Catalytic Activity of a Series of Synthesized and Newly Designed

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The catalytic activity of a series of synthesized and new-designed pyridiniumbased ionic liquids on the fixation of carbon dioxide: a DFT investigation Ya Li, Li Wang, Tengfei Huang, Jinglai Zhang, and Hongqing He Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01409 • Publication Date (Web): 04 Aug 2015 Downloaded from http://pubs.acs.org on August 11, 2015

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Industrial & Engineering Chemistry Research

Graphic Abstract

ACS Paragon Plus Environment


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The catalytic activity of a series of synthesized and new-designed pyridinium-based ionic liquids on the fixation of carbon dioxide: a DFT investigation Ya Lia, Li Wanga*, Tengfei Huanga, Jinglai Zhanga* Hongqing Heb a

Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R. China

b

Wuhan Center for Magnetic Resonance, State Key Laboratory of Magnetic Resonance and

Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China

Abstract Exploring a high-efficiency catalyst for the coupling reaction of carbon dioxide (CO2) with epoxide (PO) is still a challenging project. Ionic liquid (IL) is one of the most ideal catalysts since it could catalyze the coupling reaction in benign environment in absence of metal and organic solvent. The catalytic activity of a series of pyridinium-based ILs is theoretically investigated. The influences of the nature of cation, methylene chain length, and anion on the catalytic performance are explored. It has been proven that the catalytic activity of pyridinium-based IL is better than that of imidazolium-based and quaternary ammonium-based ILs. Since the properties of IL could be regulated by variation of cation and anion, four new ILs are designed by introduction of the -COOH, -OH, -SO3H, and -NH2 functional groups into the

*

Corresponding author. E-mail: [email protected]; [email protected]

1


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traditional pyridinium-based IL, respectively. Subsequently, the catalytic performance of four new-designed functionalized pyridinium-based ILs is compared with that of the

traditional

pyridinium-based

IL.

Only

the

carboxyl-functionalized

pyridinium-based IL has better catalytic activity than the traditional pyridinium-based IL. It is expected that the theoretical investigation might provide helpful clues for further experiments.

Keywords: Functionalized pyridinium-based ionic liquids; CO2; Chain length; Design

1. Introduction The utilization of CO2 has attracted continuous attentions owing to the increasing serious global warming together with decreasing availability of fossil fuels.1,2 The coupling reaction of epoxide (PO) with CO2 to afford the five-membered cyclic carbonates is one of the most significant approaches for the chemical fixation of CO2 because of 100% atom utilization and lower by-product. Moreover, cyclic carbonates are widely employed as electrolyte solvents for lithium batteries, aprotic polar solvents, and valuable fine chemical intermediates.3-6 Due to the thermodynamic stability and kinetic inertness of CO2, the catalyst is an essential factor to complete this process. Various catalysts have been employed to the synthesis of cyclic carbonates, such as metal oxides,7,8 transition metal complexes,9-16 organometallic compounds,17,18 and others. In the past ten years, the ionic liquid19-22 has been

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regarded as the most appropriate catalyst because of its negligible vapor pressure, excellent thermal stability, and non-flammability. More importantly, the ionic liquids are also employed as the solvent, so no additional solvent is needed in the production of cyclic carbonates. The catalytic activity of imidazolium-based and quaternary ammonium-based ILs has been widely investigated by the experimental method.23,24 Moreover, their mechanisms have been elucidated from a theoretical viewpoint. In contrast, little attention has been focused on the pyridinium-based ionic liquids. Recently, Park et al.25 employed a series of pyridinium-based ionic liquids to catalyze the fixation of the CO2. However, the mechanism is still obscure. As compared with the imidazolium-based and quaternary ammonium-based ILs, how is the catalytic activity of the pyridinium-based ILs? From the experience of previous studies, the catalytic activity would be improved when the functional group, i.e., -OH, -COOH, or others, is incorporated into the traditional ILs. Is this conclusion suitable for the pyridinium-based ionic liquids? To our best knowledge, no functionalized pyridinium-based ionic liquids are synthesized and employed as the catalysts for the coupling reaction of CO2 with PO. Conventional method, which from synthesis to measurement, is too slow to satisfy the current demand for discovery of the new catalysts. Designing new compounds and predicting their properties from theoretical viewpoint are alternative rapid routes. In this work, four new ILs are designed by addition of -OH, -COOH, -SO3H, and -NH2 on the 1-butylpyridinuim bromide (BPyBr), respectively. Their catalytic mechanism is explored. Moreover, the catalytic

