Hydroxyl-Functionalized Ionic Liquid Promoted CO2 Fixation

Article pubs.acs.org/IECR

Hydroxyl-Functionalized Ionic Liquid Promoted CO2 Fixation According to Electrostatic Attraction and Hydrogen Bonding Interaction Li Wang, Xiangfeng Jin, Ping Li, and Jinglai Zhang* Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R. China

Hongyan He and Suojiang Zhang* Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China S Supporting Information *

ABSTRACT: The mechanism of cycloaddition reaction between carbon dioxide and epoxide, catalyzed by HEMIMC (1-(2hydroxyl-ethyl)-3-methylimidazolium chloride), was investigated using the DFT (density functional theory) method. In the presence of HEMIMC, the reaction mechanism changed from single-step to multipath. Seven reaction pathways are reported here, including two steps, epoxide ring-opening and ring-closure of cyclic carbonate, or three main steps, epoxide ring-opening, carbon dioxide insertion, and ring-closure of cyclic carbonate. The catalytic activity of HEMIMC was studied, and the catalytic mechanism was elucidated. The nucleophilic attack of anion and hydrogen bond are two of the most important factors to promote the cycloaddition reaction, especially the OH functional group in HEMIMC. Finally, the influence of different anions on the catalytic activity was investigated.

1. INTRODUCTION Carbon dioxide (CO2) is one of the most abundant greenhouse gases, which has resulted in serious environmental problems, such as higher earth surface temperature and more frequent and severe climatic disturbances. However, it is also an attractive renewable carbon resource, which is nontoxic, cheap, and nonflammable.1−5 Therefore, research on the transformation of CO2 into valuable chemicals is of great importance from the environmental protection and resource utilization point of view.6−8 Few industrial processes have used CO2 as a raw material because of its high thermodynamic stability. The synthesis of five-membered cyclic carbonates via the cycloaddition of CO2 to epoxides (as shown in Scheme 1) is one of

epoxides to generate cyclic carbonates. The typical catalysts include metal oxides,13,14 alkali metal salts,15,16quaternary onium salts,17,18 ionic liquids,19−22 transition metal complexes,23−25 and functional organic compounds and polymers.26,27 Recently, the emphasis on sustainable and green chemistry has aroused great interest in developing ionic liquids as a catalyst for CO2 fixation. The mixture of ionic liquid and Lewis acid was investigated as catalyst in the past few years,28 and significant improvements have been achieved. However, the drawbacks of this catalytic system include: (1) stringent dry conditions are required because of the sensitivity of some Lewis acids; (2) the performance of the ionic liquid is substantially reduced in the absence of Lewis acid; and (3) there is a relative high cost compared to single component ionic liquid catalyst. Therefore, developing a water-stable, inexpensive, and high efficiency catalyst for CO2 chemical fixation is still highly urgent. In the past ten years, some imidazolium-based ionic liquids with different alkyl groups and anions had been developed as efficient catalysts for the cycloaddition of CO2 to epoxides.29−34 Among them, the alkylmethylimidazolium-based ionic liquids reported by Park et al. are one of the most efficient series of catalysts with high selectivities and yields. Later, Sun et al.35 introduced the hydroxyl group into the alkylmethylimidazo-

Scheme 1. Cycloaddition of CO2 with PO to Produce PC

the most prospective reactions because of the cheap reactants, high atom utilization, and low generation of byproduct.9,10 These carbonates, e.g., ethylene carbonate and propylene carbonate (PC), can serve as electrolytic elements in lithium secondary batteries, precursors of polycarbonates and other polymers, and intermediates in the production of pharmaceuticals and fine chemicals.11,12 In the past few decades, numerous catalysts have been designed for the insertion of CO2 into © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8426

March 12, 2014 April 16, 2014 April 30, 2014 April 30, 2014 dx.doi.org/10.1021/ie501063f | Ind. Eng. Chem. Res. 2014, 53, 8426−8435

