Polymer-Supported Zn-Containing Imidazolium Salt Ionic Liquids as

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Polymer-supported Zn-containing imidazolium salt ionic liquids as sustainable catalysts for the cycloaddition of CO: A kinetic study and response surface methodology 2

Dongwoo Kim, Hoon Ji, Moon Young Hur, Wonjoo Lee, Tea Soon Kim, and Deug-Hee Cho ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03296 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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ACS Sustainable Chemistry & Engineering

Polymer-supported Zn-containing imidazolium salt ionic liquids as sustainable catalysts for the cycloaddition of CO2: A kinetic study and response surface methodology Dongwoo Kim, Hoon Ji, Moon Young Hur, Wonjoo Lee, Tea Soon Kim, and Deug-Hee Cho* Advanced Industrial Chemistry Research Center, Korea Research Institute of Chemical Technology, 45, Jongga-ro, Jung-gu, Ulsan 44412, Republic of Korea

* Corresponding author E-mail address: [email protected] Tel.: +82 52 241 6040; fax: +82 52 241 6049.

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ABSTRACT

Metal-containing ionic liquids (MIL) are highly active homogeneous catalysts for the chemical fixation of CO2 into epoxides yielding five-membered cyclic carbonates. To obtain sustainability, polymer-supported metal-containing ionic liquid (MIL) catalysts were designed to overcome their previously reported inherent poor recoverability. Polymersupported Zn-containing imidazolium salts, PS-(Im)2ZnX2, possessing bifunctional properties were identified as sustainable catalysts for the activation of CO2 under solvent-free conditions. PS-(Im)2ZnX2 showed good catalytic performance and was readily recoverable and reusable in the subsequent reaction cycles for the cycloaddition of CO2 to propylene oxide (PO). Kinetic studies for the cycloaddition of CO2 to PC using batch or semi-batch systems were performed to calculate the approximate rate constants, which provide an explanation for the different reaction systems. In addition, process optimization using the response surface methodology was performed, and the interactions between the operational variables were identified. The results demonstrate the advantage of the semi-batch system, which allows the completion of the reaction in less time than when using the batch system.

KEYWORDS: Carbon dioxide, Sustainable, Immobilization, Ionic liquid, Kinetic study, RSM

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INTRODUCTION Carbon dioxide (CO2) has excellent potential as a renewable C1 feedstock to prepare valueadded chemical products in an environmentally sustainable manner because of its nontoxicity and abundance (as a major product of the chemical and power industries). The preparation of five-membered cyclic carbonates via cycloaddition of CO2 to epoxide is attractive because of the wide applications of these compounds, for example, as polar aprotic solvents, electrolytic elements in batteries, and intermediates or precursors in the production of fine chemicals, pharmaceuticals, and polymers.1-3 However, the innate stability of CO2 demands specific catalysts for its activation. Numerous kinds of catalysts, both homogeneous and heterogeneous, have been developed. One of the most notable type of catalyst among these catalyst systems in terms of turnover frequency is ionic liquids4,5 or a combination of metal salts, ionic liquids, and organometallic salen complexes,6 metal oxides,7 supportedphase catalysts,8-10 metal-organic frameworks (MOFs),11,12 and carbon materials have also been shown to have promise.13,14 This has further enriched the number of bifunctional catalysts, such as the hydroxyl-rich renewable biopolymer cellulose15 and its oligomeric analogs,16 for example, gamma and beta-cyclodextrins and cucurbituril17 with alkali metal halides for the cycloaddition of CO2 to epoxides, and the results were indicative of the synergism of halide ions and hydrogen bonding groups. According to previous study,18 metal-containing ionic liquid (MIL) catalysts based on imidazolium showed good catalytic performance because of the synergistic effects between the metal center and the nucleophilic anion. The catalytic activity of the MIL catalysts depends on the metal center: Zn-based MIL catalysts are the most efficient, among the different metal centers (in comparison to Cu and Fe). However, a homogeneous system of 3



