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Solvent-Free cycloaddition of carbon dioxide and epichlorohydrin catalyzed by surface-attached imidazolium-type poly(ionic liquid) monolayers
Journal of CO₂ Utilization 38 (2020) 168–176
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Solvent-Free cycloaddition of carbon dioxide and epichlorohydrin catalyzed by surface-attached imidazolium-type poly(ionic liquid) monolayers
T
Rong Qua, Zijie Renb,*, Na Lic, Fangfang Zhangd,*, Zhengyu Jason Zhangc, Haining Zhanga,* a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Nr. 122 Luoshi Rd., Wuhan 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, Nr. 122 Luoshi Rd., Wuhan 430070, China c School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT UK d School of Nuclear Technology and Chemistry & Biology, Hubei University of Science and Technology, Nr. 88 Xianning Avenue, Xianning 437100, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dioxide conversion Epoxy Cyclic carbonate Poly(ionic liquid) Surface-Attachment
Design and synthesis of effective catalysts for the conversion of epoxy compounds to cyclic carbonate under the presence of carbon dioxide molecules is essential for the utilization of carbon dioxide. Herein, we report that the surface-attached imidazolium-type poly(ionic liquid) bearing amine moieties is an efficient catalyst for the conversion of epoxy compound and carbon dioxide to cyclic carbonate. The effect of chemical structure of the synthesized catalyst and the reaction conditions on the catalytic efficiency is systematically studied. The results show that the catalyst with bromide anion can effectively catalyze the reaction under 1.5 MPa carbon dioxide pressure at 120 °C with the catalyst content of 10 mg mL−1, reaching the conversion rate up to 90 %. Moreover, the synthesized catalyst can be separated from the cyclic carbonate products by centrifugation. The recyclability and reversibility measurements reveal that the decay of the catalytic activity is mainly resulted from the adsorption of the cyclic carbonate products on the catalysts. The catalytic activity of the synthesized catalyst towards conversion of epichlorohydrin and carbon dioxide to cyclic carbonate can be recovered after extraction of the adsorbed cyclic carbonate products.
1. Introduction With the rapid development of industrialization and the large consumption of fossil fuels, the increased concentration of carbon dioxide (CO2) in the atmosphere results in serious environmental problems [1–5]. The atmospheric cleansing by CO2 capture and storage has been realized as an essential way to mitigate the environmental issues caused by CO2 emission [6–9]. Furthermore, carbon dioxide is an important feedstock in the synthetic process of cyclic carbonate from epoxy compounds under the existence of catalysts [10–14]. Since cyclic carbonate compounds can be applied in various industrial areas including aprotic solvents, precursors for polycarbonate synthesis, and intermediates in organic synthesis [15–18], development of effective catalysts for conversion of epoxy to cyclic carbonate is of great importance for practical applications. Typically, homogeneous catalyst exhibits good catalytic efficiency but complicated separation process of products whereas heterogeneous catalyst may have the relatively low reaction efficiency but easy separation of products from the reaction systems [19–21]. For catalyzing the formation of cyclic carbonate from epoxy and CO2, both
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homogeneous and heterogeneous catalysts have been developed to promote CO2 insertion into epoxide. Metal-organic complex and ionic liquids have been reported as efficient homogeneous catalysts for cyclic carbonate formation. Kim et al. reported that the homogenous catalysts synthesized from organophosphorus and zinc halide exhibited an exceptional catalytic performance in the reaction of ethylene oxide with carbon dioxide to from ethylene carbonate at 100 ◦C and 3.4 MPa for 1 h [22]. However, the cycloaddition reaction of carbon dioxide and epoxy compounds using metal-organic complex as catalyst was limited to the use of allyl and vinyl epoxides. Ionic liquid-based homogeneous catalysts have also been widely investigated for cycloaddition of carbon dioxide and epoxy compounds due to the unique characteristics of ionic liquids [23–27]. In the early work carried out by Peng et al., imidazolium-type ionic liquids were applied as homogeneous catalysts for the cycloaddition of CO2 and propylene oxide under 2.5 MPa at 110 °C [28]. The final products of propylene carbonate were obtained by vacuum distillation. In the recent work reported by Goodrich et al., a series of azolate-type ionic liquids were synthesized and were applied as homogeneous catalysts for cyclic carbonate formation from carbon dioxide and epoxy compounds
Corresponding authors. E-mail addresses: [email protected] (Z. Ren), zff[email protected] (F. Zhang), [email protected] (H. Zhang).
https://doi.org/10.1016/j.jcou.2020.01.022 Received 26 September 2019; Received in revised form 20 December 2019; Accepted 26 January 2020 2212-9820/ © 2020 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 38 (2020) 168–176
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obtained by centrifugation, followed by extensively washing with toluene and ethanol to remove unreacted MPS. The product was finally dried at 60 °C in a vacuum oven and the final weight of SBA16-MA is about 2.61 g.
under 1.0 MPa at 100 °C [29]. It has been found that the catalytic activity was mainly affected by the existed anions. However, the developed homogenous catalysts suffered from the high cost and difficulty in separation of products and catalysts from the reaction systems. Heterogeneous catalysts have also been widely developed for the cycloaddition of carbon dioxide with epoxy compounds and the mainly applied heterogeneous catalysts include metal oxide, supported catalysts, and polymers [30–36]. For example, Yoshihara et al. found that magnesium oxide could effectively catalyze the synthesis of cyclic carbonates [31]. However, the reaction required high temperature, high pressure, and long reaction time due to the unsatisfactory catalytic activity of magnesium oxide. Poly(ionic liquid)-based catalysts have also been applied for cycloaddition of epoxides and carbon dioxide since they bear the advantages of both ionic liquids and polymers. Xiong et al. synthesized crosslinked poly(ionic liquid) nanoparticles via copolymerization of ethylene glycol dimethacrylate and functionalized imidazolium-type ionic liquid. With the catalyst loading of 33 mg mL−1, the highest conversion of epichlorohydrin to cyclic carbonate reached 98 % under 5 MPa CO2 pressure at 160 °C for 12 h [34]. Urethane-based imidazolium-type poly(ionic liquid)s were investigated as catalysts for cycloaddition of propylene oxide with carbon dioxide and the highest conversion reached 84 % with the catalyst loading of about 140 mg mL−1 under 2.5 MPa CO2 pressure at 110 °C for 6 h [35]. In this work, we report the synthesis of cyclic carbonate from epoxy compounds and carbon dioxide by using surface-attached amine-functionalized poly(ionic liquid) monolayers on mesoporous silica particles (SBA16) as catalysts. Amine-functionalized imidazolium-type ionic liquids were selected as catalysts since both amino-groups and 2-position carbon on imidazole ring could activate the carbon dioxide molecules for the cycloaddition through Lewis acid-base interactions [37,38]. In addition, the high diffusivity of carbon dioxide in imidazolium-type ionic liquid can also facilitate the cycloaddition of carbon dioxide with epoxy compounds [39,40]. The applied silica supports can ensure the easy separation of catalysts from the reaction system and the swelling of the surface-attached polymer chains makes the reaction being locally homogenous catalysis. It is thus expected that the designed catalysts can take the advantages of both heterogeneous and homogeneous catalysts, resulting in the high catalytic efficiency and the easy separation process of catalysts from cyclic carbonate products.
