Insights into quaternary ammonium salts-catalyzed fixation carbon dioxide with epoxides

Abstract

Chemical fixation of CO₂ into useful organic compounds has attracted much attention in both academia and industry from the standpoint of environmental protection and resource utilization. In this respect, the fixation of CO₂ with epoxides catalyzed by quaternary onium salts has been successfully achieved and commercialized, however, the details of the mechanism are still unclear. In this work, we investigated the mechanism of fixation of CO₂ with ethylene oxide catalyzed by quaternary ammonium salts by experimental and density functional theory (DFT) studies. Through the study, the detailed structural and energetic information about each step of the catalytic cycle were obtained. In addition, several factors, such as the chain length and anion, affected the reaction mechanisms, and the outcomes were also identified by experimental and DFT studies. This work provides a molecular level understanding of the reaction process and forms the basis of rational design of catalytic system.

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1. Introduction

Carbon dioxide (CO₂) is a main greenhouse gas, which is also a cheap, nontoxic and abundant C1 building block. Chemical fixation of CO₂ into useful organic compounds has attracted much attention in both academia and industry from the standpoint of environmental protection and resource utilization.¹ One of the most promising methodologies in this area is the synthesis of five-membered cyclic carbonates via the fixation of CO₂ with epoxides (Scheme 1).² These carbonates, e.g. ethylene carbonate (EC) and propylene carbonate (PC), have wide applications as polar aprotic solvents, electrolytic elements of lithium secondary batteries, intermediates for organic and polymeric synthesis, and ingredients for pharmaceutical/fine chemicals in biomedical applications.³

Scheme 1 Fixation of CO₂ with epoxides into cyclic carbonates.

Many catalysts have been effectively developed for the fixation of CO₂ and epoxides to produce cyclic carbonates, such as metal oxides,⁴ alkali metal salts,⁵ quaternary onium salts,⁶ ionic liquids,⁷ transition metal complexes⁸ and functional polymers.⁹ In this respect, the quaternary onium salts, such as quaternary ammonium salts and quaternary phosphonium salts, exhibited high activity and stability for this reaction. Especially, the fixation of CO₂ and ethylene oxide (EO) to produce EC catalyzed by tetraethylammonium bromide (Et₄NBr) and tetrabutyl phosphonium iodide was commercialized by Bayer and Mitsubishi respectively. Although the fixation of CO₂ with epoxides catalyzed by quaternary onium salts has been successfully achieved, however, the details of the mechanism are still unclear. In this respect, the investigation and understanding the reaction process for fixation of CO₂ with epoxides catalyzed by quaternary onium salts are still highly desired, which would provide a molecular level understanding of the reaction process and form the basis of rational design of catalytic systems.

The mechanism for fixation of CO₂ with epoxides catalyzed by quaternary onium salts was proposed by Caló.^6a The ring of the epoxide first opened by a nucleophilic attack by the bromide ion, which led to an oxy anion species, and then afforded the corresponding cyclic carbonate after reaction with CO₂ (Scheme 2). There were also several DFT studies on the reactions of CO₂ with epoxides. Catalysts studied included heterobimetallic Ru-Mn complexes,¹⁰ alkylmethylimidazolium chlorine ionic liquids,¹¹ transition metal,^12,13 LiBr.¹⁴ As a continuous effort to fix CO₂ with epoxide,¹⁵ in this article, a detailed mechanism of conversion of ethylene oxide with CO₂ catalyzed by quaternary ammonium salt was elucidated by DFT calculations. Through the study, the detailed structural and energetic information about each step of the catalytic cycle were obtained. In addition, several factors, such as the chain length and anion, affected the reaction mechanisms, and the outcomes were also identified by experimental and DFT studies.

Scheme 2 The proposed mechanism for fixation of CO₂ with EO.

2. Results and discussion

2.1 The reaction mechanism

We initially studied the cycloaddition reaction in the presence of quaternary ammonium salt through DFT study. Et₄NBr was chosen as a representative to study the mechanism details. By performing a PES scan, we found that the reaction involved three elementary steps (Fig. 1), and the optimized structures for all intermediates and transition states involved in this pathway are shown in Fig. 2. Initially, EO and Et₄NBr interacted to form a complex A. And then this complex could be converted into intermediate B via transition state TS1 with a barrier of 29.76 kcal mol⁻¹. The unique imaginary frequency of this transition state was 306i cm⁻¹. When CO₂ entered into the reaction system, a new complex C was formed. This step proceeds via barrierless reaction pathways. Subsequently, intermediate C converted into product via an intramolecular cyclic reaction, as demonstrated by transition state TS2. The frequency calculation on TS2 gave one imaginary frequency of 399i cm⁻¹, and its direct dissociation resulted in the formation of EC and the release of Et₄NBr, and thereby a catalytic cycle was completed.

Fig. 1 Potential energy surface profiles of Et₄NBr-catalyzed process.

Fig. 2 Optimized geometries for the intermediates and transition states for fixation of CO₂ with ethylene oxide catalyzed by Et₄NBr.

