Elsevier

Carbohydrate Polymers

Volume 95, Issue 1, 5 June 2013, Pages 332-337
Carbohydrate Polymers

Improved process for the production of cellulose sulfate using sulfuric acid/ethanol solution

https://doi.org/10.1016/j.carbpol.2013.03.003Get rights and content

Highlights

  • Sulfuric acid/ethanol was successfully used as sulfating agent to prepare CS.

  • Water in system was removed to decrease the hydrolysis by adding Na2SO4.

  • The conditions on the sulfating process were investigated in detail.

Abstract

An improved process for production of cellulose sulfate (CS) was developed by using sulfuric acid/ethanol solution as sulfonating agent and Na2SO4 as water absorbent. The FTIR, SEM and TG analysis were used to characterize the CS prepared. The total degree of substitution and viscosity of the product solution (2%, w/v) were ranging from 0.28 to 0.77 and from 115 to 907 mPa s, respectively, by changing the process parameters such as the amount of Na2SO4, the reaction time, the temperature, the sulfuric acid/alcohol ratio and liquid/solid ratio. The results indicated that the product with DS (0.28–0.77) and η2% (115–907) mPa s could be produced by using this improved process and more cellulose sulfate could be produced when cellulose was sulfonated for 3–4 h at −2 °C in sulfuric acid/ethanol (1.4–1.6) solution with addition of 0.8 g Na2SO4. The 13C NMR indicated that the sulfate group of CS produced using sulfuric acid/ethanol solution was at C6 position.

Introduction

Cellulose sulfate (CS) is the derivative of cellulose with the hydroxyl groups (position 2, 3 and 6) in cellulose anhydroglucose unit (AGU) substituted by sulfate groups in partially or completely. Water-soluble cellulose sulfate (CS) is the material of choice for several medical and biotechnological applications because it is biocompatible and biodegradable (Xie et al., 2009, Zhu et al., 2010). Moreover, CS has great potential in broad-spectrum microbicides (Friend, 2010), drug delivery system (Wang, Xie, Chen, & Yao, 2010), anticoagulant agent (Wang, Li, Zheng, Normakhamatov, & Guo, 2007) and thickener (Yang, Ling, & Qu, 1988). It can also be used with polycation of poly-dimethyldiallyl-ammonium-chloride (PDMDAAC) or chitosan to prepare novel polyelectrolyte complex (PELC) membranes for salt rejection and vapor permeation (Chen et al., 2007, Li and Yao, 2009), or microcapsules for the immobilization of micro-organisms (Liu, Ren, & Yao, 2010), enzymes (Vikartovska et al., 2007), and animal cells etc. (Stiegler et al., 2007).

At present, the preparation of NaCS primary includes homogeneous and heterogeneous processes. In traditional homogeneous process, the mixture N2O4/N,N-dimethylformamide (DMF) formed nitrite ester groups, and subsequently substituted by sulfate groups using SO3 or chlorosulfonic acid as sulfonating agents (Schuldt, Wagenknecht, & Richter, 2002). It was not environment-friendly because of toxic solvent used. In a newly developed homogeneous process, cellulose was dissolved in ionic liquids, and then sulfonated by SO3-pyridine complex. CS with DS ranging from 0.14 to 1.46 was prepared in BMIMCl/DMF (Gericke et al., 2009a, Gericke et al., 2009b, Gericke et al., 2009). In heterogeneous process, CS was prepared in a mixture of isopropylalcohol and sulfuric acid as shown in Eq. (1) (Yao, 1999, Yao, 2000). The typical DS of CS was about 0.7. Compared to the homogeneous process, heterogeneous method is a direct cellulose sulfonating process. Although the acidic reaction conditions led to significant chain degradation and the various distribution of substituents, the simple and safe process without toxic reagents was an appealing process for industrial preparation of CS.

Propanol, which was used as the diluent in traditional heterogeneous processes, can decrease the carbonization of cellulose by decreasing the concentration of sulfuric acid. However, it is a low toxicity reagent with intense irritant smell, and will do harm to the human body and the environment. To overcome the problems of propanol, the cheap ethanol is used to replace the propanol in this work. Ethanol is the homologue of propanol, and is a non-toxic reagent. In addition, it is very easy to recycle by distillation. Hence, it is an ideal substitute of propanol in the heterogeneous process. The molecular weight is a very important characteristic of CS which affects the solubility, viscosity and antimicrobial. The serious degradation of cellulose, which results in the sharp decrease of product molecular weight, is another problem of the heterogeneous processes. In the reaction system, the hydrolysis of cellulose is due to the water produced in the esterification of alcohol with sulfuric acid and the cellulose sulfonating. Anhydrous Na2SO4 often used for the dehydration of organic solvents. Anhydrous sodium sulfate was added to the heterogeneous reaction system to timely remove the water generated, which did not introduce new impurity ions into system.

In this paper, an improved heterogeneous process was studied. CS was prepared by using the mixture of sulfuric acid/ethanol as sulfonating agent with addition of Na2SO4 as water absorbent. The FTIR was used to determine the groups of the product. The effect of process parameters on Ds (total degrees of substitution), η2% (the kinematic viscosity of the 2% CS aqueous solutions), Y (yield) and X (percent conversion of cellulose) were evaluated as well.

Section snippets

Materials

Native cotton with a cellulose content of 87–90% was purchased from Qipan Quilt Factory(Xiamen). Sulfuric acid (98%, pure), ethanol (>99.7%, pure) and anhydrous sodium sulfate were obtained from Sinopharm (China) Chemical Reagent Co., Ltd., sodium hydroxide was from Xilong (Shantou) Chemical Factory Co., Ltd., and deionized water was prepared in our laboratory. All other chemicals and reagents were analytical grade and used as received.

Process for the preparation of NaCS

NaCS was prepared by process of heterogeneous reaction.

FTIR of cellulose sulfate

FTIR spectroscopy was used to confirm the structure of product prepared from the improved process. The IR spectra of native cellulose and CS prepared were shown in Fig. 1. The spectrum of native cellulose was in agreement with the study (Gumuskaya et al., 2003, Nazi et al., 2012, Qua et al., 2011). The vibration band at 3398 cm−1 and 2911 cm−1 are assigned to the stretching of H-bonded OH groups and Csingle bondH stretching, respectively. The characteristic peak of the cellulose main structure was at 1637 cm

Conclusion

In this study, the CS was successfully prepared by the improved heterogeneous process with sulfuric acid/ethanol solution as a novel sulfonating agent and anhydrous sodium sulfate as water absorbent. The improved process was more inexpensive and environmentally friendly, which could be applied to the scale production of CS instead of traditional sulfuric acid/N-propanol system. The CS prepared was characterized by FTIR, SEM and TGA. The DS and Kinematic viscosity (η2%) of the CS can be

Acknowledgements

The authors express their thanks for the support from the Nature Science Foundation of China (20906035) and the Fundamental Research Funds for the Central Universities (JB-SJ1008).

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