Multi-functional phosphonium-based deep eutectic ionic liquids

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Multi-functional phosphonium-based deep eutectic ionic liquids: insights into simultaneous activation of CO2 and epoxide, and their subsequent cycloaddition Fusheng Liu, Yongqiang Gu, Hao Xin, Penghui Zhao, Jun Gao, and Mengshuai Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b04090 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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ACS Sustainable Chemistry & Engineering

Multi-functional phosphonium-based deep eutectic ionic liquids: insights into simultaneous activation of CO2 and epoxide, and their subsequent cycloaddition Fusheng Liu,† Yongqiang Gu,† Hao Xin,† Penghui Zhao,† Jun Gao,‡ Mengshuai Liu*,† †

State Key Laboratory Base of Eco-chemical Engineering, College of Chemical Engineering, Qingdao

University of Science and Technology, Qingdao 266042, P.R. China ‡

College of Chemical and Environmental Engineering, Shandong University of Science and Technology,

Qingdao 266590, P.R. China

Full Mailing Address of Authors: †

No.53, Zhengzhou Road, Shibei District, Qingdao, 266042, PR China (F. S. Liu, Y. Q. Gu,

H. Xin, P. H. Zhao, M. S. Liu) ‡

No.579, Qianwangang Road, Huangdao District, Qingdao, 266590, PR China (J. Gao)

Email Address of the Corresponding Author: *E-mail: [email protected] (M. S. Liu)

ABSTRACT: Novel phosphonium-based deep eutectic ionic liquids were well-designed and facilely synthesized; their structures and physical properties were studied in detail. As singlecomponent and sustainable media, they were successfully used for efficient cycloaddition of CO2 to epoxides under mild conditions. The effects of ionic liquid structures, reaction parameters, and substrate scope on the cycloaddition reaction were examined thoroughly. Among the designed ionic liquids, the tetra-n-butylphosphonium bromide (TBPB) combined 3-aminophenol (3-AP) in a molar ratio of 1:2 was proved to be the most active catalyst, which could afford good to excellent cyclic carbonate yields with satisfactory selectivities under the optimum conditions (80 oC, 1 MPa, 1 h). Moreover, the TBPB/3-AP was easily recovered and showed excellent reusability. The TBPB/3-AP medium realized the simultaneous -1-

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activation of CO2 and epoxide, finally the cooperative activation and reaction pathways were proposed.

KEYWORDS: CO2 conversion; Homogeneous catalysis; Deep eutectic ionic liquid; Cycloaddition; Cyclic carbonate

INTRODUCTION Carbon dioxide (CO2), as one of the main greenhouse gases that associated climate change, its emission reduction has attracted considerable attention in recent years.1 Additionally, CO2 is still an abundant, non-toxic and sustainable C1 source, the utilization of CO2 as a chemical feedstock for the production of valuable chemicals is of great significance in terms of green chemistry and atom economy.2,3 Nevertheless, the highly efficient catalytic technologies need to be developed due to the inherent inertness of CO2 species. The selective coupling of CO2 and epoxides to produce cyclic carbonates (Scheme 1) is quite promising in this area due to their wide applications (e.g. as aprotic solvents, electrolytes in lithium-ion batteries, and intermediates for polymers, pharmaceuticals and fine chemicals).4

O O R Epoxide

CO2

Catalyst

O

O

T, P

R Cyclic Carbonate

Scheme 1. Cycloaddition of CO2 to epoxide

Up to now, a variety of homogeneous catalysts, including alkali metal halides,5 metalsalen complexes,6 metalloporphyrins,6,7 and ionic liquids (ILs)8–10 have been used to catalyze the cycloaddition of CO2 to epoxide. Most of them show excellent activities for the reaction under mild conditions. Nevertheless, it is difficult to separate the homogeneous catalysts from reaction systems, being a major hurdle for their large-scale applications.11 In order to overcome this issue, many efforts have been devoted to the attractive heterogeneous catalysts, such as metal-organic frameworks,12 covalent organic frameworks,13,14 porous organic -2-


