Carbohydrate Research 342 (2007) 919–926 Homogeneous modification of sugarcane bagasse cellulose with succinic anhydride using a ionic liquid as reaction medium C. F. Liu,a R. C. Sun,a,b,* A. P. Zhang,a J. L. Ren,a X. A. Wang,a M. H. Qin,c Z. N. Chaod and W. Luod aState Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China bCollege of Material Science and Technology, Beijing Forestry University, Beijing 100083, China cShandong Key Laboratory of Pulp and Paper Engineering, Shandong Institute of Light Industry, Jinan 250100, China dCentre for Instrument and Analysis, South China University of Technology, Guangzhou 510640, China Received 14 September 2006; received in revised form 1 February 2007; accepted 5 February 2007 Available online 13 February 2007 Abstract—The homogeneous chemical modification of sugarcane bagasse cellulose with succinic anhydride using 1-allyl-3-methyl- imidazolium chloride (AmimCl) ionic liquid as a reaction medium was studied. Parameters investigated included the molar ratio of succinic anhydride/anhydroglucose units in cellulose in a range from 2:1 to 14:1, reaction time (from 30 to 160 min), and reaction temperature (between 60 and 110 °C). The succinylated cellulosic derivatives were prepared with a low degree of substitution (DS) ranging from 0.071 to 0.22. The results showed that the increase of reaction temperature, molar ratio of SA/AGU in cellulose, and reaction time led to an increase in DS of cellulose samples. The products were characterized by FT-IR and solid-state CP/MAS 13C NMR spectroscopy, and thermal analysis. It was found that the crystallinity of the cellulose was completely disrupted in the ionic liquid system under the conditions given. The data also demonstrated that homogeneous modification of cellulose with succinic anhydride in AmimCl resulted in the production of cellulosic monoester. The thermal stability of the succinylated cellulose decreased upon chemical modification. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Cellulose; Ionic liquid; Succinic anhydride; Sugarcane bagasse; Crystallinity 1. Introduction Cellulose, the most abundant of all naturally occur- ring substances, is the principal structural cell wall com- Sugarcane bagasse (SCB), an abundant agricultural lig- ponent of all major plants and accounts for about a half nocellulosic byproduct, is a fibrous residue of cane stalks of the total bagasse. It is nontoxic, renewable, bio- left over after the crushing and extraction of the juice degradable and modifiable, and has great potential as an from the sugarcane. About 54 million dry tons of excellent industrial material.3–5 Chemical modification bagasse is produced annually throughout the world.1 The of cellulose is one method for the production of value- bagasse can be used as a fuel for the boilers by the sugar added products. It is based on reactions of the free factory or as a raw material for the manufacture of pulp hydroxyl groups in the anhydroglucose units (AGU), and paper products, various types of building boards, resulting in the production of cellulose derivatives. and certain chemicals.2,3 However, the intrinsic lack of solubility of native cellu- lose in water and most organic solvent systems consti- tutes a major obstacle for cellulose homogeneous * Corresponding author at present address: College of Forestry, The North-Western University of Agricultural and Forest Sciences and modification. The efficient dissolution of cellulose is a Technology, Yangling, China. Tel.: +86 29 87082214; fax: +86 29 long-standing goal in cellulose research and develop- 6 87032276; e-mail: [email protected] ment. A number of solvent systems, such as DMAc/ 0008-6215/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carres.2007.02.006 920 C. F. Liu et al. / Carbohydrate Research 342 (2007) 919–926 LiCl, DMF/N2O4, NMNO, and some molten salt 2. Results and discussion hydrates like LiClO4Æ3H2O, have been found to be effi- cient for cellulose dissolution. Homogeneous cellulose 2.1. Modification and the degree of substitution derivatizations, such as esterification, etherification, and other reactions, have also been reported in these sol- The yield of cellulose was found to be 49.6% of the dry vents.6–10 However, there remain limitations such as tox- SCB. The neutral sugar composition of cellulose showed icity, cost, difficulty of solvent recovery, or instability in that glucose was the predominant sugar component, processing. comprising 94.8% of the total sugars, indicating a In recent years, the application of ionic liquids (ILs) relatively high content of cellulose. Small amounts of as alternative solvents and reaction media for a wide non-cellulosic sugars, such as xylose (2.8%), galactose variety of synthetic processes has received increasing (1.5%), mannose (0.8%), and arabinose (0.6%), were interest around the world.11,12 Properties of ILs, includ- also observed, indicating that the cellulosic preparation ing low melting points, wide