Cooperative Conversion of CO2 to Cyclic Carbonates in Dual-Ionic

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Cooperative Conversion of CO2 to Cyclic Carbonates in DualIonic Ammonium Salts Catalytic Medium at Ambient Temperature Fusheng Liu, Yongqiang Gu, Penghui Zhao, Jun Gao, and Mengshuai Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05997 • Publication Date (Web): 23 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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ACS Sustainable Chemistry & Engineering

Cooperative Conversion of CO2 to Cyclic Carbonates in Dual-Ionic Ammonium Salts Catalytic Medium at Ambient Temperature Fusheng Liu,† Yongqiang Gu,† 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, 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 metal-free, dual-ionic ammonium salts catalytic medium were developed, and they were used to catalyze the cycloaddition of CO2 and epoxides to yield cyclic carbonate. The effects of catalyst structures and reaction parameters on the catalytic activity

were

investigated.

The

optimized

[TEAO2][Br][DBUH]

contains

hydrogen-bond donor groups (–NH), nucleophilic [Br–] and carboxyl anions, it shows exceptional catalytic performance with satisfied product yield and selectivity at ambient temperature due to the synergy between the dual cations and anions in the system. The present catalyst exhibits good reusability and can keep excellent activity after five reaction cycles. The catalyst also shows general applicability to other epoxide substrates. Finally, an

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insight into the dual cation-anions synergetic activation pathway and catalytic mechanism were proposed.

KEYWORDS: Dual-ionic system; Carbon dioxide; Activation; Cyclic carbonate; Homogeneous catalysis

INTRODUCTION Chemical fixation of CO2 into high valuable chemicals or fuels have been attracting much attention from the viewpoints of CO2 emission reduction and energy structure reformation.1-3 Although the utilization of CO2 for production of urea, methanol, salicylic acid, polycarbonate, etc. have been industrialized, according to statistics in 2014, the global CO2 emissions amounted to 32.3 billion tons, only 0.36% of which was used as a feedstock for industrial chemicals production, there is a wide space for the development of CO2 utilization processes.4 In this regard, a widely investigated utilization strategy is the chemical conversion of CO2 into cyclic carbonate via cycloaddition with epoxide (Scheme 1). It is green for 100% atom efficiency, also the cyclic carbonates have found wide applications ranging from polar aprotic solvents to intermediates in the manufacture of fine chemicals.5,6

Metal ions or HBD

(III) Ring-closing (II) Insertion of CO2

O R

"dual functionality"

CO2

O

(I) Ring-opening

X nucleophilic group

Cat., T, P

O

O

R

Scheme 1. Cycloaddition of CO2 to Epoxide

Up to now, numerous catalysts have been developed for the selective conversion of CO2 into cyclic carbonates, such as metal oxides,7 alkali metal salts,8,9 metal-salen complexes,10,11 metal organic frameworks (MOFs),12,13 covalent organic frameworks (COF),14,15 porous organic polymers (POPs),16,17 and ionic liquids (ILs).18,19 Among these -2ACS Paragon Plus Environment

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catalysts, transition metal-based catalysts coupled with a cocatalyst (quartnary ammonium salts) showed excellent catalytic performance, and they could realize the synthesis of cyclic carbonate from CO2 and epoxide at ambient conditions.20-22 Nevertheless, the multistep and time-consuming synthesis, rigorous purification, and high cost of the most transition metal-based

catalysts

may

limit

their

preparation

and

large-scale

application.

Hydrogen-bond-donating (HBD) catalysis as a greener alternative to Lewis acid and/or transition metals has emerged and attracted much attention. The HBD-based catalysts mainly included dual organocatalyst systems23,24 and (supported/polymeric) ionic liquids.25 The

