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Effect of ionic liquids clusters microenvironment on cycloaddition reaction of carbon dioxide
Journal of Molecular Liquids 284 (2019) 68–74
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Effect of ionic liquids clusters microenvironment on cycloaddition reaction of carbon dioxide Danning Zheng, Pan Ning, Jiamin Jiang, Fang Liu, Li Wang ⁎, Jinglai Zhang ⁎ Henan Provincial Engineering Research Center of Corrosion and Protection for Magnesium Alloys, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China
a r t i c l e
i n f o
Article history: Received 23 January 2019 Received in revised form 26 March 2019 Accepted 28 March 2019 Available online 29 March 2019 Keywords: Imidazolium ionic liquids Pyrazolium ionic liquids Solvent effect ONIOM method Molecular dynamics
a b s t r a c t The mechanism for the cycloaddition of carbon dioxide and epoxides catalyzed by ionic liquids has been elucidated in lots of theoretical literature. It is well known that the calculated barrier height could not be compared with the activation energy. However, it is expected that the sequence of activity estimated by barrier height could be consistent with the experimental result. Actually, the calculated results are not accurate enough to be reliable. One of possible reasons is attributed to the improper treatment for the solvent effect. The solvent effect induced by ionic liquids is not carefully considered or neglected at all in most of previous theoretical studies. In this work, ionic liquids are really included in the catalytic system to consider the solvent effect by the ONIOM method along with the molecular dynamics simulations. When the solvent effect is considered, the theoretical reliability is greatly improved. More important, this model is suitable not only for non-protic ionic liquids but also for protic ones and not only for imidazolium ionic liquids but also for pyrazolium ionic liquids. The solvent effect aroused by epoxides is also considered by the same model. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Continuous emitted carbon dioxide (CO2) has resulted in great negative effect on the environment and human society [1,2]. On the positive side, CO2 is the abundant, non-expensive, and non-toxic C1 resource [3,4]. Catalytic chemical fixation of CO2 into propylene carbonate would be a promising method to transfer the CO2 into the valuable products [5–7]. In the past decade, great advances have been made to develop novel and high active catalysts for the coupling reaction of CO2 and propylene oxide (PO) [8–10]. Ionic liquids (ILs) stand out from various catalysts due to the high thermostability, negligible vapor pressure, and unlimited structures [11–13]. More importantly, the co-catalyst and additional organic solvent would not be required. Some popular ionic liquids include imidazolium ILs, ammonium ILs, and pyrazolium ILs [14–19]. Protic ILs are one of the most important subgroups of ionic liquids, however, less protic ILs have been applied to catalyze the cycloaddition reaction of CO2 and PO. Wu et al. [20] have synthesized some protic imidazolium ILs that present good catalytic activity for the coupling reaction of CO2 and PO. The influence of alkyl chain length substituted on imidazole ring is investigated as well as the effect of different anions and cations. According to the previous experience [21–23], the catalytic mechanism ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Zhang).
https://doi.org/10.1016/j.molliq.2019.03.165 0167-7322/© 2019 Elsevier B.V. All rights reserved.
follows the processes: first, the cation would activate the O atom of PO as electrophile. At the same time, the anion would activate the C atom of PO as nucleophile leading to the formation of energy enriches substrate; second, CO2 insertion; final, ring-closure to form the propylene carbonate (PC). It is well known for the coupling reaction of CO2 and PO promoted by imidazolium ILs. Certainly, it would also be suitable for the protic imidazolium ILs. However, there are two problems in previous studies resulting in the less reliability and inaccuracy of theoretical predicted results [21,23]. Only the catalytic effect of isolated cation and anion has been considered, however, the cooperation between cation and anion is almost omitted. In addition, the solvent effect aroused by ionic liquids is simply treated by solvent model developed for organic solvent or totally neglected. The difference among various ionic liquids is not considered. On the basis of aforementioned problems, a new “Double-IL” model is first proposed by us to consider how the interaction between cation and anion affect the catalytic activity [24,25]. Moreover, the employed ionic liquids should be included to consider the solvent effect rather than utilizing the solvent model. In this work, the catalytic mechanism of 1-Methylimidazolium bromide ([HMim]Br), 1-ethylimidazolium bromide ([HEim]Br), and 1butylimidazolium bromide ([HBim]Br) (See Scheme 1) would be theoretically studied. Moreover, They would be included in the respective catalytic system to consider the solvent effect by combination of density functional theory (DFT) and molecular dynamic simulations (MD). The reaction routes catalyzed by other five ionic liquids, i.e., 1-ethyl-3-
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Scheme 1. Schematic structures of [HMim]Br, [HEim]Br, and [HBim]Br with the key atoms labeled.
methyl imidazolium bromide ([Emim]Br) [26], 1-(2-hydroxyl-ethyl)-3methyl imidazolium chloride (HEMIMC) [27], 1,3,5trimethylpyrazolium bromide (HTMPzBr), 1,3-dimethyl-pyrazolium bromide (HMM3PzBr), and 1,5-dimethyl-pyrazolium bromide (HMM5PzBr) [28] would also be calculated by the same method to find the difference among them from the microscopic viewpoint. Besides the ionic liquids, the solvent effect of PO is also considered by the same model, which has never been reported in previous studies to our best knowledge. It is reasonable to expect that the theoretical tool would play more important role in screening out the catalysts with the further refinement of theoretical model.
