Theoretical Study of Ionic Liquid Clusters Catalytic Effect on the

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Kinetics, Catalysis, and Reaction Engineering

A theoretical study of ionic liquid clusters catalytic effect on the fixation of CO2 Xin Tan, Xiaomin Liu, Xiaoqian Yao, Yaqin Zhang, and Kun Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03947 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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Industrial & Engineering Chemistry Research

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A theoretical study of ionic liquid clusters catalytic effect on the

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fixation of CO2

3

Xin Tan†§, Xiaomin Liu‡†*, Xiaoqian Yao†, Yaqin Zhang†, and Kun Jiang†§

4

†CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Beijing Key

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Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, 1 North 2nd Street,

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Zhongguancun, Haidian District, Beijing 100190, China

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‡ School of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, Shandong, China.

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§School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences. 19A Yuquan Road, Shijingshan

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District, Beijing 100049, China.

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Abstract

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Conversion of carbon dioxide (CO2) to valuable materials is important to the academia

12

and the chemical industry. Fixation of carbon dioxide catalyzed by ionic liquids (ILs) is an

13

attractive option. A lot of works have been done in the fixation of carbon dioxide catalyzed by

14

ILs, but most of them focused on reactions catalyzed by one molecule. In this work, we

15

studied the existence of ILs clusters in a series of systems containing hydroxyl-functionalized

16

ILs (HFILs) through a series of theoretical calculations. The inter-molecule interactions and

17

microstructures of IL clusters were studied by molecular dynamics (MD) simulations. The

18

results indicated that three-ion clusters to six-ion clusters exist in systems, while the number

19

of clusters decreased as the cluster size increased. And, instead of studying the single ion or

20

ionic pair catalytic processes, we gained more insight into the catalytic effects of ionic liquid

21

clusters on the fixation of CO2 by using density functional theory (DFT) calculations. Results

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suggested that the activity of ILs could be enhanced to a certain extent in the presence of

23

clusters.

24 25

Keywords:

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Ionic Liquid, Ionic Clusters Catalysis, CO2 Conversion

27 28 29

1. Introduction In recent decades, the ionic liquids (ILs) , with unique physical and chemical

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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

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properties, have become a class of new functional materials and designable green solvent, and

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have been broadly used in biomass utilization [1], catalytic synthesis [2-6], electrochemical

3

research [7-9], material preparation [10-12], extraction and gases capture [13-17]. ILs have

4

been used as a cleaning medium for kinds of catalytic reactions and chemical synthesis

5

process, replacing the traditional organic solvents and acid-base catalyst, and reducing the

6

industrial pollution from the sources. As the solvent and the catalyst, the zwitterionic,

7

functional, and acid ionic liquids were used in some organic reactions [18, 19]. The important

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applications are that the ionic liquids used in catalytic reactions, for instance, in the

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hydroformylation reactions [20], carbonylation reactions [21], hydrogenation reactions [22]，

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Heck reactions [23] ， and oxidation reactions [24]. Accordingly, the catalytic reaction

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mechanisms based on ILs have been investigated. However, most of the IL catalytic reaction

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mechanisms reported were for a single ion or an ionic pair [25-28]. However, some chemical

13

reactions are not only promoted by catalyst molecule individually, but also effected by other

14

catalyst molecules or ions around [29].

15

It is known that the ionic clusters existed in ILs systems [30]. Models of liquid clusters

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were proposed at least a century ago, which provided a basic understanding of the liquids

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structure and behavior. Aggregates/clusters in liquid metals [31, 32], inert gases [33], water

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[34-36], and organic solvents [37, 38] have also been studied by many spectroscopic methods

19

and theoretical calculations. In neat ionic liquids, clustering was first reported by MD studies,

20

and deep insights into the fluids behavior could be provided at molecular level. In 2005, Voth

21

et al. identified the segregated domains in ILs by a coarse-grained model [39]. After

22

performing an all-atom simulation, Lopes et al. indicated the existence of nanosegregation

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between nonpolar and polar domains [40]. Triolo et al. reported the direct experimental

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evidences for the existence of clusters in bulk ILs through X-ray scattering [41]. The

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aggregation of ionic liquids in solution was also found. structures of amphiphilic association

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was reported in the solution composed of laureth, water and [BMIM][PF6] by small angle

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X-ray diffraction [42]. Zhao et al. studied the aggregation of ILs and phase separation in

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phosphonium-based ionic liquids/H2O systems [43]. In 2015, a correlation between viscosity

29

and ionic liquid clusters in aqueous solutions of three kinds of ILs was established by Chaban

30

and Fileti [44]. Recently, several papers reported the cationic clusters in ILs, in which the


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cooperative hydrogen bonds between cations could overcome the charge repulsions [45-50].