reported

ascorbic

acid/TBAI,26

DBU/BnBr,27

2-pyridinemethanol/TBAI,28

squaramide/TBAI,29 graphene oxides/DMF etc.30 catalyzed conversion of CO2 into cyclic carbonate with good to excellent product yield at ambient CO2 pressure or even at ambient temperature, while these processes usually needed additional additives. Despite these advances, only very few metal-free catalyst can realize efficient synthesis of cyclic carbonate from CO2 and epoxide under ambient and cocatalyst-free conditions. Hence, the development of new types of catalysts with high-activity under mild and metal/additive-free conditions is highly desirable from the sustainability and energy-saving standpoints. Herein, several kinds of dual-ionic ammonium salts were facilely synthesized (Scheme 2). The detailed synthesis process and characterization results were listed in Supporting Information. They were applied to the cycloaddition of CO2 and epoxides as catalytic medium to yield cyclic carbonates. The effects of catalyst structures and reaction parameters on the catalytic activities were investigated. The catalyst recyclability and general applicability to different epoxides were also evaluated. Moreover, an insight into a cooperating activation process and catalytic mechanism deriving from the dual cation-anions were provided. Compared with the mono-ionic counterparts and other reported mono-ionic catalytic systems, the dual-ionic ammonium medium were proved to be highly active, inexpensive and readily available catalysts for chemical fixation of CO2 to cyclic carbonate under mild, metal-/cocatalyst-free conditions.



HO

HO

OH

N

O

CH3CN Br

HO

Br N

25 oC, 12 h

HO

OH

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OH

HO

O

25 oC, 8 h

OH

OH

Br

HO N

O

N

HO

HO

O

O

N H

OH N O H2N

N

O

[TEOAO2][Br][TMGH] Br

Br N

Br

HO

[TEOAO2][Br][DBUH]

N


N

N H

OH O

N

[O2TEOA][Br] HO

Br

DBU, H2O

N O O

N

N H

O HN 2 O

N N

[TEAO2][Br][TMGH]


Scheme 2. Synthesis and Structures of Dual-Ionic Ammonium Salts

RESULTS AND DISCUSSION Catalyst Screening The cycloaddition of CO2 and propylene oxide (PO) was conducted in different dual-ionic ammonium salts medium to evaluate their catalytic activities, the typical catalytic process was described in Supporting Information. As shown in Table 1, no product was detected without a catalyst (entry 1). The DBU, [HO2TEOA][Br] and [HO2TEA][Br] showed low activities because they could not realize the synergistic activation toward CO2 and PO (entries 2–4). As reported in previous study, the activating abilities of catalytic medium to CO2 and epoxide played vital roles in the synthesis of cyclic carbonate under ambient conditons.31 For the present dual-ionic catalytic medium, as shown in Scheme 2, their structures consist of hydrogen-bond donor groups (–OH and N-H), nucleophilic Br–, and Lewis basic COO– group. The –OH/N-H cooperated with Br– can promote the ring-opening of PO substrate, and Lewis basic COO– can activate CO2 species, which has been proved by Coutinho et al.32 It is in this sense that the synergistic effects can allow the coupling reaction to proceed under the applied conditions. Unexpectedly, the [TEOAO2][Br][DBUH] and [TEOAO2[Br][TMGH] only -4ACS Paragon Plus Environment

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exhibited low activities (entries 5 and 6). It could be that the multiple –OH groups in [TEOAO2 formed the stronger intermolecular hydrogen bond with Br–, which restrained the nucleophilic attack of Br– toward less hindered β-C atom of PO substrate, resulting in the decrease of catalytic activity.29,33 To support this speculation, [TEAO2][Br][DBUH] and [TEAO2][Br][TMGH] were examined to catalyze the coupling reaction, respectively (entries 7 and 8). It was noteworthy that the catalytic performance was obviously improved compared with [TEOAO2-based medium. Especially for [TEAO2][Br][DBUH], a 85% PC yield with 99% selectivity was achieved at ambient temperature. To our delight, an exceptional catalytic activity was shown for the transformation of CO2 into PC at the higher temperature (entries 9–11). It was notably better than the mono-ionic system (i.e. typical ILs) already reported in the literature.25 Considering the energy-saving process, an ambient temperature of 30 oC was chosen for the CO2 transformation.