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polymers,15,16 metal zeolitic imidazolate frameworks,17 nitrogen/metal-doped porous carbon18 and polymeric/supported ILs.19,20 Although these catalysts are easily recycled by filtration or centrifugation, they often require harsh reaction conditions and tedious preparation processes. In most of the cases, both the homogeneous and heterogeneous catalysts need transition metal ions, additives and/or nucleophilic cocatalyst (n-Bu4NBr, KI, etc.) to promote ring-opening of epoxide and coupling with CO2 except for ionic liquids.9 Therefore, the development of low cost, single-component, eco-friendly, and easily recyclable catalysts that can efficiently catalyze the coupling reaction under mild and metal/solvent-free conditions is still very desirable. Task-specific ILs as green and promising reaction media recently have drawn much attention and shown significant advance in the efficient conversion of CO2 into cyclic carbonates. Various hydrogen bond donors (HBDs)-based or metal-containing task-specific ILs have been developed, and the HBD groups (such as –OH, –COOH, –NH2) or metal ions play imperative roles in activating epoxide.9,21 However, the high cost, complicated synthesis and purification, and the use of toxic metals for substrate activation are still drawbacks that need to be improved. Deep eutectic solvents (DESs) have similar characteristics with the ILs.22 Moreover, they have preferable advantages, such as low cost, facile synthesis without purification and no by-products generation, and are widely used in CO2 capture,23 extraction separation,24 nanotechnology,25 analytical chemistry,26 and organic synthesis.27 By designing different structure of the DESs, they show superior abilities for CO2 capture and activation.28,29 Many efforts have been devoted to choline chloride-based DESs for the coupling of CO2 and epoxide to yield cyclic carbonate. Nevertheless, harsh reaction conditions are needed to obtain satisfactory product yield and selectivity.30–32 It may be due to the weak activation ability of the choline chloride-based DESs toward epoxide and CO2, and the poor nucleophilicity of Cl– anion for ring-opening of the epoxide. Previous reports have evidenced that either the ring-opening of the epoxide or activation of CO2 may be a rate-determining step of the reaction.33,34 These recent advances prompt our interest in design and synthesis of multi-functional DESs, which lead to the ring-opening of epoxide and simultaneous activation of CO2. Herein, several multi-functional phosphonium-based DESs were developed by using -3-



readily available tetra-n-butylphosphonium bromide (TBPB) as hydrogen-bonding acceptor (HBA) and various aromatic compounds as hydrogen-bonding donors (HBDs). The structures of the DESs as-obtained were shown in Scheme 2. The formation of the phosphonium-based DES through hydrogen bond was clearly demonstrated, and partial physical properties were provided. The influence of DES structures, reaction parameters, and substrate scope on the coupling reaction was investigated in detail. The possible interactions between the catalyst and reactants (i.e., epoxide and CO2) were studied to support the reaction mechanism well. As compared to the phosphonium-based ILs and other reported DESs, the catalysts presented here show comparable and superior performance under the milder conditions. More importantly, the single-component DESs are easily recovered by solvent extraction, and exhibit excellent reusability, which make the process more economical in industry application.

Scheme 2. Design of multi-functional catalysts in this work

RESULTS AND DISCUSSION Characterization and Physical Properties of Phosphonium-Based DESs The tetra-n-butylphosphonium bromide (TBPB) and 3-aminophenol (3-AP) were respectively selected as the HBA and HBD, and we initially confirmed the formation of the phosphonium-based DES by FT-IR, 1H NMR, 13C NMR and 31P NMR analyses. As shown in Figure 1A, the TBPB/3-AP integrated the characteristic peaks of both TBPB and 3-AP in the FT-IR spectra, the –OH and –NH2 vibration bands of 3-AP became broader and had -4-


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obvious red-shifts after interacting with TBPB due to the formation of the hydrogen bond.35 By 1H NMR analysis in Figure 2A, both of the H atom signals deriving from –OH (δ = 8.821 ppm) and –NH2 (δ = 4.876 ppm) groups in 3-AP conducted slight downfield shifts, also indicating the hydrogen bond formation.36 The results were consistent with the FT-IR analysis. The

31P

NMR spectra were almost the same before and after formation of the

TBPB/3-AP (Figure S4). In addition, the DSC data showed that the TBPB/3-AP (Tm = 73 oC) with molar ratio of 1:2 gave a lower melting point than both TBPB (Tm = 100 oC) and 3-AP (Tm = 121 oC) alone (Figure 1C). To our delight, the TBPB/3-AP could keep the eutectic liquid state when the temperature was reduced to ambient 20 oC, and no obvious exothermic peak was observed. By choosing different HBDs, various phosphonium-based DESs were obtained, such as TBPB/aniline and TBPB/phenol. While according to the 1H NMR characterization as shown in Figure 2B and Figure 2C, slight upfield shifts of the –NH2 (δ = 4.979 ppm) and –OH (δ = 9.432 ppm) proton signals appeared after forming the DESs, these phenomena might also result from the complicated hydrogen bond interaction. Other characterization results of the phosphonium-based DESs were shown in the Supporting Information (Figure S2-S11). As shown in Figure 1B, the TBPB/3-AP has excellent thermal stability when the temperature was lower than 200 oC, and it could be used to catalyze the cycloaddition of CO2 to epoxide. Furthermore, the physical properties of the phosphoniumbased DESs, including melting points, water contents, UV-vis absorption, and solubility were studied in detail, and the results were provided in the Supporting Information.

