A novel approach of utilizing quaternized chitosan as a catalyst for the eco-friendly cycloaddition of epoxides with CO₂

Abstract

A novel attempt of using quaternized chitosan (QCHT) as a catalyst for the cycloaddition reaction of allyl glycidyl ether (AGE) and CO₂ under solvent-free conditions has been made. The surface quaternization of CHT has been characterized by means of various physicochemical methods, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and elemental analysis. The effects of reaction parameters like catalyst amount, reaction time, reaction temperature, and CO₂ pressure have been investigated. The catalyst system was recyclable and was reused. It has been demonstrated that the inherently present hydroxyl group in the catalyst had a synergistic effect with halide anions, and a high yield of cyclic carbonates and excellent selectivity could be obtained under optimum conditions. These results reveal that QCHT is an efficient and environmentally friendly catalyst for the cycloaddition reaction.

---

Introduction

The chemistry of CO₂ and CO₂ fixation have received much attention from both economical and environmental points of view.¹ Conversion of CO₂ into carbonates is a potentially significant transformation that could help decrease carbon emissions. Cyclic carbonates can be used as alternative nonflammable solvents, monomers for polycarbonate synthesis, or solvents in secondary and fuel-cell batteries for polymer and gel electrolytes.² To synthesize cyclic carbonates, a wide range of catalysts such as alkali metal salts,^3a metal oxides,^3b–d zeolites,^3e organic bases,^3f titanosilicates,^3g transition metal complexes,^3h and ionic liquids (ILs)^3i,j have been employed. Owing to their low viscosities and negligible vapor pressures, ILs have been employed widely as the most efficient catalysts for the cycloaddition of CO₂ and epoxides. Unfortunately, because of their homogenic nature, product separation and catalyst recovery are difficult. Recent years have witnessed the generation of heterogeneous IL catalysts prepared by grafting ILs onto solid supports. This approach retains the important physical and chemical features of ILs, such as their nonvolatility, nonflammability, and good thermal stability, as well as their high catalytic activity and selectivity. Therefore, supported ILs are considered promising catalysts for cycloaddition reactions. The quaternary ammonium alkyl halides and imidazolium alkyl halides immobilized on silica^4a–f and polymer supports^4g have been extensively studied for use as heterogeneous catalysts for the synthesis of carbonates.

Chitosan (CHT), the deacetylated derivative of chitin, is the second most abundant polysaccharide found on Earth after cellulose.^5a It is found in the structures of a wide number of invertebrates (crustaceans' exoskeletons and insects' cuticles) and in the cell walls of fungi. As a natural renewable resource, CHT has attracted attention for its physicochemical characteristics and bioactivities.^5b In addition to its biocompatibility, biodegradability, and nontoxicity, CHT can be easily modified chemically or physically, which makes it a versatile supporting material.^5c The CHT/IL system has been used by a few groups as a supporting matrix for palladium catalyzed Heck's and allylation reactions.^6a–c A few studies discussing the supportive role played by CHT in the cycloaddition reaction have been published. Xie et al.^7a demonstrated that a CHT/IL solution is an efficient, reversible fixing system for CO₂. Xiao et al.^7b reported a chitosan-supported zinc chloride catalyst system with 1-butyl-3-methylimidazole halide as a cocatalyst. Even though this catalyst was active and recyclable, fresh cocatalyst needed to be added during each catalytic cycle, resulting in a complicated system. Recently, Zhao et al.^5c and Sun et al.^7c succeeded in covalently immobilizing quaternary ammonium and imidazolium based ILs, respectively, onto the chitosan moiety. Albeit their high efficiency, being infamous for their air and moisture sensitiveness makes the IL containing catalyst synthesis possibly cumbersome. Hence arises the demand for a pertinent and stable system which could overcome these shortcomings.