Table 1. Catalyst Screeninga Entry

Temperature

Reaction resultsb

(oC)

Yield (%)

Selectivity (%)

Catalyst

1

none

30

—

—

2

[HO2TEOA][Br]

30

11

99

3

[HO2TEA][Br]

30

13

99

4

DBU

30

16

98

5


30

28

99

6

[TEOAO2[Br][TMGH]

30

19

99

7


30

85

99

8

[TEAO2][Br][TMGH]

30

54

99

9


40

96

99

10c


50

63

99

11c


60

98

99

a

Reaction conditions: n (PO) = 34.5 mmol, cat. loading 15 mol%, P (CO2) = 1.5 MPa, t = 12.0 h.


b


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Determined by GC analysis. c t = 4.0 h.

Effects of Reaction Parameters Using the desirable [TEAO2][Br][DBUH] catalytic medium, the effects of other reaction parameters on the PC yield were examined at ambient temperature (Figure 1). Initially,

the

PC

yield

was

significantly

improved

with

increasing

the

[TEAO2][Br][DBUH] loading from 1 mol% to 15 mol% (towards to PO amount). When the catalyst loading was further raised, it could not markedly promote the reaction (Figure 1A). In consideration of process economy, the catalyst loading of 15 mol% was chosen. Figure 1B shows the kinetic plot for PC synthesis catalyzed by [TEAO2][Br][DBUH]. A longer reaction time was better for a higher PC yield, and a quantitative 95% PC yield could be reached within 15 h. When the reaction time was further prolonged, almost the same product yield was obtained. Hence, the reaction time of 15 h was sufficient. Figure 1C gives the influence of initial CO2 pressure on the reaction. A moderate 66% PC yield was obtained at 30 oC with CO2 pressure of 0.5 MPa. When the CO2 pressure was increased from 0.5 MPa to 1.5 MPa, the PC yield was smoothly improved to 95% due to the favorable contact between CO2 molecules and substrate (PO) phase.34 When CO2 pressure was kept in the range of 1.5–2.5 MPa, no obviously positive impact was observed. Therefore, a mild CO2 pressure of 1.5 MPa is optimal for the coupling reaction.


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Figure 1. Effects of different reaction parameters. (A) cat. loading, PO 34.5 mmol, 30 oC, 1.5 MPa, 12.0 h; (B) reaction time, PO 34.5 mmol, 30 oC, 1.5 MPa, cat. loading 15 mol%; (C) CO2 pressure, PO 34.5 mmol, 30 oC, cat. loading 15 mol%, 15.0 h.

Catalyst Reusability After the reaction, the [TEAO2][Br][DBUH] catalyst could be easily recycled by extraction, and reused for the cycloaddition of CO2 to PO under the optimum conditions. As shown in Figure 2A, both of the PC yield and selectivity kept almost unchanged after five consecutive runs. To better support the results, the recycled [TEAO2][Br][DBUH] was characterized by FT-IR and NMR, then was compared with the fresh one. As shown in Figure 2B, almost all the characteristic peaks of the recycled [TEAO2][Br][DBUH] were reserved except for weakening of carboxyl absorption peak at 1730 cm-1, possibly attributed to partial overlap with the peak of absorbed CO2 at 1790 cm-1. This was consistent with the NMR analysis (Figure S3-S6) and CO2 activation analysis as described in Figure 3. It should be noted that the activity of [TEAO2][Br][DBUH] did not obviously affected, indicating an excellent reusability of the catalyst.



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Figure 2. (A) Recyclability of [TEAO2][Br][DBUH], and (B) FT-IR spectra of the recycled catalyst. Conditions: PO 34.5 mmol, [TEAO2][Br][DBUH] 15 mol%, 30 oC, 1.5 MPa, 15.0 h.

General Applicability To evaluate the general applicability of the dual-ionic ammonium catalytic medium, the cycloaddition

of

various

epoxides

with

CO2

were

examined

by

using

[TEAO2][Br][DBUH] catalyst at ambient temperature. As shown in Table 2, the terminal epoxides could be selectively transformed into the corresponding cyclic carbonates with satisfied product yields (entries 1-6). Especially for the halogenated epoxides, they were

completely

converted

with

nearly

100%

product

yields

due

to

the

electron-withdrawing effects of their substituent (entries 3 and 4).35 For an internal epoxide with high steric hindrance, such as cyclohexene oxide, a moderate product yield with excellent selectivity was also obtained by prolonging the reaction time (entry 7). The novel dual-ionic ammonium medium presents excellent catalytic performance, reusability and versatility, which make it a good candidate for catalyzing the cycloaddition of CO2 and epoxides to form cyclic carbonates.