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Figure 1. (A) FT-IR spectra, (B) TGA and (C) DSC curves of TBPB, 3-AP and TBPB/3-AP.

Figure 2. 1H NMR spectra of (A) 3-AP and TBPB/3-AP, (B) aniline and TBPB/aniline, (C) phenol and TBPB/phenol in d6-DMSO at 298 K.

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Catalyst Screening The catalytic activities of various phosphonium-based DESs were investigated towards the cycloaddition of CO2 to propylene oxide (PO). As shown in Table 1, the TBPB and various HBDs including phenol, aniline, benzoic acid, and aminophenol did not afford satisfactory propylene carbonate (PC) yields when they were used alone (entries 1–7). While the deep eutectic IL that combined TBPB with 3-AP could effectively catalyze the cycloaddition reaction with 96% PC yield and 99% selectivity under the same conditions of 80 oC and 1 MPa CO2 for 1 h (entry 8), indicating the synergistic effects between TBPB and 3-AP. For a comparison, the HBDs with one substituent, namely phenol, aniline and benzoic acid were also screened (entries 9–11), but they all showed the lower activities than 3-AP. The results indicated that both the –OH and –NH2 groups in 3-AP showed positive effects on the coupling reaction, as they could activate the substrate and CO2, respectively. By changing the HBD component to 2-AP or 4-AP, no significant decrease of activity was observed, and the spatial position of the two active groups (–OH and –NH2) slightly affected the catalytic activity when combining with TBPB (entries 12 and 13). Notably, the TBPB/3-AP could also afford desirable results with 95% PC yield and 99% selectivity at the lower reaction temperature of 30 oC, although longer reaction time was required (entry 14). As new catalytic media for the conversion of CO2 into cyclic carbonate, the phosphonium-based DESs showed a great advance in the catalytic activity under mild and metal-free conditions.

Table 1. Catalyst Screeninga entry

HBA

HBD

1

TBPB

2

reaction resultsb PC yield (%)

PC selectivity (%)

—

35

≥98

—

phenol

trace

—

3

—

aniline

trace

—

4

—

benzoic acid

trace

—

5

—

2-AP

5

≥98

6

—

3-AP

11

≥98

7

—

4-AP

2

≥98

8

TBPB

3-AP

96

≥99 -7-



a

9

TBPB

phenol

81

≥99

10

TBPB

aniline

79

≥99

11

TBPB

benzoic acid

93

≥99

12

TBPB

2-AP

94

≥99

13

TBPB

4-AP

92

≥99

14c

TBPB

3-AP

95

≥99

Conditions: PO 15 mmol, catalyst loading 0.9 mmol (4.5 mol% to PO), 80 oC, 1 MPa, 1 h. b Determined

by GC analysis. c T = 30 oC, t = 24 h.

Effects of Reaction Parameters Next, we optimized the reaction conditions and examined the reusability of TBPB/3-AP catalyst (Figure 3). The temperature had a prominently positive impact on the coupling of CO2 and PO in the range of 40–80 oC, at which the PC selectivity was almost perfect (Figure 3A). When the temperature was increased to 90 oC, only a slight enhancement in the PC yield was observed. Hence, a mild temperature of 80 oC was chosen. Figure 3B depicted the influence of CO2 pressure on the catalysis reaction. When the CO2 pressure was 0.5 MPa, a passable 80% PC yield was obtained. It smoothly increased to 96% when the CO2 pressure was raised to 1.0 MPa. While further increasing the CO2 pressure was insignificant, and comparable high PC yields were obtained. The PC yield strongly depended on the catalyst loading (Figure 3C). When the catalyst loading was increased from 0.5 to 4.5 mol% (toward to PO amount), the PC yield was sharply improved from 35% to 96%. When the catalyst loading was further increased to 6 mol%, the PC yield was not obviously changed. Under the optimized reaction conditions, we repeated the cycloaddition of CO2 to PO catalyzed by the recycled TBPB/3-AP (Figure 3D). The PC yield still reached above 90% in the fifth cycle, and only a slight decrease of the catalytic activity was observed due to the loss of TBPB/3AP during the recycling process. Moreover, the recycled TBPB/3-AP was characterized by using FT-IR technique (Figure S12), and it indicated that the recycled TBPB/3-AP was identical to its pristine form.

