Novel Silica Gel Supported Polyether Ionic Liquids: Efficient Catalysts Applied in Transformation of Carbon Dioxide

Guo Liying; Jin Xianchao; Wang Yirong; Wang Haozhi

(School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang, 111003)

Abstract: Three new catalysts based on the silica gel supported polyether ionic liquids (ILs), i.e., [HO-PECH-MIM]Cl-Si, [H2N-PECH-MIM]Cl-Si, and [HOOC-PECH-MIM]Cl-Si, were prepared, and their chemical structures were characterized by infrared (IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. Thermogravimetric analyzer (TG), X-ray diffractometer (XRD) and scanning electron microscope (SEM) were used to evaluate their thermal stability, crystalline structure and apparent morphology, respectively. Surface areas of the prepared catalysts were calculated by the Brunauer-Emmett-Teller (BET) method. The catalytic reaction for the synthesis of propylene carbonate (PC) using CO2 and propylene oxide (PO) in the presence of the prepared catalysts was studied. The influences of times of recycling and catalyst structure on catalytic performance were also investigated. The experimental results showed that the silica gel supported polyether ILs catalysts successfully prepared under mild condition could possess the advantages of high activity, excellent thermal stability, good selectivity and easy recycling, while the phase transition of the liquid polyether ILs catalysts was also achieved. When the reaction temperature was 90 °C, the CO2 pressure was 2.0 MPa and the dosage of the catalyst was 2.5%, [HOOC-PECH-MIM]Cl-Si was found to have the best catalytic performance in the catalytic process, with the conversion rate reaching 100% and the selectivity equating to 98.2%. The conversion rate and selectivity still could reach more than 90% even after the catalyst was reused for 15 times.

Key words: polyether ionic liquids; silica gel; catalyst; CO2

1 Introduction

Ionic liquids (ILs) with excellent catalytic properties have been widely applied in organic synthesis, biomass dissolution, catalysis, preparation of composite materials, and other areas[1-5], and a great number of investigations on their applications in new catalytic systems have been reported. The first approach using ILs as catalyst in the cycloaddition reaction was reported in 2001[6]. Studies on the synthesis of ethylene carbonate using [BMIM]Cl as catalyst[7-8]showed that ILs had brilliant advantages in the catalytic conversion of CO2. However, traditional ILs are in liquid state along with high viscosity, making ILs difficult to be reused and purified so that ILs could only be used in the tank reactor. Meanwhile, in the industrial conversion of CO2, the application of ILs catalysts is also limited.

In recent years, structural designing concept in connection with the supported ILs catalysts has made a significant breakthrough in solving the above problems and has been attracting considerable attention. Zhang’s group published an innovative review on imidazole ILs grafted on chitosan, which showed excellent catalytic performance in the synthesis of propylene carbonate (PC) with CO2and propylene oxide (PO)[9]. Yin’s group immobilized 3-(2-hydroxyl-ethyl)-1-propylimidazolium bromide (HEPIMBr) onto SBA-15, Al-SBA-15 and SiO2to catalyze the synthesis of PC at a reaction temperature of 120 °C and a pressure of 2.0 MPa[10]. In 2015, the ILs catalysts immobilized onto activated cocoanut charcoal were prepared for continuous conversion of CO2to epichlorohydrin, however, the catalytic performance began to degrade after 50 h[11]. The ILs immobilized onto a molecular chain of polystyrene was applied in the cycloaddition of CO2into epoxide[12], leading to a conversion rate of 100% and a yield of 91%. In addition, ILs with polyether macromolecular chains and functional groups showed obvious advantages in catalytic synthesis of cyclic carbonates from CO2and epoxides[13-14]. In this work, we report on our preparation of polyether ILs catalysts supported on silica gel by the sol-gel technique, and the achievements related with the phase transitions of liquid polyether ILs and the catalytic conversion of CO2. Our present work will also provide necessary information for industrial production.

2 Experimental

2.1 Reagents

Tetraethyl orthosilicate (TEOS) (containing 28% of SiO2) was purchased from the Tianjin Damao Chemical Reagent Factory. Other chemicals were obtained from the Sinopharm Chemical Reagent Co., Ltd. Polyether ILs [HO-PECH-MIM]Cl, [H2N-PECH-MIM]Cl and [HOOCPECH-MIM]Cl were prepared according to published procedures[13-15]. All reagents were used without further purification.