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It is known that clusters exist in many ILs, and play a key role in many reactions and

3

processes. However, it is still unclear that how the IL clusters affect the reactions, and there is

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still lack of the understanding of the catalytic mechanism of ion cluster reaction. There may

5

be more than one pair of ions to promote the reactions, and there are many complex

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interactions between the ion clusters, such as static electricity and hydrogen bonds and etc.

7

Among all of the reactions, the produce of cyclic carbonates through the cycloaddition

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reactions of carbon dioxide and epoxides is one of the most important reactions. This reaction

9

is attractive because it is environmentally friendly, and the cyclic carbonates are important

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chemicals which are widely used in many aspects. In addition, polycarbonates are produced in

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the process of ring opening. Recently, some ILs are found to be efficient catalysts for CO2

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cycloaddition to epoxides [51]. At the first time, Peng et al. reported the conversion of

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propylene oxide to propylene carbonate catalyzed by 1-butyl-3-methylimidazolium

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(BMIMBF4) IL [52]. Steinbauer et al. studied the mechanism of the addition of carbon

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dioxide to the epoxides catalyzed by ILs [53]. Thereafter, imidazolium- [54, 55], ammonium-

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[56], phosphonium- [57], guanidinium- [58], pyridinium- [59, 60], amino acid- [61], and

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Lewis base- [62] ILs were reported. Li Wang et al. illustrated the hydrogen bonding

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interactions in the CO2 fixation catalyzed by hydroxyl-functionalized ILs [63]. Besides, it is

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reported that a great improvement of the cyclic carbonate yields when using

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hydroxyl-functionalized ionic liquids [64]. However, there are some questions: do IL clusters

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exist in these reaction systems? What are the IL cluster structures and their characteristics in

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these solutions? What are the properties and performance of the IL clusters in the solutions?

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To obtain probable answers of the above questions, a mechanism study of the reaction by MD

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simulations and density functional theory calculations could provide us a detailed insight into

25

the reaction and process.

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In our work, we studied the cycloaddition of PO and CO2 catalyzed by HEMIMBr

27

(1-(2-hydroxylethyl)-3-methylimidazolium bromide), a kind of hydroxyl-functionalized IL.

28

The MD simulations were used to study these systems, and ionic cluster structures were found

29

in the reactions. In the systems, 3-ions, 4-ions, 5-ions, and 6-ions clusters existed, while the

30

number of clusters decreased as the cluster size increased. It is supposed that the ionic clusters



1

play a role in the reactions. Then, the quantum chemistry calculations were carried out to give

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a further view of this assume. The mechanisms of one-pair-catalyst reaction and ions clusters

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catalytic reactions are explored. There are three steps in the above reactions: ring-opening of

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propylene oxide, addition of carbon dioxide, and ring-closure of propylene carbonate. The

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calculations indicate that the activity of step one and three in multiple catalyst reactions could

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be enhanced, showing the synergistic effects of ionic clusters on increasing the activity.

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2. Computational details

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2.1. Molecular dynamics simulations

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A series of molecular dynamics simulations containing HEMIMBr, propylene oxide

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(PO), CO2 and propylene carbonate (PC) were performed with GROMACS 5.1.1. We

12

employed the GAFF [65] force field including the parameters of bonds, angles, dihedrals, and

13

Lennard-Jones interactions for the all components. While Restrained Electrostatic Potential

14

(RESP) method at HF/6-31+G(d,p) level was adopted for the partial charges . The

15

electrostatic interactions for the systems were simulated using Particle Mesh Ewald (PME)

16

method. Leapfrog integration algorithm with a time step of 2.0 fs was used in the equations of

17

motion. 1.2 nm cutoff distance was used for van der Waals interactions and electrostatic

18

interactions. An interpolation order of 4 and Fourier grid spacing of 0.10 nm was used in the

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Particle Mesh Ewald summation method.

