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Heterogenization of a green homogeneous catalyst: synthesis and characterization of imidazolium ionene/Br-Cl- @SiO2 as an efficient catalyst for cycloaddition of CO2 with epoxides Zahra Akbari, and Mehran Ghiaci Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02803 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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Industrial & Engineering Chemistry Research

Heterogenization of a green homogeneous catalyst: synthesis and characterization of imidazolium ionene/Br-Cl-@SiO2 as an efficient catalyst for cycloaddition of CO2 with epoxides

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Zahra Akbaria, Mehran Ghiacia,*

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a

Department of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran *

[email protected]

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Abstract

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A new and promising strategy for immobilization of a water soluble oligomeric ionic liquid

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through coating the ionic liquid with mesoporous silica was established. The immobilized ionic

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liquid (IL@SiO2) exhibited a good catalytic activity for the production of cyclic carbonates

13

through cycloaddition of different epoxides with CO2 in the solvent-free conditions. The

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IL@SiO2 catalyst was characterized by different techniques such as FTIR, XRD, TG, FE-SEM-

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EDX, map analysis, BET and TEM. By using 30 mg of the IL@SiO2 catalyst which contains

16

0.023 mmol of oligomeric ionic liquid, IL-3, the yield of cyclic carbonate with styrene oxide was

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above 95% and the selectivity of cyclocarbonate was almost 100%. We realized that the

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hydroxyl groups on the surface of silica could play an important role in the reaction through the

19

hydrogen bonding interactions between the epoxides and the hydroxyl groups. Moreover, the

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prepared catalyst was very stable in the reaction conditions and reusable for at least five runs

21

without considerable change in the yield of the reaction. The influence of different factors was

22

investigated on cyclic carbonates formation such as pressure, time, the amount of catalyst and

23

temperature.

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Keywords: Oligomeric ionic liquid, Styrene oxide, Styrene carbonate, Heterogenization.

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1. Introduction

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As an established phenomenon, the climate changes are because of human’s activities.1-3

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Scientifically, it is clear that climate changes is underway, but the important question is that

35

these changes “at what rate” are occurring and what can we globally do about it. The most

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important chemical that influences this phenomenon is carbon dioxide (CO2) which is mostly

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generated through burning fossil fuels.2,4-6

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Perhaps a significant stage in regard to sustainable development is the carbon dioxide

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fixation which has attracted a lot of interest in academic and industrial sectors.7-10 This nontoxic,

40

non-flammable and cheap chemical has been involved in more than twenty reactions10-13 as

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starting material that among them the synthesis of cyclic carbonates might be the most promising

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reaction because of its atom economy.14-16 As a matter of fact, the cyclic carbonates have been

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widely used as aprotic solvents, intermediate for synthesis of fine chemicals and pharmaceutical

44

products.7,17-19

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Variety of catalytic systems has been introduced for the synthesis of cyclic carbonates

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through the reaction of CO2 with epoxides.20-22 In this regard, we could name reactions which

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have used metal oxides,23-25 ionic liquids,26-28 organometallic compounds,29,30 etc. For example,

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Hua, et al.31 used the organometallic catalyst Re(CO)5Br for the synthesis of propylene carbonate

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under the PCO2 = 5.5 MPa at 110 °C in a period of 24 h with 97% yield. In another work,

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Cu(II)porphyrin/DMAP was used by Srinivas and his co-workers32 for the synthesis of propylene

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carbonate which during 4 h, PCO2 = 6.9 bar and temperature of 120 °C they reported yield of 78%

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for the reaction. Deng et al. demonstrated that anionic counterpart of the ionic liquids could also

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influence on the yield of the reaction between propylene oxide and CO2 for production of

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propylene carbonate.33 Hesemann et al. by grafting different ionic liquids on SBA-15 also

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studied the base-catalyzed cycloaddition of carbon dioxide with epoxides.34 Other efficient

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catalytic systems such as ionic liquid/ZnBr2,35 Co(TPP)X/PTAT,36 Co(salen)X/PTAT,37 and

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Ru(TPP)X/PTAT36 are reported by Jing et al. for synthesis of cyclic carbonates. As a general

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view, it has been shown that metal containing catalysts exhibit very good performance compared

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with the metal-free catalysts.

