Task-Specific Ionic Liquid Functionalized-MIL- 101(Cr) as a

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Task-Specific Ionic Liquid Functionalized-MIL-##101(Cr) as a Heterogeneous and Efficient #Catalyst #for the Cycloaddition of CO2 with Epoxides Under #Solvent Free Conditions Mehrnaz Bahadori, Shahram Tangestaninejad, Marko Bertmer, Majid Moghadam, Valiollah Mirkhani, Iraj Mohammadpoor-Baltork, Reihaneh Kardanpour, and Farnaz Zadehahmadi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05226 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Sustainable Chemistry & Engineering

Task‒Specific

Ionic

Liquid

Functionalized‒MIL‒101(Cr) as a Heterogeneous and Efficient Catalyst for the Cycloaddition of CO2 with Epoxides Under Solvent Free Conditions Mehrnaz Bahadori†, Shahram Tangestaninejad†,*, Marko Bertmer‡, Majid Moghadam†,*, Valiollah Mirkhani†,*, Iraj Mohammadpoor‒Baltork†, Reihaneh Kardanpour†, Farnaz Zadehahmadi§ †

Department of Chemistry, Catalysis Division, University of Isfahan, Hezarjerib Street,

Isfahan 81746‒73441, Iran; *E‒mail addresses: [email protected] (S. Tangestaninejad), [email protected] (M. Moghadam), [email protected] (V. Mirkhani) ‡

Leipzig University, Felix Bloch Institute for Solid State Physics, Linnéstr. 5, 04103 Leipzig,

Germany §

La Trobe Institute for Molecular Sciences, La Trobe University, Bundoora VIC 3086,

Australia

ACS Paragon Plus Environment

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Keywords: MIL‒101(Cr), Task‒Specific Ionic Liquid, Cycloaddition of CO2 with Epoxides, Solvent‒Free

Abstract:

A novel heterogeneous catalyst was synthesized by immobilization of a carboxylic acid‒ and imidazolium‒based ionic liquid on the mesoporous MIL‒101(Cr) (MIL‒101(Cr)‒TSIL) and used to convert abundant, non‒toxic, economical and renewable CO2 gas to cyclic carbonates without the need for a co‒catalyst or a solvent. The catalyst was characterized in detail by multiple techniques such as XRD, TEM, SEM, EDX, DR‒FTIR, solid‒state NMR, as well as N2 and CO2 adsorption measurement. The catalytic properties were studied by varying different parameters including amount of catalyst and epoxide, temperature, pressure, and reaction time. Under optimal conditions (100 mg catalyst, 15 mmol epoxide, 2.0 MPa CO2 pressure, 110 °C and 2 h reaction time) various cyclic carbonates were obtained with high yield and selectivity. MIL‒101(Cr)‒TSIL catalyst displayed good thermal stability and could be reused after simple separation without a significant decrease in its catalytic activity. Due to synergetic effect of the hydrogen bond from the carboxylic acid group for activation of the C–O bond of the epoxide, adsorption of CO2 by the imidazolium moiety and high concentration of CO2 around the task specific ionic


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liquid (TSIL), arisen from the mesoporous framework, MIL‒101(Cr)‒TSIL is highly effective catalytic system for the solvent‒free cycloaddition of CO2 to epoxides.


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Introduction: Metal‒organic frameworks (MOFs) are a class of porous crystalline compounds composed of 1D, 2D or 3D networks resulting from the bonding of metal nodes with polyfunctional organic linkers. High surface area, purely crystalline nature, tunable porosity, and structural diversity make them promising candidates for diverse applications in membrane separation,1 heterogeneous catalysis,2 encapsulation,3 as hosts for metal colloids or nanoparticles,4‒6 chemical separation,7 gas storage,8 drug delivery,9 and sensor technology.10‒11 Due to their potential in modification of pore size, aperture size, surface area, topology, and ability to incorporate of task‒specific functional groups into the framework, MOFs have also shown catalytic properties for fine chemical synthesis.12‒15 Carbon dioxide is a greenhouse gas and its rising level of emissions from anthropogenic sources, such as burning of fossil fuels, industrial waste gases, and motor vehicle exhaust emissions, is a worldwide environmental concern.16‒17 Despite being notorious for global warming, carbon dioxide is an abundant, non‒toxic, economical, and renewable C1 carbon source to produce useful organic compounds.18‒20 An important reaction of CO2 fixation is the coupling reaction of epoxides with CO2 to afford five‒membered cyclic carbonates.21 Cyclic carbonates are widely employed as polar aprotic solvents,22 electrolytic elements for lithium batteries,23 valuable fine chemical intermediates in the production of pharmaceuticals,24 and fine and agricultural chemicals.25‒28 Due to the high inherent thermodynamic stability and kinetic inertness of CO2, the catalyst is an essential


