Nanoconfined Ionic Liquids - ACS Publications - American Chemical

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Nanoconfined Ionic Liquids Shiguo Zhang,*,†,‡ Jiaheng Zhang,§ Yan Zhang,† and Youquan Deng*,‡ †

College of Materials Science and Engineering, Hunan University, Changsha 410082, China Center for Green Chemistry and Catalysis, State Key Laboratory for Oxo Synthesis & Selective Oxidation, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, No.18, Tianshui Middle Road, 730000 Lanzhou, China § School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China ‡

ABSTRACT: Ionic liquids (ILs) have been widely investigated as novel solvents, electrolytes, and soft functional materials. Nevertheless, the widespread applications of ILs in most cases have been hampered by their liquid state. The confinement of ILs into nanoporous hosts is a simple but versatile strategy to overcome this problem. Nanoconfined ILs constitute a new class of composites with the intrinsic chemistries of ILs and the original functions of solid matrices. The interplay between these two components, particularly the confinement effect and the interactions between ILs and pore walls, further endows ILs with significantly distinct physicochemical properties in the restricted space compared to the corresponding bulk systems. The aim of this article is to provide a comprehensive review of nanoconfined ILs. After a brief introduction of bulk ILs, the synthetic strategies and investigation methods for nanoconfined ILs are documented. The local structure and physicochemical properties of ILs in diverse porous hosts are summarized in the next sections. The final section highlights the potential applications of nanoconfined ILs in diverse fields, including catalysis, gas capture and separation, ionogels, supercapacitors, carbonization, and lubrication. Further research directions and perspectives on this topic are also provided in the conclusion.

CONTENTS 1. Introduction 2. Ionic Liquids in the Bulk State 2.1. Complex Interactions in Ionic Liquids 2.2. Heterogeneous Structure 2.3. Solvation Ability and Polarity 2.4. Thermal Properties 2.5. Electrochemical Window 3. Covalent Grafting vs Physical Confinement 4. Synthetic Methodologies for Nanoconfined Ionic Liquids 4.1. In Situ Introduction of Ionic Liquids 4.2. Post Impregnation 4.3. “Ship-in-Bottle” Method 5. Methods To Investigate the Nanoconfined Ionic Liquids 5.1. Experimental Techniques 5.2. Computational Simulations 6. Investigation of the Nanoporous Matrices for Ionic Liquids 7. Structure and Properties of Ionic Liquids Confined in Nanoporous Matrices 7.1. Porous Silica 7.1.1. Structure 7.1.2. Dynamics 7.1.3. Thermal Properties 7.1.4. Other Properties

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7.2. Carbon Materials 7.2.1. Structure 7.2.2. Dynamics 7.2.3. Thermal Properties 7.3. Metal−Organic Frameworks (MOFs) 7.4. Porous Metals 7.5. Polymers 7.6. Other Matrices 8. Applications of Nanoconfined Ionic Liquids 8.1. Catalysis 8.1.1. Introduction of Confined Ionic Liquid Catalysis 8.1.2. Classification of Confined Ionic Liquid Catalysts 8.1.3. Reactions Catalyzed by Confined Ionic Liquid Catalytic Systems 8.2. CO2 Capture and Separation 8.2.1. CO2 Adsorption by Ionic Liquids 8.2.2. CO2 Adsorption by Nanoconfined Ionic Liquids 8.3. Ionogels 8.3.1. Ionogels as Solid Electrolytes 8.3.2. Ionogels as Optical Materials

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Special Issue: Ionic Liquids Received: July 31, 2016

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Chemical Reviews 8.4. Supercapacitors 8.5. Carbonization 8.6. Lubrication 9. Conclusion and Perspective Author Information Corresponding Authors ORCID Author Contributions Notes Biographies Acknowledgments Abbreviations References

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liquid.40−42 The incorporation of ILs into the pore structure results in nanoconfined ILs with improved mechanical integrity and ionic conductivity, constituting a new class of hybrid materials with the intrinsic properties of ILs and the original functions of a solid matrix.43,44 The use of nanoconfined ILs can overcome the major drawbacks of bulk ILs, such as high cost, high viscosity, and slow gas diffusivity. For example, the confinement of a well-stabilized thin layer of IL in a highly porous matrix may significantly reduce the quantity of IL necessary for a certain scale of application while making it possible to exploit all the mass of the incorporated IL, because the viscosity and diffusion effects are reduced in such ultrathin layers.45 Many microporous, mesoporous, or hierarchical porous matrices, particularly inorganic solids, such as nanoporous silicas (SiO2) and carbons, metal−organic frameworks (MOFs), covalent organic frameworks (COFs), carbon nanotubes (CNTs), and zeolites, have been widely used for the nanoconfinement of ILs (Figure 1). Nanoconfined ILs are

BA BC BE BF BH BH BH BH BH BH BH BH BJ

1. INTRODUCTION Ionic liquids (ILs) are salts of organic cations and inorganic or organic anions. The presence of bulky and asymmetric ions of ILs compared to simple ions of classical inorganic molten salts such as NaCl prevents them from easy crystallization and results in melting points below 100 °C. When the melting points are near or below room temperature, the ILs are known as room-temperature ILs (RTILs). ILs have noteworthy properties, such as low vapor pressure, wide liquidus range, high chemical and thermal stability, wide electrochemical window, nonflammability, high ionic conductivity, acceptable biocompatibility, and good capability of dissolving various organic/inorganic materials. The properties of ILs, including conductivity, density, viscosity, melting point, polarity, solvation power, hydrophilicity/hydrophobicity, and acid/base character, can be readily modified by judiciously coupling selected cations and anions, by changing the length of the alkyl chain of the cation, or by the covalent tethering of task-specific functionalities to one or both of the constituent ions.1−6 Therefore, ILs are frequently referred to as tunable, tailored, task-specific, or designer solvents or media. Earle and Seddon estimated the number of potential cation/anion combinations to be of the order of one billion,7 and >1000 RTILs have been synthesized to date, already outnumbering all the conventional inorganic salts.8 The unique properties of ILs make them particularly promising candidates as an environmentally benign or “green” alternative to organic solvents for chemical synthesis,9 catalysis,10−18 separation,19,20 and extraction,21 as well as safe and versatile electrolytes for electrochemistry (lithium batteries, supercapacitors, and fuel cells) and photovoltaics (dye-sensitized solar cells (DSSCs)),2,22−26 and as novel functional materials27−30 for lubrication,31 microfludics,32−37 propellants,38 and sensor.39 However, the advantageous liquid state in most cases hinders the development of ILs as advanced solvents, electrolytes, and materials, particularly for applications in devices requiring a solid shape. For example, the liquid nature of an IL causes difficulties, such as packaging, leakage, and portability. Other drawbacks include rather specific properties, such as high viscosity and low diffusion coefficients, difficulties in product purification and recycling, and high cost, resulting in tedious handling procedures. One way to circumvent these problems, particularly the fluidic problem, is to confine ILs in nanoporous matrices with one or more of their spatial dimensions subjected to a geometrical restriction. Considering that the filling process is usually carried out under vacuum conditions, the extremely low vapor pressure of an IL makes it more feasible to prepare a genuinely confined liquid than a conventional molecular