liquid ranges, and lack of contained small quantities of residual hemicelluloses. vapor pressure, have encouraged researchers to explore The intrinsic viscosity, degree of polymerization, and known processes or chemical reactions using ILs in molecular weight of the native cellulosic preparation place of volatile organic solvents. In 2002, Swatloski were determined to be 378 mL gÀ1, 1277, and et al.13 firstly reported that 1-butyl-3-methylimidazo- 206,800 g molÀ1, respectively. In comparison, treatment lium chloride ([C4mim]Cl) could be used as a nonderiv- of the cellulosic preparation in AmimCl at 80 °C for atizing solvent for cellulose, and the high chloride 10 h with a cellulose to AmimCl weight ratio of 2.0% concentration and activity in ILs was found to play an resulted in a substantial degradation of cellulose macro- important role in cellulose dissolution. However, the rel- molecules under the dissolving process given. This was atively high melting point of [C4mim]Cl (approximately shown by a reduction in its intrinsic viscosity, degree 80 °C) limits its application as a solvent or reaction of polymerization, and molecular weight to 286 mL gÀ1, medium. In 2003, 1-allyl-3-methylimidazolium chloride 934, and 151,280 g molÀ1, respectively. In this case, the (AmimCl) IL, which has a low melting point (17 °C), high chloride concentration and activity in ILs are was synthesized and proved to have outstanding capa- highly effective in breaking the extensive hydrogen- bility for dissolving cellulose.14 The chemical structure bonding network present in cellulose, and play an of AmimCl is shown in Chart 1. It is worth noticing that important role in cellulose dissolution. Above the criti- one substituent on nitrogen is alkenyl instead of satu- cal temperature, the ion pair in AmimCl dissociated into rated alkyl. As a result this IL has a lower melting point individual ClÀ and Amim+ ions. Free ClÀ ions, which and maintains a higher thermal stability compared to are nonhydrated and in a concentration of approxi- other ILs substituted by saturated alkyl containing the mately 20%, then bonded to the cellulose hydroxyl pro- same number of carbon atoms.10 These properties make tons, and the free cations associated with the cellulose AmimCl a promising reaction medium for cellulose hydroxyl oxygen. This disrupted the hydrogen bonding functionalization. Homogeneous acetylation of cellulose network of cellulose and promoted its dissolution.15,16 with acetic anhydride in AmimCl has been successfully 13C NMR studies of cellulose in ILs also showed that investigated.10 As far as the authors are aware, succinyl- the cellulose was disordered in this medium, indicating ation of cellulose using ILs as reaction media has not that its hydrogen bonding network was indeed dis- been reported. We therefore investigated the possibility rupted.17 In addition, we also found that the DP of of homogeneous cellulose succinoylation in ILs. In this the cellulose decreased from 1310 to 751 (data not work, succinoylation conditions of SCB cellulose in shown) when [C4mim]Cl was used as a solvent, suggest- AmimCl IL, including the molar ratio of succinic anhy- ing that the extent of degradation of cellulose in dride (SA)/AGU in cellulose between 2:1 and 14:1, reac- AmimCl was lower than in [C4mim]Cl. A similar tion temperature from 60 to 110 °C, and reaction time observation on the degradation of cellulose in ILs has from 30 to 160 min, were investigated. The extent of suc- also been reported by Ren et al.14 The authors reported cinoylation was measured by the DS of cellulose deriva- that the DP of regenerated cellulose decreased from 640 tives. The modified cellulosic samples were then to 460 in AmimCl and to 360 in [C4mim]Cl. characterized by FT-IR and solid-state CP/MAS 13C Modification of cellulose with linear chain anhydrides NMR as well as thermal analysis. produces an undesired by-product, namely the corre- sponding carboxylic acid. This acid must be removed from the reaction following modification. However, Cl modification with cyclic anhydrides such succinic anhy- N N dride does not yield a by-product. Furthermore, the CH2=CHCH2 CH3 reaction results in a pendant carboxylic moiety attached Chart 1. Chemical structure of the ionic liquid (1-allyl-3-methylimid- to the cellulose via a covalent ester bond, providing a azolium chloride). site upon which further reactive chemistry is possible. C. F. Liu et al. / Carbohydrate Research 342 (2007) 919–926 921 More importantly, since acylation of cellulose with a leads to cellulose laurates with DS decreasing to 0.34– cyclic anhydride would esterify one hydroxyl group 1.54 under the same conditions in ILs.21 In other words, per anhydride reached, the net result would be the gen- a steric effect on the uncatalyzed acylation reaction eration of one carboxylic acid group of equal or greater should be indicated, where increasing alkyl chain length hydrophilicity of hydrogen bonding water molecules.