Novel Cellulose Products Prepared by Homogeneous

Chapter 15

Downloaded by DUKE UNIV on June 26, 2012 | http://pubs.acs.org Publication Date (Web): February 23, 2010 | doi: 10.1021/bk-2010-1033.ch015

Novel Cellulose Products Prepared by Homogeneous Functionalization of Cellulose in Ionic Liquids Susann Dorn1, Michael Schöbitz1,2, Kerstin Schlufter1,3 and Thomas Heinze1,* 1Centre

of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstraße 10, D-07743 Jena, Germany 2Thuringian Institute for Textile and Plastics Research, Breitscheidstraße 97, D-07407 Rudolstadt, Germany 3Forschungszentrum für Medizintechnik und Biotechnologie GmbH, Geranienweg 7, D-99947 Bad Langensalza, Germany *[email protected], Member of the European Polysaccharide Network of Excellence, EPNOE (www.epnoe.eu).

The paper deals with the application of ionic liquids (ILs) as solvents and as reaction media. Ils, namely 1-butyl-3methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium acetate, 1-butyl2,3-methylimidazolium chloride and 1-allyl-2,3-dimethylimidazoliumbromide, dissolve cellulose and can easily be applied as media for cellulose functionalization. We investigated the homogeneous acylation of cellulose with a degree of polymerization the range from 330 to 6500. Under mild conditions and within short reaction time, at low temperature and low molar ratio, different cellulose esters and dendronized cellulose were obtained.

Introduction Cellulose is an important raw material of the future. Cellulose is not only the most abundant renewable biopolymer but also a very uniform macromolecule. It consists of β-(1→4) linked anhydroglucose repeating units, which form a very © 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.


compact structure including hydrogen bonding. Cellulose is stereo regular, chiral, biocompatible and reactive. Based on cellulose, a brood variety of chemical products may be prepared. Mostly, chemical modifications of cellulose come along with heterogeneous synthesis pathways. Homogeneous paths open new opportunities for the design of cellulose products with unconventional functional groups and with controlled functionalization pattern (1, 2). By applying a homogeneous synthesis, the degree of substitution (DS) can easily be controlled and more options to introduce novel functional groups with bulky and exotic functions are available (3). Typical cellulose solvents like N,N-dimethylacetamide (DMAc)/LiCl (4) and dimethyl sulfoxide (DMSO)/tetrabutylammonium fluoride (5, 6) are effective homogeneous media for cellulose functionalization in lab scale synthesis. Never the less, some problems appear, especially for industrial application. The system consisting of organic solvent and salt are very difficult to handle because of the combustibility of the organic compound and the recycling is proved to be expensive. In the case of DMSO, some side reactions, like the Swern oxidation may occur and can lead to undesired structures in the products. Thus, there is an increasing interest in new cellulose solvents that are efficient and recyclable. Ionic liquids (IL) can be an alternative solvent for cellulose, a media for homogeneous cellulose functionalization. ILs are low-melting salts that consist only of cations and anions. ILs are non-flammable, highly thermal stable and have no measurable vapour pressure. So-called designer solvent are more and more in the focus of cellulose chemistry. The first useful IL, ethyl ammonium nitrate, investigated by Walden, generated little interest and until the early 1980s, no detailed information about physical and chemical properties were described. The discovery of tetraalkylammonium ILs, which are air- and moisture stable, leads to an increasing interest for application, especially in spectroscopy, synthesis, and electrochemistry (7). ILs are in the focus of the interest in various fields of research and development (8). At the present, there are several articles concerning the concept of ILs. Next to the synthesis, the properties and different application of ILs, the carbohydrate chemistry in ILs were discussed intensively (9–18). Up to now, the broad variety of structures of ILs was not really estimated in the field of cellulose modification, mainly imidazolium based ILs were studied. Swatlowski et al. Found that these ILs, in particular 1-butyl-3-methylimidazolium in combination with several anions, e.g., halides, are able to dissolve cellulose (19, 20). 1-Butyl-3-methylimidazolium chloride (BMIMCl) is announced to be the most efficient solvent for cellulose. Other studies show that 1-allyl-3-methylimidazolium chloride can be used as medium for cellulose acetylation. Cellulose acetates with a high degree of substitution (DS) were obtained (21). In the course of our studies, we investigated different imidazolium based ILs as reaction medium for cellulose chemistry, on the one hand. Especially the application of IL as reaction medium for the acylation of cellulose will be studied.