MIL catalysts for the synthesis of cyclic carbonates shows poor catalyst separation. The exploration of highly efficient heterogeneous catalysts still remains a challenge. Polymersupported catalysts are widely used in the field of catalysis due to the advantageous properties such as facile catalyst recovery relative to homogeneous catalysts.19,20 Recently, polymer-supported ionic liquids have been utilized in the preparation of cyclic carbonates and have been found to combine the merits of homogeneous ionic liquids and heterogeneous catalysts.21,22 In the design of a reactor for chemical production, it is critical to select whether batch- or continuous-type system should be applied. Continuous flow methods enable technologies for a wide range of chemical transformations, including gas–liquid biphasic reactions. The use of continuous-flow reactions is one of the main strategies used to increase productivity because of its advantages in the purification/separation steps, the high surface-to-volume ratio, enhanced mass transport, straightforward scaling-up, and improved safety. Despite the numerous advantages of the continuous flow system, many reactions are first investigated in batch lab-scale reactors because batch and semi-batch systems are more amenable to the research and development (R&D) practice and attitude.23 In this study, a structurally modified polystyrene based on a Merrifield peptide resin was used to support Zn-containing imidazolium salts, PS-(Im)2ZnX2 (X is Cl, Br, or I), which is a bifunctional and heterogeneous catalyst, were prepared and characterized via various physicochemical methods. The catalytic performance of the PS-(Im)2ZnX2 catalysts for the cycloaddition of CO2 to epoxides in the solvent-free system was investigated, and the supported catalyst was subjected to recycling tests to examine its stability. We have attempted to compare the effects of batch and semi-batch systems for the cycloaddition of 4


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CO2 to PO using preliminary kinetic studies at various temperatures and reaction times. In addition, the optimization of the reaction conditions for the batch and the semi-batch systems has been studied using the response surface methodology (RSM).

EXPERIMENTAL SECTION Materials. High-purity (> 97%) 1-(2-hydroxyethyl)imidazole (HEIm), zinc halides (ZnCl2, ZnBr2, and ZnI2), and Merrifield’s peptide resin (MPR, 1% divinylbenzene, 3.5–4.5 mmol Cl/g) were purchased from Sigma–Aldrich. Propylene oxide (PO, > 99%), propylene carbonate (PC, > 99%), and other epoxides (> 98%) were obtained from Sigma–Aldrich. CO2 with 99.999% purity was used without further purification. Catalysts synthesis. The polymer-supported Zn-containing imidazolium salt (PS(Im)2ZnX2) catalysts were prepared in two steps following our previously reported method (Scheme 1).24 In the first step, the synthesis of bis[1-(2-hydroxyethyl)imidazolium]zinc halide, (HEIm)2ZnX2, was carried out by metal insertion. To a 500-mL round-bottom flask equipped

with

a

condenser,

an

ethanol

solution

(100

mL)

containing

1-(2-

hydroxyethyl)imidazole (HEIm, 40 mmol) was added to an ethanolic solution (100 mL) of ZnX2 (20 mmol). This mixture was stirred for 2 h at 50 °C and, subsequently, the solvent was removed at 60 °C under vacuum. A viscous liquid, (HEIm)2ZnX2, was obtained after washing the residue with ethanol and drying (yield: > 90%). To immobilize (HEIm)2ZnX2 on Merrifield’s peptide resin (MPR) by alkoxylation, a mixture of MPR (5 g, 20 mmol Cl), (HEIm)2ZnX2 (10 mmol), and acetonitrile (100 mL) was heated at 80 °C for 48 h in a 250-mL round-bottomed flask equipped with a condenser. After cooling the reaction mixture to room temperature, the solid was collected by filtration and washed several times with ethanol. The solid was dried at 80 °C for 24 h under vacuum. The 5