2.3. Synthesis of surface-attached poly(ionic liquid) monolayers on SBA16 To a dispersion of SBA16-MA (0.50 g) and AIBN (0.06 g) in 10 mL N, N-dimethylformamide, the ionic liquid monomer solution (6.0 g in 3 mL deionized water) was added under magnetic stirring. The reaction system was degassed through three freeze-thaw cycles under vacuum to remove trance of oxygen and was heated to 70 ℃ under protection of nitrogen atmosphere for polymerization. After 4 h polymerization, the mixture was allowed to cool down to room temperature. The mixture was centrifuged and extensively washed with deionized water to remove non-attached polymer chains. The resulted white powder was then re-dispersed in a solution of 12 mL triethylamine in 40 mL deionized water and was kept stirring for 48 h at room temperature to remove hydrobromic acid. The final product (0.55 g) was obtained by centrifugation and extensively washing with deionized water, followed by drying at 60 °C under vacuum. The product was denoted as SBA16PIL-Br, where Br refers to bromide counterions of PIL. The surface-attached PIL monolayers with different counterions (CH3COO− and PF6−) were synthesized by ion exchange process according to literature [43–45]. Typically, 0.10 g of SBA16-PIL-Br was dispersed in deionized water (100 mL) at room temperature under magnetic stirring. After addition of 20 mL of the according salt aqueous solution (0.1 mol L-1, CH3COONa or KPF6), the reaction mixture was kept stirring continuously for 48 h. The product was then centrifuged and extensively washed with deionized water, followed by drying under vacuum at 75 °C. The products were denoted as SBA16-PIL-AC and SBA16-PIL-PF6, respectively. 2.4. Formation of cyclic carbonates from epichlorohydrin and CO2 Epichlorohydrin (5 mL) and surface-attached PIL catalysts (0.05 g) were successively added into a 50 mL autoclave equipped with a magnetic particle and heating devices, as shown in Fig. S1. The autoclave is connected to a high-pressure carbon dioxide gas cylinder to deliver carbon dioxide at a constant pressure during the catalytic process. After CO2 was charged at room temperature, the outlet valve of the autoclave was closed and the inlet valve remained open. The reaction system was heated up to the desired temperature and the pressure was adjusted to the desired value through continuous aeration during the reaction. After a desired reaction time, the reaction system was allowed to cool down to room temperature. The inlet valve was then closed and the outlet valve was slowly opened to vent the unreacted CO2. The products of cyclic carbonate were obtained after the catalyst was filtered. The re-collected catalyst was washed with ethanol and can be directly re-used after drying at 75 °C in a vacuum oven for 12 h unless otherwise stated.
2. Experimental section 2.1. Materials 1-Vinylimidazole, 2-bromoethylamine hydrobromide, and 3-methacryloylpropyl trimethoxysilane (97 %, MPS) were purchased from Alfa Aesar. Azobisisobutyronitrile (AIBN) was received from Aladdin (Shanghai, China) and was recrystallized from methanol prior to polymerization. Water was deionized using a Ulupure-H ultrapure water generator (Ulup, China) with resistivity of 18.2 MΩ cm. Toluene was distilled over sodium using benzophenone as indicator and stored with molecular sieves. All the other chemicals including tetraethoxysilane (TEOS–98 %, Sinopharm, China), pluronic F-127 (BASF, Germany), and epichlorohydrin (ECH, Aladdin, China) were used as received. The SBA16 was synthesized through acid catalyzed sol-gel chemistry according to literature [41] and the synthesis of 1-aminoethyl-3-vinylimidazolium bromide hydrobromide monomer was reported elsewhere [42].
2.5. Characterization Fourier transform infrared (FTIR) spectra were recorded on 60SXB spectrometer (Nicolet) with a resolution of 4 cm−1 to determine the chemical composition of samples. X-ray diffraction (XRD) patterns of mesoporous silica particles and modified silica were recorded on a Rigaku D/MAX-RB diffractometer using a Cu Kα radiation at 40 kV and 50 mA. Elemental analysis was carried out on a Vario EL Cube analyzer (Elementar, Germany) to calculate the grafted amounts of polymers for the catalysts. Nuclear magnetic resonance (1H NMR) was applied to investigate the chemical structure on Bruker AVANCE DRX-500.spectrometer using tetramethylsilane as internal standard. Solid-state 29Si and 13C nuclear magnetic resonance spectra were recorded on Varian Infinity spectrometer. Magic angle spinning (MAS) was applied at a
2.2. Synthesis of methacrylate modified SBA16 To a dispersion of SBA16 (2.50 g) in anhydrous toluene (20 mL), a solution of MPS (0.96 g MPS in 10 mL anhydrous toluene) and 3 mL triethylamine were added. The mixture was heat to 80 °C and continuously stirred for 24 h under protection of nitrogen atmosphere. The methacrylate-modified SBA16, denoted as SBA16-MA, was then 169
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CH3COO- and PF6-. The observed absorption bands at 1567 cm−1 for the symmetric vibration of carbonyl groups and the characteristic absorption bands at 850 cm−1 for PF6- indicate the successful formation of SBA16-PIL-AC and SBA16-PIL-PF6 [50]. Solid-state NMR spectroscopy was further applied to determine the grafting of imidazolium-type poly (ionic liquid) onto SBA16. The relatively broad resonance peaks at 123 and 137 ppm in 13C NMR spectrum (Fig. S4a) are attributed to the carbon atoms in imidazole ring and signals between -50 and -70 ppm in 29 Si NMR spectrum (Fig. S4b) can be assigned to O-Si-C, suggesting the successful grafting of imidazolium-type poly(ionic liquid) onto silica particles. In addition, SEM images of SBA16 (Fig. 2c) and SBA16-PIL-Br (Fig. 2d) revealed that the aggregation of the formed SBA16-PIL-Br is more evident than that of pristine SBA16. Fig. 3 shows TEM images and the associated elemental mapping images of carbon and nitrogen atoms for the synthesized SBA16-PIL-Br. The TEM image of SBA16 (Fig. 3a) evidenced that the synthesized silica substrates have the well-arranged mesopores with ordered structure. After the surface-attachment of polymeric layers on SBA16, the wellordered porous structure was retained, as shown in Fig. 3b. Such wellordered meso-porous structure of the designed catalysts cannot only lead to a large surface area for gaseous adsorption, but also facilitate the transport of carbon dioxide during the catalytic process. The associated elemental mapping images of carbon atoms (Fig. 3c) and nitrogen atoms (Fig. 3d) revealed that the attached polymer chains are well-located on the SBA16 substrates as evidenced by the well-distribution of carbon and nitrogen atoms. Moreover, XRD patterns of SBA16 and SBA16-PIL-Br were also recorded to further determine the ordered structure, as shown in Fig. S5. It can be seen that only one intense diffraction peak was observed at 2θ of about 0.9° for SBA16, suggesting the long-range periodicity of mesopores derived from the characteristic pore connectivity of hexagonal morphology [41]. After grafting of polymers, the intense diffraction peak remained with slightly decrease in the intensity, indicating the maintained periodic structure, which agrees with the TEM results. Since the microstructure of the formed catalysts can affect the gas transport and the according catalytic efficiency, surface area and porous structure of SBA16-PIL-Br was investigated by using nitrogen adsorption–desorption isotherms as shown in Fig. 4a. It can be seen that the isotherms of SBA16, SBA16-MA, and SBA16-PIL-Br exhibit type IV isotherms with distinct hysteresis loops, suggesting the ordered mesoporous structure [51,52]. The accordingly derived pore size distribution curves were displayed in Fig. 4b. It is apparent that the size distribution of generated pores is quite narrow with an average pore size of about 3−5 nm. The calculated structural parameters including surface area, pore volume, and average pore size from nitrogen adsorption-desorption isotherms were listed in Table 1. It is evident that both the surface area and pore volume decreased significantly after immobilization of methacrylate moieties and they further decreased with the grafting of polymer chains on the surface. This can be understood since the small functionalized methacrylate molecules could penetrate into the pores and react with surface hydroxyl groups on both the inner and outer surface, resulting in a significant decrease in surface area and pore volume. With the subsequent grafting of polymers, the pre-
spinning rate of 10 kHz and all spectra are based on tetramethylsilane (TMS) as the chemical shift reference. Thermogravimetric analysis (TGA) and differential scan calorimetry (DSC) were carried out using STA449F3 thermal analyzer (Netzsch) with a dynamic heating mode at ramp rate of 5 ℃ per minute in air over the temperature range of 30–1000 ℃. Prior to measurements, samples were dried in a vacuum oven at 100 °C for 6 h. Field emission transmission electron microscopyEnergy Dispersive Spectroscopy (TEM, jem2100 F/TEM-EDS, Talos F200S) and scanning electron microscope (SEM, Hitachi S4800) with an accelerating voltage of 5 kV were perform to characterize the morphology and distribution of elements of the synthesized catalysts. Nitrogen adsorption-desorption isotherms were recorded on a Micromeritic 2020 to determine the porous structure of samples. Prior to the measurements, samples were degassed at 120 ℃ for 6 h. The Brunauer-Emmet-Teller (BET) model was applied to calculate specific surface area and The Barret-Joyner-Halenda (BJH) approach was taken to obtain the pore size distribution. The conversion of epichlorohydrin was analyzed by 1H NMR on Ascend 400 and gas chromatography–mass spectroscopy (GCMS) on Atomx P&T-Agilent 7890B-5977B with the inlet temperature of 260 °C, detector temperature of 280 °C, and the oven temperature of 80 °C (programming temperature: 80 °C for 1 min, 60 °C per minute to 240 °C for 2 min).
