Preparation of 1-butyl-3-methylimidazolium dodecatungstophosphate and its catalytic performance for esterification of ethanol and acetic acid

Abstract

1-Butyl-3-methylimidazolium dodecatungstophosphate catalyst ([bmim]₃PW₁₂O₄₀) with high water tolerance was prepared from 1-butyl-3-methylimidazolium bromide ([bmim]Br) and phosphotungstic acid (H₃PW₁₂O₄₀). The catalyst was characterized by means of Fourier transform infrared spectroscopy, thermogravimetry-differential scanning calorimetry, n-BuNH₂ potentiometric titration, elemental analysis and so on. Its catalytic activity for esterification of ethanol and acetic acid to ethyl acetate was measured. The results show that there were three crystal-water molecules in the [bmim]₃PW₁₂O₄₀ catalyst, and it preserved the primary Keggin structure and acid strength of H₃PW₁₂O₄₀. The acid amount of [bmim]₃PW₁₂O₄₀ catalyst was less than that of H₃PW₁₂O₄₀. The [bmim]₃PW₁₂O₄₀ catalyst exhibited higher catalytic activity and reusability in the esterification of ethanol and acetic acid to ethyl acetate.

Keywords

1-butyl-3-methylimidazolium bromide / 12-phosphotungstic acid / heteropoly acid / 1-butyl-3-methylimidazolium dodecatungstophosphate / ethanol / acetic acid / esterification / ethyl acetate

Heteropolyacids (or their salts) and supported heteropolyacids (or their salts), as one kind of environmentally friendly catalysts, have attracted greater attention because they possess many advantages, such as, simple composition, definite structure, easily modified component,strong acidic and redox properties, unique reaction fields such as pseudoliquid, high-activity in low temperature and high thermal stability [1, 2]. Most of the heteropolyacids (or their salts) homogeneous catalysts are difficult to recover, which can lose the expensive heteropolyacids (or their salts) and pollute the environment, and are hence not suitable for industrial applications. The heteropolyacids (or their salts) supported on different porous supporters can resolve the problem of catalyst recovery, increase the surface area and enhance the catalytic performance, consequently improving the product quality and reducing the production cost [3, 4]. However, when the supported heteropolyacids (or their salts) catalysts are prepared by a physical adsorption method, there is a desorption problem of the active components from the supporter, and the catalysts are not ideal for recycled use. Therefore, the preparation of new heteropolyacids (or their salts) catalysts has attracted great attention. Chiang M-H et al. [5] reported the redox properties of 1-pentyl-3-methylimidazolium dodecatungstophosphate characterized by electrochemistry. So far, the application of 1-butyl-3-methylimidazolium dodecatungstophosphate in catalytic reactions has not yet been reported. Based on our earlier work, we prepared 1-butyl-3-methylimidazolium dodecatungstophosphate catalyst in order to realize the heterogenizing application of H₃PW₁₂O₄₀ homogeneous catalyst, which is insoluble in water, alcohol and ester. The catalyst was characterized by means of Fourier transform infrared spectroscopy (FT-IR), thermogravimetry-differential scanning calorimetry (TG-DSC), n-BuNH₂ potentiometric titration and elemental analysis, and its catalytic activity for esterification of ethanol and acetic acid to ethyl acetate was measured.

Experimental

Preparation of Catalyst

0.2 mole of N-methylimidazole (Linhai Kaile Chemical Factory) was dissolved into 0.2 mole of 1-bromobutane (Shanghai Wulian Chemical Factory) and then heated to 110°C and stirred for 12 h. The mixture was cooled to room temperature, and a yellow and viscous 1-butyl-3-methylimidazolium bromide ([bmim]Br) was obtained [6].

0.01 mole of dodecatungstophosphoric acid was dissolved into 100 mL water, and 0.06 mole of ([bmim]Br) was added into the above solution. The mixture was stirred at room temperature for 12 h. The white solid precipitated was filtered and washed with water until no detectable bromide ions were present in the solution. The white solid was dried at 80°C for 24 h, and white [bmim]₃PW₁₂O₄₀ catalyst powder was obtained.

