Conversion of CO2 into Organic Carbonates over a Fiber-Supported

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Conversion of CO into organic carbonates over a fiber-supported ionic liquids catalyst in impellers of the agitation system Xian-Lei Shi, Yongju Chen, Peigao Duan, Wenqin Zhang, and Qianqian Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01051 • Publication Date (Web): 15 Apr 2018 Downloaded from http://pubs.acs.org on April 15, 2018

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ACS Sustainable Chemistry & Engineering

Conversion of CO2 into organic carbonates over a fiber-supported ionic liquids catalyst in impellers of the agitation system Xian-Lei Shi,*,†,‡ Yongju Chen,† Peigao Duan,† Wenqin Zhang,‡ Qianqian Hu*,† †

Henan Key Laboratory of Coal Green Conversion, Henan Polytechnic University, No. 2001 Century Avenue, Jiaozuo, Henan 454003, P. R. China. E-mail: [email protected], [email protected]; Tel./fax: +86-0391-3987823 ‡

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, No. 92 Weijing road, Tianjin 300072, P. R. China

ABSTRACT: Herein, a fiber-supported imidazolium-based ionic liquids catalyst in impellers of the agitation system was developed for efficient cycloaddition of CO2 with epoxides under metal- and solvent-free conditions. The fiber-supported ionic liquids catalyst was designed collectively and synthesized systematically, and characterized detailedly on the physicochemical properties via various technologies during both of the preparation and utilization processes. Moreover, the influence of ionic liquids structures and reaction conditions on the cycloaddition was inspected, and the fiber catalyst-mediated CO2 conversion proceeded smoothly (100 °C and 1 MPa) in gram-scale for the synthesis of organic carbonates in good to quantitative yields. Notably, the novel fiber catalyst in impellers of the agitation system also displayed prominent recyclability (21 cycles), and the protocol could be operated concisely with good prospect for industrial applications.

KEYWORDS: CO2 conversion; fiber catalyst; organic carbonates; gram-scale; recyclability

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INTRODUCTION Carbon dioxide (CO2), commonly be known as the main component of the greenhouse gases, which is also expected to be a potentially abundant, cheap, nontoxic and renewable C1 feedstock in organic synthesis.1,2 From this standpoint, the conversion of CO2 into valuable chemicals of academic or industrial interest is of concern not only to environmental protection but also to sustainability in chemical production and processing.3,4 Therefore, many efforts have been devoted to chemical fixation of CO2 in the past decade, while the molecule of CO2 has a high thermodynamic stability, and exploiting new catalytic systems for more efficient CO2 conversion processes has received increasing attention recently.5,6 It is worthy noting the atom-efficient cycloaddition of CO2 with epoxides to produce organic carbonates that find use as good aprotic polar solvents, fine chemical intermediates, and precursors for polycarbonate.7,8 Actually, There were considerable researches on the catalytic systems for CO2 cycloaddition, which including many homogeneous catalysts such as metal catalysts7,9-13 or organocatalysts.14-19 While with the objective of ‘‘green chemistry’’, it is more inclined to use the easily separated and efficient heterogeneous catalysts. Among those, the supported catalysts, which heterogenized homogeneous catalysts onto solid supports and combined the advantages of homogenous and heterogeneous catalysts for such cycloaddition have been considered to be of more prospect for industrial applications.20,21 Up to now, a variety of materials such as polymers,22-25 silica,26-28 zeolite,29 and metal-organic framework,30,31 etc. have been served as catalyst supports in CO2 cycloaddition for the synthesis of organic carbonates, especially for supported ionic liquids.32-36 Although good catalytic activities were often acquired, the potential industrialized large-scale manufacturing processes were frequently absence, namely, a majority of reported supported catalysts were merely merit superiorities in fundamental researches, and the suitable catalytic systems for mass production are still need to be paid more attention.37,38 As such, the application of novel materials for new, efficient supported-catalyst systems with the potential for large-scale processing, which could quickly transfer from the academic laboratories to chemical plant would be of great value and significance. In recent years, the fibrous catalysts have being recognized as one of the most promising catalyst forms from the point of view of reaction engineering, since their excellent flexibility and good practical performance, and have been used in several reactions ranging from organic synthesis to fuel processing.39,40 A series of inorganic fibers such as metal-fiber,41 silica-fiber,42 ACS Paragon Plus Environment