Figure 1. Concept of nanoconfined ILs in various porous solid matrices.

different from bulk ILs because either or both the porous matrix (rigid host) and IL (soft guest) can be suitably modified. The structures and properties of the hybrid systems can be tuned by varying the geometric structures, pore sizes, and surface functionalities of porous hosts, the structure and loading fraction of ILs, and external conditions, such as electric field46 and temperature, to satisfy the requirements of any specific application. Owing to the effects of spatial confinement as well as the interactions between ILs and pore walls, the structures and physicochemical properties exhibited by ILs confined in nanopores would be significantly different from the corresponding bulk systems. Many striking phenomena have been reported, concerning changes in the well-known behavior of ILs, such as the local structure (ionic orientation and layer structure), dynamics (diffusion coefficient, viscosity, dielectric relaxation, and ionic conductivity), thermal properties (phase transition and thermal stability), optical properties (Fourier transform infrared spectroscopy (FTIR), Raman, light scattering, and fluorescence), and even the chemical reactivity. For example, either a decrease47 or an increase48 in the melting point of ILs compared to the bulk state was experimentally observed under confinement. The solubility or adsorption ability of CO2 in IL could be enhanced through confinement.49−53 A recent study demonstrated that a nanoconfined IL showed ionic conductivity, 1 order of magnitude higher than its unconstrained analog.54 The simulation of an IL, 1,3B

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dimethylimidazolium chloride ([MMIm][Cl]), confined between two noncorrugated walls, also showed faster diffusion of ions under confinement compared to the bulk IL.55 Moreover, the ionic structure and dynamics of nanoconfined ILs are very complex and can be described by heterogeneous characteristics. The attractive characteristics of nanoconfined ILs make their potential applications feasible in diverse fields, including catalytic processes,56,57 lubricants,31 electrochemistry,58,59 and others.60,61 The study of ILs in a confined state thus presents an exciting challenge and is of paramount importance from both the scientific and application perspectives. This review summarizes the recent advances in the past 12 years of research on nanoconfined ILs, with a particular emphasis on their structure and properties related to the potential applications. This review is organized as follows: First, a brief introduction of ILs in their bulk state is provided, and the supported strategies for ILs including covalent grafting onto supports and physical confinement into porous matrices are compared. Second, the synthetic strategies as well as the experimental and theoretical methods for investigating nanoconfined ILs are presented. Third, the nanoconfinement of ILs in diverse porous systems is discussed in more detail, focusing on the local structure and peculiar physicochemical properties of nanoconfined ILs, such as the interplays between the ion pairs of ILs and the pore wall, dynamics, phase transitions, and thermal stabilities. Further, the potential applications of nanoconfined ILs in catalysis, gas capture and separation, ionogels, supercapacitors, carbonization, and lubrication are explored. Finally, this review is concluded by providing an overview of possible directions for future research in this field. Note that the ILs supported onto the pore walls of porous matrices by the “grafting” method rather than physical confinement are not covered in this review because of the presence of only a monolayer of free degree-limited ILs, which are actually not “true” ILs. Moreover, because at least one dimension is restricted in nanoconfined ILs, the chemistry of ILs located only on the surface is not discussed here.

Figure 2. Some common cations and anions used in ionic liquids and their abbreviations.

[CH3SO3], tetrafluoroborate ([BF4]), hexafluorophosphate ([PF 6 ]), trifluoromethanesulfonate ([TfO]), and bis(trifluoromethanesulfonyl)imide ([NTf2]). Because nanoconfined imidazolium ILs have been extensively studied, mainly the bulk state of the [CnMIm][X] ILs with variable anions is discussed in the following sections. 2.1. Complex Interactions in Ionic Liquids

Unlike water and other traditional molecular liquids, ILs are single-component systems in which the cations and anions likely play independent roles in determining the liquid behavior.62 ILs are composed of organic cations and organic/ inorganic anions, generally asymmetric and flexible with delocalized electrostatic charges. This results in more complex interactions of Coulombic forces (mutual electrostatic attraction or repulsion of charged particles), van der Waals interactions, polarization, π−π interactions, dipole−dipole, hydrogen-bonding, and solvophobic interactions (Figure 3);

2. IONIC LIQUIDS IN THE BULK STATE This section describes the study of ILs in their bulk state. This is the first fundamental step for understanding the confinement effects observed in different systems reviewed herein. Similar to molten salts, such as molten NaCl obtained by heating it above its melting point of 801 °C, ILs generally consist of cations and anions in a liquid phase. However, the main difference between ILs and simple molten salts is that the former is composed of bulky and asymmetrical cations, such as imidazolium, pyrrolidinium, pyridinium, ammonium, and phosphonium ,that only loosely fit together; thus, they do not crystallize and only remains in the liquid state at or near room temperature. Figure 2 shows the most widely studied families of ILs, i.e., 1-alkyl-3methylimidazolium cations ([CnMIm] or [RMIm], n = 2−16, R = E, B, H, O, and D) with variable alkyl chain lengths such as 1-ethyl-3-methylimidazolium ([EMIm]), 1-butyl-3-methylimidazolium ([BMIm]), 1-hexyl-3-methylimidazolium ([HMIm]), 1-octyl-3-methylimidazolium ([OMIm]), 1-decyl-3-methylimidazolium ([DMIm]), 1-dodecyl-3-methylimidazolium ([C12MIm]), 1-hexadecyl-3-methylimidazolium ([C16MIm]), and numerous different inorganic or organic anions, such as halides and others (chloride [Cl], bromide [Br]), acetate ([AcO]), nitrate ([NO3]), tetrachloroaluminate [AlCl4], dicyanamide ([DCA]), thiocyanate [SCN], ethyl sulfate ([EtSO4]), n-octyl sulfate ([OcSO4]), methylsulfate

Figure 3. Schematic representation of the different types of interactions present in imidazolium-based ILs. Reproduced with permission from ref 83. Copyright 2010 Elsevier.

these interactions seldom occur together in other materials.63 The understanding of the specific role and relative importance of these interactions in determining the (bulk and surface) structure and properties of ILs is still one of the most challenging tasks. For example, the imidazolium cation, one of the most studied cations for ILs, is amphiphilic: Only a part of the ion structure holds the charge and serves as the hydrophilic domain, while another part (or parts) of the ion is relatively hydrophobic. The cationic headgroup (imidazolium ring) exhibits strong electrostatic interactions with counterions. The intrinsic electrical fields in imidazolium ILs determined by vibrational Stark effect spectroscopy and molecular dynamics (MD) simulation are slightly higher (on average 3.0 MV cm−2) than but still comparable to those in common molecular solvents.64,65 Moreover, the aromaticity of the headgroup makes it possible to form clathrates when mixed with aromatic hydrocarbons such as benzene66−68 and benzyl methacrylate,69 C