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factor for its conversion to proceed. A great many catalytic systems have been investigated for CO2 fixation including homo‒ and heterogeneous catalysts of various type ranging from metal oxides,29‒30 transition metal complexes.31‒33 and organometallic compounds 34‒35 to designed organometallic salen complexes,36 supported ionic liquids,37 and most recently metal‒organic frameworks.38‒40 Among these, ionic liquids (IL) have been regarded as an effective and environmental catalyst, because of negligible vapor pressure, excellent thermal stability, tunable structure, and nonflammability.25,

41‒44

Modification of ionic liquids by covalent attachment of functional groups to either their cations or anions can create task‒specific ionic liquids (TSILs), which defined a subclass of ionic liquids designed for particular applications.45‒46 Selection of the appropriate functional groups plays a key role in the architecture of excellent and stable TSILs. From the experience of previous studies, imidazolium‒based ILs with hydroxyl (―OH) and carboxylic groups (―COOH) showed a more efficient catalytic activity in the CO2 fixation with epoxides due to hydrogen bonding.47‒48 However, on the basis of economic criteria such as catalyst recovery and product purification, supported ionic liquids are highly desirable because their physical and chemical properties, catalytic activity, and selectivity are retained. Until now, immobilization of IL catalysts on various support materials has been successfully achieved using silica,49 chitosan,50 cross‒linked polymers,51 magnetic nanoparticles,52 carbon nano tubes,53 and metal‒organic frameworks.54‒55 Functionalized


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metal‒organic frameworks with porous structure are uniquely appropriate to combine the high level of reactivity of homogeneous catalysts with the ease of separation and recycling of a heterogeneous catalyst. An alternative and simple strategy for installation of catalytic moieties into MOFs is post synthetic modification (PSM) which has been recently used to introduce functionalities that are difficult to obtain via de novo solvothermal synthesis. In particular, MIL‒101(Cr),56 Cr3O(F/OH)(H2O)2[C6H4(CO2)2], with a rigid zeotype crystal structure consisting of 2.9 and 3.4 nm mesoporous cages accessible through window sizes of ca. 1.2 and 1.6 nm, respectively, is one of the most stable, tunable, and porous materials among the many MOFs that have been studied. The use of MIL‒101(Cr) as support for immobilization of homogeneous catalysts by chemical grafting has been regarded due to the high surface area and temperature stability.57‒58 In the present work, we have designed a novel catalytic system composed of MIL‒101(Cr) as a structural support associated with a new “task‒specific” ionic liquid for cycloaddition of CO2 to epoxides under solvent free conditions. The cation of this task‒specific ionic liquid comprised of an imidazolium ion with a covalently bound carboxylic acid moiety. The imidazolium part of the ionic liquid leads to activation of the CO2 molecule due to electrostatic interactions and the carboxylic acid moiety activates the epoxide via hydrogen bonding. On the other hand, MIL‒101(Cr) with its mesoporous structure enhances the concentration of carbon dioxide around the task‒specific ionic liquid. In order to synthesize the products under green chemistry strategy,59‒60 we


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performed the reactions under solvent free conditions. The catalyst displayed high catalytic activity, high selectivity, and excellent reusability over various substrates for the solvent free cycloaddition of CO2. MIL‒101(Cr)‒TSIL was characterized and evaluated for the synthesis of cyclic carbonates without use of any organic solvent and co‒catalyst. Experimental: Materials and Methods All reagents were analytical grade purchased from Sigma–Aldrich Co. Ltd. and used without further purification. Diffuse reflectance Fourier transform infrared (DR‒FTIR) spectroscopy was performed on a JASCO FT/IR 6300 spectrometer, in the range 4000 ‒ 400 cm‒1. Powder X‒ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8‒Advance diffractometer using Co Kα radiation (λ= 1.7890 Å) in the range 0.5 to 70°. Nitrogen gas adsorption isotherms at ‒196 °C and CO2 gas adsorption isotherms at 0 °C were obtained with a surface area and porosity analyzer (Micromeritics TriStar II Plus) after evacuation of the samples at 100 °C for 12 h. Field emission scanning electron microscopy (FE‒SEM) images and energy dispersive analysis of X‒ray (EDX) were recorded on a MIRA3 instrument from TESCAN. For further investigations on the morphology, transmission electron microscopy (TEM) was performed on a Zeiss‒EM10C‒100 kV instrument. TGA curve was obtained on a SDT Q600 thermogravimetric analyzer under dynamic N2 atmosphere in the range 30‒1000 °C using heating rate of 20 °C/min.


