Metal-Containing Ionic Liquids as Synergistic Catalysts for the

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Metal-containing ionic liquids as synergistic catalysts for the cycloaddition of CO2: A density functional theory and response surface methodology corroborated study Dongwoo Kim, Yeji Moon, Dahye Ji, Hyeongook Kim, and Deug-Hee Cho ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00711 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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ACS Sustainable Chemistry & Engineering

Metal-containing ionic liquids as synergistic catalysts for the cycloaddition of CO2: A density functional theory and response surface methodology corroborated study Dongwoo Kim, Yeji Moon, Dahye Ji, Hyeongook Kim, and Deughee Cho* Center for Chemical Industry Development, Korea Research Institute of Chemical Technology, 45, Jongga-ro, Jung-gu, Ulsan 681-802, Republic Korea KEYWORDS Carbon dioxide; Ionic liquid; Density Functional Theory; Box-Behnken design

ABSTRACT

Metal-containing ionic liquids possessing bifunctional moieties were identified as efficient catalysts for the synthesis of propylene carbonate (PC) from CO2 and propylene oxide under moderate and solvent free conditions. Density functional theory calculations was performed to rationalize the difference in catalytic activities of the metal-containing ionic liquid catalysts (MeIm)2MCl2, where M was Fe, Cu, or Zn. (MeIm)2ZnCl2 was the most efficient catalyst for the chemical fixation of carbon dioxide in terms of efficiency and environmental 1

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benignity. Process optimization using response surface methodology was performed, and the interactions between the operational variables were elucidated. The optimum reaction conditions for maximum PC yield were 4.6 h, 124 °C and 9 bar which were obtained by a Box-Behnken design with the minimum number of reaction tests. Under the optimum reaction conditions, the predicted and validated yields of PC were 98.37 and 97.91 ± 0.02%, respectively.

INTRODUCTION The increasing levels of carbon dioxide in the atmosphere have widely been considered detrimental to the carbon balance of the biosphere, a major effect being global warming. However, CO2 holds the tag for one of the most non-toxic, non-flammable and nearly inexhaustible source of C1 for the production of a range of compounds such as dimethyl carbonate, N,N′-disubstituted ureas, cyclic carbonates, cyclic urethanes, and formic acid.1-3 The cycloaddition reaction of CO2 with highly reactive substrates such as epoxides and oxetane substrates to yield five and six membered cyclic carbonates respectively4,5, has attracted wide interest among researchers, owing to its various applications. Five membered cyclic carbonates possess very industrially valuable properties such as polar aprotic solvents, electrolytes for Li- ion batteries, de greasing agents, cosmetics, and intermediates in the production of fine chemicals, pharmaceuticals, and polymers.6 The innate stability of CO2 demands specific catalysts for its activation. Numerous kinds of catalysts belonging to both homo and heterogeneous phase have been developed through active research. One of the most notable among these catalyst systems, in terms of turn over frequency are ionic liquids,7,8 or a combination of metal salts-ionic liquids and organometallic salen complexes9; even though metal oxides,10 supported phase catalysts,11,12 2


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metal–organic frameworks (MOFs)13,14 and carbon materials have also contributed positively.15,16 The mechanistic pathway of CO2-epoxide cycloaddition, studied using analytical, experimental and computational studies revealed the excellent role of halide ions and hydrogen bonding groups in the catalyst materials, to work in synergism for delivering high selectivity towards cyclic carbonates.1,17,18 This has further enriched the number of bifunctional catalysts, such as the hydroxyl-rich renewable biopolymer cellulose19 and its oligomeric analogues,20 such as gamma and beta cyclodextrins, and cucurbit,21 with alkali metal halides to effectuate the CO2-epoxide cycloaddition and the results supplemented the aforementioned cooperative effect of halide ions and hydrogen bonding groups. Ionic liquids are widely acclaimed environmentally benign chemicals for catalysis and chemical extraction processes, ascribable to its unique properties such as negligible vapor pressure, high thermal stability, which distinguishes them from conventional organic and inorganic solvents.22-24 Since, by the judicious choice of the cationic part and anionic part, the acido-basic properties of ionic liquids can be devised, metal containing ionic liquids have recently received increased research attention.25-27 Imidazolium based metal-containing ionic liquids possessing a weak base as the anion have been successfully used as catalysts for Friedel-Crafts alkylation and glycerolysis.28,29 However, Only few reports exits that discuss the utilization of metal containing ionic liquids as promising catalysts towards CO2 fixation30,31 and hence, we envisioned it would be worthwhile to examine the potential of metal containing ionic liquids for CO2 fixation, experimentally and theoretically. The use of ab initio quantum chemical calculations to fathom the key intermediates and transition states of a particular catalytic reaction at the atomic scale has gained ever increasing interest.32 Density functional theory (DFT), developed by Hohenberg and Kohn,33 is one of the mostly applied quantum mechanical model used for examining catalytic 3



