Structural and thermal characterization of sugarcane bagasse cellulose succinates prepared in ionic liquid
Introduction
Cellulose is a uniform, linear glucose polymer photosynthesized by solar energy in various plants and is the most abundant of all naturally occurring substances [1]. That is, the cellulosic polymer is the principal structural cell wall component of all major plants. It is non-toxic, renewable, biodegradable and modifiable, and has great potential for use as an excellent industrial material [2]. At present, cellulose is widely used as a raw material in numerous industrial applications, e.g. in paper, paint, textile, food and pharmaceutical industries [2], [3].
As environmental requirements have become of great importance in today's society, there is an increasing interest in the industrial use of renewable resources [4], [5], and considerable efforts are now being made in the research and development of cellulose as the basic material in industrial applications. Chemical modification of cellulose is one method for the production of value-added products. It is based on reactions of the free hydroxyl groups in the anhydroglucose units (AGU), resulting in the production of cellulose derivatives.
The intrinsic lack of solubility of native cellulose in water and most organic solvent systems constitutes a major obstacle for cellulose homogeneous modification. The efficient dissolution of cellulose is a long-standing goal in cellulose research and development [6]. A number of solvent systems, such as DMAc/LiCl, DMF/N2O4, NMNO, and some molten salt hydrates like LiClO4·3H2O, have been found efficient for cellulose dissolution [6], [7], [8], [9], [10]. However, there remain limitations such as toxicity, cost, difficulty for solvent recovery, or instability.
In recent years, the application of ionic liquids (ILs) as alternative solvents and as reaction media for a wide variety of synthetic processes is an area of intense research [11], [12], [13], [14]. Properties of ILs, including low melting points, wide liquid ranges, and lack of vapor pressure, have encouraged researchers to explore known processes or chemical reactions using ILs in place of volatile organic solvents. In 2002, Swatloski et al. [15] firstly reported that ILs including 1-butyl-3-methylimidazolium ([C4mim]+) with different anions could be used as non-derivatising solvents for cellulose. It is considered that the high chloride concentration and activity in ILs play an important role in cellulose dissolution. In 2003, another IL, 1-allyl-3-methylimidazolium chloride (AmimCl), was reported to have outstanding capability for dissolving cellulose [16]. The utilization of ILs in the research of cellulose and cellulosic products such as paper and fiber has been increasing over the past two years [17], [18], [19]. On the other hand, the investigation into the use of ILs as reaction media for cellulose functionalisation has been reported. Homogeneous acetylation of cellulose in different ILs in absence of any catalyst was accomplished [6], [10], [20], [21]. In 2005, carboxymethylation of cellulose in 1-butyl-3-methylimidzolium chloride ([C4mim]Cl) was also successfully investigated [6]. As far as the authors are aware, succinylation of cellulose using ILs as reaction media has not been reported. We therefore investigated the possibility of cellulose succinylation in ILs.
In this work, succinoylation conditions of sugarcane bagasse (SCB) cellulose in homogeneous solutions of [C4mim]Cl/DMSO systems, including molar ratio of succinic anhydride (SA)/AGU in cellulose between 1:1 and 12:1, reaction temperature 85–105 °C, and reaction time 5–120 min, were investigated. The extent of succinoylation was measured by the DS (degree of substitution) of cellulose derivatives. The modified cellulosic samples were then characterized by FT-IR and solid-state CP/MAS 13C NMR as well as thermal analysis. In addition, the effects of ILs on the degradation and the degree of crystallinity of the cellulosic polymer under the conditions used were also investigated.
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Materials
Sugarcane bagasse (SCB) was obtained from a local sugar factory (Guangzhou, China). It was dried in sunlight and then cut into small pieces. The cut SCB was ground and screened to prepare 20–40 mesh size particles (450–900 μm). The ground SCB was further dried in a cabinet oven with air circulation for 16 h at 50 °C before use. [C4mim]Cl was purchased from the Institute of Chemistry and Chemical Engineering, Heibei Normal University, China, and used as received. All other chemicals were of
Degradation of cellulose and degree of substitution
The yield of crude cellulose was found to be 52.4% of the dry SCB. The neutral sugar composition of the crude cellulose showed that glucose was the major sugar component in the cellulosic sample, comprising 55.7% of the total sugars. On the other hand, it should be noted that the cellulose preparation also contained noticeable amounts of non-cellulosic sugars, such as xylose (25.8%), arabinose (12.8%), galactose (3.2%), and mannose (1.2%), indicating that the cellulosic preparation contained
Conclusions
The above results showed that the succinoylation of cellulose with succinic anhydride using 1-butyl-3-methylinidazolium chloride ionic liquid/DMSO system as reaction medium was successfully accomplished, and carboxylic groups were introduced into cellulose. During dissolution and derivatisation, the cellulosic polymer was substantially degraded and was completely converted from its crystalline structure into amorphous state. The DS of cellulose derivatives ranged from 0.037 to 0.53, and
Acknowledgements
The authors are grateful for the financial support of this research from National Natural Science Foundation of China (No. 30430550), Guangdong Natural Science Foundation (No. 05103548) and Ministry of Education, China.
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