supported catalysts are denoted PS-(Im)2ZnCl2, PS-(Im)2ZnBr2, and PS-(Im)2ZnCI2, depending on the (HEIm)2ZnX2 employed. Catalyst characterization. 1H NMR spectra were obtained with Bruker 300 MHz spectrometer. Fourier transform infrared (FT-IR) spectra were acquired on Thermo Nicolet 6700 spectrophotometer with a resolution of 4 cm-1. Raman spectra were recorded using a Thermo DXR Raman microscope employing 780 nm excitation radiation from a diodepumped, solid-state (DPSS) laser. X-ray photoelectron spectroscopy (XPS) of the catalysts was conducted with a Theta Probe AR-XPS System X-ray source using monochromatic Al K radiation (hv = 1486.6 eV). Elemental analysis (EA) of the samples was carried out using Thermo Scientific Flash 2000 instrument. Thermogravimetric analysis (TGA) was performed from room temperature to 600 °C on TA Instruments AutoTGA Q500 apparatus under nitrogen flow at a heating rate of 10 °C min−1 under an N2 flow of 100 mL/min. Field emission scanning electron microscopy (FE-SEM) micrographs were acquired using TESCAN MIRA 3 equipped with Thermo Fisher NORAN System 7 EDS system. Cycloaddition of CO2 to PO. The cycloaddition of CO2 to epoxide (Scheme 2) using PS(Im)2ZnX2 catalysts was performed in a 50-mL stainless steel autoclave equipped with a magnetic stirrer (Figure S1). For each typical batch operation, epoxides and the desired amount of catalyst were placed into the reactor without solvent. The reactor was then purged several times with CO2 without PO and then pressurized with CO2 at room temperature, which was stopped or maintained in the batch or semi-batch systems, respectively. The reactor was then heated, and, at the desired temperature, the reaction was started by stirring the reaction mixture. After a preset reaction time, the cycloaddition was stopped by cooling the reaction mixture to room temperature and venting the remaining CO2. The reactants and

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products were analyzed by 1H-NMR (Bruker 300 MHz spectrometer). The conversion and selectivity were calculated based on the assumption that epoxide was the limiting reactant.

RESULTS AND DISCUSSION Characterization of PS-(Im)2ZnX2 catalysts. (HEIm)2ZnX2: The chemical shifts for the proton resonances of the HEIm and (HEIm)2ZnX2 are as follows (Figure S2). 1-(2-hydroxyethyl)imidazole (HEIm) 1H-NMR (300 MHz, DMSO-d6): δ = 3.63 (t, CH2, 2H), 3.98 (m, CH2, 2H), 5.10 (s, OH, 1H), 6.87 (d, NCH,

1H),

7.15

(d,

NCH,

1H),

7.59

(s,

NCH,

1H).

Bis[1-(2-

hydroxyethyl)imidazolium]zinc chloride [(HEIm)2ZnCl2] 1H-NMR (300 MHz, DMSOd6): δ = 3.67 (m, CH2, 2H), 4.10 (t, CH2, 2H), 5.05 (s, OH, 1H), 7.05 (d, NCH, 1H), 7.41 (d, NCH, 1H), 8.06 (s, NCH, 1H). Bis[1-(2-hydroxyethyl)imidazolium]zinc bromide [(HEIm)2ZnBr2] 1H-NMR (300 MHz, DMSO-d6): δ = 3.68 (m, CH2, 2H), 4.12 (t, CH2, 2H), 5.05 (s, OH, 1H), 7.09 (d, NCH, 1H), 7.45 (d, NCH, 1H), 8.12 (s, NCH, 1H). Bis[1-(2hydroxyethyl)imidazolium]zinc iodide [(HEIm)2ZnI2] 1H-NMR (300 MHz, DMSO-d6): δ = 3.68 (m, CH2, 2H), 4.15 (t, CH2, 2H), 5.03 (s, OH, 1H), 7.14 (d, NCH, 1H), 7.52 (d, NCH, 1H), 8.19 (s, NCH, 1H). According to 1H-NMR results, all protons on the imidazole ring of the HEIm after metal insertion were shifted downfield because of the electron withdrawing effect on the imidazolium ring of Zn, which has Lewis acid characteristics.25 This clearly indicates the coordinate bonding of N-Zn-N in (HEIm)2ZnX2. To investigate the bonding between the ligands and Zn in (HEIm)2ZnX2, FT-Raman spectroscopy measurements were also carried out, and the spectra show bands at ca. 240 cm-1 arising from v(Zn-N) stretching (Figure 1).26,27 The FT-IR spectra of the HEIm and (HEIm)2ZnX2 are shown in Figure 2;