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reactions.34,35 However, due to the high computational power requirements, the simulation of complex catalytic systems (systems with large number of atoms) remains tedious. Not only that the understanding of mechanism of a reaction is important, but the optimization of a reaction process can also be cumbersome since it involves a large number of sets of reactions to be conducted at different reaction conditions such as temperature, pressure, catalyst amount, time of reaction etc. Recently, response surface methodology (RSM) has become an effective tool to optimize the reaction parameters of a production processes, and allows the users to gather large amounts of information from a small number of experiments.36 The influence of individual variables and their combinations of interactions shall also be predicted using RSM, which has the potential to minimize the laboriousness in optimizing a reaction process. In this study, the imidazolium-based metal-containing ionic liquid (RIm)2MX2 (where R is methyl (Me), ethyl (Et), or butyl (Bu); M is Fe, Cu, or Zn; and X is Cl, Br, or I), which is a bifunctional catalyst in which the metal ions serve as the acidic center and the anion as nucleophile, is prepared by a metal insertion reaction. The catalytic performances of (RIm)2MX2 catalysts for the synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO) under solvent free conditions are investigated (Scheme 1). We have attempted to compare the reactivity of three different metal-containing (Fe, Cu, and Zn) ionic liquid catalysts using quantum mechanical calculations (i.e., DFT) for the synthesis of PC by cycloaddition of CO2 to PO. In addition, a response surface methodology using Box-Behnken design was applied to minimize the number of reaction tests aimed to deduce the optimum reaction conditions.

EXPERIMENTAL SECTION 4


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Materials. 1-alkylimidazoles (1-methyl, 1-ethyl, and 1-butyl imidazole) and metal halides (iron chloride, copper chloride, zinc chloride, zinc bromide, and zinc iodide) with high purity (> 99%) were purchased from Sigma-Aldrich. Propylene oxide (PO, > 99%) and propylene carbonate (PC, > 99%), were obtained from Sigma-Aldrich. CO2 with 99.999% purity was used without further purification. Synthesis of (RIm)2MX2 catalysts. The metal-containing ionic liquid catalysts based on 1alkylimidazoles, i.e., (RIm)2MX2, were prepared by a metal insertion reaction as illustrated in Scheme 2.25 An ethanol solution (100 mL) containing the 1-alkylimidazole (40 mmol) was added to an ethanol solution (100 mL) containing the metal halide (20 mmol). This mixture was stirred for 2 h at 50 °C, after which the mixture was filtered. A solid powder was obtained after drying at 80 °C for 24 h under vacuum. The products were denoted (MeIm)2FeCl2 (FeCl2, 1-methylimidazole), (MeIm)2CuCl2 (CuCl2, 1-methylimidazole), (MeIm)2ZnCl2 (ZnCl2, 1-methylimidazole), (MeIm)2ZnBr2 (ZnBr2, 1-methylimidazole) (MeIm)2ZnI2 (ZnI2, 1-methylimidazole), (EtIm)2ZnCl2 (ZnCl2, 1-ethylimidazole), and (BuIm)2ZnCl2 (ZnCl2, 1-butylimidazole). Characterization of (RIm)2MX2 catalysts. The 1H NMR spectra were obtained with a Bruker 300 MHz spectrometer. Fourier transform infrared (FT-IR) spectra were obtained with a Thermo Nicolet 6700 spectrophotometer at a resolution of 4 cm-1. Raman spectra were recorded using a Thermo DXR Raman microscope employing 532-nm excitation radiation from a diode-pumped, 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). Cycloaddition of CO2 to PO. The synthesis of the cyclic carbonate from PO and CO2 using (RIm)2MX2 catalysts was performed in a 50-mL stainless steel autoclave equipped with 5



