Dissolution of Cellulose in Ionic Liquids and Its ... - ACS Publications

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Dissolution of Cellulose in Ionic Liquids and Its Application for Cellulose Processing and Modification Liu ChuanFu1,*, Sun RunCang1,2, Zhang AiPing1 and Li WeiYing1 1State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, China 2Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing, China *[email protected]

Cellulose is the most abundant renewable bioresource in the world and the most promising feedstock for industry in the future. Ionic liquids as novel and designable solvents have attracted much attention. In this review, the progress of the dissolution of cellulose in ionic liquids is illustrated. As suitable and environmentally friendly media, Ionic liquids also provide a new platform for cellulose processing and modification. The preparation of composite materials, cellulosic derivatives, and biofuels from cellulose in ionic liquids is summarized.

Introduction Cellulose, the principal structural cell wall component of all territory plants, is the most abundant renewable bioorganic substance on the earth. This polymer consists of a chain of β-(1→4)-linked glucose residues (1, 2). It is non-toxic, renewable, biodegradable and modifiable, which make it one of the most promising feedstock for industry in the future (3–5). Utilization of cellulose has a long history and there have been well-established technologies for the traditional applications of cellulose in industries including paper, paint, textile, food and pharmaceutical (5, 6). However, due to the stiff molecules and close chain packing via the numerous inter- and intra-molecular hydrogen bonds, it is extremely hard to dissolve cellulose in water and in most common © 2010 American Chemical Society In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


organic solvents, which constitutes a major obstacle for cellulose application. The efficient dissolution of cellulose is a long-standing goal in cellulose research and development. To date, only a limited number of solvent systems, such as DMAc/LiCl (7), DMF/N2O4 (8), NMNO (9), DMSO/TBAF (10), and some molten salt hydrates like LiClO4·3H2O (11), have been found efficient for cellulose dissolution. However, these solvent systems currently used for cellulose dissolution suffer drawbacks such as volatility or generation of poisonous gas, difficulty for solvent recovery, or instability in application and processing (12). The low melting points organic salts known as Ionic liquids (ILs) have attracted increasing attention as novel and designable solvents in recent years (13). ILs are defined as materials that are composed of cations and anions which melt at or below 100°C. These properties make them non-volatile and interesting substitutes for many applications in which the volatile traditional organic solvents causes problems. Today, this is generally accepted that ILs are the promising novel green solvents for most organic and inorganic substances and can be used as green media in many processing and derivatization.