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Epoxide activation through hydrogen bonding and subsequent ring-opening by

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nucleophile has been reported in the literature.7,38 In this respect, in an interesting work Sakakura 2 ACS Paragon Plus Environment

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et al. reported that immobilization of phosphonium halides on silica noticeably increased the

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catalytic activity of the catalyst compared to the corresponding homogeneous phosphonium

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salt.39 They demonstrated that silica itself does not show catalytic activity even at high

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temperature but it shows a synergistic effect in combination with onium salt for cyclic carbonate

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production.

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As a matter of fact and as mentioned in the previous paragraph there are many reports on

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using ionic liquid for preparing cyclic carbonates. Therefore, a question rises that what is the

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novelty of the present work? Immobilization of an ionic liquid on different supports is an old

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story and leaching of the catalyst has always been a drawback for such systems. We thought if

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one prepares a polymeric ionic liquid and traps it inside a shell of an inorganic porous material

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the leaching of the ionic liquid should be decreased. Therefore, in continuation of our previous

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works40-43 in regard to increasing the life time of a catalyst by coating it with an inorganic

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mesoporous metal oxide we have used this methodology for immobilization of a new oligomeric

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ionic liquid in mesoporous silica as environmentally gentle media and investigated its usability

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in the synthesis of cyclic carbonates.

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2. Experimental

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2.1. Materials and analysis

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All necessary chemicals such as 1,4-dibromobutane, benzyl chloride, imidazole, styrene

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oxide and other epoxides were purchased from Merck Company. CO2 with 99.999% was

81

obtained from Argon Company (Iran). Acetonitrile was obtained from Aldrich and used as

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received.

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FTIR spectra of the prepared intermediates and the catalyst were recorded by FTIR

84

spectrophotometer (Jasco 680-plus) using KBr pellets. 1H NMR spectra were obtained on a

85

Bruker Avance 400 MHz spectrometer in D2O, CD3OD or DMSO-d6 as solvent. The XRD

86

pattern of the catalyst was obtained using a Philips X’pert diffractometer (Netherland) with Cu

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anode. Thermogravimetric analysis of the IL-3@SiO2 catalyst was performed in the Argon

88

atmosphere at a heating rate of 10 °C min−1 using BAHR-STA/TGA-503. Adsorption–desorption

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isotherms of nitrogen at 77.3 K were measured using a NOVAWin2, version 2.2 (Quantachrome

90

instruments) and also the specific surface area of the catalyst was determined by the BET

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technique. FESEM equipped with an EDX spectrometer and transmission electron microscopy 3 ACS Paragon Plus Environment


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(TEM) were performed with the Mira 3-XMU FESEM (Germany) and Philips CM120

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transmission electron microscope with accelerator voltage of 100 kV (Germany), respectively.

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2.2. Preparation of IL-3@SiO2 catalyst

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2.2.1. Synthesis of 1,4-di(1H-imidazol-1-yl)butane(1). NaH (60% in oil, 40 mg, 1 mmol) was

96

added to an acetonitrile solution of imidazole (68 mg, 1 mmol), and the resulting suspension was

97

stirred at 0 ºC to room temperature. Then, 0.5 mmol (0.06 mL) of 1,4-dibromobutane was added

98

and the mixture was reflux for 3 h. After completion of the reaction which was monitored by

99

TLC, the produced sodium bromide was separated by filtration. The solvent was evaporated at

100

reduced pressure and the product was purified by column chromatography on silica get (solvent:

101

cyclohexane-EtOAc). 1H NMR (400 MHz, D2O): δ1.68 (m, 4H, CH2), 3.99 (m, 4H, CH2), 6.99

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(s, 1H, CH), 7.11 (bs, 1H, CH), 7.70 (bs, 1H, CH) (Figure S1).