One of the most widely modified forms of CHT is quaternized chitosan (QCHT). QCHT is a well-established material in biomedical fields and has many applications in antibacterial agents,^5b antioxidant agents,^5a,8a and antifungal agents.^8b Herein we report for the first time the use of QCHT as a catalyst in which the quaternization is done on the primary amine group present inherently on the CHT itself forming a cationic biopolymer (Scheme 1), rather than covalently tethering an already quaternized species such as quaternary ammonium or imidazolium salts into the CHT backbone. The results from our previous study^4a revealed the role of –COOH and –OH groups in enhancing the catalytic activity. Delighted by this, we foresee that QCHT possessing hydroxyl groups capable of forming hydrogen bonding would enhance the catalysis by its own rather than acting merely as a support. The process offers a simple, ecologically safer, cost-effective, and energy-saving route for synthesizing cyclic carbonates with high product quality, as well as easy catalyst recycling.

Scheme 1 Preparation of quaternized chitosan (QCHT).

Experimental

Materials

Chitosan, iodomethane (99%), sodium iodide (99.5%), sodium hydroxide (97%), and 1-methyl-2-pyrrolidinone (99%) were purchased from Aldrich and used without further purification. Allyl glycidyl ether (AGE) and other epoxides were also obtained from Aldrich and used as received. CO₂ of 99.999% purity was used without further purification. CH₂Cl₂ was obtained from SK Chemicals, Korea, and used as received.

Catalyst synthesis

QCHT was synthesized as follows (Scheme 1).^8b First, 3 g of chitosan was dispersed into 150 mL of 1-methyl-2-pyrrolidinone (NMP) for 12 h at room temperature. To this mixture, 0.35 mL of NaOH (1 M), 4.49 g of NaI, and 12 mL of CH₃I were added, and the reaction was carried out under stirring at 40 °C for 20 h. The solution was precipitated by excess acetone, and the precipitate was thoroughly washed 3–4 times with acetone, filtered, and dried at 60 °C for 24 h to obtain QCHT.

Characterization

The catalyst was characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), elemental analysis (EA), and field emission electron microscopy (FE-SEM). The FT-IR spectra were obtained on an AVATAR 370 Thermo Nicolet spectrophotometer with a resolution of 4 cm⁻¹. The X-ray diffraction (XRD) patterns were obtained on a Philips PANalytical X'pert PRO Model power diffractometer operating at 40 kV and 30 mA using Ni-filtered Cu Kα radiation (λ = 1.5404 Å). The diffractograms were recorded in the 2θ range of 10–80°. XPS analyses were performed using an X-ray photoelectron spectrometer (VG, ESCALAB 250) with monochromatic Al Kα radiation (hν = 1486.6 eV). Thermogravimetric analysis (TGA) was performed with 1.72 and 1.61 mg of CHT and QCHT, respectively, on an AutoTGA 2950 apparatus under a nitrogen flow of 100 mL min⁻¹ using a heating rate of 10 °C min⁻¹ from room temperature to 600 °C. Elemental analyses of the catalyst were carried out using a Vario EL III analyzer. The surface features of CHT and QCHT were observed using an S-4200 field emission electron microscope (FE-SEM, Hitachi).

Cycloaddition of AGE with CO₂

The synthesis of the cyclic carbonate from allyl glycidyl ether (AGE) and CO₂ (Scheme 2) using the QCHT catalyst was performed in a 50 mL stainless steel autoclave equipped with a magnetic stirrer. For each typical batch operation, QCHT catalyst (0.1 g) and AGE (18.6 mmol) were charged into the reactor without solvent and then purged several times with CO₂. The reactor was then pressurized with CO₂ to a preset pressure, 0.34–1.17 MPa, at room temperature. The reactor was heated to the desired temperature, and then the reaction was started by stirring the reaction mixture at 600 rpm. The reactor pressure increased by about 0.03–0.10 MPa, depending on the reaction temperature, because of the vapor pressure of the reactants. After the completion of the reaction time, the cycloaddition was stopped by cooling the reaction mixture to room temperature and venting the remaining CO₂. The product (4-[(allyloxy)methyl]-1,3-dioxolan-2-one) was dissolved in dichloromethane and filtered to remove the catalyst. Product analysis was carried out via gas chromatography/mass spectrometry (GC-MS, Micromass, UK) analysis, ¹H NMR (500 MHz, CDCl₃) δ (ppm): 5.23–5.14 (2H), 5.86 (1H), 4.82–4.76 (1H), 4.49–4.45 (2H), 4.5 (2H), 4.82–4.9 (2H) and ¹³C NMR (75 MHz, CDCl₃) δ (ppm): 154.2, 61.7, 71.8, 72, 75.1, 131.7, 115.4. The conversion of AGE was obtained from gas chromatography (GC, HP 6890, Agilent Technologies, Santa Clara, CA, USA) data.