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All the systems were started from initial molecular structures with randomly mixed in a

21

cubic box using PACKMOL [66, 67]. The energetically minimization with a steepest descent

22

algorithm for 100000 MD steps were used. Then, the NVT ensembles and the NPT ensembles

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were adopted. Temperatures were controlled by a V-rescale thermostat and pressures were

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controlled by Parrinello-Rahman with the temperature at 398.15 K and pressure at 20 atm. To

25

stabilize these systems, 12 ns simulations were performed in the NVT ensemble and the NPT

26

ensemble. Then, 100 ns simulations in canonical ensemble were adopted for a further

27

analysis, and the trajectories were recorded with 100 fs interval. The components of

28

simulation systems are listed in Table 1.

29

Table 1. The components of the simulation systems


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System

PO (nmol)

PC (nmol)

CO2 (nmol)

HEMIMBr (nmol)

PC (kg/m3)

System 1

22000

0

3000

352

0

System 2

19250

2750

3000

352

137.10

System 3

16500

5500

3000

352

278.83

System 4

13750

8250

3000

352

420.15

System 5

11000

11000

3000

352

558.67

System 6

8250

13750

3000

352

696.24

System 7

5500

16500

3000

352

823.30

System 8

2750

19250

3000

352

953.25

System 9

0

22000

3000

352

1089.48

1 2

2.2. Quantum chemistry calculations

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The reactants, products, intermediates, and transition states equilibrium geometries were

4

optimized using the B3LYP method and 6-31+G(d,p) basis set (B3LYP/6-31+G(d,p)). We

5

carried out vibrational frequency calculations at the same level to confirm the nature of the

6

extreme points of the potential energy surface. Vibrational frequency with only one imaginary

7

frequency stands for the transition state. Then, the intrinsic reaction coordinate theory with

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B3LYP/6-311++G(d,p) method and basis set was employed to construct the minimum-energy

9

path (MEP). All the above calculations were performed by Gaussian 09 program.

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3. Results and discussions

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3.1.1. IL clusters in systems.

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Clustering was found in systems in this work, and the number of 3-ions (two cations and

14

one anion), 4-ions (two cations and two anions), 5-ions (three cations and two anions), and

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6-ions (three cations and three anions) clusters in nine systems are shown in Figure 1. The

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typical configures of 3-ions, 4-ions, 5-ions, and 6-ions clusters obtained from MD simulations

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were shown in Figure 2. The clusters are defined with the single-linkage approach in

18

GROMACS simulation suite, and the cutoff distance is 0.4 nm. From Figure 1, it could be

19

found that the clusters contain three to six ions exited in nine systems. Meanwhile, we could



1

see that the number of clusters decreased as the cluster size increased.

2

Figure 1. Number of (a) 3-ions, (b) 4-ions, (c) 5-ions, and (d) 6-ions clusters in systems.

3

A

B

C

D

Figure 2. The typical configures of clusters in systems. Configures A, B, C, and D represent


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the 3-ions, 4-ions, 5-ions, and 6-ions clusters.

2 3

3.1.2. The microstructures of IL clusters.

4

The molecular dynamics (MD) studies were provided in this work to give insights of the

5

nine systems, and then the distributions of ILs were obtained to discuss the microstructures of

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IL clusters. To obtain the changes of the inter-molecule distances in different system,

7

pair-pair radial distribution function (RDF) and site-site radial distribution function analyses

8

were calculated. The description of the atom sites can be found in Figure S1 in SI.

(a)

(b)

(c) 9

Figure 3. Center-of-mass RDFs of (a) cation-anion, (b) cation-cation and (c) anion-anion.



1 2

Figure 4. Site-site intermolecular RDFs of hydroxyl hydrogen between cations.

3 4

Figure 5. Site-site RDFs of methyl hydrogen between cations.

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Radial distribution functions. In order to explore structures of the nine systems, the

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center-of-mass RDFs of these systems are shown in Figure 3. The maximum position and

7

peak height for cation-anion, cation-cation and anion-anion RDFs are listed in Table S6.

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Comparing the maximum position for these systems, it can be seen that the distances between

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the center cations to adjacent anions for these systems are about 0.49 nm, the distances

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between the center cations to adjacent cations are about 0.49 nm, and the distances between

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the central anions to adjacent anions are about 0.79 nm. It indicates short-ranged interactions

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in these ions and illustrates the existence of the aggregation effect of cations. However, the

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increase of PC resulted in the decrease of the first peak intensity. It indicates the correlation

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between ions decreases with the extent of the reactions.

