Protic Quaternary Ammonium Ionic Liquids for Catalytic Conversion of

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Kinetics, Catalysis, and Reaction Engineering

Protic quaternary ammonium ionic liquids for catalytic conversion of CO2 into cyclic carbonates: A combined ab initio and MD study Huiqing Yang, Danning Zheng, Jingshun Zhang, Ke Chen, Junfeng Li, Li Wang, Jinglai Zhang, Hong-Yan He, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01148 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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Industrial & Engineering Chemistry Research

Protic quaternary ammonium ionic liquids for catalytic conversion of CO2 into cyclic carbonates: A combined ab initio and MD study Huiqing Yanga, Danning Zhenga, Jingshun Zhanga, Ke Chena, Junfeng Lia,b, Li Wanga*, Jinglai Zhanga* a

College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R. China

b

Division of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Techn ology, SE-106 91 Stockholm, Sweden

Hongyan Hec, Suojiang Zhangc† c

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

Abstract The mechanism of CO2 fixation catalyzed by protic hydroxyl-functionalized quaternary ammonium ionic liquids (ILs) is investigated by two different models, Single-IL model and Double-IL model. The relative sequence of catalytic activity calculated by Single-IL model is contradictory with the experimental result. The situation is totally varied when Double-IL model is utilized. In this system, ILs are not limited to the catalyst but solvent. The ILs are incorporated into the catalytic system to consider the solvent effect rather than by the existed solvent model. When the solvent effect is included, it is better to distinguish the catalytic activity of three ILs. *

Corresponding author E-mail: [email protected] Corresponding author E-mail: [email protected] † Corresponding author E-mail: [email protected] 1 *

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

According to the non-covalent interaction and the atoms in molecules analysis, the highest catalytic activity of tris(2-hydroxyethyl)ammonium bromide ([HTEA]Br) is attributed to its strongest nucleophilic attack and moderate hydrogen bond interaction between IL and reactant. It is necessary to consider the interaction between ILs to get a reliable result. Moreover, the solvent effect aroused by ILs should be carefully considered.

Keywords: protic quaternary ammonium ionic liquid; ONIOM; hydrogen bond; molecular dynamics; solvent effect

1. Introduction As one of the most classic and hot topics, catalysts have attracted extensive attentions from both academic and industrial communities. The involvement of catalyst would vary the reaction mechanism, activate the substrate, and others with the ultimate goal to promote the reaction. Taken the coupling reaction of carbon dioxide (CO2) and epoxides as an example, it is difficult to be performed in benign condition since the C atom in CO2 is at the highest oxidation state.1 However, this reaction is still one of the most efficient pathways to accomplish the chemical fixation of CO2, which is attributed to the following distinct features, such as, ameliorating the negative influence of CO2 on the atmosphere, exploring new abundant and cheap C1 resources, negligible by-product, and others.2-4 Various catalysts have been developed in past two decades to promote the title reaction including metal oxides,5 zeolitic 2


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imidazolate

frameworks

(ZIFs),

metal-organic

frameworks

(MOFs),6,7

melamine-derived graphitic carbon nitrides,8 and functional polymers.9 Although the reaction is promoted in different extent, there are some common deficiencies, such as, harsh reaction condition, requirement of organic solvent and co-catalyst, and introduction of new pollution. As compared with previous catalysts, especially for catalysts including metal, the pollution caused by ionic liquids (ILs) has been greatly reduced. The negligible vapor pressure of ionic liquids would be regarded as no contribution to air pollution.10 Moreover, additional solvent is not necessary since ionic liquids are the excellent solvent for most of organic and inorganic compounds.11-13 Certainly, toxicity and pollution are inevitable to synthesize the ionic liquids. Other green catalysts with less pollution are expected to be developed in the future. However, ionic liquids are still a good choice among various catalysts. Consequently, the available investigations on ionic liquids to be catalyst have increased exponentially from traditional ionic liquids to task-functionalized ionic liquids,14,15 including tetrabutyl ammonium bromide (TBAB),