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a magnetic stirrer. For each typical batch operation, PO (50 mmol) and the desired amount of catalyst were placed into the reactor without solvent. The reactor was then purged several times with CO2 (at zero degree Celsius) and then pressurized with CO2 to a preset pressure at room temperature. The reactor was then heated, and when the desired temperature was attained, the reaction was initiated by stirring the reaction mixture at 600 rpm. After a preset reaction time, the cycloaddition was stopped by cooling the reaction mixture to room temperature and venting off the remaining CO2 (at zero degree Celsius). The products and reactants were analyzed by a gas chromatography (Bruker 450-GC) equipped with a flame ionization detector and a capillary column (HP-5, (5%-phenyl)-methylpolysilosan). Biphenyl was used as the internal standard. The conversion and selectivity were calculated based on the assumption that PO was the limiting reactant. Acetalization of benzaldehyde and ethanol. Typically, 10 mmol benzaldehyde and 20 mmol ethanol without any solvent was chosen as the model substrates for conducting the acetalization and the reaction was carried out for 2 h at 60 °C using 1 wt% of (MeIm)2MCl2 catalysts to yield 1,1-diethoxymethylbenzene. Gas chromatography using a Bruker 450-GC, was utilized to determine the initial rate (ro) of acetalization. Experimental design for RSM. The Box-Behnken design (BBD) was employed to analyze the cycloaddition of CO2 and to obtain optimized reaction conditions with a high yield of PC under moderate reaction conditions. The variables considered important were reaction time (X1), temperature (X2), and initial pressure of CO2 (X3). These variables were coded to three levels, and the experimental range and the central points for each factor are shown in Table 1. A model equation was used to predict the optimum values and to elucidate the interactions between the variables. The quadratic equation used to predict the optimal point was expressed as follows:36 6


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Y=

λ + λ X + λ

+ λ

where λ0 (constant term), λi (linear effect term), λii (squared effect term), and λij (interaction effect term) are regression coefficients, and Y is the predicted response value (i.e., yield of PC).37 All data were analyzed with the assistance of the Expert Design 9 software package, and the significant second-order coefficients were selected by regression analysis with backward elimination from the analysis of variance (ANOVA).

RESULTS AND DISCUSSION Characterization of (RIm)2MX2 catalysts. The NMR data of the (RIm)2ZnX2 samples are as follows: Bis(1-methylimidazolium)iron chloride [(MeIm)2FeCl2] 1H NMR (300 MHz, DMSO-d6): δ = 3.74 (s, NCH3, 3H), 7.03 (s, CH, 1H), 7.37 (s, CH, 1H), 8.06 (s, CH, 1H). Bis(1-methylimidazolium)copper chloride [(MeIm)2CuCl2] 1H NMR (300 MHz, DMSOd6): δ = 3.74 (s, NCH3, 3H), 7.03 (s, CH, 1H), 7.37 (s, CH, 1H), 8.06 (s, CH, 1H). Bis(1methylimidazolium)zinc chloride [(MeIm)2ZnCl2] 1H NMR (300 MHz, DMSO-d6): δ = 3.74 (s, NCH3, 3H), 7.03 (s, CH, 1H), 7.37 (s, CH, 1H), 8.06 (s, CH, 1H). Bis(1methylimidazolium)zinc chloride [(MeIm)2ZnBr2] 1H NMR (300 MHz, DMSO-d6): δ = 3.74 (s, NCH3, 3H), 7.03 (s, CH, 1H), 7.37 (s, CH, 1H), 8.06 (s, CH, 1H). Bis(1methylimidazolium)zinc chloride [(MeIm)2ZnI2] 1H NMR (300 MHz, DMSO-d6): δ = 3.74 (s, NCH3, 3H), 7.03 (s, CH, 1H), 7.37 (s, CH, 1H), 8.06 (s, CH, 1H). Bis(1ethylimidazolium)zinc chloride [(EtIm)2ZnCl2] 1H NMR (400 MHz, DMSO-d6): δ = 1.36 (t, CH3, 3H), 4.10 (m, NCH3, 3H), 7.06 (s, CH, 1H), 7.48 (s, CH, 1H), 8.14 (s, CH, 1H). Bis(1-butylimidazolium)zinc chloride [(BuIm)2ZnCl2] 1H NMR (400 MHz, DMSO-d6): δ 7