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many of the bands observed from 1600 to 600 cm-1 are associated with various vibrational modes of the imidazole ring.28 PS-(Im)2ZnX2: The successful covalent immobilization of the Zn-containing imidazolium salt on the surface of the MPR support was verified via FT-IR (Figure 3). In the FT-IR spectral analyses of PS-(Im)2ZnX2 and MPR, the characteristic peak corresponding to the stretching of the CH2Cl functional group (1265 cm-1) disappeared in each spectrum of the PS(Im)2ZnX2, suggesting the complete immobilization of (HEIm)2ZnX2 onto MPR.21,29 In addition, new peaks were observed for PS-(Im)2ZnX2 in the regime of 1600–700 and 3450 cm-1, which are absent from the MPR spectrum; these peaks are related to the stretching frequencies of the (HEIm)2ZnX2.28 The amount of Zn-containing imidazolium salt in the catalysts was analyzed by elemental analysis, and the amount of (Im)2ZnX2 supported on the Merrifield’s peptide resin is summarized in Table 1. The amounts of (Im)2ZnX2 were 0.26, 0.84, and 0.69 mmol/g-cat, corresponding to PS-(Im)2ZnCl2, PS-(Im)2ZnBr2, and PS(Im)2ZnI2, respectively. XPS analyses of PS-(Im)2ZnX2 catalysts were performed to verify the state of N and halide anions (Cl-, Br- and I-), and the results are shown in Figure 4. In the N 1s spectra of the PS-(Im)2ZnX2 catalyst (Figure 4(a)), the (Im)2ZnX2 catalysts contain two nitrogen species that originate from the two nitrogen atoms of the imidazolium ring, resulting in signals at 401.7 and 399.3 eV. The strong peak around 401.7 eV can be regarded as arising from the positively charged nitrogen atom of the quaternary ammonium moiety, and the other peak can be attributed to the imidazolium nitrogen attached to the alkyl group of the amine species.30,31 Moreover, the Cl 2p, Br 3d, and I 3d spectra of PS-(Im)2ZnX2 were acquired to identify the bonding nature of the halogen associated with the metal species, as depicted in Figure 4(b and c), and (d), respectively. The Cl 2p spectrum of PS-(Im)2ZnCl2 (Figure 4(b)) 8


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showed a peak near 197.5 eV, confirming that the bonds between chlorine and zinc are ionic, not covalent.32 The Br 3d spectrum of PS-(Im)2ZnBr2 showed a peak at ca. 68.6 eV (Figure 4(c)), which is assigned to the negatively charged bromide ion.33 An I 3d peak was observed at 619.3 eV in the case of PS-(Im)2ZnI2 (Figure 4(d)), which indicates the anionic state of iodine.34 The aforementioned results are a clear indication of the successful immobilization of the Zn-containing imidazolium salts on the support, as shown in Scheme 1. The TGA of the PS-(Im)2ZnX2 catalysts (Figure S3) reveals that all the PS-(Im)2ZnX2 catalysts show good thermal stability up to 200 °C, which is much higher than the reaction temperature for the synthesis of PC and another cyclic carbonates by the cycloaddition of CO2 to epoxide. The SEM images of MPR and PS-(Im)2ZnX2 are presented in Figure S4. The images show that the diameter of the polymer beads was approximately 80 µm, and the spherical shape of the beads was retained, even after immobilization. Reactivity of PS-(Im)2ZnX2 catalysts. The effects of the counteranions of the PS(Im)2ZnX2 catalysts on the synthesis of PC were examined at 60 °C under a 10 bar CO2 pressure (semi-batch) for 4 h (Table 2). In the absence of a catalyst (entry 1) and using MPR or HEIm alone (entry 2–3), the conversion of PO was less than 0.5%. However, the (HEIm)2ZnX2 (entries 4–6) and PS-(Im)2ZnX2 (entries 7–9) catalysts showed appreciable activity because of the presence of halide anions (Cl-, Br-, and I-). It is well known that halide anions have a significant effect on the catalytic reactivity for the ring-opening nucleophilic attack on epoxide, which is the step with the highest activation energy for the cycloaddition of CO2.35 As shown in Table 2, the (HEIm)2ZnX2 and PS-(Im)2ZnX2 catalysts having different anions show the following order of conversion: (HEIm)2ZnCl2 (10.1%) < (HEIm)2ZnBr2 (62.4%) < (HEIm)2ZnI2 (99.9%) and PS-(Im)2ZnCl2 (4.5%) < PS-(Im)2ZnBr2 (52.4%) < PS-(Im)2ZnI2 (85.4%). These results are consistent with the order of the halide ion 9