Characterization of catalysts

Analysis of C, H and N elements in the sample was performed using Flash EA1112 Elemental analysis instrument (Thermofinigan). P and W elements in sample were measured using IRIS full spectrum direct reading ICP spectrometer (Fisher Scientific). The TG-DSC patterns were recorded at 20°C/min on TG-50 thermal analysis instrument (Shimadzu) in N₂ (20 mL/min). The FT-IR spectra of H₃PW₁₂O₄₀, [bmim]Br and [bmim]₃PW₁₂O₄₀ were recorded on a Nicolet 5700 FT-IR at room temperature in KBr pellets over the range of 400-4000 cm^-1 under atmospheric conditions. The acidity of [bmim]₃PW₁₂O₄₀ catalyst was measured by means of potentiometric titration. The [bmim]₃PW₁₂O₄₀ catalyst (0.5 g) was added to 30 mL of acetonitrile and the mixture was agitated until electrode potential stabilization. The suspension was titrated with 50 mmol/L n-butylamine in acetonitrile at 0.05 mL/min. The electrode potential variation was measured with a digital pH meter. According to the titration jump, the acid amount of the catalyst was measured.

Synthesis of ethyl acetate

Acetic acid (AR, Sinopharm Chemical Reagent Co. Ltd), ethanol (AR, Anhui Ante Biochemistry Co. Ltd), [bmim]₃PW₁₂O₄₀ catalyst and toluene (AR, Hangzhou Chemical Reagent Co. Ltd) were added to a flask with a temperature controller, water separator, magnetic stirrer and condenser. The mixture was stirred at the reflux temperature for required time. Then, the mixture was cooled and filtered, and the concentrations of ethanol and ethyl acetate in reaction solution were determined by gas chromatography. The catalytic activities were evaluated on the base of the yield of ethyl acetate.

Results and discussion

Characterization of catalysts

Figure 1 shows the FT-IR spectra of different catalyst samples. The four absorption bands at 1081, 983, 890 and 805 cm^-1 can be observed for H₃PW₁₂O₄₀, which are the fingerprints of the Keggin structure of H₃PW₁₂O₄₀, usually assigned to P—O, W═O, W—O_b1—W in corner shared octahedral and W—O_b2—W in edge shared octahedral associated with the asymmetric vibrations, respectively [7]. Similarly, there are four characteristic bands of Keggin structure at 1079, 982, 890 and 804 cm^–1 for [bmim]₃PW₁₂O₄₀, indicating that the [bmim]₃PW₁₂O₄₀ catalyst preserves the primary Keggin structure of H₃PW₁₂O₄₀. The absorption bands at 3141, 2960, 1571 and 1169.7 cm^-1 are observed in [bmim]₃Br catalyst as shown in Fig. 1, which are assigned to C—H stretching vibration in the imidazole ring, C—H stretching vibration of substitute group, —C═N— stretching vibration and imidazole ring stretching vibration, respectively. Similar observations have been reported in reference [6]. For the [bmim]₃PW₁₂O₄₀ catalyst, there are characteristic bands of [bmim]Br at 3147, 2958, 1568 and 1165 cm^-1 respectively, indicating that there is [bmim]⁺ ion in the [bmim]₃PW₁₂O₄₀ catalyst.

Figure 2 shows TG-DSC profiles of the different catalyst samples. It can be seen that H₃PW₁₂O₄₀ shows two endothermic peaks accompanied by the loss of water at 82°C and 200°C, respectively, and one exothermic peak associated to the decomposition of H₃PW₁₂O₄₀ into corresponding single oxides at 613^oC. The [bmim]₃PW₁₂O₄₀ catalyst shows one broad endothermic dehydration peak at the range of 40-350°C (1.57% weight loss) and four exothermic peaks associated to the decomposition at 396°C, 472°C, 590°C and 608°C, respectively, which are accompanied by the weight loss. It indicates that the thermal decomposition of [bmim]₃PW₁₂O₄₀ catalyst is carried out gradually. The [bmim]⁺ ion is firstly decomposed in the range of 400-600°C, the dodecatungstophosphoric acid anion is decomposed at near 610°C.

The elemental analysis results show that the contents of C, H, N, P and W in the [bmim]₃PW₁₂O₄₀ catalyst are 9.08%, 1.48%, 2.47%, 0.94% and 66.26%, respectively. Therefore, the structural formula of [bmim]₃PW₁₂O₄₀ catalyst possesses three molecules of crystal water (the calculated contents of C, H, N, P and W are 8.61%, 1.54%, 2.51%, 0.92% and 65.88%, respectively).

According to the above deduction, the weight loss of [bmim]₃PW₁₂O₄₀ catalyst should be about 14%. However, the weight loss from Fig. 2 (about 12% in N₂) is less than the theory value. It is possibly due to the carbonization in the decomposition process of [bmim]⁺ ion in N₂.