2.2 Instruments

The chemical structures and apparent morphology of the prepared catalysts were described by IR (MAGNA-IR750), NMR (AVANCE AV-400) and SEM (TM3000) techniques. The crystalline structures of the catalysts were measured by XRD (D/max-2400) technique. TG (TGA-4000) was employed to measure the thermal stability of products. A surface area analyzer (JW-BK100) was used to investigate the porous structure parameters of catalysts. An experimental device (PARR-4523) was used in the catalytic reaction. The purity of PC was measured by gas chromatography (1790F). In addition, a CO2mass f lowmeter (D08-8C), a rotary evaporator (SFX-2L), a magnetic stirrer (DF-101S), a vacuum drying oven (DZF-6050), and a rotary vane vacuum pump (2-XZ-4) were also used in the experiments.

2.3 Synthesis of silica gel supported polyether ionic liquids catalysts

First, the TEOS-ethanol solution (at a volume ratio of 3:2) was poured into a round-bottomed f lask (250 mL). The stirred reactor was turned on when the temperature reached 60 °C. Three prepared samples of polyether ILs and TEOS (at a mass ratio of 4:1) were added in the f lasks, respectively, and then the oil-like suspended material gradually appeared upon stirring the solution. Then, hydrochloric acid (2 times of the polyether ILs volume) was added to the suspension. The color of the reaction solution gradually faded, an oily material disappeared, and the viscosity of the solution increased. Upon continuous stirring, the reaction mixture solidified into a colloidal form. Finally, the products were subject to ageing for 8 h at 60 °C and were dried in vacuum for 24 h at 150 °C, and then the products turned into solids. Three products were ground into powder to form the materials of silica gel supported polyether ILs catalysts, namely [HO-PECH-MIM]Cl-Si, [H2N-PECH-MIM]Cl-Si, and [HOOC-PECH-MIM]Cl-Si, which were studied in further catalytic reactions. Their structures are shown in Figure 1.

Figure 1 Chemical structure of three catalysts

2.4 Catalytic experiments

A specified amount of the catalyst was added into autoclave (300 mL). The internal air was replaced by nitrogen after the reactor was sealed. CO2was fed into the autoclave through the intake bypass, when the pressure reached 1 MPa, and then PO (150 mL) was added via a metering pump. The reaction temperature, pressure, and stirring rate (200 r/min) were measured. The system was cooled to the room temperature by cooling water, and the pressure was relieved when the flow of CO2was terminated. To purify the product, we added crude product into a distillation f lask for vacuum distillation under the conditions covering a temperature of 135 °C and a pressure of 0.09 MPa, while the residual catalyst was reused. The obtained colorless liquid product was PC. The purity of PC was determined by gas chromatography. Finally, we calculated the conversion rate, selectivity and turnover frequency (TOF)[16-17].

3 Results and Discussion

3.1 Chemical structures of catalysts

The chemical structures of three prepared catalysts were characterized by IR and1H NMR spectroscopy. The results are shown in Figure 2 and Figure 3. In Figure 2, characteristic absorption peaks of -OH, -COOH and -H2N could be observed at 3 437 cm-1, 1 754 cm-1and 3 452 cm-1, respectively[18]. [HO-PECH-MIM]Cl-Si, [HOOC-PECH-MIM]Cl-Si and [H2N-PECHMIM]Cl-Si had all the above characteristic peaks. Compared with polyether ILs, the peak width of -OH at 3 437 cm-1increased since the new silicon hydroxyl appeared. In addition, new antisymmetric stretching vibration peaks of Si-O and symmetric stretching vibration absorption peaks appeared at 1 188 cm-1and 826 cm-1, respectively, and the polyether peak increased at 1 099 cm-1because of the immobilization of polyether ILs[18-20]. All above data indicated that three silica gel supported polyether ILs catalysts were successfully prepared.

Figure 2 Infrared spectra of the catalysts

The1HNMR results of three catalysts are shown in Figure 3. Referring to chemical structures of catalysts in Figure 1, the NMR spectrum of [HO-PECH-MIM]Cl-Si (HNMR, DMSO) is analyzed as: 3.63 (d, 1H, OH-a), 3.56 (d, 2H, CH2-b), 3.87 (d, 1H, CH-c), 4.73 (s, 2H, CH2-d), 7.29 (s, 1H, CH-e), 7.79 (s, 1H, CH-f), 3.56 (s, 3H, CH3-g), 9.27 (s, 1H, CH-h), and 2.50 (DMSO).