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ceramic-fiber,43 glass-fiber,44 and carbon-fiber45 or activated carbon fiber46 have been served as materials for supported catalysts and shown prominent activities. However, as far as we are concerned, the fiber-mediated systems for efficient and large-scalable cycloaddition of CO2 has not been reported, and exploiting effective fibrous materials with preferable performance for chemical fixation of CO2 in more practical industrial tactics, tend to be given great interests. It is worth mentioning that polyacrylonitrile fiber (PANF), one of the artificial synthetic polymer materials, has been widely used in industry and our daily life, moreover, given the high strength and good flexibility, as well as the molecular chains of the fiber contain significant amounts of cyano groups which could be functionalized to various active groups,47,48 PANF reveals a substantial advantage over many other polymers as the starting material for supported catalysts. In our previous work, the merits and details of PANF-supported acid,49 base50 and metal catalysts51,52 in various different types of organic reactions had been investigated meticulously, and following our pioneering studies on fiber catalysts and the increasingly interest in green chemistry,53,54 in this work, the PANF was further served as an effective support material, to report a new fiber-supported ionic liquids catalyst in impellers of the agitation system for efficient and large-scalable conversion of CO2 into organic carbonates.

EXPERIMENTAL SECTION Synthesis of the fiber-supported ionic liquids catalysts Step 1, Imidazole functionalization. N-(3-Aminopropyl)-imidazole (20 g) was dissolved in water (30 g) in a three-necked flask, and the solution was stirred and preheated to reflux. Then, PANF (2.00 g) was introduced into the above solution, and the mixture was stirred under refluxing (105 °C) for 12 h. The fiber sample was taken out and rinsed repetitiously with water (60-70 °C) until the lotion was neutral, and next dried to constant weight at 60 °C under vacuum to obtain imidazole functionalized PANF (PANF-IM, weight 2.40 g and weight gain 20%). Step 2, Formation of ionic liquids. A typical procedure for the synthesis of PANF-supported ionic liquids catalyst is as follows: 3-Bromopropionic acid (3.06 g, 20 mmol) and acetonitrile (50 mL) were mixed in a flask, and the solution was stirred and preheated to reflux. Whereafter, dried PANF-IM (2.00 g) was added to the solution, and the mixture was stirred under refluxing (83 °C) for 12 h. The fiber was taken out and washed with acetonitrile (3×15 mL), and next dried to ACS Paragon Plus Environment


constant weight at 60 °C under vacuum to acquire PANF-supported 3-carboxyethylimidazolium bromides (PANF-CEIMBr, 2.82 g, the content of ionic liquids was 1.90 mmol g-1). The preparation of other PANF-supported imidazolium-based ionic liquids catalysts was similar to the above procedures with the substituted substrates. General procedure for CO2 cycloaddition A stainless-steel autoclave (100 mL) equipped with an automatic temperature control system was used for the cycloaddition experiment. The moderate amounts of PANF-supported ionic liquids catalyst was intertwined in the impellers of the reactor, and known amounts of epoxide was introduced into the autoclave at room temperature. Next, the reactor was heated to a specified temperature with an addition of CO2 from a high-pressure reservoir tank to maintain a specified constant pressure for a desired period of time. After reaction finished, the apparatus was cooled with an ice-salt bath below 5 °C, then slowly depressurized and the reaction mixture was let out through discharge spout and the autoclave reactor was wished with diethyl ether (20 mL) which was merged into the above mixture. Finally, the combined mixture was distilled in vacuo to obtain organic carbonates (for solid products, by recrystallization). The fiber catalyst in impellers was washed with diethyl ether, dried with hairdryer and reused without any further activation.