Industrial & Engineering Chemistry Research

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lium-based ionic liquids. Several hydroxyl-functionalized ionic liquids have shown higher catalytic activity to the synthesis of cyclic carbonate without any additional Lewis acid reagent and solvent. Several theoretical studies have been performed with different kinds of catalyst and in particular ionic liquids and salts (C(2,4,6)mimCl, 36 DMimBr Me 2 PO 4 +ZnBr 2 , 37 TBABr,38,39 TEA(Br−Cl),39 LiBr,40 TBDHBr,41 KI/glycerol,42 and TBAI/pyrogallol43). In particular, the three last studies have been devoted to the understanding of the role of hydrogen bonding in the reaction mechanism. On the basis of the previous reports, a hypothetic mechanism was reported by the group based on the previous reports. However, it was only a qualitative analysis without any data support, and the intermediates had not yet been detected or captured in the experiments. Thus, the precise mechanism is still ambiguous. There is no doubt that elucidating the mechanism is of great importance, not only for unraveling the obstacles associated with the poor performance but also for developing high efficient catalyst. Here, a thorough theoretical study, using density functional theory44−46 on the reaction of carbon dioxide with propylene oxide (PO) in the presence of HEMIMC (1-(2-hydroxylethyl)-3-methylimidazolium chloride), was reported. The detailed intermediate structure and energy information in each step of the reaction were obtained. The objective is to understand the reasons for high reactivity discovered using hydroxyl-functionalized ionic liquids.

Figure 1. Optimized geometries for CO2, HEMIMC, PO, and PC along with labeling the key atoms. Distances are in angstroms, and angles are in degrees. The values in parentheses are experimental results.51,52 The values in square brackets are theoretical values calculated at the B3PW91/6-31G(d,p) level.36

2. COMPUTATIONAL DETAILS The equilibrium geometries of all the stationary points, including the reactants, products, minima, and transition states were optimized using the B3LYP47 method (Becke’s threeparameter nonlocal-exchange functional with the nonlocal correlation of Lee−Yang−Parr method) with 6-31G(d,p)48 basis set (B3LYP/6-31G(d,p)). Vibrational frequency calculations were carried out at the same level to determine the nature of critical points and to derive the zero point energy (ZPE). The number of imaginary frequency indicates whether a minimum or a transition state has been located: all positive frequencies for a minimum and one imaginary frequency for a transition state. Starting from the saddle-point geometries and going downhill to two desired minima, the minimum-energy path (MEP) was constructed by intrinsic reaction coordinate (IRC) theory.49 To improve the reaction enthalpies and refine the barrier heights, the energies were corrected at the B3LYP/ 6-311+G(2d,2p) level48 based on the optimized geometries. All electronic structure calculations were performed by the Gaussian 09 program.50

Zhang et al.,36 indicating that B3LYP method can be taken as a reliable tool to describe the present system. Schematic potential energy profiles associated with the corresponding optimized structures of intermediates and the transition states without catalyst are plotted in Figure 2. Two reaction pathways (path a and path b) are located for the cycloaddition of CO2 with PO in the absence of HEMIMC, which correspond to the nucleophilic attack by the oxygen atom of CO2 to the C1 atom (substituted carbon) and C2 atom (unsubstituted carbon) of PO, respectively. For path a, a

3. RESULTS AND DISCUSSION 3.1. Cycloaddition Reaction without Catalyst. The geometries of stationary points, including the reactants (CO2 and PO), catalyst (HEMIMC), and product (propylene carbonate, PC) optimized at the B3LYP/6-31G(d,p) level, are plotted in Figure 1 together with the available experimental values51,52 of CO2 and PO. As shown in Figure 1, the calculated geometric parameters are identical to the experimental results: the maximum deviation for bond length is 0.01 Å, and the angle is 1.4°. Moreover, the theoretical structures of reactants and products are in reasonable agreement with the results performed at the B3PW91 level with the same basis set by

Figure 2. Potential energy surface profiles with the optimized geometries of stable points (intermediates and transition states) on PESs for the cycloaddition of CO2 with PO in the absence of catalyst. 8427

dx.doi.org/10.1021/ie501063f | Ind. Eng. Chem. Res. 2014, 53, 8426−8435


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Figure 3. Potential energy profiles for the cycloaddition reaction along routes 1−7 calculated at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d,p) level. Red line represents route 6.