3


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activities among different traditional ILs and between functionalized and traditional ILs are compared to determine their performance. It is expected to provide the useful clues and to improve the efficiency to discover new catalysts. 2. Computational details The equilibrium geometries of all the stationary points including reactants, products, intermediates, and transition states, were optimized by the B3PW9126,27 function with the 6-31G(d,p)28 basis set. The properties of stationary points were characterized by the vibrational frequencies calculated at the same level. The number of imaginary frequency indicates whether a minimum or a transition state has been located: all positive frequencies for a minimum and only one imaginary frequency for a transition state. Starting from the saddle-point geometries, the minimum-energy path (MEP) was constructed by intrinsic reaction coordinate (IRC) theory to determine that two desired minima were connected.29 On the basis of the optimized geometries, the energies were refined at the M06/6-311+G(2d,2p) level of theory.28,30 The atomic charge distributions were calculated by the natural bond orbital (NBO) analysis31,32 at the B3PW91/6-31G(d,p) level to better understand the catalytic activity. 3. Results and Discussion 3.1. Comparison of different methylene chain length in pyridinium-based ILs It is well known that the coupling reaction of CO2 with PO to afford PC is almost impossible to happen in absence of the catalyst in benign environment.33 The catalyst is employed to activate CO2 or to obtain the energy-rich substrate. The anion of IL could play a role as nucleophile to attack the C atom of CO2 leading to an activated

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CO2 or to attack the C atom of PO resulting in an energy-rich substrate. The latter is more competitive with the lower barrier height, which has been testified in the previous literature.34,35 The mechanism is that the anion attacks the C atom of PO to form a ring-opening substrate; next, the CO2 reacts with the activated ring-opening substrate; finally, the cyclic carbonate is formed by the subsequent intramolecular cyclization and simultaneously the catalyst is released.36-41 In general, the whole catalytic cycle follows a three-step mechanism, i.e., ring-opening, CO2 insertion, and ring-closure. Furthermore, learning from our previous work or literature,34,35 the rate-determining step is the ring-opening or ring-closure step. Alternatively, they are competitive steps with the similar barrier heights. Normally, the barrier height of CO2 insertion is small since there is no bond cleavage or bond formation in this step. In some cases there is no transition state as the CO2 insertion is not systematically activated, which has been proven in the previous literature.24,33 So the CO2 insertion is not stated out in this work. To explore the catalytic performance of different methylene chain length in pyridinium cations, i.e., CH3CnH2nPyCl n=1, 2, 3, and 5 corresponding to catalysts 1a, 2a, 3a, and 4a along the routes 1, 2, 3, and 4, the most favorable three-step route is studied. However, the CO2 insertion step is omitted because it is not the rate-determining

step.

The

energy

profiles

obtained

at

the

M06/6-311+G(2d,2p)//B3PW91/6-31G(d,p) level are plotted in Figure 1, in which the CO2 insertion step is represented by a straight dashed line. Note that the sum of energies of isolated reactants (CO2+PO) and catalyst is taken as zero. In the following discussions the relative energy is calculated with respect to the sum of isolated

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reactant and catalyst corrected by ZPE, that is, ∆E(relative energy)=E(transition state/complex)-[E(reactant)+E(catalyst)]+∆ZPE. And the energy of corresponding complex is set to be zero point to evaluate the barrier height, which is also corrected by