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Figure 3. Influence of (A) temperature, (B) CO2 pressure, (C) catalyst loading, and (D) recyclability of the TBPB/3-AP catalyst. Standard conditions: PO 15 mmol, catalyst loading 4.5 mol%, 80 oC, 1 MPa, 1 h.

Based on the results as mentioned above, the simple TBPB/3-AP realized the efficient conversion of CO2 into cyclic carbonate under mild conditions. Then we compared the reaction conditions and catalytic performance of the TBPB/3-AP with various reported phosphonium-based ILs and DESs, as shown in Table 2. Notably, the TBPB/3-AP showed superior or comparable activities to the reported catalysts at a lower temperature for shorter reaction time. Moreover, the present TBPB/3-AP was facilely synthesized, and it avoided the use of metals and solvents in the catalysis.

Table 2. Comparison of Various Phosphonium-Based ILs and DESs for the Cycloaddition of CO2 to PO T

P

t

yield

selectivity

(oC)

(MPa)

(h)

(%)

(%)

catalyst

ref.

-9-



a

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[Ph3PC2H4OH]Br

120

1

4

66

—

37

[Bu3PC2H4OH]Br

120

1

4

99

—

37

(C6F13C2H4)3MePI

100

14

24

93

99

38

[Ph3PC2H4COOH]Br

130

2.5

3

97.3

99.8

39

Ph4PI/ZnBr2 (6:1)

120

2.5

1

88.7

99.0

40

ChCl/PEG400 (1:2)

130

1.2

5.0

89.4

99.4

30

Urea/ZnI2 (3:1)

120

1.5

3.0

95.0

98.0

41

ChCl/urea (1:2)

110

—a

10.0

99.0

—

31

TBPB/3-AP (1:2)

80

1

1

96.0

99.0

this work

Molar ratio of CO2/substrate was 2.45.

General Applicability The substrate scope of the coupling reaction catalyzed by TBPB/3-AP was investigated. As shown in Figure 4, the terminal epoxides were successfully converted to their corresponding products with satisfactory yields under mild conditions (80 oC, 1 MPa, 1 h). The terminal epoxides with an electron-withdrawing substituent and smaller steric hindrance were more liable to conduct the coupling reaction with CO2. For the internal epoxide, such as cyclohexene oxide, only 19% product yield was detected due to the larger steric hindrance, which limited its ring-opening reaction and the subsequent CO2 insertion step.

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O O

TBPB/3-AP

CO2

R 1 O O

R

O

2a, 96%

O O

O

O O

Cl

O

O

O O

Br 2d, 99%

O

2c, 89%

2b, 93%

O

O 2

O O

O

O

80 oC, 1.0 h 1.0 MPa CO2

O

O

O

O

Ph 2e, 97%

2f, 87%

2g, 19%

Figure 4. Cycloaddition of CO2 to various epoxides using TBPB/3-AP catalyst. Conditions: epoxide 15 mmol, catalyst 4.5 mol%, product yields were determined by GC analysis with dodecane as an internal standard.

Plausible Reaction Mechanism To better support the reaction mechanism, we initially studied the interaction between the TBPB/3-AP and CO2 species. As shown in Figure 5A, a new FT-IR absorption peak was observed at 1790 cm-1, it belonged to C=O vibration of carbamate salt formed between 3-AP and CO2. The

13C

NMR spectra were further collected to evidence the activation of CO2

(Figure 5B), new peaks centered at 75.6 and 71.7 ppm were detected, indicating CO2 activation by TBPB/3-AP. The results were consistent with the previous reports about CO2 activation by free amino grafted metal-organic frameworks and ionic liquids.42–44

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OH

CO2 NH2

3-AP

OH

3-AP

OH

OH

NHCOO

NHCOOH

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R-NH-COO

NH3 R-NH3

Figure 5. (A) FT-IR and (B) 13C NMR spectra of TBPB/3-AP before and after interacting with CO2.