Dissolution of Cellulose in Ionic Liquids The most common ILs in use are those containing alkylammonium, alkylphosphonium, 1-alkylpyridinium, and 1,3-dialkylimidazolium cations, as seen in Fig. 1 (13). The acidic proton of the imidazolium ring plays an important role in the imidazolium salts, which are the dominant ILs. Anions such as Cl‾, Br‾, NO3‾, [BF4]‾, [PF6]‾, CF3SO3‾, and CF3COO‾ can be used in combination with the above cations to form low melting temperature ILs (14). The nature of the anion is largely responsible for the chemical properties of ILs. The dissolution of cellulose in IL, molten N-ethylpyridinium chloride in the presence of nitrogen-containing bases, was firstly reported in 1934 (15). However, there had not been the concept of ILs and this finding was thought to be of little practical value at that time. Until recently, the application of ILs in cellulose chemistry has regained attention based on the understanding of ILs. In 2002, Rogers and his co-workers (16) reported ILs including 1-butyl-3-methylimidazolium chloride ([C4mim]Cl, Fig. 2a) could be used as non-derivatizing solvents for cellulose. They carried out comprehensive studies on cellulose dissolution in ILs and its regeneration to produce advanced cellulose-based materials (17–22). Because of his great contribution, Rogers has become a winner of the 2005 US Presidential Green Chemistry Challenge Awards, indicating the importance of this work in society. Since then, cellulose dissolution of in ILs and its application have attracted the increasing attention. Many kinds of functionalized ILs, especially room-temperature ionic liquid (RTIL), with higher solubility for cellulose were reported. In 2003, Zhang and his co-workers (12, 23) synthesized a novel RTIL, 1-allyl-3-methylimidazolium chloride (AmimCl, Fig. 2b), which has outstanding capability for dissolving cellulose. This RTIL is a nonderivatizing solvent with higher solubility for cellulose than [C4mim]Cl. Solution containing 5 wt% cellulose in AmimCl 288 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Structure of common cations in ionic liquids could be formed within only 15 min at 100°C without any pretreatment or activation. In addition, it is more convenient for the application of AmimCl than [C4mim]Cl because this IL is liquid at room temperature. In 2005, another RTIL, 1-(2-hydroxylethyl)-3-methylimidazolium chloride (HemimCl, Fig. 2c), was reported as the nonderivatizing solvent for cellulose by Luo and his co-workers (24). This IL has relatively high solubility for cellulose. Solutions containing 5% cellulose in HemimCl could be formed at 60°C, and 6.8% at 70°C. However, the relatively low thermal stability of HemimCl limits its application. In 2008, Ohoo and his group (25) reported that a series of RTILs, alkylimidazolium salts containing dimethyl phosphate, methyl methylphosphonate, or methyl phosphonate (26), had the potential to solubilize cellulose under mild conditions. Especially, 1-ethyl-3-methylimidazolium mehtylphosphonate (Fig. 2d) could enable the preparation of 10 wt% cellulose solution at 45°C within 30 min and 2-4 wt% cellulose solution even at room temperature without any pretreatment and hearing. These functionalized ILs exhibit higher solubility for cellulose and more convenience for processing and application. However, the present reports on cellulose dissolution in ILs mainly focus on the synthesis of functionalized ILs and the effect of dissolution conditions on cellulose solubility. The mechanism of cellulose dissolution in ILs was investigated only in a few publications. It is generally accepted that the high concentration and activity of chloride anion, which is strong hydrogen bond acceptor and highly effective in breaking the extensive hydrogen-bonding network present in cellulose, is responsible for the dissolution of cellulose (16). In addition, the presence of water in ILs significantly decreases the solubility of cellulose through competitively hydrogen-bonding to the cellulose microfibrils which inhibits solubilization. On the other hand, ILs containing “noncoordinating” anions such as [BF4]‾ and [PF6]‾ are not suitable solvents for cellulose, whereas they can be used as biocides (27, 28) and anti-electrostatic agents for wood (29). However, Navard (30) pointed out that the swelling and dissolution mechanism in ILs were entirely due to the way cellulose fibers are structured, not depending on the type of solvent. It was accepted that there was no decomposition of cellulose occurred during dissolution in ILs and the regenerated cellulose had almost the same degree of polymerization and polydispersity as the initial one (16, 31). However, the recent studies have indicated that degradation of cellulose during dissolution does occur and the degree of polymerization of regenerated cellulose decreases (32–34). Moreover, the microstructure of cellulose is changed after regeneration from ILs. There are two opinions on the crystalline structure of the regenerated cellulose. Zhai (35) investigated the structural differences between cellulose regenerated from [C4mim]Cl and untreated cellulose using X-ray diffraction. The data showed 289 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Chemical structure of ionic liquids for cellulose dissolution

Figure 3. X-ray diffraction patterns for original cellulose (A) and cellulose regenerated from [C4mim]Cl (B) and AmimCl (C). that the crystalline form of wood pulp cellulose was transformed completely from cellulose I to cellulose II after regeneration from IL solution, as shown in Fig. 3. The similar results were reported by Zhang (22). Whereas the crystalline structure was found to be completely destroyed during dissolution of cellulose in ILs and the crystalline cellulose I was totally transformed to amorphous cellulose, as seen in Fig. 4, according to our research (36, 37). Similar results were reported in many publications (20, 38–40). However, the mechanism of the transformation of cellulose crystallinity is still not clear.