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Solid‒state NMR measurements were performed using a Bruker Avance 750 MHz (magnetic field 17.6 T) and a Bruker Avance 400 MHz spectrometer (magnetic field 9.4 T) for

1H

and

13C

spectra, respectively. The

1H

spectra were recorded with a

rotor‒synchronized Hahn‒echo sequence and a spinning frequency of 13.333 kHz. The 90° pulse length was 2 µs. For the

13C

spectra, the DEPTH sequence61 was applied to

remove probe background signal. The spinning frequency was 12 kHz and about 200k scans were recorded. The 90° pulse length was 2.7 µs. In all measurements, the recycle delay was 100 ms. Spectra are referenced to TMS at 0 ppm for both 1H and 13C. Synthesis of MIL‒101(Cr)‒CH2Cl After

synthesis

of

MIL‒101(Cr)

(presented

in

Supporting

information),

chloromethylation was carried out based on a synthesis reported by Goesten et al.62 A mixture of MIL‒101(Cr) (1 g), AlCl3·6H2O (1.9 g, 7.8 mmol), nitromethane (80 g, 1.31 mol) and methoxyacetylchloride (0.4 g, 3.7 mmol) was heated at 100 ºC for 5 h. The green solid sample was treated with water at 100 ºC for 24 h and in THF at 67 ºC for 3 h. The resulting material was activated at 100 ºC.

Synthesis of MIL‒101(Cr)‒CH2‒imi


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To

synthesize

MIL‒101(Cr)‒CH2‒imi,

imidazole

(300

mg,

4.41

mmol)

and

MIL‒101(Cr)‒CH2Cl (150 mg) were added to toluene and the mixture was stirred at 110 ºC under inert gas for 48 h. After cooling to room temperature, MIL‒101(Cr)‒CH2‒imi was obtained. The toluene was removed and the solid material was washed in DMF at 100 ºC for 24 h. Synthesis of MIL‒101(Cr)‒TSIL MIL‒101(Cr)‒TSIL was synthesized by adding 4‒(bromomethyl)benzoic acid (300 mg, 1.4 mmol) to MIL‒101(Cr)‒CH2‒imi (150 mg) in ethanol (15 mL) and stirring at 80 ºC for 24 h under inert gas. The product was separated by filtration and washed with ethanol and dichloromethane several times. Finally, the supported task‒specific ionic liquid was obtained after heat treatment in vacuum at 100 ºC for 24 h. General procedure for the coupling reaction of epoxides with CO2 The cycloaddition of carbon dioxide to epoxides was carried out in a 75 mL stainless steel autoclave equipped with a magnetic stirrer and immersed into an oil bath. The autoclave was first filled with the required amount of the catalyst and epoxide. Subsequently, CO2 was filled into the reactor to reach the desired pressure. The autoclave with its content was allowed to heat to the selected temperature and stirred for a predetermined period of time. At the end of the reaction, the reactor was cooled down to 0 °C in an ice‒water bath, and the remaining CO2 was slowly vented. The product was


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diluted with ethyl acetate and filtered before GC analysis (Agilent 7890A GC‒FID). In the catalyst recycling process, MIL‒101(Cr)‒TSIL was separated by centrifugation, washed using ethyl acetate and DMF, and then dried under vacuum at 150 °C. Results and discussion: Characterization of MIL‒101(Cr)‒TSIL As illustrated in Scheme 1, MIL‒101(Cr)‒CH2Cl was synthesized by treating methoxyacetyl chloride with MIL‒101(Cr) and then reacting with imidazole to produce MIL‒101(Cr)‒CH2‒imi. In the last step, MIL‒101(Cr)‒TSIL was obtained by reacting the resulting product from the previous step with 4‒(bromomethyl)benzoic acid.

O

O

O

O

(i)

O

Scheme

O

1.

O Cl

O

Synthesis

methoxyacetylchloride,

(ii)

O

O

O

Br N

O

procedure nitromethane,

N

N

(iii)

O

O

for

O

O

MIL‒101(Cr)‒TSIL. (ii)

N

imidazole,

COOH

(i)

AlCl3.6H2O,

toluene,

(iii)

4‒(bromomethyl)benzoic acid, ethanol. The effects of modification reactions on the structural integrity and phase purity of MIL‒101(Cr) were evaluated by XRD. The powder patterns of MIL‒101(Cr),


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MIL‒101(Cr)‒CH2Cl, MIL‒101(Cr)‒CH2‒imi and MIL‒101(Cr)‒TSIL are shown in Figure 1. The main diffraction peaks of MIL‒101(Cr) (2θ = 3.25°, 3.8°, 9.8°, 10.57°)56 and in the simulated pattern, were observed in all samples and therefore confirm preservation of the structure of MIL‒101(Cr) during the reactions.