RESULTS AND DISCUSSIONS The design and preparation of fiber-supported ionic liquids catalyst According to our previous work49,53 and referring to the literatures,16,36 the conceive and design in this study for the preparation of fiber-supported ionic liquids catalysts were based on the following two points. 1) Supported material selection. The molecular chains of polyacrylonitrile fiber contain an abundance of cyano groups, on the one hand, these groups can be acted as the active sites for immobilization, on the other hand, the surplus cyano groups surrounding with catalytic active sites in the fiber surface possess good soakage behavior for the reaction reagents, which could promote the contacts between the catalytic active sites and the reaction substrates, and further improving the catalytic performance. 2) Catalyst structure construction. The imidazolium-based ionic liquids have been verified in CO2 cycloaddition and generally shown high efficiency,33,35 moreover, after the ionic liquids rooted in PANF, the imidazoliums equipped with longer chains could swing along with ACS Paragon Plus Environment

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the reagents which affording a “half-flowing” interphase to construct a very unique “catalytic microenvironment” that were very in favor of the molecular collision of the reaction substrates and further beneficial to the catalytic activity. Based on the above, the fiber-supported ionic liquids catalyst was synthesized with the strategy of immobilizing imidazolium-based ionic liquids on the support of PANF (Scheme 1).

Scheme 1. Synthesis of the fiber-supported imidazolium-based ionic liquids catalyst. As shown in Scheme 1, the first step is the imidazole functionalization. In this step, N-(3-aminopropyl)-imidazole was used as the aminating reagent for amination to immobilize the imidazole groups onto PANF, and the degree of functionality was evaluated by weight gain (Weight gain [%]= [(W2−W1)/W1]× 100, where W1 and W2 are the weight of fiber sample before and after functionalization, respectively). Through optimization, a weight gain of 20% imidazole functionalized PANF (PANF-IM) was obtained. Step 2 is the ionic liquids construction, and different types of halides were served as the salinization reagents to form fiber-supported imidazoliums, and five species of PANF-supported imidazolium-based ionic liquids catalysts were acquired. PANF-supported 3-carboxyethylimidazolium bromides (PANF-CEIMBr, Scheme 1, D) was chose as a model in consideration of its better activities than other fiber catalysts for CO2 cycloaddition, and the loading of ionic liquids was 1.90 mmol g-1 calculated by weight. The characterization of fiber-supported ionic liquids catalyst To confirm dependability of this strategy for the preparation of fiber-supported ionic liquids catalyst and to examine the changes of fiber samples at different stages, a detailed characterization on the fibers was conducted. The original PANF, PANF-IM, PANF-CEIMBr, and the fiber-supported ionic liquids catalyst in impellers recovered after the first cycle (PANF-CEIMBr-1) in model cycloaddition of CO2 with propylene oxide (conditions as shown in Table 4), as well as the fiber catalyst recovered after the 21th cycle (PANF-CEIMBr-21) were all through the characterization of elemental analysis, mechanical properties, Fourier transform infrared (FTIR) spectroscopy and ACS Paragon Plus Environment


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scanning electron microscopy (SEM), respectively. Moreover, the physical photographs and solid state 13C NMR spectra of fiber samples could be seen in the Supporting Information. Elemental analysis. Firstly, elemental analysis was used to test the composition of fiber samples, and the results of C, H and N contents of PANF, PANF-IM, PANF-CEIMBr, PANF-CEIMBr-1, and PANF-CEIMBr-21 are summarized in Table 1. Elemental analysis data of the original PANF (Table 1, entry 1) was comparable to our previous study.50 After the immobilization of imidazole groups onto the fiber, the C, H and N amounts of PANF-IM were all decreased (Table 1, entry 2) due to the formation of amide bonds and the immobilized N-(3-aminopropyl)-imidazole has less C, H and N than the original PANF. Moreover, with the subsequent salinization with 3-bromopropionic acid to form fiber-supported imidazoliums, the C, H and N contents of fiber declined again (Table 1, entry 3), perhaps due to the incoming 3-bromopropionic has no N, and less C and H than front PANF-IM. However, with the subsequent catalytic applications in CO2 cycloaddition for synthesizing carbonates, the contents of C, H and N in PANF-CEIMBr-1 (Table 1, entry 4) and PANF-CEIMBr-21 (Table 1, entry 5) had no obvious change from the fresh fiber-supported ionic liquids, and the slight C and N decrease, and H increase perhaps due to some cycloaddition products were adsorbed onto the fiber. Furthermore, these results above also proved the dependability of the catalyst synthetic strategy and the stability of the fiber-supported imidazolium-based ionic liquids in catalytic applications. Table 1. Elemental analysis data of PANF, PANF-IM, PANF-CEIMBr, PANF-CEIMBr-1, and PANF-CEIMBr-21 Entry