Acidity of the catalysts

The acidity of the catalysts were measured by means of potentiometric titration with n-butylamine because the [bmim]₃PW₁₂O₄₀ catalyst decomposes easily during heating. Fig. 3 shows the potentiometric titration curves of different samples with acetonitrile solution of n-butylamine. Generally, the initial electrode potential indicates the maximum acid strength of the sites, and the titration jump indicates the total number of acid sites when the acidity of solid acid catalysts is measured by the mean of n-butylamine potentiometric titration. The acid strength of these sites may be classified according to the following scale: E>100 mV (very strong sites), 0 mV<E<100 mV (strong sites), -100 mV<E<0 mV(weak sites) and E<-100 mV (very weak sites) [8]. Figure. 3 shows that the initial electrode potentials of [bmim]₃PW₁₂O₄₀ and H₃PW₁₂O₄₀ are 561 and 567 mV respectively, and their titration jump electrode potentials are 368 and 328 mV, respectively. It indicates that the acid sites of [bmim]₃PW₁₂O₄₀ and H₃PW₁₂O₄₀ catalysts are very strong and the acid strengths of them are about the same, but the numbers of acid sites are obviously different. The acid amount of H₃PW₁₂O₄₀ (1.0 mmol/g) is greater than that of [bmim]₃PW₁₂O₄₀ catalyst (0.08 mmol/g). It is due to the different manners of producing acidity. The H₃PW₁₂O₄₀ catalyst is a typical Bronsted’s acid, and the latter’s acidity can be from H⁺ produced by hydrolysis.

Catalytic activity of the catalysts

Table 1 shows the catalytic activity of different samples for esterification of ethanol and acetic acid to ethyl acetate. As can be seen from Table. 1, because of the autocatalysis of acetic acid, a certain amount of ethyl acetate was obtained (yield 57.8%) in the absence of catalyst. The yield of ethyl acetate in the presence of [bmim]Br is about the same as in the absence of catalyst, indicating that the [bmim]Br has no catalytic activity for the esterification ethanol and acetic acid. As shown in Table 1, H₃PW₁₂O₄₀ and [bmim]₃PW₁₂O₄₀ catalysts exhibit catalytic activity. The catalytic activity of H₃PW₁₂O₄₀ is a little higher than that of [bmim]₃PW₁₂O₄₀, because both the catalysts have the same acid strength and the acid amount of the former is greater than that of the latter. In addition, the solubilities of [bmim]₃PW₁₂O₄₀ catalyst in water, ethanol, acetic acid and ethanol acetate are very little (0.39, 0.38, 0.29 and 0.40 mg/L, respectively). The solubility of [bmim]₃PW₁₂O₄₀ catalyst in the reaction solution is only 0.40 mg/L. In other words, the weight loss of catalyst after being used once was about 0.004%. It indicates that the [bmim]₃PW₁₂O₄₀ catalyst is separated from reaction solution in this reaction , thus realizing the heterogenizing application of the H₃PW₁₂O₄₀ homogeneous catalyst.

Stability of [bmim]₃PW₁₂O₄₀ catalyst

Table 2 shows the recycle performance of [bmim]₃PW₁₂O₄₀ catalyst. It can be seen from Table 2 that, the yield of ethyl acetate is 99.7% when the fresh catalyst is used. When the catalyst is reused, the catalytic activity decreases. Then, the catalytic activity is invariable in subsequent uses. It indicates that the [bmim]₃PW₁₂O₄₀ catalyst exhibits good stability in the esterification of ethanol and acetic acid to ethyl acetate. The reason why the activity of the second used [bmim]₃PW₁₂O₄₀ catalyst decreases is that ethyl acetate is strongly adsorbed on the surface of the catalyst. The absorption bands of C═O and C—O—C at near 1740 cm^-1 and 1238 cm^-1 on the used catalyst were observed by IR determination. The ester on the used catalyst cannot be removed by washing with water and alcohol, indicating that the adsorption of ester on the surface of catalyst is very strong.

Conclusions

There are three crystal-water molecules in the [bmim]₃PW₁₂O₄₀ catalyst which preserves the primary Keggin structure of H₃PW₁₂O₄₀. The catalytic activities of H₃PW₁₂O₄₀ and [bmim]₃PW₁₂O₄₀ in the synthesis of ethyl acetate are almost the same because they have the same acid strength. The [bmim]₃PW₁₂O₄₀ catalyst can be reused with high catalytic activity and the recycling use of H₃PW₁₂O₄₀ homogeneous catalyst is realized.

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