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Figure 4. (A) Spatial heterogeneity observed in snapshots of the MD simulation presenting the snapshots of (a) [C2MIm][PF6] CPK coloring, (b) [C2MIm][PF6] in the same configuration as in part a with red/green (charged/nonpolar) coloring, (c) [C4MIm][PF6], (d) [C6MIm][PF6], (e) [C8MIm][PF6], and (f) [C12MIm][PF6]. Reproduced with permission from ref 91. Copyright 2006 American Chemical Society. (B) Schematic of proposed 3D ordering of imidazolium cations with the spiropyran probe in [C6MIm][NTf2] showing the preferred residence of the MC (lower) and SP (upper) forms of the probe in the polar and nonpolar regions, respectively. Reproduced with permission from ref 108. Copyright 2009 American Chemical Society.

wherein the imidazolium cation strongly interacts with the benzene moiety through π−π interactions located above and below the plane of the benzene aromatic ring. Hydrogen bonding in ILs has been extensively studied, because it plays an important role in cation−anion and solvent−solute interactions.70−75 For example, the hydrogen-bonding characteristics of ILs were reported to be vital in designing ILs as the potential solvents for cellulose.76,77 The acidic hydrogens in the imidazolium ring, particularly the C(2)-H, may serve as hydrogen-bonding donors, forming hydrogen bonds with negatively charged species.78−80 Furthermore, in the presence of a base, the C(2)-H can be easily deprotonated, resulting in the formation of N-heterocyclic carbene that can stabilize transition-metal nanocolloids.81,82 The above-mentioned abundant different interactions acting together in ILs make them very miscible with polar substances. At the same time, the presence of a long alkyl chain on the cation could lead to van der Waals interactions with less polar molecules. In contrast, the anions may serve as hydrogen-bonding acceptor and Lewis base depending on their nature, which codetermines the structure and properties of ILs. For example, ILs were designed to be hydrophobic by selecting specific fluorinated anion species such as [PF6] and [NTf2] or long alkyl chains on the cation and/or anion.1

Both experimental and theoretical studies demonstrated the remarkable structural heterogeneity of ILs. Imidazolium-based ILs exhibit aggregation behavior as a function of alkyl chain length, as determined by atomistic/coarse-grained MD simulation.91−94 The experimental evidence of the existence of a well-defined spatial scale associated with the long-range order in bulk imidazolium ILs was provided by small/wideangle X-ray scattering (S/WAXS).86,95 This technique using a low-momentum-transfer amorphous halo (denoted as low-Q peak) allows the probing of electron density fluctuations over a spatial scale from angstroms to several nanometers. Small-angle neutron scattering (SANS) shows that the increased heterogeneity mainly stems from the increasing asymmetry of the cation as the chain length is increased.96 Neat [HMIm][PF6] molecules would self-organize into a nanoscale segregation in the bulk, leading to an observable low-Q peak at scattering vector q = 3.67 nm−1 in the WAXS profile and a characteristic length of 1.71 nm.97 The existence of structural heterogeneities in imidazolium ILs is a common feature of almost all the ILs containing other cations, such as phosphonium,98 ammonium,99 and piperidinium.100 Systematic studies have also been conducted by comparing asymmetric/symmetric101,102 or aromatic/nonaromatic cations,100 comparing functionalization of cations, 103 and varying the anion86,104 alkyl chain length.91,105 Aromatic and nonaromatic cations exhibited differences. For example, although low-Q peaks were observed in the SWAXS patterns for piperidinium-based ILs with side alkyl chains ranging from ethyl to heptyl, the corresponding peak position was distinctly higher than that in imidazoliumbased ILs, indicating a smaller characteristic size of the heterogeneity domains in nonaromatic ILs than that in aromatic ILs.100 Studies on ILs show that the alkyl chain length affects the nanostructural organization of ILs.91,102,105 [CnMIm][PF6] ILs with a variable alkyl chain length have been investigated by both MD simulation91 and S/WAXS.105 The simulation snapshots shown in Figure 4A, particularly those rendered under the red/green polar/nonpolar convention, provide a powerful visual insight into the nature and evolution of the observed structures as the length of the nonpolar chain is increased. The polar domain has a 3D network of ionic channels, whereas the nonpolar domain is arranged as a dispersed microphase for ethylmethylimidazolium ILs and as a

2.2. Heterogeneous Structure

Driven by the combined long-range (Coulombic) and shortrange (van der Waals, hydrogen-bonding, dipole−dipole, and solvophobic) interactions present between ions, ILs in the bulk state often exhibit richer ordering than conventional liquids, and many form well-defined nanostructures with segregated regions of polar and nonpolar domains in the bulk phase.84−89 Generally, the binary cation−anion interaction in ILs imposes a degree of short-range order, and the amphiphilic combination of polar and nonpolar components results in other effects at larger scales. The resulting spatial heterogeneity represents one of the most peculiar properties of ILs and is also useful for tuning their desirable chemical and physical properties for numerous potential applications. In many respects, the spatial heterogeneity at the nanoscale is reminiscent of the selfassembly in aqueous surfactant mesophases, but with dimensions at least an order of magnitude smaller.90 D

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of ILs may help to explain their solvating power for both polar and nonpolar solutes.10,126 The polar or nonpolar organic solutes probably segregate into the structural domains for which they have larger affinity; for example, nonpolar compounds will be distributed in the nonpolar domains, while polar molecules will reside in the charged network, developing extensive hydrogen-bonding interactions with the charged moieties. By modifying ILs, such as tethering amine or carboxyl groups, to the anion or cation or introducing novel functional anions, task-specific ILs can also be designed to highly adsorb CO2,127,128 or selectively dissolve metal oxides129,130 and even nonmetallic solid elements (sulfur, selenium, tellurium, and phosphorus).131 The strong solvating ability of ILs makes them a new generation of solvents for catalysis, synthesis, and separation.19,126,132−135 Polarity is one of the most important parameters often used to predict solvent effects and solvent strength in ILs, because it is a general term that refers to the interplay of electrostatic, inductive, dispersive, charge-transfer, and hydrogen-bonding forces.136 The empirical polarity scales (ET(30)) of ILs were usually obtained from the UV absorption of Reichardt’s betaine, one of the most commonly used solvatochromic dyes.137 Fundamental studies showed that the polarity of ILs varies but slightly depends on the cation, anion, and alkyl chain length. Most common ILs, such as [CnMIm][X], exhibit almost anionindependent polarity with comparable ET(30) values in the narrow range of 50−54 kcal mol−1, indicating that they have polarities similar to those of short-chain alcohols (e.g., ethanol, ET(30) = 51.1 kcal mol−1) and other polar aprotic solvents (e.g., DMSO, ET(30) = 55.4 kcal/mol−1), but much lower than that of water (ET(30) = 63.1 kcal mol−1). However, the presence of some functional groups significantly affects the polarity of ILs. The attachment of a hydroxyl group on the tail of the alkyl chain of imidazolium cations exhibited a dramatic differentiating effect on the polarity.138,139 The ET(30) values of hydroxyl ILs cover a rather wide range (51.2−61.7 kcal mol−1) and very strongly depend on the anion. Noticeably, hydroxyl ILs containing anions [PF6], [NTf2], and [ClO4] possess unusual “hyperpolarity” (ET(30) = 60.3−61.7 kcal mol−1), even close to protic ILs140 and water. The polarity of ILs has a powerful influence on the outcome of chemical reactions,63,137,141 particularly for polarity-sensitive reactions and catalysis.142,143 For example, the high polarity of a Brønsted acidic pyrrolidinium IL was an important factor for the oxidative desulfurization of diesel fuel in the presence of H2O2.143 Diels−Alder reactions carried out in highly polar ILs produced a much higher endo/exo ratio compared to others.142 The solvatochromism, photochromism, and thermal reversion of spiropyran in ILs were clearly polarity-dependent.138,144