Fiber simple

C (%)

H (%)

N (%)

1

PANF

67.53

5.86

23.65

2

PANF-IM

60.06

5.75

23.30

3

49.38

4.97

16.45

4

PANF-CEIMBr PANF-CEIMBr-1

49.26

5.01

16.31

5

PANF-CEIMBr-21

49.14

5.06

16.19

Mechanical properties. Next, the breaking strengths of different stages of fibers were measured by an electronic single fiber strength tester, and their mechanical properties are listed in Table 2. The original PANF with a breaking strength of 10.61 cN (Table 2, entry 1). After imidazole immobilization in refluxed water, the obtained PANF-IM with an abridged breaking strength of 8.79 cN, that is, 83.2% of the primal fiber mechanical properties were retained (Table 2, entry 2). After PANF-IM salinization with 3-bromopropionic acid in acetonitrile, the breaking strength of the ACS Paragon Plus Environment

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freshly prepared PANF-CEIMBr only decreased to 7.93 cN, i.e. 75.1% of the original fiber strength was maintained (Table 2, entry 3). Moreover, after the firstly served as catalyst in impellers for the cycloaddition of CO2, there was almost no change in the breaking strength of PANF-CEIMBr-1 from the fresh fiber catalyst (Table 2, entry 4). What is more, comparing with PANF-CEIMBr, the breaking strength of PANF-CEIMBr-21 only decreased 0.29 cN after the catalytic application for 21 cycles, namely, the PANF-CEIMBr-21 reserved 72.3% of the original mechanical properties (Table 2, entry 5). It is can be inferred from the above data that the selected fiber possesses superior mechanical properties served as the support material, and the fiber-supported ionic liquids catalyst in impellers held sufficient strength even in the vigorous temperature and pressure conditions for the cycloaddition of CO2. Table 2. Mechanical properties of PANF, PANF-IM, PANF-CEIMBr, PANF-CEIMBr-1, and PANF-CEIMBr-21

a

Entry

Fiber simple

Breaking strength (cN)

Retention ratea (%)

1

PANF

10.56

100

2

PANF-IM

8.79

83.2

3

7.93

75.1

4

PANF-CEIMBr PANF-CEIMBr-1

7.91

74.9

5

PANF-CEIMBr-21

7.64

72.3

Based on PANF. FTIR spectroscopy. Then, different stages of fiber samples were minced to prepare the KBr

pellets for FTIR detection, and their spectra are displayed in Figure 1. Similar results as expected in the FTIR spectrum of PANF (Figure 1a), and had been expounded in our previous work.49 However, after imidazole functionalization, the major difference between PANF-IM and PANF is the two strong absorptions around 3350 and 1591 cm-1 (Figure 1b), which are characterized as the N−H stretching vibration of the formed amide bond and skeletal vibration of imidazole ring, respectively. With the following salinization process, the PANF-CEIMBr presented a stronger absorption band at 1765 cm-1 (Figure 1c), which are assigned to the added C=O vibrational modes of 3-bromopropionic acid. Moreover, after the succedent utilization of the fiber catalyst in CO2 conversion, the FTIR spectra of the recovered PANF-CEIMBr-1 (Figure 1d) and PANF-CEIMBr-21 (Figure 1e) were very similar to the spectrum of fresh PANF-CEIMBr, these results indicate that the imidazoliums were still supported on the fiber after several catalytic cycles and also suggest that the fiber catalyst could be reused more runs. ACS Paragon Plus Environment


Figure 1. FTIR spectra of (a) PANF, (b) PANF-IM, (c) PANF-CEIMBr, (d) PANF-CEIMBr-1, and (e) PANF-CEIMBr-21. SEM images. Finally, the SEM technique was used to observe the morphology of fiber samples, and their SEM images are shown in Figure 2. A relatively cicatricial surface was exhibited by PANF (Figure 2a). Moreover, the surface roughness of PANF-IM and PANF-CEIMBr was extended with the two-step preparation process (Figure 2b and c). Furthermore, after catalytic applications, increasingly coarser and more flecks were emerged in the surface of the recycled catalysts (Figure 2d and e), perhaps due to the pressure effect to result some cycloaddition products were adsorbed to the fiber. However, there was no substantial flaw and breakage on the cicatricial surface of fiber, and all of the fiber samples retained their regular fibrous structure.