the HF/3-21G level [39,40]. All the central QM species and eight ionic liquids (ten PO molecules) around QM region were optimized again in the ONIOM calculations. It should be noted that the effect of spin-orbit coupling (SOC) is not considered for Br− anion or CO2 ion. Although the influence of SOC is large for Br atom and CO2 ion even in the ground state [41–43]. It is more important for the spectrum properties or some dissociation pathway. In this work, the sequence is more critical than the absolute accurate values. The SOC effect is considered to be same for each route, which would not affect the comparison among them even if it is neglected.
2. Computational details
3. Results and discussion
2.1. Molecular dynamics simulations
3.1. Importance of synergetic effect of cation and anion
Molecular dynamics simulations were performed using the GROMACS 5.1.2 software package [29] with the General Amber Force Field (GAFF) [30] for both pure ionic liquids and complex system of ionic liquids and PO. In order to prove the applicability of GAFF force field, the simulated density should be comparable with the experimental value. Due to the absence of experimental density data for three reported protic imidazolium ILs, it is difficult to compare them. The density of a similar non-protic imidazolium ionic liquid, i.e., [Emim]Cl, was simulated with the value of 1.189 g cm−3 at 298 K under 0.1 MPa, which agrees well with the experimental result with the deviation of 0.25% indicating the reliability of GAFF. 256 pairs of ionic liquids or 256 PO molecules are put in a grid box with dimension 5 × 5 × 5 nm, respectively. After 10,000 energy minimization steps were performed using the steepest descent method, the system was equilibrated for 2 ns and then allowed to 5 ns production run with a timestep of 2 fs. The configuration is saved every 5000 timesteps. These MD trajectories ran under the isothermal and isobaric ensemble (NPT) at T = 298 K and P = 0.1 MPa, where the V-rescale thermostat and Parrinello-Rahman barostat were used with relaxation constants of 1.0 and 4.0 ps, respectively.
The catalytic mechanism of three protic imidazolium ILs are calculated at the B3PW91/6-31G(d,p) level following the classical threestep mechanism. Only the ring-opening step is plotted in Fig. 1 since it is the rate-determining step [44–46]. Both the electrophilic activation and nucleophilic activation result in the ring-opening of PO. Since the protic hydrogen atom presents the stronger acidity than other hydrogen atoms in cation, it is taken as the electrophile. The calculated barrier heights decrease in the sequence of TSS-HB (22.4 kcal/mol) N TSS-HE (22.27 kcal/mol) N TSS-HM (21.48 kcal/mol) (S indicating “Single-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) suggesting that the catalytic activity of [HMim]Br is the best and that of [HBim]Br is the least. The activity determined by the “SingleIL” model is consistent with the experimental result, i.e., the product yield of [HMim]Br (94.5%) N [HEim]Br (85.8%) N [HBim]Br (77.7%). However, the maximum difference is as small as 0.9 kcal/mol for three transition states, which falls into the acceptable computational error. Therefore, it is not reliable to predict the activity for a series of ionic liquids without experimental results. To improve the theoretical accuracy, the ring-opening step catalyzed by aforementioned three protic ionic liquids are also calculated by “Double-IL” model [24], in which the effect of interaction between cation and anion is included. There are various catalyzed routes due to the different electrophilic activation. Only protic hydrogen atom is taken as the electrophile since it presents more active than other hydrogen atoms. One situation is that both protic hydrogen atoms from two cations are employed as electrophile to activate the O atom of PO, i.e., Route D-HM-1 (See Fig. S1). The other situation is that one cation is utilized to activate the O atom of PO, while the other cation is utilized to stabilize the Br− anion, i.e., Route D-HM-2 and Route D-HM-3 (See Fig. S1). Other possible routes, which are taken other hydrogen atom as electrophile, are omitted because of the less activity. Route D-HM-3 is the most feasible pathway because of the smallest barrier height. The transition states catalyzed by [HEim]Br and [HBim]Br are also confirmed following the same ring-opening step. The potential energy profiles for routes D-HM (i.e., D-HM-3 in aforementioned section, the corresponding transition state is renamed by TSD-HM), D-HE, and D-HB (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) are plotted in Fig. 2 along with the schematic structures of corresponding transition states. The sequence of barrier heights calculated by “Double-IL” model is also the same with the experimental results. However, the difference between two transition states is not enlarged as
2.2. Quantum chemistry calculations The electronic calculations were performed using the Gaussian 09 program [31]. The geometric parameters of stationary points were optimized by Becke's three-parameter exact exchange functional combined with the Perdew and Wang (B3PW91) [32,33] method with the 6-31G (d,p) basis set [34]. Starting from the transition state, the minimum energy path (MEP) is constructed by intrinsic reaction coordinate (IRC) method, which is employed to determine that the transition state is connected with two desired minima, reactant and product or the corresponding intermediates [35]. On the basis of the optimized structures, the energies of three stationary points, including transition state and two minima, were corrected at the M06/6-311+G(2d,2p) level [36]. The solvent effect is estimated by the polarized continuum model (PCM) in ethyl ether (Et2O) [37,38]. Two-layer ONIOM (QM:MM) calculations were carried out for each selected structure in the Gaussian 09 program [31]. Because the MM region is often large but inexpensive to calculate, while the QM region is small but expensive, so the central QM species (where reaction occurs) were treated at the B3PW91/6-31G(d,p) level to ensure the accuracy of calculation, while the surrounding solvent molecules were treated at
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Fig. 1. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes S-HM, S-HE, and S-HM (S indicating “Single-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) calculated at the M06/6–311 + G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level.