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2.2.2. Synthesis of 1,1’-(butane-1,4-diyl)bis(3-(4-bromobutyl)-1H-imidazol-3-ium bromide (IL-

104

1). 0.47 mL (4 mmol) 1,4-dibromobutane was added to a solution of 1,4-di(1H-imidazol-1-

105

yl)butane (190 mg, 1 mmol) in acetonitrile (15 mL). The resulting mixture was stirred under

106

reflux at 80 ºC for 8 h. After completion of the reaction by monitoring it with TLC, the solvent

107

was evaporated under reduced pressure and then by addition of 20 mL H2O, the mixture

108

extracted with chloroform to remove the unreacted dibromobutane. The ionic liquid IL-1 was

109

extracted from the aqueous solution by ethyl acetate (2x25 mL) and purified by column

110

chromatography on silica gel (solvent: cyclohexane-EtOAc) (Scheme 1). 1H NMR (400 MHz,

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CD3OD): δ 1.93 (m, 4H, CH2), 2.01 (m, 8H, CH2), 4.31 (m, 4H, CH2), 4.36 (m, 8H, CH2), 7.76

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(bs, 4H, CH), 9.29 (bs, 2H, CH). 13C NMR (125 MHz, D2O) IL-1: δ (ppm) 26.5, 28.2, 28.9, 33.7,

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49.0, 122.6, 135.5. M.p. 95 ºC (Figure S2).

114

2.2.3. Synthesis of 1-(4-(1H-imidazol-1-yl)butyl)-3-benzyl-1H-imidazol-3-ium chloride (IL-2).

115

Benzyl chloride (0.117 mL, 1 mmol) was added to a solution of 190 mg of 1,4-di(1H-imidazol-1-

116

yl)butane in acetonitrile (20 mL), and then mixture was reflux for 12 h. After completion of the

117

reaction by monitoring it with TLC, 25 mL water was added and after extraction with chloroform

118

(2x25 mL), the crude ionic liquid IL-2 was extracted from aqueous phase with ethyl acetate (2x

119

25 mL) that was purified with column chromatography (EtOAc-CH3OH). 1H NMR (400 MHz,

120

DMSO-d6): δ1.83 (m, 4H, CH2), 4.28 (m, 4H, CH2), 5.49 (s, 2H, CH2), 7.40-7.49 (m, 8H, CH),

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7.89-7.90 (bs, 2H, CH), 9.58 (bs, 1H, CH). 13C NMR (125 MHz, D2O): δ (ppm) 26.4, 49.1, 53.2,

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122.7, 129.0, 129.6, 133.8, 135.5. M.p. 150 ºC (Figure S3).

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2.2.4. Synthesis of 1,1’-(butane-1,4-yl)bis(3-(4-(1-(4-(3-benzyl-1H-imidazol-3-ium-1-yl)butyl)-

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1H-imidazol-3-ium-3-yl)butyl)-1H-imidazol-3-ium)

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preparation of the ionic liquid IL-3, in a 50 mL round-bottom flask 621 mg (1 mmol) of IL-1 and

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632 mg (2 mmol) of IL-2 and 30 mL dry DMF was poured and the mixture was refluxed for 24

127

h. The progress of the reaction monitored by TLC and by completion of the reaction, solvent was

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evaporated under vacuum at 100 °C. The IL-3 ionic liquid was precipitated by adding ethanol to

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the aqueous solution of crude IL-3 and then characterized by 1H NMR spectroscopy. 1H NMR

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(400 MHz, DMSO-d6): δ 2.32-2.49 (m, 20H, CH2), 3.34 (m, 20H, CH2), 5.43 (s, 4H, CH2), 7.41-

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7.42 (m, 12H, CH), 7.72-7.73 (m, 10H, CH), 7.83 (d, 4H, CH), 9.42 (s, 2H, CH). 13C NMR (125

132

MHz, D2O): δ (ppm) 26.4-26.5, 49.1, 53.2, 122.7, 122.9, 128.9, 129.5, 129.6, 134.3, 135.6

133

(Figure S4). Anal. Calcd. For C51H72Br4Cl2N12: C 49.43, H 5.82, N 13.57%. Found: C 49.52, H

134

5.88 and N 13.45%. (m.p. 230 ºC).