3. Results and discussion To make the synthesis of catalysts simple and suitable for large-scale production, the reported “grafting through” technique was applied [46,47]. For this approach, methacrylate groups were first immobilized onto SBA16 substrate through silane chemistry. Mesoporous silica SBA16 was selected as substrates since the suitable pore size and large surface area can enhance the grafted amounts of polymers and can facilitate CO2 adsorption and transport. Both polymer chains and monomers were then integrated with surface-attached methacrylate groups with the presence of initiators and monomers, leading to grafting of polymer chains on the surface of SBA16, as schematically shown in Fig. 1. The chemical structure and the synthetic conditions for SBA16-PIL-Br were shown in Fig. S2. From the nitrogen content derived from elemental analysis (Table S1), the grafted amounts of monomeric units were calculated to be 43.5 μmolg−1. It should also be mentioned that the polymerization was performed in mixed solvent of DMF and deionized water since AIBN does not dissolved in water. FTIR spectroscopy was first applied to monitor the surface-attachment of methacrylate groups and the PIL chains, as displayed in Fig. 2a and Fig. S3. Compared with SBA16, the appeared absorption bands at 2800, 1724, and 1300 cm−1 in the FTIR spectrum of SBA16-MA are corresponded to C–H stretching vibration, C]O stretching vibration, and in-plane C–H vibration of vinyl groups, respectively, confirming the successful self-assembly of methacrylate groups on SBA16. After polymerization, the disappearance of the absorption band at 1300 cm−1 and the appearance of the characteristic absorption bands for imidazolium rings at 1569, 1450, and 649 cm−1 indicated the grafting of imidazolium moieties on the surface of SBA16 [48,49]. Fig. 2b shows FTIR spectra of SBA16-PIL-Br and the samples after ion exchange with
Fig. 1. Schematic illustration of the synthetic process of surface-attached poly(ionic liquid) monolayers via “grafting through” technique. 170
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Fig. 2. (a) FTIR spectra of SBA16, SBA16-MA, and SBA16-PIL-Br as indicated in the figure. (b) FTIR spectra of SBA16-PIL-Br, SBA16-PIL-AC, and SBA16-PIL-PF6. SEM images of SBA16 (c) and SBA16-PIL-Br (d). The scale bars in SEM images are 500 nm.
carbonate was further determined by 1H NMR spectroscopy and gas chromatography–mass spectroscopy. As a typical example, Fig. 6b shows the 1H NMR spectrum of the reaction mixture of ECH (5 mL) and SBA16-PIL-Br (0.05 g) after catalysis for 4 h under the CO2 pressure of 1.5 MPa at 120 °C, followed by removal of SBA16-PIL-Br. The detailed assignment of resonance peaks was displayed in the figure. The resonance peaks in the 1H NMR spectra confirmed that the resulted product after the reaction of epichlorohydrin with CO2 is cyclic carbonate compounds rather than the polycarbonates [56]. From the integration of the accordingly assigned resonance peaks and the according number of H-atom (5.10 ppm for cyclic carbonate and 3.25 ppm for ECH), the conversion was calculated to be about 90 %, that agrees well with the GCMS result (91 %). The effect of the according counterions on the catalytic activity towards cycloaddition of ECH with CO2 was further investigated. The solvent-free catalysis was carried out in 5 mL of ECH with 0.05 g of catalysts under the CO2 pressure of 1.5 MPa at 120 °C for 4 h and the corresponding conversion using different catalysts were collected in Table 2. It is apparent that the pristine SBA16 had almost no catalytic activity whereas the PIL-functionalized SBA16 materials exhibited a promising catalytic activity towards cycloaddition of ECH with CO2. Moreover, it can be seen that the SBA16-PIL-Br displayed the highest catalytic activity among all the tested samples under the same conditions for catalysis, as evidenced by the conversion of 90 % for SBA16PIL-Br, 63 % for SBA16-PIL-PF6, and 55 % for SBA16-PIL-AC. The effect of counterions on the catalytic activity has also been reported in homogenous catalysis for hydrogenation and cycloaddition of epoxy compounds using ionic liquid as catalysts due to the difference in electrophilic ability and steric hindrance of anions, which can in turn affect the catalysis [28,57]. Thus, the catalyst SBA16-PIL-Br was selected for the detailed investigation of the optimized reaction condition in the following sections. The effect of the CO2 pressure, reaction temperature, reaction time, and the ratio between ECH and catalyst on the catalytic efficiency was evaluated to determine the optimized condition for conversion of ECH
attached polymer chains can hinder further penetration of other polymer chains, resulting in the less-significant decrease in surface area and pore volume compared with the step of methacrylate attachment. Thermal properties of the synthesized SBA16-PIL-Br were analyzed to determine the safe temperature range for the following catalysis process of CO2 and epichlorohydrin. Fig. 5 displays the TGA and DSC curves of SBA16-PIL-Br synthesized by polymerization time of 4 h. For the TGA curve, the continuous weight loss of 5.4 wt% at temperature below 250 °C can be attributed to the evaporation of both physically adsorbed and structured water molecules, as also evidenced by the observed endothermal peak at 182.5 °C in the DSC curve. The relatively large water content in catalyst was attributed to the incorporation of water molecules from humid environment driven by the osmotic pressure of the counterions in the surface-attached hydrophilic poly(ionic liquid) monolayers [53–55]. Decomposition of the attached PIL layers seemed to be started around 250 °C with a significant exothermal peak centered at 360.3 °C. Thus, the synthesized SBA16-PIL-Br is thermally stable below 250 °C, under which the catalytic reaction of CO2 with epichlorohydrin can be performed. Moreover, the weight loss can be related to the loading of grafted polymers [44]. The weight loss of SBA16-PIL-Br beginning at around 250 °C was around 23 % that is slightly higher than the value calculated from elemental analysis (20 %). The catalytic reaction of CO2 and epichlorohydrin was carried out under a solvent-free condition, with which the separation of catalysts from final products is relatively simple and cost-effective. FTIR spectrum of the mixture in the reaction system was recorded to qualitatively determine whether the catalytic reaction occurred, as shown in Fig. 6a. For comparison, the spectrum of the raw material ECH was plotted in the same figure. It is apparent that the appeared absorption band at 1810 cm−1 is the characteristic absorption of C]O groups in carbonate, indicating the successful conversion of ECH to cyclic carbonate. In addition, the appearance of absorption band at 1164 cm−1 for stretching vibration of CeO in the carbonate also suggests the existence of carbonate compounds. The conversion of ECH and CO2 to cyclic 171
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Fig. 3. TEM images of SBA16 (a) and SBA16-PIL-Br (b). Elemental mapping images of carbon atoms (c) and nitrogen atoms (d) for the sample of SBA16-PIL-Br. Scale bars in all the images are 50 nm.
to cyclic carbonate. Fig. 7a displays the conversion as a function of CO2 pressure operated at 120 °C for 4 h with the catalyst content of 10 mg mL−1. It can be seen that the conversion increased from 13 % to 90 % with the increase in the CO2 pressure from 0.1 MPa to 1.5 MPa due to the increased concentration of CO2. However, with further increase in the CO2 pressure to 2.5 MPa, the conversion slightly decreased to 86 %. The optimized intermediate pressure for catalytic efficiency has also been observed in other catalytic systems [58–60]. Although the exact mechanism is still in argument, we proposed that such a slight decrease in the conversion could be possibly attributed to the shrinkage of the surface-attached polymer chains and the blockage of the epoxides to reach the active sites under relatively high CO2 pressure, which results in the decreased number of active sites for catalysis.
