Ionic liquid transported into metal–organic frameworks

Accepted Manuscript Title: Ionic liquid transported into metal–organic frameworks Author: Kazuyuki Fujie Hiroshi Kitagawa PII: DOI: Reference:

S0010-8545(15)00284-2 http://dx.doi.org/doi:10.1016/j.ccr.2015.09.003 CCR 112136

To appear in:

Coordination Chemistry Reviews

Received date: Revised date: Accepted date:

19-8-2015 6-9-2015 6-9-2015

Please cite this article as: K. Fujie, H. Kitagawa, Ionic liquid transported into metalndashorganic frameworks, Coordination Chemistry Reviews (2015), http://dx.doi.org/10.1016/j.ccr.2015.09.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ionic liquid transported into metal–organic frameworks

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Kazuyuki Fujie*,1,2 and Hiroshi Kitagawa*,2,3,4,5 R&D Center Kagoshima, Kyocera Corporation, 1-4 Kokubuyamashita-cho,

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Kirishima-shi, Kagoshima 899-4312, Japan

Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa

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Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan

Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida,

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Sakyo-ku, Kyoto 606-8501, Japan

INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku,

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Keywords

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Fukuoka 819-3095, Japan

metal–organic framework; porous coordination polymer; ionic liquid; phase behavior;

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ionic conductivity

Highlights

 First review on ionic liquids supported by metal–organic frameworks.  Tunable properties of ionic liquids by nanosized effect and interaction with metal–organic frameworks.

 Possible applications such as gas absorbents, catalysts, precursors of carbons, and ionic conductors.

Contents 1.

Introduction ......................................................................................................... 2

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Strategies for impregnation of ILs into MOFs ...................................................... 4

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Absorption of carbon dioxide and sulfide gases.................................................... 6

4.

Catalysis for organic synthesis ............................................................................. 7

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Templates for synthesis of nanoporous carbon ..................................................... 9

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Phase behavior and ionic conductivity ............................................................... 10

7.

Conclusion......................................................................................................... 11

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References ......................................................................................................... 13

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ABSTRACT Ionic liquids (ILs) show promise as new green solvents for chemical reactions, extractions, catalysis, gas absorption, and electrolytes in electrochemical devices. For

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the widespread use of ILs, the concept of ILs impregnated with porous supports has

recently been established. Metal–organic frameworks (MOFs) have great potential as

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new host materials for ILs; they are able to decrease the amounts of ILs and tune the

properties of ILs via host–guest interactions. In this review, we will present an overview

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of the studies carried out to date on MOF-supported IL systems, and their possible applications, such as in gas absorption, catalysis, templates for synthesis of nanoporous

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carbons, and ionic conductors.

1. Introduction

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Solvents called molten salts or ionic liquids (ILs) consist of liquid phase ions and are free from any electrically neutral molecular solvents. The melting points of ILs are significantly lower than those of classical salts as a result of the many efforts that have

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been made to design the structures of component ions [1,2]. ILs whose melting points

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are as low as ambient temperature are specifically referred to as room-temperature ionic liquids (RTILs). Many RTILs have been reported to date. Since the 1980s, research into

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ILs has attracted much interest, largely due to their suitable properties for various applications, such as their negligible volatility, nonflammability, high thermal and chemical stability, and high ionic conductivity. ILs therefore show promise as new green solvents for chemical reactions [3–5], extractions [6,7], catalysis [8–12], and gas absorption [13,14]. Moreover, ILs are promising candidate materials for electrolytes in electrochemical devices, such as lithium ion batteries [15–17], electric double layer capacitors (EDLCs) [18,19], fuel cells [20], and dye-sensitized solar cells [21]. For the widespread use of ILs, the structures of component ions have been modified to tune the physical or chemical properties of ILs to suit the various intended purposes. Because of their high designability, ILs are called “designer solvents.” Despite the advantages of ILs, their actual applications are limited due to their drawbacks, such as high cost, high viscosity, water absorbability, and strong interaction with other ions. Over the past decade, the concept of ILs impregnated with porous supports, referred to as “supported

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ionic liquid phase (SILP),” has been established [22–27]. Various porous materials have been used as supports in SILPs, such as silica gels, mesoporous silica, activated carbons, or porous glasses. SILPs have potential to suppress the costs of ILs because SILPs can

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decrease the amount of ILs per surface area. Moreover, the properties of ILs can be tuned or enhanced because of the nanosizing of ILs or interactions between ILs and host materials.