continuous microphase for longer side chains, such as hexyl, octyl, or dodecyl. In these systems, the butyl side chain marks the onset of the transition from one type of structure to the other. The obtained structure function from S/WAXS also exhibits an increased prepeak at 0.3 Å−1 as the prolongation of the alkyl chain, associated with the enhanced aggregation of ILs.105 The structural heterogeneity of ILs was even observed in shorter alkyl chain ILs.86,104,105 Atkin et al. reported the existence of a low-Q peak and thus long-range order in protic ILs of ethyl- and propylammonium nitrate ILs with such short alkyl chains, namely, ethyl and propyl.84 Such a nanostructure was distinct from the general identification of a low-Q peak for chains longer than butyl and was proposed to be a consequence of electrostatic and hydrogen-bonding interactions between ions.85 The presence of chemical units such as hydroxyl or ether groups on the side chains led to the disappearance of the low-Q peak or weakened the scattering intensity in the mentioned Q range, even lower than that observed for the [EMIm] salts,103 indicating that the functionalization of the alkyl chain has the overall effect of disrupting (or partially doing so) the mesoscopic order. This is probably because of the combination of the increased polarity of side chains, the more flexible behavior of the alkoxy chain than the alkyl chains, and preferable interaction of the ether portions with the imidazolium group though either intra- or intermolecular hydrogen-bonding interactions.103 MD simulations showed that the use of an external electric field significantly affects the spatial heterogeneity of ILs. With increasing external electric field strength, the IL structure of [C12MIm][NO3] was first disordered from spatially heterogeneous to spatially homogeneous under an external electric field of 2 × 109 N/C and then reordered to be nematic-like under an external electric field of 1010 N/C because of the competition between the external electric field and the electrostatic interactions between ions.106 The structural heterogeneities of ILs are expected to produce an inhomogeneous distribution of solutes in these solvents depending on their polarity. Molecular photoswitches such as spiropyran that undergo reversible structural transformation between the thermodynamically more stable closed and less polar form (SP) and open colored and highly polar zwitterionic form (MC)107 have been used to investigate the possibility of nanostructured polar and nonpolar domains in ILs.108 Kinetics and thermodynamic parameters indicate that the molecule may be dynamically transferred between the polar and nonpolar nanostructured domains in the IL [C6MIm][NTf2], wherein the SP form is likely to locate in the nonpolar region where side groups extend into space, whereas the MC form tends to locate in the polar region where the headgroup (ion charge) resides (Figure 4B), mainly because of the “like dissolves like” rule.108 The “red-edge effects” of the fluorescence spectroscopy of the organic probe molecule, 2-amino-7-nitrofluorene, in ILs are also related to the structural and dynamical heterogeneity.109,110

2.4. Thermal Properties

Compared to conventional molecular liquids and inorganic molten salts, ILs are characterized by a rather complex phasetransition behavior. Large molecular volumes and a subtle competition between van der Waals and Coulombic forces result in frustrated ionic packing that prevents the crystallization of ILs at a relatively low temperature, thus resulting in relatively low melting points. In many cases, the crystallization process is kinetically inhibited, and supercooling phenomena can be observed over extended periods of time (up to several weeks). Thus, the measurements have to be checked carefully by differential scanning calorimetric (DSC) analysis before reporting a melting point. Sometimes, pour points offer good

2.3. Solvation Ability and Polarity

Depending on the nature of the cation and anion, as well as the intermolecular interaction between ions, ILs exhibit interesting solvation and coordination properties. For example, ILs are good solvents that can dissolve various substances, including both organic and inorganic compounds, such as gases,111,112 polar or nonpolar organic compounds,113 biomass (e.g., cellulose, lignin, and directly wood as well as other renewable polymers, such as starch and chitin),114−120 metals,121,122 metal salts,123,124 and proteins.125 The heterogeneous nanostructure E

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between the hydrocarbon chains and increased structural ordering.146 Branching of the alkyl chains or the introduction of functional groups that can form hydrogen bonds also increased the melting point.147,148 Notably, accurate values for the melting and glass transition temperatures of ILs are scarce because the phase-transition behavior can be strongly affected by the presence of impurities such as water, organic solvents, and halide ions.149 Since ILs have little measurable vapor pressure, they do not boil under normal atmospheric conditions. The upper limit of the liquidus range of ILs is therefore determined by the thermal decomposition temperature (Td), whereas the lower limit is determined by the Tm or Tg. The liquid range exhibited by most imidazolium ILs at ambient pressure is significantly higher than those of common molecular solvents, such as water (100 °C). The thermal stability of ILs significantly depends on the structure of the constituent ions.150,151 Among the cations, phosphonium salts are the most stable, followed by imidazolium salts and ammonium salts. The thermal decomposition of imidazolium-based aprotic ILs occurs mainly through the dealkylation reaction (SN2 reaction) by the nucleophilic attack of the anion on the cation. Therefore, the thermal stability of ILs significantly depends on the nucleophilicity of the anion. The activation energy and reaction rate for the SN2 dealkylation of the cation for [BMIm][X] have been calculated using quantum chemical (QM) calculations; the results are consistent with experimental decomposition temperatures.152 The halide-based anions of imidazolium salts exhibit poor thermal stability, whereas fluorinated anions such as [PF6], [NTf2], and [BF4] are thermally more stable. ILs containing [NTf2] usually showed the highest decomposition temperatures, up to 455 °C for [EMIm][NTf2]. Their thermal stabilities were inversely proportional to the length of the alkyl chain.153 Protic ILs usually show much lower thermal stabilities than aprotic ILs, because their thermal decomposition occurs most easily at lower temperatures by proton transfer from the salt form to the parent acid/base pairs. The decomposition temperatures are related to the ΔpK a values of the corresponding acids and bases.154 Notably, the long-term thermal stability of ILs is >50 °C lower than the decomposition temperatures obtained by step-tangent TGA experiments.155

indications for the melting point behavior under the operating conditions. Some other ILs may completely lack a melting point and show a glass-transition temperature (Tg) instead.5 As intensively described in a very large number of wellcharacterized imidazolium ILs,1,145−148 the melting point (Tm) of an IL is a function of both the cation and anion: more specifically, the substitution pattern and symmetry on the cation and the choice of the anion. Generally, symmetric ions with a localized charge and strong interactions between ions exhibit a good packing efficiency and hence a high melting point; one of the extreme cases is NaCl (Tm = 801 °C). Conversely, larger and more asymmetric cations with a delocalized charge afford ILs with lower melting points. The replacement of the C(2)-H in the imidazolium ring with an alkyl or alkoxy group will improve the melting point. An increase in the anion size or electron delocalization in the anion leads to lower melting points because of reduced Coulombic attraction contributions to the lattice energy of the ionic crystal. The manipulation of the alkyl chain substitution produced major changes in the melting points. Figure 5 shows the phase