Scheme 2. Reaction Pathway of Route 1a

a

A character is assigned underneath each structure as a notation of the species. TSnm is used to denote the transition state connecting the structures n and m.

the van der Waals radii of C atom and O atom (3.22 Å).53 It is a barrier-free process from reactants to IMa, since the transition state can not be located. Subsequently, the reaction overcomes the transition state (tsa) leading to the product PC. In tsa, the

complex denoted as IMa is initially formed between CO2 and PO via the weak van der Waals interaction because of the distance 2.796 Å between O3−C3, which is much longer than the covalent bond length of C−O but shorter than the sum of 8428



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a



a


breaking bond (C1−O3) is elongated to 2.057 Å, and the two forming bonds (C1−O1 and C3−O3) are shortened to 2.396 and 1.659 Å, respectively. The unique imaginary frequency (596.67i cm−1) of the transition state tsa corresponds to the simultaneous stretching of the breaking bond (C1−O3) and contracting of the forming bonds (C1−O1 and C3−O3). PC is formed via an elementary step with concerted mechanism. The property of path b is analogous to path a, except that the oxygen atom of the CO2 attacks the unsubstituted C2 atom of PO. From an energy point of view, path a is more favorable by requiring almost 6.3 kcal/mol energy less than path b. However, the relative energies of both paths (52.69 kcal/mol for tsa and 58.99 kcal/mol for tsb) are too high for the noncatalyzed cycloaddition to happen spontaneously, which is consistent with the experimental observation. Thus, it is necessary to find green and efficient catalyst for the

cycloaddition of CO2 with PO. In the following section, we will focus on the mechanism of cycloaddition reaction in the presence of HEMIMC. 3.2. Cycloaddition Reaction with HEMIMC. For the reaction of CO2 with PO in the presence of HEMIMC, seven reaction pathways, including 25 intermediates and 16 transition states, are located. The energy profiles along with the reaction coordination, obtained at the B3LYP/6-311+G(2d,2p) level, are depicted in Figure 3, where the sum of energies of isolated reactants (CO2+PO) and catalyst (HEMIMC) is taken as zero. It is worthy to note that in this work the relative energy is calculated according to the reactant energies, which are set as zero, while the barrier height is measured from the corresponding intermediate energy level. For brevity, the relative energies are listed in Figure 3, and barrier height for every route is tabulated in Table S1, Supporting Information. 8429



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a


Figure 4. Optimized geometries for the intermediates and transition states exclusively involved in route 1. Distances are in angstroms.

a2 is generated. It is clearly shown in Figure 3 that this process is endothermic (19.90 kcal/mol) and needs to overcome the barrier energy of 40.30 kcal/mol. In complex a2, C2−O3 bond is broken, and C2−O1 and C3−Cl bonds are formed. From a2, a less stable species a3 is formed via the rotation of −CH3 group of PO to facilitate the following ring-closure step. Although the transition state fails to be located from a2 and a3, it is reasonable to expect that the barrier height from a2 to a3 is small, since this process involves only a torsional deformation, rather than the bond formation or rupture.36 On the basis of the a3, the addition of O3 to C3 is accompanied by proton transferring from O3 back to C4 and the departure of Cl− anion simultaneously via tsa3a4. Finally, the complex a4 is produced, and its direct dissociation leads to the formation of cyclic carbonate and release of HEMIMC. Thereby, a catalytic cycle is completed. Overall, the ring-opening of PO is the ratedetermining step with the barrier energy of 40.30 kcal/mol. As compared with the reaction of CO2 with PO without

On the basis of the key elementary steps involved in the reaction, the seven routes can be divided into two classes: (I) two-step mechanism for routes 1−3 and (II) three-step mechanism for routes 4−7. The reaction mechanisms of four representative routes 1, 2, 4, and 6 are depicted in Schemes 2, 3, 4, and 5, which will be discussed in detail in the following sections. 3.2.1. Route 1. Figure 4 presents the optimized structures of the intermediates and transition states involved in route 1. When two isolated reactants and catalyst are close to each other, a trimolecule complex a1 is first formed via hydrogen bonds with the relative energy of −8.32 kcal/mol. Starting from complex a1, the nucleophilic attack of Cl− anion to C atom of CO2 and O1 atom of CO2 to C2 atom of PO happens simultaneously. In this step, the transition state tsa1a2 has been located, which is characterized by ring-opening of PO and proton migration from C4 (carbon atom of HEMIMC) to O3 (oxygen atom of PO). After the transition state tsa1a2, complex 8430