Figure 1. XRD powder patterns of (a) simulated MIL‒101(Cr), (b) MIL‒101(Cr), (c) MIL‒101(Cr)‒CH2Cl, (d) MIL‒101(Cr) ‒CH2‒imi, (e) MIL‒101(Cr)‒TSIL, and (f) reused MIL‒101(Cr)‒TSIL. Figure S1 depicts the DR‒FTIR spectra of MIL‒101(Cr), MIL‒101(Cr)‒CH2Cl, MIL‒101(Cr) ‒CH2‒imi and MIL‒101(Cr)‒TSIL. In the spectrum of MIL‒101(Cr), the high intensity bands at 1400–1600 cmˉ1 were ascribed to the stretching vibrations of C═C in the aromatic ring


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and the band at 1620 cm‒1 is related to asymmetric stretching vibrations of the carboxylic group in the framework. Vibrations of Cr‒O bond at 550‒610 cm‒1, which were retained during modification, proved the synthesis of the metal‒organic framework. The broad band at 3400‒3350 cm‒1 can be assigned to the acidic OH of the carboxylic group or crystalline water. Also the absorption band at 1700 cm‒1 was attributed to non-ionized carboxyl groups of unreacted terephthalic acid. The unreacted terephthalic acid is removed from the structure during the modification steps. The existence of a weak absorption band at 2866 cm‒1, related to stretching vibrations of benzylic CH in the spectrum of MIL‒101(Cr)‒CH2Cl, confirmed chloromethylation of MIL‒101(Cr). The bands at 3000‒3200 cm‒1 and 1305‒1338 cm‒1 are respectively ascribed to the CH and CN stretching modes in the imidazole moiety attached to MIL‒101(Cr)‒CH2Cl. The characteristic band of non‒ionized carboxyl groups was observed at 1700 cmˉ1 in the spectrum of MIL‒101(Cr)‒TSIL after introduction of 4‒(bromomethyl)benzoic acid, illustrating the successful synthesis of TSIL on the MOF surface. To survey the surface morphology and chemical composition, MIL‒101(Cr) and MIL‒101(Cr)‒TSIL were characterized by SEM and EDX (Figure 2). As depicted in Figure 2a, the SEM image of pristine MIL‒101(Cr) show cubic shape crystals. After chemical modification, the structural integrity of the framework is still maintained although the surface of MIL‒101(Cr)‒TSIL tends to be rough (Figure 2b). Also, EDX mapping (Figure 3) illustrated homogeneous dispersion of Br and Cr and the EDX spectrum displayed the.


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Accordingly, the loading of task‒specific ionic liquid was 0.12 mmol/g as determined by Br elemental analysis.

(a)

(c)

(b)

300 nm

300 nm

300 nm

Figure 2. SEM images of (a) MIL‒101(Cr), (b) MIL‒101(Cr)‒TSIL, and (c) reused MIL‒101(Cr)‒TSIL.


13


Br

Cr

5 μm

5 μm

Page 14 of 41

Figure 3. EDX spectrum and EDX mapping of MIL‒101(Cr)‒TSIL. Transmission Electron Microscopy (Figure S2) showed good agreement between MIL‒101(Cr) and MIL‒101(Cr)‒TSIL, indicating the stability of the framework during post‒synthetic modification. The effect of the grafted ionic liquid on the surface area and pore volume was studied using the Brunauer‒Emmett‒Teller (BET) method. The N2 adsorption at ‒196 °C and CO2 adsorption at 0 °C were carried out for MIL‒101(Cr) and MIL‒101(Cr)‒TSIL and the results are summarized in Table 1. The results indicated that the BET surface area and pore volume decreased because of the presence of the ionic liquid moiety on the surface of the metal‒organic framework. Table 1: Sorption data for MIL‒101(Cr) and MIL‒101(Cr)‒TSIL


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Sample

SBET (m2 g‒1) (from N2 SBET (m2 g‒1) (from CO2 adsorption at ‒196 °C) adsorption at 0 °C)

Vpore (mL g‒1)

MIL‒101(Cr)