hydroxyethyltriethyl

ammonium

bromide

(NEt3(HE)Br),

and

1-(2-hydroxyl-ethyl)-3-methylimidazolium bromide (HEMIMB).16 The catalytic efficiency is refreshed step by step. Normally, the product yields would be higher than 98% with the selectivity larger than 99% under the following reaction condition (PO: 0.2 mol, catalyst: 3.2 mmol, temperature: 125 ℃, CO2 pressure: 2.0 MPa, reaction time: 1 h). Besides the experimental investigations, the coupling reaction of CO2 and epoxides catalyzed by ionic liquids has also achieved the focus from theoretical 3



viewpoint. As early as 2007, its mechanism catalyzed by alkylmethylimidazolium chlorine ([Cnmim]Cl, n = 2, 4, and 6) was uncovered by Zhang et al..17 The three-step mechanism, ring-opening of PO, CO2 insertion, and ring-closure, is confirmed. Due to the more acidity of hydrogen atom in hydroxyl group, hydroxyl-functionalized imidazolium ionic liquids present the better activity than imidazolium ionic liquids without functional group.18 Except for the pure theoretical study, involvement of theoretical study in the experimental investigation has also become a popular trend.19-23 However, there are two common problems in almost all previous theoretical studies. One is that the prediction for catalytic activity is not reliable, which is attributed to the ignorance of interactions between ionic liquids. Last year, a new Double-IL model was built by us to successfully predict the activity for a series of hydroxyl-functionalized quaternary ammonium ILs.24 The other is that the solvent effect aroused by ILs is not sufficiently considered. The solvent model included in the commercial program is suitable for the organic medium rather than ionic liquids. The ONIOM method is employed to treat this problem accompanied with by molecular dynamic (MD) simulation.25 The ionic liquids would be treated as the solvent to consider their influence on the central catalytic region. According to the conclusion reported in previous literature, the more active hydrogen atom included in the ionic liquid is favorable to promoting the ring-opening of epoxides leading to the more energy-rich substrate. Consequently, protic ionic liquids have one more active proton in ionic liquids, which is expected to be more 4


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efficient to promote the ring-opening of epoxides than inert ionic liquids. However, fewer studies have been performed to explore the protic ionic liquids, especially for the task-functionalized protic ionic liquids. Inspired by Zhang’s work, three protic hydroxyl-functionalized

quaternary

ammonium

N,N-diethyl-2-hydroxyethanaminium

iodide

N-ethyl-2-hydroxy-N-(2-hydroxyethyl)ethanaminium

iodide

ionic

liquids, ([HMEA]I),

([HDEA]I),

and

tris(2-hydroxyethyl)ammonium iodide ([HTEA]I), are synthesized by Sun et al. with excellent catalytic activity for the coupling reaction of CO2 and propylene oxide (PO) (See Scheme 1).26 Owing to the appearance of one more active proton, the catalytic mechanism of [HMEA]I is more complicated than the hydroxyethyltriethyl ammonium bromide (NEt3(HE)Br). More possible routes and more promoting method should be considered. In this work, the mechanism of coupling reaction of CO2 and PO catalyzed by N,N-diethyl-2-hydroxyethanaminium

bromide

N-ethyl-2-hydroxy-N-(2-hydroxyethyl)ethanaminium

bromide

([HMEA]Br), ([HDEA]Br), and

tris(2-hydroxyethyl)ammonium bromide ([HTEA]Br) is investigated by both Single-IL and Double-IL models (Single-IL model means that one IL involves in the catalytic mechanism. Double-IL model means that two ILs involves in the catalytic mechanism.) to compare the difference between them. Furthermore, the effect of ionic liquids as solvent is considered by joint of ONIOM method and MD simulation. It is expected that the accuracy of predicted activity is favorable to design ILs with the desired properties. 5



2. Computational details 2.1. Quantum chemistry calculations The structures of Single-IL model and Double-IL model including reactants, products, intermediates, and transition states were performed by the B3PW91 method27,28 combined with the 6-31G(d,p) basis set.29 All species were fully optimized and their structures were confirmed by the existence of a single (for transition state structure) or none (for equilibrium structure) imaginary frequency via vibrational analysis. Zero-point energy (ZPE) corrections were taken into account in the calculation of the energy barrier. The minimum-energy path (MEP) was obtained by the intrinsic reaction coordinate (IRC) method30 starting from the transition state. The energies were corrected at the M06/6-311+G(2d,2p) level29,31 based on the optimized geometries. And the solvent effect was taken into consideration by the polarized continuum model (PCM)32,33 in ethyl ether (Et2O) solvent. The dielectric constant of Et2O is similar to that of epoxide that plays a role as solvent, which is used in several previous literature.34,35 The 6-311G(d) basis set36 was employed for the optimization of I- anion, while the 6-311G(d,p) basis set36 was utilized to correct the energy. Aforementioned electronic calculations were performed out in the Gaussian 09 program.37 2.2. Molecular Dynamics Simulations All the simulations reported here were performed using the GROMACS 5.1.2 software package.38 The simulation details were similar to those described in our previous work.39 The GAFF force field40 was used to describe the ILs. The density of 6