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= 0.87 (t, CH3, 3H), 1.23 (m, CH2, 2H), 1.72 (m, CH2, 2H), 4.08 (t, NCH3, 3H), 7.06 (s, CH, 1H), 7.46 (s, CH, 1H), 8.14 (s, CH, 1H). The FT-IR spectra of (MeIm)2FeCl2, (MeIm)2CuCl2, (MeIm)2ZnCl2, (MeIm)2ZnBr2, (MeIm)2ZnI2, (EtIm)2ZnCl2, and (BuIm)2ZnCl2, (Figure 1) confirm that different alkyl groups are attached to the ring nitrogen in the (RIm)2ZnCl2 catalysts. Clearly, the C-H stretching frequency at ~3110 cm-1 and many of the bands observed from 1600 to 600 cm-1 are associated with various vibrational modes of the ring.38 Differences in the alkyl chain length in the (RIm)2ZnCl2 catalysts are apparent in the region from 2885 to 2980 cm-1 by differences in the transmittance. In order to investigate bonding between the ligands and metals in the (RIm)2MX2 catalysts, FT-Raman spectroscopy was performed, and the spectra are shown in Figure 2. The spectra show bands at ~220 cm-1 due to N-Fe-N stretching in (MeIm)2FeCl2,39 265 cm-1 due to N-Cu-N stretching in (MeIm)2CuCl2,40 and 240 cm-1 due to N-Zn-N stretching in (RIm)2ZnX2.41 XPS analyses of the (RIm)2MX2 catalysts were performed to confirm their structure. The N 1s spectra of (RIm)2MX2 are depicted in Figure 3. The (RIm)2MX2 catalysts contain two nitrogen species that originate from the two imidazolium nitrogens, resulting in signals at 399.3 and 400.7 eV. The characteristic peak for amine nitrogen species appearing around 399.3 eV can be attributed to the imidazolium nitrogen attached to the alkyl group, and the other peak can be regarded as a signal from the positively charged nitrogen atom of the quaternary ammonium moiety.18 The Cl 2p, Br 3d, and I 3d spectra of the (RIm)2MX2 catalysts were also acquired to identify the bonding nature of the halogen associated with the metal species, as illustrated in Figure 4-6. The Cl 2p spectrum of (RIm)2MCl2 (Figure 4) shows a peak near 197.5 eV, confirming that the bonding between chlorine and the metals (Fe, Cu, or Zn) is ionic and not covalent.42 The Br 3d spectrum of (MeIm)2ZnBr2 shows a 8


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peak at ~68 eV (Figure 5), which is assigned to the negatively charged bromide ion.43 An I 3d peak was observed at 619 eV in the spectrum of (MeIm)2ZnI2 (Figure 6), which indicates the anionic state of iodine.43,44 These results are a clear indication of the successful preparation of metal-containing imidazolium salt catalysts, as shown in Scheme 2. Catalytic activity of (RIm)2MX2 catalysts. In order to explore the catalytic activity of metal-containing ionic liquids, we have examined the influence of 1-methylimidazole alone, ZnCl2 alone, and a mechanical mixture of 1-methylimidazole and ZnCl2 on the cycloaddition of CO2 to PO (Table 2). In the absence of catalyst (entry 1) and with 1-methylimidazole alone (entry 2) no reaction occurs, while ZnCl2 (entry 3) and 1-methylimidazole/ZnCl2 (entry 4) afford PC in low yields with similar catalytic activities. Thus, the zinc, which can interact with the oxygen of PO, probably acts as the catalytically active site, and the lack of a free nucleophilic anion (Cl−) must be the reason for the low catalytic activity.45 However, the (MeIm)2ZnCl2 catalyst (entry 7) exhibits significant activity toward the selective conversion of PO to PC. Hence, the Cl anion of (MeIm)2ZnCl2 functions synergistically with the zinc active site for the synthesis of PC from CO2 and PO.3 The synergy observed in bifunctional catalytic systems, such as MOFs, double-metal-cyanide complexes, and organic base/alkali metal systems, in the catalysis of CO2-PO coupling has been discussed previously.46-52 To rationalize our experimental results for the activity of the (MeIm)2ZnCl2 catalyst in the cycloaddition of CO2 to PO, we propose the following plausible mechanism: The O atom of the epoxide binds strongly with the zinc active site through Zn⋯O interactions, while the chloride anion nucleophile simultaneously attacks the least sterically hindered β-carbon atom of the PO molecule, forming the ring-opened intermediate and synergistically promoting the cycloaddition (Scheme 3).

