Majid Masteri-Farahani, Samaneh Shahsavarifar

Research Institute of Green Chemistry, Faculty of Chemistry, Kharazmi University, Tehran, Iran

Keywords:Chitosan Magnetite Nanostructure Sulfonic acid Catalysis Esterification

A B S T R A C T Chemical functionalization of chitosan biopolymer and chitosan-magnetite nanocomposite was performed with sulfonic acid functional groups to achieve new solid acid materials. The sulfonic acid functional groups were created through the ring opening nucleophilic reaction of amine groups of chitosan with 1,4-butane sultone. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopies (XPS) verified the successful sulfonic acid functionalization of chitosan. The obtained sulfonic acid functionalized chitosan-magnetite nanocomposite showed superparamagnetic properties according to the vibrating sample magnetometry analysis and exhibited magnetic separation feature from dispersed mixtures.Nitrogen adsorption-desorption analysis indicated the increase in surface area after formation of chitosan-magnetite nanocomposite and functionalization with sulfonic acid. Both of the prepared solid acids exhibited high catalytic activities in the acid-catalyzed acetic acid esterification with n-butanol and benzaldehyde acetalization with ethylene glycol as model reactions.Furthermore,they can be reused several times without considerable loss of their activities.

1. Introduction

Chitosan is an inexpensive natural biopolymer which can be used as an appropriate support for designing new heterogeneous catalysts. It has some desirable properties such as non-toxicity,biocompatibility, and biodegradability [1-4]. These properties as well as its facile functionalizability make chitosan as an attractive support for the immobilization of catalytically active species[5-11]. Furthermore, organic-inorganic nanocomposites based on chitosan and magnetite nanoparticles have shown important contributions to the current researches.In this context,due to its simple and fast recovery,chitosan-magnetite nanocomposite has been utilized as an interesting support in various catalytic reactions[12-15].

Esterification of carboxylic acids is one of the important organic reactions generally performed in liquid phase.Among them,acetic acid esterification withn-butanol is industrially employed to preparen-butyl acetate with wide applications [16-21]. On the other hand,acetalization reactions with ethylene glycol is a key reaction in multi-step organic syntheses [22]. Cyclic acetals, which are resistant in various conditions are utilized for protecting the carbonyl groups during synthetic organic reactions. Furthermore,acetals have been utilized in various applications such as solvents,polymers, fragrances, and biofuels [23].

Preparation of acetals and esters requires the utilization of strong Br?nsted acid catalysts such as phosphoric acid and sulfuric acid which are encountered by some disadvantages such as environmental hazards, equipment corrosion, and difficult catalyst recycling. To overcome these issues, several attempts have been done to substitute the liquid phase acid catalysts by solid acids.Zeolites[24,25],resins[26,27],heteropoly acids[28-32],and functionalized mesoporous silicas [33-36], have been exploited as heterogeneous catalysts in acetalization and esterification reactions.The support structure and its interaction with active species have important effect on the catalytic activity and acidity of the heterogeneous catalysts.There are also some reports on the preparation of solid supported sulfonic acids with various applicationse.g.in acid-catalyzed reactions [37-46].

Herein, we wish to report two new supported sulfonic acid solid catalysts through the reaction of 1,4-butane sultone with chitosan and chitosan-magnetite nanocomposite. To our knowledge, there is no any report on the preparation of chitosanmagnetite supported sulfonic acid catalyst and investigation of its catalytic properties. Thus, the catalytic properties of the prepared heterogeneous catalysts is investigated in the acidcatalyzed acetalization and esterification reactions. Some parameters affecting catalytic efficiencies such as time, temperature,and catalysts amount are studied to determine the optimized conditions.