Scheme 2 Cycloaddition of allyl glycidyl ether and CO₂.

Results and discussion

Catalyst synthesis

In general, the procedure for direct alkylation of primary amines to their quaternary ammonium salts involves the use of an organic base rather than an inorganic one. However, because of their insolubility in the medium and lower pK_a values than those of CHT, organic bases are replaced with an inorganic base for this quaternization reaction. When an inorganic base is used, complications arise from the fact that the physical properties of quaternary ammonium salts resemble those of inorganic salts. To mitigate this effect, the solution of NaOH must be as weak as possible to limit the degradation of CHT, while at the same time strong enough to maintain a global pK_a higher than that of CHT over the whole synthesis time. It has been found that the degree of quaternization obtained differs only slightly when an inorganic base is used, and therefore a stoichiometric equivalent of the base to the NH₂ group is sufficient.^9a

It has also been found that the degree of quaternization increases with increasing temperature.^9b Since the boiling temperature of CH₃I is 42.5 °C, synthesis is carried out at 40 °C. As the synthesis advances, the charge density increases, and the rate of quaternization is affected by the electrostatic hindrance of the molecule. Thus, it is judicious to screen the charges through the presence of an inorganic salt. Since NaOH binds with the iodhydric acid formed and produces NaI, it is preferable to use the same salt, i.e., NaI. After the synthesis of QCHT, its activity as a catalyst was tested by performing the cycloaddition reaction of AGE and CO₂ under solvent-free conditions.

Characterization of catalyst

FT-IR spectra of the CHT and QCHT samples obtained are shown in Fig. 1. The IR peaks of CHT and QCHT around 890 and 1155 cm⁻¹ were assigned to the saccharine structure.^5a Characteristic peaks (3500 cm⁻¹) ascribed to O–H stretching were observed in both the CHT and QCHT spectra.^10a The same peak could be also due to the N–H stretching vibrations. The spectra of QCHT revealed the presence of the characteristic band at 1475 cm⁻¹ that corresponds to the asymmetric angular bending of methyl groups of quaternary nitrogen atoms. This peak belonged to the methyl groups of the –N⁺Me₃ residues.^10b The above mentioned results confirm that the quaternization in QCHT was successful.

Fig. 1 FT-IR spectra of CHT and QCHT.

The XRD patterns (Fig. 2) of CHT exhibit two peaks. One is at 2θ = 20°, and the other is a broad peak around 2θ = 13°, and these peaks are ascribed to the diffraction at the plane of the crystal region in the chitosan structure.^11a At the same time, QCHT contained only one broad diffraction peak centered at 2θ = 22°, suggesting an amorphous structure. In CHT, the crystallization capacity is enhanced because of the intramolecular hydrogen bonding between the NH₂ and OH groups in the repeating hexosaminide residue.^11b However, quaternization resulted in the breakage of the intramolecular hydrogen bonding, leading to the decrease in crystallinity.

Fig. 2 XRD patterns of CHT and QCHT.

XPS analysis further supports the success of the surface quaternization on QCHT. The presence of quaternary ammonium groups after surface modification can be confirmed by comparing the N 1s spectrum (Fig. 3) of QCHT with that of the virgin CHT. The N 1s peak of the CHT can be fitted with one peak at 397 eV. On the other hand, the N 1s peak of QCHT can be split into two peaks, one that appears at the same binding energy as that of the CHT and another that has a higher binding energy at 400 eV. The latter peak can be regarded as a signal from the positively charged nitrogen atom of the quaternary ammonium moiety.¹²

Fig. 3 XPS N 1s spectra of CHT and QCHT.