Figure 2. SEM images of (a) PANF, (b) PANF-IM, (c) PANF-CEIMBr, (d) PANF-CEIMBr-1, and (e) PANF-CEIMBr-21. Scale bars=200 µm (insets: 5 µm). ACS Paragon Plus Environment

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Fiber-supported ionic liquids catalyst in impellers of the agitation system for CO2 conversion to synthesize organic carbonates The development of cost-efficient and eco-friendly catalytic systems to maximize reaction efficiency and minimize waste production has become one of the main themes of contemporary green chemistry, and this work was also from this point of view. The selected fiber material owns higher mechanical properties, which make it not easy to be broken in the catalytic process, and the good flexibility contributes to knit into various shapes, which are easy to be applied in different types of reactors. Therefore, we devote to make full use of the superiority of PANF to utilize the fiber-supported ionic liquids catalyst in impellers of the agitation system (Figure 3) to verify a new catalytic reaction apparatus for efficient and gram-scalable conversion of CO2 into organic carbonates.

Figure 3. A diagrammatic sketch of the agitation system with the fiber catalyst in impellers. Screening fiber-supported ionic liquids catalyst in impellers of the agitation system for the cycloaddition of CO2. The performance of the fiber-supported ionic liquids catalysts in impellers of the agitation system was fully inspected on the CO2 conversion. Initially, the condition optimization was conducted on model cycloaddition of CO2 with propylene oxide, and the screening results are summarized in Table 3. In the preliminary optimization, the reaction condition was set at 120 °C with 1.0 MPa pressure of CO2 for 2 h, and as a control, the conversion without ACS Paragon Plus Environment


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any catalyst or with PANF and PANF-IM was all invalid for the synthesis of organic carbonate (Table 3, entries 1-3). Whereafter, the fiber-supported ionic liquids catalysts with different structures were employed to the reaction (Table 3, entries 4-8). As expected, and the results were consistent with the references,35,36 the PANF-supported ethylimidazolium bromide (PANF-EIMBr) had less efficient on the cycloaddition of CO2 (Table 3, entry 4). However, the catalytic activity was greatly improved using hydroxyethyl replaced ethyl on the imidazoliums, and the PANF-supported hydroxyethylimidazolium bromide (PANF-HEIMBr) gave a higher carbonate yield of 93% (Table 3, entry

5).

Moreover,

both

the

PANF-supported

carboxymethylimidazolium

bromide

(PANF-CMIMBr) and PANF-CEIMBr were effective for this conversion, and the yields of carbonate were 90% and 95% (Table 3, entries 6 and 7), respectively. Even so, when Cl‾ substituted Br‾ as the halide anion of imidazolium, the corresponding product yield declined markedly (Table 3, entry 8). From the above results, it can be suggested that OH group was active on the catalytic process, which accelerated the ring-opening reaction of epoxides by forming hydrogen bonds,16,36 and the opportune chain length between the OH group and imidazolium might be also favorable to the formation of the ring-open intermediate (Scheme 2). Additionally, the anion types in ionic liquids have a remarkable influence on the catalytic performance, and the activity order of anion was Br‾ > Cl‾, which is consistent with their nucleophilicity order. Therefore, the PANF-CEIMBr was used as the catalyst for further optimization. Next, the effect of catalyst dosage on CO2 cycloaddition was inspected (Table 3, entries 9-11). The results demonstated that the catalyst amount decreased to 1 mol%, the CO2 conversion still kept its reactivity (Table 3, entry 10), while further to cut down the amount of catalyst was unfavorable for the cycloaddition (Table 3, entry 11). Then, with the catalyst amount of 1 mol%, the influence of reaction temperature and CO2 pressure on the conversion of CO2 were inquired concisely (Table 3, entries 12-17). The experiments revealed that raised the temperature or increased the pressure were adverse to the carbonate yield, which may cause more carbonate polymerization or other side reactions (Table 3, entries 12 and 16), while excessively reduce the reaction temperature and pressure, the CO2 conversion was inhibited and with lower yields of carbonate (Table 3, entries 15 and 17). Besides, the optimization on reaction time indicated that overlong time would give rise to side reactions (Table 3, entry 20) and shorter time against the complete conversion (Table 3, entry 18), and with 1 mol% catalyst dosage at 100 °C under 1.0 MPa pressure of CO2 for 2.5 h, the organic carbonate yield of CO2 cycloaddition with propylene oxide could reach to 98% (Table 2, entry 19). ACS Paragon Plus Environment