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Reactivity of (RIm)2MX2 catalysts. The effects of the molecular structure and composition (i.e., the identity of the metal and anion, and the alkyl chain length) of the (RIm)2MX2 catalysts on the synthesis of PC were studied using a reaction time of 6 h at 140 °C under a 5 bar initial CO2 pressure, and the results are shown in Table 2 (entries 5-11). The catalytic activity of the (RIm)2MX2 catalysts differ depending on the metal center, anion nucleophile, and alkyl chain length. (MeIm)2ZnCl2 (entry 7) is the most efficient of the metalcontaining ionic liquid catalysts, with a PO conversion of 78.1% and PC selectivity of 91.5% under the employed reaction conditions. (MeIm)2FeCl2 (entry 5) and (MeIm)2CuCl2 (entry 6) are also active, with PO conversions of 67.4% and 51.2%, respectively. In order to investigate the difference in the catalytic performances of the (MeIm)2MCl2, where M was Fe, Cu, or Zn, we conducted theoretical studies using DFT calculations. All the quantum mechanical calculations were performed with Beck’s three-parameter B3LYP hybrid exchange-correlation function using the Gaussian 03 software package. For the geometry optimizations and frequency calculations, the standard 6-31G(d) basis set was used. The activation energy (∆Ea) required for a non-catalyzed cycloaddition of PO to CO2 to produce PC has been previously reported to be 55–56 kcal/mol.48,53-55 This energy barrier is too high for the reaction to proceed spontaneously, necessitating the addition of a catalyst. The Zn-containing ionic liquid catalyst (MeIm)2ZnCl2 was found to activate the cycloaddition of CO2 to PO efficiently; therefore, the potential energy surfaces (PESs) of the cycloaddition with (MeIm)2FeCl2, (MeIm)2CuCl2, and (MeIm)2ZnCl2 were compared (Figure 7). Of the three catalysts, (MeIm)2ZnCl2 exhibits the lowest activation energy (∆Ea = 37.6 kcal/mol) for the ring-opening Cl- attack on the epoxide, which is the highest activation energy step of the mechanism.56 The activation energies of the ring opening process using (MeIm)2MCl2 catalysts are in the order (MeIm)2ZnCl2 (37.6 kcal/mol) < (MeIm)2FeCl2 (38.8 10


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kcal/mol) < (MeIm)2CuCl2 (41.3 kcal/mol). These results are in reasonable agreement with the experimentally obtained activity order for (MeIm)2MCl2 catalysts. Moreover, the reactive performance of the catalysts was examined based on the acetalization57 of benzaldehyde with ethanol, a reaction catalyzed solely by acidic centers. The observed results of the aforementioned test conducted using the different metal containing ionic liquid catalysts are displayed in Figure 8. The initial rates (ro) of reaction for (MeIm)2ZnCl2, (MeIm)2FeCl2, and (MeIm)2CuCl2 were 1.45, 1.27 and 0.92 mmol min−1 g−1, respectively which is in accordance with the results obtained from the cycloaddition of CO2 and PO. It is well known that counter anions have a significant effect on catalytic performance during the cycloaddition of epoxides to CO2.11,58 As shown in Table 2, the three catalysts that differ only in their counterion exhibit similar conversion rates, but the order of the selectivities is (MeIm)2ZnCl2 (92.1%) < (MeIm)2ZnBr2 (97.1%) < (MeIm)2ZnI2 (98.1%), (entries 7, 8, and 9). These results are consistent with the order of the counterion nucleophilicity (Cl- < Br- < I-).58 The effect of the alkyl chain length in the (RIm)2ZnCl2 catalysts, where R is methyl, ethyl, or butyl, was investigated, and the results are shown in Table 2 (entries 7, 10, and 11). The conversion of PO decreases with increasing alkyl chain length. This could be explained in terms of the positive inductive effect (+I), which increases moving from methyl to butyl, resulting in a decrease in the effective positive character of the quaternized species and a commensurate decrease in the activity of the (RIm)2ZnCl2 catalysts.58,59 Also, the steric hindrance of the bulky butyl group may inhibit the approach of the PO to the metal center. Effect of various epoxides using (MeIm)2ZnCl2 catalyst. The versatility of (MeIm)2ZnCl2 in catalyzing epoxide-CO2 cycloadditions was investigated by conducting the 11