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Site-site RDFs are also presented and compared to obtain more information about the

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interaction between cations. Curves in Figure 4 show the RDFs for the central hydroxyl

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hydrogen at (p) site of cations (refer to Figure 1 in SI) and the adjacent hydroxyl hydrogen,

18

and curves in Figure 5 show the RDFs for the central methyl hydrogen at (m) site of cations


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(refer to Figure 1 in SI) and the adjacent methyl hydrogen. The H atoms in the hydroxyl and

2

methyl groups are the chain terminal atoms of butyl-chain on HEMIMBr, and we could use

3

them to determine whether there is aggregation or not. All the RDFs for hydroxyl of cations

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in Figure 4 exhibit three peaks—a first peak at approximately 0.24 nm, and then a peak at

5

0.47 nm and a peak at approximately 0.80 nm. The position and height of each peak for

6

site-site RDFs of hydroxyl hydrogen are listed in Table S7. The three peaks may indicate

7

three probable space distributions of cations around the central cation. All the RDFs for

8

methyl of cations in Figure 5 exhibit one peak at about 0.8 to 0.9 nm, and the position and

9

height of peak for site-site RDFs of methyl hydrogen are listed in Table S8. Similarly, this

10

peak may indicate a probable space distribution of cations around the central cation. The

11

RDFs above could infer that there were interactions between cations, and there was

12

considerable chance that cations distributed around each other and formed clusters.

13

Spatial distribution functions. From the RDF calculations, we guessed that the ILs could

14

gather together, and further formed clusters. To prove this conjecture, we calculated the

15

typical spatial distribution functions (SDFs) of hydroxyl hydrogen in cations around a central

16

cation, which could stand for the probability of finding the HEMIM+ cations around the

17

center HEMIM+ ions. In this work, the SDFs for hydroxyl hydrogen (blue surface) in cations

18

around cations in systems were visualized by gOpenMol software package and are shown in

19

Figure 6(a) and in SI. The spatial distribution functions (SDFs) of methyl hydrogen (blue

20

surface) in cations around a central cation in systems was also calculated and shown in Figure

21

6(b) and in SI.

22 23

(a)

(b)

(c)

24

Figure 6. Spatial distribution functions (SDFs) for the (a) hydroxyl hydrogen (blue surface)

25

(b) methyl hydrogen (blue surface) in HEMIM+ cations around the center HEMIM+ cation in

26

the ionic liquids system 2, (c) Br (red surface) and hydroxyl hydrogen (yellow surface) in



1

cations around the center PO molecule in the ionic liquids system 3. The counter levels are (a)

2

37.0, (b) 23.0 and (c) 2.2. The distributions of hydroxyl and methyl hydrogen atoms around

3

the central HEMIM+ cation in the all nine systems are drawn in SI.

4 5

From the Figure 6(a), it is found that the hydroxyl hydrogen of cations were most likely

6

around the hydrogen atoms at (j), (k), (l) sites (refer to Figure 1 in SI). It is also found that,

7

there is no obvious change in the spatial distribution position of cations around the central

8

cation, however, the distribution density of cations decreases with increasing PC. The

9

distribution density of cations around the hydrogen atoms indicates the interactions between

10

the cations, and as the reaction proceeded, the interactions decreased. The Figure 6(b)

11

indicated that the methyl group distributed symmetrically in both sides of imidazole ring, and

12

similar to the distributions of hydroxyl groups, no obvious change was found in the spatial

13

distribution position of cations around the central cation, but the distribution density of

14

cations decreases with increasing PC.

15

The SDFs of Br (red surface) and hydroxyl hydrogen (yellow surface) atoms in cations

16

around the center PO molecule were calculated to find the Br anions and HEMIM+ cations

17

around the center PO molecule. The SDFs are shown in Figure 6(c). As showed in the figure,

18

the carbon atom and oxygen atom of PO are attacked by the Br anion and the hydroxyl group.

19 20

3.2. Effects of clusters on the ionic liquids catalytic reactions.

21

Recently, a lot of attentions have been paid to the ionic liquids in catalytic reactions.