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tetrabutylammonium bromide (TBABr) is calculated to be 1.048 g/cm-3 at 298 K and 0.1 MPa, which is in good accordance with the corresponding experimental value of 1.039 g/cm-3 with an error of 0.87%.24 Therefore, it is reasonable to believe that the GAFF force field is also sutiable to describe the ILs although it is developed for the protein. Initially, an IL mixture of 256 ion pairs was randomly prepared in a cubic box of approximate 5×5×5 nm. After that, 10,000 energy minimization steps were performed using the steepest descent method and then allowed to equilibrate for 2 ns. The production simulations were carried out for 5 ns, using a timestep of 2 fs and saving a configuration every 5000 timesteps. All simulations were conducted in the isothermal–isobaric ensemble (NPT) at 1 bar using a V-rescale thermostat and Parrinello-Rahman barostat with relaxation times of 1 and 4 ps, respectively. In addition to the simulation for pure ion pairs, the MD simulation was performed to obtain the solvated structures. For each transition state and its corresponding intermediate optimized in the gas phase by QM method, 256 pairs of surrounding ILs were added respectively to build MD structures. The simulation steps were the same as the MD simulation for pure ILs. In this simulation, the central QM structures were fixed and only 256 pairs of cations and anions were allowed to move. The MD simulations were run for 2 ns equilibrium and 5 ns production, respectively. After that, the solvated structures were extracted from MD simulations. The central QM species and six closest neighbor ionic liquid pairs were involved in a solvated structure. Since the radial distribution function from MD simulations showed a coordination number of 4, 4, and 3 for cations and anions of [HMEA]Br, [HDEA]Br, 7



and [HTEA]Br, respectively (See Fig. S1), six closest IL pairs were enough to include the first solvation shell and provide a reasonable solvation environment for the central QM species for further calculations. To have a more reliable initial guess, six solvated molecules were extracted from MD simulations’ 5 ns production session for further ONIOM calculations. 2.3. ONIOM calculations The ONIOM calculations were carried out for each solvated structure in the Gaussian 09 program.37 In these calculations, the central QM species (where reaction occurs) were treated at the B3PW91/6-31G(d,p) level, while the surrounding solvent molecules were treated at the HF/3-21G41,42 level. The central QM species were optimized again in the ONIOM calculations with six ion pairs around. 3. Results and discussion On the basis of the previous study, the hydrogen atom with more acidity in cation would play the more important role in activating the O atom of PO to promote the ring-opening, such as the H atom in hydroxyl group and the H atom in C2 position in imidazole ring. Certainly, other hydrogen atoms, H atom in -CH2 group and H atom in -CH3 group, would also have a possibility to be taken as the electrophiles. They are not considered because of the less activity. Since there is one more active hydrogen atom in protic [HMEA]Br, the mechanism is different from hydroxyethyltriethyl ammonium bromide (NEt3(HE)Br). It is considered again by both Single-IL model and Double-IL model. 3.1. Single-IL model 8