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Furthermore, the ionic liquids of CEIMBr was also used for the control experiment to confirm the active groups, and the result showed that the single ionic liquids was also valid for the cycloaddition of CO2 and afforded a product yield of 89% (Table 3, entry 21). Finally, a “hot filtration test” was investigated on the model reaction to figure out the heterogeneous nature of the catalytic performance. After the normal manner reaction was performed for 0.5 h, the fiber catalyst in impellers of the agitation system was quickly removed from the impellers. Then, the remaining neat mixture was reverted to the initial state and the reaction could not proceed smoothly, only got a very low yield of 29% (Table 3, entry 22), which means that traces of the catalytic activity of catalyst was assigned to a homogeneous way. Table 3. Performance optimization for the cycloaddition of CO2 over fiber catalysts in impellersa

Entry

Cat.

Cat. dosageb (mol%)

Temperature (°C)

Pressure (MPa)

Time (h)

Yieldc (%)

1

-

-

120

1.0

2.0

Traces

d

2

PANF

-

120

1.0

2.0

Traces

3

PANF-IM

2.0

120

1.0

2.0

Traces

4

PANF-EIMBr (A)

2.0

120

1.0

2.0

65

5

PANF-HEIMBr (B)

2.0

120

1.0

2.0

93

6

PANF-CMIMBr (C)

2.0

120

1.0

2.0

90

7

PANF-CEIMBr (D)

2.0

120

1.0

2.0

95

8

PANF-CEIMCl (E)

2.0

120

1.0

2.0

61

9

PANF-CEIMBr (D)

1.5

120

1.0

2.0

94

10

PANF-CEIMBr (D)

1.0

120

1.0

2.0

94

11

PANF-CEIMBr (D)

0.5

120

1.0

2.0

87

12

PANF-CEIMBr (D)

1.0

130

1.0

2.0

92

13

PANF-CEIMBr (D)

1.0

110

1.0

2.0

95

14

PANF-CEIMBr (D)

1.0

100

1.0

2.0

96

15

PANF-CEIMBr (D)

1.0

90

1.0

2.0

81

16

PANF-CEIMBr (D)

1.0

100

1.5

2.0

94

17

PANF-CEIMBr (D)

1.0

100

0.5

2.0

82

18

PANF-CEIMBr (D)

1.0

100

1.0

1.5

77

19

PANF-CEIMBr (D)

1.0

100

1.0

2.5

98

20

PANF-CEIMBr (D)

1.0

100

1.0

3.0

93

21

CEIMBr

1.0

100

1.0

2.5

89

22

PANF-CEIMBr (D)

1.0

100

1.0

2.5

29e

a

Reaction conditions: propylene epoxide (0.2 mol), with the corresponding catalyst, reaction temperature, pressure and time. b The catalyst dosage was based on the ionic liquids content. c Isolated yield. d With PANF 0.5 g. e Hot filtration test. ACS Paragon Plus Environment


Scheme 2. The possible mechanism of the cycloaddition of CO2 with epoxides.

