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These unique IL–H2O systems are successfully used for facile one-step synthesis .... First, tetraglyme was selected as a destroyer, which could comp...
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Temperature-Controlled Reaction−Separation for Conversion of CO2 to Carbonates with Functional Ionic Liquids Catalyst Xianglei Meng,† Hongyan He,† Yi Nie,*,† Xiangping Zhang,† Suojiang Zhang,*,† and Jianji Wang‡ †

Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China

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S Supporting Information *

ABSTRACT: Carboxylic acid-based ionic liquids (CAILs) were designed and synthesized. They displayed temperature-dependent dissolution−precipitation transitions in propylene carbonate (PC) by controlling the chain length of the carboxyl group. It was found that it could be attributed to the temperature-controlled hydrogen-bond formation/broken between the CAILs components and PC by NMR investigations and DFT calculations. Such a unique phase behavior was successfully utilized for the cycloaddition reaction of CO2 with epoxides. Due to the activation of epoxide assisted by the hydrogen bond, the optimal ILs could show a 92% yield of PC within 20 min and quickly precipitate out from a homogeneous system at room temperature for easy recycling. The reversible phase transition phenomenon supplied an efficient way to combine activity and recovery of homogeneous catalysts, which is a benefit for energy-saving and industrial applications. KEYWORDS: Ionic liquids, Temperature-controlled, Reaction−separation, CO2 conversion, Cyclic carbonates

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INTRODUCTION The combination of activity and recovery of homogeneous catalysts is recognized as a great challenge in catalysis research from an industrial point of view.1 The reported homogeneous catalysts are usually more efficient, while it is costly to reclaim them from reaction products. By contrast, heterogeneous catalysts can be easily separated from the products and reused. But they are inefficient, and at the same time, high temperatures and pressures are often demanded, which make them expensive. This has promoted the development of efficient strategies for homogeneous catalysis and heterogeneous separation.2,3 Recently, Wang et al.4 have reported on the interesting finding of the reversible liquid−liquid phase transition of IL−H2O systems by bubbling/removing CO2. These unique IL−H2O systems are successfully used for facile one-step synthesis of Au porous films. The group of Xiong5 has prepared IL-based crosslinked polymeric nanogels (CLPNs) and found that they display reversible, temperature-driven nanogel−macrogel transitions in methanol. The reversible phase transition phenomenon supplied a novel strategy for the efficient combining of activity and recovery of homogeneous catalysts. The efficient utilization of carbon dioxide (CO2) also remains an important issue, as this may reduce greenhouse gas and produce various organic chemical commodities.6−9 One of the typical viable routes is the synthesis of five-membered cyclic carbonates via the cycloaddition of CO 2 with epoxides.10−12 Cyclic carbonates can be extensively used as © 2017 American Chemical Society

polar aprotic solvents, raw materials, and intermediates in the synthesis of pharmaceuticals and fine chemicals,13−15 which made this process very attractive and important. To this end, various catalysts have been developed.16−32 Ionic liquids (ILs) have been demonstrated as efficient homogeneous catalysts for the synthesis of cyclic carbonates.33 Many functional ILs containing a Lewis acid and a nucleophile have been reported. The He group34 supported phosphonium chloride on a fluorous polymer and used it as a recyclable homogeneous CO2-soluble catalyst for conversion of CO2 with epoxides under supercritical CO2 conditions. The catalyst could be easily recovered by simple filtration after reaction. The process demonstrated a simpler access of homogeneous catalyst recycling. Cokoja et al.35 synthesized hydroxy-functionalized mono- and bisimidazolium bromide catalysts for the cycloaddition of CO2 and epoxides. The most active catalyst showed a high conversion at very mild reaction conditions (70 °C, 0.4 MPa CO2). The Han group36 prepared carboxylic acid groupfunctionalized ammonium salts for the fixation of CO2. North21 proposed a new catalytic cycle that fully explains the role of the tetraalkylammonium bromide co-catalyst. Park et al.37 exploited carboxyl-group-functionalized imidazolium-based ionic liquids (CILs) and grafted them onto silica gel for cycloaddition of Received: November 16, 2016 Revised: January 21, 2017 Published: February 8, 2017 3081

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ACS Sustainable Chemistry & Engineering CO2. Zhang et al.38 provided a boronic acids/onium salts system as efficient organocatalysts for the conversion of CO2 with epoxides in H2O under mild conditions. We also reported four kinds of carboxylic acid-based bifunctional catalysts (ABBCs) for the reaction of epoxides with CO2.39 Although, the reported homogeneous systems provide higher activity, it is often inconvenient for separation. On the basis of the problems in homogeneous catalytic processes, some heterogeneous catalysts have been developed for CO2 chemical conversion.8,9 In contrast, heterogeneous systems are easier to separate, but they are inefficient (the most efficient metal-free heterogeneous catalyst turnover frequencies (TOFs) are 90 h−1),40 and hard reaction conditions are always needed. Hence, it is highly necessary to explore a convenient approach to realize both high activity and quick separation of catalysts.