Table 1 Structural parameters of SBA16, SBA16-MA, and SBA16-PIL-Br. Samples
Surface area (m2 g−1)
Pore volume (cm3 g−1)
Pore size (nm)
SBA16 SBA16-MA SAB16-PIL-Br
712.3 404.5 336.7
0.67 0.46 0.39
3.3 4.6 4.5
The effect of the reaction temperature on the conversion of ECH to cyclic carbonate was investigated under CO2 pressure of 1.5 MPa for 4 h with the catalyst content of 10 mg mL−1, as illustrated in Fig. 7b. It is evident that the catalysis did not occur at 30 °C and the conversion is only 5 % at 60 °C. With the increase in the reaction temperature to 60
Fig. 4. Nitrogen adsorption-desorption isotherms (a) and the according pore size distribution (b) of SBA16, SBA16-MA, and SBA16-PIL-Br. 172
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Table 2 Catalytic conversion of ECH to cyclic carbonate using different catalysts. Entry
Catalyst
Conversion (%)*
1 2 3 4
SBA16 SBA16-PIL-Br SBA16-PIL-PF6 SBA16-PIL-AC
4 90 63 55
* Reaction conditions: ECH: 5 mL, catalyst: 0.05 g, CO2 pressure: 1.5 MPa, reaction temperature: 120 °C, reaction time: 4 h.
it decreased to 81 % and 75 % for the third and the fourth cycles, respectively, implying the reduction of catalytic activity existed. To elucidate the reduction of the catalytic activity for the conversion of ECH and CO2 to cyclic carbonate after the reusage of the catalyst, FTIR spectra of the synthesized SBA16-PIL-Br catalyst and the catalysts after the first and fourth cycles of catalysis were recorded, as shown in Fig. 8b. It can be clearly seen that the FTIR spectrum of the catalyst after the first cycle for catalysis is almost identical to the spectrum of the initial catalyst, indicating there is no detectable change on the chemical structure of catalyst. However, the FTIR spectrum of the catalyst after the four catalytic cycles showed a significant absorption band at 1810 cm−1 that was assigned to stretching vibration of C]O groups in cyclic carbonate. This result suggests that the continuous adsorption of the cyclic products on the catalysts is probably the reason for the reduction of the catalytic activity towards conversion of ECH and CO2 to cyclic carbonate since the adsorbed products can cover and accordingly can reduce the catalytic active sites. The fuzzy TEM image (Fig. S6) of the catalyst after four catalytic cycles compared with the initial image of the catalyst may also be an indication of the adsorption of organic molecules on the surface of catalyst. To further confirm the hypothesis, the recycled catalysts after the fourth cycle were thoroughly extracted by using Soxhlet extraction in ethanol for 12 h and were further applied for cycloaddition. The disappearance of the absorption band at 1810 cm−1 in FTIR spectrum (Fig. S7) revealed that the adsorbed cyclic carbonate can be removed from catalysts. The slightly increased carbon contents and the accordingly slight decrease in nitrogen content in elemental analysis results (Table S2) after the reaction suggested that small amount cyclic carbonate may still adsorb on the catalyst. In addition, the elemental analysis results indicate that the grafted polymer layers on SBA16 is quite stable. As shown in Fig. S8, the conversion using the re-treated catalysts reached 84 %, close to the initial conversion value in the first cycle (87 %) and much higher than that using the recycled catalysts without Soxhlet extraction, demonstrating that the catalytic activity can be recovered by removal of the adsorbed cyclic carbonate. It was proposed that the mechanism for the conversion of ECH and CO2 to cyclic carbonate using the amine-functionalized imidazoliumtype ionic liquid as catalysts includes the following steps (Fig. 8c), i.e.
Fig. 5. TGA (black line) and DSC (red line) curves of SBA16-PIL-Br in air.
and 120 °C, the conversion increased significantly to 40 % and 90 %, respectively. This can be understood that the increase in reaction temperature can lead to the enhanced reaction kinetics, which accordingly results in an increased conversion at the higher reaction temperature within the same reaction time. While the reaction temperature further increased to 140 °C, the conversion only slightly increased to 93 %. The dependence of the conversion on reaction time was shown in Fig. 7c using reaction conditions of 1.5 MPa of CO2 pressure at 120 °C with the catalyst content of 10 mg mL−1. It can be seen that the conversion continuously increased with the increase in the reaction time. Specifically, the conversion increased from 60 % to 90 % with the increase in the reaction temperature from 2 h to 4 h. With the further increase in the reaction time, the according increase in the conversion is less significant, for example 8 h for 94 %, due to the decreased concentration of the ECH in the reaction system. Considering the energy consumption for the catalytic reaction and the purity of cyclic carbonate required as feedstock, the optimized reaction condition could be set to 1.5 MPa of CO2 pressure at 120 °C for 4 h. Under this reaction condition, the influence of the catalyst content on the conversion efficiency was finally evaluated, as shown in Fig. 7d. It is apparent that the conversion initially increased from 66 % to 90 % with the increase in the catalyst content from 5 to 10 mg mL−1 and it kept constant with further increase in the amount of catalyst applied, suggesting that the catalyst content of 10 mg mL−1 could already provide enough active sites for the catalytic reaction. The recyclability and reversibility of the synthesized catalysts SBA16-PIL-Br for the conversion of ECH and CO2 to cyclic carbonate were evaluated under the operated CO2 pressure of 2.5 MPa at 120 °C for 4 h with the catalyst content of 10 mg mL−1 and the results were shown in Fig. 8a. It should be noted that the catalyst of SBA16-PIL-Br was just washed with deionized water and ethanol prior to reuse to simplify the whole process. It can be seen that the conversion in the first two cycles have no obvious change (with the values of 87 %). However,
Fig. 6. FTIR spectrum (a) and 1H NMR spectrum of the reaction mixture after catalysis for 4 h under the CO2 pressure of 1.5 MPa at 120 °C, followed by removal of SBA16-PIL-Br. 173
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Fig. 7. The dependence of conversion of ECH to cyclic carbonate on the reaction conditions of CO2 pressure (a), reaction temperature (b), reaction time (c), and the content of catalyst (d). The reaction conditions were described in the figure.
difference in the electrophilicity and steric hindrance of the associated counterions. The catalysis can be easily carried out under medium pressure (1.5 MPa) and elevated temperature of 120 °C with the conversion up to 90 %. The conversion slightly decreased with the increase in the cycle numbers of catalysis due to the adsorption of the cyclic carbonate products on the synthesized catalysts. However, the conversion could be recovered if the adsorbed cyclic carbonate products are removed from the catalysts by Soxhlet extraction. The results demonstrate that the surface-attached imidazolium-type poly(ionic liquid) monolayers bearing amino groups could be an effective catalyst for the conversion of epichlorohydrin and carbon dioxide to cyclic carbonate.
the formation of carbamate salt through reversible chemical reaction of amino group with CO2 to activate CO2 molecules, the interaction of active H-atoms in ammonium carbamate with O-atoms in ECH through hydrogen bonding to polarize the CeO bonds, the attack of counterions (bromide ions here) to the β-carbon atoms of ECH to promote the ring opening reaction of ECH, and the CO2 insertion and intramolecular ring closure to produce cyclic carbonate [61,62]. In addition, it has been reported that the 2-position carbon atoms on imidazole rings could activate epoxide, which in turn also improves the catalytic activity of imidazolium-type ionic liquid for cycloaddition of epoxy compounds and CO2 [63]. It should be noted that the proposed intermediates of ammonium carbamate were not detected in FTIR spectrum (Fig. S7), indicating that the reaction of amino group with CO2 is reversible. The major advantage of the synthesized surface-attached PIL layers as catalysts is the easy separation process of the catalyst from the reaction (Fig. S9), which is essential for reusage of catalyst in practical application. Moreover, the swelling of the PIL layers in the ECH of the reaction can result in large numbers of active sites exposed to the reactants, accordingly improving the catalytic efficiency for conversion of ECH and CO2 to cyclic carbonate. Finally, we compared the results in this work with the recently published works using imidazolium-type ionic liquid/poly(ionic liquid) as homogeneous catalysts for cycloaddition of epoxides and CO2 molecules as listed in Table S3. It is evident that the designed heterogeneous catalyst in this work exhibited comparable or even higher catalytic activity towards cycloaddition of epoxides and CO2 molecules under the very similar reaction conditions.
CRediT authorship contribution statement Rong Qu: Methodology, Investigation, Data curation, Writing original draft. Zijie Ren: Conceptualization, Methodology. Na Li: Data curation. Fangfang Zhang: Investigation, Data curation, Writing original draft. Zhengyu Jason Zhang: Writing - review & editing, Supervision. Haining Zhang: Conceptualization, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest 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.
4. Conclusions Surface-attached poly(ionic liquid) monolayers bearing amino groups on mesoporous silica substrate were synthesized through a “grafting through” technique and were applied as catalysts for the solvent-free conversion of epichlorohydrin to cyclic carbonate with the existence of carbon dioxide. The swelling of the surface-attached polymer layers in the reaction system leads to large numbers of active sites exposed to reactants and the applied silica substrate can result in an easy separation process of the products by centrifugation. It has been found that the counterions associated with the poly(ionic liquid) molecules has strong influence on the catalytic efficiency due to the
Acknowledgements This work was supported by the Natural Science Foundation of China under grant numbers of 21576216 and 21878239.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2020.01.022. 174
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Fig. 8. (a) Conversion of ECH to cyclic carbonate using SBA16-PIL-Br as catalyst at different cycles. (b) Comparison of FTIR spectra of the initial SBA16-PIL-Br and the catalyst after different catalytic cycles. (c) Proposed mechanism for the SBA16-PIL-Br catalyzed cycloaddition of ECH and CO2.