135

2.2.5. Preparation of IL-3@SiO2. 0.1 g of the IL-3 was added to 20 mL of dry n-hexane under

136

vigorous stirring (800 rpm) at room temperature to be dispersed completely. Then calculated

137

amount of tetraethyl orthosilicate (TEOS) dissolved in 5 mL of n-hexane and added dropwise to

138

the mixture. After 1 h, for hydrolysis of TEOS, the required amount of DI water was added to the

139

mixture. After 24 h, the solid was filtered and dried.

tetrabromide

140


dichloride

(IL-3).

For


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141

142

Scheme 1. Preparation process for synthesis of IL-3.

143 144

2.2.6. Coupling reaction

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Styrene carbonate (SC) was synthesized by coupling reaction between styrene oxide (SO)

146

and CO2 in the presence of the IL-3@SiO2 catalyst (Scheme 2). In a typical reaction, a 50 mL

147

stainless-steel batch reactor was charged with the catalyst (50 mg) and 1 mL of styrene oxide.

148

The reaction was carried out under a preset pressure of carbon dioxide at a selected temperature.

149

After completion of the reaction reactor was cooled to 0°C, and unreacted carbon dioxide was

150

released. Ethanol was added to dissolve the products and unreacted epoxide. The catalyst was

151

separated by centrifuge, and washed thoroughly by ethanol and chloroform, dried under vacuum

152

at 40°C to be ready for the next run. The products were determined quantitatively by gas

153

chromatograph (Agilent HP6890A) using internal standard (biphenyl). 1H NMR (400 MHz,

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CDCl3): δ 4.33-4.42 (t, 1H, CH2), 4.71-4.83 (t, 1H, CH2), 5.67-5.71 (t, 1H, CH), 7.26-7.38 (m,

155

3H, CH), 7.44-7.46 (m, 2H, CH) (Figure S5). GC-Mass data of the product is presented in Fig.

156

S6.

157 158

Scheme 2. Synthesis of styrene carbonate from styrene epoxide and CO2.


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3. Results and discussion

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3.1. Characterization of Catalyst

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3.1.1. FT-IR analysis

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The FTIR spectra of IL-1, IL-2, IL-3 and IL-3@SiO2 are shown in Figure 1. In the

163

spectrum of IL-1, the broad band appeared in the range of 3600-3000 cm-1 is attributed to the O-

164

H stretching of the adsorbed water. The peaks at 3051cm-1, 2907 and 2850 cm-1 are due to the C-

165

H stretching vibrations in the imidazolium ring and the alkyl chains, respectively. The peak

166

appeared at 1630 cm-1 is assigned to the O-H bending vibration. The appeared peaks at 1558 and

167

1455 cm-1 are due to the stretching vibrations of the imidazolium skeleton. The sharp peak at

168

1250 cm-1 was assigned to C-N stretching vibrations.44 The C-Br stretching vibration was also

169

appeared at 635 cm-1. Furthermore, in the spectrum of IL-2, in addition to the peaks which were

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belonged to the imidazolium and alkyl functional groups, the C-Br stretching vibration was

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substituted with the C-H out-of-plane bending vibrations of the phenyl group in IL-2. In the

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spectrum of the IL-3 the peaks corresponding to the imidazolium moieties, the alkyl chains and

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phenyl groups have intensified. The FTIR spectrum of the IL-3@SiO2 hybrid composite is

174

shown in Figure 1d. According to the observed characteristic peaks for SiO2, the broad peak

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appearing at 1064 cm−1 in this spectrum can be attributed to the stretching vibration of the

176

Si−O−Si bonds,6 and the band appeared at 450 cm-1 is due to bending vibration of the Si-O-Si

177

bonds.45



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Figure 1: FTIR spectra of (a) IL-1, (b) IL-2, (c) IL-3 and (d) IL-3@SiO2

180 181

3.1.2. TG, XRD and TEM analyses

182

The thermal behavior of IL-3 and catalyst IL-3@SiO2 were investigated by TG analysis.