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reactions using terminal and internal epoxides and the results are as summarized in Table 3. The internal epoxide, cyclohexene oxide (entry 1, conversion = 10.3%) afforded the least conversion, mainly due to the steric hindrance occurred in nucleophilic attack imposed by the bulky nature of the cyclohexene ring. The relatively low conversion of styrene oxide is due to the low reactivity at its β-carbon center (entry 2, conversion = 32.5%). However, aliphatic terminal epoxides (entries 3-5) got cyclized efficiently with CO2 under (MeIm)2ZnCl2 catalysis. Statistical analysis of RSM and optimization of reaction conditions. RSM is a statistical technique with various uses for the optimization of multiple experimental variables to predict the most efficient sets of conditions with a minimum numbers of tests. Herein, since the (MeIm)2ZnCl2 catalyst can be considered as an efficient catalyst for the cycloaddition of PO to CO2, BBD of the RSM was applied to model the yield of PC using (MeIm)2ZnCl2 with three reaction parameters; reaction time (X1), temperature (X2), and initial pressure of CO2 (X3). To the best of our knowledge, this is the first attempt to utilize BBD for CO2-epoxide cycloaddition reactions. A BBD center-united design was employed for the experiments, and the corresponding response values are shown Table 4. Based on BBD, 15 runs were used for deriving an objective function for the yield of PC. Table 5 shows the analysis of variance (ANOVA) of the responses. The correlation coefficient (R2) for the yield of PC is 0.9849 with the derived model, which demonstrates that the theoretical values are in good accordance with the experimental values. The polynomial model for the yield of PC (Y) obtained from the experimental design is shown as follows: Y = 70.33 + 5.14·X1 + 5.21·X2 + 40.75·X3 − 6.55·X1·X2 − 0.23·X1·X3 − 1.93·X2·X3 − 7.84·X12 − 8.14·X22 − 9.22·X32

12


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X3 has the main linear effect on Y, and the terms that have minus and plus signs have a negative and positive effect, respectively, on Y. The magnitude of the effect of X is related to the value of λ in Y. In order to check the significance of each coefficient, the p-value can be used as an indicator, where p-values less than 0.05 indicate that the model terms are significant. From Table 5, it is evident that the linear term of X3 and quadratic term of X32 are significant. The response surfaces and contour plots are generally used to evaluate the relationships between parameters and to predict the result under given conditions. The RSM profiles between three parameters were constructed for the above-mentioned model for the yield of PC, and they were plotted as a function of two of the factors while the remaining parameter was maintained at a constant center level (Figure 9). As show in Figure 9 (a), PC is generated at the maximum yield at reaction time and temperature levels of ~0.2. When the initial pressure of CO2 is used as the most significant term, a high level of initial CO2 pressure shows the highest yield of PC at 0.2 levels of reaction time (Figure 9 (b)) and temperature (Figure 9 (c)). The programming showed that the optimal operational conditions under a moderate reaction environment are 4.6 h, 124 °C and 9 bar levels of reaction time, temperature, and initial CO2 pressure, respectively. The theoretically predicted yield of PC under the above conditions is 98.37%. To validate the model, three independent experiments for the cycloaddition of CO2 to PO were carried out under the established optimal reaction conditions. The average yield of PO was 97.91 ± 0.02%, which is close to the predicted value. Thus, RSM with appropriate experimental design can be effectively applied to optimize the cycloaddition of CO2 using the (MeIm)2ZnCl2 catalyst. The effect of the catalyst loading on the yield of PC was investigated using (MeIm)2ZnCl2 under the optimized reaction conditions. As shown in Table 6, the yield of PC and the PO 13



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conversion increases as the catalyst loading increases to 0.3 mmol (entry 3). However, the yield remains almost constant on increasing the catalyst loading further to 0.5 and 1.0 mmol (entry 4 and 5).

CONCLUSIONS Metal-containing ionic liquid catalysts based on imidazolium, i.e., (RIm)2MX2, were successfully synthesized and characterized by various physicochemical methods and were utilized for the cycloaddition of CO2 to PO. The (RIm)2MX2 catalysts showed good catalytic activity due to the synergism between the metal centers and the nucleophilic anion. To further corroborate the proposed mechanism, a DFT investigation into the cycloaddition of CO2 to PO using different metal-containing ionic liquid catalysts was carried out. Among the (MeIm)2MCl2 catalysts, (MeIm)2ZnCl2 afforded the lowest activation energy and exhibited better catalytic activity in the synthesis of PC. The optimum values for the maximum yield of PC were obtained using a BBD with a minimum of experimental runs. Under the optimum reaction conditions using the (MeIm)2ZnCl2 catalyst, the predicted yield of PC was 98.37% and the experimental results were 97.91 ± 0.02%, which is in good agreement with the predicted value.