nucleophilicity: Cl- < Br- < I-.36 Moreover, the reactivity of the homogeneous catalysts, (HEIm)2ZnX2 were compared with that of the (HEIm)2ZnX2 immobilized on MPR (Table 2). The results indicate that PS-(Im)2ZnI2 was the most efficient catalyst with the highest turnover number (TON), although conversion of PO remained higher for the (HEIm)2ZnI2 catalyst. The adaptability of PS-(Im)2ZnI2 in catalyzing the cycloaddition of CO2 was studied by conducting the reactions using various epoxides at 60 °C at 10 bar CO2 pressure (semibatch) for 4 h, and the results are summarized in Table 3. The cyclohexene oxide as the internal epoxide showed no conversion under the employed reaction conditions, and the reactivity of CHO was limited to 21.6% conversion at higher temperatures, pressures, and longer reaction times (entries 1–2), mainly because of the greater steric hindrance during nucleophilic attack. However, the reactivity of PO was the most efficient for the cycloaddition of CO2 using PS-(Im)2ZnI2 among the aliphatic terminal epoxides (entries 3–7). The poor conversion of styrene oxide and allyl glycidyl ether can be attributed to their bulky nature, resulting in low reactivity at the β-carbon arising from steric hindrance (entries 3–4). The lower conversion of epichlorohydrin and glycidol than PO could be explained on the basis of the electron withdrawing effects of the CH2Cl and CH2OH groups, which reduce the electron density of the epoxide oxygen system (entries 5 and 6).37 The effect of the catalyst loading on the synthesis of PC was tested using PS-(Im)2ZnI2 at 60 °C at 10 bar CO2 pressure (semi-batch) for 4 h. As shown in Table 4, the yield of PC and the PO conversion increased as the catalyst loading increased to 0.3 g (entry 3). However, the yield remained almost constant as catalyst loading increased. The stability of the PS-(Im)2ZnI2 catalyst was evaluated in recycling experiments. For each cycle, the used catalyst was separated by filtration, washed with ethanol to remove the products adhering to the surface of the catalyst, dried at room temperature, and then reused directly for the next run without regeneration. The 10


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activity of the reused PS-(Im)2ZnI2 catalyst is summarized in Table 5. The conversion of PO with high selectivity toward PC was almost the same as the activity of the fresh catalyst. To determine whether the active (HEIm)2ZnI2 center underwent any leaching from the immobilized MPR, the XPS spectra of the fresh PS-(Im)2ZnI2 catalyst and that used for three cycles were measured. As shown in Figure 5, the spectra of the recycled and fresh catalysts are very similar. The peak at 401.7 eV corresponds to the imidazolium nitrogen, although the intensities decreased slightly after use. FT-IR analysis of the fresh PS-(Im)2ZnI2 catalyst and that used for three cycles was also carried out (Figure 6). The characteristic peaks of the PS(Im)2ZnI2 catalyst were present in the spectra of both the recycled and fresh catalysts. Therefore, it can be concluded that the PS-(Im)2ZnI2 catalyst could be recycled for up to three consecutive cycles without any considerable loss of its initial activity. Comparative effect of batch and semi-batch system. To compare the effects of batch and semi-batch systems for the cycloaddition of CO2 to PO, preliminary kinetic studies were carried out. The approximate rate constant, which affects the half-life of the first-order reaction, was obtained under various temperature for both reaction systems. The general rate formula for the cycloaddition of CO2 to PO using the catalyst is given by Eq. (1). For this reaction, the rate equation can be simplified to Eq. (2) because the large excesses of CO2 and the high concentration of the catalyst can be considered constant.

rate = rate =

= kC C C .