2. Experimental

2.1. Catalysts preparation

2.1.1. Preparation of chitosan supported sulfonic acid, CS-SO3H

1 g of chitosan(dried at 100 °C in vacuum oven overnight)was added to a solution of 1,4-butane sultone (5 mmol) in 30 ml of toluene and the mixture refluxed for 18 h under N2atmosphere.Afterwards, the solid was separated by filtration, soxhlet washed with chloroform to eliminate the unreacted species, and dried in vacuum oven at 80 °C. Sulfonate groups of the product were converted into-SO3H by addition of 30 ml of H2SO4(0.01 mol·L-1)followed by stirring at ambient temperature for 2 h.The solid catalyst was isolated by filtration, soxhlet washed with deionized water,and dried in vacuum oven at 80 °C.

2.1.2.Preparation of the chitosan-magnetite nanocomposite supported sulfonic acid, MNPs@CS-SO3H

MNPs@CS nanocomposite was prepared similar to reported method [47]. First, 1.5 g of chitosan was dissolved in 100 ml of 0.05 mol·L-1acetic acid solution. Then, FeCl2·4H2O (1.29 g) and FeCl3.6H2O (3.51 g) were added and the solution stirred for 5 h at 80 °C under N2atmosphere. 6 ml of ammonia solution 25%was added dropwise and the reaction mixture stirred for 30 min.The obtained MNPs@CS was separated by applying a magnet and washed with distilled water, ethanol, and finally dried in vacuum oven at room temperature. In order to immobilize sulfonic acid groups, MNPs@CS was treated with 1,4-butane sultone followed by sulfuric acid solution similar to that described for the preparation of CS-SO3H.

2.1.3. Determination of the acid capacity of the prepared solid acid catalysts

In order to measure the acid capacity of the prepared solid acid catalysts, an acid-base reaction between supported sulfonic acid groups and triethylamine were employed. Briefly, the solid acid catalysts were immersed in aqueous solution of triethylamine(20 ml, 2 ml) and stirred at room temperature for 24 h. The catalysts were then filtered, washed several times with methanol and dried in vacuum oven overnight. The nitrogen content of catalysts were determined with CHN elemental analysis which correlates with the amount of sulfonic acid groups in the catalysts.

2.2. Catalytic tests

2.2.1. Esterification reaction

The prepared CS-SO3H and MNPs@CS-SO3H were exploited as catalysts in the acetic acid esterification withn-butanol. First,0.1 g of the catalyst was dispersed in a solution ofn-butanol(45 mmol) and acetic acid (30 mmol) and stirred at 100 °C for 3 h. Samples were taken out from the mixture at specific times and analyzed by gas chromatography (GC).

Recycling tests were conducted as follows; At the end of reactions, the catalyst was isolated from the mixture with centrifugation or magnetic separation, washed with methanol in order to remove the remaining reactants, dried in vacuum oven at 80 °C overnight, and reused in the next esterification reaction in the same conditions. Blank test was performed by reacting acetic acid(30 mmol)andn-butanol(45 mmol)without the utilization of catalyst at 100 °C.

2.2.2. Acetalization reaction

In a typical acetalization reaction, 45 mmol of ethylene glycol,30 mmol of benzaldehyde, 60 mmol of cyclohexane, and 0.1 g of catalyst were mixed in a round bottom flask equipped with a condenser.The reaction mixture was stirred at 100°C for 8 h and samples were taken out from the reactor at specific times and analyzed by GC. At the end of reactions, the CS-SO3H and MNPs@CS-SO3H catalysts were isolated from the reactions mixture by centrifugation and external magnet, respectively. The blank acetalization reaction without the catalyst was conducted in the same conditions.

3. Results and Discussion

3.1. Preparation of CS-SO3H and MNPs@CS-SO3H catalysts

According to the Fig.1,the nucleophilic reaction of amine group located at the C-2 position of chitosan chain with 1,4-butane sultone produced chitosan-N-butyl sulfonateviathe 1,4-butane sultone ring opening. Then, with addition of diluted sulfuric acid,the sulfonate group was converted to sulfonic acid.

Fig. 1. Preparation of CS-SO3H (a) and MNPs@CS-SO3H (b) catalysts.