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FIGURES.

Figure 1. FT-IR Spectra of (RIm)2MX2: (a) (MeIm)2FeCl2, (b) (MeIm)2CuCl2, (c) (MeIm)2ZnCl2, (d) (MeIm)2ZnBr2, (e) (MeIm)2ZnI2, (f) (EtIm)2ZnCl2, and (g) (BuIm)2ZnCl2.

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Figure 2. FT-Raman Spectra of (RIm)2MX2: (a) (MeIm)2FeCl2, (b) (MeIm)2CuCl2, (c) (MeIm)2ZnCl2, (d) (MeIm)2ZnBr2, (e) (MeIm)2ZnI2, (f) (EtIm)2ZnCl2, and (g) (BuIm)2ZnCl2.

16


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Figure 3. N 1s XPS spectra of (RIm)2MX2: (a) (MeIm)2FeCl2, (b) (MeIm)2CuCl2, (c) (MeIm)2ZnCl2, (d) (MeIm)2ZnBr2, (e) (MeIm)2ZnI2, (f) (EtIm)2ZnCl2, and (g) (BuIm)2ZnCl2.

17



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Figure 4. Cl 2p XPS Spectra of (RIm)2MCl2: (a) (MeIm)2FeCl2, (b) (MeIm)2CuCl2, (c) (MeIm)2ZnCl2, (d) (EtIm)2ZnCl2, and (e) (BuIm)2ZnCl2.

18


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Figure 5. XPS Br 3d spectra of (MeIm)2ZnBr2.

19



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Figure 6. XPS I 3d spectra of (MeIm)2ZnI2.

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Figure 7. Energetics profile for the cycloaddition of CO2 to PO using (MeIm)2MCl2 catalysts.

21



2.0

1.5

ro (mol/min.g)x103

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

1.0

0.5

0.0

(a)

(b)

(c)

Figure 8. The initial rate (ro) of (MeIm)2MCl2 catalysts on the acetalization of benzaldehyde and ethanol: (a) (MeIm)2ZnCl2, (b) (MeIm)2FeCl2 and (c) (MeIm)2CuCl2. [Conditions: 10 mmol of benzaldehyde with 20 mmol of ethanol using 1 wt% of catalyst at 60 °C for 2 h]

22 Environment ACS Paragon Plus

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(a)

(b)

(c)

Figure 9. 2D and 3D RSM plots of the yield of PC; (a) reaction time and temperature, (b) initial pressure of CO2 and temperature, (c) initial pressure of CO2 and reaction time. The remaining parameter was held at its mean level.



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SCHEMES.

Scheme 1. Synthesis of PC from PO and CO2.

Scheme 2. Preparation of (RIm)2MX2.


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Scheme 3. The proposed mechanism for cycloaddition of CO2 to PO using (MeIm)2ZnCl2. O

O

Cl

O N

O

N M N

N

PO

Cl

PC

(MeIm)2ZnCl2 nucleophilic attack

Cl

N N N M

O

N

cycloaddition

O N M N

O

O

Cl Cl

N N O N M N Cl

N


N


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TABLES.

Table 1. Coded levels for independent factors used in the experimental design.

Coded levels No.

Symbol -1

0

1

Reaction time (h)

X1

1

4

7

Temperature (°C)

X2

100

120

140

Initial pressure of CO2 (bar)

X3

1

5

9


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Table 2. Activities of (RIm)2MX2 in the synthesis of PC from CO2 and PO.