Table 2. Cycloaddition of CO2 to Various Epoxidesa Reaction results (%)b Entry

Epoxide

Product

T (°C)

P (MPa) Yield


Selectivity

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O O

1

O

30

1.5

95

99

30

1.5

92

99

O

30

1.5

99

99

O

30

1.5

99

99

30

1.5

88

99

40

1.5

90

99

40

2.0

65

98

O

O O

2

O

O

O

O

3

O

Cl Cl

O O

4

O

Br

Br O

O

O

5

O

O O

6

O

O

O

7c a

O

O O

Reaction conditions: epoxide 20.0 mmol, catalyst 15 mol%, 15.0 h.

b

Determined by GC analysis,

dodecane as an internal standard. c 24.0 h.

Plausible Reaction Mechanism To better support the reaction mechanism, we first investigated the activation pathway of [TEAO2][Br][DBUH] to CO2 molecules. As reported by Coutinho and Lu et al.,32,36 acetate anions could bond the acid carbon of the CO2 molecules. According to the FT-IR characterization result (Figure 3), assuredly, a new peak at ṽ = 1790 cm-1 appeared for the [TEAO2][Br][DBUH] after interacting with CO2 due to the formation of acid anhydride derivative. The detailed activation pathway was shown in Figure 3. The activation of CO2 molecules in the reaction medium played an important role for its subsequent catalytic -9ACS Paragon Plus Environment


conversion.

Br N

N O O

Br

CO2 N H

activation

N

N O O C O O

N H

Figure 3. The activation pathway of [TEAO2][Br][DBUH] to CO2 molecules.

In Scheme 3, a feasible reaction mechanism was proposed. Initially, the epoxy substrate is activated by forming hydrogen bonds with the N-H groups in [DBUH+], which makes C-O bond of the epoxide polarized. Then the Br– attacks at the less hindered β-C atom of epoxide, resulting in ring-opening of the epoxide to yield an anion intermediate, which is stabilized by the [DBUH+] groups. Subsequently, the formed intermediate spontaneously interacts with the activated CO2 molecule through nucleophilic attack to realize the CO2 insertion. Following an intramolecular ring-closure step, it can form the cyclic carbonate and regenerate the catalyst. The synergistic actions of dual cation-anions provided by [TEAO2][Br][DBUH] promote the coupling reaction of CO2 and epoxide effectively.


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Scheme 3. Proposed Mechanism for the Catalysis Reaction

CONCLUSION The novel dual-ionic ammonium salts catalytic medium were facilely synthesized, and applied for promoting the chemical fixation of CO2 to cyclic carbonate. By optimizing the catalyst structures and reaction parameters, the [TEAO2][Br][DBUH] was proved to be a promising catalyst, and it could afford good to excellent product yield with high selectivity. The protocol could allow the reaction to proceed under mild conditions (30-50 oC,

0.5-1.5 MPa) in the absence of metal and cocatalyst, showing great advance towards the

reported mono-ionic liquid system. The synergistic activation of the dual cation-anions to epoxy substrate and CO2 promoted the reaction efficiently. Besides, the novel dual-ionic ammonium salt catalytic medium is eco-friendly, easily available and reusable, which make it an interesting candidate for the synthesis of cyclic carbonate from CO2 and epoxide.

ASSOCIATED CONTENT Supporting Information General information, synthesis and characterization of the dual-ionic ammonium salts, and typical catalytic process.



AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M. S. Liu) ORCID Mengshuai Liu: 0000-0001-9467-1427 Fusheng Liu: 0000-0002-4909-1252 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), the Science and Technology Research Project of Shandong Province (2016GSF116005), a Project of Shandong Province Higher Educational Science and Technology Program (J18KA065), and the Scientific Research Foundation of Qingdao University of Science and Technology (0100229019).

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For Table of Contents Use Only Novel dual-ionic ammonium salts show excellent activity for conversion of CO2 into cyclic carbonates at ambient temperature without metal, cocatalyst and solvent.


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Cooperative Conversion of CO2 to Cyclic Carbonates in Dual-Ionic

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