183

As shown in Fig. 2, both samples might be stable up to 250 °C where IL-3 because of its

184

hydrophilic nature lose more adsorbed water than the IL-3@SiO2 catalyst. In the IL-3 sample

185

upon heating under argon from 250 to ~700 °C, decomposition and probably reconstruction of

186

organic moieties has occurred, and above 700 °C it experiences a complete decomposition. The

187

IL-3@SiO2 sample also under argon tolerates approximately the same behavior with some 8 ACS Paragon Plus Environment

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188

degree of stability because of silica presence. To have a clear idea in regard to the percentage of

189

silica in the IL-3@SiO2 composite the thermal behavior of the composite under air showed that

190

the entrapped ionene in the composite is around 16 wt%.

191 192

Figure 2: TG curves of (a) IL-3, (b) IL-3@SiO2 under Ar, and (c) IL-3@SiO2 under air.

193

Figure 3 shows the XRD patterns of the IL-3 and the IL-3@SiO2 hybrid composite. The

194

peaks at 2θ = 33°, 38° and 50° demonstrate that IL-3 has some crystalline zones. Clearly, as can

195

be seen in Figure 3b, by coating the IL-3 with an amorphous SiO2 layer the crystallinity of the

196

IL-3 has improved appreciably by appearing new diffraction peaks. It is not unexpected that by

197

surrounding the IL-3 with amorphous silica it will develop more crystalline arrangement.

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TEM images of the IL-3 and IL-3@SiO2 clearly show that the IL-3 before coating with

199

silica is not really a dense material and the particles which are aggregates of oligomeric ionic

200

liquid molecules dispersed to some extent and by coating these particles with SiO2 the particles

201

packed together appreciably (Figure 4).



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Figure 3: XRD spectra of (a) IL-3 and (b) IL-3@SiO2

204 205

Figure 4: TEM images of IL-3 (a) 200 nm, (b) 60 nm and IL-3@SiO2 (c) 200 nm, (d) 60 nm.

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3.1.3. FESEM, EDX and mapping analyses 10 ACS Paragon Plus Environment

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Morphology of the IL-3@SiO2 catalyst is shown through FESEM images in different

209

magnifications (1 µm, 2 µm, 200 nm and 500 nm) in Figure 5. As mentioned in section 2.2.5, IL-

210

3 was dispersed as a very fine powder in n-hexane and then coated with silica; images clearly

211

evoke the process. There could be seen particulates were buried in the silica coating. By

212

formation of the porous silica around an aggregate of IL-3 molecules we have been able to

213

inhibit escaping of the molecules from the silica matrix and in other words, we were succeeded

214

to heterogenize such a highly ionic molecule. On the basis of TG analysis of the composite under

215

air (section 3.1.2), we concluded that the percentage of silica in the composite was about 60%

216

which is in contradiction with the EDX data. The accuracy of EDX analysis depends on various

217

factors such as the nature of the sample, and accurate measurement of the sample composition

218

needs the application of quantitative procedures known as matrix corrections. Therefore, the

219

reported silica percentage from EDX analysis could not be reliable quantitatively. In our

220

synthesis system, the oligomeric chains of silica interact with the IL-3 via coulombic forces and

221

a mesoporous structure is formed. Actually, data of EDX mapping show a uniform distribution

222

of most of the elements in the sample (Figure 6). However, it might be interesting to compare the

223

distribution of Cl- and Br- in these maps and observe a much more uniformity in distribution of

224

bromide anions! Clearly, the nature of the anion Cl- or Br- has a strong impact on the

225

physicochemical properties of imidazolium rings present in the IL-3 chains, and such diversity of

226

interactions when shows itself in an aggregate of ionene molecules leads to some crystalline

227

zones that there are stronger electrostatic interactions and more chloride anions.