2.2 Effect of chain length and anion

Furthermore, to understand the effects of cation structure and the anion on the fixation of CO₂, we also performed the PES scans for the fixation of CO₂ with EO catalyzed by Bu₄NBr and Et₄NCl respectively. The potential energy surface profiles process and optimized geometries for the intermediates and transition states were shown in Fig. 3–6. The rate-determining steps were all found to be the first step with a bigger barrier of around 29 kcal mol⁻¹, which is easily overcome at the experimental temperature of 100 °C¹⁶ compared with the uncatalyzed reaction (Table 1, entry 4 vs. entry 5) involving one elementary step with barriers as high as 58.92 kcal mol⁻¹ (Fig. S1, see ESI†). In addition, the quaternary ammonium salts with different chain length and anion had little influence on the relative activation energy of the rate-determining step (Fig. 7), the relative activation energy slightly increased in the order of Et₄NCl > Et₄NBr > Bu₄NBr. Moreover, it is found that the barrier of each elementary step slightly decreased with enlarging the chain length (Fig. 1vs.Fig. 3), and the order of the barrier for anions was Cl > Br (Fig. 1vs.Fig. 5). However, the above barriers vary at most by about 1 kcal mol⁻¹, which is even smaller than the error bars of the present B3PW91/6-31++G(d,p) calculation. Further experimental study will be performed to investigate the effects of cation structure and the anion on the fixation of CO₂ compared with DFT results.

Fig. 3 Potential energy surface profiles of Bu₄NBr-catalyzed process.

Fig. 4 Optimized geometries for the intermediates and transition states for fixation of CO₂ with ethylene oxide catalyzed by Bu₄NBr.

Fig. 5 Potential energy surface profiles of Et₄NCl-catalyzed process.

Fig. 6 Optimized geometries for the intermediates and transition states for fixation of CO₂ with ethylene oxide catalyzed by Et₄NCl.

Table 1 Fixation of CO₂ with EO and PO^a

Entry	Substrate	Catalyst	Conv. (%)^b	Yield (%)^b
a Reaction conditions: Epoxide (14.3 mmol), catalyst (0.143 mmol, 1% mmol), pressure (3 MPa), temperature (100 °C), time (2 h). b Conversion and yield were determined by GC using biphenyl as the internal standard. Every experiment was repeated 3 times.
1	EO	Bu4NCl	81	81
2	EO	Bu4NBr	78	78
3	EO	Bu4NI	76	76
4	EO	Et4NBr	77	77
5	EO	None	0	0
6	PO	Bu4NCl	73	72
7	PO	Bu4NBr	56	56
8	PO	Bu4NI	54	53
9	PO	Et4NBr	57	57

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Fig. 7 Comparison of relative activation energy.

2.3 Catalytic activity towards various quaternary ammonium salts

Additionally, in order to study the influence of chain length and anion for fixation of CO₂ with epoxide, the corresponding experiments were also performed using EO as substrate. The results revealed that activities of anions increased in the order of Cl⁻ > Br⁻ > I⁻ with the same cation (Table 1, entries 1–3), which was not in accord with the relative energy of the rate-determining step. However, this phenomenon could be explained by the stronger nucleophilicity and the smaller steric hindrance of Cl⁻ than Br⁻ and I⁻, which easily attacked the EO and led to ring-opening of EO. In addition, the quaternary ammonium salts with different chain length had little influence on EO conversion (entry 2 vs. entry 4), which was in accord with the relative energy of the rate-determining step. However, previous research by others found the reaction favoured a longer alkyl chain length especially for the epoxides with big steric hindrance,¹⁷ which might be due to bulk solvent effects, which is not taken into account in the present calculations.¹¹ Moreover, the same trend could be found when using propylene oxide (PO) as a substrate (entries 6–9), and the activity of epoxides was EO > PO (entry 1 vs. entry 6 and entry 2 vs. entry 7).

2.4 A comparison of substrate activity

In order to examine the substrate activity of the catalytic system, a 1 : 1 molar mixture of epoxides was tested. The results also indicated the activity of epoxides was EO > PO, the yields were 62% and 48% respectively (Scheme 3, eqn (1)). The styrene oxide and cyclohexene oxide had relatively lower substrate activity, and needed a longer reaction time to achieve high yield (Scheme 3, eqn (2) and (3)). It is important to note that the selectivity to corresponding cyclic carbonates was near 100%, no undesired by-products were detected in the catalytic system.

Scheme 3 Comparison of epoxide activity.