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Metal–organic frameworks (MOFs), which are also called porous coordination polymers (PCPs), comprise a new group of porous materials. MOFs are crystalline

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materials composed of metal centers and organic ligands, infinitely connected by coordinative bonds. MOFs have numerous uniformly sized micropores or mesopores

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originating from the crystal structures of MOFs. A large variety of MOFs have been reported, and various properties of MOFs have been studied, such as gas adsorption [28–30], catalysis [31,32], and ionic conductivity [33–35]. The extraordinary advantage

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of MOFs as host materials is the ability to create tunable host–guest interactions. We can tune the properties of MOFs, such as pore size, surface area, framework topology,

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and polarity of the inner surface, by appropriate selection of the metal centers and organic ligands. Because this high designability of MOFs enables tunable host–guest

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interactions, MOFs have great potential as new host materials for SILPs, which, in turn, could tune or enhance the properties of ILs.

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ILs incorporated within MOFs (symbolized as IL@MOFs) were first obtained by

ionothermal synthesis [5,36–38]. Ionothermal synthesis is a form of solvothermal synthesis, where ILs are used as solvents. Ionothermally synthesized MOFs usually have negatively charged frameworks, and the cations of ILs are left in the MOFs as counterions to maintain electric neutrality. The cations are embedded in the MOFs with ordered structure because of strong host–guest interactions. However, the cations in ionothermally synthesized MOFs are considered not to exhibit the same useful properties as bulk ILs, because the cations of ILs are strongly bound to the MOFs. Moreover, there are a limited number of options for ILs and MOFs in ionothermal synthesis. This may be a disadvantage for the widespread application of IL@MOFs. Recently, several groups have reported IL@MOFs obtained by a postimpregnation strategyILs are incorporated in MOFs after the synthesis of the MOFs. The postimpregnation strategy maximally utilizes the advantage of the IL@MOF system, 4 Page 4 of 28

because a large number of ILs and MOFs can be selected as the components of the IL@MOFs. Here, we present an overview of the studies of IL@MOF systems obtained by the postimpregnation method and their possible applications, such as in gas

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absorption, catalysis, templates for synthesis of nanoporous carbons, and ionic conductors.

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2. Strategies for impregnation of ILs into MOFs

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Three strategies have been reported to prepare the IL@MOFs by the

postimpregnation. In the first strategy, the solution of ILs were used [39–42]. Amino-functionalized basic ionic liquid (ABIL-OH) was introduced into a MOF,

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HKUST-1 (Cu3(BTC)2, H3(BTC) = 1,3,5-benzenetricarboxylic acid) by this strategy shown in Fig. 1 [39]. The powder of HKUST-1 was dispersed in the ethanol solution of

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ABIL-OH, and the mixture was stirred at ambient temperature. HKUST-1 has coordinatively unsaturated sites (CUSs), which can lead to pinning of the ABIL-OH ions by their Lewis acidity. The solvent was separated by filtration and the excess

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ABIL-OH was removed by washing with solvents. After the solvents were removed by

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drying under vacuum, ABIL-OH@HKUST-1 was obtained. This strategy is the most widely used to introduce the ILs into the MOFs with CUSs.

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[Insert Fig. 1]

The second strategy is the tandem postsynthetic modification. A Brönsted acidic IL

(BAIL) was confined in the mesopores of Cr-MIL-101 (Cr3O(F, OH)(H2O)2(BDC)2) by this strategy (Fig. 2) [43]. Cr-MIL-101 was first modified by N-Cr coordinate covalent bonds between the CUSs and the N-heterocyclic compounds (triethylene diamine or imidazole) that contain two nitrogen atoms. Subsequently, 1,4-butane sultone was added and reacted with the N-heterocyclic compounds to construct ILs. Finally, the anions of ILs were exchanged with HSO4– by the addition of H2SO4. The tandem postsynthetic modification strategy makes it possible to incorporate the ILs with larger ions than pore aperture diameter into the pores of MOFs. This ship-in-bottle process can effectively confine the ILs inside the pores of MOFs. [Insert Fig. 2] In the third strategy, the ILs are introduced into the pores of MOFs through 5 Page 5 of 28

capillary action [44–46]. An IL, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMI-TFSA; Fig. 3a) was introduced into the micropores of a MOF, ZIF-8 (Zn(MeIM)2, H(MeIM) = 2-methylimidazole; Fig. 3b) by

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this strategy [45,46]. The ZIF-8 powder was dried to remove guest molecules from the micropores, and was mixed with EMI-TFSA. The mixture was heated and stored to enhance the diffusion of EMI-TFSA through capillary action. The advantage of the

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capillary action is that this strategy can widely be applied to various types of ILs and

[Insert Fig. 3]

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MOFs such as ZIF-8, which has no CUSs.