compared with the results calculated by “Single-IL” model. It would be attributed to two possible factors. One is that the relative position of two ionic pairs are unreasonable. The pure [HMim]Br is simulated by MD method and the corresponding radial distribution functions of N1 atom in two [HMim]Br ion pairs is plotted in Fig. S2. The most feasible distance between two nitrogen atoms in two neighboring [HMim]Br ion pairs is 6.7 Å. In TSD-HM, the distance between two N1 atoms in two cations are 5.1 Å, which is consistent with the MD result. The relative position of two ionic pairs in TSD-HM is reliable. The other possible reason to arouse the inaccuracy is that the solvent effect is not treated accurately. The solvent effect is treated by the PCM model along with
ethyl ether as the solvent rather than in the environment of ionic liquids or PO. 3.2. Influence of solvent The ionic liquids are included to consider the influence of solvent on the catalytic activity. How many ionic liquids should be included in the catalytic system is the first problem deserved to be considered. The MD simulations testify that there are four [HMim]Br ion pairs around the TSD-HM. (See Fig. S3) Eight [HMim]Br ion pairs are taken as the solvent, which is enough to include the ionic liquids around the central catalytic
Fig. 2. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes D-HM, D-HE, and D-HB (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) calculated at the M06/6–311 + G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level.
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region. Other ionic liquids far away from the center region are neglected because of the less effect. Both transition state and solvent are optimized again including 150–240 atoms. The similar methods are employed to study other two routes, route D-HE and route D-HB. The corresponding potential energy profiles are plotted in Fig. 3 along with the schematic structures of transition states. The barrier heights are 5.45 kcal/mol for TSD-HM-SOL(HM), 9.16 kcal/mol for TSD-HESOL(HE), and 11.84 kcal/mol for TSD-HB-SOL(HB) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, while (HM), (HE) and (HB) indicating the corresponding ionic liquids are employed as solvent, respectively). The difference between two transition states is enlarged to be 2.68–3.71 kcal/mol, which is helpful to distinguish the catalytic activity. Since the PO is excess in the reaction system, it would also be taken as the solvent. The number of PO taken as the solvent is confirmed by the same method described in the above section. (See Fig. S4) Ten PO is considered since they are close to the central catalytic region. The calculated barrier heights are 4.45 kcal/mol for TSD-HM-SOL(PO), 8.35 kcal/mol for TSD-HE-SOL(PO), and 12.51 kcal/mol for TSD-HBSOL(PO) (See Fig. 4) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, PO indicating the propylene oxide is employed as solvent). It is similar with the result that ionic liquids are utilized as solvent except that the difference between two transition states is further enlarged. When the PO is taken as the solvent, the computational cost is less, which is deserved to be popularized. The barrier heights calculated by different models are plotted in Fig. 5 along with the corresponding experimental result. It is obvious that the results including the solvent effect are more close to the experimental result. 3.3. Comparison with other ionic liquids The “Double-IL” model along with the solvent effect has been successfully applied to judge the catalytic activity for a series of ionic liquids
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with similar structure in previous literature [24,45]. The catalytic activity for [Emim]Br, HEMIMC, and [HEim]Br is compared to uncover the suitability of “Double-IL” model for different catalysts. The corresponding calculated barrier heights are tabulated in Table 1, in which route DHEM-SOL(HEM) (i.e., route 1-D-SOL in the literature [46], the corresponding transition state is renamed by TSD-HEM-SOL(HEM) in this work) is cited from the literature [46] and route D-EM-SOL(EM) and route D-HE-SOL(HE) are calculated first time to compare with them (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, while (HM), (HE), and (HB) indicating the corresponding ionic liquids employed as solvent, respectively). The calculated barrier heights are 26.24 kcal/mol for TSD-EM-SOL(EM), 25.1 kcal/mol for TSD-HEM-SOL (HEM), and 9.16 kcal/mol for TSD-HE-SOL(HE). The corresponding experimental product yields are 52.7% for [Emim]Br, 78.0% for HEMIMC, and 85.8% for [HEim]Br. Although the catalytic activity determined by the sequence of barrier heights is the same with the experimental results. However, the barrier heights between TSD-EM-SOL (EM) and TSD-HEM-SOL(HEM) is too close to distinguish them. It is related with their different experimental conditions. The higher product yield for HEMIMC should be resulted by the enhancement of experimental conditions including reaction temperature, pressure, and amount of catalyst. If they are performed by the same condition, the product yield would be very close. The barrier height calculated by “Double-IL” model along with the solvent effect is suitable for both non-protic imidazolium ILs and protic imidazolium ILs. Since the pyrazolium ionic liquids have the similar structure with imidazolium ionic liquids, their catalytic activity would be probably compared with each other by “Double-IL” model along with solvent effect. The ring-opening of PO catalyzed by HTMPzBr, HMM3PzBr, and HMM5PzBr is also calculated by the same model, i.e., route D-HT-SOL (HT), route D-HM3-SOL(HM3) and route D-HM5-SOL(HM5). The barrier height of route D-HM3-SOL(HM3) is the higher than that of route D-HTSOL(HT) suggesting the less catalytic activity for the HMM3PzBr. Moreover, the barrier height of TSD-HT-SOL(HT) (16.46 kcal/mol) is the
Fig. 3. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes D-HM-SOL(HM), route D-HE-SOL(HE) and route D-HB-SOL(HB) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, while (HM), (HE) and (HB) indicating the corresponding ionic liquids are employed as solvent, respectively) calculated by the ONIOM (B3PW91/6-31G(d,p):HF/3-21G) method.