ZPE,

that

is,

∆E(barrier

height)=E(transition

state)-E(corresponding

complex)+∆ZPE. The structures of the transition states related with the ring-opening and ring-closure process catalyzed by 1a-4a catalysts are plotted in Figure S1 along with labeling the key atoms. Hydrogen bond is formed between H atom of methylene group of the cation and O atom of PO related with the ring-opening step of all the transition states. So the ring-opening is the cooperative result of both the weak hydrogen bond interaction between the O atom of PO and an H atom of the butyl group of the cation and nucleophilic attack from the anion. For all routes, the ring-opening step is the rate-determining step with the higher barrier height as compared with that of the ring-closure step. The catalytic activity increases with the increase of methylene number from 1 to 3 and decreases when the number is above 3. When the number of methylene group is lower than 3, the anion activity is increased with the elongation of the alkyl chain length. The distances between cation and anion are longer with the increase of the methylene chain length resulting in a higher anion activation capacity.42 The charge of Cl- anion is increased from 1a to 3a with the values

of

-0.527e

for

1-ethylpyridinium

chloride

(EPyCl),

-0.533e

for

1-propylpyridinium chloride (PPyCl), and -0.815e for 1-butylpyridinium chloride (BPyCl), which is calculated by NBO method in isolated IL. The more negative charge of Cl- anion would facilitate the nucleophilic attack resulting in the higher activity. In contrast, the charge of anion is decreased when the number of methylene group is higher than 3. Moreover, the steric hindrance of bulky cation with the longer methylene chain length is unfavorable to the weak hydrogen bond interaction between

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the O atom of the PO and one of the H atoms of the butyl group. In addition, it would also decrease the catalytic activity. Consequently, the pyridinium-based ILs with the moderate methylene chain length (n=3) has the best catalytic activity, which is consistent with the result of phosphonium-based ILs.43 The choice of a suitable methylene chain length is an essential factor to refine the catalytic activity for a series of ILs with the same skeleton. 3.2 Comparison of different anions Since the BPyCl has the best catalytic activity in the abovementioned ILs, it is chosen to compare with other catalysts. The ring-opening step is the rate-determining step, so the choice of an anion with stronger nucleophilic ability would improve the catalytic activity. When the halide anion is changed from Cl- to Br-, the barrier height of ring-opening step is decreased by 0.10 kcal/mol resulting in a higher activity. So the strong nucleophilic anion with moderate methylene chain length in cation would have a better catalytic performance. The energy profiles for route 3 and route 5 catalyzed by BPyCl and BPyBr are plotted in Figure 2. The structures of transition states involved in the two routes are similar because of the same cation, so structures of route 5 are not plotted for clarity. 3.3 Comparison of different cations One of the most important advantages of ILs is that their property is easy to be regulated by employment of different cations and/or anions. How about the catalytic performance for cyclic carbonate synthesis of CO2 with epoxides if the Cl- anion is combined with other cations? The most favorable route of the CO2 fixation catalyzed by other ILs, including 1-butyl-4-methylpyridinium chloride (BMPyCl) (route 6), 7


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1-butyl-3-methylimidazolium

chloride

(BMImCl)

(route

7),

and quaternary

ammonium chloride (Bu4NCl) (route 8) is explored, which has been proven that they have the similar three-step mechanism. The energy profiles are presented in Figure 3 along with that of route 3 catalyzed by BPyCl for comparison. The same methylene chain

length,

i.e.,

butyl

group,

is

employed

in

the

pyridinium-based,

4-methylpyridinium-based, imidazolium-based, and quaternary ammonium-based ILs, respectively. The catalytic activity decreases in the following sequence, BMPyCl → BPyCl → BMImCl → Bu4NCl, according to the barrier height of ring-opening step (rate-determining step). Thus, the 4-methylpyridinium-based IL has the best catalytic activity in all the studied ILs. 3.4 Design new ILs In above sections, the mechanism is studied and the catalytic activity is compared among different traditional ILs by the theoretical method. The theoretical results show that the combination of the moderate methylene chain length (n=3) and bromide anion (Br-) is the best choice to obtain the higher activity for the pyridinium-based ILs, which is consistent with the experimental result.25 However, the theoretical study should play a more important role to provide helpful clues for discovery of new ILs. The new ILs should be designed and their catalytic activity should be determined by theoretical method before they are synthesized in laboratory, which would greatly improve the ratio of performance/cost to explore the new ones. Sun et al.44 have proven that the catalytic activity of traditional IL is enhanced for the coupling reaction of CO2 with PO by introduction of the hydroxyl functional group