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Figure 5: FESEM images of IL-3@SiO2 (a) 1 µm, (b) 2 µm, (c) 200 nm and (d) 500 nm

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Figure 6: EDX mapping of IL-3@SiO2 (a) Carbon, (b) Nitrogen, (c) Oxygen, (d) Silicon, (e) Bromide and (f) Chloride.

233 234

3.1.4. N2 adsorption–desorption isotherms

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The N2 adsorption–desorption isotherm of the IL-3@SiO2 catalyst is presented in Figure

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7. The prepared hybrid composite has a specific surface area of 236 m2/g and presents a type II

237

adsorption isotherm which shows a H4 hysteresis loop on the basis of IUPAC definition. The

238

average pore size of the composite and the total pore volume were 2.06 nm and 2.56 cm3/g,

239

respectively. Therefore, the prepared hybrid composite could be categorized as a macro-

240

mesoporous solid.46

241 242

Figure 7. N2 adsorption–desorption isotherms of IL-3@SiO2.

243 244

3.2. Catalytic activity

245

Encouraged by the efficient heterogeneous catalysis of the cycloaddition of CO2 to

246

epoxide, and this fact that cycloaddition of carbon dioxide to epoxides is promoted by especially

247

imidazolium based ILs when supported on silica,39 we tested this idea by trapping a water 13 ACS Paragon Plus Environment


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soluble imidazolium based ionene (IL-3) in mesoporous silica and investigated the synergistic

249

effect between the supported ionic liquid and the silica support where leading to a appreciable

250

acceleration of the reaction compared to the reaction carrying with IL-3 without supporting on

251

silica.

252

The influence of reaction parameters such as temperature, time, pressure of carbon

253

dioxide, and amount of catalyst on the conversion were investigated (Tables 1 and 2). A broad

254

temperature range was tested from 100 to 125 °C on the conversion of styrene oxide. The

255

conversion enhanced progressively with an increase in reaction temperature to 120°C. No further

256

increase in conversion was observed by raising the temperature to 125 °C; therefore, 120 °C was

257

chosen as the optimum temperature (Table 1, entries 1-5). It was observed that the reaction

258

almost completed within 5 h and by increasing the time of reaction to 7 h the conversion did not

259

change (Table 1, entries 6-8).

260 261

Table 1. Effect of temperature and time on conversion and selectivity T t Conversion Selectivity (%) TONc Entry (°C) (h) (%) SCa SGb 1449 1 100 5 85 87 13 2

105

5

88

90

10

1500

3

115

5

95

96

4

1619

4

120

5

98

98

2

1670

5

125

5

98

98

2

1670

6

120

1.5

68

98

2

1159

7

120

3

71

98

2

1210

8

120

7

98

98

2

1670

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Effect of carbon dioxide pressure was examined, and by increasing PCO2 the conversion

268

increased and reached to a maximum of 98% when the pressure was 20 bars (Table 2; entries 1-

269

4). The amount of IL-3@SiO2 catalyst used in this work was between 0-50 mg that the reaction

270

did not occur without the catalyst and the conversion reached to a maximum of 98% when 30 mg

Reaction conditions: styrene oxide 1 mL; CO2 pressure 20 bars; catalyst: 30 mg (equivalent to 0.25x10-2 mmole IL-3/mmol of styrene oxide). aStyrene carbonate. b Styrene glycol. cTON: moles of epoxide converted/moles of imidazolium active sites (mol % of halide ions, Br- and Cl-, in x mg of catalyst)