After the impregnation of ILs into MOFs by the strategies described above,

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nitrogen gas adsorption or infrared absorption (IR) measurements have been conducted to confirm the existence of ILs inside the pores of MOFs [39–43,45,46]. For example,

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the impregnation of 1-butyl-3-methylimidazolium chloride (BMI-Cl; Fig. 4) into Cr-MIL-101 was confirmed by these measurements [40]. As shown in Fig. 5a, there is no change in the crystal structure of Cr-MIL-101 after the loading of the BMI-Cl. The

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nitrogen gas adsorption isotherms and the pore size distributions (Fig. 5b and 5c,

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respectively) showed the decrease in the pore volume of Cr-MIL-101 with the amount of BMI-Cl. This result indicates that the existence of BMI-Cl molecules in the pores of

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Cr-MIL-101. Moreover, the observed IR spectra of BMI-Cl@Cr-MIL-101 showed IR bands assigned to C=N, C=C, and C–N stretching, which indicated the existence of BMI-Cl.

[Insert Fig. 4] [Insert Fig. 5]

To obtain detailed structural information on IL@MOFs, it is a useful technique to

analyze observed X-ray diffraction (XRD) patterns by the maximum entropy method (MEM) and Rietveld refinement [46]. It is well known that MEM analysis make it possible to visualize the electron density of guest molecules included in porous materials from the observed XRD patterns [48–50]. Fig. 6 shows the results of the MEM/Rietveld analysis, where ZIF-8 and EMI-TFSA were used as the MOF and the IL, respectively. As shown in Fig. 6b, there was no obvious charge density within the micropores of ZIF-8, which suggested that the ZIF-8 contained no guest molecules. In 6 Page 6 of 28

contrast, apparent electron density peaks were found within the micropores of EMI-TFSA@ZIF-8 samples (Fig. 6d). The results of the MEM/Rietveld analysis strongly suggest that the EMI-TFSA was successfully included inside the micropores of

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the ZIF-8 framework. The result of the MEM analysis in Fig. 6d also shows that the charge density originating from the EMI-TFSA units was low at the center of the

micropores, suggesting that EMI-TFSA units interacted attractively with the host ZIF-8

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framework.

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3. Absorption of carbon dioxide and sulfide gases

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[Insert Fig. 6]

ILs show promise as absorbents for gases such as carbon dioxide or sulfides [13,14]. MOFs show promising applications for gas adsorption because of their large surface

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areas and tunable interactions with gas molecules, which could provide selective gas adsorption properties [28–30]. Recently, gas absorption properties of MOF-supported ILs have been investigated mainly by means of computational simulations [51–55].

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1-Butyl-3-methylimidazolium (BMI+)-based ILs supported by IRMOF-1

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(Zn4O(BDC)3, H2(BDC) = 1,4-benzenedicarboxylic acid) were investigated for CO2 capture by molecular computation [51,52]. Because the anions of ILs, such as

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hexafluorophosphate (PF6–), tetrafluoroborate (BF4–), and thiocyanate (SCN–), prefer to locate near the metal cluster in IRMOF-1, interactions between anions and BMI+ cations are weakened (Fig. 7). CO2 is more strongly adsorbed than N2 from the CO2/N2 mixture in the IL@IRMOF-1 system, where the anions are the most favorable site for CO2 adsorption. This result indicates that selectivity of gas absorption by ILs could be enhanced or tuned by the interaction between ILs and MOFs. CO2/N2 permeation selectivity and CO2 permeability in BMI-SCN@IRMOF-1 membrane were also studied. While BMI-SCN@IRMOF-1 exhibits comparable CO2/N2 permeation selectivity with polymer membranes and polymer-supported ILs, BMI-SCN@IRMOF-1 has substantially higher CO2 permeability (Fig. 8). This result suggests that IL@MOF membrane could outperform the polymer membranes and the SILP membranes in CO2/N2 separation. [Insert Fig. 7]