Preparation of Regenerated Cellulose Materials in Ionic Liquids The dissolution of cellulose in novel green solvents, ILs, has been providing a new platform for cellulose applications. Cellulose could be precipitated from the IL solution by the addition of anti-solvents such as water, ethanol, acetone, isopropanol, etc. After dissolution and regeneration from ILs, cellulose morphology is significantly changed and a relatively homogeneous macrostructure is obtained. The physicochemical properties of regenerated cellulose are affected by the dissolution and regeneration processes. Different structural forms of 290 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 4. X-ray powder diffraction patterns for untreated Avicel (A) and regenerated Avicel from IL solution with deionized water after incubation at 130°C for 2 h (B) or 30 min (C). regenerated cellulose such as fibers, films, powders, beads and membranes can be prepared by changing processing conditions, especially regeneration conditions (21). It was reported that cellulose fiber obtained from ILs by dissolution and regeneration in ILs such as [C4mim]Cl, called Ionicell fiber, has good properties similar to those of Lyocell fiber (41). Kosan (42) also successfully prepared cellulose dopes using ILs, which could be shaped by a dry-wet spinning process to manufacture cellulose fibers. Ionicell fiber would be a new kind of environmentally friendly promising fiber following the Lyocell fiber. Cellulose blended or composite materials can be prepared using ILs. The incorporated functional additives can be dissolved or dispersed in ILs before and after dissolution of the cellulose. With this simple approach, many kinds of cellulose composite materials with different structural forms can be easily obtained. Rogers’ group (21) proposed a new method for introducing enzymes into cellulosic matrixes which can be used to produce membranes, films, or beads using a cellulose-in-IL-dissolution and regeneration process. Zhang and his co-workers (43) also developed a method to prepare wool keratin/cellulose blended materials by the dissolution and regeneration of wool keratin fibers in [C4mim]Cl. Pletnev (44) prepared cellulose films containing entrapped analytical reagents suitable for metal-ion detection by joint dissolution of cellulose and the reagents in IL followed by precipitated with water. This method provides a new technology for quantitative determination of transition metal cations. Kadokawa (1, 45) reported a facile method for preparation of composites composed of cellulose and a polystyrene-type polymeric IL using an imidazolium-type polymerizable IL. Cellulose-nanohydroxyapatite composite scaffolds with high and open porosity were successfully prepared by poly(methyl methacrylate) particulate leaching with [C4mim]Cl as cellulose solvent (46). The preparation of cellulose blended or composite materials using ILs would broaden the conventional cellulose application scope.

291 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Table 1. DS of cellulose succinylated with succinic anhydride in [C4mim]Cl Succinylation conditions


Catalyst

Catalyst/SA (%)

Succinylated cellulose

Temp. Reaction time (°C) (min)

Sample No.

DS

0

100

60

1

0.24

I2

8

100

60

2

1.28

DMAP

5

100

60

3

2.34

NBS

5

100

60

4

2.31

Preparation of Cellulose Derivatives Using Ionic Liquids as Reaction Media Although homogeneous derivation of cellulose has been one focus in cellulose chemistry for a long time, the number of cellulose derivatives commercially available is limited because only heterogeneous synthesis paths are carried out in large-scale processes. Recently, ILs as the nonderivatizing solvents for cellulose have drawn much attention on the potential application of ILs as the novel green homogeneous reaction media for preparation of cellulose derivatives. Cellulose acetate is one of the most commercially important cellulose derivatives with a wide application in the fields of coating, film, membrane, textile, and cigarette industries (47). Cellulose acetylation with aqueous acetic anhydride or acetyl chloride has been extensively studied because of easily established homogeneous reaction system. Zhang and his co-workers (47–49) reported that the homogeneous acetylation of cellulose with acetic anhydride could be carried out in AmimCl without any catalyst and cellulose acetates with a wide range of degree of substitution (DS) could be obtained under different conditions. Heinze and his co-workers (31, 50, 51) found that it was very easy to synthesize cellulose acetate with high DS in good yield within a short time using different ILs such as [C4mim]Cl as reaction media and using acetyl chloride or acetic anhydride as acetylation reagent without any catalysts. The reaction of three free hydroxyl groups in cellulose at the C2, C3, and C6 positions all occur during homogeneous modification in ILs, and the order of reactivity is C6-OH>C3-OH>C2-OH (48), similar to that observed in acetylation in DMAc/LiCl system (52). The efficient O-acetylation of cellulose was accomplished using a zinc based ionic liquid by Handa (53). Furthermore, ILs can also be used as reaction medium for homogeneous Carbanilation of cellulose with phenyl isocyanate (50, 51), acylation with lauroyl chloride (51) and perpropionylation with propionic anhydride (50). However, it should be noted that it is much difficult to achieve homogeneous modification media for cellulose with solid derivatizing reagents. Carboxymethylation of cellulose in [C4mim]Cl was proposed by Heinze (31). In addition, carboxymethylated cellulose (CMC) obtained according to this method had relatively low DS and an increase of the 292 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 5. FT-IR spectra of unmodified cellulose (spectrum 1), succinylated cellulose without any catalyst (spectrum 2, sample 1) and with iodine as a catalyst (spectrum 3, sample 2).