Fig. 2. FT-IR spectra of CS (a), CS-SO3H (b), and MNPs@CS-SO3H (c).

Fig. 3. FE-SEM images and EDX spectra of CS-SO3H (a,c) and MNPs@CS-SO3H (b,d); inset of part (b) shows the TEM image of MNP@CS-SO3H.

3.2. Characterization of CS-SO3H and MNPs@CS-SO3H catalysts

In order to get the evidence for sequential functionalization of chitosan, the FT-IR spectra of CS, CS-SO3H, and MNPs@CS-SO3H were acquired and given in Fig. 2. In addition to the presence of characteristic peaks of CS, the FT-IR spectrum of CS-SO3H(Fig. 2b) exhibited intense bands at 1034 and 1192 cm-1assigned to the O=S=O symmetric and asymmetric stretching vibrations of the sulfonic acid groups, respectively [48,49] indicating the successful sulfonation of the chitosan. The observation of similar peaks in the FT-IR spectrum of MNPs@CS-SO3H (Fig. 2c) also verified the sulfonation of MNPs@CS nanocomposite. In addition, the MNPs@CS-SO3H showed additional peaks at 570 and 590 cm-1due to the Fe-O stretching vibrations in the magnetite nanoparticles [50].

For investigating the morphology of functionalized CS and MNPs@CS, FE-SEM analysis was conducted. Figs. 3a and 2b show the FE-SEM images of CS-SO3H and MNPs@CS-SO3H, respectively.The morphology of MNPs@CS-SO3H is spherical and well distributed with some aggregation.On the other hand,the TEM image of MNPs@CS-SO3H in Fig.3b reveals its core-shell structure.In the case of CS-SO3H, a different morphology is appeared and a nonuniform and irregular shape can be seen.

The elemental compositions of CS-SO3H and MNPs@CS-SO3H were studied by EDX analysis. Fig. 3c shows the existence of C,N,O,and S elements in CS-SO3H.The EDX spectrum in Fig.2d indicates the existence of C, N, O, S, and Fe elements in MNPs@CSSO3H. The observation of S peak is due to the sulfonated chitosan in both materials, while the Fe peak comes from the magnetite nanoparticles in MNPs@CS-SO3H.

XPS analysis further approved the success of chitosan sulfonation process. In the S 2p XPS spectrum of CS-SO3H (Fig. 4a), the position of S 2p peaks indicated the existence of sulfur in the oxidation state of 6+in the sulfonate groups(S 2p3/2at 168 eV).Due to the spin-orbit coupling,the S 2p was splitted into the S 2p3/2and S 2p1/2components with binding energy difference of 1.18 eV.

The X-ray diffraction (XRD) was employed to determine the crystal phase of the MNPs@CS-SO3H. The XRD pattern of the MNPs@CS-SO3H in Fig. 4b indicated that the Fe3O4structure was remained intact in the sulfonation process. The positions of the diffraction peaks were corresponded with standard XRD pattern of Fe3O4inverse spinel structure (JCPDS No.19-0629) [12].

Vibrating sample magnetometry (VSM) curves of MNPs and MNPs@CS-SO3H catalyst were shown in Fig. 4c. The saturation magnetization of 60 emu·g-1was observed for bare MNPs. However, the saturation magnetization of MNPs@CS-SO3H catalyst decreased to 42 emu·g-1owing to the existence of chitosan shell on the external surface of MNPs.However,the MNPs@CS-SO3H catalyst was efficiently attracted toward a permanent magnet (inset of Fig.4c).This observation indicated that the magnetic properties of MNPs@CS-SO3H allowed it to separate with applying a magnet from a dispersed mixture.

Fig. 5. TGA and DTG curves of (a) CS-SO3H and (b) MNPs@CS-SO3H.