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[Insert Fig. 8] Following the investigations carried out using computational simulations, an experimental study of gas absorption of IL@MOF was recently reported [40]. An IL,

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BMI-Cl, was introduced to a widely studied MOF, Cr-MIL-101. Cr-MIL-101-supported IL showed 71% higher absorption capacity of the organosulfur compound,

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benzothiophene (BT), compared with the virgin Cr-MIL-101. The importance of

Cr-MIL-101 for the absorption of BT was indicated by the fact that the bulk BMI-Cl

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itself without any support material showed negligible absorption. The improved

absorptive performance could be explained by the acid–base interactions between the acidic IL and basic BT. This study also showed the importance of the porosity of

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IL@MOFs. As shown in Fig. 9, the quantities of the absorbed BT were increased by increasing the contents of BMI-Cl up to 33%. Even though the 50%

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BMI-Cl@Cr-MIL-101 had the highest BMI-Cl content, it showed lower absorptive performance than the virgin Cr-MIL-101 and the other BMI-Cl@Cr-MIL-101 absorbents. The low absorptive performance of 50% BMI-Cl@Cr-MIL-101 can be

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explained by a too high degree of pore filling with excess amounts of BMI-Cl. This

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result indicates that the free space is necessary for the gas molecules to diffuse in the pores of IL@MOFs, although the larger amount of ILs lead to higher uptake of

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absorbed gas molecules.

[Insert Fig. 9]

4. Catalysis for organic synthesis Heterogeneous catalysis has been thoroughly investigated as an area of application

of MOFs, as well as gas adsorption [31,32,56]. MOFs have large surface areas and uniformly sized pores, similar to zeolites (which are among the most important catalysts used in many industrial processes). Therefore, MOFs are expected to perform well as a new type of catalyst, which could provide high catalytic activity and selectivity. ILs also exhibit various catalytic activities, such as for esterification, alkylation, Michael addition, and the Diels–Alder reaction [8–12]. The catalytic activities of SILPs were recently studied in an effort to overcome some of the drawbacks associated with ILs, such as high cost and difficulties associated with extraction. Several studies on the 8 Page 8 of 28

catalysis of IL@MOF have recently been reported, with findings of improved catalytic activity, selectivity, and reusability [39,41–43]. In the earliest work on IL@MOF catalysis, Brönsted acidic ILs (BAILs) were

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confined in the mesopores of Cr-MIL-101 by tandem postsynthetic modification, where the BAILs were bound to the CUSs of Cr-MIL-101 (Fig. 2) [43]. Two types of BAILs were used in this study; imidazole-based BAIL (IMIZ-BAIL) and triethylene

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diamine-based BAIL (TEDA-BAIL). The loading amounts of IMIZ-BAIL and

TEDA-BAIL were 0.15 mmol g–1 and 1.30 mmol g–1, respectively. The low amount of

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IMIZ-BAIL can be explained by the weak coordination ability of the nitrogen atoms to CUSs of Cr-MIL-101 due to the delocalization of the lone pair (Fig. 10). In contrast, the

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TEDA-BAIL system possesses the nitrogen atoms with coordination ability, and is more appropriate for constructing the BAIL@MOF catalyst. In the microenvironment of the mesopores, the TEDA-BAIL@Cr-MIL-101 demonstrated notably higher catalytic

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performance than the IMIZ-BAIL@Cr-MIL-101 for the acetalization of glycol with benzaldehyde because of the higher loading amount of BAIL. This result can be

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explained by the difference in the loading amount of ILs. Furthermore, TEDA-BAIL@Cr-MIL-101 catalyst exhibited comparable catalytic performance with

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that of the benchmark catalyst, organic homogeneous p-toluenesulfonic acid. In the reusability test, the TEDA-BAIL@Cr-MIL-101 catalyst could be recycled six times

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without marked loss of catalytic activity. This result indicated that the active species of the catalyst was stable and hardly leaked from the mesopores during the reaction and recovery processes. This is due to the coordinative bonds between the TEDA-BAIL and CUSs of Cr-MIL-101.