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271

of the catalyst was used (Table 2; entries 5-8). Albeit for comparison we have studied the

272

catalytic activities of the IL-1, IL-2 and IL-3 in pure states. As expected and shown in Table 2

273

(entries 9-11), TONs of the reaction when using IL-3, IL-2, and IL-1 were 259, 52 and 35,

274

respectively which are consistent with the number of catalytic sites in these ionic liquids. It is

275

interesting to compare the TONs of the IL-3 without coating and IL-3@SiO2 catalyst. As shown

276

by TG analysis the IL-3@SiO2 catalyst contains 16 wt% of IL-3 and naturally has much lower

277

catalytic sites than IL-3 but its TON is at least six times higher than that of pure IL-3. This

278

behavior could be related to role of silica participation in the reaction through hydrogen bonding,

279

and crystallinity of IL-3 in pure state and when it is dispersed in the silica matrix.

280 281

282 283 284 285 286

Table 2. Effect of carbon dioxide pressure and amount of catalyst on conversion and selectivity Amount of PCO2 Conversion Selectivity (%) Entry catalyst TONc catalyst a b (bar) (%) SC SG (mg) 1534 1 IL-3@SiO2 5 30 90 90 10 2

IL-3@SiO2

10

30

94

92

8

1602

3

IL-3@SiO2

15

30

95

96

4

1619

4

IL-3@SiO2

20

30

98

98

2

1670

5

IL-3@SiO2

20

Blank

-

-

-

-

6

IL-3@SiO2

20

10

65

99

1

3287

7

IL-3@SiO2

20

30

98

98

2

1670

8

IL-3@SiO2

20

50

98

98

2

1009

9

IL-1

20

30

78

100

-

35

10

IL-2

20

30

57

100

-

52

11

IL-3

20

30

95

98

2

259

Reaction conditions: styrene oxide 1 mL; 120 °C and 5 h. a30 mg catalyst is equivalent to 0.25x10-2 mmole IL-3/mmol of styrene oxide. bStyrene carbonate. c Styrene glycol. dTON: moles of epoxide converted/moles of imidazolium active sites (mol % of halide ions, Br- and Cl-, in x mg of catalyst).

3.2.2. Reusability of IL-3@SiO2 catalyst



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Page 16 of 26

287

The main purpose of this work was to establish an efficient methodology for

288

heterogenization of a water soluble catalyst. Moreover, in order to demonstrate the success of

289

this methodology, reusability of the IL-3@SiO2 catalyst was studied. Therefore, the reactor was

290

charged with 30 mg (0.043 mmol) of IL-3@SiO2 catalyst, 1 mL of styrene oxide and CO2 with

291

an initial pressure of 20 bars at room temperature. The reaction was conducted at 120 °C for 5 h

292

and by completion of the reaction, solid product was dissolved in ethanol and then catalyst was

293

separated, washed with chloroform and dried for the next run. The used catalyst in accompanied

294

with styrene oxide and CO2 was charged into the reactor and reaction was performed again at

295

120°C for 5 h. This procedure was repeated five times and the results are shown in Figure 8.

296

After 5 runs, the catalyst was stable and its catalytic activity declined a few percent that should

297

be in the range of experimental error. It should be mentioned that the leaching of the IL-3 from

298

the catalyst was argued but no ionic liquid was detected in the reaction media. This demonstrates

299

that our technique for heterogenization of a water soluble ionic liquid has been successful. 98

97

Conversion (%) Selectivity (%)

96

100

96

96

80 98 60 1

98 2

96

95

3

Cycle

4

95

5

300 301

Figure 8. Reusability test of the catalyst.