(1)

= k C (where: k = kC C . )

(2)

Assuming that the cycloaddition is an elementary and first-order reaction (α = 1) and the concentration of PO can be described in terms of PO conversion (C = C X ) after 11



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reaction time t, Eq. (2) can be written as Eq. (3). The integration of Eq. 3 from t = 0 to t gives Eq. (4).

= k (1 − X ) (where: k = kC C . )

− ln|1 − X | = k t

(3) (4)

The slope of the linear plot of the left-hand side of Eq. (4), f(XPO) versus time, can be used to estimate the approximate reaction rate constant kapp. The cycloaddition of CO2 was achieved using 0.2g of PS-(Im)2ZnI2 catalyst and 50 mmol of PO at 40–60 °C for 1–3 h under 10 bar CO2 pressure for the batch and semi-batch systems. The variations in the conversion of PO for two different systems at various temperatures and reaction times are listed in Table S1. Figures 7 and 8 show plots of f(XG) versus reaction time at different temperatures for the batch and semi-batch systems, respectively. The experimental results verify that the reaction shows a linear dependence on the PO conversion at different temperatures, which strongly suggests a pseudo-first-order reaction.38,39 Therefore, the approximate rate constant (kapp) can be computed using Eq. (4), and the results are summarized in Table 6. The approximate rate constant of the semi-batch system is higher than that of the batch system at three different temperatures (40, 50, and 60 °C). As listed in Table 2 (entries 9 and 10), when using the semi-batch system for the preparation of PC, the PS-(Im)2ZnI2 catalyst displayed better activity than the batch system under the same reaction conditions. Thus, the relative reactivities of PS-(Im)2ZnI2 for both systems are consistent with those derived approximate rate constant (kapp) from the kinetic studies for the cycloaddition of CO2 to PO. Optimization of reaction conditions using response surface methodology (RSM). The Box–Behnken design (BBD) as a RSM was applied to optimize the reaction conditions to obtain a high yield of PC in the batch and semi-batch system. The optimal reaction conditions 12


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under a moderate environment for the batch and semi-batch systems are summarized in Table S6. The optimized reaction conditions for the batch system are 60 °C (level 0.5), 15 bar (level 1) CO2 pressure, and 17.6 h (level 0.7) reaction time, and the maximum yield of PC for the semi-batch system was obtained at 60 °C (level 0.5) at 15 bar (level 1) after 12.8 h (level 0.1). The predicted yields of PC in the batch and semi-batch systems under the optimized conditions are 99.67% and 99.69%, respectively. The results demonstrate the advantage of the semi-batch system, which allows for the completion of the reaction in less time (12.8 h, Table S6) than the batch system. These results are in reasonable agreement with the obtained kinetic studies. For details, see the Supporting Information.

CONCLUSIONS Zn-containing imidazolium salt catalysts supported on Merrifield’s peptide resin, PS(Im)2ZnX2, were successfully synthesized and characterized by various physicochemical methods. For the cycloaddition of CO2 to PO, PS-(Im)2ZnI2 was found to be the most efficient catalyst with a high TON. Furthermore, the catalyst can be easily recovered and reused without any considerable loss of the initial activity. When using a semi-batch system for the preparation of PC, the PS-(Im)2ZnI2 catalyst displayed better activity than the batch system at the same temperature and reaction time. Thus, the relative reactivities of PS(Im)2ZnI2 for both systems are consistent with the approximate rate constants (kapp) derived from the kinetic studies. Furthermore, the optimal reaction conditions with a maximum yield of PC for the batch and semi-batch systems were obtained using an RSM with a minimum of experimental runs. Studies of the application of the developed methodology to continuous flow systems and the utilization of the resulting five-membered cyclic carbonate in the synthesis of polyether carbonate polyols are ongoing. 13



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SUPPORTING INFORMATION The supporting Information includes the apparatus for the cycloaddition of CO2 and epoxide, additional characterization results of catalyst and statistical analysis of RSM and optimization of reaction conditions.