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Solvent-Free cycloaddition of carbon dioxide and epichlorohydrin catalyzed by surface-attached imidazolium-type poly(ionic liquid) monolayers
T
Rong Qua, Zijie Renb,*, Na Lic, Fangfang Zhangd,*, Zhengyu Jason Zhangc, Haining Zhanga,* a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Nr. 122 Luoshi Rd., Wuhan 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, Nr. 122 Luoshi Rd., Wuhan 430070, China c School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT UK d School of Nuclear Technology and Chemistry & Biology, Hubei University of Science and Technology, Nr. 88 Xianning Avenue, Xianning 437100, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon dioxide conversion Epoxy Cyclic carbonate Poly(ionic liquid) Surface-Attachment
Design and synthesis of effective catalysts for the conversion of epoxy compounds to cyclic carbonate under the presence of carbon dioxide molecules is essential for the utilization of carbon dioxide. Herein, we report that the surface-attached imidazolium-type poly(ionic liquid) bearing amine moieties is an efficient catalyst for the conversion of epoxy compound and carbon dioxide to cyclic carbonate. The effect of chemical structure of the synthesized catalyst and the reaction conditions on the catalytic efficiency is systematically studied. The results show that the catalyst with bromide anion can effectively catalyze the reaction under 1.5 MPa carbon dioxide pressure at 120 °C with the catalyst content of 10 mg mL−1, reaching the conversion rate up to 90 %. Moreover, the synthesized catalyst can be separated from the cyclic carbonate products by centrifugation. The recyclability and reversibility measurements reveal that the decay of the catalytic activity is mainly resulted from the adsorption of the cyclic carbonate products on the catalysts. The catalytic activity of the synthesized catalyst towards conversion of epichlorohydrin and carbon dioxide to cyclic carbonate can be recovered after extraction of the adsorbed cyclic carbonate products.
1. Introduction With the rapid development of industrialization and the large consumption of fossil fuels, the increased concentration of carbon dioxide (CO2) in the atmosphere results in serious environmental problems [1–5]. The atmospheric cleansing by CO2 capture and storage has been realized as an essential way to mitigate the environmental issues caused by CO2 emission [6–9]. Furthermore, carbon dioxide is an important feedstock in the synthetic process of cyclic carbonate from epoxy compounds under the existence of catalysts [10–14]. Since cyclic carbonate compounds can be applied in various industrial areas including aprotic solvents, precursors for polycarbonate synthesis, and intermediates in organic synthesis [15–18], development of effective catalysts for conversion of epoxy to cyclic carbonate is of great importance for practical applications. Typically, homogeneous catalyst exhibits good catalytic efficiency but complicated separation process of products whereas heterogeneous catalyst may have the relatively low reaction efficiency but easy separation of products from the reaction systems [19–21]. For catalyzing the formation of cyclic carbonate from epoxy and CO2, both
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homogeneous and heterogeneous catalysts have been developed to promote CO2 insertion into epoxide. Metal-organic complex and ionic liquids have been reported as efficient homogeneous catalysts for cyclic carbonate formation. Kim et al. reported that the homogenous catalysts synthesized from organophosphorus and zinc halide exhibited an exceptional catalytic performance in the reaction of ethylene oxide with carbon dioxide to from ethylene carbonate at 100 ◦C and 3.4 MPa for 1 h [22]. However, the cycloaddition reaction of carbon dioxide and epoxy compounds using metal-organic complex as catalyst was limited to the use of allyl and vinyl epoxides. Ionic liquid-based homogeneous catalysts have also been widely investigated for cycloaddition of carbon dioxide and epoxy compounds due to the unique characteristics of ionic liquids [23–27]. In the early work carried out by Peng et al., imidazolium-type ionic liquids were applied as homogeneous catalysts for the cycloaddition of CO2 and propylene oxide under 2.5 MPa at 110 °C [28]. The final products of propylene carbonate were obtained by vacuum distillation. In the recent work reported by Goodrich et al., a series of azolate-type ionic liquids were synthesized and were applied as homogeneous catalysts for cyclic carbonate formation from carbon dioxide and epoxy compounds
Corresponding authors. E-mail addresses: [email protected] (Z. Ren), zff[email protected] (F. Zhang), [email protected] (H. Zhang).
https://doi.org/10.1016/j.jcou.2020.01.022 Received 26 September 2019; Received in revised form 20 December 2019; Accepted 26 January 2020 2212-9820/ © 2020 Elsevier Ltd. All rights reserved.
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obtained by centrifugation, followed by extensively washing with toluene and ethanol to remove unreacted MPS. The product was finally dried at 60 °C in a vacuum oven and the final weight of SBA16-MA is about 2.61 g.
under 1.0 MPa at 100 °C [29]. It has been found that the catalytic activity was mainly affected by the existed anions. However, the developed homogenous catalysts suffered from the high cost and difficulty in separation of products and catalysts from the reaction systems. Heterogeneous catalysts have also been widely developed for the cycloaddition of carbon dioxide with epoxy compounds and the mainly applied heterogeneous catalysts include metal oxide, supported catalysts, and polymers [30–36]. For example, Yoshihara et al. found that magnesium oxide could effectively catalyze the synthesis of cyclic carbonates [31]. However, the reaction required high temperature, high pressure, and long reaction time due to the unsatisfactory catalytic activity of magnesium oxide. Poly(ionic liquid)-based catalysts have also been applied for cycloaddition of epoxides and carbon dioxide since they bear the advantages of both ionic liquids and polymers. Xiong et al. synthesized crosslinked poly(ionic liquid) nanoparticles via copolymerization of ethylene glycol dimethacrylate and functionalized imidazolium-type ionic liquid. With the catalyst loading of 33 mg mL−1, the highest conversion of epichlorohydrin to cyclic carbonate reached 98 % under 5 MPa CO2 pressure at 160 °C for 12 h [34]. Urethane-based imidazolium-type poly(ionic liquid)s were investigated as catalysts for cycloaddition of propylene oxide with carbon dioxide and the highest conversion reached 84 % with the catalyst loading of about 140 mg mL−1 under 2.5 MPa CO2 pressure at 110 °C for 6 h [35]. In this work, we report the synthesis of cyclic carbonate from epoxy compounds and carbon dioxide by using surface-attached amine-functionalized poly(ionic liquid) monolayers on mesoporous silica particles (SBA16) as catalysts. Amine-functionalized imidazolium-type ionic liquids were selected as catalysts since both amino-groups and 2-position carbon on imidazole ring could activate the carbon dioxide molecules for the cycloaddition through Lewis acid-base interactions [37,38]. In addition, the high diffusivity of carbon dioxide in imidazolium-type ionic liquid can also facilitate the cycloaddition of carbon dioxide with epoxy compounds [39,40]. The applied silica supports can ensure the easy separation of catalysts from the reaction system and the swelling of the surface-attached polymer chains makes the reaction being locally homogenous catalysis. It is thus expected that the designed catalysts can take the advantages of both heterogeneous and homogeneous catalysts, resulting in the high catalytic efficiency and the easy separation process of catalysts from cyclic carbonate products.
2.3. Synthesis of surface-attached poly(ionic liquid) monolayers on SBA16 To a dispersion of SBA16-MA (0.50 g) and AIBN (0.06 g) in 10 mL N, N-dimethylformamide, the ionic liquid monomer solution (6.0 g in 3 mL deionized water) was added under magnetic stirring. The reaction system was degassed through three freeze-thaw cycles under vacuum to remove trance of oxygen and was heated to 70 ℃ under protection of nitrogen atmosphere for polymerization. After 4 h polymerization, the mixture was allowed to cool down to room temperature. The mixture was centrifuged and extensively washed with deionized water to remove non-attached polymer chains. The resulted white powder was then re-dispersed in a solution of 12 mL triethylamine in 40 mL deionized water and was kept stirring for 48 h at room temperature to remove hydrobromic acid. The final product (0.55 g) was obtained by centrifugation and extensively washing with deionized water, followed by drying at 60 °C under vacuum. The product was denoted as SBA16PIL-Br, where Br refers to bromide counterions of PIL. The surface-attached PIL monolayers with different counterions (CH3COO− and PF6−) were synthesized by ion exchange process according to literature [43–45]. Typically, 0.10 g of SBA16-PIL-Br was dispersed in deionized water (100 mL) at room temperature under magnetic stirring. After addition of 20 mL of the according salt aqueous solution (0.1 mol L-1, CH3COONa or KPF6), the reaction mixture was kept stirring continuously for 48 h. The product was then centrifuged and extensively washed with deionized water, followed by drying under vacuum at 75 °C. The products were denoted as SBA16-PIL-AC and SBA16-PIL-PF6, respectively. 2.4. Formation of cyclic carbonates from epichlorohydrin and CO2 Epichlorohydrin (5 mL) and surface-attached PIL catalysts (0.05 g) were successively added into a 50 mL autoclave equipped with a magnetic particle and heating devices, as shown in Fig. S1. The autoclave is connected to a high-pressure carbon dioxide gas cylinder to deliver carbon dioxide at a constant pressure during the catalytic process. After CO2 was charged at room temperature, the outlet valve of the autoclave was closed and the inlet valve remained open. The reaction system was heated up to the desired temperature and the pressure was adjusted to the desired value through continuous aeration during the reaction. After a desired reaction time, the reaction system was allowed to cool down to room temperature. The inlet valve was then closed and the outlet valve was slowly opened to vent the unreacted CO2. The products of cyclic carbonate were obtained after the catalyst was filtered. The re-collected catalyst was washed with ethanol and can be directly re-used after drying at 75 °C in a vacuum oven for 12 h unless otherwise stated.