[Insert Fig. 10]

A composite of IL, polyoxometalate (POM), and MOF was reported as another

approach to encapsulating ILs in the pores of MOFs [41]. The system studied consisted of [SO3H-(CH2)3-HIM][HSO4] (HIM = imidazole), phosphotungstic acid (H3(PW12O40)) and Fe-MIL-100 (Fe3O(F, OH)(H2O)2(BTC)2; Fig. 11) as the IL, POM and MOF, respectively. The POM included Fe-MIL-100 and was synthesized by using the direct hydrothermal method. The sulfonic acid group-functionalized IL was modified on the POM through the anion-exchange method to encapsulate the IL in the

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mesopores of the Fe-MIL-100 (Fig. 12). The obtained [SO3H-(CH2)3-HIM]3PW12O40@Fe-MIL-100 catalyst has both Lewis and Brönsted acid sites, which led to high catalytic activity comparable with other SILP catalysts for the

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esterification of oleic acid with ethanol. Moreover, the [SO3H-(CH2)3-HIM]3PW12O40@Fe-MIL-100 catalyst could be easily recovered and reused six times without marked loss of activity. This result indicated that the

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IL-modified POM was effectively confined in the cages because the POM clusters cages. [Insert Fig. 11]

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[Insert Fig. 12]

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(12–14 Å) are larger than the pore aperture diameter (5.5 and 8.6 Å) of the Fe-MIL-100

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5. Templates for synthesis of nanoporous carbon

ILs are ideal precursors for carbon synthesis by direct carbonization because of

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their liquid state at ambient temperature and negligible vapor pressure. Nitrogen-doped mesoporous carbons are synthesized by carbonization of ILs, including the nitrogen

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atoms, where mesoporous silica or silica nanoparticles are used as templates [58,59]. The nitrogen-doped mesoporous carbons are suitable for applications such as catalysts

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and electrode materials for EDLCs. MOFs are ideal templates for the synthesis of nanoporous materials, which have large surface areas and uniform nanostructures [60–62].

Uniformly nitrogen (N)- and boron–nitrogen (BN)-decorated microporous carbons

with large surface areas were obtained by impregnation of ILs within a MOF, Al-MIL-100 (Al3O(F, OH)(H2O)2(BTC)2); structurally same as Fe-MIL-100, but the metal center is Al), followed by carbonization (Fig. 3) [44]. 1-Ethyl-3-methylimidazolium dicyanamide and 1-ethyl-3-methylimidazolium tetracyanoborate were used as precursors for the synthesis of N- and BN-decorated carbons, respectively. The synthesized carbons have the microporous nature with pore sizes mainly centered at 1–3 nm. The N- and BN-decorated carbons demonstrated high CO2 and H2 uptake capacities and high selectivity for CO2 adsorption over N2. This high selectivity can be explained by the interaction between high charge density at the N 10 Page 10 of 28

sites and polarized CO2 molecules. [Insert Fig. 13]

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6. Phase behavior and ionic conductivity

Ionic conducting materials have increasingly gained importance in recent years for

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applications in electrical energy storage and generation. They are used as electrolytes in electrochemical devices, such as secondary batteries, EDLCs, and fuel cells. These

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electrochemical devices should operate at low temperatures of 253 K or below for automotive applications, such as electric or hybrid electric vehicles, since vehicles may

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be exposed to these temperatures [63]. Therefore, these devices, particularly lithium ion batteries and EDLCs, contain volatile and flammable organic solvents as electrolytes to avoid freezing of the electrolyte and a decrease in ionic conductivity at low

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temperatures [64–66].

In order to avoid the use of these flammable solvents, ILs are promising candidate materials for safe electrolytes in electrochemical devices [15–20]. However, the ionic

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conductivity of ILs is very low at low temperatures [67,68], where the mobility of the

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ions decreases markedly due to the formation of intermolecular interactions [69–72], which are particularly strong below the freezing point of the IL. Being able to tune the