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b), where they could react effectively. As the mixture was cooled, the solubility of CABFILs in PC dramatically decreased. At the same time, the catalysts precipitated spontaneously, and the phases were separated again (Scheme 1). So, we could simply filtrate to recover the catalysts precipitated from the products. Besides, the optimal ILs displayed high productivity and stability because of the activation of epoxide and CO2. Nearly 92% PC yield and 99% PC selectivity were obtained within 20 min under metal-free and solvent-free conditions (TOFs = 276 h−1). Such temperature-dependent dissolution−precipitation transitions behavior helped to avoid further waste of the subsequent separation step, which could be used to realize the coupling of the reaction and separation in energy savings and industrial applications. A series of these CABFILs (Figure S1) were obtained through a one-step synthesis process at room temperature. Their solubility in PC at 25 °C is shown in the Supporting Information (Figure S2). It could be observed that the carboxylic chain length of CABFILs had great influence on the solubility of CABFILs in PC: the solubility decreased sharply from 14.8% to 0.4% with an increase in chain length from 2 to 5 (1a−4a) at room temperature. The possible reason for the different solubility was mainly attributed to the effects of the different nonpolarity of CABFILs.41 Then, 3a was used as a representative example to investigate the temperature-dependent solubility. The solubility of 3a in PC was as low as 0.27% at 10 °C and increased obviously as the temperature increased (Figure S3). Hence, when 3a (8 wt %) was added into PC, such a liquid/solid mixture remained separated at room temperature. When the temperature went up, 3a readily dissolved in PC. As the mixture was cooled to room temperature, the catalysts precipitated spontaneously, and the phases separated again. We reasoned that the temperature-dependent solubility of CABFILs in PC was mainly attributed to the intermolecular hydrogen bonding of the ILs components.41,42 Then, nuclear magnetic resonance (NMR) studies were performed in order to gain insight into the mechanism. First, tetraglyme was selected as a destroyer, which could compete in the formation of a hydrogen bond,43 to investigate the intermolecular hydrogen bonding of the ILs components on the effect of the solubility. The 1H NMR spectra of 3a with and without tetraglyme is shown in Figure S4. The signal at 14.47 ppm was associated with the carboxylic proton, and the signal of the carboxylic proton decreased after mixed with tetraglyme. Such a trend suggests a weakened interaction energy.44 This was mainly because the hydrogen bond of O−H···O between carboxylic protons was broken. A similar result was found in the C2/C6 proton of the pyridine ring, which indicated the breaking of the hydrogen bond C−H···Br−. Then, we studied the intermolecular hydrogen

EXPERIMENTAL SECTION

Taking account of these issues, we exploited a series of carboxylic acidbased functional ILs (CABFILs) (Scheme S1, 1a−4a) as singlecomponent, metal-free homogeneous catalysts for cycloaddition of CO2 with epoxides into cyclic carbonates. Interestingly, these types of ILs displayed temperature-dependent phase transitions in propylene carbonate (PC) by controlling the chain length of carboxyl group. (The solubility in PC is shown in the Supporting Information, Figures S2 and S3). Thus, these catalysts offered an efficient way to recover the homogeneous catalysts. When the catalysts were added into PC, such a liquid/solid phase remained separated at room temperature (Scheme 1, step a). When heated, they readily dissolved in PC (Scheme 1, step

Scheme 1. Photograph Showing Temperature-Controlled Phase Transitions in PC (3a = 8 wt % and PC = 92 wt %)

Figure 1. (a) Molecular structure of 3a and the intermolecular hydrogen bonding of the ILs components. (b) SEM images of ILs 3a. 3082