2729.0

129.3

1.358

MIL‒101(Cr)‒TSIL

577.6

57.2

0.325

Figure 4. (a) N2 isotherm and (b) CO2 isotherm for MIL‒101(Cr) and MIL‒101(Cr)‒TSIL (● = sorption, ○ = desorption). Figure 4a shows the N2 adsorption‒desorption isotherm at ‒196 °C for MIL‒101(Cr) and MIL‒101(Cr)‒TSIL. The N2 adsorption displayed two uptakes at P/P0=0‒0.15 and P/P0=0.15‒0.22 resulting from two kinds of nanoporous windows. Also, the


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adsorption‒desorption isotherm of CO2 at 0 °C confirmed that grafting of the ionic liquid decreases SBET (Figure 4b and Table 1). TGA analysis has been carried out for MIL‒101 (Cr) and MIL‒101 (Cr)‒TSIL and the result illustrated at the Figure S3. At the both plot of MIL‒101 (Cr) the first step (up to 130 °C) corresponds to the removal of the physically adsorbed water or organic solvents and the weight loss between 400 to 630 °C related to decomposition of framework of MIL‒101(Cr). At the plot of MIL‒101 (Cr)‒TSIL, the weight loss in the second step (130‒400 °C) was due to removal of the organic moieties from the surface which was about 0.11 mmol/g which demonstrates very good agreement with obtained result from elemental analysis. 1H

and

13C

solid‒state NMR were applied to confirm the covalent bond between TSIL

and the MOF framework. It is worth mentioning that strong shift effects (leading to unusual shift values) and line broadening of the NMR signals are observed due to the paramagnetic character of chromium(III) by coupling of the unpaired electron spin with the nuclear spin. This complicates signal assignment, even though there are strategies for the assignment.63 However, in our case, some conclusions can already be derived from just the MAS spectra. As shown in Figure 5, MIL‒101(Cr) and MIL‒101(Cr)‒TSIL show significant differences in their 1H and

13C

spectra. This indicates that TSIL interacts with

the surface of MIL‒101(Cr). Furthermore, the fact that similar linewidths are observed for all the signals indicates chemisorption of TSIL because interaction with the paramagnetic


16

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chromium in close vicinity will increase the linewidth for the ionic liquid signals as compared to the mobile ionic liquid in the pore system. In more detail, the 1H spectrum (the manifold of spinning sidebands are not shown) of the pure MIL‒101(Cr) shows a major signal around 2 ppm that is assigned to the aromatic protons. Similar values have been found in a recent publication using very fast MAS together with DFT calculations.63‒64 Minor signals at 13‒15 ppm are probably due to residual hydroxyl protons from incomplete bonding to chromium. For MIL‒101(Cr)‒TSIL, the 2 ppm signal is still present but accompanied by signals at 4.5 and 7.5 ppm, resulting from grafting of the ionic liquid to the MOF. The

13C

spectra of two samples also show significant differences. Multiple

spinning sidebands are identified, originating mainly from the hyperfine coupling between electron and nuclear spin that cannot be eliminated at 10 kHz MAS. However, for MIL‒101(Cr), major signals can be found around 130 ppm for the protonated aromatic carbons together with a signal at ‒434 ppm. This unusual shift is a result of the proximity of the quaternary aromatic carbons to the paramagnetic metal. Strong shifts also have been observed in other cases such as Cu3btc2 MOF.65‒66 From DFT‒calculations64 signals at 129, ‒412, and 1680 ppm are expected for the protonated aromatic, quaternary aromatic and carboxylic carbon atoms, respectively, and shows quite good agreement with our data. The signal at 1680 ppm was not detected because of large linebroadening from the unpaired electron spin at chromium (also experimentally not detected in Ref. 60 at higher MAS frequencies). Additional signals are apparent for MIL‒101(Cr)‒TSIL. These


17


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include signals at 15 ppm, between 40 and 70 ppm, and at about 180 ppm. These fall partly in the range expected for carbons of the ionic liquid. Due to the larger distance to the paramagnetic chromium the hyperfine shift seems to be smaller.

Figure 5. 1H (left) and

13C

(right) solid‒state NMR spectra of MIL‒101(Cr) (bottom) and

MIL‒101(Cr)‒TSIL (top). For the 1H spectrum only the isotropic signals are given. In the 13C

spectrum the isotropic signals (marked by arrows) were identified by measurements

at different MAS frequencies, the isotropic ones are not shifting at different rotation frequencies. Furthermore, shift changes for the terephthalate carbons from the MOF are visible as well being due to the chemical modification from the grafting process. These include the signal for the quaternary aromatic carbon now resonating at ‒296 ppm. These results together with the observation that the signals have practically the same spin‒lattice