The NMR spectrum of [H2N-PECH-MIM]Cl-Si (HNMR, DMSO) is analyzed as: 3.81 (d, 1H, COOH-a), 3.78 (d, 2H, CH2-b), 2.35 (d, 1H, CH-c), 3.66 (s, 2H, CH2-d), 7.68 (s, 1H, CH-e), 7.76 (s, 1H, CH-f ), 3.37 (s, 3H, CH3-g), 9.08 (s, 1H, CH-h), and 2.50 (DMSO).

The NMR spectrum of [HOOC-PECH-MIM]Cl-Si (HNMR, DMSO) is analyzed as: 5.09 (d, 1H, COOH-a), 3.83 (d, 2H, CH2-b), 2.95 (d, 1H, CH-c), 3.97 (s, 2H, CH2-d), 7.40 (s, 1H, CH-e), 7.72 (s, 1H, CH-f), 3.63 (s, 3H, CH3-g), 8.32 (s, 1H, CH-h), and 2.50 (DMSO).

Figure 3 1H NMR spectra of the catalysts

3.2 Thermal stability of catalysts

The thermal stability of three prepared catalysts was determined by the thermogravimetric analyzer. The system was programmed in the course of increasing the temperature from 25 °C to 500 °C in 47.5 minutes. The TGA curves of three prepared catalysts are showed in Figure 4. The first loss appeared owing to the evaporation of water and solvent in the course of temperature rise from room temperature to 120 °C. The initial decomposition temperature of [HO-PECH-MIM]Cl-Si, [H2N-PECHMIM]Cl-Si and [HOOC-PECH-MIM]Cl-Si was 181 °C, 156 °C, and 177 °C, respectively. The corresponding residual rate of these samples was 44.2%, 40.5%, and 35.7%, respectively. Thus, it could be realized that the initial decomposition temperature and residual rate of the samples were different. In contrast to other two catalysts, [HO-PECH-MIM]Cl-Si had better thermal stability and higher residual rate.

Figure 4 TGA curves of three catalysts

3.3 Crystalline properties of catalysts

The crystal structures of three catalysts were detected by the X-ray diffractometer. Judging from the XRD patterns shown in Figure 5, we could observe wide diffraction peaks with strong dispersion from 20°—30°, which were amorphous diffraction peaks of silica gel, while no new crystallization phases appeared. The distribution of polyether ILs on the surface of the catalyst carrier was more dispersive, which was difficult to form a crystalline phase. The destruction of the crystalline structure might also be caused by the release of hydrochloric acid during the reaction of polyether ILs with Si-OH.

Figure 5 XRD patterns of three catalysts

3.4 Apparent morphology of catalysts

The apparent morphology of [HO-PECH-MIM]Cl-Si was measured by the camera and the scanning electron microscope, with pictures presented in Figure 6. Pictures (a) and (b) are ordinary photos of polyether ILs [HOPECH-MIM]Cl and the silica gel supported polyether ILs catalyst [HO-PECH-MIM]Cl-Si, while pictures (c) and (d) are SEM photos of the silica gel supported polyether ILs [HO-PECH-MIM]Cl-Si, which are amplified by 5 000 times and 20 000 times, respectively. As we can see from picture (a), [HO-PECH-MIM]Cl was shown as a pale-yellow transparent viscous liquid. In picture (b), we can observe that the polyether ILs changed from liquid to pale-yellow solid particles after having been supported on silica gel. The change showed that the phase transition of the polyether ILs catalyst was achieved. Picture (c) and (d) showed that the morphology of the prepared catalyst [HO-PECH-MIM]Cl-Si was shown as particles with irregular shapes. The results of measurements showed that the grain size of particle was between 1 μm to 6 μm. The above analyses showed that the connection between polyether ILs and the silicon atomic group was successfully achieved by covalent bond, and the phase transition was also implemented after being supported on silica gel. Thus it can be seen that the product prepared by the sol-gel method was the target catalyst, and the inorganic silicon particles were successfully immobilized.