22

however, only a little works have preliminary revealed the clustering effects of ionic liquids

23

in some catalytic reactions [68]. In this work, we used the DFT method to provide an insight

24

into the synergistic catalytic effect of ILs clusters on reactions. DFT method is a popularly

25

accepted method to explore the mechanism and compare the activity qualitatively with a high

26

accuracy, and we used the DFT calculations to investigate the CO2 cycloaddition process

27

catalyzed by one pair of HEMIMBr ions and by HEMIMBr ion clusters.

28 29 30

3.2.1. Cycloaddition reaction without catalyst. The cycloaddition reaction of carbon dioxide with PO without catalyst was calculated to


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1

compare with the reaction catalyzed by HEMIMBr. The calculated potential energy surface

2

(PES) profiles with the minima and transition states optimized structures were shown in

3

Figure 7. Firstly, a complex between CO2 and PO, denoted as IM1 and IM2, was generated by

4

the van der Waals interaction. There is a long distance (2.896 nm and 2.882 nm) and a small

5

binding energy (1.83 kcal·mol-1 and 1.92 kcal·mol-1) between CO2 and PO. From IM1 and

6

IM2, two similar transition states, TS1 and TS2, appeared in the potential energy surface,

7

where a C-O bond in PO is broken and the C-O bonds between PO and carbon dioxide

8

formed. The broken C-O bond in TS1 is 1.986 nm and the two C-O bonds are 1.655 and

9

2.310 nm, and in TS2 the corresponding ones are 2.074, 1.636, and 2.434 nm, respectively.

10

From the IRC calculations, we know that not only TS1 but also TS2 could converge to

11

propylene carbonate, and it

12

However, the energy barriers from IM to TS1 and TS2, 58.96 and 53.01 kcal·mol-1, are quite

13

high, which may indicating the difficulty of the reaction without the catalyst HEMIMBr.

is more stable with 16.28 kcal·mol-1 than the reactants.

14 15

Figure 7. Potential energy surface profiles of reactant, intermediates, transition states, and

16

product in the reaction of carbon dioxide with propylene oxide. Distances are in angstroms.

17 18

3.2.2. Reactions catalyzed by one pair of HEMIMBr ions.

19

Energy profiles over the HEMIMBr catalysts are shown in Figure 8. Figure 9 displays

20

optimized structures of reactants, transition states, intermediates and products, and geometries



1

were calculated at B3LYP/6-311++G(d,p) level. When isolated PO and HEMIMBr get close,

2

hydrogen bonds form in the bimolecular complex A. Starting from complex A, as the

3

nucleophilic Br− anion attacked the C2 atom and the electrophilic H1 atom attacked the O3

4

atom in PO, the C2−O3 bond of PO is ruptured. After the TS-AB, complex B form. It is seen

5

in Figure 8 that the barrier energy of this process is 21.6 kcal·mol-1. Studies found a favorable

6

interaction between the carbon dioxide and the imidazolium cation alkyl chains [69, 70].

7

Thus, it is convenient for carbon dioxide being introduced into the reaction. After the

8

formation of C, C3−O3 bond form as a result of the electrophilic attack of carbon dioxide to

9

O3. The CO2 addition process is a reaction with 1.7 kcal·mol-1. Then, intermediate D form.

10

The C2 atom adds to O2 and Br− departs simultaneously via TS-DE. In the end, the complex E

11

is obtained, and it directly leads to the formation of the product and the release of HEMIMBr.

12

In the reaction, the ratedetermining step is the ring-opening of PO, and its barrier energy is

13

21.6 kcal·mol-1. Comparing the above reaction without catalyst, barrier height with

14

HEMIMBr is decreased by 31.4−37.3 kcal·mol-1.

15 16 17 18 19 20 21

Figure 8. Energy profiles of minimum energy path (MEP) at the B3LYP/6-311++G(d,p) level for the reaction of PO with CO2 catalyzed by one pair of HEMIMBr ions and by HEMIMBr ion clusters. Energy profiles were shown in different colors, black for one-pair reaction, red for three-ion reactions, blue for four-ion reactions, green for five-ion reactions, and purple for six-ion reactions.


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A

TS-AB

B

C

TS-CD

D



TS-DE

Page 14 of 26

E

1 2 3 4

Figure 9. The optimized geometries of reactants, products, intermediates and transition states at b3lyp/6-31+G(d,p) level for the cycloaddition reaction of carbon dioxide with PO catalyzed by one pair of HEMIMBr ions.