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The protic H atom and the H atom in hydroxyl group would not only activate the O atom of PO but also stabilize the anion via the hydrogen bond. As a result, three different routes are confirmed. One is that both protic H atom and H atom in hydroxyl group activate the PO (See Table S1, route S-SOH-1, S indicating Single-IL model, SOH indicating [HMEA]Br, 1 indicating the first route in this model, other routes are named following the same rule); another one is that protic H atom activates the PO and H atom in hydroxyl group stabilizes the Br- anion (See Table S1, route S-SOH-2); and the third situation is to inverse the role of two hydrogen atoms (See Table S1, route S-SOH-3). The corresponding plots of transition states along with barrier heights are listed in Table S1. The barrier heights of routes S-SOH-2 and S-SOH-3 are lower than that of route S-SOH-1 indicating that the stabilization of anion plays a critical role to lower the barrier height. Normally, other ILs would follow the same optimal route with the similar IL. However, the situation would be more complicated with the increase of hydroxyl groups. Thus, the mechanisms taken [HDEA]Br and [HTEA]Br as catalysts are considered again. The corresponding plots of transition states are listed in Table S2 and Table S3 together with barrier heights. There are three active H atoms, which are named as protic H, H1 atom in primary hydroxyl group, and H2 atom in secondary hydroxyl group. First, H1, H2, and protic H atoms are all utilized to activate the PO (See Table S2, route S-DOH-1, S indicating Single-IL model, DOH indicating [HDEA]Br, 1 indicating the first route in this model, other routes are named following the same rule). Second, H1 atom and protic H atom are taken as the electrophile while 9



the H2 atom is to stabilize the Br- anion (See Table S2, route S-DOH-2). According to the same analysis, there is the other situation, i.e., both H1 and H2 atoms activate the PO and protic H atom to form hydrogen bond with Br- anion (See Table S2, route S-DOH-3). However, it is unsuccessfully confirmed because of the large steric hindrance. Next, only one hydrogen atom is employed to activate the PO, while another hydrogen atom is to stabilize the Br- anion. Routes S-DOH-4, S-DOH-5, S-DOH-6, and S-DOH-7 are located in Table S2 with the barrier heights of 22.19, 21.85, 27.00, and 28.56 kcal/mol. It is found that route S-DOH-1 has the highest barrier height although it has the most H atoms to activate the PO. It is attributed that the electrophilic attack is not the sole element to determine the ring-opening of PO, which is the synergetic result of both electrophilic and nucleophilic attack. Even if anion is the same, its nucleophilic ability would be further refined by formation of hydrogen bonds between anion and ionic liquids. It is better to make a compromise between electrophilic and nucleophilic ability. The mechanism of [HTEA]Br is the same as that of [HDEA]Br (See Table S3). Only the ring-opening step is considered because it is the rate-determining step.43,44 3.2. Double-IL model Due to the incorporation of one more ionic liquid, the situation becomes more complicated. Here, two [HMEA]Br are taken as an example to consider various possible routes. They are classified into three types: one is that two ionic liquids are employed to activate the PO (See Table S4, route D-SOH-a1 and route D-SOH-a2, D indicating Double-IL model, SOH indicating [HMEA]Br, a1 indicating the first route 10


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in the first situation, other routes are named following the same rule); one ionic liquid is to activate the PO and the other one is to stabilize the attacking ionic liquid (See Table S4, route D-SOH-b1, route D-SOH-b2, route D-SOH-b3, and route D-SOH-b4); one ionic liquid is to activate the PO and the other one is to stabilize the Br- anion (See Table S4, route D-SOH-c1, route D-SOH-c2, route D-SOH-c3, and route D-SOH-c4). The corresponding schematic structures of transition states and barrier heights are shown in Table S4. Only the third situation is detail stated out due to their relative lower barrier heights. One Br- anion locates in the catalytic central region, which plays the more important role in determining the barrier heights. The other Br- anion is far away from the catalytic central region, which is not discussed. The O atom of PO is activated by protic H atom from the primary IL and the Br- anion is stabilized by protic H atom from the secondary IL (route D-SOH-c1). The attacking H atom is fixed, then, the Branion is stabilized by H atom in hydroxyl group (route D-SOH-c2). Other two routes are confirmed by variation of two H atoms position (route D-SOH-c3 and route D-SOH-c4). The barrier heights of routes D-SOH-c1 and D-SOH-c2 are 15.26 and 13.87 kcal/mol, which are two lowest ones in all routes calculated by Double-IL model. The deviation between them is as small as 1.39 kcal/mol, which is difficult to distinguish them. Moreover, they have the similar attacking model. Therefore, both of them are considered as possible routes. It testifies again that the more electrophilic attack is not the insurance to lower the barrier height. Stabilization of Br- anion and a ring-shaped transition state are more favorable to promote the ring-opening. 11