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


Figure 1. Structures of the ionic liquids used in this studies.

ILs in Cellulose Chemistry In the context of our investigations, imidazolium based IL with chloride as anion were utilized. Next to BMIMCl, 1-ethyl-3-methylimidazolium chloride (EMIMCl), 1-butyl-2,3-dimethylimidazolium chloride (BDMIMCl), 1-allyl-2,3dimethyl-imidazolium bromide (ADMIMBr) and 1-ethyl-3-methylimidazolium acetate (EMIMAc), were applied (Figure 1). The ILs possess a purity of 98%. Because impurities may act as catalyst or inhibitor and/or side reactions may occur.

ILs as Solvent for Cellulose ILs can dissolve cellulose (19, 20, 22, 23). Depending on the type of cellulose, very high concentrated solution can be achieved. Microcrystalline cellulose (MCC) with a degree of polymerization (DP) of 330 can easily be dissolved in IL as well as high molecular weight bacterial cellulose (BC) with a DP of 6500. The amount of cellulose in the solution depends on the type of IL; BMIMCl is the most efficient solvent for cellulose. EMIMCl and ADMIMBr dissolve less cellulose (22). For dissolving MCC in different ILs of the homologous series of a 1-alkyl-3-methylimidazolium based IL and chloride as anion, unexpected results were found. The solubility of the cellulose depends on the alkyl chain length of the cation. For the methylimidazolium cation, no solubility of the cellulose was found. By applying IL with ethyl, butyl or hexyl as alkyl chain, cellulose can be dissolved without any residue. Longer alkyl chain, like octyl leads to an extensive swelling of the biopolymer. Our studies showed the ILs are non-derivatizing solvents for cellulose (23). The DP values both of the starting material and the regenerated samples were determined by the instrinsic viscosity. No degradation during the dissolution process appears independent of the cellulose type (22). Using the IL as solvent for cellulose, the IL can easily be recycled as hygroscopic liquid. This liquid can be reused without additional purification after 24 h freeze-drying.



Figure 2. Values of the degree of substitution of cellulose acetate, cellulose pentanoate, cellulose hexanoate and cellulose benzoate, synthesized in 1-butyl-3-methylimidazolium chloride (2 h, 80°C).

IL as Reaction Media To investigate the reactivity of the cellulose dissolved in IL, the acylation with several reagents was studied (22, 23). The conversion of cellulose in BMIMCl with acid chlorides, like acetyl chloride, pentanoyl chloride, hexanoyl chloride and benzoyl chloride was successfully carried out within 2 h at 80°C. Cellulose esters with a DS in the range from 0.3 to 3.0 were obtained (Figure 2). This reaction requires the addition of 2.5 mol pyridine to the medium. On the one hand pyridine activates the acid chloride to form the reactive acylium ion and, on the other, it neutralizes the liberated HCl. Hexanoyl chloride exposed to be the less reactive reagent used in these studies. Applying a molar ratio of 1/1 (mol anhydroglucose unit (AGU)/mol reagent), a conversion of 30% was achieved. With an increasing molar ratio, the DS obtained increases as well. Thus, a completely substituted cellulose hexanoate was obtained by applying a molar ratio of 1/5. Acetyl chloride and pentanoyl chloride were more reactive than hexanoyl chloride; 1 mol reagent per mol AGU, leads to a cellulose ester with a DS of 0.9 for acetyl chloride and 0.8 for pentanoyl chloride. The DS can be increased up to 3.0 with an input of reagent of 5 mol per mol AGU. In the course of our studies, benzoyl chloride seems to be the most efficient reagent. There was a nearly complete conversion of the reagent, independent of the amount of benzoyl chloride used. Regarding the cellulose esters synthesized, solubility appears in DMSO at a DS> 0.3 and additionally in acetone and chloroform at a DS over 2.0.



Figure 3. Values of the degree of the substitution of the cellulose derivatives, obtained with different reagent and different molar ratio (mol anhydroglucose unit per mol reagent) in 1-butyl-3-methylimidazolium chloride at 80°C within 2 h.