Fig. 4 shows the TGA analyses of CHT and QCHT. Both showed a similar weight loss at around 100 °C, which is attributed to the removal of adsorbed water molecules. The second weight loss occurred at 300 °C for CHT and 240 °C for QCHT, and it is ascribed to the decomposition of the polysaccharide chain.^7a This weight loss occurred at a lower temperature for QCHT than for CHT. This could be explained by the fact that the thermal stability of CHT was slightly weakened after quaternization. This result is in agreement with the XRD analysis, which showed that a destruction of the crystallinity of CHT is brought about by quaternization.

Fig. 4 TGA analysis of CHT and QCHT.

The elemental analyses of CHT and QCHT are shown in Table 1. The amount of iodide ion in the QCHT is calculated to be 2.6 mmol I⁻ per g of CHT as revealed from the elemental analysis. The presence of iodide ions in QCHT is confirmed from the energy-dispersive X-ray spectroscopy (EDS) spectra shown in Fig. S1 in the ESI.† The SEM images of CHT and QCHT are given in Fig. S2 in the ESI.†

Table 1 Elemental analysis of CHT and QCHT

Sample	%C	%H	%N	%O
CHT	40.3	6.92	7.2	42.88
QCHT	27.41	5.33	4.69	29.45

---

The SEM image of QCHT shows that it is more uniform and has a lower crystallinity than CHT, which is also in accordance with the successful quaternization of CHT.¹³

Synthesis of cyclic carbonate from CO₂ and epoxides

The scope of its ability to accept epoxide substrates was investigated by using QCHT as a catalyst. From the results summarized in Table 2, we can conclude that this catalyst system exhibits high efficiency for almost all of the monosubstituted terminal epoxides (entries 1–5). Among all the terminal epoxides, AGE (entry 4) showed highest conversion and yield. An epoxide with an electron-withdrawing CH₂–Cl group (entry 2) showed lower activity, which is probably due to the reduced electron density of the epoxide oxygen atom.^14a When styrene oxide (entry 5) was used, the conversion was slightly lower than that for other monosubstituted terminal epoxides, probably because of the low reactivity of the β-carbon atom of styrene oxide. The disubstituted epoxide, cyclohexene oxide (entry 6), exhibited the lowest activity for the production of the corresponding cyclic carbonate, probably because of the high steric hindrance produced by the cyclohexene ring.^14b In all cases, cyclic carbonates were formed with very high selectivity (>99%).

Table 2 Cycloaddition of CO₂ and various epoxides by QCHT

Entry	Epoxide	Cyclic carbonate	Time/h	Conversion (%)	Yield (%)
Reaction conditions: epoxides = 18.6 mmol, QCHT catalyst amount = 0.1 g, PCO2 = 1.17 MPa, temp = 120 °C.
1			6	87	86
2			6	84	83
3			6	88	87
4			6	91	90
5			6	81	80
6			24	64	63

---

Effect of reaction parameters

Since the QCHT catalyst system showed good catalytic activity, the effects of the reaction parameters were also examined for cycloaddition reactions using the AGE epoxide substrate. Fig. 5 shows the relationship between the AGE conversion and the amount of catalyst. The conversion increased with increasing the amount of catalyst up to 0.1 g. Further increases in the amount of catalyst did not produce any significant increase in the conversion, probably because of the hindrance of the mass transfer between the active site and reagent caused by the low dispersity of the excess catalyst in the reaction mixture.^15a The influence of the reaction time on the AGE conversion is given in Fig. 6. The results indicate that the reaction proceeded rapidly within the first 6 h, with 91% conversion and 99% selectivity. Thereafter, the reaction rate did not vary significantly. Thus, a reaction time of 6 h was required for maximum AGE conversion.