5

3.2.3. Reactions catalyzed by 3-ion clusters.

6

To gain an insight into the effect of 3-ion clusters, the DFT calculations were performed

7

to investigate the 3-ion-catalyzed CO2 cycloaddition process, respectively. Figure 8 shows the

8

calculated reaction pathways of PO and CO2 catalyzed by clusters of three HEMIMBr ions.

9

And the optimized structures of reactants, products, intermediates and transition states

10

calculated at B3LYP/6-31+G(d,p) level of theory are displayed in Figure 10. From a complex

11

F formed via hydrogen bonds, the nucleophilic attack of the Br− activates the C2 atom of PO

12

and the O3 atom is activated by the electrophilic attack of H1 atom from hydroxyl group. In

13

the transition state TS-FG, the imidazolium cations and anions are thought to jointly activate

14

the reactant PO. Owing to the steric hindrance, only one hydrogen atom from hydroxyl group,

15

the H1 atom, would attack the O3 atom of PO to activate the C2–O3 bond of PO through the

16

H-bond interaction between them. Meanwhile, the adjacent C2 atom in PO suffers from the

17

nucleophilic attack of bromide anion, and the H2 atom from another hydroxyl group would

18

form a hydrogen bond with this Br− anion. After the transition state TS-FG, complex G was

19

generated. It can be seen that the added ions realized an enhancement in the reactivity of the

20

ring-opening of PO, and the energy barrier of this ring-opening is 11.5 (F–TS-FG)

21

kcal·mol-1 for 3-ion-catalyzed reactions, respectively. This energy barrier is lower than 21.6

22

kcal·mol-1 for one-pair-catalytic reaction. The second step is the addition of CO2 to the

23

activated PO. Following the introducing of CO2 into G, a more stable complex H was formed.

24

Via the transition state TS-HI, the carbon atom of CO2 could easily bond to the activated O3

25

to form the C3 −O3 bond with a quite low energy barrier of 1.4 (H–TS-HI) kcal·mol-1. As a

26

result, the complex I was generated. The third step is the formation of PC and the

27

regeneration of HEMIMBr. On the basis of the I, the addition of C2 to O2 is accomplished via

28

TS-IJ, and the Br− separated from the C2 atom simultaneously. Finally, the product J

29

generates. The energy barriers is respectively decreased to 10.2 kcal·mol-1. The ring-opening

30

reaction of PO is also the rate-determining step, and the ring-opening barrier heights and


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1

ring-closure barrier heights are both decreased compared with the one-catalyst reaction. This

2

phenomenon revealed that the combination of 3-ion HEMIMBr clusters could realize the

3

improvement in the activity compared with the single pair of HEMIMBr ions, showing the

4

synergistic effects of ion liquid clusters on the enhancement in reaction activities.

F

TS-FG

G

H

TS-HI

I



TS-IJ 1 2 3 4

Page 16 of 26

J

Figure 10. The b3lyp/6-31+G(d,p) optimized geometries of reactants, products, intermediates and transition states for the cycloaddition reaction of carbon dioxide with PO catalyzed by 3-ion clusters.

5 6


7

Figure 8 shows the calculated reaction pathways of carbon dioxide and PO catalyzed by

8

clusters of four HEMIMBr ions. And the optimized structures of reactants, products,

9

transition states and intermediates calculated at B3LYP/6-31+G(d,p) level of theory are

10

displayed in Figure 11. From a complex k formed via hydrogen bonds, the C2 atom of PO is

11

activated by the nucleophilic attack of the Br− and the O3 atom is activated by the

12

electrophilic attack of H1 atom in hydroxyl group. In the transition state TS-KL, the

13

imidazolium cations and anions in the HEMIMBr cluster are thought to jointly activate the

14

reactant PO. Owing to the steric hindrance, only one hydrogen atom from hydroxyl group, the

15

H1 atom, would attack the O3 atom of PO to activate the C2–O3 bond of PO. Meanwhile, the

16

adjacent C2 atom in PO suffers from nucleophilic attack of bromide, and H2 atom from

17

another hydroxyl group would form a hydrogen bond with this Br− anion. After the transition

18

state TS-KL, complex L generates. The energy barriers of this process is 14.2 (K–TS-KL)