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Fig. 4. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes D-HM-SOL(PO), route D-HE-SOL(PO) and route D-HB-SOL(PO) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, PO indicating the propylene oxide is employed as solvent) calculated by the ONIOM (B3PW91/6-31G(d,p):HF/3-21G) method.
higher than that of TSD-HM5-SOL(HM5) (9.24 kcal/mol), which is consistent with the experimental results. (See Table 1) The protic pyrazolium ionic liquids, HTMPzBr, HMM3PzBr, and HMM5PzBr, are theoretically designed and its catalytic performance is evaluated by the “Double-IL” model without solvent effect [28]. Then, they are synthesized and applied to catalyze the coupling reaction of CO2 and PO. The good agreement between theoretical and experimental result indicates the possibility to apply theoretical screening out before experiment. However, it is only applied to the ionic liquids with the same cation and different substituted alkyl group. As compared with the imidazolium ionic liquids, the “Double-IL” model including solvent effect could be successfully applied to the ionic liquids with similar cations, such as, HMM5PzBr and [HEim]Br. They have the similar barrier
heights, which is consistent with their close product yields. Note that although the experimental results are performed in different conditions, the ONIOM could still discover the catalytic difference for various ionic liquids. 4. Conclusion The mechanism of cycloaddition of CO2 with PO catalyzed by three protic imidazolium ILs, [HMim]Br, [HEim]Br, and [HBim]Br, is theoretically investigated by three different models including “Single-IL” model, “Double-IL” model, and “Double-IL” model along with the solvent effect. The barrier heights calculated by three models agree well with the experimental product yields. However, the difference between
Fig. 5. The barrier heights (in kcal/mol) calculated by three models and experimental yields.(ONIOM(IL) model indicating the “Double-IL” model along with the solvent effect and the corresponding ionic liquids are employed as solvent; ONIOM(PO) model indicating the “Double-IL” model along with the solvent effect and the corresponding propylene oxide are employed as solvent).
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Table 1 The barrier heights (in kcal/mol) calculated at B3PW91/6-31G(d,p):HF/3-21G level and experimental yields. Entry
Catalyst structure
Name
Catalytic conditions
Yield/%
Route
Barrier height (kcal/mol)
1
[Emim]Br
1 mol%, 120 °C, 1.5 MPa, 1.5 h
52.7 [26]
route D-EM-SOL(EM)
26.24
2
HEMIMC
1.6 mol%, 125 °C, CO2: 2 MPa, 1 h
78 [17]
route D-HEM-SOL(HEM)
25.1
3
[HEim]Br
1 mol%, 120 °C,1.5 MPa, 2 h
85.8 [20]
route D-HE-SOL(HE)
9.16
4
HTMPzBr
1 mol%, 130 °C,1.5 MPa, 4 h
83.1 [27]
route D-HT-SOL(HT)
16.46
5
HMM3PzBr
1 mol%, 130 °C, 1.5 MPa, 4 h
65 [27]
route D-HM3-SOL(HM3)
21.55
6
HMM5PzBr
1 mol%, 130 °C, 1.5 MPa, 4 h
86.2 [27]
route D-HM5-SOL(HM5)
9.24
two catalysts is too small to distinguish them for calculated results by “Single-IL” model and “Double-IL” model. When the solvent effect induced by ionic liquids is considered by ONIOM method, the calculated barrier heights are easier to be distinct with the enough difference among them. The suitability of ONIOM method is tested for other five ionic liquids including non-protic imidazolium ILs and protic pyrazolium ILs. Even when the experiments are performed under different reaction conditions, the relative sequence determined by ONIOM model is also reliable. It is very important to include the ionic liquids as solvent to calculate the barrier height rather than utilizing the simple solvent model.