Entry

Catalyst

Conversion of PO (%)

Selectivity* to PC (%)

1

None

0

-

2a

1-methylimidazole

0

-

3b

ZnCl2

16.4

48.8

4c

1-methylimidazole / ZnCl2

16.7

49.2

5

(MeIm)2FeCl2

67.4

89.7

6

(MeIm)2CuCl2

51.2

91.5

7

(MeIm)2ZnCl2

78.1

92.1

8

(MeIm)2ZnBr2

78.8

97.1

9

(MeIm)2ZnI2

79.1

98.1

10

(EtIm)2ZnCl2

74.1

91.5

11

(BtIm)2ZnCl2

71.7

90.4

12d

EMImCl

50.4

91.3

13e

TMACl

46.2

90.6

Reaction conditions: 50 mmol PO, 0.5 mmol of (RIm)2MX2, Reaction time = 4 h, Temp. = 120 °C, Initial pressure of CO2 = 5 bar. The Conversion of PO was analyzed by GC using biphenyl as internal standard. * The major bye-product was propylene glycol as detected by the GC, possibly formed from the reaction of the small amounts of moisture in the epoxide as well as from the hygroscopic nature of ZnCl2. a

1.0 mmol

b

0.5 mmol

c

1.0 mmol 1-methylimidazole + 0.5 mmol ZnCl2

d

0.5 mmol 1-Ethyl-3-methylimidazolium chloride (Sigma-Aldrich)

e

0.5 mmol Tetramethy ammonium chloride (Sigma-Aldrich)



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Table 3. Cycloaddition of CO2 with various epoxides using (MeIm)2ZnCl2.

Entry

Epoxide

Cyclic carbonate

Conversion (%)

Selectivity (%)

1

10.3

91.1

2

32.5

92.2

3

79.5

91.8

4

80.4

91.2

78.1

92.1

O

5

O

O

Reaction conditions: 50 mmol epoxides, 0.5 mmol of (MeIm)2ZnCl2, Reaction time = 4 h, Temp. = 120 °C, Initial pressure of CO2 = 5 bar.


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Table 4. Experimental design and results of the RSD.

No.

X1

X2

X3

Yield of PC (%)

1

-1

-1

0

30.2

2

1

-1

0

61.8

3

-1

1

0

60.0

4

1

1

0

65.4

5

-1

0

-1

12.0

6

1

0

-1

14.5

7

-1

0

1

92.5

8

1

0

1

94.1

9

0

-1

-1

7.5

10

0

1

-1

15.5

11

0

-1

1

94.3

12

0

1

1

94.6

13

0

0

0

70.3

14

0

0

0

68.8

15

0

0

0

71.9



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Table 5. Coefficients of the model and ANOVA.

Term

Coefficient

P-value (Prob. > F)

Intercept

70.33

0.0005

X1

5.14

0.0816

X2

5.21

0.0784

X3

40.75

< 0.0001

X1·X2

-6.55

0.1071

X1·X3

-0.23

0.9489

X2·X3

-1.93

0.5893

X1 2

-7.84

0.0737

2

-8.14

0.0662

X3 2

-9.22

0.0453

R2

0.9849

X2


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Table 6. The effect of catalyst amount on the cycloaddition of PO to CO2.

Entry

Amount of catalyst (mmol)

Conversion of PO (%)

Selectivity to PC (%)

1

0.1

63.7

99.9

2

0.2

75.5

99.9

3

0.3

89.8

99.9

4

0.5

98.0

99.9

5

1.0

98.3

99.8

Reaction conditions: 50 mmol PO, x mmol of (MeIm)2ZnCl2, Reaction time = 4.6 h, Temp. = 124 °C, Initial pressure of CO2 = 9 bar.



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AUTHOR INFORMATION

Corresponding Author *E-mail address: [email protected] ACKNOWLEDGMENT This study was supported by the Korea Research Institute of Chemical Technology program (SI-1609-01) and the Korea Evaluation Institute of Industrial Technology (10049173). REFERENCES (1) Roshan, K. R.; Mathai, G.; Kim, J.; Tharun, J.; Park, G. A.; Park, D. W. A biopolymer mediated efficient synthesis of cyclic carbonates from epoxides and carbon dioxide. Green

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Table of Contents Graphic and Synopsis Here

Metal-containing ionic liquids as synergistic catalysts for the cycloaddition of CO2: A density functional theory and response surface methodology corroborated study

Dongwoo Kim, Yeji Moon, Dahye Ji, Hyeongook Kim, and Deughee Cho

Synopsis: Metal-containing ionic liquid catalysts possessing bifunctional active sites have confirmed its catalytic potential for the synthesis of propylene carbonate from CO2 and propylene oxide.


Page 40 of 40

Metal-Containing Ionic Liquids as Synergistic Catalysts for the

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