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into the cation of imidazolium-based and quaternary ammonium-based ILs. The presence of -OH group is the impetus to enhance the catalytic activity because of the cooperative action of electrostatic interaction and hydrogen bond. Furthermore, the role of the –OH group has been clearly testified in a binary-catalyst system of phenolic compounds/nBu4NI.45 Therefore, the functional group is introduced into the traditional IL to design the high-efficiency catalyst. It is expected that the hydrogen bond is formed between the functional group and the substrate to promote the ring opening without the organic solvent. Inspired by it, four functionalized pyridinium-based ILs are designed by addition of four functional groups, i.e., -COOH (1b), -OH (2b), -SO3H (3b), and -NH2 (4b), on the BPyBr, respectively. And their catalytic activity is determined to compare with the BPyBr. The mechanism of the cycloaddition of CO2 catalyzed by the functionalized IL is similar with that catalyzed by the corresponding traditional IL, which has been reported in our previous work.34 However, the most favorable route is totally different. Only the most favorable route is stated out. For carboxyl-functionalized pyridinium-based IL, a complex cp9-1 with the relative energy of -10.87 kcal/mol is firstly formed via the hydrogen bond between the hydrogen atom of carboxyl and the oxygen atom of PO (H5-O1). Next, complex cp9-1 converts into intermediate cp9-2 via ts9-1 with the relative energy of 7.84 kcal/mol. The imaginary vibration of ts9-1 corresponds to the stretching of H5-O5 bond and C2-O1 bond and the shrinking of H5-O1 bond, which indicates that the transfer of proton from O5 to O1 and the ring open of PO are completed with a concerted mechanism. Subsequently, the

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intermediate cp9-2 is formed with the bond distances of H5-O5 and H5-O1 being 1.81 Å and 0.99 Å, respectively, indicating that the proton has transferred from the O5 atom of IL 1b to the O1 atom of PO. The proton transfer is a common phenomenon that has been reported.34,46 The rupture of C2-O1 bond should be attributed to the cooperatively promoted by the weak hydrogen bond interaction between the O atom of PO and an H atom of the functional group and nucleophilic attack from the anion. Subsequently, the CO2 is introduced into the reaction system. Finally, the PC is formed and the catalyst 1b is regenerated by an intramolecular cyclic SN2-type reaction with ts9-3 as the transition state, in which the Br- anion departs from C2 atom as a result of the backside nucleophilic attack of O3 atom. The most favorable routes of the cycloaddition reaction catalyzed by 2b and 3b are the same, which are not discussed one by one. As to the amino-functionalized IL, its catalytic mechanism is totally different

from

all

other

functionalized

and

traditional

ILs.

Two

1-(3-aminopropyl)pyridinium bromide (4b) and one CO2 molecule firstly react to form carbamic acid. The addition of C1 atom to N2 atom is accompanied with shift of the H1 atom from N2 atom to N3 atom and migration of the H2 atom from N3 atom to O1 atom. As a result, the carbamic acid is formed with the barrier height of 17.12 kcal/mol, indicating that the formation of the carbamic acid is easy to be done at room temperature. Starting from the carbamic acid, the following mechanism is the same with that catalyzed by the catalyst 1b. The energy profiles for routes 9-12 catalyzed by 1b, 2b, 3b, and 4b are shown in Figure 4. The related barrier heights are tabulated in Table 1. And the structures of the