ACKNOWLEDGMENTS This study was supported by the Korea Electric Power Corporation Open R&D program (KEPCO-R17XH03) and the Korea Research Institute of Chemical Technology program (SI1809-01).

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21



FIGURES.

imidazole ring

v(Zn-N)

(d)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

(c)

(b) (a)

1800

1600

1400

1200

1000

800

600

400

200

Raman Shift (cm-1)

Figure 1. FT-Raman Spectra of (a) HEIm, (b) (HEIm)2ZnCl2, (c) (HEIm)2ZnBr2, and (d) (HEIm)2ZnI2.

22


Page 23 of 37

(a)

(b)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)

(d)

C-H =C-H C-N C=C C=C, C=N 4000

3500

3000

2500

2000

1500

C-H C-N-C

1000

Wavenumber (cm-1)

Figure 2. FT-IR Spectra of (a) HEIm, (b) (HEIm)2ZnCl2, (c) (HEIm)2ZnBr2, and (d) (HEIm)2ZnI2.

23



(a) CH2Cl

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

(b)

(c)

(d)

-OH C=C, C=N, C-N in ring

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Figure 3. FT-IR Spectra of (a) Merrifield’s peptide resin (MPR), (b) (HEIm)2ZnCl2, (c) (HEIm)2ZnBr2 and (d) (HEIm)2ZnI2.

24


Intensity (c) PS-(Im)2ZnI2 (b) PS-(Im)2ZnBr2 (a) PS-(Im)2ZnCl2

406

404

402

400

398

396

Binding Energy (eV)

204

202

200

198

196

194

(b)

Binding Energy (eV)

(a)

Intensity

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity

Page 25 of 37

74

72

70

68

66

64

Binding Energy (eV)

624

(c)

622

620

618

Binding Energy (eV)

616

614

(d)

Figure 4. XPS spectra of (a) N 1s PS-(Im)2ZnX2, (b) Cl 2p PS-(Im)2ZnCl2, (c) Br 3d PS(Im)2ZnBr2 and (d) I 3d PS-(Im)2ZnI2.

25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

Intensity


(b)

(a)

406

404

402

400

398

Binding Energy (eV) Figure 5. N 1s XPS spectra of (a) three-times reused and (b) fresh PS-(Im)2ZnI2.

26


396

Page 27 of 37

(a)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

-OH C=C, C=N, C-N in ring

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Figure 6. FT-IR spectra of (a) three-times reused and (b) fresh PS-(Im)2ZnI2.

27


500


1.0 40 oC, y=0.0666x+0.0100, R2=0.9821 50 oC, y=0.1602x+0.0139, R2=0.9943 60 oC, y=0.3052x+0.0323, R2=0.9923

0.8

- ln(1-XPO)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

0.6

0.4

0.2

0.0 0

1

2

3

Reaction Time (h)

Figure 7. Linear plots of [ln(1-XPO)] versus reaction time with different temperature at 10 bar CO2 pressure in the batch system.

28


Page 29 of 37

2.0 40 oC, y=0.0925x+0.0070, R2=0.9962 50 oC, y=0.2627x+0.0443, R2=0.9806 60 oC, y=0.5475x+0.0955, R2=0.9756

1.8 1.6 1.4

- ln(1-XPO)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

1

2

3

Reaction Time (h)

Figure 8. Linear plots of [ln(1-XPO)] versus reaction time with different temperature at 10 bar CO2 pressure in the semi-batch system.

29 Environment ACS Paragon Plus


SCHEMES.

Scheme 1. Preparation of polymer supported Zn-containing imidazolium salt ionic liquid catalysts.

Scheme 2. Synthesis of cyclic carbonate from epoxide and CO2.


Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TABLES.

Table 1. Elemental analysis results of PS-(Im)2ZnX2.

CHNO from elemental analysis Catalyst

Amount of (Im)2ZnX2a (mmol/g)

C (wt%)

H (wt%)

N (wt%)

O (wt%)

MPR

78.73

6.64

-

0.45

PS-(Im)2ZnCl2

72.99

6.37

1.45

1.42

0.26

PS-(Im)2ZnBr2

56.98

5.39

4.73

3.05

0.84

PS-(Im)2ZnI2

56.74

5.31

3.88

3.21

0.69

a

Amount of metal containing imidazolium salt immobilized onto Merrifield’s peptide resin.