In order to obtain information on the thermal stabilities of the prepared CS-SO3H and MNPs@CS-SO3H, TGA analysis was performed and the results were shown in Fig. 5. Both materials showed two-step decomposition in their TGA curves.The first step below 200 °C can be attributed to the elimination of the physisorbed water molecules. Combustion and decomposition of the organic compoundsi.e.chitosan polymer and sulfonic acid groups from 250 to 600°C results in the second weight loss.It can be seen that the weight loss of MNPs@CS-SO3H is lower than CS-SO3H due to the existence of MNPs@CS nanocomposite. Furthermore, it is clear that the decomposition temperature of the MNPs@CS-SO3H is higher than that of CS-SO3H. This difference in the thermal stabilities is likely due to the formation of chitosan shell at the surface of the magnetite core and the formation of hybrid nanomaterial with a well-developed polymeric structure and enhanced thermal stability compared to the free chitosan.

Fig.6. (a)NH3-TPD profiles of CS-SO3H and MNPs@CS-SO3H,(b)The N2 adsorptiondesorption isotherms of CS, CS-SO3H, and MNPs@CS-SO3H.

The acid strengths of the catalysts were examined with NH3temperature programmed desorption(TPD).In the NH3-TPD curves depicted in Fig. 6a, the CS-SO3H catalyst shows medium acid sites at above 220 °C and does not show any peak in the strong acidic region at above 400 °C due to the decomposition of propyl sulfonate species at this temperature according to the TGA analysis.The MNPs@CS-SO3H sample shows medium and strong acid sites at about 200 and 400 °C, respectively. The observation of strong acid sites at above 400 °C is a result of the thermal stability of MNPs@CS-SO3H in higher temperatures which is consistent with its corresponding TGA curve.

The amount of acidic sites of the prepared solid acid catalysts was also measuredviatheir interaction with triethylamine and analysis by CHN elemental analysis. The results showed that the acidic sites of CS-SO3H and MNPs@CS-SO3H are 0.71 and 0.82 mmol·g-1, respectively.

In order to estimate the specific surface areas of the chitosan and prepared catalysts, N2adsorption-desorption analysis was conducted. The obtained surface area (BET method) of chitosan,CS-SO3H, and MNPs@CS-SO3H catalysts were 2, 1, and 28 m2·g-1,respectively. According to the obtained results, functionalization of the chitosan surface with sulfonic acid groups results in decreasing the surface area. On the other hand, due to the existence of MNPs and the formation of core-shell structure, the surface area of the MNPs@CS-SO3H is higher than that of chitosan. According to Fig. 6b,the isotherm of MNPs@CS-SO3H is of type IV and shows a hysteresis loop owing to the existence of mesoporosity in this material. However, the isotherms of chitosan and CS-SO3H are of type II without appearance of hysteresis loops and show no porosity in these materials.

3.3. Catalytic performance of CS-SO3H and MNPs@CS-SO3H

Catalytic activities of the CS-SO3H and MNPs@CS-SO3H catalysts were evaluated in two acid-catalyzed reactionsi.e.acetic acid esterification withn-butanol and benzaldehyde acetalization with ethylene glycol.

3.3.1. Esterification of acetic acid with n-butanol

The results of using the prepared catalysts in the acetic acid esterification withn-butanol were given in Table 1. According to the results obtained from using different amounts of CS-SO3H and MNPs@CS-SO3H in esterification reaction,an increase in acetic acid conversion is observed by increasing the amount of catalysts from 0.05 to 0.1 g.This can be explained with the presence of more proton species available to promote the catalytic esterification reaction. Furthermore, Table 1 reveals that in the absence of any catalyst, the acetic acid conversion is low.

Table 1 Catalytic effect of the CS-SO3H and MNPs@CS-SO3H catalysts on the acetic acid esterification with n-butanol with different amount of catalysts

Table 2 Catalytic effects of the CS-SO3H and MNPs@CS-SO3H catalysts on the benzaldehyde acetalization with ethylene glycol in cyclohexane

The effect of reaction temperature on the on acetic acid conversion in the presence of CS-SO3H and MNPs@CS-SO3H is shown in Fig. 7. An increase can be seen in the production of butyl acetate with increasing the temperature from 40 to 120 °C.