Fig. 5 Effect of catalyst amount on the reactivity of QCHT. Reaction conditions: AGE = 2.2 mL (18.6 mmol); P_CO2 = 1.17 MPa; temperature = 120 °C; time = 6 h.

Fig. 6 Effect of reaction time on the reactivity of QCHT. Reaction conditions: AGE = 2.2 mL (18.6 mmol); P_CO2 = 1.17 MPa; temperature = 120 °C; catalyst amount = 0.1 g.

As shown in Fig. 7, the temperature had a pronounced positive effect on the coupling reaction when the temperature was varied from 60 to 160 °C. The highest conversion of 91% with 99% selectivity was obtained at 120 °C; thereafter, further increases in temperature produced only slight increases in conversion. Thus, it can be seen that higher reaction temperatures increase the reactivity, but the solubility of the CO₂ gas phase in the reaction system decreases with increasing temperature. The effect of CO₂ pressure was also investigated. As shown in Fig. 8, when the CO₂ pressure increased from 0.34 MPa to 0.97 MPa, the AGE conversion also increased from 20% to 67%. Upon a further increase in the pressure, to 1.17 MPa, the conversion increased to its maximum, 91%. Similar effects of the CO₂ pressure on the catalytic activity have been observed in other catalytic systems. During such catalytic reactions, a higher CO₂ pressure can effectively increase the solubility of CO₂ in AGE and thus result in an acceleration of the cyclic carbonate formation.^15b However, when the catalytic reaction was carried out at a CO₂ pressure of 1.6 MPa, the AGE conversion decreased slightly. It has been reported that too high a CO₂ pressure could decrease the conversion as a result of dilution by excessive quantities of CO₂.^15c Remarkably, the selectivity remained >99% over the entire course of the reaction.

Fig. 7 Effect of reaction temperature on the reactivity of QCHT. Reaction conditions: AGE = 2.2 mL (18.6 mmol); P_CO2 = 1.17 MPa; time = 6 h; catalyst amount = 0.1 g.

Fig. 8 Effect of CO₂ pressure on the reactivity of QCHT. Reaction conditions: AGE = 2.2 mL (18.6 mmol); time = 6 h; temperature = 120 °C; catalyst amount = 0.1 g.

Proposed mechanism of coupling reaction

The DFT studies by Sun et al.^7c established the presence of hydrogen bonding interaction between the oxygen atom of the epoxide and the hydroxyl group of the CHT. Based on this and other reports,^4a,f,16 the following mechanism is proposed. As shown in Scheme 3, first, the hydrogen atom from the OH group of QCHT forms a hydrogen bond with the oxygen atom of the epoxide molecule. Simultaneously, the halide anion of QCHT makes a nucleophilic attack on the less sterically hindered β-carbon atom of the epoxide, furnishing the open-ring intermediate.^7c,16 Then, this intermediate further reacts with CO₂ to form the corresponding cyclic carbonate and regenerate the catalyst. Thus, in this catalytic system, the coexistence of hydrogen bond donors (–OH), the positively charged nitrogen of the quaternary ammonium moiety (–N + Me₃), and halide anions showed a synergistic effect in promoting the coupling reaction.^4a This synergistic effect may be the reason for this system's high catalytic activity and selectivity.

Scheme 3 Proposed mechanism of the cycloaddition reaction.

Reusability of catalyst

The spent catalyst was regenerated by washing it with acetone. It was then filtered and dried at 60 °C for 24 h. The regenerated catalyst was again used for the cycloaddition of AGE and CO₂ under similar conditions. The results in Fig. 9 show that the catalyst could be reused for at least 5 cycles without much loss in its activity, while the selectivity of the product remained the same throughout.

Fig. 9 Recycle test. Reaction conditions: AGE = 2.2 mL (18.6 mmol); time = 6 h; temperature = 120 °C; catalyst amount = 0.1 g; P_CO2 = 1.17 MPa.