Page 32 of 37

Table 2. Activities of PS-(Im)2ZnX2 in the synthesis of PC from CO2 and PO.

Entry

Catalyst

Conversion of PO (%)

Selectivity to PC (%)

TONa

1

None

0

-

-

2b

MPR

0

-

-

3b

HEIm

0.5

-

-

4b

(HEIm)2ZnCl2

10.1

> 99

9.9

5b

(HEIm)2ZnBr2

62.0

> 99

61.4

6b

(HEIm)2ZnI2

99.9

> 99

98.9

7

PS-(Im)2ZnCl2

4.5

> 99

42.9

8

PS-(Im)2ZnBr2

52.4

> 99

154.4

9

PS-(Im)2ZnI2

85.4

> 99

306.3

10c

PS-(Im)2ZnI2

63.4

> 99

227.4

Reaction conditions: 50 mmol PO, 0.2 g of PS-(Im)2ZnX2, Temperature = 60 °C, CO2 Pressure = 10 bar (semi-batch), Reaction time = 4 h. a

TON = (mole of product)/(mole of active site)

b

0.5 mmol

c

CO2 Pressure = 10 bar (batch)


Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Synthesis of various cyclic carbonates with different epoxides using PS-(Im)2ZnI2.

Conversion of epoxide (%)

Selectivity to cyclic carbonate (%)

1

-

-

2a

21.6

> 99

3

35.6

> 99

4

45.4

97.3

5

60.8

95.0

6b

55.1

86.4

7

85.4

> 99

Entry

Epoxide

Cyclic carbonate

Reaction conditions: 50 mmol epoxide, 0.2 g of PS-(Im)2ZnI2, Temperature = 60 °C, CO2 Pressure = 10 bar (semi-batch), Reaction time = 4 h. a

Temperature = 120 °C, Pressure of CO2 = 15.5 bar (semi-batch), Reaction time = 15 h.

b

The major byproduct was glycerol as detected by the 1H-NMR.



Page 34 of 37

Table 4. The effect of PS-(Im)2ZnI2 amount on the cycloaddition of CO2 to PO.

Entry

Amount of catalyst (g)



1

0.1

65.7

> 99

2

0.2

85.4

> 99

3

0.3

87.2

> 99

4

0.5

87.5

> 99

5

1.0

88.1

> 99

Reaction conditions: 50 mmol epoxide, Temperature = 60 °C, CO2 Pressure = 10 bar (semi-batch), Reaction time = 4 h.


Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5. Recycle test of PS-(Im)2ZnI2 on the cycloaddition of CO2 to PO.

Entry



1

Fresh

85.4

> 99

2

1st

85.2

> 99

3

2nd

84.9

> 99

4

3rd

83.7

> 99

Reaction conditions: 50 mmol PO, 0.2 g of PS-(Im)2ZnI2, Temperature = 60 °C, CO2 Pressure = 10 bar (semi-batch), Reaction time = 4 h.



Page 36 of 37

Table 6. The values of approximate reaction rate constants for the batch and semi-batch system.

rate constant, kapp (h-1) Temperature (°C)

batch system

R2

semi-batch system

R2

40

0.0666 ± 0.0063

0.9821

0.0925 ± 0.0041

0.9962

50

0.1602 ± 0.0086

0.9943

0.2627 ± 0.0261

0.9806

60

0.3052 ± 0.0190

0.9923

0.5475 ± 0.0612

0.9756

Reaction conditions: 50 mmol PO, 0.2 g of PS-(Im)2ZnI2.


Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TABLE OF CONTENTS (TOC) GRAPHIC

Synopsis: Polymer-supported Zn-containing imidazolium salts, PS-(Im)2ZnX2, possessing bifunctional properties were identified as sustainable catalysts for the activation of CO2 under solvent-free conditions.


Polymer-Supported Zn-Containing Imidazolium Salt Ionic Liquids as

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