Fig.7. The effect of reaction temperature on acetic acid conversion in the presence of CS-SO3H and MNPs@CS-SO3H. Reaction conditions: n-butanol, 45 mmol; acetic acid, 30 mmol; catalyst amount, 0.1 g; time, 3 h.

At the end of catalytic reactions, the MNPs@CS-SO3H and CSSO3H solid acid catalysts were isolated from the reaction mixtures and after washing with ethanol and drying in vacuum oven,reused in the same esterification reaction.The reusing tests indicated that the catalytic activities of the recovered MNPs@CS-SO3H and CSSO3H catalysts have not changed considerably after six successive recycling tests (Fig. 8).

Fig. 8. Results of the acetic acid esterification with n-butanol in the presence of recycled catalysts. Reaction conditions: n-butanol, 45 mmol; acetic acid, 30 mmol;Temp.,100 °C.

3.3.2. Benzaldehyde acetalization with ethylene glycol

To further explore the catalytic activities of CS-SO3H and MNPs@CS-SO3H catalysts, the benzaldehyde acetalization with ethylene glycol was carried out as another model reaction. The results of the catalytic acetalization reaction showed the increase of benzaldehyde conversion with increasing the reaction time and temperature, and the conversion reached 87% at 100 °C for 8 h over CS-SO3H and 63% for MNPs@CS-SO3H at the same conditions(Table 2).Table 2 shows that low conversion of benzaldehyde(14%) was obtained in the absence of catalysts suggesting the essential role of the catalysts in this reaction.

It is interesting to note that in both esterification and acetalization reactions,the activity of MNPs@CS-SO3H catalyst is lower than that of CS-SO3H catalyst. The lower activity of MNPs@CS-SO3H with respect to CS-SO3H in spite of its higher amount of SO3H groups can be attributed to the presence of magnetite nanoparticles which decreases the mobility of this catalyst.

Comparative results for the acetic acid esterification withnbutanol and benzaldehyde acetalization with ethylene glycol with some earlier reported solid acid catalysts [51-55] are given inTable 3.Comparison of the results suggests that our novel catalysts have comparable performance in these acid-catalyzed reactions.

Table 3 Comparison of the efficiency of CS-SO3H and MNPs@CS-SO3H catalysts with other sulfonic acid catalysts in acetic acid esterification with n-butanol and benzaldehyde acetalization with ethylene glycol

The reaction mechanisms of acetic acid esterification withn-butanol and benzaldehyde acetalization with ethylene glycol in the presence of MNPs@CS-SO3H as a typical catalyst have been shown in Fig. 9. In the first step the carbonyl groups of acetic acid or benzaldehyde is protonated by uptaking a proton from the solid acid. Then, nucleophilic attack ofn-butanol or ethylene glycol to the protonated carbonyl group followed by removing the generated water molecule occurs. Finally, the butyl acetate or 2-phenyl-1,3-dioxolane products with high yields are produced and the proton is returned to the catalyst structure to begin the next catalytic cycle.

Fig. 9. The mechanisms of catalytic esterification and acetalization reactions in the presence of MNPs@CS-SO3H.

4. Conclusions

In summary, simple chemical functionalization of chitosan biopolymer and chitosan-magnetite nanocomposite with sulfonic acid achieved new supported sulfonic acid catalysts. The catalyst was characterized various techniques which confirmed the successful attachment of sulfonic acid group to the surface of chitosan and chitosan-magnetite nanocomposite. The prepared solid acid catalysts were employed in the acid-catalyzed acetic acid esterification withn-butanol and benzaldehyde acetalization with ethylene glycol as model reactions. The catalysts showed high activities for the production of butyl acetate and acetal product in the mentioned reactions. Moreover, the formation of the products was found to increase with temperature and catalyst loading.The catalysts were reused five times without loss of activities in the acetic acid esterification withn-butanol.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge financial support(H/4/361)from Kharazmi University.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.04.037.