19

kcal·mol-1 for 4-ion-catalyzed reactions, respectively. The second step is the addition of CO2

20

to the activated PO. Following the introducing of CO2 into L, a more stable complex M

21

generates. Via the TS-MN, the carbon atom of CO2 could easily bond to the activated O3 atom

22

of PO to form the C3−O3 bond, the energy barrier was only 2.4 (M–TS-MN) kcal·mol-1. As a

23

result, the complex N generates. The third step is the formation of PC and the regeneration of


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


1

HEMIMBr. On the basis of the N, the addition of C2 to O2 is accomplished via TS-NO, and

2

the bromide anion separated from the C2 atom simultaneously. Finally, the product O

3

generates. The energy barriers is respectively decreased to 5.2 kcal·mol-1. The

4

rate-determining step is the ring-opening step of PO, and the barrier heights of ring-opening

5

and ring-closure are both decreased compared with the one-catalyst reaction.

K

TS-KL

L

M

TS-MN

N



TS-NO 1 2 3

Page 18 of 26

O

Figure 11. The optimized geometries of reactants, intermediates, transition states and products calculated at b3lyp/6-31+G(d,p) level for the cycloaddition reaction of carbon dioxide with PO catalyzed by 4-ion clusters.

4 5

3.2.3. Reactions catalyzed by HEMIMBr 5-ion clusters.

6

Figure 8 shows the calculated reaction pathways of CO2 and PO catalyzed by clusters of

7

five HEMIMBr ions. And the optimized structures of reactants, products, transition states,

8

and intermediates calculated at B3LYP/6-31+G(d,p) level are displayed in Figure S8. From a

9

complex P formed via hydrogen bonds, the C2 atom of PO is activated by the nucleophilic

10

attack of the Br− anion and the O3 atom is activated by the electrophilic attack of H1 atom

11

from hydroxyl group. In the transition state TS-PQ, the imidazolium cations and anions are

12

thought to jointly activate the reactant PO. Owing to the steric hindrance, only one hydrogen

13

atom from hydroxyl group, the H1 atom, would attack the O3 atom of PO to activate the C2–

14

O3 bond of PO. Meanwhile, the adjacent C2 atom in PO suffers from nucleophilic attack of

15

Br−, and H2 atom from another hydroxyl group would form a hydrogen bond with this Br−.

16

After the transition state TS-PQ, complex Q generates. The energy barriers of the

17

ring-opening is 15.5 (P–TS-PQ) kcal·mol-1 for 5-ion-catalyzed reactions, respectively. The

18

second step is the addition of CO2 to the activated PO. After introducing of CO2 into Q, a

19

more stable complex R generates. Via the TS-RS, the carbon atom of CO2 could easily bond

20

to the activated O3 atom of PO to form the C3−O3 bond, the energy barrier was only 2.8 (R–

21

TS-RS) kcal·mol-1. As a result, the complex S generates. The third step is the formation of PC

22

and the regeneration of HEMIMBr. On the basis of the S, the addition of C2 to O2 is

23

accomplished via TS-ST, and Br− separated from the C2 simultaneously. Finally, the product


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


1

T generates. The energy barriers is respectively decreased to 5.3 kcal·mol-1. The

2

rate-determining step of the reaction is still the ring-opening reaction of PO, and the barrier

3

heights of this step and the ring-closure step are both decreased compared with the

4

one-catalyst reaction.

5 6


7

Figure 8 also shows the calculated reaction pathways of propylene oxide and carbon

8

dioxide catalyzed by 6-ion HEMIMBr clusters. The optimized structures of reactants,

9

products, transition states, and intermediates calculated at B3LYP/6-31+G(d,p) level are

10

displayed in Figure S9. Similar to reactions catalyzed by 3-ion, 4-ion and 5-ion clusters, after

11

a complex U was formed, the C2 atom of PO is activated by the nucleophilic attack of the Br−

12

and the O3 atom is activated by the electrophilic attack of H1 atom in hydroxyl group. In the

13

transition state TS-UV, the collective effect of cations in HEMIMBr was thought to exist in

14

activating the reaction. Due to the steric hindrance, only one hydrogen atom from hydroxyl

15

group, the H1 atom, would attack O atom of propylene oxide to activate the C2–O3 bond of PO

16

through the H-bond interaction between them. Meanwhile, the adjacent C2 atom in PO suffers