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transition states are plotted in Figure S2. Only the ring-opening and ring-closure steps are presented. For the 1-(3-hydroxypropyl)pyridinium bromide (2b), the barrier height of ring-opening step is the higher than that of ring-closure step by 6.85 kcal/mol to be the rate-determining step. In contrast, the ring-closure step is the rate-determining step with the higher barrier height for route 11 catalyzed by 3b. As to other two catalysts 1b and 4b, the ring-opening and ring-closure are competitive steps with a similar barrier height. As compared with the BPyBr, the catalytic activity increases in the order of 3b < 4b < 2b < BPyBr < 1b with the rate-determining barrier heights of 25.49, 23.63, 23.11, 22.77, and 18.71, respectively. Only the catalytic activity of the carboxyl-functionalized pyridinium-based IL is better than that of the traditional pyridinium-based IL. The functionalized IL is not a guarantee to obtain a higher efficiency as compared to the traditional IL with the same cation. The choice of a suitable functional group is also an important factor to obtain a high efficient catalyst. 4 Conclusions In this contribution, the effects of the methylene chain length, the nature of cation, and the anion on the catalytic activity of a series of catalysts are compared to determine the best combination. Then, four new functionalized ILs are designed and their catalytic performance is determined. On the basis of above studies, we get the following conclusions:

The moderate methylene chain length, i.e., n=3, for the pyridinium-based IL

is the best choice to achieve the higher catalytic activity.

The catalytic activity of Br- anion is better than that of Cl- anion because of

11


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the stronger nucleophilic ability.

As compared with the quaternary ammonium-based and imidazolium-based

traditional ILs with the same methylene chain length, the catalytic performance of the pyridinium-based IL is better.

Four functionalized pyridinium-based ILs are designed with addition of

-COOH, -OH, -SO3H, and –NH2 functional groups, respectively. Only the carboxyl-functionalized pyridinium-based IL has a better catalytic activity than the traditional pyridinium-based IL. In contrast, other three functionalized ILs have poor performance. The catalytic activity of the hydroxyl-functionalized pyridinium-based IL is even lower than that of the traditional pyridinium-based IL, which is contrary to the hydroxyl-functionalized imidazolium-based IL.

To design a new catalyst with the higher activity, the following factors

should be considered, including the methylene chain length, the anion, the nature of cation, and the functional group. Acknowledgements We thank the Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences for providing computational resources. This work was financial supported by the National Natural Science Foundation of China (21376063, 21476061), Program for He’nan Innovative Research Team in University (15IRTSTHN005), Natural Science Foundation of He’nan Province of China (144300510032, 142300410120), and Science Foundation of Henan Province (14A150034).

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Figure 1 Potential energy profiles for the cycloaddition reaction along routes 1-4 calculated at the M06/6-311+G(2d,2p)//B3PW91/6-31G(d,p) level. Route 3 is the most favorable reaction route.

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Figure

2

Potential

energy

profiles

for

the

cycloaddition

reaction

along

route

Page 22 of 25

3

and

M06/6-311+G(2d,2p)//B3PW91/6-31G(d,p) level. Route 5 is the most favorable reaction route.

21


route

5

calculated

at

the

Page 23 of 25

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Figure


3

Potential

energy

profiles

for

the

cycloaddition

reaction

along

route

3

and

M06/6-311+G(2d,2p)//B3PW91/6-31G(d,p) level. Route 6 is the most favorable reaction route.

22


routes

6-8

calculated

at

the


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

Figure 4 Potential energy profiles for the cycloaddition reaction along routes 9-12 calculated at the M06/6-311+G(2d,2p)//B3PW91/6-31G(d,p) level. Route 9 is the most favorable reaction route.

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Table 1 The characters of routes 9-12 involved in this work. Route

Route 9

Mechanism

Reactants

CO2

O

Three-step

Catalyst

1b

N

Br

Rate-determining step

Barrier height (kcal/mol)

Ring-opening step

18.7

Ring-opening step

23.1

Ring-closing step

25.5

Ring-closing step

23.6

COOH

Route 10

CO2

O

Three-step

2b

N

Br OH

Route 11

CO2

O

Three-step

3b

N

Br SO3H

Route 12

CO2

O

Four-step

4b

N

Br NH 2

24


Catalytic Activity of a Series of Synthesized and Newly Designed

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