2. Experimental section 2.1. Materials 1-Vinylimidazole, 2-bromoethylamine hydrobromide, and 3-methacryloylpropyl trimethoxysilane (97 %, MPS) were purchased from Alfa Aesar. Azobisisobutyronitrile (AIBN) was received from Aladdin (Shanghai, China) and was recrystallized from methanol prior to polymerization. Water was deionized using a Ulupure-H ultrapure water generator (Ulup, China) with resistivity of 18.2 MΩ cm. Toluene was distilled over sodium using benzophenone as indicator and stored with molecular sieves. All the other chemicals including tetraethoxysilane (TEOS–98 %, Sinopharm, China), pluronic F-127 (BASF, Germany), and epichlorohydrin (ECH, Aladdin, China) were used as received. The SBA16 was synthesized through acid catalyzed sol-gel chemistry according to literature [41] and the synthesis of 1-aminoethyl-3-vinylimidazolium bromide hydrobromide monomer was reported elsewhere [42].
2.5. Characterization Fourier transform infrared (FTIR) spectra were recorded on 60SXB spectrometer (Nicolet) with a resolution of 4 cm−1 to determine the chemical composition of samples. X-ray diffraction (XRD) patterns of mesoporous silica particles and modified silica were recorded on a Rigaku D/MAX-RB diffractometer using a Cu Kα radiation at 40 kV and 50 mA. Elemental analysis was carried out on a Vario EL Cube analyzer (Elementar, Germany) to calculate the grafted amounts of polymers for the catalysts. Nuclear magnetic resonance (1H NMR) was applied to investigate the chemical structure on Bruker AVANCE DRX-500.spectrometer using tetramethylsilane as internal standard. Solid-state 29Si and 13C nuclear magnetic resonance spectra were recorded on Varian Infinity spectrometer. Magic angle spinning (MAS) was applied at a
2.2. Synthesis of methacrylate modified SBA16 To a dispersion of SBA16 (2.50 g) in anhydrous toluene (20 mL), a solution of MPS (0.96 g MPS in 10 mL anhydrous toluene) and 3 mL triethylamine were added. The mixture was heat to 80 °C and continuously stirred for 24 h under protection of nitrogen atmosphere. The methacrylate-modified SBA16, denoted as SBA16-MA, was then 169
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CH3COO- and PF6-. The observed absorption bands at 1567 cm−1 for the symmetric vibration of carbonyl groups and the characteristic absorption bands at 850 cm−1 for PF6- indicate the successful formation of SBA16-PIL-AC and SBA16-PIL-PF6 [50]. Solid-state NMR spectroscopy was further applied to determine the grafting of imidazolium-type poly (ionic liquid) onto SBA16. The relatively broad resonance peaks at 123 and 137 ppm in 13C NMR spectrum (Fig. S4a) are attributed to the carbon atoms in imidazole ring and signals between -50 and -70 ppm in 29 Si NMR spectrum (Fig. S4b) can be assigned to O-Si-C, suggesting the successful grafting of imidazolium-type poly(ionic liquid) onto silica particles. In addition, SEM images of SBA16 (Fig. 2c) and SBA16-PIL-Br (Fig. 2d) revealed that the aggregation of the formed SBA16-PIL-Br is more evident than that of pristine SBA16. Fig. 3 shows TEM images and the associated elemental mapping images of carbon and nitrogen atoms for the synthesized SBA16-PIL-Br. The TEM image of SBA16 (Fig. 3a) evidenced that the synthesized silica substrates have the well-arranged mesopores with ordered structure. After the surface-attachment of polymeric layers on SBA16, the wellordered porous structure was retained, as shown in Fig. 3b. Such wellordered meso-porous structure of the designed catalysts cannot only lead to a large surface area for gaseous adsorption, but also facilitate the transport of carbon dioxide during the catalytic process. The associated elemental mapping images of carbon atoms (Fig. 3c) and nitrogen atoms (Fig. 3d) revealed that the attached polymer chains are well-located on the SBA16 substrates as evidenced by the well-distribution of carbon and nitrogen atoms. Moreover, XRD patterns of SBA16 and SBA16-PIL-Br were also recorded to further determine the ordered structure, as shown in Fig. S5. It can be seen that only one intense diffraction peak was observed at 2θ of about 0.9° for SBA16, suggesting the long-range periodicity of mesopores derived from the characteristic pore connectivity of hexagonal morphology [41]. After grafting of polymers, the intense diffraction peak remained with slightly decrease in the intensity, indicating the maintained periodic structure, which agrees with the TEM results. Since the microstructure of the formed catalysts can affect the gas transport and the according catalytic efficiency, surface area and porous structure of SBA16-PIL-Br was investigated by using nitrogen adsorption–desorption isotherms as shown in Fig. 4a. It can be seen that the isotherms of SBA16, SBA16-MA, and SBA16-PIL-Br exhibit type IV isotherms with distinct hysteresis loops, suggesting the ordered mesoporous structure [51,52]. The accordingly derived pore size distribution curves were displayed in Fig. 4b. It is apparent that the size distribution of generated pores is quite narrow with an average pore size of about 3−5 nm. The calculated structural parameters including surface area, pore volume, and average pore size from nitrogen adsorption-desorption isotherms were listed in Table 1. It is evident that both the surface area and pore volume decreased significantly after immobilization of methacrylate moieties and they further decreased with the grafting of polymer chains on the surface. This can be understood since the small functionalized methacrylate molecules could penetrate into the pores and react with surface hydroxyl groups on both the inner and outer surface, resulting in a significant decrease in surface area and pore volume. With the subsequent grafting of polymers, the pre-
spinning rate of 10 kHz and all spectra are based on tetramethylsilane (TMS) as the chemical shift reference. Thermogravimetric analysis (TGA) and differential scan calorimetry (DSC) were carried out using STA449F3 thermal analyzer (Netzsch) with a dynamic heating mode at ramp rate of 5 ℃ per minute in air over the temperature range of 30–1000 ℃. Prior to measurements, samples were dried in a vacuum oven at 100 °C for 6 h. Field emission transmission electron microscopyEnergy Dispersive Spectroscopy (TEM, jem2100 F/TEM-EDS, Talos F200S) and scanning electron microscope (SEM, Hitachi S4800) with an accelerating voltage of 5 kV were perform to characterize the morphology and distribution of elements of the synthesized catalysts. Nitrogen adsorption-desorption isotherms were recorded on a Micromeritic 2020 to determine the porous structure of samples. Prior to the measurements, samples were degassed at 120 ℃ for 6 h. The Brunauer-Emmet-Teller (BET) model was applied to calculate specific surface area and The Barret-Joyner-Halenda (BJH) approach was taken to obtain the pore size distribution. The conversion of epichlorohydrin was analyzed by 1H NMR on Ascend 400 and gas chromatography–mass spectroscopy (GCMS) on Atomx P&T-Agilent 7890B-5977B with the inlet temperature of 260 °C, detector temperature of 280 °C, and the oven temperature of 80 °C (programming temperature: 80 °C for 1 min, 60 °C per minute to 240 °C for 2 min).