17

from nucleophilic attack of Br−. H2 atom in another hydroxyl group would form a hydrogen

18

bond with this Br−. After the transition state TS-UV, complex V generates. It can be seen that

19

addition of HEMIMBr ions realizes an enhancement in the reactivity of ring-opening of PO,

20

and the energy barriers is 18.1 (U–TS-UV) kcal·mol-1 for six-ion-catalyzed reactions This

21

energy barrier is lower than 21.6 kcal·mol-1 for one-pair-catalytic reaction. Then, the addition

22

of CO2 to the activated PO occurs. Following the introducing of CO2 into V, a more stable

23

complex W generates. Via the TS-WX, the carbon atom of CO2 could easily bond to the

24

activated O3 atom and form the C3 − O3 bond with a quite low energy barrier of 1.7 (W–

25

TS-WX) kcal·mol-1. As a result, the complex X generates. The third step is the formation of

26

PC and the regeneration of HEMIMBr. On the basis of the X, the addition of C2 to O2 is

27

accomplished via TS-XY, and the Br− anion separated from the C2 atom simultaneously.

28

Finally, the product Y was generated. The energy barriers is respectively decreased to 3.2

29

kcal·mol-1. It could be found that the rate-determining step is still the ring-opening step , and

30

it is lower than that of separate HEMIMBr. It could indicate that six ion clusters could



1

enhance the reaction significantly relatively.

2

From the above calculations we could find that the mechanisms of 3-ions, 4-ions, 5-ions,

3

and 6-ions catalytic reactions are analogous to one-pair-catalyst reaction, involving three

4

steps: ring-opening of propylene oxide, addition of carbon dioxide, and ring-closure of

5

propylene carbonate. Besides, the barrier heights of the ring-opening step and the ring-closure

6

step catalyzed by clusters of HEMIMBr are both lower than that of one pair of HEMIMBr’s.

7 8

4. Conclusions

9

MD simulations were performed for nine systems in the fixation of CO2 catalyzed by

10

using HEMIMBr ILs. Starting from a random solution, the IL clusters were observed in

11

systems. There were 3-ions, 4-ions, 5-ions, and 6-ions clusters in systems, while the number

12

of clusters decreased as the cluster size increased. From the RDFs and SDFs for the nine

13

systems, it is observed that the cations in clusters were most likely around the hydrogen atoms

14

in the imidazole ring of a central cation. It was guessed that the ionic clusters play a role in

15

the reactions.

16

Reaction mechanisms of cycloaddition reaction catalyzed by 3-ions, 4-ions, 5-ions, and

17

6-ions clusters were investigated by the DFT calculation. From the calculations, we could find

18

that the mechanisms of ions clusters catalytic reactions are analogous to one-pair-catalyst

19

reaction, involving three basis steps: ring-opening of propylene oxide, addition of carbon

20

dioxide, and ring-closure of propylene carbonate. Besides, the following remark is concluded:

21

The barrier heights of the ring-opening of propylene oxide step and the ring-closure of

22

propylene carbonate step catalyzed by clusters of HEMIMBr ions are both lower than that of

23

one pair of HEMIMBr’s. This finding indicates the synergistic effects of ionic clusters on the

24

enhancement in reaction activities.

25

The above results about the IL clusters in the reaction could provide new insights into

26

the reaction mechanism of IL in solutions. We hope that this research is helpful to the further

27

applications of ionic liquids in chemical engineering processes.

28 29

ASSOCIATED CONTENT

30

Supporting Information


Page 20 of 26

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


1

The Supporting Information is available free of charge on the Internet.

2

AUTHOR INFORMATION

3

Corresponding Author

4

*E-mail [email protected]

5

Notes

6

The authors declare no competing financial interest.

7 8

ACKNOWLEDGMENT

9

This work was supported by National Science Fund for Excellent Young Scholars

10

(21722610), National Key R&D Program of China (2017YFB0307303), Natural Science

11

Foundation of Beijing of China (No.2182073), National Natural Scientific Fund of China

12

(91434203)

13

QYZDB-SSW-SLH022 and QYZDY-SSW-JSC011).

and

Key

Research

Program

of

Frontier

Sciences,

CAS

(NO.

14 15 16

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Table of Contents (TOC) Graphic:

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Theoretical Study of Ionic Liquid Clusters Catalytic Effect on the

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