Figure 5. Phase behavior for [CnMIm][BF4] ILs as a function of chain length showing the melting points (red closed square), glass transitions (open square), and clearing (blue circle) transitions. Reproduced with permission from ref 146. Copyright 1999 Royal Society of Chemistry.

behavior for [CnMIm][BF4] ILs as a function of linear alkyl chain length. Only the salts with n = 1 (n is the number of carbon atoms in the alkyl chain) and n > 9 crystallized on cooling, whereas the salts with 1 < n < 9 showed the major trend toward glass formation on cooling. The reduction in the melting points and glass formation with the initial lengthening of the substituent can be explained by a decrease in the effective Coulombic force between the more asymmetric ions and an impediment to efficient crystal packing. However, on extending the alkyl chain lengths beyond a certain point (n = 10), the melting points of the salts started to increase again with increasing chain length, as van der Waals interactions between the long hydrocarbon chains contribute to the local structure by inducing microphase separation between the covalent, hydrophobic alkyl chains and charged ionic regions of the molecules. Normally, diverse ILs form liquid crystalline phases by increasing the amphiphilic character of the cation through substitution with longer, linear alkyl groups. Noticeably, structured liquid crystalline materials (Smectic A mesophase) with re-emergence of higher melting points can be obtained when the chain lengths were further increased to >12 carbon atoms, because of the increased attractive van der Waals forces

2.5. Electrochemical Window

Because ILs have intrinsic ionic conductivity, wide liquid range, negligible volatility, nonflammability, etc., they represent very interesting solvent-free electrolytes for electrochemical applications, including batteries, electrochemical sensors, photovoltaic devices, electrodeposition, and capacitors.1,23,39,156−158 One of the key advantages that enables ILs to overcome the limits imposed by aqueous solutions or organic media is their wide electrochemical windows (EWs).2,159 For example, a number of elements, such as the light and refractory metals, as well as elemental and compound semiconductors that previously could not be deposited from conventional water baths, can be electrodeposited from ILs with EWs larger than 4 V.160 The wide EWs of IL electrolytes could also undoubtedly improve the performance of supercapacitors, because both their energy and power densities are proportional to the square of the operating voltage of the cell.158 The EW is a common expression of the electrochemical stability of ILs, which is defined as the potential range between which the IL undergoes no faradaic processes and is bounded by the cathodic and anodic limits. The cathodic limit is often F

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determined by the reduction of the cationic components, whereas the anodic limit is set by the oxidation of the anionic components. Since both anions and cations are extremely resistant to electrochemical reduction and oxidation, ILs usually have wide windows of electrochemical stability (up to 5−6 V for certain cation−anion combinations).2 The EWs of ILs strongly depend on the type of constituent ions. Generally, [BF4], [PF6], and [NTf2] anions are likely to give wide EWs for most ILs, while little variation is found in the anodic potential limit because the oxidation potentials of ions all lie in the same 0.5 V range. In contrast, the cations play an important role in limiting the EWs of ILs because the cathodic limit is more variable. ILs based on tetraalkylammonium and dialkylpyrrolidinium cations are around a volt more stable to reduction as compared to imidazolium cations, mainly because the acidic proton of the imidazolium ring limits the cathodic stability potential.156 The presence of impurities in ILs, in particular halide and water, has a profound impact on the anodic or cathodic potential limits and the corresponding EWs. For example, the accessible EW of ILs is decreased in the presence of halide impurities, since halides are oxidized much easier than many organic anions whose negative charge is delocalized over larger volume, and thus, they reduce the observed anodic potential limit.1,161 In addition, water impurities in ILs have been shown to significantly reduce both the anodic and the cathodic potential limits. It was reported that the EW of [BMIm][BF4] was dramatically reduced from 4.10 to 1.95 V upon the addition of 3 wt % water.162

Figure 6. Incorporation of ILs into the pore structure by two different strategies: (A) covalent grafting of monolayer IL on the pore wall and (B) physical confinement of multilayers of ILs into nanopores.

generally refers to the self-assembly of a monolayer of IL onto a support, resulting in a low density of ions. Therefore, supported-multi-ILs containing two or more imidazolium moieties in one grafted side chain are often required to provide more catalytic active centers in organic reactions.169 Third, covalent grafting involves functionalized ILs as well as complex chemical reactions. This is only applicable to solids with “active” surfaces such as silica, whereas for porous matrices with a relatively inert surface, such as porous carbon and metal, additional pretreatment of the supports is required to render them with a large number of linking groups, such as hydroxyl. Even so, the chemical oxidation of CNTs under harsh conditions affords only carboxylic acid groups on their outer surface for the covalent anchoring of functionalized ILs, while it does not take advantage of the enhanced nanoconfinement effect offered by the inner space of CNTs. Compared to the tedious covalent grafting method, the concept of physical confinement of ILs into nanoporous matrices is highly attractive because of the following advantages: First, the extremely low vapor pressure of ILs makes it easy to achieve physical confinement with permanent coating of ILs onto the pore surface through either in situ chemistry or post-treatment of the porous materials (for details, see the following section). In contrast, previous studies showed that the thin film of the loaded molecular liquids, such as water, evaporated quickly during the operation.170,171 Second, unlike the monolayer of ILs resulting from the covalent grafting method, the multilayers of ILs are immobilized onto the pore wall of a support, and the specific bulk properties of the ILs can be retained to some extent depending on the strength of the confinement effect. Third, from a practical perspective, the ILs physically entrapped in nanoporous matrices in some cases can be removed by extraction with miscible organic solvents for separate recycling of the porous materials and ILs.172 Fourth, the properties of nanoconfined ILs can be easily modulated to fulfill the special requirements of a given application by deliberately varying the structure and composition of both ILs and porous matrices. Considering these characteristics, in the following sections of this review, only the concept of nanoconfined ILs is discussed.