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between two routes is that the different C−O bonds (C2−O3 bond for route 2 and C1−O3 bond for route 3) are cleaved in the ring-opening process as a result of the Cl− anion attacking the different C atom of PO. Other processes involved in route 3 are analogous to route 2, which will not be discussed in detail in this paper. Although the energy barrier of the ring-opening step (tsb1c1) is still higher than the ring-closure step, the difference between them is 0.51 kcal/mol, which is too small to determine the absolute rate-determining step. The energy barrier of the ring-opening step in route 3 (25.96 kcal/mol) is slightly affected by the larger steric hindrance of the methyl group on C1 atom. However, the energy barrier of the ring-closure step is greatly increased to be the other rate-limiting step. Thus, the influence of steric bulk/substitution on the ring-closure step is much larger. Moreover, the difference is expected to be enlarged with the augmentation of the substitution group on C1 in PO. Therefore, attacking the substituted C1 atom is expected to be less favored for the catalyzed cycloaddition reaction of CO2 with PO, especially for the large substitution group. 3.2.3. Route 4 and Route 5. In the three reaction pathways discussed above, CO2 enter into the system in the first step. The idea is sparked to elucidate the mechanism that CO2 is not incorporated from the starting point. Two more pathways (route 4 and route 5) are identified including three main steps. First, complex d1 with relative energy of −6.13 kcal/mol is formed via the hydrogen bond (C5−H1···O3) as illustrated in Figure 6. Next, complex d1 will convert into intermediate d2 via tsd1d2 with the energy barrier of 22.78 kcal/mol. The imaginary vibration of tsd1d2 corresponds to the shrinking of H2−O3 bond and stretching of C2−O3 bond, which indicates that the proton transferring from C4 to O3 and epoxy ringopening of PO are completed with a concerted mechanism. The electronegativity of O3 is larger than C4, which facilitates the H2 atom to migrate from C4 to O3, while the rupture of C2−O3 bond is also attributed to the nucleophilic attack of Cl− anion to C2 atom of PO. When CO2 is introduced into the reaction system, complex d3 is formed via the electrostatic attraction between O3 and C3 atom of CO2. The CO2 insertion process is an exergonic reaction with 0.8 kcal/mol. Following

catalyst, the barrier height is decreased by 12−19 kcal/mol. Thus, the reaction of CO2 with PO becomes more favorable in the presence of the HEMIMC as the catalyst. However, the barrier height of 40.30 kcal/mol is still too high to be overcome in the experimental temperature of 125 °C. Do other possible routes with lower barrier height exist? 3.2.2. Route 2 and Route 3. In route 1, the completion of ring-opening is promoted by the Cl− anion attacking C atom of CO2. Alternatively, C atoms of PO can also be attacked. Consequently, route 2 and 3 are discovered, which also present a two-step mechanism. The corresponding optimized structures are plotted in Figures 5 and S1 of the Supporting Information, respectively. Similar to route 1, a trimolecule complex b1 is formed in the first step of route 2. Subsequently, Cl− anion will attack C2 atom (least substituted C atom) of PO; then, the C3 atom of CO2 is joined itself to O3 atom of PO simultaneously via tsb1b2 leading to intermediate b2. As a result, intermediate b2 with the relative energy of −11.60 kcal/mol is formed, which is stabilized by two hydrogen bonds C3−O2···H3 and C4−H2··· O1. Next, intermediate b2 will convert to the other less stable configuration b3 through the rotation of −CH2Cl group with a small energy difference (1.30 kcal/mol), which is easy to surpass. In b3, the distance of C2 and O1 is shortened to favor the cyclic reaction, which will happen in the next step. The following step is an intramolecular cyclic SN2-type reaction with tsb3b4 as the transition state, where the Cl− anion departs from C2 atom as a result of the backside nucleophilic attack of O1 atom. Another intermediate b4 is generated through the formation of C2−O1 bond and cleavage of C2−Cl bond. Ultimately, PC is formed with the regeneration of HEMIMC. The calculated barrier for intramolecular cyclic step, 18.37 kcal/ mol, is lower than the preceding epoxy ring-opening step. Hence, the epoxy ring-opening step is the rate-limiting step of route 2 with the energy barrier of 25.48 kcal/mol. Since Cl− anion can attack the nonsubstituted C2 atom to promote the ring-opening of PO, it is the group which determines whether Cl− anion will attack the substituted C1 atom. In accordance with the idea, route 3 is developed from the adduct b1 with the alike mechanism. The main difference 8431