Acknowledgements We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) and the National Supercomputing Center in Changsha (Changsha Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21476061, 21503069, 21676071). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.03.165.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Effect of ionic liquids clusters microenvironment on cycloaddition reaction of carbon dioxide Danning Zheng, Pan Ning, Jiamin Jiang, Fang Liu, Li Wang ⁎, Jinglai Zhang ⁎ Henan Provincial Engineering Research Center of Corrosion and Protection for Magnesium Alloys, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China
a r t i c l e
i n f o
Article history: Received 23 January 2019 Received in revised form 26 March 2019 Accepted 28 March 2019 Available online 29 March 2019 Keywords: Imidazolium ionic liquids Pyrazolium ionic liquids Solvent effect ONIOM method Molecular dynamics
a b s t r a c t The mechanism for the cycloaddition of carbon dioxide and epoxides catalyzed by ionic liquids has been elucidated in lots of theoretical literature. It is well known that the calculated barrier height could not be compared with the activation energy. However, it is expected that the sequence of activity estimated by barrier height could be consistent with the experimental result. Actually, the calculated results are not accurate enough to be reliable. One of possible reasons is attributed to the improper treatment for the solvent effect. The solvent effect induced by ionic liquids is not carefully considered or neglected at all in most of previous theoretical studies. In this work, ionic liquids are really included in the catalytic system to consider the solvent effect by the ONIOM method along with the molecular dynamics simulations. When the solvent effect is considered, the theoretical reliability is greatly improved. More important, this model is suitable not only for non-protic ionic liquids but also for protic ones and not only for imidazolium ionic liquids but also for pyrazolium ionic liquids. The solvent effect aroused by epoxides is also considered by the same model. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Continuous emitted carbon dioxide (CO2) has resulted in great negative effect on the environment and human society [1,2]. On the positive side, CO2 is the abundant, non-expensive, and non-toxic C1 resource [3,4]. Catalytic chemical fixation of CO2 into propylene carbonate would be a promising method to transfer the CO2 into the valuable products [5–7]. In the past decade, great advances have been made to develop novel and high active catalysts for the coupling reaction of CO2 and propylene oxide (PO) [8–10]. Ionic liquids (ILs) stand out from various catalysts due to the high thermostability, negligible vapor pressure, and unlimited structures [11–13]. More importantly, the co-catalyst and additional organic solvent would not be required. Some popular ionic liquids include imidazolium ILs, ammonium ILs, and pyrazolium ILs [14–19]. Protic ILs are one of the most important subgroups of ionic liquids, however, less protic ILs have been applied to catalyze the cycloaddition reaction of CO2 and PO. Wu et al. [20] have synthesized some protic imidazolium ILs that present good catalytic activity for the coupling reaction of CO2 and PO. The influence of alkyl chain length substituted on imidazole ring is investigated as well as the effect of different anions and cations. According to the previous experience [21–23], the catalytic mechanism ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (J. Zhang).
https://doi.org/10.1016/j.molliq.2019.03.165 0167-7322/© 2019 Elsevier B.V. All rights reserved.
follows the processes: first, the cation would activate the O atom of PO as electrophile. At the same time, the anion would activate the C atom of PO as nucleophile leading to the formation of energy enriches substrate; second, CO2 insertion; final, ring-closure to form the propylene carbonate (PC). It is well known for the coupling reaction of CO2 and PO promoted by imidazolium ILs. Certainly, it would also be suitable for the protic imidazolium ILs. However, there are two problems in previous studies resulting in the less reliability and inaccuracy of theoretical predicted results [21,23]. Only the catalytic effect of isolated cation and anion has been considered, however, the cooperation between cation and anion is almost omitted. In addition, the solvent effect aroused by ionic liquids is simply treated by solvent model developed for organic solvent or totally neglected. The difference among various ionic liquids is not considered. On the basis of aforementioned problems, a new “Double-IL” model is first proposed by us to consider how the interaction between cation and anion affect the catalytic activity [24,25]. Moreover, the employed ionic liquids should be included to consider the solvent effect rather than utilizing the solvent model. In this work, the catalytic mechanism of 1-Methylimidazolium bromide ([HMim]Br), 1-ethylimidazolium bromide ([HEim]Br), and 1butylimidazolium bromide ([HBim]Br) (See Scheme 1) would be theoretically studied. Moreover, They would be included in the respective catalytic system to consider the solvent effect by combination of density functional theory (DFT) and molecular dynamic simulations (MD). The reaction routes catalyzed by other five ionic liquids, i.e., 1-ethyl-3-
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Scheme 1. Schematic structures of [HMim]Br, [HEim]Br, and [HBim]Br with the key atoms labeled.
methyl imidazolium bromide ([Emim]Br) [26], 1-(2-hydroxyl-ethyl)-3methyl imidazolium chloride (HEMIMC) [27], 1,3,5trimethylpyrazolium bromide (HTMPzBr), 1,3-dimethyl-pyrazolium bromide (HMM3PzBr), and 1,5-dimethyl-pyrazolium bromide (HMM5PzBr) [28] would also be calculated by the same method to find the difference among them from the microscopic viewpoint. Besides the ionic liquids, the solvent effect of PO is also considered by the same model, which has never been reported in previous studies to our best knowledge. It is reasonable to expect that the theoretical tool would play more important role in screening out the catalysts with the further refinement of theoretical model.
the HF/3-21G level [39,40]. All the central QM species and eight ionic liquids (ten PO molecules) around QM region were optimized again in the ONIOM calculations. It should be noted that the effect of spin-orbit coupling (SOC) is not considered for Br− anion or CO2 ion. Although the influence of SOC is large for Br atom and CO2 ion even in the ground state [41–43]. It is more important for the spectrum properties or some dissociation pathway. In this work, the sequence is more critical than the absolute accurate values. The SOC effect is considered to be same for each route, which would not affect the comparison among them even if it is neglected.