3. COVALENT GRAFTING VS PHYSICAL CONFINEMENT The incorporation of ILs into the pore structure of solid matrices such as silica or porous carbon is of key importance for developing supported ILs that can overcome major drawbacks related to the bulk ILs, such as high viscosity, slow gas diffusivity, and high cost. At the same time, the immobilization of “soft” ILs on the surface of “hard” solids can further modify and improve the properties of the supports, such as wettability, lubricating property, and separation efficiency. The significantly improved recoverability and reusability make feasible the largescale application of supported ILs as solvents, catalyst dispersion agents, and absorbents in the fields of organic synthesis, heterogeneous catalysis, and gas separation.163−165 For example, supported ILs have dual effects on catalysis: On one hand, the ILs supply a special microenvironment for affecting the reaction pathway; on the other hand, the micropores of the porous material control the selectivity of the product. Generally, supported ILs can be obtained by either chemical coupling involving the covalent grafting of ILs on the pore surface or physical confinement of ILs into nanopores, as shown in Figure 6. The covalent strategy is usually accomplished by the reaction between silica walls and imidazolium-based ILs with special functional groups, such as the trimethoxysilyl group.166−168 The immobilization of ILs on porous matrices by covalent anchoring is a highly attractive strategy to circumvent the leaching of ILs, thus minimizing the amount needed. However, the chemical coupling method has several drawbacks: First, once bound to a solid support, the IL becomes a part of the support material, and its free degree is limited so that the cation−anion pair no longer constitutes a “true” IL, thus losing certain bulk-phase properties, such as solvation strength and conductivity. Second, chemical coupling

4. SYNTHETIC METHODOLOGIES FOR NANOCONFINED IONIC LIQUIDS Depending on the type of the porous matrices, there are several methods to introduce ILs into a nanoconfined environment, e.g., in situ introduction by a sol−gel method for silica gels or porous metal oxides;57,173−175 post-impregnation for nearly all materials including porous silica,48,176−178 porous carbon, G

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found that although [BMIm][NTf2] was insoluble in water, it can be easily removed from the gels when immersed in water. Incorporation of hydrophobic methyl groups by adding some amount of methyltrimethoxysilane into TMOS could make the ionogels completely stable toward water.174 The complicated synthetic procedures involving very long time (up to several weeks) for aging, however, limited its use for nanoconfined ILs. Ionothermal synthesis, developed by Cooper et al. in 2004, refers to the use of an IL as both the solvent and the structure directing agent (sometimes also known as a template) to synthesize zeolites and other porous materials.199 Recently, such a concept was reported to encapsulate ILs into MOFs by a one-pot ionothermal reaction of the precursors of MOFs (e.g., zinc nitrate and 2-methylimidazole for ZIF-8) in ILs.60,182 Attributed to the in situ crystallization, both the cationic and anionic components of ILs were found to be located within cavities; however, they could not go out of the framework because of the small aperture size. Therefore, ILs in this concept serve as not only a solvent and template, but also an “inert” reactant which participates in the reaction. Addition of polymer such as poly(vinyl alcohol) (PVA) at the commencement of the in situ ionothermal synthesis led to the fabrication of flexible composite membranes with MOF-encapsulated IL nanocrystals uniformly dispersed in the PVA polymer matrix.182

CNTs, and MOFs by soaking a previously prepared host network in an IL; the so-called “ship-in-bottle” method specific to zeolites or MOFs with large cavities but small pore apertures;52,179−181 ionothermal synthesis;60,182 and precipitation of metal in ILs.183 In this review we mainly focused on the former three methods because of their wide range of applications. 4.1. In Situ Introduction of Ionic Liquids

One of the most common methods for the synthesis of nanoconfined ILs is the well-established one pot sol−gel method. This process can lead to the confinement of the ILs within an oxide matrix in situ during the synthesis process, resulting in composite materials called ionogels or ion gels.173,184 It is noteworthy that ionogels are different from normal xerogels (or aerogel) obtained by the classical sol−gel process. In the latter case, the liquid phase from the “sol” after gelification is removed during the drying process, yielding the target material: the porous (oxide) solid. However, in ionogels, the dopant IL without measurable vapor pressure remains entrapped in the pores of the gelled oxide matrix, yielding a stable solid−liquid system as a material.185−187 In general, two sol−gel methods, including nonhydrolytic and hydrolytic processes, are used to synthesize ionogels, thus confining ILs into porous oxide matrices. Since the pioneering work of Dai et al.,186 formic acid solvolysis (nonhydrolytic sol−gel route) was reported to give access to ionogels using various ILs.174,175,188−194 Moreover, the hydrolytic sol−gel synthesis of ionogels from ILs was reported by Deng et al. and others using hydrochloric acid.50,57,195,196 The typical procedure of the ILs-involved sol−gel method is as follows: liquid alkoxysilane precursors such as tetraethoxysilane or tetramethoxysilane (TEOS or TMOS), acidic reagent such as formic acid or hydrochloric acid, and target ILs were first mixed in short chain alcohols in a stoichiometric molar ratio. Gelation occurred after several hours via hydrolysis or solvolysis to form siloxane bridges by inorganic polymerization, and gels were aged for a few days at room temperature, or for a few minutes upon exposure to ultrasound.175,197 Investigation of the gelation mechanism by time-resolved Raman and proton nuclear magnetic resonance (1H NMR) spectroscopy indicates that the sol−gel reaction involves hydroxylation, esterification, and condensation.190 The structure and loadings of ILs and the type of acidic reagent in the sol−gel process were found to have effects on the change in the gelation time, pore parameters (e.g., specific surface area, pore size and distribution, and pore volume), and morphological structure.97,190−192 A nonhydrolytic sol−gel process has been used extensively for synthesis of ionogels due to its advantages over the conventional sol−gel process, e.g. the absence of water in the initial reactant, rapid gelation, and ease of giving monolith ionogels.174,198 The IL sol−gel route should result in a more intimate biphasic system than the simple impregnation of oxide particles with an IL, and the obtained ionogels were shown to be endowed with both the transparency and mechanical properties of silica and the conductivity performances of ILs.175 Ionogels thereby act as promising candidates for potential applications, such as temperature-resistant electrolyte membranes in electrochemical devices.174 Moreover, ionogels are even stable when they are immersed into an organic solvent as long as the IL was insoluble in this solvent. Conversely, the IL could be extracted from the gel by polar solvents that can dissolve the IL. However, the case of water was reported to be specific. It was