Conclusions

A promising new approach of using QCHT as a heterogeneous catalyst was found to be efficient for the cycloaddition of CO₂ and AGE under solvent-free conditions. Investigation of various reaction parameters indicated that the AGE conversion reached its maximum when the reaction was carried out with 0.1 g of catalyst at 120 °C under 1.17 MPa of CO₂ pressure for 6 h. The naturally occurring OH groups in QCHT showed a synergistic effect with the halide anions and helped to improve the catalytic activity. The catalyst was separated and effectively used for up to 5 cycles without much loss in its activity under identical conditions. In summary, the environmentally benign catalyst presented here has great potential for industrial applications because of its advantages in terms of stability, low cost, easy preparation from renewable biopolymers, and simple separation from the product.

Acknowledgements

This study was supported by the Ministry of Knowledge and Economy through the Wide Economy Region Cooperative Program (2010) and the Brain Korea 21 Project. The authors are grateful to KBSI for performing characterization studies.

Notes and references

1. (a) M. North, R. Pasquale and C. Young, Green Chem., 2010, 12, 1514; (b) M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975; (c) J. Melendez, M. North and P. Villuendas, Chem. Commun., 2009, 2577; (d) T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365; (e) S. Klaus, M. W. Lehenmeier, C. E. Anderson and B. Rieger, Coord. Chem. Rev., 2011, 255, 1460.
2. (a) A. G. Shaikh and S. Sivaram, Chem. Rev., 1996, 96, 951; (b) M. Mikkelsen, M. Jorgensen and F. C. Krebs, Energy Environ. Sci., 2010, 3, 43; (c) W. L. Dai, S. L. Luo, S. F. Yin and C. T. Au, Appl. Catal., A, 2009, 366, 2.
3. (a) N. Kihara, N. Hara and T. Endo, J. Org. Chem., 1993, 58, 6198; (b) T. Yano, H. Matsui, T. Koike, H. Ishiguro, H. Fujihara, M. Yoshihara and T. Maeshima, Chem. Commun., 1997, 1129; (c) B. M. Bhanage, S. I. Fujita, Y. Ikushima and M. Arai, Appl. Catal., A, 2001, 219, 259; (d) H. Yasuda, L. N. He and T. Sakakura, J. Catal., 2002, 209, 547; (e) M. Tu and R. J. Davis, J. Catal., 2001, 199, 85; (f) H. Kawanami and Y. Ikushima, Chem. Commun., 2000, 2089; (g) R. Srivastava, D. Srinivas and P. Ratnasamy, Catal. Lett., 2003, 91, 133; (h) H. Xie, S. Li and S. Zhang, J. Mol. Catal. A: Chem., 2006, 250, 30; (i) J. J. Peng and Y. Q. Deng, New J. Chem., 2001, 25, 639; (j) F. Ono, K. Qiao, D. Tomida and C. Yokoyama, J. Mol. Catal. A: Chem., 2007, 263, 223.
4. (a) L. Han, H. J. Choi, S. J. Choi, B. Liu and D. W. Park, Green Chem., 2011, 13, 1023; (b) L. Han, S. W. Park and D. W. Park, Energy Environ. Sci., 2009, 2, 1286; (c) S. Udayakumar, M. K. Lee, H. L. Shim, S. W. Park and D. W. Park, Catal. Commun., 2009, 10, 659; (d) S. Udayakumar, M. K. Lee, H. L. Shim and D. W. Park, Appl. Catal., A, 2009, 365, 88; (e) H. L. Shim, S. Udayakumar, J. I. Yu, I. Kim and D. W. Park, Catal. Today, 2009, 148, 350; (f) K. Motokura, S. Itagaki, Y. Iwasawa, A. Miyaji and T. Baba, Green Chem., 2009, 11, 1876; (g) L. Han, H. J. Choi, D. K. Kim, S. W. Park, B. Liu and D. W. Park, J. Mol. Catal. A: Chem., 2011, 338, 58.