2. Computational details
3. Results and discussion
2.1. Molecular dynamics simulations
3.1. Importance of synergetic effect of cation and anion
Molecular dynamics simulations were performed using the GROMACS 5.1.2 software package [29] with the General Amber Force Field (GAFF) [30] for both pure ionic liquids and complex system of ionic liquids and PO. In order to prove the applicability of GAFF force field, the simulated density should be comparable with the experimental value. Due to the absence of experimental density data for three reported protic imidazolium ILs, it is difficult to compare them. The density of a similar non-protic imidazolium ionic liquid, i.e., [Emim]Cl, was simulated with the value of 1.189 g cm−3 at 298 K under 0.1 MPa, which agrees well with the experimental result with the deviation of 0.25% indicating the reliability of GAFF. 256 pairs of ionic liquids or 256 PO molecules are put in a grid box with dimension 5 × 5 × 5 nm, respectively. After 10,000 energy minimization steps were performed using the steepest descent method, the system was equilibrated for 2 ns and then allowed to 5 ns production run with a timestep of 2 fs. The configuration is saved every 5000 timesteps. These MD trajectories ran under the isothermal and isobaric ensemble (NPT) at T = 298 K and P = 0.1 MPa, where the V-rescale thermostat and Parrinello-Rahman barostat were used with relaxation constants of 1.0 and 4.0 ps, respectively.
The catalytic mechanism of three protic imidazolium ILs are calculated at the B3PW91/6-31G(d,p) level following the classical threestep mechanism. Only the ring-opening step is plotted in Fig. 1 since it is the rate-determining step [44–46]. Both the electrophilic activation and nucleophilic activation result in the ring-opening of PO. Since the protic hydrogen atom presents the stronger acidity than other hydrogen atoms in cation, it is taken as the electrophile. The calculated barrier heights decrease in the sequence of TSS-HB (22.4 kcal/mol) N TSS-HE (22.27 kcal/mol) N TSS-HM (21.48 kcal/mol) (S indicating “Single-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) suggesting that the catalytic activity of [HMim]Br is the best and that of [HBim]Br is the least. The activity determined by the “SingleIL” model is consistent with the experimental result, i.e., the product yield of [HMim]Br (94.5%) N [HEim]Br (85.8%) N [HBim]Br (77.7%). However, the maximum difference is as small as 0.9 kcal/mol for three transition states, which falls into the acceptable computational error. Therefore, it is not reliable to predict the activity for a series of ionic liquids without experimental results. To improve the theoretical accuracy, the ring-opening step catalyzed by aforementioned three protic ionic liquids are also calculated by “Double-IL” model [24], in which the effect of interaction between cation and anion is included. There are various catalyzed routes due to the different electrophilic activation. Only protic hydrogen atom is taken as the electrophile since it presents more active than other hydrogen atoms. One situation is that both protic hydrogen atoms from two cations are employed as electrophile to activate the O atom of PO, i.e., Route D-HM-1 (See Fig. S1). The other situation is that one cation is utilized to activate the O atom of PO, while the other cation is utilized to stabilize the Br− anion, i.e., Route D-HM-2 and Route D-HM-3 (See Fig. S1). Other possible routes, which are taken other hydrogen atom as electrophile, are omitted because of the less activity. Route D-HM-3 is the most feasible pathway because of the smallest barrier height. The transition states catalyzed by [HEim]Br and [HBim]Br are also confirmed following the same ring-opening step. The potential energy profiles for routes D-HM (i.e., D-HM-3 in aforementioned section, the corresponding transition state is renamed by TSD-HM), D-HE, and D-HB (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) are plotted in Fig. 2 along with the schematic structures of corresponding transition states. The sequence of barrier heights calculated by “Double-IL” model is also the same with the experimental results. However, the difference between two transition states is not enlarged as
2.2. Quantum chemistry calculations The electronic calculations were performed using the Gaussian 09 program [31]. The geometric parameters of stationary points were optimized by Becke's three-parameter exact exchange functional combined with the Perdew and Wang (B3PW91) [32,33] method with the 6-31G (d,p) basis set [34]. Starting from the transition state, the minimum energy path (MEP) is constructed by intrinsic reaction coordinate (IRC) method, which is employed to determine that the transition state is connected with two desired minima, reactant and product or the corresponding intermediates [35]. On the basis of the optimized structures, the energies of three stationary points, including transition state and two minima, were corrected at the M06/6-311+G(2d,2p) level [36]. The solvent effect is estimated by the polarized continuum model (PCM) in ethyl ether (Et2O) [37,38]. Two-layer ONIOM (QM:MM) calculations were carried out for each selected structure in the Gaussian 09 program [31]. Because the MM region is often large but inexpensive to calculate, while the QM region is small but expensive, so the central QM species (where reaction occurs) were treated at the B3PW91/6-31G(d,p) level to ensure the accuracy of calculation, while the surrounding solvent molecules were treated at
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Fig. 1. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes S-HM, S-HE, and S-HM (S indicating “Single-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) calculated at the M06/6–311 + G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level.