Synthesis of organic carbonates over fiber-supported ionic liquids catalyst in impellers of the agitation system. Afterwards, to survey the feasibility of this technology and the scope of this method, the fiber-supported ionic liquids catalyst in impellers of the agitation system was investigated via different epoxides for the cycloaddition with CO2 in gram-scale under the optimized conditions (Table 4). Epoxides with different types of substituted functional groups including electron withdrawing and electron donating all proceeded smoothly to give high isolated yields of organic carbonates. However, it should be noted that the steric hindrance had a significant influence on the reaction activity, which perhaps due to the obstruction on the nucleophilic attack of bromine anion in the catalytic process, and the epoxides with bulky substituents such as styrene oxide and cyclohexene oxide obtained relatively low yields with longer reaction time and higher CO2 pressure (Table 4, 3d and f), and from the NMR analysis of product 3f, it can be conclude that the cyclic carbonate with a nearly 100% cis-structure. As a whole, the outcome demonstrate the fiber-supported ionic liquids catalyst in impellers of the agitation system was provided with high catalytic activity, broad applicability and gram-scalable for the synthesis of a series of organic carbonates, which are of more potential prospect for industrial applications.


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Table 4. Conversion of CO2 into organic carbonates over fiber-supported ionic liquids catalyst in impellers of the agitation systema

O

O O

O

O

O

Cl

3a, 98%

3b, 96%

3c, 99% O

O O O

O

O

Ph

3d, 96%b

3e, 97%

3f, 81%c

a

Reaction conditions: epoxide (0.2 mol), with PANF-CEIMBr loading 1.0 mol% and CO2 pressure 1.0 MPa at 100 °C for 2.5 h. b Reaction time 6 h. c Reaction time 24 h with 2.0 MPa of CO2 pressure, product with nearly 100% cis-structure. Recyclability and storage performance of the fiber-supported ionic liquids catalyst in impellers of the agitation system. Subsequently, to test the recyclability of the fiber catalytic system, the fiber-supported ionic liquids catalyst was reused in gram-scale cycloaddition of CO2 and propylene epoxide. In the whole process, fiber catalyst was intertwined in impellers of the reactor to accomplish the reaction, then the apparatus was cooled and depressed, and the reaction vessel was washed with diethyl ether after reaction mixture was discharged from the spout, next, the reactor was employed for the following cycle without further treatment. The results indicated all the catalytic cycles proceeded smoothly, and there was no significant drop in the activity of PANF-CEIMBr and the yield of carbonate (Figure 4). However, it must be point out that after more than 20 cycles, the catalytic performance has a slight decrease (yield 89% in run 20), while only with the reaction time extended to 3 h, a high yield of 97% could be obtained (run 21), which still abreast of the first cycle. Nevertheless, the catalytic activities for more runs would be altered along with the geometric structure changes, this because the possible structuration of fiber catalyst into pads is multifactorial, and the geometry of fiber’s length, thickness distribution, and specific surface area would change during the reaction cycles,20,21 while in this study the fiber-supported ionic liquids catalyst in impellers glisten over 20 cycles, and there almost no loss in the contents of ionic liquids. Moreover, during the study, the storage performance of fiber-supported ionic liquids ACS Paragon Plus Environment


catalyst was also examined, and the PANF-CEIMBr stored on shelves without special protection at least for three months, equally active of the fiber catalyst was remained.

Figure 4. Recyclability of fiber-supported ionic liquids catalyst for the conversion of CO2 into organic carbonates.

Comparison of CO2 cycloaddition in various supported-catalyst systems At the end of this study, the performance of various supported catalysts for the cycloaddition of CO2 was compared under their optimized conditions (Table 5). By contrast, the fiber-supported ionic liquids catalyst in impellers of the agitation system still revealed its superiorities in the total performance, especially for the catalyst reusability, which compare favorably with many different reported systems. Although some reported catalytic systems with lower reaction temperature less than 100 °C, the corresponding cycloadditions generally need longer reaction time (Table 5, entries 3, 5 and 14). In addition, the easily available PANF provided with a convenient method for catalyst preparation, and the reusable fiber-supported ionic liquids catalyst in impellers of the agitation system offered a simple operational approach, which prompt the fiber catalyst system a highly attractive alternative to many reports in the past. However, the development of novel catalysts for CO2 conversion under more mild conditions, such as at room temperature and atmosphere pressure still needs further explorations and studies. ACS Paragon Plus Environment

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Table 5. Comparison of CO2 cycloaddition in various supported-catalyst systems

Entry

Catalyst

Catalyst amount (mol%)

T (°C)

Pressure (MPa)

Time (h)

Yielda (%)

Run

Ref.