3. Experimental

3.1 Materials

The quaternary ammonium salts were provided by Beijing zhongke anyin technology Co. Ltd. with a stated purity of >98%, and dried in vacuum for 48 h before use. The ethylene oxide and propylene oxide were purchased from J&K CHEMICA without further purification. GC-MS were measured on a Finnigan HP G1800 A. GC analyses were performed on an Agilent GC-6890 equipped with a capillary column (DB-624, 30 m × 0.32 μm) using a flame ionization detector. NMR spectra were recorded on a Bruker 300 or Varian 400 spectrometer in CDCl₃. ¹H and ¹³C NMR chemical shifts (δ) were given in ppm relative to TMS. ¹H and ¹³C positive chemical shifts (δ) in ppm were downfield from tetramethylsilane (CDCl₃: δ_C = 77.0 ppm; residual CHCl₃ in CDCl₃: δ_H = 7.26 ppm). The calculations were carried out by performing DFT by use of the B3PW91functional with the 6-311++G(d,p) basis set as implemented in Gaussian 09 program package. All geometries for the isolated reactants, products, intermediates, and transition states involved in the cycloaddition reaction have been fully optimized. Vibrational frequency calculations, from which the zero-point energies (ZPEs) were derived, have been performed for each optimized structure at the same level to identify the nature of all the stationary points (local minimum or first-order saddle point). The intrinsic reaction coordinate (IRC) pathways have been traced in order to verify that each saddle point links two desired minima.

3.2 Experimental details

The reactions of CO₂ fixation with epoxides were conducted in a 25 mL stainless steel reactor equipped with a magnetic stirrer and automatic temperature control system. Typically, in the reactor, an appropriate volume of CO₂ (1.0 MPa) was added to a mixture of epoxide (14.3 mmol), quaternary ammonium salt (0.143 mmol) at room temperature. The temperature was then raised to 100 °C while more CO₂ was added from a reservoir tank to maintain a constant pressure (3 MPa). After the reaction had proceeded for 2 h, the reactor was cooled to 0 °C in an ice water bath, and the remaining CO₂ was removed slowly. The products were analyzed by GC using biphenyl as the internal standard and identified by GC-MS by comparing retention times and fragmentation patterns with authentic samples. The product was purified by distillation or silica gel column chromatography if necessary.

The typical experiments of the comparison of substrate activity were conducted in a 25 mL stainless steel reactor. Typically, in the reactor, an appropriate volume of CO₂ (1.0 MPa) was added to a mixture of EO (14.3 mmol), PO (14.3 mmol) and quaternary ammonium salt (0.286 mmol) at room temperature. The temperature was then raised to 100 °C while more CO₂ was added from a reservoir tank to maintain a constant pressure (3 MPa). After the reaction for 2 h, the reactor was cooled to 0 °C in an ice water bath, and the remaining CO₂ was removed slowly. The products were analyzed by GC using biphenyl as the internal standard and identified by GC-MS by comparing retention times and fragmentation patterns with authentic samples. The product was purified by silica gel column chromatography if necessary.

Spectral characteristics of the cyclic carbonates, EC: ¹H NMR (CDCl₃, TMS, 400 MHz): 4.2 (t, J = 10 Hz, 4H); ¹³C NMR (CDCl₃, TMS, 100.4 MHz): 63.3, 155 (C=O). PC: ¹H-NMR (CDCl₃, TMS, 400 MHz):1.49 (d, J = 6.0 Hz, 3H); 4.05 (t, J = 8.8 Hz, 1H); 4.60 (t, J = 8.0 Hz, 1H); 4.86–4.94 (m, 1H); ¹³C NMR (CDCl₃, TMS, 100.4 MHz): 18.95, 70.46, 73.51, 154.95 (C=O). 4, 5-tetramethylene-1,3-dioxolan-2-one: ¹H-NMR (CDCl₃, TMS, 400 MHz): 1.80–1.86 (m, 2H); 1.97–2.05 (m, 2H); 2.28 (d, 4H, J = 4.8 Hz); 5.06–5.11 (m, 2H); ¹³C NMR (CDCl₃, TMS, 100.4 MHz): 19.00, 26.61, 75.65, 155.27 (C=O). 4-phenyl-1, 3-dioxolan-2-one: ¹H NMR (CDCl₃, TMS, 400 MHz): 4.34 (t, 1H, J = 8.4 Hz); 4.80 (t, 1H, J = 8.4 Hz); 5.68 (t, 1H, J = 8.0 Hz); 7.35–7.44 (m, 5H); ¹³C NMR (CDCl₃, TMS, 100.4 MHz): 71.10, 77.92, 125.81, 129.12, 129.63, 135.70, 154.81 (C=O).

4. Conclusions

In conclusion, we thoroughly elucidated the detailed mechanism of conversion of ethylene oxide with CO₂ catalyzed by quaternary ammonium salts by experimental and DFT studies. We found that the reaction involved three elementary steps and obtained all intermediates and transition states. In addition, the chain length and anion of quaternary ammonium salt affected the reaction mechanisms, and the outcomes were also identified. This work reported will help to understand and design catalysts for fixation of CO₂ with epoxides at a molecular level.

Acknowledgements

Support of the grant from the National Basic Research Program of China (2009CB219901) and National Natural Sciences Foundation of China (No. 21003129, 20936005) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Supporting figures, and cartesian coordinates for the optimized geometries of all species. See DOI: 10.1039/c2cy20103h

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