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intermolecular interactions of the ions is a significant issue in controlling the ionic conductivity and phase behavior of ILs. Because the high designability of MOFs enables tunable host–guest interactions [38,73,74], MOFs have great potential as desirable host materials to control the conductivity and phase behavior of ILs. We introduced EMI-TFSA into the micropores of ZIF-8, and studied the resulting

phase behavior and ionic conductivity [45,46]. EMI-TFSA was introduced into the micropores of ZIF-8 through capillary action by mixing EMI-TFSA and ZIF-8. To detect the possible phase transitions of nanosized EMI-TFSA in the micropores, solid-state 19F static nuclear magnetic resonance (NMR) measurements were made. The observed temperature dependences of the 19F spectra and line widths in the bulk and nanosized EMI-TFSA are shown in Fig. 14, indicating the motional state of the TFSA– anions. In the bulk EMI-TFSA spectra (Fig. 14b), only a broad line was observed in the process of heating from 123 K to 213 K. A sharp line appeared superimposed on the

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broad line at 243 K. Only a sharp line was obtained at 273 K, and the broad line disappeared. This sharpening of the line is explained by “motional narrowing,” that is, the line sharpening arising from free rotation and diffusion of the TFSA– anions. This

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implies that the bulk EMI-TFSA partly melted at approximately 243 K and completely turned into the liquid phase after further heating. The phase-transition behavior of the

nanosized EMI-TFSA (Fig. 14a) differed significantly from that of the bulk EMI-TFSA.

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Gradual and continuous narrowing occurred in the temperature range 123–303 K. This

result indicated that the nanosized EMI-TFSA in the micropores of ZIF-8 was prevented

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from freezing in this temperature range. Using the micropore volume of ZIF-8 (817 Å3) [47] and the van der Waals volumes of the EMI+ cations (116 Å3) and TFSA– anions

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(147 Å3) [75], we confirmed that the storage capacity of each ZIF-8 micropore was only three or fewer ion pairs. This small number of ions is not sufficient to construct an ordered crystal structure, and no phase transition between the solid and liquid phases is

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observed.

[Insert Fig. 14]

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Fig. 15 shows an Arrhenius plot of the ionic conductivity. Bulk EMI-TFSA

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exhibited a sharp decrease in conductivity below 264 K. EMI-TFSA@ZIF-8 at a volumetric occupancy of 125%, which has the excess EMI-TFSA outside of the

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micropores of ZIF-8, also showed a decrease in conductivity, with the inflection point at around 257 K. These temperatures correspond to the melting point of bulk EMI-TFSA (257 K). In contrast, the ionic conductivity of EMI-TFSA@ZIF-8 at volumetric occupancies of 50, 75, and 100% showed no sharp decrease corresponding to the phase transition on freezing between 228 and 341 K. The EMI-TFSA inside the ZIF-8 micropores is thus thought to remain liquid, even in the low-temperature region. Therefore, the ionic conductivity is maintained in the temperature range where bulk EMI-TFSA is frozen. EMI-TFSA@ZIF-8 at a volumetric occupancy of 100% showed a higher ionic conductivity compared with bulk EMI-TFSA below 250 K. This result indicated that IL@MOF could be used as an electrolyte for electrochemical devices that operate in the low-temperature region. [Insert Fig. 15]

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7. Conclusion In this review, we have discussed studies carried out to date on the ILs supported by MOFs and their possible applications, such as in gas absorption, catalysis, templates for

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the synthesis of nanoporous carbons, and ionic conductors. IL@MOFs have mainly been studied as gas absorbents and catalysts. Several reports have shown the great

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potential of IL@MOFs as absorbents for CO2 and sulfides, mainly determined by computational simulations.

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Catalysis has been the area of IL@MOFs most actively studied experimentally by researchers. Some IL@MOFs demonstrate higher catalytic activity and selectivity than the bulk ILs, which indicates the enhancement of catalytic activity that is derived from

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the combined effects of ILs and MOFs. It is expected that future research will include investigations into the mechanism of the enhanced catalytic activity in efforts to

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develop commercially usable IL@MOF catalysts.

Other applications of ILs have recently been proposed, such as templates for the synthesis of nanoporous carbons, and ionic conductors. N- or BN-decorated nanoporous

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carbons, which are synthesized from an IL@MOF precursor, demonstrate high CO2 and

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H2 adsorption properties. This synthesis method has great potential, and could be applied to the preparation of new nanoporous carbon materials as catalysts or electrodes

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for EDLCs.