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relaxation time (data not shown) let us conclude that indeed the ionic liquid is chemically linked to the MOF MIL‒101(Cr). The MIL‒101(Cr)‒NH2, a porous MOF decorated with pendant amine (‒NH2) groups, provides large surface area and accessible mesoporous cages. It should also be noted that the amine groups of MIL‒101(Cr)‒NH2 favors the adsorption of CO2, and thus enhance the catalytic activity for cycloaddition of CO2 to epoxide.67 To prove the successful synthesis of the catalyst MIL‒101(Cr)‒NH2, DR‒FTIR was conducted and the results are shown in Figure S4. Also, the powder X‒ray diffraction pattern of MIL‒101(Cr)‒NH2 was recorded (Figure S5) and consistent with the literature data67 which indicated that the MIL‒101(Cr)‒NH2 was successfully synthesized. Catalytic Activity The catalytic activity of MIL‒101(Cr)‒TSIL was evaluated in the cycloaddition of epoxides with CO2 under solvent free conditions. The results are summarized in Table 2. Cycloaddition of styrene oxide was selected as a model substrate to fix CO2 at reaction conditions of CO2 pressure of 2 MPa, temperature of 110° C, 20 mmol styrene oxide, and 2 mmol TBAB (tetra butyl ammonium bromide). No desired product was detected in the absence of MOF after 6 h and only 5% conversion with no selectivity was observed after 24 h. MIL‒101(Cr) and NH2‒MIL‒101(Cr) displayed a conversion of 60% and 74% with selectivities of 85% and 82%, respectively. This illustrates the important role of the framework for enhancement of the increasing CO2 concentration and reaction conversion. Better performance of


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NH2‒MIL‒101(Cr) rather than MIL‒101(Cr) is expected because the amine groups increase the CO2 concentration in the pores.68 A significant increase in the conversion and selectivity was achieved in the presence of MIL‒101(Cr)‒TSIL under the same conditions after 2 hours without adding TBAB. The influence of different parameters (CO2 pressure, temperature, amount of the catalyst and amount of styrene oxide) was investigated. Increasing the amount of catalyst from 50 to 100 mg increased the conversion from 76% to 100% with high selectivity toward cyclic carbonate (Table S1, entries 1‒3).

Table 2: Cycloaddition of CO2 to styrene oxidea O

[CO2]

O

O

R

Catalyst

‒

R

Amount (mg)

O

time

Conversionb (%)

Selectivityb (%)

6h

0

0

24 h

5

0

‒

MIL‒101(Cr)

110

6h

60

85

NH2‒ MIL‒101(Cr)

110

6h

74

82

MIL‒101(Cr)‒TSILc

110

6h

95

98


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aReaction bYields cSame

conditions: 20 mmol styrene oxide, 2 mmol TBAB, PCO2 = 2.0 MPa, T = 110° C

were determined by GC analysis. conditions without TBAB

A further increase in the amount of catalyst up to 150 mg did not change the conversion (Table S1, entries 4 and 5) but decreased selectivity due to limiting mass transfer in the pores of the metal‒organic framework in the presence of excess catalyst.25 The dependence of styrene oxide conversion and product selectivity on the amount of styrene oxide was evaluated. An increase in conversion and selectivity was observed from 5 to 15 mmol styrene oxide in the presence of 100 mg MIL‒101(Cr)‒TSIL (Table S1, entries 6‒8). There was no considerable change from 5 to 15 mmol of styrene oxide, indicating high capacity of the catalyst at these reaction conditions. Subsequently, the effect of reaction temperature on the synthesis of cyclic carbonate was investigated. The improvement of the catalytic activity was demonstrated in the range of 90 to 110 °C (Table S2, entries 1‒3). A further increase in temperature up to 120 °C showed no variation of conversion but represented a slight decrease in cyclic carbonate selectivity.At higher temperature (130 oC) both the conversion and selectivity significantly decreased (Table S2, entries 4 and 5). A decrease in cyclic carbonate selectivity at higher reaction temperature has also been reported previously.69‒71 Such decrease may be due to the occurrence of side reactions like polymerizations at high temperatures.72 It should


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be also noted that the concentration of CO2 in the liquid phase gradually decreases with rising temperature. The influence of reaction time on epoxide conversion and cyclic carbonate selectivity is summarized in Table S2, entries 6‒10. The conversion and selectivity are more or less constant for reaction times of 2 h and longer but the highest turnover frequency (TOF) was observed after 2 h. After selection of 2h for optimized time reaction, other catalytic reaction, under other temperature for 2h was carried out and the results presented same trend (Table S3) and confirmed that 110 °C is suitable for optimized temperature. The effect of initial CO2 pressure over MIL‒101(Cr)‒TSIL was also investigated in the range 0.8 to 2.8 MPa. As depicted in Figure 6, an increase in CO2 pressure led to a moderate increase in conversion and a significant improvement in cyclic carbonate selectivity. The solubility of CO2 in the liquid phase enhances with increasing pressure, resulting in raising conversion and selectivity up to 2 MPa. But both conversion and selectivity decreased slightly at higher pressures. Such unexpected results have also been reported in the literature and were related to a CO2‒epoxide complex produced in the reaction mixture.29,