Figure 6. Solid state CP/MAS 13C-NMR spectra of unmodified cellulose (spectrum a), succinylated cellulose without any catalysts (spectrum b, sample 1) and with NBS as a catalyst (spectrum c, sample 3). dosage of carboxymethylating reagent did not increase the DS. The reason for this unusual result is still unclear. Schubert (54) accomplished homogeneous tritylation of cellulose with trityl chloride in [C4mim]Cl using pyridine as base. A DS of around 1 was obtained after 3 h reaction time using a six fold excess of trityl chloride, indicating the slow reaction speed. Cellulose derivatives obtained with solid reagents such as cyclic anhydrides have also been widely used in various applications such as water absorbents for soil in agriculture, natural absorbents for the removal of heavy metal ions in waste water treatment, medicine for drug delivery systems, and thermoplastic materials (55, 56). The esterification of cellulose with cyclic anhydrides does not yield an undesired corresponding carboxylic acid by-product, which is generally 293 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


produced with linear chain acylation reactants. These cellulose products also provide a reactive site upon which further modification for other potential utilization such as in pharmaceutical industries is possible. In addition, because carboxylic groups are introduced to cellulose by ester linkages which are more susceptible to hydrolysis and biodegradation than ether linkages, these derivatives are more suitable for the design of biodegradable materials than CMC, the most representative cellulosic derivatives containing carboxylic groups linked by ether linkages (56). In our earlier work, the homogeneous modification of cellulose with solid cyclic anhydrides such as phthalic anhydride (57, 58) and succinic anhydride (32, 34) was investigated without any catalysts. However, phthalation and succinoylation is more difficult to occu than acetylation and the obtained cellulose derivatives have relatively low DS. Many kinds of novel catalysts such as N-bromosuccinimide (NBS) (59), 4-dimethylaminopyridine (DMAP) (60), and iodine were investigated to improve modification efficiency of cellulose. The results in Table 1 showed that DS of succinylated cellulosic derivatives increased from 0.24 to 1.28 at 100°C for 60 min catalyzed with 8% iodine, to 2.34 with 5% DMAP, and to 2.31 with 5% NBS. FT-IR and CP/MAS 13C-NMR spectroscopies further provided evidence of catalyzed succinoylation, as shown in Figs 5 and 6. The possible mechanism of succinoylation and the actual role of catalysts in ionic liquids are under study. The catalyzed modification of cellulose with other solid modifying reagents using ILs as homogeneous media should be further investigated.

Application of Ionic Liquids in the Preparation of Biofuel from Cellulosic Materials Besides cellulose composite materials and cellulose derivatives, biofuel such as bioethanol will be another promising application of cellulose in industries in the future. Hydrolysis of cellulose to glucose in aqueous media catalyzed by the cellulase or acid suffers from slow reaction rates in industry due in large part to the highly crystalline structure of cellulose and inaccessibility of cellulase or acid adsorption sites. In order to make the hydrolysis process viable for producing simple sugars for fermentation to produce ethanol fuel and other bio-based products, this highly ordered structure of cellulose has to overcome. Schall (38) made an attempt to disrupt the cellulose structure in [C4mim]Cl. The results indicated that the initial enzymatic hydrolysis rates were approximately 50-fold higher for regenerated celluloses as compared to untreated cellulose, which was due to the conversion of crystalline cellulose to amorphous cellulose during pretreatment with IL. Kamiya (61) established an enzymatic saccharification of cellulose in an imidazolium type IL with an alkylphosphate anion, which will be very useful in integrated bioprocesses such as bioethanol production from cellulosic materials. A novel method for cellulose hydrolysis catalyzed by mineral acids in [C4mim]Cl was developed by Zhao (62, 63). This method could facilitate the hydrolysis of cellulose with dramatically accelerated reaction rates at 100°C under atmospheric pressure and without pretreatment. 294 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


In short, ionic liquids’ the excellent capability of dissolving cellulose make them a promising substitutes to classic solvents for processing cellulose and carrying out reactions in cellulose derivatization chemistry, providing a new platform for cellulose utilization as composite materials, cellulose derivatives, and biofuels. However, many fundamentals have not been totally understood. More comprehensive studies on the fundamentals, such as the mechanism of dissolution and degradation of cellulose macromolecules, transformation of cellulose crystalline structure, the homogeneous chemical modification of cellulose with solid reagents, and the processing of cellulose in ionic liquids, have to be investigated to develop new biopolymers and further prosper the industry.

Acknowledgements The authors are grateful for the financial support of this research from National Natural Science Foundation of China (Nos. 30871994, 30972325, and 30710103906), Guangdong National Science Foundation (Nos. 07118057 and 8451064101000409), Specialized Research Fund for the Doctoral Program (No. 20070561040) of Higher Education (111 project), and National Basic Research Program of China (No. 2010CB732201).

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