4.2. Post Impregnation

Compared to the in situ sol−gel method, simple physisorption is a more straightforward method for the synthesis of nanoconfined ILs.48,176−178,200,201 Fortunately, the liquid nature at room temperature as well as the extremely low vapor pressure of ILs makes it feasible to prepare the genuine nanoconfined ILs via post-impregnation of the as-obtained porous matrices under vacuum condition. In a typical filling experiment, nanoporous materials such as silica or CNTs were first put into a flask and then the gas present inside the pores was drawn out under vacuum. IL was then transferred into the flask through a syringe, and the mixture was ultrasonically vibrated for several hours at high temperature. IL is incorporated into the interior channels or pores by vacuumassisted capillarity. Next, the filled samples were separated from the mixture and further purified through washing with appropriate low-boiling solvents such as methanol to remove the surface adsorbed ILs (the absence of a surface liquid film after washing was visualized by scanning electron microscopy (SEM)178). Nanoconfined ILs were finally obtained by overnight drying under high vacuum. Notably, prior to impregnation, ILs in some cases can be diluted by organic solvents to reduce their viscosity and facilitate the diffusion process. The vacuum conditions during the filling process were shown to be extremely important during the final state of ILs in the porous supports. Considering the post-impregnation of [BMIm][PF6] into mesoporous silica as an example, if the filling process is performed at atmospheric pressure, the IL is found to be immobilized onto the outer surface of silica particles. However, if the same process is conducted under ultrahigh vacuum (1 × 10−5 Pa), the compressed air present in the cavities of the SiO2 is completely removed by evacuation and ILs enter inside the nanopores of SiO2 instead of getting adsorbed on the surface. The observed results may also explain the inconsistent observations of melting point variation for confined liquids often revealed by different research groups.176 The filling process in the preparation of nanoconfined IL membranes can also be improved via combined vacuum- and H

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(ZIFs), MOFs, and zeolites because of their large porosity and 3D pore structure;52,179−181,207 however, this concept is inapplicable to porous matrices, such as mesoporous silica or carbons, which have no special supercages. Moreover, the reported types of ILs and anions available are very limited because, in general, ILs are obtained by quaternization reaction of imidazole and haloalkane. For example, NaY zeolite, which consists of supercages and channels of 1.2 nm diameter and 0.74 nm diameter, respectively,211 has been employed to confine imidazolium ILs with side alkyl chains longer than butyl for efficient heterogeneous catalysis and CO2 capture.52,179,181 It is noteworthy that another “ship”, such as a catalytically active metal complex, can also be introduced into the nanoconfined ILs via the ship-in-bottle method.179

pressure-assisted infiltration, wherein a high pressure difference applied on both sides of the membrane acts as the driving force.202 Depending on the original morphology of SiO2, the obtained nanoconfined ILs via post-impregnation can be either powder178 or monolith.203 The post-impregnation method is available to nearly all of the porous matrices, including SiO2,176 CNTs,48 porous carbons,204 and MOFs,61 and the relative ratio between ILs and porous host and thus the filling ratio (also denoted as pore loading or loading fraction) can be more easily controlled than the complicated in situ process. 4.3. “Ship-in-Bottle” Method

Even though the methods mentioned above, including in situ introduction and post-impregnation, have been widely used for physical confinement of ILs, such composite materials still suffer from one key issue, that is, instability and lack of longevity due to the leaching of the ILs, in particular, under rigorous reaction conditions.205 To this end, a concept of “shipin-bottle”206 has been recently shown to be a promising approach for the preparation of stable nanoconfined ILs.52,179−181,207−210 This concept is illustrated in Figure 7,

5. METHODS TO INVESTIGATE THE NANOCONFINED IONIC LIQUIDS 5.1. Experimental Techniques

It is worth stressing that experimental observations of nanoconfined ILs are still a significant challenge compared to ILs in the bulk phase due to enormous difficulties encountered in accessing the interior confinement region in well-defined model experiments. However, the negligible vapor pressure of ILs makes it possible to be investigated in the confined state by many advanced technologies that require vacuum conditions for sample analyses and material manufacturing. Many experimental techniques have been employed to investigate the physicochemical properties of nanoconfined ILs. The frequently used technologies include DSC,177,212 TGA, X-ray powder diffraction (XRD),213 nitrogen adsorption isotherms,51,97,213−215 NMR spectroscopy,175,191,216−222 highresolution transmission electron microscopy (HRTEM),177,223,224 and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM),176,200,225 atomic-force microscopy (AFM),226−230 sum frequency generation vibrational spectroscopy (SFG),231,232 small-angle neutron scattering (SANS),45,213,233,234 S/WAXS,97,235,236 quasielastic neutron scattering (QENS),234,237 X-ray absorption fine structure (XAFS) analysis, broad band dielectric spectroscopy (BDS),220,222 surface force apparatus (SFA),235,238−245 high-energy X-ray reflectivity,8 and electrochemical technologies.246,247 For example, DSC is one of the most frequently used technologies to characterize the dramatic change in the phase behavior of the confined ILs when compared to the bulk one. The basic principle of DSC is that when the sample (ILs) undergoes phase transitions, more (e.g., melting) or less (e.g., crystallization) heat resulting from an exothermic or endothermic process needs to flow to the sample than the reference to maintain both at the same temperature. By virtue of the difference in heat flow between sample and reference, DSC is able to measure the amount of heat absorbed or released during these transitions, thus revealing the melting point (Tm, the first-order endothermic phase-transition temperature), crystallization temperature (Tc, the first-order exothermic phase-transition temperature), glass transition temperature (Tg, the second-order transition), and specific heat capacities (Cp, the heat capacity per unit mass of a material). TGA, XRD, and nitrogen adsorption isotherms were used to measure the thermal stability (Td, decomposition temperature), the crystal structures, and the porous structure (specific surface area, pore volume, and pore size and distribution) of nanoconfined ILs, respectively. However, HRTEM and HAADF-STEM can

Figure 7. (A) Schematic diagram of the concept of the “ship-in-bottle” technique and (B) its application for synthesis of nanoconfined ILs.

wherew the small precursors of ILs are first diffused into the porous hosts (“bottle”) in sequence through the pore apertures, and then the desired guest product (“ship”) is generated via in situ chemical reaction inside the “bottle”, which, however, is entrapped inside the cavities because of its larger size than the pore apertures of hosts. Therefore, problems related to the instability/leaching of active guest species could be resolved by application of the ship-in-bottle protocol.180 However, the structure and size of ILs (guest) and the pores (host) have to be carefully considered before conducting the ship-in-bottle process. According to the sizes of precursors and IL candidates, the ship-in-bottle protocol is, in principle, only applicable to microporous materials, such as zeolitic imidazolate frameworks I

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Figure 8. (A) Schematic diagram of the SFA setup for normal and shear force measurements. (B) Typical result of measured normal force normalized by the radius of curvature between sheet surfaces across ILs as a function of surface separation. The inset shows the “three layers” and “one layer” ion packing corresponding to the adjacent energy minimum at 1.08 and 0.23 nm, respectively. Reproduced with permission from ref 239. Copyright 2010 Royal Society of Chemistry.