Figure 6 The SEM photographs of [HO-PECH-MIM]Cl-Si

3.5 BET results of catalysts

The porous structure parameters of original silica gel and three prepared catalysts were determined by the BET method, with the results listed in Table 1. Compared with the original silica gel, the pore size, the pore volume, and the BET surface area of prepared catalysts decreased evidently, because the pore size reduced from 13.62 nm to 7.92 nm, 7.31 nm, and 7.75 nm, the pore volume decreased from 1.745 cm3/g to 0.930 cm3/g , 0.988 cm3/g , and 0.962 cm3/g, and the BET surface area declined from 570.3 m2/g to 178.5 m2/g, 148.0 m2/g , and 137.2 m2/g, respectively, which indicated that a certain amount of polyether ILs entered the pores of silica gel and the surface of silica gel was covered by other parts of polyether ILs, which was consistent with the SEM pictures presented in Figure 6.

Table 1 Porous structure parameters of original silica gel and three catalysts

3.6 Catalytic performance of catalysts

3.6.1 Effects of different catalysts on catalytic performance

Three prepared catalysts were used in the synthesis of PC from CO2and PO under reaction conditions covering a temperature of 90 °C, a CO2pressure of 2.0 MPa, a catalyst mass fraction of 2.5%, and a stirring speed of 190 r/min. The purity, conversion rate, selectivity and turnover frequency (TOF) were used to evaluate the catalytic performance of three catalyst samples, with the results shown in Table 2. The data in Table 2 show that the catalytic performance of three catalyst samples does not differ so much. In comparison with other two catalysts, the catalytic performance of [HOOC-PECH-MIM]Cl-Si was better, since the polyether ILs with -COOH group had strong acidity and hydrogen-bond donor, which could form synergistic catalytic effect with chloride ions to achieve the activation and ring opening of PO. Polyether chains also had the basal isolation effect, which could inhibit the formation of by-products and increase the selectivity, leading to better catalytic performance of [HOOC-PECH-MIM]Cl-Si catalyst.

Table 2 Catalytic performance of three catalyst samples

3.6.2 Effects of recycle times on the catalytic performance

The synthesis of PC from PO and CO2using [HOOCPECH-MIM]Cl-Si as catalyst was investigated. The reaction conditions were the same as shown above. The effects of recycle times on the catalytic performance were investigated, with the results presented in Figure 7. The catalytic performance of the catalyst changed very little after being reused in 6 cycles. However, when the catalyst was recycled 9 times, the conversion rate dropped from 100% to 99.5%, and the selectivity dropped from 98.2% to 93.8%, and when the catalyst was recycled 15 times, the conversion rate and selectivity were equal to 96.9% and 90.2% respectively. As the recycle times increased, the reaction time was prolonged. The result indicated that the catalyst had steadily active components, so we could prolong the reaction time appropriately and increase the recycle times, because these changes would not affect the catalytic activity. This approach was possible because cations of polyether ILs were complexed with oxygen atoms of PO in the catalytic process, and the anion attacked β carbon atom and broke the C-O bond of the PO to cause the ring opening. The silica gel carrier and the terminal carboxyl could form synergistic catalytic effect with the anion, and the polyether chains had basal isolation effect which could inhibit the formation of by-products to improve the selectivity. In the catalytic process, anions and cations of the catalyst still had high activity, so that the structure of the catalyst was not destroyed and the catalytic performance was stable. Therefore, the prepared catalyst could be reused many times to still retain high catalytic activity and selectivity.

Figure 7 Effects of recycle times on the catalytic performance

4 Conclusions

In this article, three silica gel supported polyether ILs catalysts were synthesized, with their catalytic performance studied. The structures and morphology of the catalysts showed that the silica gel supported polyether ILs catalysts were prepared successfully, and the results of thermogravimetric analysis showed good thermal stability of the catalysts, among which [HO-PECH-MIM]Cl-Si had the best thermal stability and the highest residual rate. The catalytic process of the reaction between PO and CO2using the silica gel supported polyether ILs as catalysts could be completed efficiently under the mild reaction conditions. The catalysts had advantages of high activity, good selectivity, easy recovery and recycling. The catalytic performance of [HOOC-PECH-MIM]Cl-Si was the best among three prepared catalysts under the reaction conditions covering a temperature of 90 °C, a pressure of 2.0 MPa, and a catalyst dosage of 2.5% to achieve a conversion rate of 100%, a selectivity of 98.2%, and a TOF of 3420. The conversion rate and selectivity still reached more than 90% when the catalyst was recycled 15 times.

Acknowledgements:This work was supported by the National Natural Science Foundation of China (NSFC 21706163) and the Liaoning Province Department of Education Foundation (LQGD2017020).