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the formation of d3, the electrophilic attack of CO2 to O3 atom results in the C3−O3 bond formation associated with the H2migration from O3 atom back to C4 atom via tsd3b2. As a result, intermediate b2 is formed. Starting from b2, route 4 falls into the mechanism of route 2. According to the experience obtained from route 2 and route 3, it is easy to extend route 4 to route 5, which is from attacking the nonsubstituted C2 atom to the substituted C1 atom. Certainly, route 4 and route 5 are alike in mechanism except that route 5 will fall into route 3 in the ring-closure step. The ring-opening of PO is concluded to be the rate-determining step of route 4. On the contrary, the ring-closure step is the rate-determining step in route 5. The steric bulk/substitution plays a more important role in the ring-closure step than the ring-opening or CO2 insertion step. The corresponding optimized structures are plotted in Figure S2, Supporting Information. The mechanisms of the above-discussed five pathways are analogous to the same reaction with [C2mim][Cl] as a catalyst reported by Zhang et al.36 Furthermore, the barrier height of the rate-determining step in the most favorable path is also comparable. However, Sun et al.35 stated that a series of hydroxyl-functionalized ionic liquids including HEMIMC synthesized by the group showed more efficient reactivity. According to their description, a more competitive route should exist with much lower barrier or with different reaction mechanism. Our interest is aroused to find the novel reaction pathway that is different from the reported ones. 3.2.4. Route 6 and Route 7. Before exploring the new reaction route, it is essential to analyze the common features of the above five routes for theorization. One prominent point is that there are hydrogen-bonding interactions formed between H3 atom/H2 atom of HEMIMC and O3 atom of PO in the precursor state of the epoxide ring-opening step. On the other hand, H2 atom will migrate from C4 atom of HEMIMC to O3 atom of PO in the transition state of the ring-opening step for route 1, route 4, and route 5. Therefore, the hydrogen-bonding interaction and proton migration are two vital elements to design a new route. Compared with the [C2min][Cl], the introduction of the hydroxyl in ionic liquid HEMIMC is the most distinct point. Therefore, we speculate an alternative process, in which the H-bonding interaction is formed between H3 atom of hydroxyl and O3 atom of PO (O4−H3···O3), and H3 atom of hydroxyl is transferred. As expected, route 6 is confirmed. As shown in Figure 7, the H3 atom is coordinated with O3 atom of PO through a hydrogen bond, and Cl− anion

Figure 7. Optimized geometries for the intermediates and transition states involved in route 6. Distances are in angstroms.

is located by three hydrogen bonds (C2−H5···Cl, C6−H4···Cl, and C5−H1···Cl) in the initial complex of route 6. Then, the C2−O3 bond of PO is easily ruptured as a result of the nucleophilic attack of Cl− anion to the nonsubstituted C2 atom and the migration of H3 atom from hydroxyl to O3 atom of PO. The mechanisms of route 6 and route 4 are analogous, including three elementary steps: epoxide ring-opening, carbon dioxide insertion, and ring-closure of cyclic carbonate. However, there are three distinguishable properties: (1) H3 atom of hydroxyl is transferred rather than H2 atom of imidazolium ring; (2) the ring-opening step and ring-closure step are competitive with the barrier heights of 16.83 and 17.63 kcal/mol, respectively. The difference between them is smaller than the error bar of the B3LYP level, so both steps are ratelimiting steps, which are different from those of routes 1−4; (3) the barrier heights are significantly decreased by about 5 kcal/ mol compared with route 4, which is induced by the cooperation of both the electrostatic attraction and hydrogen bonding interaction in single ionic liquid molecule. According to experience, it is easy to conjecture another similar route in which the Cl− anion attacks the substituted C1 atom of PO, that is, route 7, which will not be discussed again. The difference is that the ring-closure of PO is the rate-determining step of route 7 compared with route 6. 3.3. Experiment Implications and Reaction Mechanism. The plausible reaction mechanisms for the cycloaddition of epoxides with CO2 catalyzed by hydroxyl-functionalized ionic liquids (HFILs) or other catalysts including hydroxyl have been portrayed in the literature.35,54−56 The H3 atom of hydroxyl will coordinate with O3 atom of PO through a hydrogen bond in the initial step, which is the ordinary proposed mechanism. Our calculation is in line with the experimental assumption. The hydrogen bond between hydroxyl and PO is confirmed in the complex f1 that is located at the entrance of route 6. However, this hydrogen bond is broken because of the migration of H3 atom of hydroxyl to O3 atom of PO in the next step, which is contrary to the experimental proposed mechanism that the hydrogen bond is 8432



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Figure 8. Potential energy profiles for the cycloaddition reaction along routes 6 and 8 calculated at the B3LYP/6-311+G(2d,2p)//B3LYP/631G(d,p) level. Red line represents route 6.