5. (a) Z. Guo, R. Xing, S. Liu, Z. Zhong and P. Li, Carbohydr. Polym., 2008, 73, 173; (b) W. Sajomsang, U. R. Ruktanonchai, P. Gonil and C. Warin, Carbohydr. Polym., 2010, 82, 1143; (c) Y. Zhao, J. S. Tian, X. H. Qi, Z. N. Han, Y. Y. Zhuang and L. N. He, J. Mol. Catal. A: Chem., 2007, 271, 284.
6. (a) V. Calò, A. Nacci, A. Monopoli, A. Fornaro, L. Sabbatini, N. Cioffi and N. Ditaranto, Organometallics, 2004, 23, 5154; (b) R. Moucel, K. Perrigaud, J. M. Goupil, P. J. Madec, S. Marinel, E. Guibal, A. C. Goumont and I. Dez, Adv. Synth. Catal., 2010, 352, 433; (c) N. Clousier, R. Moucel, P. Naik, P. J. Madec, A. C. Gaumont and I. Dez, C. R. Chim., 2011, 14, 680.
7. (a) H. Xie, S. Zhang and S. Li, Green Chem., 2006, 8, 630; (b) L. F. Xiao, F. W. Li and C. G. Xia, Appl. Catal., A, 2005, 279, 125; (c) J. Sun, J. Wang, W. Cheng, J. Zhang, X. Li, S. Zhang and Y. She, Green Chem., 2012, 14, 654.
8. (a) Z. Guo, H. Liu, X. Chen, X. Ji and P. Li, Bioorg. Med. Chem. Lett., 2006, 16, 6348; (b) Z. Guo, R. Xing, S. Liu, Z. Zhong, X. Ji, L. Wang and P. Li, Int. J. Food Microbiol., 2007, 118, 214.
9. (a) A. Domard, M. Rinaudo and C. Terrassin, Int. J. Biol. Macromol., 1986, 8, 105; (b) M. Rinaudo and J. Desbrieres, Makromol. Chem., 1981, 182, 373.
10. (a) E. A. Stepnova, V. E. Tikhonov, T. A. Babushkina, T. P. Klimova, E. V. Vorontsov, V. G. Babak, S. A. Lopatin and I. A. Yamskov, Eur. Polym. J., 2007, 43, 2414; (b) J. Luo, X. Wang, B. Xia and J. Wu, J. Macromol. Sci., Part A: Pure Appl. Chem., 2010, 47, 952.
11. (a) S. Lu, X. Song, D. Cao, Y. Chen and K. Yao, J. Polym. Sci., 2004, 91, 3497; (b) I. Aranaz, M. Mengíbar, R. Harris, I. Paños, B. Miralles, N. Acosta, G. Galed and Á. Heras, Curr. Chem. Biol., 2009, 3, 203.
12. N. Vallapa, O. Wiarachai, N. Thongchul, J. Pan, V. Tangpasuthadol, S. Kiatkamjornwong and V. P. Hoven, Carbohydr. Polym., 2011, 83, 868.
13. J. Wu, Z. G. Su and G. H. Ma, Int. J. Pharm., 2006, 315, 1.
14. (a) T. Sakai, Y. Tsutsumi and T. Ema, Green Chem., 2008, 10, 337; (b) A. Zhu, T. Jiang, B. Han, J. Zhang, Y. Xie and X. Ma, Green Chem., 2007, 9, 169.
15. (a) J. Sun, W. Cheng, W. Fan, Y. Wang, Z. Meng and S. Zhang, Catal. Today, 2009, 148, 361; (b) Y. Xie, Z. F. Zhang, T. Jiang, J. L. He, B. X. Han, T. B. Wu and K. L. Ding, Angew. Chem., Int. Ed., 2007, 46, 7255; (c) L. F. Xiao, F. W. Li, J. J. Peng and C. G. Xia, J. Mol. Catal. A: Chem., 2006, 253, 265.
16. J. Sun, S. Zhang, W. Chang and J. Ren, Tetrahedron Lett., 2008, 49, 3588.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20137b

---