Figure 4. 13C NMR spectrum of a cellulose furoate (DS 3.0) synthesized in 1-butyl-3-methylimidazolium chloride within 3 h at 65°C. Compared to the results of the acylation with acid anhydrides in BMIMCl (2 h at 80°C), it was found that the acid chlorides were more efficient than the acid anhydrides (acetic anhydride, propionic anhydride, butyric anhydride, pentanoic anhydride and hexanoic anhydride) applied. For the reaction with acetic anhydride the addition of pyridine is not necessary. As shown in Figure 279 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


3, propionic anhydride and butyric anhydride lead to low DS (DSpropionate= 0.9; DSbutyrate= 0.4). The DS values achieved with pentanoic anhydride and hexanoic anhydride are much higher (DSpentanoate= 2.4; DShexanoate= 2.7) at comparable molar ratio of 1/3 (mol AGU/mol reagent). Expect for cellulose butyrate, the DS of the cellulose esters increases with increasing molar ratio. In the case of pentanoic anhydride and hexanoic anhydride, a complete substitution of the cellulose occurred at a molar ratio of 1/5 (AGU/reagent). All products with a DS higher than 0.9 are soluble in DMSO and at DS= 2.3 additional soluble in acetone and chloroform. The DS of the cellulose butyrate (DS= 0.4) could not be enhance by changing the molar ratio, the reaction temperature or the reaction medium considering the ILs studied. Obviously, there is a relation between the length of the alkyl chain of the acid anhydride and the alkyl chain of the imidazolium based cation of the IL. Synthesis of cellulose furoate can be realized in BMIMCl as reaction medium. Within 3 h at 65°C, it was possible to prepare soluble cellulose furoates with a DS in the range from 0.5 to 3.0. The DS can be reached independent of the type of cellulose used (MCC, DP 330 or cotton linters, CL, DP 1800). The cellulose furoates are soluble in DMSO and with a DS higher than 2.4, additional soluble in DMA (Table 1). Applying a molar ratio of 1 mol 2-furoyl chloride (FC) per mol AGU, a conversion of 46% of the reagent occurred. The addition of the equimolar amount of pyridine results in a product of a higher DS of 0.62. Pyridine acts in two ways; formation of the reactive acylium ion and as base to bind the HCl liberated. Increasing the amount of FC up to 3 or 5 mol per mol AGU, an increase of the DS up to 3.0 of the cellulose furoates were obtained. The reactivity of the FC depends on the starting biopolymer. Comparing the reaction of MCC and CL with FC, it was found that the CL of comparable high molecular weight is less reactive. At similar molar ratio (1/5, AGU/FC) the cellulose furoate based on CL has the lower DS of 0.67. Cellulose furoate can be synthesized without any impurities in BMIMCl (Figure 4). The 13C NMR spectrum shows the typical peaks of a completely substituted cellulose furoate. The peaks of the AGU can be found in the range from 60 to 100 ppm. In the range from 110 to 150 ppm, the peaks of the carbons of the furoyl moiety and at 159 ppm the carbonyl carbon can be assigned.

Functionalization of Bacterial Cellulose in IL Next to 1-ethyl-3-methylimidazolium acetate, BMIMCl is able to dissolve MCC, spruce sulphite pulp, cotton linters and even bacterial cellulose (BC) with a very high DP up to 6500. The dissolution process in BMIMCl runs very fast, within 20 min, a clear solution can be achieved (Figure 5). The acetylation of BC with acetic anhydride and the carbanilation with phenyl isocyanate was studied. Within 2 h at 80°C, very high substituted cellulose esters can be obtained. The one pot synthesis occurs under mild conditions without an additional base. With an increasing molar ratio from 1/1 to 1/10 (AGU/reagent) different DS values can be generated (Figure 6). The excess of 3 mol acetic anhydride per mol AGU yields to cellulose acetate with a DS= 2.5. Increasing the molar ratio up to 1/10 280 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Table 1. Degree of substitution and solubility of the cellulose furoates prepared in 1-butyl-3-methylimidazolium chloride within 3 h at 65°C

Figure 5. Microscopic images of bacterial cellulose (BC): (1) native BC, (2) after contact with solvent 1-butyl-3-methylimidazolium chloride, (3) after 10 min, (4) after 15 min, and (5) after 20 min dissolution time.

