strong absorption peaks at 1745 and 1748 cm-1 are observed, which confirms the existence of carboxylic acid ester. Furthermore, the 1H NMR and 13C NMR spectra of
Figure 2.
13C
NMR spectrum of product C1.
products show that the chemical shift of methyl hydrogen is in the range from 1.8 to 2.2, and the chemical shift of carbonyl carbon is in the range of 169-171. These FTIR and NMR data are in accordance with those reported in the previous literature.19 Commercially, cellulose acetates are produced by reaction of cellulose with an excess of acetic anhydride in the presence of sulfuric acid or perchloric acid as the catalyst. However, due to the heterogeneous nature of the reaction, it is impossible to synthesize partially substituted cellulose acetates directly. Generally, commonly used cellulose diacetate esters are obtained by hydrolyzing fully substituted cellulose acetate. In homogeneous solution, acetylation of cellulose can be carried out by using highly active reagents such as acetyl chloride, whereas effective acetylation of cellulose with aliphatic anhydrides needs a catalyst, such as pyridine, sulfuric acid, or perchloric acid. An uncatalyzed reaction of cellulose with acetic anhydride in DMAC/LiCl often obtains only low DS-(acetyl).3 Our results clearly show that one-step acetylation of cellulose having a wide range of DS is accomplished under homogeneous reaction conditions in the absence of any catalysts. It is reasonable to speculate that homogeneous acylation uncatalyzed will be much more valuable, due to less chain cleavage of cellulose, absence of catalyst residues in product, and minimization of some side reactions. Investigation has been conducted on the effect of reaction parameters, such as reaction time, temperature, and the molar ratio of acetic anhydride/AGU in cellulose, in the acylation of cellulose. The DS of the products increases as reaction time prolongs. Table 1 shows that, under conditions of 80 °C and acetic anhydride of 5:1, the DS of the acylated product reaches 0.94 within only 15 min, 1.86 within 3 h, and 2.74 in 23 h. The reaction can be accelerated through raising the temperature. For example, a DS of 2.21 was achieved after 4 h at 80 °C, whereas with the same molar ratio, a DS of 2.3 was reached in only 3 h at 100 °C. Even at ambient temperature (20 °C), homogeneous acetylation
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Figure 3. Distribution of acetyl moiety among the three OH groups of the AGU in cellulose acetates.
of cellulose in AMIMCl for 2 days with a molar ratio of 7.5:1 produced cellulose acetate with a low DS (0.64) which was fully soluble in water (this result is not listed in Table 1). Increasing the molar ratio of acetic anhydride/AGU generally increases the DS of product (C1 to C3 and B1 to B2). However, when the molar ratio increased from 6.5 to 8, the DS decreased from 2.43 to 2.38 (B2, B3). This is deemed as an evidence that AMIMCl has the property in favor of acetylation and too much acetic anhydride can weaken the property. These results show that it is possible to control the DS value by stoichiometric method, temperature, and reaction time, especially the latter two methods. However, stirring had little effect on the reaction proceeding, which should be attributed to greater accessibility of the reagent in homogeneous solution. Table 1 also includes the solubility of the samples. All of the samples were soluble in DMSO. In acetone, samples with a DS greater than 1.86 could dissolve rapidly, except that sample A8 (DS 2.74) dissolved slowly. In chloroform, samples began to dissolve slowly when the DS was greater than 2.30, and only sample A8 could dissolve rapidly. The influence of freshness of AMIMCl on the acetylation was also studied, such as reaction in recycled AMIMCl with acetic anhydride as acetylating reagent. As a result, cellulose acetate with a similar DS was obtained under comparable reaction conditions. After only a simple distillation, AMIMCl with high purity was obtained. Because of negligible vapor pressure and good thermostability, the recycling and isolation of this IL are very easily done. This advantage makes this IL potentially attractive for industrial applications. The three hydroxyl groups at the C2, C3, and C6 positions, exhibit various reaction activities. Figure 2 shows a 13C NMR spectrum of sample C1 with a DS of 1.99 in which the signal at 170.6 ppm was attributed to the carbonyl carbon at C-6, 169.8 ppm to that at C-3, and 169.5 ppm to that at C-2. The distribution of the moiety among the three OH groups was calculated from the integration of the 13C NMR spectra, and the results are presented in Figure 3. It can be seen that, for instance, a sample with a total DS of 0.94 shows a partial DS at C-6 of acetic acid ester of 0.71, while the partial DS at C-3 of 0.14 and at C-2 of 0.01, respectively. Obviously, the acetylation reaction is preferred