1

Fluorous polymers-R3P+X−

1.0

150

8

8

93

7

[22]

2

ABMDFP

1.5

130

2

4

98

6

[23]

3

poly(NHC-Zn)

1.0

80

1

10

56

8

[25]

4

Silicon-poly-imidazolium salts

0.9

110

1

2

94

6

[26]

5

PS-phosphonium salts

2

90

1.0

6

86

15

[27]

6

LMFI-I

0.15

140

2

4

97

4

[29]

7

PSIL

0.68

110

6

7

97

5

[32]

8

PS-HEIMBr

1.6

120

2.5

4

98

6

[34]

9

CILBr-Si

0.45

110

1.62

3

99

5

[35]

10

FDU-HEIMBr

0.5

110

1

3

99

5

[36]

1.8

160

8

4

96

4

[55]

1.7

160

4

6

98

5

[56]

11

+

-

([C4-mim] [BF4] /SiO2 +

−

12

CS-N Me3Cl

13

[Smim]OH/SiO2

1.8

120

2

4

94

5

[57]

14

[BMIm]Br-GO

2.5

80

1

6

99

5

[58]

15

PANF-CEIMBr

1.0

100

1

2.5

98

21

This work

a

Yields under their optimal experimental conditions.

CONCLUSION In summary, we have designed and synthesized a new type of fiber-supported ionic liquids catalysts for CO2 cycloaddition, and the novel catalytic apparatus with the fiber catalyst in impellers of the agitation system exhibited high efficient, broad applicability, prominent recyclability and large-scalability for the conversion of CO2 into organic carbonates. Moreover, the fiber-supported ionic liquids catalysts were prepared via a concisely two-step to acquire a higher amount of ionic liquids loading, and characterized detailedly with elemental analysis, mechanical properties, FTIR spectroscopy and SEM throughout the study to insure the reliability of this new technology. In addition, the influence of ionic liquids-structure and reaction conditions on the cycloaddition was examined comprehensively, and the fiber-supported ionic liquids catalyst in impellers of the agitation system-mediated the cycloaddition of CO2 under metal- and solvent-free conditions to gain many substituted organic carbonates in good to quantitative yields. What is more, the novel fiber-supported ionic liquids catalyst in impellers of the agitation system could be recycled over 21 ACS Paragon Plus Environment


runs in gram-scale without marked loss of its activities, and the fiber catalyst also exhibited good mechanical properties and flexibility to be potentially utilized in various reactors, which are very advantageous to practical industrial applications. To the best of our knowledge, this is the first research on the utilization of polyacrylonitrile fiber for a novel supported-catalyst system in the practice of CO2 conversion research, and further work on the exploitation of fiber catalysts for CO2 conversion is now in progress.

ASSOCIATED CONTENT Supplementary Information Experimental details, the method for testing the breaking strength of fiber samples, the physical photograph and solid state 13C NMR spectra of fiber samples at different stages, the 1H NMR and 13

C NMR data and copies of spectra of the synthesized organic carbonates.

AUTHOR INFORMATION Corresponding Author *E-mail for X.-L Shi: [email protected], Q. Hu: [email protected] ORCID Xian-Lei Shi: 0000-0003-4703-6601 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for the financial support by the Science and Technology Research Project of Henan Province (172102210282), Henan Province Office of Education (17A150012), Fundamental Research Funds for the Universities of Henan Province (NSFRF170910), Technological Innovation Team in University of Henan Province (18IRTSTHN010), Natural Science Foundation of Henan Province (182300410143), and National Natural Science Foundation of China (21776063).


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For Table of Contents Use Only Synopsis An efficient and gram-scale fiber-supported ionic liquids catalyst in impellers of the agitation system for the conversion of CO2.


Conversion of CO2 into Organic Carbonates over a Fiber-Supported

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