Experimental and theoretical studies suggested that HBDs cooperated with halide anions could have a positive effect on the cycloaddition reaction.9,45 Then we provided an insight into the PO activation by forming hydrogen bonds with TBPB/3-AP. As shown in Figure 6, upon addition of PO to the TBPB/3-AP, slight upfield shifts of the –OH proton (from δ = 8.823 to 8.811 ppm) and –NH2 proton (from δ = 4.885 to 4.868 ppm) signals in TBPB/3-AP were observed. It indicates that the PO forms hydrogen bonds with both the –OH and –NH2 groups.46 The result was inconsistent with the silanediol- and boronic acids-based binary catalytic systems for PO activation that reported by Mattson and Zhang et al.,36,47 it might be due to the complicated hydrogen-bond interaction in the DES. The newly formed hydrogen bond weakened the internal interactions between bromine anions (Br–) and the HBD groups -12-


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in TBPB/3-AP system, which improved the flexibility and nucleophilic attack ability of Br–, then further promoted ring-opening of the epoxide. A similar activation pattern was obtained by Hirose et al. using 2-pyridinemethanol and n-Bu4NI as two-component catalysts.46

Figure 6. 1H NMR spectra of TBPB/3-AP (A) before and (B) after interacting with PO.

Based on the above analyses, a feasible reaction mechanism catalyzed by TBPB/3-AP is proposed (Scheme 3). The epoxide is activated by the –OH and –NH2 groups in 3-AP through hydrogen bonds (Step I). Simultaneously, the less sterically hindered β-C atom of epoxide was attacked by the Br− anion to form a ring-opened oxyanion intermediate, which is stabilized by the –OH and –NH2 groups (Step II). Subsequently, a nucleophilic addition allows to proceed between the oxyanion intermediate and activated CO2 species, producing a new alkyl carbonate anion (Step III). Finally, an intramolecular ring-closure step takes place to form the cyclic carbonate along with the catalyst regeneration.

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HO O C4H9

P

C4H9

C4H9 C4H9

Br

HO

R

H NH H O

NH H

O H O

Step I R

O O

C4H9 Br C H 4 9

P

C4H9 C4H9

H2N

O

Step II

Step IV

R

HO

NH2

NH H

O H O

Br O

C

NH2 O

C4H9 C4H9

P

R

HO

Step III

C4H9 C4H9

NH H

O H O

R

O N C R NH3 H O activated CO2

R Br

NH2 C4H9 C4H9

P

C4H9 C4H9

Scheme 3. Proposed reaction pathway for cyclic carbonate formation catalyzed by TBPB/3-AP

CONCLUSIONS Novel phosphonium-based deep eutectic ILs are successfully developed, and used for efficient catalyzing the cycloaddition of CO2 to various epoxides under mild and metal-free conditions. Different catalysts structure and reaction parameters are optimized. The unique design of phosphonium-based deep eutectic ILs endows the catalytic media with Lewis base property, hydrogen bond donor ability, and nucleophilicity. The protocol realizes the ringopening of epoxide and simultaneous activation of CO2, avoiding the use of an additional cocatalyst. Compared with the reported phosphonium-based and other deep eutectic ILs, the optimum TBPB/3-AP exhibits a comparable or superior catalytic activity under the milder conditions even at ambient temperature. The single-component catalytic media presented here are easily recovered by solvent extraction, and show excellent catalyst reusability and economic practicality. In addition, the well-designed phosphonium-based deep eutectic ILs are low cost, facilely synthesized and eco-friendly, showing great potential for practically efficient CO2 conversion.

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ASSOCIATED CONTENT Supporting Information General information, typical synthesis, detailed characterization, and physical properties of phosphonium-based DESs. Typical procedure for synthesis of cyclic carbonate from epoxide and CO2.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M. S. Liu) ORCID Fusheng Liu: 0000-0002-4909-1252 Jun Gao: 0000-0003-1145-9565 Mengshuai Liu: 0000-0001-9467-1427 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21805154, 51673106), the Natural Science Foundation of Shandong Province (ZR2018BB009), a Project of Shandong Province Higher Educational Science and Technology Program (J18KA065), the China Postdoctoral Science Foundation (2019M652343), Opening Project of Shandong Eco-chemical Engineering Collaborative Innovation Center (XTCXQN01) and the Scientific Research Foundation of Qingdao University of Science and Technology (0100229019).

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For Table of Contents Use Only Specially designed phosphonium-based deep eutectic ILs show efficient activation and catalytic performance for conversion of CO2 into cyclic carbonates under mild and metal/cocatalyst-free conditions.

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Multi-functional phosphonium-based deep eutectic ionic liquids

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