Following the similar mechanism of route D-SOH-c2, the mechanism catalyzed by two [HDEA]Br (See Table S5, route D-DOH-c2, D indicating Double-IL model, DOH indicating [HDEA]Br) and two [HTEA]Br (See Table S5, route D-TOH-c2, D indicating Double-IL model, TOH indicating [HTEA]Br) are determined with the barrier heights of 12.51 and 20.41 kcal/mol, respectively. It is interesting that the calculated barrier heights decreased in the order of [HTEA]Br (20.41 kcal/mol) > [HMEA]Br (13.87 kcal/mol) > [HDEA]Br (12.51 kcal/mol), which are not consistent with the experimental catalytic activity. The product yields catalyzed by [HMEA]I, [HDEA]I, and [HTEA]I are 65%, 90%, and 91%, respectively. In the other word, the experimental catalytic activity is [HTEA]I > [HDEA]I > [HMEA]I. Although the Ianion is employed in the experimental study, it is replaced by Br- anion in the theoretical study because of the acceptable computational cost. Neither the relative sequence of catalytic activity nor catalyzed mechanism would not be varied from Ianion to Br- anion. In contrast, the barrier heights are 14.83 and 12.14 kcal/mol for route D-DOH-c1 catalyzed by two [HDEA]Br and route D-TOH-c1 catalyzed by two [HTEA]Br following the same model of route D-SOH-c1. The barrier heights increased in the sequence of [HTEA]Br (12.14 kcal/mol) → [HDEA]Br (14.83 kcal/mol) → [HMEA]Br (15.26 kcal/mol), which obeys the experimental catalytic activity. The corresponding energy profiles are plotted in Fig. 1 along with the schematic structures of transition states. Following the same mechanism, routes D-SOH-I, D-DOH-I, and D-TOH-I (D indicating Double-IL model, SOH, DOH, and TOH indicating [HMEA]I, [HDEA]I, and [HTEA]I, respectively, I indicating the I12


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anion) are calculated with the catalysts of two [HMEA]I, two [HDEA]I, and two [HTEA]I. The corresponding barrier heights are 15.95, 15.11, and 11.41 kcal/mol, respectively. And the corresponding schematic structures of transition states and barrier heights are shown in Table 1. It testifies that the relative sequence of barrier heights determined by Br- anion could also be applied to ILs with the same cation along with I- anion. It has been widely testified that the ring-opening step is the rate-determining step. Whether it is still right in the Double-IL model should be carefully checked. For route D-SOH-c1, the ring-closure step is also calculated with the barrier height of 14.81 kcal/mol. (See Fig. S2). The ring-opening step is still the rate-determining step in Double-IL model. Since there is no bond formation or rupture in the CO2 insertion, its barrier height would not be large. Therefore, the CO2 insertion is not considered. The Double-IL have been successfully applied for other ILs including hydroxyl-functionalized quaternary ammonium ILs,24 protic pyrazolium ILs,45,46 and amino-functionalized imidazolium ILs.47 The barrier heights evaluated by Double-IL model is more reliable than that predicted by Single-IL model. The ring-shape transition state is more favorable than other transition state in Double-IL model. More importantly, the Double-IL could study the mechanism of reaction catalyzed by mixed catalysts rather than single-component catalyst.47 3.3. Solvent effect Although the sequence of barrier heights is consistent with the experimental result, the deviation between route D-SOH-c1 (two [HMEA]Br) and route D-DOH-c1 13



(two [HDEA]Br) is as small as 0.43 kcal/mol, which is not large enough to differentiate them since it is in the limitation of acceptable computational error. It is attributed to that the solvent effect of ionic liquids is omitted in the aforementioned calculation. Although there is various solvent model incorporated in the Gaussian program, it is suitable for the organic solvent rather than ionic liquids. Ionic liquids are scarcely considered as the solvent in previous literature due to both the absence of existed model and expensive computational cost. According to the description in computational details, six [HMEA]Br, [HDEA]Br, and [HTEA]Br ionic liquids are regarded as solvent molecules, respectively, which are around the central catalytic region that is taken from the above optimized structure of route D-SOH-c1, route D-DOH-c1, and route D-TOH-c1. The central catalytic region (QM) is calculated by B3PW91/6-31G(d,p) level and the surrounding six ion pairs (MM) is calculated by HF/3-21G level as solvent. All of them are fully optimized although it is very expensive including 1064-1208 basis functions. The barrier heights are 18.58, 12.73, 8.97 kcal/mol for route D-SOH-O (D indicating the QM structure calculated by Double-IL model, SOH indicating [HMEA]Br, O indicating ONIOM), route D-DOH-O (D indicating the QM structure calculated by Double-IL model, DOH indicating [HDEA]Br, O indicating ONIOM), and route D-TOH-O (D indicating the QM structure calculated by Double-IL model, TOH indicating [HTEA]Br, O indicating ONIOM), which is totally consistent with the experimental result. Moreover, the deviation between two neighbor catalysts is further enlarged, which is enough to distinguish them. The corresponding energy profiles are plotted in Fig. 2 14