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ACS Sustainable Chemistry & Engineering bonding of ILs b without a carboxyl group by 1H NMR spectra. As shown in Figure S5, when mixed with the tetraglyme, we found an increasing trend of the signal associated with the C2/C6 proton, which demonstrated very weak interaction energy between ILs b. Temperature-dependent NMR was shown in Figure S6 to provide further evidence for the effect of the temperature on the solubility of ILs in PC. The signal of the carboxylic proton and the C2/C6 proton of the pyridine ring also decreased when the temperature increased from 30 to 80 °C, which may be caused by the breaking of the hydrogen bonds O−H···O and C−H···Br−.45 The DFT calculations are shown in Figure 1 for further study of the intermolecular hydrogen bonding. All calculations were carried out by the use of the DFT functional with the 6-311++G (d, p) basis set as implemented in the Gaussian 09 program package. The result indicated that molecules of 3a showed a tendency to form a class of clusters with self-aggregation through a hydrogen-bonding network. Of all the clusters, tetramolecular clusters were relatively more stable, and the structure of the tetramolecular clusters is shown in Figure 1(a). Four carboxyl groups were forming tetramers that were connected by strong hydrogen bonds (O−H···O distance of 1.572−1.656 Å). Besides, several C−H···Br contacts between Br− anions and the cations can be observed, ranging from 2.36 to 2.87 Å. The hydrogen bonds of C−H···Br led to the four molecule aggregation and formed a tetramolecular cluster. Strong hydrogen bonds were also found between 3a and PC with an O−H···O distance of 1.93−1.94 Å and a C−H···O distance of 2.40−2.74 Å, respectively (Figure S7). We also studied the intermolecular hydrogen bonding of ILs b without carboxyl group. As shown in Figure S8, the molecules of b could hardly form stable tetramolecular clusters because of the weak interaction of the hydrogen bonding. Therefore, based on the above results, a possible mechanism of reversible temperature-dependent phase transitions behavior of CABFILs was proposed. The nonpolarity of CABFILs with a longer carboxylic chain was higher than those with a shorter chain, as well as a stronger intermolecular hydrogen bonding, which made them hardly dissolve in polar solvent PC at room temperature. When heated, the intermolecular hydrogen-bond network of O−H···O and C−H···Br was broken. Then, the molecules of ILs diffused into PC. Finally, the formation of a hydrogen bond between CABFILs and PC led to the dissolution of CABFILs. The dissolution process ended as precipitation−dissolution reached equilibrium. SEM images of ILs 3a was also shown in Figure 1, and the surface profile of 3a was like a brain wrinkle (Figure 1 b) due to the tetramolecular cluster structure caused by the hydrogen-bonding network. This particular shape could provide increased surface area and was useful in catalytic processes.

Table 1. Catalysts Screening for Synthesis of Propylene Carbonatea entry

catalysts

PO conv. (%)b

PC select. (%)b

1 2 3 4 5 6 7c 8c 9c 10d 11d

[BPy]Br [CEDMAPy]Br (1a) [CPrDMAPy]Br (2a) [CBDMAPy]Br (3a) [CPeDMAPy]Br (4a) [BDMAPy]Br (b) [CBDMAPy]Br (3a) [{(CH2)3CO2H}inic]Br39 [{(CH2)3CO2H}bpy]Br39 [CBDMAPy]Br (3a) Poly imidazolium salt a40

47 89 94 90 89 75 99 73 83 99 90

99 99 99 99 99 99 99 96 99 99 90

a

Reaction conditions: PO (14.3 mmol), catalyst (1.0 mol %), CO2 pressure (2.0 MPa), reaction time (1.0 h) and temperature (120 °C). b Conversion and selectivity were determined by GC using biphenyl as the internal standard. cTemperature (125 °C). dPO (21.4 mmol), catalyst (60 mg), CO2 pressure (1.0 MPa), reaction time (1.5 h), temperature (110 °C).

Figure 2. Reuse of catalyst 3a. Reaction conditions: PO (14.3 mmol), catalyst (1.0 mol %), CO2 pressure (2.0 MPa), temperature (130 °C), and reaction time (20 min).

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RESULTS AND DISCUSSION This unique phase behavior and structure were useful in the cycloaddition reaction of CO2 with epoxides. Due to the ringopening of epoxides assisted by the hydrogen bond, all of CABFILs exhibited higher PC yield than the butylpyridinium bromide ([BPy]Br) without a carboxyl group (Table 1, entries 2−5 vs 1). Compared with the other CABFILs, 2a was observed as the highest activity. Considering that 2a could only partly transform to precipitate at room temperature, 3a was chosen as the optimal catalyst for further investigation. The comparison of the catalytic activity of 3a with our previously reported acid-based bifunctional ILs16 is displayed in Table 1, and 3a exhibited much higher activty than the bipyridiniumand isonicotinic-based functionalized ILs (entry 6 vs 7, 8). We also compared catalytic activity of 3a with the most efficient metal-free heterogeneous catalysts at the same conditions. Then, we investigated the effect of some parameters on the catalytic activity of 3a (Figure S9). The results indicated that nearly 92% yield of PC together with 99% selectivity was obtained at 130 °C, 2.0 MPa in 20 min (TOFs = 276 h−1), which was much higher than the most efficient metal-free

Figure 3. Reaction conditions: epoxides (14.3 mmol), catalyst (1.0 mol %), CO2 pressure (2.0 MPa), and temperature (130 °C).