Communications
at C-6, and the order of reactivity is C6-OH > C3-OH > C2-OH. This result is similar to that observed in DMAc/ LiCl solution20 but different from that in other solution, such as LiCl/1,3-dimethyl-2-imidazolidinone, in which the order of reactivity is C6-OH > C2-OH > C3-OH.21 It is worthy to note that there are more free hydroxyls at C6 than at C2 and C3 for partially substituted cellulose acetate prepared by commercial method because of deacetylation procedure.22 In conclusion, homogeneous acetylation of cellulose in a new ionic liquid, AMIMCl, has been successfully accomplished. This reaction possesses several obvious advantages, such as catalyst-free, rapid, DS value-controllable, and solvent recyclable. So this reaction system could be applied conveniently in laboratories. The authors predict that other cellulose esters, such as propionates, butyrates, inorganic acid esters, and/or mixed cellulose esters, also can be obtained through homogeneous esterification of cellulose by using corresponding acylating agents. Furthermore, other cellulose functionalization, including etherification, alkylation, silylation, grafting, and halogenation, are also possible in such homogeneous cellulose/ionic liquid solutions. Related studies are in progress. Regarding the potential of ionic liquids as promising green solvents, homogeneous functionalization of cellulose in ionic liquids is expected to attract more attention in the future. Acknowledgment. The authors thank the National Natural Science Foundation of China (NSFC) for financial support (No. 50103011). Dr Ruigang Liu is thanked for providing dissolving pulp samples and beneficial suggestions. References and Notes (1) Heinze, T.; Liebert, T. Prog. Polym. Sci. 2001, 26, 1689. (2) Heinze, T.; Liebert, T. Biomacromolecules 2001, 2, 1124. (3) Edgar, K. J.; Arnold, K. M.; Blount, W. W.; Lawniczak, J. E.; Lowman, D. W. Macromolecules 1995, 28, 4122. (4) Heinze, T.; Schaller, J. Macromol. Chem. Phys. 2000, 201, 1214. (5) Regiani, A. M.; Frollini, E.; Marson, G. A.; Arantes, G. M.; El Seoud, O. A. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1357. (6) Dawsey T. R.; McCormick, C. L. J. Macromol. Sci. ReV. Macromol. Chem. Phys. 1990, C30, 405. (7) Fischer, S.; Voigt, W.; Fischer, K. Cellulose 1999, 6, 213. (8) Fischer, S.; Thu¨mmler, K.; Pfeiffer, K.; Liebert, T.; Heinze, T. Cellulose 2002, 9, 293. (9) Welton, T. Chem. ReV. 1999, 99, 2071. (10) Holbrey, J. D.; Seddon, K. R. Clean Prod. Processes 1999, 1, 223. (11) Earle, M.; Seddon, K. R. In Clean SolVents: AlternatiVe Media for Chemical Reactions and Processing; Abraham, M. A., Moens, L., Eds.; ACS Symposium Series 819; American Chemical Society: Washington, DC, 2002; p 10. (12) Gordon, C. M. Appl. Catal. A: General 2001, 222, 101. (13) Forsyth, S.; MacFarlane, D. R.; Thomson, R. J.; Itzstein, M. Von. Chem. Commun. 2002, 714. (14) Graenacher, C. U.S. Patent 1,943,176, 1934. (15) Husemann, E.; Siefert, S. Makromol. Chem. 1969, 128, 288. (16) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2002, 124, 4974. (17) Ren, Q.; Wu, J.; Zhang, J.; He, J. S. Acta Polym. Sin. 2003, 3, 448. (18) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermochim. Acta 2000, 357-358, 97. (19) Kowsaka, K.; Okajima, K.; Kamide, K. Polym. J. 1986, 18, 843. (20) El Seoud, O. A.; Marson, G. A.; Ciacco, G. T.; Frollini, E. Macromol. Chem. Phys. 2000, 201, 882. (21) Takaragi, A.; Minoda, M.; Miyamoto, T.; Liu, H. Q.; Zhang, L. N. Cellulose 1999, 6, 93. (22) Ott, E.; Spurlin, H. M.; Grafflin, M. W. Cellulose (Part 2); WileyInterscience: New York, 1954; p 673.
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