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along with the structures for transition states. The predicted result is more reliable when the solvent effect of ionic liquids is considered, which would be helpful to quantitatively determine the catalytic activity for an unknown catalyst. 3.4. Non-covalent interaction (NCI) and the atoms in molecules (AIM) analysis Besides the electrostatic interaction, the weak interaction between catalyst and substrate also plays a vital role in catalytic activity.48,49 Three transition states in QM region of ONIOM model are analyzed by the non-covalent interaction (NCI) and the atoms in molecules (AIM) methods to investigate the effect of weak interactions. The detailed definitions please refer to refs. 50-53. Only N-H…O or O-H…O and C…Br interactions are focused on in the following analysis, which are two most determined items related with the electrophilic attack and nucleophilic attack, respectively. While other hydrogen bonds far away from the central region, N-H…Br1, O-H…Br1, and O-H…Br2, are neglected. There is bond critical point (BCP) between H and O for all three transition states, which obeys the basic requirement for the hydrogen bond and/or covalent bond. However, no one is classic hydrogen bond since the ρ and ▽2ρ values of normally hydrogen bond should be in a range of 0.002-0.035 a.u. and 0.024-0.139 a.u., respectively. The interaction of N-H … O in TSD-DOH-O ([HDEA]Br) is already the covalent bond owing to its negative ▽2ρ value. The corresponding electrophilic attack is also the strongest. While the N-H…O or O-H… O interactions in TSD-SOH-O ([HMEA]Br) and TSD-TOH-O ([HTEA]Br) have both hydrogen bond and covalent bond properties. Moreover, the N-H…O interaction is stronger than the O-H…O interaction. The ρ value of N-H…O in TSD-SOH-O 15



([HMEA]Br) is 0.10421 a.u., which is larger than that in TSD-TOH-O ([HTEA]Br) (0.07846 a.u.). Similarly, the ρ value of N-H…O (0.18826 a.u.) in TSD-DOH-O ([HDEA]Br) is larger than that (0.10421 a.u.) in TSD-SOH-O ([HMEA]Br). The larger ρ value indicates the stronger electrophilic interaction. The electrophilic ability of [HMEA]Br is stronger than [HTEA]Br and that of [HDEA]Br is larger than of [HMEA]Br. In general, there is not clear relationship between electrophilic ability and experimental result/number of hydroxyl groups. Consequently, electrophilic ability is not the sole item to determine the final activity. The C…Br interaction in TSD-DOH-O belongs to the halogen bond with the ρ value of 0.03705 a.u. and the▽2ρ value of 0.07323 a.u., while C…Br interactions in other two transition states are stronger with the larger ρ value that is over the upper limitation of halogen bond.54 The C…Br interaction in TSD-TOH-O is the strongest, which is favorable to promote the ring-opening. Additionally, there is one more hydrogen bond, O-H…O, is involved to activate the PO, which is absence in other two transition states. One more hydrogen bonds along with the strongest C…Br interaction finally results in the lowest barrier height for TSD-TOH-O. Although the C…Br in TSD-DOH-O is the weakest, its N-H…O interaction is the strongest with the totally covalent property leading to the mediate barrier height. The deviation of barrier height between TSD-TOH-O and TSD-DOH-O is smaller than that between TSD-DOH-O and TSD-SOH-O, which is consistent with the reported product yields. The highest barrier height for TSD-SOH-O is attributed to that there is no prominent item in them including both electrophilic interaction and nucleophilic interaction. The 16


