Study on the heterogeneous degradation of chitosan with hydrogen peroxide under the catalysis of phosphotungstic acid
Introduction
Chitosan, (1 → 4)-2-amino-2-deoxy-β-d-glucan, is a natural polymer generally obtained by full or partial deacetylation of chitin. Due to many unique properties such as biocompatibility, biodegradability, bioactivity and non-toxicity (Carmen and Roland, 1997, Cheng et al., 2003), chitosan has broad range of actual and potential applications. Much attention is focused on the use of chitosan in many areas such as biotechnology, pharmaceuticals, wastewater treatment, cosmetics, agriculture, food science and textiles, etc. (Li, Dunn, Grandmaison, & Goosen, 1997).
However, chitosan has a high molecular weight resulting in its low solubility in most solvents, which limits its wide applications especially in medicine and the food industry. To improve its solubility and biological, chemical and physical properties, several methods have been tried to prepare a water-soluble low molecular weight chitosan (LWCS) without altering its chemical structure, some of which show advantages, but also disadvantages (Rege and Block, 1999, Tanioka et al., 1996, Terbojevich et al., 1996, Vårum et al., 2001, Wang et al., 2005, Zhang and Neau, 2002).
H2O2 has long been used in the treatment of chitosan because it is easy to handle, easily available and environmentally friendly (Chang et al., 2001, Qin et al., 2002, Shao et al., 2003). This technique is based on the formation of free radicals, which can attack the β-d-(1 → 4) glucosidic linkages of chitosan. However, the formation of radical groups is inefficient when H2O2 is used alone. Recently, to improve the efficiency, other degradation patterns of chitosan with H2O2 have been reported (Shao et al., 2003, Wang et al., 2005). But to date there have been few reports about the degradation with H2O2 under the catalysis of heteropoly acids in heterogeneous phase.
Heteropoly acids with Keggin anion structures have received considerable attention due to many advantages (Okuhara, 2002, Okuhara et al., 2001), such as simple preparation, high reactivity, non-corrosive, non-pollutive and excellent stability. Specifically, phosphotungstic acid is among the most extensively studied (Dias et al., 2003, Hu and Xu, 2004, Kozhevnikova and Kozhevnikov, 2004) since it possesses high acidic strength and relatively high thermal stability (Devassy et al., 2005), which can be used as acid, oxidative and bifunctional catalysts in homogeneous or heterogeneous phase.
Usually the oxidative degradation of chitosan with H2O2 occurs in homogeneous phase, such as in acetic acid solution. However, in this paper, the degradation in heterogeneous phase was studied, which avoided using acetic acid, furthermore, made the precipitation process of LWCS convenient. The effect of volume of H2O2, dosage of phosphotungstic acid, reaction temperature and time on the degradation was discussed by orthogonal tests. The degradation mechanism was also discussed by FTIR, DRS and XRD analysis.
Section snippets
Materials
Original chitosan, obtained from Yuhuan Biology Engineering (Zhejiang, China), whose degrees of acetylation is 95.54%, its viscosity-average molecular weight (Mv) is about 700,000, determined based on viscosity measurements (Wang et al., 2005). Phosphotungstic acid, phosphomolybdic acid, tungstosilicic acid, hydrogen peroxide and other reagents used, supplied by Fuzhou Chemical Agent Corporation (Fujian, China), were utilized without further purification.
Heterogeneous degradation of chitosan
Chitosan 1.5000 grams was put into 50 mL
Oxidative degradation of chitosan under the catalysis of different heteropoly acids
In order to reveal the contrast of catalysis between phosphotungstic acid and other heteropoly acids, four experiments on the degradation of chitosan were designed. In these experiments, catalyst was tungstosilicic acid, phosphotungstic acid and phosphomolybdic acid in experiment 1, 2 and 3, respectively, the quantity of each catalyst was 0.02 g. In experiment 4, catalyst was not used. The other reaction conditions were determined as follows: chitosan 1.5000 g, 30% (wt%) H2O2 3 mL, H2O 17 mL,
Conclusions
In heterogeneous phase, chitosan was effectively degraded with H2O2 under the catalysis of phosphotungstic acid. The optimum reaction conditions determined by orthogonal tests were as follows: 30% (wt%) H2O2 3 mL, amount of phosphotungstic acid 0.02 g, reaction temperature 70 °C, reaction time 30 min.
By FTIR, DRS and XRD analysis, it was presumed that the mechanism of degradation be attributed to free radical formed by the reaction of phosphotungstic acid and H2O2, which result in the rupture of
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