two catalysts have the same cation, it is reasonable to infer that they have a similar mechanism. Thus, only the most favorable reaction route (route 6) is studied with the HEMIMB as a catalyst. As shown in Figure 8, since the barrier height of the ring-opening step (the first step) is higher than the ring closure step (the third step), it is the absolute rate-determining step. In the presence of HEMIMB, the reaction activity is slightly improved with less barrier height compared with HEMIMC, which is consistent with the experimental report.35

kept in the whole process. As a result, a new hydrogen bond (O3−H3···O4) is formed between the migrated H3 atom and O4 atom of hydroxyl, which is not reported in the previous theoretical and experimental studies, to our best knowledge. Finally, the H atom transfers back to the O atom of hydroxyl to complete the catalyst recycle. The migrations of H atom and accurate hydrogen bond information have never been proposed in the experimental studies.35,54−56 It is the advantage of the theoretical study to demonstrate the explicit mechanism that is an impossible mission for experimentalist. The barrier height of the cycloaddition of CO2 with PO in HEMIMC is as low as 17 kcal/mol, which is easy to be overcome. Our calculation testified again that HEMIMC is a high efficient catalyst in the cycloaddition of CO2 with PO without any cocatalyst and organic solvent. In brief, seven routes are identified for the cycloaddition of CO2 with PO in HEMIMC. Compared to the uncatalyzed reaction, the barrier heights are reduced remarkably by about 13−42 kcal/mol. Thus, the HEMIMC plays an important catalytic role in the cycloaddition process. The nucleophilic attack of the Cl− anion to C atom in the PO or CO2 is one of the most important factors to facilitate the ring-opening. On the other hand, the intermediates and transition states are stabilized by the hydrogen bonding interaction, which is also helpful to promote the reaction. Except for the above routine conclusions that have been reported by others,36 there are three novel points: (1) A new hydrogen bonding interaction is identified (as shown in Figure 7), which is formed between O3 atom of PO and H3 atom of hydroxyl. (2) The barrier height is further decreased compared to the reaction in the presence of similar ionic liquid without the hydroxyl group, thus the incorporation of hydroxyl is a crucial factor to refine the catalytic activity of ionic liquid. (3) Both ring-opening and ring closure steps should be regarded as the rate-determining step with comparable barrier heights. 3.4. Effect of the Different Anion in Ionic Liquid. To explore the influence of different anions in ionic liquid on the reaction activity, the mechanism of cycloaddition of CO2 with PO in the presence of HEMIMB (1-(2-hydroxyl-ethyl)-3methylimidazolium bromide) was investigated (route 8). Since

4. CONCLUSION The reaction mechanism of cycloaddition of CO2 with PO catalyzed by the HEMIMC is explored by the DFT (density functional theory) method. On the basis of the theoretical results, the following remarks are concluded: (1) In the presence of HEMIMC, the one-step cycloaddition is changed to two (routes 1−3), i.e., the epoxy ring-opening step and intramolecular cyclic step, or three elementary steps (routes 4− 7), i.e., the epoxy ring-opening step, CO2 electrophilic attack step, and intramolecular cyclic step. (2) For the most favorable reaction route (route 6), both the ring-opening and ring-closure step have the possibility to be the rate-determining step with a similar energy barrier. (3) The high catalytic activity originates from the cooperative action of electrostatic interaction and hydrogen bond, where the nucleophilic attack of the Cl− anion promotes the ring-opening, and the hydrogen bond facilitates the stabilization of the intermediates and transition states, especially the OH functional group in HEMIMC. The reaction barrier is slightly affected by the different types of anions.

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ASSOCIATED CONTENT

S Supporting Information *

Geometries of stationary points involved in routes 3, 5, and 7 and the barrier heights associated with each route. This material is available free of charge via the Internet at http://pubs.acs. org/. 8433



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-371-23881589. Fax: +86-371-23881589. *E-mail: [email protected]. Tel: +86-10-82627080. Fax: +86-10-82627080. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Beijing Key Laboratory of Ionic Liquids Clean Process and State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, for providing computational resources. This work was supported by the National Natural Science Foundation of China (21376063), the Fund of State Key Laboratory of Multiphase Complex Systems, IPE, CAS (No. MPCS-2012-A10), Natural Science Foundation of He’nan Province of China (134300510008, 144300510032), Science Foundation of Henan Province (14A150034), and the Foundation for University Key Teachers from the He’nan Educational Committee.

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Hydroxyl-Functionalized Ionic Liquid Promoted CO2 Fixation

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