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NCI analysis that is coincident with the AIM (See Table 2) is presented in Fig. 3. Actually, the number of solvent and structure of solvent would also affect the final result. Moreover, both of them are greatly related to the initial guess. The further effort should be made to improve the reliability to calculate the solvent effect, which is our next goal. However, this work would provide the more reliable result than that predicted by Single-IL model/Double-IL model with PCM model. 4. Conclusions The

mechanism

of

CO2

with

PO

catalyzed

by

three

protic

hydroxyl-functionalized quaternary ammonium ILs is thoroughly elucidated by Single-IL model and Double-IL model. On the basis of the optimized results by Double-IL model, the solvent effect of ILs is considered by the ONIOM calculations along with MD simulation rather than PCM model incorporated in Gaussian program with the ultimate goal to compare the difference among different model. Moreover, it is found that the catalytic activity sequence, [HMEA]Br → [HDEA]Br → [HTEA]Br, predicted by Double-IL model is totally consistent with the experimental result. The difference between two routes is further refined when the solvent effect is considered by ONIOM model. However, the Single-IL model along with the PCM model is not an advisable choice to study the catalytic activity for a series of ionic liquids because of the absence of interaction between ILs. As indicated in AIM and NCI analytic results, the nucleophilic attack is the more important item to promote the ring-opening of PO. Moreover, it is better to stabilize the anion via hydrogen bond between anion and cation. The sole anion without any fixation presents the less nucleophilic attack. 17



Supporting Information Supporting Information Available: Radial distribution functions and integrated coordination numbers with respect to the center of mass (c.o.m.) of [HMEA]+, [HDEA]+, and [HTEA]+ cation and the center of mass (c.o.m.) of Br− anion. The corresponding schematic structures of transition states and barrier heights of Single-IL model and Double-IL model. Potential energy profile and sketch structures of transition states for the ring-opening step and ring-closure step along route D-SOH-c1. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21476061, 21503069, 21676071), Program for He’nan Innovative Research Team in University (15IRTSTHN005).

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into the magnitude and nature of halogen bonding interactions. J. Phys. Chem. A 2007, 111, 10781-10788.

27



Fig. 1 Potential energy profiles and sketch structures of transition states for the ring-opening step along route D-SOH-c1, route D-DOH-c1, and route D-TOH-c1 calculated at the M06/6-311+G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level.

28


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Fig. 2 Potential energy profiles and schematic structures of transition states for the ring-opening step along route D-SOH-O, route D-DOH-O, and route D-TOH-O calculated by the ONIOM (B3PW91/6-31G(d,p):HF/3-21G) method.

29



Fig. 3 NCI plots for the central QM structures of TSD-SOH-O, TSD-DOH-O, and TSD-TOH-O in ONIOM model. The corresponding 3D plots are displayed left with blue regions representing strong electrostatic interactions and green regions representing more dispersion attractive interactions.

30


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Scheme 1 Sketch structures of catalysts synthesized in experiment.

31



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Table 1 The corresponding schematic structures of transition states and barrier heights catalyzed

by

two

[HMEA]I,

two

[HDEA]I,

and

two

[HTEA]I

at

the

M06/6-311+G(2d,2p)-6-311G(d,p) (PCM)//B3PW91/6-31(d,p)-6-311G(d) level. routes

schematic structures of transition states

barrier heights (kcal/mol)

route D-SOH-I

15.95

route D-DOH-I

15.11

route D-TOH-I

11.41

32


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Table 2 Selected topological parameters of the BCP in the central QM structures of the ONIOM model calculated at the B3PW91/6-31G(d,p) level. Transition states

X-Y…Z

sign(λ2)ρ

ρ

▽2 ρ

N-H…O

-0.10421

0.10421

0.10067

C…Br1

-0.05731

0.05731

0.06443

N-H…O

-0.18826

0.18826

-0.51406

C…Br1

-0.03705

0.03705

0.07323

N-H…O

-0.07846

0.07846

0.15779

O-H…O

-0.04284

0.04284

0.12420

C…Br1

-0.06674

0.06674

0.05326

TSD-SOH-O ([HMEA]Br)

TSD-DOH-O ([HDEA]Br)

TSD-TOH-O ([HTEA]Br)

33



Graphic abstract

34


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Protic Quaternary Ammonium Ionic Liquids for Catalytic Conversion of

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