302

In regard to the mechanism of the reaction we think that the SiO2 and polarity of the

303

imidazolium moiety in this catalyst could play an important role in the cycloaddition of carbon

304

dioxide to styrene oxide (Scheme 3). As previously reported,47,48 this reaction occurs in three

305

steps (1) ring opening of epoxide, (2) insertion of CO2 and finally (3) an intramolecular

306

cyclization. Generally, the ring opening step is the rate-determining step which could be

307

promoted through hydrogen bonding that oxygen atom of epoxide can form with the OH groups 16 ACS Paragon Plus Environment

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308

present on the surface of SiO2 coating.49,50 Therefore, it is clear that the barrier height of ring-

309

opening step by using IL-3@SiO2 as catalyst should be lower than that of corresponding ionic

310

liquid, IL-3, as result of strong hydrogen bonding between hydroxyl groups and epoxide. In this

311

respect, the theoretical calculations reported by Wang, et al.51 demonstrated that the presence of

312

such hydrogen bonding play an important role in increasing the rate of the cycloaddition reaction

313

between epoxide and carbon dioxide. Another point that should be mentioned is that IL-3 which

314

is a crystalline compound loses some of its catalytic sites because of difficulty of substrate

315

diffusion for reaching to the active sites of the ionic liquid. Therefore, silica not only lowers the

316

barrier of ring-opening step through hydrogen bonding but the active sites of the catalyst would

317

be more open to the substrate molecules.

318 319

Scheme 3. Proposed mechanism for the reaction.

320 321

3.2.2. Cycloaddition reaction of CO2 with other epoxides catalyzed with IL-3@SiO2

322

To demonstrate success of the methodology, various epoxides were used and reacted with

323

carbon dioxide under the optimized condition obtained for the reaction of styrene oxide with CO2

324

(Table 3). Reaction of cyclohexene oxide with CO2 is reported in two conditions (Table 3;

325

entries 2 and 3). In condition appropriate for other epoxides the yield of cyclohexene carbonate 17 ACS Paragon Plus Environment


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Page 18 of 26

326

was low (7%) because of the steric effects. However, by increasing the time of reaction to 20 h,

327

the yield of cyclohexene carbonate increased to a large extent (82%).

328 329

Entry

a

330 331 332 333 334

Table 3. Catalytic tests for different epoxidesa Conversion Selectivity Substrate TONc (%) (%)

1

98

98

1670

2

7

100

119

3b

82

100

1398

4

100

100

1704

5

99

98

1687

6

97

99

1653

Reaction conditions: epoxide1 mL;120 °C; CO2 pressure 20 bars; catalyst 30 mg and 5 h. b Reaction conditions: epoxide1 mL; 120 °C; CO2 pressure 20 bars; catalyst 30 mg and 20 h. c TON: moles of epoxide converted/moles of imidazolium active sites (mol (%) Br- and Cl-)

4. Conclusion

335

A new methodology for heterogenization of a very soluble oligomeric ionic liquid was

336

developed and as a green catalyst which exhibited a desire activity and selectivity for the

337

synthesis of different cyclic carbonate with various epoxides without using any solvent. The

338

product could be separated very easily from the catalyst. This green catalyst was reused five

339

times without losing its activity and selectivity.

340 341

Supporting information

342

The Supporting Information file contains: Figures S1-S5 reporting 1H and

343

1,4-di(1H-imidazol-1-yl)butane, IL1, IL2, IL3 and styrene carbonate. Fig. S6 contains GC-Mass

344

information of the main products. Fig. S7 contains N2 adsorption-desorption isotherm data.

345

AUTHORS INFORMATION 18 ACS Paragon Plus Environment

13

C NMR spectra of

Page 19 of 26

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346

Corresponding Author

347

*Tel. and FAX +98-031-33913254 (2350)

348

E-Mail: [email protected]

349

WEB site: www.ghiaci.iut.ac.ir

350

ORCID

351

The authors declare no competing financial interest.

352

ACKNOWLEDGMENTS

353 354

Thanks are due to the Research Council of Isfahan University of Technology for supporting of this work.

355 356 357 358 359 360 361 362 363 364 365 366 367 368



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369

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framework

via

immobilization


of

oxodiperoxo


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