3. Results and discussion To make the synthesis of catalysts simple and suitable for large-scale production, the reported “grafting through” technique was applied [46,47]. For this approach, methacrylate groups were first immobilized onto SBA16 substrate through silane chemistry. Mesoporous silica SBA16 was selected as substrates since the suitable pore size and large surface area can enhance the grafted amounts of polymers and can facilitate CO2 adsorption and transport. Both polymer chains and monomers were then integrated with surface-attached methacrylate groups with the presence of initiators and monomers, leading to grafting of polymer chains on the surface of SBA16, as schematically shown in Fig. 1. The chemical structure and the synthetic conditions for SBA16-PIL-Br were shown in Fig. S2. From the nitrogen content derived from elemental analysis (Table S1), the grafted amounts of monomeric units were calculated to be 43.5 μmolg−1. It should also be mentioned that the polymerization was performed in mixed solvent of DMF and deionized water since AIBN does not dissolved in water. FTIR spectroscopy was first applied to monitor the surface-attachment of methacrylate groups and the PIL chains, as displayed in Fig. 2a and Fig. S3. Compared with SBA16, the appeared absorption bands at 2800, 1724, and 1300 cm−1 in the FTIR spectrum of SBA16-MA are corresponded to C–H stretching vibration, C]O stretching vibration, and in-plane C–H vibration of vinyl groups, respectively, confirming the successful self-assembly of methacrylate groups on SBA16. After polymerization, the disappearance of the absorption band at 1300 cm−1 and the appearance of the characteristic absorption bands for imidazolium rings at 1569, 1450, and 649 cm−1 indicated the grafting of imidazolium moieties on the surface of SBA16 [48,49]. Fig. 2b shows FTIR spectra of SBA16-PIL-Br and the samples after ion exchange with
Fig. 1. Schematic illustration of the synthetic process of surface-attached poly(ionic liquid) monolayers via “grafting through” technique. 170
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Fig. 2. (a) FTIR spectra of SBA16, SBA16-MA, and SBA16-PIL-Br as indicated in the figure. (b) FTIR spectra of SBA16-PIL-Br, SBA16-PIL-AC, and SBA16-PIL-PF6. SEM images of SBA16 (c) and SBA16-PIL-Br (d). The scale bars in SEM images are 500 nm.
carbonate was further determined by 1H NMR spectroscopy and gas chromatography–mass spectroscopy. As a typical example, Fig. 6b shows the 1H NMR spectrum of the reaction mixture of ECH (5 mL) and SBA16-PIL-Br (0.05 g) after catalysis for 4 h under the CO2 pressure of 1.5 MPa at 120 °C, followed by removal of SBA16-PIL-Br. The detailed assignment of resonance peaks was displayed in the figure. The resonance peaks in the 1H NMR spectra confirmed that the resulted product after the reaction of epichlorohydrin with CO2 is cyclic carbonate compounds rather than the polycarbonates [56]. From the integration of the accordingly assigned resonance peaks and the according number of H-atom (5.10 ppm for cyclic carbonate and 3.25 ppm for ECH), the conversion was calculated to be about 90 %, that agrees well with the GCMS result (91 %). The effect of the according counterions on the catalytic activity towards cycloaddition of ECH with CO2 was further investigated. The solvent-free catalysis was carried out in 5 mL of ECH with 0.05 g of catalysts under the CO2 pressure of 1.5 MPa at 120 °C for 4 h and the corresponding conversion using different catalysts were collected in Table 2. It is apparent that the pristine SBA16 had almost no catalytic activity whereas the PIL-functionalized SBA16 materials exhibited a promising catalytic activity towards cycloaddition of ECH with CO2. Moreover, it can be seen that the SBA16-PIL-Br displayed the highest catalytic activity among all the tested samples under the same conditions for catalysis, as evidenced by the conversion of 90 % for SBA16PIL-Br, 63 % for SBA16-PIL-PF6, and 55 % for SBA16-PIL-AC. The effect of counterions on the catalytic activity has also been reported in homogenous catalysis for hydrogenation and cycloaddition of epoxy compounds using ionic liquid as catalysts due to the difference in electrophilic ability and steric hindrance of anions, which can in turn affect the catalysis [28,57]. Thus, the catalyst SBA16-PIL-Br was selected for the detailed investigation of the optimized reaction condition in the following sections. The effect of the CO2 pressure, reaction temperature, reaction time, and the ratio between ECH and catalyst on the catalytic efficiency was evaluated to determine the optimized condition for conversion of ECH
attached polymer chains can hinder further penetration of other polymer chains, resulting in the less-significant decrease in surface area and pore volume compared with the step of methacrylate attachment. Thermal properties of the synthesized SBA16-PIL-Br were analyzed to determine the safe temperature range for the following catalysis process of CO2 and epichlorohydrin. Fig. 5 displays the TGA and DSC curves of SBA16-PIL-Br synthesized by polymerization time of 4 h. For the TGA curve, the continuous weight loss of 5.4 wt% at temperature below 250 °C can be attributed to the evaporation of both physically adsorbed and structured water molecules, as also evidenced by the observed endothermal peak at 182.5 °C in the DSC curve. The relatively large water content in catalyst was attributed to the incorporation of water molecules from humid environment driven by the osmotic pressure of the counterions in the surface-attached hydrophilic poly(ionic liquid) monolayers [53–55]. Decomposition of the attached PIL layers seemed to be started around 250 °C with a significant exothermal peak centered at 360.3 °C. Thus, the synthesized SBA16-PIL-Br is thermally stable below 250 °C, under which the catalytic reaction of CO2 with epichlorohydrin can be performed. Moreover, the weight loss can be related to the loading of grafted polymers [44]. The weight loss of SBA16-PIL-Br beginning at around 250 °C was around 23 % that is slightly higher than the value calculated from elemental analysis (20 %). The catalytic reaction of CO2 and epichlorohydrin was carried out under a solvent-free condition, with which the separation of catalysts from final products is relatively simple and cost-effective. FTIR spectrum of the mixture in the reaction system was recorded to qualitatively determine whether the catalytic reaction occurred, as shown in Fig. 6a. For comparison, the spectrum of the raw material ECH was plotted in the same figure. It is apparent that the appeared absorption band at 1810 cm−1 is the characteristic absorption of C]O groups in carbonate, indicating the successful conversion of ECH to cyclic carbonate. In addition, the appearance of absorption band at 1164 cm−1 for stretching vibration of CeO in the carbonate also suggests the existence of carbonate compounds. The conversion of ECH and CO2 to cyclic 171
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Fig. 3. TEM images of SBA16 (a) and SBA16-PIL-Br (b). Elemental mapping images of carbon atoms (c) and nitrogen atoms (d) for the sample of SBA16-PIL-Br. Scale bars in all the images are 50 nm.
to cyclic carbonate. Fig. 7a displays the conversion as a function of CO2 pressure operated at 120 °C for 4 h with the catalyst content of 10 mg mL−1. It can be seen that the conversion increased from 13 % to 90 % with the increase in the CO2 pressure from 0.1 MPa to 1.5 MPa due to the increased concentration of CO2. However, with further increase in the CO2 pressure to 2.5 MPa, the conversion slightly decreased to 86 %. The optimized intermediate pressure for catalytic efficiency has also been observed in other catalytic systems [58–60]. Although the exact mechanism is still in argument, we proposed that such a slight decrease in the conversion could be possibly attributed to the shrinkage of the surface-attached polymer chains and the blockage of the epoxides to reach the active sites under relatively high CO2 pressure, which results in the decreased number of active sites for catalysis.