compared with the results calculated by “Single-IL” model. It would be attributed to two possible factors. One is that the relative position of two ionic pairs are unreasonable. The pure [HMim]Br is simulated by MD method and the corresponding radial distribution functions of N1 atom in two [HMim]Br ion pairs is plotted in Fig. S2. The most feasible distance between two nitrogen atoms in two neighboring [HMim]Br ion pairs is 6.7 Å. In TSD-HM, the distance between two N1 atoms in two cations are 5.1 Å, which is consistent with the MD result. The relative position of two ionic pairs in TSD-HM is reliable. The other possible reason to arouse the inaccuracy is that the solvent effect is not treated accurately. The solvent effect is treated by the PCM model along with
ethyl ether as the solvent rather than in the environment of ionic liquids or PO. 3.2. Influence of solvent The ionic liquids are included to consider the influence of solvent on the catalytic activity. How many ionic liquids should be included in the catalytic system is the first problem deserved to be considered. The MD simulations testify that there are four [HMim]Br ion pairs around the TSD-HM. (See Fig. S3) Eight [HMim]Br ion pairs are taken as the solvent, which is enough to include the ionic liquids around the central catalytic
Fig. 2. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes D-HM, D-HE, and D-HB (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br) calculated at the M06/6–311 + G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level.
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region. Other ionic liquids far away from the center region are neglected because of the less effect. Both transition state and solvent are optimized again including 150–240 atoms. The similar methods are employed to study other two routes, route D-HE and route D-HB. The corresponding potential energy profiles are plotted in Fig. 3 along with the schematic structures of transition states. The barrier heights are 5.45 kcal/mol for TSD-HM-SOL(HM), 9.16 kcal/mol for TSD-HESOL(HE), and 11.84 kcal/mol for TSD-HB-SOL(HB) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, while (HM), (HE) and (HB) indicating the corresponding ionic liquids are employed as solvent, respectively). The difference between two transition states is enlarged to be 2.68–3.71 kcal/mol, which is helpful to distinguish the catalytic activity. Since the PO is excess in the reaction system, it would also be taken as the solvent. The number of PO taken as the solvent is confirmed by the same method described in the above section. (See Fig. S4) Ten PO is considered since they are close to the central catalytic region. The calculated barrier heights are 4.45 kcal/mol for TSD-HM-SOL(PO), 8.35 kcal/mol for TSD-HE-SOL(PO), and 12.51 kcal/mol for TSD-HBSOL(PO) (See Fig. 4) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, PO indicating the propylene oxide is employed as solvent). It is similar with the result that ionic liquids are utilized as solvent except that the difference between two transition states is further enlarged. When the PO is taken as the solvent, the computational cost is less, which is deserved to be popularized. The barrier heights calculated by different models are plotted in Fig. 5 along with the corresponding experimental result. It is obvious that the results including the solvent effect are more close to the experimental result. 3.3. Comparison with other ionic liquids The “Double-IL” model along with the solvent effect has been successfully applied to judge the catalytic activity for a series of ionic liquids
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with similar structure in previous literature [24,45]. The catalytic activity for [Emim]Br, HEMIMC, and [HEim]Br is compared to uncover the suitability of “Double-IL” model for different catalysts. The corresponding calculated barrier heights are tabulated in Table 1, in which route DHEM-SOL(HEM) (i.e., route 1-D-SOL in the literature [46], the corresponding transition state is renamed by TSD-HEM-SOL(HEM) in this work) is cited from the literature [46] and route D-EM-SOL(EM) and route D-HE-SOL(HE) are calculated first time to compare with them (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, while (HM), (HE), and (HB) indicating the corresponding ionic liquids employed as solvent, respectively). The calculated barrier heights are 26.24 kcal/mol for TSD-EM-SOL(EM), 25.1 kcal/mol for TSD-HEM-SOL (HEM), and 9.16 kcal/mol for TSD-HE-SOL(HE). The corresponding experimental product yields are 52.7% for [Emim]Br, 78.0% for HEMIMC, and 85.8% for [HEim]Br. Although the catalytic activity determined by the sequence of barrier heights is the same with the experimental results. However, the barrier heights between TSD-EM-SOL (EM) and TSD-HEM-SOL(HEM) is too close to distinguish them. It is related with their different experimental conditions. The higher product yield for HEMIMC should be resulted by the enhancement of experimental conditions including reaction temperature, pressure, and amount of catalyst. If they are performed by the same condition, the product yield would be very close. The barrier height calculated by “Double-IL” model along with the solvent effect is suitable for both non-protic imidazolium ILs and protic imidazolium ILs. Since the pyrazolium ionic liquids have the similar structure with imidazolium ionic liquids, their catalytic activity would be probably compared with each other by “Double-IL” model along with solvent effect. The ring-opening of PO catalyzed by HTMPzBr, HMM3PzBr, and HMM5PzBr is also calculated by the same model, i.e., route D-HT-SOL (HT), route D-HM3-SOL(HM3) and route D-HM5-SOL(HM5). The barrier height of route D-HM3-SOL(HM3) is the higher than that of route D-HTSOL(HT) suggesting the less catalytic activity for the HMM3PzBr. Moreover, the barrier height of TSD-HT-SOL(HT) (16.46 kcal/mol) is the
Fig. 3. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes D-HM-SOL(HM), route D-HE-SOL(HE) and route D-HB-SOL(HB) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, while (HM), (HE) and (HB) indicating the corresponding ionic liquids are employed as solvent, respectively) calculated by the ONIOM (B3PW91/6-31G(d,p):HF/3-21G) method.