(AGU/reagent), completely substituted products were obtained. The BC acetates possess an unexpected distribution of groups in the order O-6>O-3>O-2 compared to cellulose acetates based on MCC. The cellulose esters are soluble in DMSO at a DS higher 0.7, but they are insoluble in acetone. The carbanilation could also be successfully carried out. With molar ratio from 1/1 to 1/10 (AGU/ reagent), cellulose carbanilates with a DS in the range from 0.4 to 3.0 can be achieved. Applying 10 mol phenyl isocyanate per mol AGU, a completely substituted cellulose carbanilate was obtained within 4 h at 80°C. At a DS> 0.8 solubility in DMSO and at value higher than 2.2, even solubility in DMSO, THF and DMF appeared. The synthesis of cellulose carbanilates in BMIMCl can be used to prepare samples for studying the molecular weight of polymers by the means of SEC. During the reaction, no degradation of the polymer chain occurs (24). 281 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 6. Reaction scheme of the synthesis of cellulose acetate and cellulose carbanilate based on bacterial cellulose in 1-butyl-3-methylimidazolium chloride (BMIMCl) within 2 h (* 4 h) at 80°C and the values of the degree of substitution of the products. Table 2. DS of dendritic PAMAM-triazolo-cellulose derivatives synthesized homogeneously in EMIMAc by reaction of propargy-polyamidoamin dendrons of 1st, 2nd, and 3rd generation via Copper-Catalysed Huisgen Reaction

Preparation of Special Cellulose Derivatives in IL Besides esterification reactions, unconventional types of cellulose derivatization were investigated. Namely the preparation of dendronized cellulose applying 1, 3-dipolar cycloaddition. Therefore, 6-azido-6-deoxycellulose (ADC) was dissolved in EMIMAc and the respective dendron was added followed by CuSO4 and sodium ascorbate as catalysts. The dissolution of ADC occurs within 4 h at 80°C. 282 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 7. DEPT 135 NMR spectrum of 2nd generation PAMAM-triazolo cellulose, recorded in DMSO-d6 at 25°C. The correlation between reaction parameters and DS was investigated for the preparation of first generation of PAMAM-triazolo-cellulose. Using conventional solvents, the Huisgen reaction shows higher efficiency at lower temperature. However, using EMIMAc as a solvent, the DS is higher at ambient temperature. This may be caused by the increased viscosity of ADC solutions in the IL at lower temperature. Optimal conditions are a ratio of ADC and reagent of 1/1 and a reaction time of 24 h. Further increase of both the molar ratio and the reaction time leads to a minor DS increase only. It is well known that the reactivity of the dendrons decreases with increasing bulkiness of the dendron, i.e., with increasing generation. This results from the fact that functional groups along the cellulose backbone may be buried and are not easily accessible as a result of random coil conformation. Moreover, it is possible that the focal points of the dendrons may be buried inside the dendritic branches, which is even more pronounced with increasing generation (25). Therefore, for the attachment of second generation dendrons, the molar ratio was increased to 1/3 and for third generation, due to the high molar mass of the dendron, the reaction time was increased to 72 h (Table 2). Although the increase of time and molar ratio, the DS values for 2nd and 3rd generation PAMAM-triazolo-cellulose derivatives were lower than those of the 1st generation. All samples obtained were soluble in polar aprotic solvents like DMSO and DMAc. The 3rd generation derivative is also soluble in water. Full structure characterization was carried out by elemental analysis and DEPT 135 NMR analysis (Figure 7). Due to the bulkiness of the dendrons the intensity of the signals of the AGU atoms is significantly reduced in NMR spectra. All signals can be assigned confirming the structure of the products.

Conclusion ILs dissolve cellulose with a DP in the range from 330 to 6500 without degradation, if the right IL and procedure is provided. Using ILs as reaction media, the purity of the IL must be evaluated. In the course of our studies 283 In Cellulose Solvents: For Analysis, Shaping and Chemical Modification; Liebert, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


various cellulose derivatives were investigated, especially cellulose esters and even unconventional dendronized cellulose sample. Different DS values were achieved by varying the molar ratio of the agent to biopolymer and the reaction time. Depending on the product in question, the reaction begins homogeneously and may end heterogeneously. During the etherification, gels formation and precipitation occurs quite often. BMIMCl can be used as reaction media for the modification of bacterial cellulose. Highly substituted cellulose acetates and cellulose carbanilates can be synthesized with low excess of reagent and without an additional catalyst. IL may be reactive yielding unexpected product. Further studies will be focused on novel products based on cellulose, synthesized homogeneously in Ilon the one hand and, on the other, on the design of IL for particular modification.

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Novel Cellulose Products Prepared by Homogeneous

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