Table 1 Structural parameters of SBA16, SBA16-MA, and SBA16-PIL-Br. Samples
Surface area (m2 g−1)
Pore volume (cm3 g−1)
Pore size (nm)
SBA16 SBA16-MA SAB16-PIL-Br
712.3 404.5 336.7
0.67 0.46 0.39
3.3 4.6 4.5
The effect of the reaction temperature on the conversion of ECH to cyclic carbonate was investigated under CO2 pressure of 1.5 MPa for 4 h with the catalyst content of 10 mg mL−1, as illustrated in Fig. 7b. It is evident that the catalysis did not occur at 30 °C and the conversion is only 5 % at 60 °C. With the increase in the reaction temperature to 60
Fig. 4. Nitrogen adsorption-desorption isotherms (a) and the according pore size distribution (b) of SBA16, SBA16-MA, and SBA16-PIL-Br. 172
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Table 2 Catalytic conversion of ECH to cyclic carbonate using different catalysts. Entry
Catalyst
Conversion (%)*
1 2 3 4
SBA16 SBA16-PIL-Br SBA16-PIL-PF6 SBA16-PIL-AC
4 90 63 55
* Reaction conditions: ECH: 5 mL, catalyst: 0.05 g, CO2 pressure: 1.5 MPa, reaction temperature: 120 °C, reaction time: 4 h.
it decreased to 81 % and 75 % for the third and the fourth cycles, respectively, implying the reduction of catalytic activity existed. To elucidate the reduction of the catalytic activity for the conversion of ECH and CO2 to cyclic carbonate after the reusage of the catalyst, FTIR spectra of the synthesized SBA16-PIL-Br catalyst and the catalysts after the first and fourth cycles of catalysis were recorded, as shown in Fig. 8b. It can be clearly seen that the FTIR spectrum of the catalyst after the first cycle for catalysis is almost identical to the spectrum of the initial catalyst, indicating there is no detectable change on the chemical structure of catalyst. However, the FTIR spectrum of the catalyst after the four catalytic cycles showed a significant absorption band at 1810 cm−1 that was assigned to stretching vibration of C]O groups in cyclic carbonate. This result suggests that the continuous adsorption of the cyclic products on the catalysts is probably the reason for the reduction of the catalytic activity towards conversion of ECH and CO2 to cyclic carbonate since the adsorbed products can cover and accordingly can reduce the catalytic active sites. The fuzzy TEM image (Fig. S6) of the catalyst after four catalytic cycles compared with the initial image of the catalyst may also be an indication of the adsorption of organic molecules on the surface of catalyst. To further confirm the hypothesis, the recycled catalysts after the fourth cycle were thoroughly extracted by using Soxhlet extraction in ethanol for 12 h and were further applied for cycloaddition. The disappearance of the absorption band at 1810 cm−1 in FTIR spectrum (Fig. S7) revealed that the adsorbed cyclic carbonate can be removed from catalysts. The slightly increased carbon contents and the accordingly slight decrease in nitrogen content in elemental analysis results (Table S2) after the reaction suggested that small amount cyclic carbonate may still adsorb on the catalyst. In addition, the elemental analysis results indicate that the grafted polymer layers on SBA16 is quite stable. As shown in Fig. S8, the conversion using the re-treated catalysts reached 84 %, close to the initial conversion value in the first cycle (87 %) and much higher than that using the recycled catalysts without Soxhlet extraction, demonstrating that the catalytic activity can be recovered by removal of the adsorbed cyclic carbonate. It was proposed that the mechanism for the conversion of ECH and CO2 to cyclic carbonate using the amine-functionalized imidazoliumtype ionic liquid as catalysts includes the following steps (Fig. 8c), i.e.
Fig. 5. TGA (black line) and DSC (red line) curves of SBA16-PIL-Br in air.
and 120 °C, the conversion increased significantly to 40 % and 90 %, respectively. This can be understood that the increase in reaction temperature can lead to the enhanced reaction kinetics, which accordingly results in an increased conversion at the higher reaction temperature within the same reaction time. While the reaction temperature further increased to 140 °C, the conversion only slightly increased to 93 %. The dependence of the conversion on reaction time was shown in Fig. 7c using reaction conditions of 1.5 MPa of CO2 pressure at 120 °C with the catalyst content of 10 mg mL−1. It can be seen that the conversion continuously increased with the increase in the reaction time. Specifically, the conversion increased from 60 % to 90 % with the increase in the reaction temperature from 2 h to 4 h. With the further increase in the reaction time, the according increase in the conversion is less significant, for example 8 h for 94 %, due to the decreased concentration of the ECH in the reaction system. Considering the energy consumption for the catalytic reaction and the purity of cyclic carbonate required as feedstock, the optimized reaction condition could be set to 1.5 MPa of CO2 pressure at 120 °C for 4 h. Under this reaction condition, the influence of the catalyst content on the conversion efficiency was finally evaluated, as shown in Fig. 7d. It is apparent that the conversion initially increased from 66 % to 90 % with the increase in the catalyst content from 5 to 10 mg mL−1 and it kept constant with further increase in the amount of catalyst applied, suggesting that the catalyst content of 10 mg mL−1 could already provide enough active sites for the catalytic reaction. The recyclability and reversibility of the synthesized catalysts SBA16-PIL-Br for the conversion of ECH and CO2 to cyclic carbonate were evaluated under the operated CO2 pressure of 2.5 MPa at 120 °C for 4 h with the catalyst content of 10 mg mL−1 and the results were shown in Fig. 8a. It should be noted that the catalyst of SBA16-PIL-Br was just washed with deionized water and ethanol prior to reuse to simplify the whole process. It can be seen that the conversion in the first two cycles have no obvious change (with the values of 87 %). However,
Fig. 6. FTIR spectrum (a) and 1H NMR spectrum of the reaction mixture after catalysis for 4 h under the CO2 pressure of 1.5 MPa at 120 °C, followed by removal of SBA16-PIL-Br. 173
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Fig. 7. The dependence of conversion of ECH to cyclic carbonate on the reaction conditions of CO2 pressure (a), reaction temperature (b), reaction time (c), and the content of catalyst (d). The reaction conditions were described in the figure.
difference in the electrophilicity and steric hindrance of the associated counterions. The catalysis can be easily carried out under medium pressure (1.5 MPa) and elevated temperature of 120 °C with the conversion up to 90 %. The conversion slightly decreased with the increase in the cycle numbers of catalysis due to the adsorption of the cyclic carbonate products on the synthesized catalysts. However, the conversion could be recovered if the adsorbed cyclic carbonate products are removed from the catalysts by Soxhlet extraction. The results demonstrate that the surface-attached imidazolium-type poly(ionic liquid) monolayers bearing amino groups could be an effective catalyst for the conversion of epichlorohydrin and carbon dioxide to cyclic carbonate.
the formation of carbamate salt through reversible chemical reaction of amino group with CO2 to activate CO2 molecules, the interaction of active H-atoms in ammonium carbamate with O-atoms in ECH through hydrogen bonding to polarize the CeO bonds, the attack of counterions (bromide ions here) to the β-carbon atoms of ECH to promote the ring opening reaction of ECH, and the CO2 insertion and intramolecular ring closure to produce cyclic carbonate [61,62]. In addition, it has been reported that the 2-position carbon atoms on imidazole rings could activate epoxide, which in turn also improves the catalytic activity of imidazolium-type ionic liquid for cycloaddition of epoxy compounds and CO2 [63]. It should be noted that the proposed intermediates of ammonium carbamate were not detected in FTIR spectrum (Fig. S7), indicating that the reaction of amino group with CO2 is reversible. The major advantage of the synthesized surface-attached PIL layers as catalysts is the easy separation process of the catalyst from the reaction (Fig. S9), which is essential for reusage of catalyst in practical application. Moreover, the swelling of the PIL layers in the ECH of the reaction can result in large numbers of active sites exposed to the reactants, accordingly improving the catalytic efficiency for conversion of ECH and CO2 to cyclic carbonate. Finally, we compared the results in this work with the recently published works using imidazolium-type ionic liquid/poly(ionic liquid) as homogeneous catalysts for cycloaddition of epoxides and CO2 molecules as listed in Table S3. It is evident that the designed heterogeneous catalyst in this work exhibited comparable or even higher catalytic activity towards cycloaddition of epoxides and CO2 molecules under the very similar reaction conditions.
CRediT authorship contribution statement Rong Qu: Methodology, Investigation, Data curation, Writing original draft. Zijie Ren: Conceptualization, Methodology. Na Li: Data curation. Fangfang Zhang: Investigation, Data curation, Writing original draft. Zhengyu Jason Zhang: Writing - review & editing, Supervision. Haining Zhang: Conceptualization, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest 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.
4. Conclusions Surface-attached poly(ionic liquid) monolayers bearing amino groups on mesoporous silica substrate were synthesized through a “grafting through” technique and were applied as catalysts for the solvent-free conversion of epichlorohydrin to cyclic carbonate with the existence of carbon dioxide. The swelling of the surface-attached polymer layers in the reaction system leads to large numbers of active sites exposed to reactants and the applied silica substrate can result in an easy separation process of the products by centrifugation. It has been found that the counterions associated with the poly(ionic liquid) molecules has strong influence on the catalytic efficiency due to the
Acknowledgements This work was supported by the Natural Science Foundation of China under grant numbers of 21576216 and 21878239.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2020.01.022. 174
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Fig. 8. (a) Conversion of ECH to cyclic carbonate using SBA16-PIL-Br as catalyst at different cycles. (b) Comparison of FTIR spectra of the initial SBA16-PIL-Br and the catalyst after different catalytic cycles. (c) Proposed mechanism for the SBA16-PIL-Br catalyzed cycloaddition of ECH and CO2.
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