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Fig. 4. Potential energy profiles and schematic structures of transition states for the ring-opening step along routes D-HM-SOL(PO), route D-HE-SOL(PO) and route D-HB-SOL(PO) (D indicating “Double-IL” model, HM indicating [HMim]Br, HE indicating [HEim]Br, HB indicating [HBim]Br, SOL indicating solvent, PO indicating the propylene oxide is employed as solvent) calculated by the ONIOM (B3PW91/6-31G(d,p):HF/3-21G) method.
higher than that of TSD-HM5-SOL(HM5) (9.24 kcal/mol), which is consistent with the experimental results. (See Table 1) The protic pyrazolium ionic liquids, HTMPzBr, HMM3PzBr, and HMM5PzBr, are theoretically designed and its catalytic performance is evaluated by the “Double-IL” model without solvent effect [28]. Then, they are synthesized and applied to catalyze the coupling reaction of CO2 and PO. The good agreement between theoretical and experimental result indicates the possibility to apply theoretical screening out before experiment. However, it is only applied to the ionic liquids with the same cation and different substituted alkyl group. As compared with the imidazolium ionic liquids, the “Double-IL” model including solvent effect could be successfully applied to the ionic liquids with similar cations, such as, HMM5PzBr and [HEim]Br. They have the similar barrier
heights, which is consistent with their close product yields. Note that although the experimental results are performed in different conditions, the ONIOM could still discover the catalytic difference for various ionic liquids. 4. Conclusion The mechanism of cycloaddition of CO2 with PO catalyzed by three protic imidazolium ILs, [HMim]Br, [HEim]Br, and [HBim]Br, is theoretically investigated by three different models including “Single-IL” model, “Double-IL” model, and “Double-IL” model along with the solvent effect. The barrier heights calculated by three models agree well with the experimental product yields. However, the difference between
Fig. 5. The barrier heights (in kcal/mol) calculated by three models and experimental yields.(ONIOM(IL) model indicating the “Double-IL” model along with the solvent effect and the corresponding ionic liquids are employed as solvent; ONIOM(PO) model indicating the “Double-IL” model along with the solvent effect and the corresponding propylene oxide are employed as solvent).
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Table 1 The barrier heights (in kcal/mol) calculated at B3PW91/6-31G(d,p):HF/3-21G level and experimental yields. Entry
Catalyst structure
Name
Catalytic conditions
Yield/%
Route
Barrier height (kcal/mol)
1
[Emim]Br
1 mol%, 120 °C, 1.5 MPa, 1.5 h
52.7 [26]
route D-EM-SOL(EM)
26.24
2
HEMIMC
1.6 mol%, 125 °C, CO2: 2 MPa, 1 h
78 [17]
route D-HEM-SOL(HEM)
25.1
3
[HEim]Br
1 mol%, 120 °C,1.5 MPa, 2 h
85.8 [20]
route D-HE-SOL(HE)
9.16
4
HTMPzBr
1 mol%, 130 °C,1.5 MPa, 4 h
83.1 [27]
route D-HT-SOL(HT)
16.46
5
HMM3PzBr
1 mol%, 130 °C, 1.5 MPa, 4 h
65 [27]
route D-HM3-SOL(HM3)
21.55
6
HMM5PzBr
1 mol%, 130 °C, 1.5 MPa, 4 h
86.2 [27]
route D-HM5-SOL(HM5)
9.24
two catalysts is too small to distinguish them for calculated results by “Single-IL” model and “Double-IL” model. When the solvent effect induced by ionic liquids is considered by ONIOM method, the calculated barrier heights are easier to be distinct with the enough difference among them. The suitability of ONIOM method is tested for other five ionic liquids including non-protic imidazolium ILs and protic pyrazolium ILs. Even when the experiments are performed under different reaction conditions, the relative sequence determined by ONIOM model is also reliable. It is very important to include the ionic liquids as solvent to calculate the barrier height rather than utilizing the simple solvent model.
Acknowledgements We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) and the National Supercomputing Center in Changsha (Changsha Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21476061, 21503069, 21676071). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.03.165.
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