In the study of the IL@MOF as an ionic conductor, the IL in the micropores of

MOF showed no marked decrease in conductivity even at low temperature, and showed higher conductivity than the bulk IL because the nanosized IL in the micropores was prevented from freezing transition. This result provides a route towards developing new electrolytes for electrochemical devices such as secondary batteries and EDLCs that could operate in the low-temperature region. The IL@MOF systems have great potential for various applications because of the nanosizing of ILs and the tunable interaction between ILs and MOFs. We are optimistic that further studies will lead to widespread applications of IL@MOFs.

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Fig. 1. The schematic illustration for synthesizing ABIL-OH@HKUST-1. Adapted from

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te

d

M

Ref. [39].

Fig. 2. The generic tandem procedure for synthesizing BAIL confined in Cr-MIL-101 nanocages by tandem postsynthetic modification. Adapted from Ref. [43].

19 Page 19 of 28

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Fig. 3. Structure of EMI-TFSA (a) and crystal structure of ZIF-8 [47] (b), where blue

an

tetrahedra, gray balls, and yellow spheres indicate ZnN4, C, and micropores,

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d

M

respectively. Hydrogen atoms are omitted for clarity.

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Fig. 4. Structure of BMI-Cl.

20 Page 20 of 28

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Fig. 5. XRD patterns (a), nitrogen adsorption isotherms (b), and pore size distributions (c) of the virgin and IL-supported Cr-MIL-101. Adapted from Ref. [40].

21 Page 21 of 28

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Fig. 6. Rietveld fitting result (a) and MEM equi-charge density surface (b, isosurface

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level: 0.15 eÅ–3) of the virgin ZIF-8. Rietveld fitting result (c) and MEM equi-charge density surface (d, isosurface level: 0.15 eÅ–3) of EMI-TFSA@ZIF-8, where the volumetric occupancies of EMI-TFSA to the pore volume of ZIF-8 are 25%. Adapted from Ref. [46].

22 Page 22 of 28

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Fig. 7. Simulation snapshot of CO2/N2 mixture in BMI-PF6@IRMOF-1 at WIL/IRMOF-1 =

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te

d

M

0.4. Adapted from Ref. [51].

Fig. 8. CO2/N2 permeation selectivity versus CO2 permeability. The filled circles are in the BMI-SCN@IRMOF-1 membrane. The red circles are experimental data in polymer membranes. The data in polymer-supported ILs are also illustrated. Adapted from Ref. [52].

23 Page 23 of 28

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Fig. 9. Contact time dependence (a) and IL content dependence (b) of the absorbed amount or kinetic constant for the absorption of BT in BMI-Cl@Cr-MIL-101. Adapted

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from Ref. [40].

Fig. 10. Electronic structures of two different types of electron-rich N-heterocyclic compounds. Adapted from Ref. [43]. 24 Page 24 of 28

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Fig. 11. Crystal structure of Fe-MIL-100. (A) A trimer of iron octahedra and trimesic

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acid. (B) Schematic view of one unit cell of Fe-MIL-100. (C) The two types of cages in polyhedral mode. (D) Pentagonal and hexagonal windows in balls and sticks (Fe: grey;

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O: red; C: black). Adapted from Ref. [57].

Fig. 12. The schematic illustration for synthesizing [SO3H-(CH2)3-HIM]3PW12O40@Fe-MIL-100. Adapted from Ref. [41].

25 Page 25 of 28

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Fig. 13. Schematic illustration of the synthetic procedure for N- and BN-decorated

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nanoporous carbons with molecular structures of the ILs used. Adapted from Ref. [44].

26 Page 26 of 28

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Fig. 14. Solid-state 19F static NMR spectra of EMI-TFSA@ZIF-8 at a volumetric occupancy of 25% (a) and bulk EMI-TFSA (b) and their full widths at half maximum

M

(FWHM; c). Open red circles indicate EMI-TFSA@ZIF-8, and open and closed blue squares indicate sharp and broad lines, respectively, of the bulk EMI-TFSA in the panel

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te

d

(c). Adapted from Ref. [45].

27 Page 27 of 28

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Fig. 15. Arrhenius plots of the ionic conductivity on heating in bulk EMI-TFSA (black)

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and EMI-TFSA@ZIF-8, where the volumetric occupancies of EMI-TFSA to the pore volume of ZIF-8 are 50 (blue), 75 (green), 100 (red), and 125% (purple). The solid lines

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are provided as guides for the eye. Adapted from Ref. [46].

28 Page 28 of 28