heterogeneous catalysts turnover TOFs (90 h−1). Recycling experiments showed that 3a could be reused four times with 3083

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were carried out under atmospheric pressures of CO2 (0.1 MPa) with a catalyst loading of 1 mol %, and the reaction was run at 50 °C. As shown in Figure 4, a 95% yield of 6e could be obtained within 20 h at this CO2 pressure and temperature. The yield of 6c could reach to 96% after 44 h reaction. The results encouraged us to design a high activity catalyst for the conversion of CO2 at mild conditions in the next study. According to the reports34−39 and results above, a possible mechanism for fixation of CO2 with epoxides was illustrated in Scheme 2, and we chose 3a as a representative to study the mechanism details through DFT study (Figure 5). As shown in Scheme 2 and Figure 5, first, 3a and PO interacted to form a complex A, and then, the epoxy ring-opening step made A convert into intermediate B via transition state TS1 with a barrier of 14.0 kcal/mol. A new complex C was formed when CO2 was added into the reaction system. Then, nucleophilic attack of the intermediate on the complex C produced the alkyl carbonate D through TS2 with a lower energy barrier of 2.9 kcal/mol. Finally, the cyclic carbonate was obtained by intramolecular ring-closure, and the catalyst was regenerated (TS3). The ring-closure step was the rate-limiting step with the highest energy barrier of 21.4 kcal/mol (TS3). The difference between the ring-closure and ring-opening step was 7.4 kcal/ mol. This could be attributed to the hydrogen bond-assisted ring-opening of epoxide, which made the ring-opening much easier. Besides, there was some negative effect of the hydrogen bonding on the ring-closing event.46

Figure 4. Reaction conditions: PO (14.3 mmol), catalyst (1.0 mol %), CO2 pressure (0.1 MPa), and temperature (50 °C).

high activity and selectivity (Figure 2). During the cycle experiments, the catalysts were easily recovered by simple filtration at room temperature and then directly used for the next cycle. The thermogravimetric curve (Figure S10) proved the high stability of the catalyst activity. A battery of epoxide substrates were examined under the optimized conditions for the synthesis of corresponding cyclic carbonates to test the adaptability of 3a to different epoxides. As shown in Figure 3, 3a was suitable for a variety of terminal epoxides and exhibited high yield of the corresponding cyclic carbonates. Among all the epoxides surveyed, the activity of 5e was the highest, with 99% yield of 6e achieved in 20 min. To achieve the goal of finding room-temperature and atmospheric pressure CO2 activity catalysts,24,35 5c/CO2 and 5e/CO2 cycloadditions

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CONCLUSIONS In summary, a series of carboxylic acid-based functional ionic liquids (CABFILs) were facilely prepared by a one-step synthesis method. They displayed reversible temperaturecontrolled phase transitions in PC, which were useful for the coupling reaction and separation processes. CABFILs were

Scheme 2. Proposed Mechanism for Cyclic Carbonates Synthesis Catalyzed by 3a

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Figure 5. Potential energy surface profiles of the 3a-catalyzed process and the optimized geometries for the intermediates and transition states. See the Supporting Information for computational details.

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ACKNOWLEDGMENTS The authors are grateful for the support from the 973 Program (2015CB251403) and National Natural Science Foundation of China (21210006, 21576262, and 21406230). We further thank Dr. Jian Sun, Jun Ma, Jinquan Wang, and Baohua Xu for their helpful discussions.

successfully used in reaction−separation for conversion of CO2 to carbonates under metal-free and solvent-free conditions. Besides, IL 3a displayed high activity, and nearly 92% PC yield was obtained within 20 min and could be readily separated by filtration at room temperature for easy recycling. More importantly, 3a was suitable for a variety of terminal epoxides. This approach helps to avoid further waste of the subsequent separation step, as well as a highly effective way to combine the benefits of heterogeneous and homogeneous catalysts, which is of great interest from the viewpoint of energy savings and chemical engineering.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02612. Preparations of ILs, NMR and FT-IR of ILs, NMR of products, procedure of the cycloaddition reaction, Cartesian coordinates, solubility of IL in PC, 1H NMR of IL 3a with and without tetraglyme, temperaturedependent 1H NMR spectrum of IL 3a, DFT study of hydrogen-bonding interaction between 3a and PC, DFT study of intermolecular hydrogen bonding of ILs b, dependencies of PC yields on different parameters, and TGA of ILs (fresh and reused four times). (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Nie). *E-mail: [email protected] (S. Zhang). ORCID

Xianglei Meng: 0000-0002-3685-2243 Suojiang Zhang: 0000-0002-9397-954X Notes

The authors declare no competing financial interest. 3085

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