73‒74

At very high pressures, CO2 dissolves in epoxide and the

CO2‒epoxide interactions overcome those of the catalyst‒epoxide ones.34,

50, 75‒76

Accordingly, a very high CO2 pressure is not suitable for cycloaddition by this heterogeneous catalyst and therefore, a pressure of 2.0 MPa was selected as the optimal pressure for CO2 in the presence of MIL‒101(Cr)‒TSIL. Consequently, a reaction time of 2


22

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h, CO2 pressure of 2.0 MPa, and temperature of 110 °C, together with 15 mmol epoxide and 100 mg catalyst were optimal for the cycloaddition of CO2 to styrene oxide in this study and also used for the conversion of other epoxides.

Figure 6. Conversion and selectivity of styrene oxide reaction with CO2 as a function of pressure. Reaction conditions: 15 mmol styrene oxide, 100 mg MIL‒101(Cr)TSIL, temperature 110 °C, reaction time 2 h. Cycloaddition of CO2 to other Epoxides The catalytic activity of MIL‒101(Cr)‒TSIL for the chemical fixation of CO2 to various epoxides was investigated using the above optimized conditions. The results shown in Table 3 indicate that the prepared catalyst has potential to convert a variety of terminal


23


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epoxides bearing electron‒withdrawing and electron‒donating substituents to their corresponding carbonates. Table 3: Cycloaddition of CO2 to various epoxides catalyzed by MIL‒101(Cr)‒TSILa

O R

entry

MIL-101(Cr)-TSIL

Epoxide O

1

O

2 3

4 5 6

O

O

O

Cl

O

O

7

8

O

O

O

O

9c

O

[CO2]

O R

O

Conversionb (%)

Selectivityb (%)

>99

83

98

79

>99

48

>99

81

>99

95

89

95

98

97

96

40

26

24

O


24

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aReaction

conditions: 100 mg catalyst, 15 mmol epoxide, PCO2 = 2.0 MPa, T = 110 °C, reaction time 2 h bYields c

were determined by GC analysis.

Additional solvent 12 mL ethyl acetate

It is obvious that the steric hindrance of the epoxide is an effective factor in the selectivity and conversion of the cycloaddition reaction due to drawback of nucleophilic attack of bromide and difficult mass transfer in the bulky substrate. On the other hand, an obstacle in the escape of epoxides from pores of the framework is an advantage for cyclohexene oxide (Table 3, entry 8) which requires long reaction times with other catalytic systems. The proximity of molecule and active site at the pore in the appropriate position enhanced conversion of cyclohexene oxide in comparison to similar catalytic systems.37, 77‒78

Catalyst Recovery From an economical point of view, the reusability of a catalyst is an important parameter which should be well investigated. Therefore, fixation of CO2 with styrene oxide with the recovered MIL‒101(Cr)‒TSIL was carried out under the optimized conditions. After each cycle, the catalyst was filtered, washed with ethyl acetate and activated in the vacuum oven at 100 °C for 24 h. As depicted in Figure 7, the catalyst was recovered for 5 runs with only a minor decrease in the conversion and selectivity, indicating high stability of MIL‒101(Cr)‒TSIL. Additionally, the nature of the catalyst after the last run was


25


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characterized by XRD (Figure 1f) and FE‒SEM (Figure 2c). No significant difference between the reused and the fresh catalyst, indicated that the basic lattice structure is well maintained. It should be noted that while the immobilized task‒specific ionic liquid displayed activity even after five runs, free task‒specific ionic liquid decomposed at 90 °C. Nucleophilic attack of bromide leads to an instability of the free ionic liquid while immobilization of ionic liquid leads to a more difficult attack of the bromide ion. Generally, the 'good side' for an attack of the anion is blocked by the immobility of the ionic liquid cation.

Figure 7. Recycling of MIL‒101(Cr)‒TSIL in the cycloaddition of CO2 with styrene oxide under optimized conditions. Proposed Mechanism of the Cycloaddition Reaction


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Based on previous reports,37, 79 a plausible mechanism is suggested for the cycloaddition of CO2 to epoxides using the new MIL‒101(Cr)‒TSIL catalyst (Scheme 2). The influence of hydrogen bond formation with oxygen from the epoxide to facilitate cycloaddition reaction has been proven.80‒81 Therefore, intermediate I produced through hydrogen bonding between epoxide and carboxylic acid group results in polarization of the C−O bond of the epoxide. This follows with the nucleophilic attack of Brˉ (as Lewis base) on the less sterically hindered carbon atom of the epoxide to generate intermediate II with epoxy ring opening. Because of the presence of the imidazolium moiety, CO2 is activated via interaction with the cation which locates it in a good position, resulting in convenient insertion of CO2 into intermediate II. Hydrogen bonding in the intermediate III can also affect stabilization of the synthesized linear alkyl carbonate anion. Finally, the corresponding cyclic carbonate is formed by the simultaneous intramolecular ring‒closure and release of Brˉ with regeneration of the catalyst. Accordingly, the synergic effect of hydrogen bonding and imidazolium cation are extremely desirable for a smooth progress of the cycloaddition reaction.


27


N

Page 28 of 41

N

Br

O

C

O

O O H

O

R

O O

O

R

N N

O

III

O

O

H

O

Br

N

N

O

Br

O

O

C

O H

O

O

R

I

R N O

C O

Br

N

O H

O

O

R

II Scheme 2. Proposed mechanism of cycloaddition of CO2 with epoxides catalyzed by MIL‒101(Cr)‒TSIL.

Comparison of MIL101(Cr)‒TSIL with other Catalysts Despite the fact that the metal plays a key role at most of the heterogeneous catalytic systems for CO2 fixation, researchers have focused on catalytic systems without metal.


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This is in due to environmental pollution by the metal and to reduce the price of the catalyst. Few heterogeneous catalysts are known to exhibit good non-metal performance for cycloaddition of epoxides with CO2. Some of them are shown in Table 4 and compared to MIL‒101(Cr)‒TSIL. In these catalysts, mesoporous catalysts and ionic liquids with appropriate functional groups have been considered as the important factor. Short reaction time, excellent conversion and suitable temperature and efficiency in the presented MIL‒101(Cr)‒TSIL are clearly advantageous in comparison to the other catalytic systems. Table 4: Comparison of various catalysts proposed for cycloaddition of CO2 to epoxides.

Catalyst

Styrene oxide

Solvent

Time

T (ºC)

Conversio n (%)

CO2 pressure

polyILs@MIL‒101 (100 mg)82

1 mmol

acetonitril e

2d

70

81

0.1 MPa

10 mmol

‒

24 h

120

46

0.1 MPa

143 mmol

‒

5h

140

67.8

2 MPa

15 mmol

‒

2h

110

98

2 MPa

(Iˉ)MeimUiO‒66 mg)83

(50

BCP‒IMCl (Organic Polymer grafted IL) (300 mg)84 MIL‒101(Cr)‒TSIL (100 mg) (this work)


29


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Conclusion A metal‒organic framework covalently supported by a task‒specific ionic liquid (MIL‒101(Cr)‒TSIL) was synthesized and utilized as a catalyst for solvent‒free cycloaddition of CO2 to epoxides in the absence of solvent or any co‒catalyst. Excellent performance of the designed catalyst under green conditions emerged from the special arrangement of three effective factors: 1) carboxylic acid group, 2) imidazolium cation, and 3) mesoporous framework, led to a remarkable improvement in catalytic activity compared to MIL‒101(Cr). The carboxylic acid group in the ionic liquid played an important role in the acceleration of the reaction under solvent free condition. This is due to the strong hydrogen bond which resulted in stabilization of the intermediate and activation of the C–O bond in the epoxy ring of the epoxide. Additionally, the mesopore cages of the MIL‒101(Cr) in cooperation with the grafted imidazolium cation exhibited a high carbon dioxide concentration in the vicinity of the active sites. High activity and selectivity, easy work up and convenient reusability are other advantages of the designed catalyst. Associated Content Supporting information


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Synthetics procedure, characterization of MOF and functionalized MOF (DR-FTIR, TEM images, TGA and XRD), experimental details of optimization of reaction conditions. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents



Synthesis of a new task‒specific ionic liquid containing an imidazole and a carboxylic acid functionality



Covalent immobilization of the task‒specific ionic liquid onto MIL‒101(Cr)



Excellent performance of MIL‒101(Cr)‒TSIL in solvent‒free cycloaddition of CO2



Simple recovery and good reusability of heterogeneous catalyst


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Task-Specific Ionic Liquid Functionalized-MIL- 101(Cr) as a

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