radius of curvature, R. The separation between the surfaces, D, is determined by white-light multiple beam interferometry and fast spectral correlation with angstrom resolution. The normal interaction force, FN, across the IL film is normalized by R. The resultant FN/R is proportional to the interaction energy per unit area between parallel plates, so as to allow quantitative comparison between different contacts and different experiments. Shear forces (FS) are measured as a function of surface separation to characterize the shear properties of the film by applying lateral motion to one surface. Measuring the normal and shear forces, respectively, as a function of surface separation allows elucidation of the structural and the dynamic properties of ILs in spaces from the submicrometer down to the molecular scale. For example, surface force measurement by SFA revealed the existence of an oscillating solvation force resulting from the arrangement of ions in discrete layers adjacent to the solid surfaces, with ion packing and dimensions of layered structures depending on the structure of ILs, and charged/uncharged state and surface chemistry of the sheet.235,239−245 The dynamic properties of nanoconfined ILs (e.g., viscosity and friction) as a function of surface separation distance have also been evaluated quantitatively by SFA-based resonance shear measurements.238

directly visualize the state and morphology of IL encapsulated in channels or pores. It is noteworthy that ILs containing heavy atoms, such as bromine and iodine, are desirable to achieve a high resolution and contrast for TEM imaging. NMR spectroscopy, such as pulsed-field gradient (PFG) and magicangle spinning (MAS), has been used extensively to gain a molecular-level understanding of the dynamic properties (i.e., reorientational and translational motions and diffusion) of ILs in porous matrices by T1- and T2-relaxation measurements and line shape analysis.212,216,217 SANS has been proved to be an essential tool for elucidating the microstructure of nanoporous materials and the properties of confined ILs within the pores. For example, SANS has the advantage over conventional gas adsorption methods that it can provide information about the pores inaccessible to gas molecules, such as closed pores.45,213 S/WAXS, a technique which adopts elastic scattering of X-rays and records the structural information on a sample with heterogeneities at very low angles (typically 0.1−10°), was employed to investigate the structural heterogeneity in nanoconfined ILs. The low-Q peaks in the S/WAXS, which are related to the self-ordering of ILs as a consequence of the segregation of the alkyl tails into mesoscopic domains, have been confirmed to be an extremely powerful tool to directly observe the packing status of ILs confined within the nanopores.97,236 QENS is one of the best techniques to probe multiple diffusion processes in ILs due to its capability of extracting the information related to the spatial characteristics of diffusion jumps through the analysis of the Q-dependence of the scattering signal.248,249 By analyzing the white line peak and R-space, XAFS is also employed to acquire information about the nanoconfined ILs, including the local structure, the distance between the anion and the cation, and the interactions and charge transfer between ILs and porous matrices. AFM can be effectively used to observe the nanoscale morphological and structural properties of confined IL nanofilms and measure the near surface oscillating solvation force profiles and liquid structure of ILs confined between solid surfaces and an AFM tip. SFA has been proven to be a powerful technique to probe the behavior of ultrathin IL films. Figure 8 shows the schematic representation of the SFA experimental setup. The IL under study is placed between two atomically smooth and curved mica sheets glued onto crossed cylindrical lenses with the local

5.2. Computational Simulations

Even though extensive experimental efforts have been devoted to the study of the behaviors of nanoconfined ILs, it is still very difficult to gain a molecular-level picture for fundamental understanding of the confinement effect of ILs in nanopores from experiments alone, primarily due to the complexity of these systems and difficulties in monitoring conformational changes and spatial reorientation. Moreover, from experimental studies, it is difficult to determine the individual effect of each variable, such as pore size, shape, and interconnectivity, on the properties of the nanoconfined IL systems. As a complement to experimental observations, theoretical simulations, such as QM calculations and, in particular, MD simulations, have been carried out to gain insight into these questions.250−253 A reliable computational model can assist in the effective interpretation of the experimental results and provide accurate predictions of the behavior of nanoconfined ILs, thus eliminating the need for expensive trial-and-error experiments. For example, QM was used to understand the interaction between confined ILs and the pore wall surface and, thus, the preferred geometries of J

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Table 1. Summary of the Characteristics of Representative Porous Materials as Well as the Infiltration Method for Nanoconfined ILs

K

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Table 1. continued

ILs.178,197,254−257 MD methods are more promising and can provide a molecular-scale description of the conformation and properties of the confined ILs. MD simulations are well suited to perform a systematic study of the relevant physical variables of ILs confined in a model porous material that is completely characterized, and to determine their individual effects on the system. Another advantage of MD simulations is that they can also be used to study the behavior of confined ILs over experimentally relevant size scales. MD simulations used for ILs are, in general, classified into three types, i.e., ab initio MD, classical MD, and coarse-grained MD. Ab initio MD simulations provide significant insight into atomic level interactions and electronic structure and can broaden knowledge of electric dipole moments, polarization processes, or possible charge-transfer effects. However, ab initio MD is computationally expensive and can be propagated to only relatively small systems of tens of ion pairs over short time periods of tenths of picoseconds.251 In contrast, classical MD simulation solving the Newton equations using a finite step approximation is a choice to study larger systems on longer time scales, such as liquid bulk-phase and multiphase systems. The development and refinement of force fields for ILs ensure the predictive power of classical MD methods for various nanoconfined IL properties.252 The multiscale coarse-graining methodology has also been employed to increase the simulation speed by a factor of 100 or more, thereby making it possible to study the mesoscopic behavior of ILs by computer simulations.250

MD simulations have been utilized to study the behavior of nanoconfined ILs inside various nanopores of different geometries and materials (namely CNTs, graphitic or silica slit-like pores, and ordered mesoporous carbons, such as CMK3). The simulated results indicate that variables, such as wall distance or pore size, pore geometry, pore surface roughness, and pore loading, have a profound influence on the global and local structure and dynamics (e.g., local densities and orientations, radial distribution functions (RDFs), and mean square displacement (MSD)) of the confined ILs, and thus on the ion distribution, phase transition, self-diffusivity, viscosity, and gas solubility of ILs in the systems. For example, the ion distribution and configuration can be observed in the 2D RDF; however, the solid−liquid phase transition of confined ILs can be detected by monitoring the potential energy profiles. The self-diffusivity of ions, which is a single molecule property and is by far the easiest macroscopic dynamical property to compute from simulation, can be determined by calculating the slope of the MSD of the molecules versus time over a sufficiently long period of time.55,258 The effects of the external variables, such as temperature and pressure, as well as the presence of cosolvent, on the structural, dynamical, and thermodynamic properties of nanoconfined ILs have also been analyzed and discussed. In most cases, MD simulations can yield surprisingly accurate predictions in comparison to experimental data for both thermodynamic and transport properties of nanoconfined ILs.219 L

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Table 2. Experimentally Determined Change in Structure and Properties of Representative ILs after Confinement in Various Matricesa ILs [EMIm][Cl]

[EMIm][Br] [EMIm][DCA]

[EMIm][SCN]

[EMIm][BF4]

[EMIm][TfO]

[EMIm][EtSO4]

[EMIm][NTf2]

[BMIm][Cl] [BMIm][Br]

Confining conditions

Structure and properties of confined ILs

CNTs (internal diameters: 2 nm), post-impregnation CNTs (internal diameters: 1−3 nm, 967−1540 m2 g−1, 0.79− 1.23 cm3 g−1), post-impregnation Mesoporous silica (3.7 and 7.1 nm), 31.1 and 29.7 wt % IL, post impregnation (under high vacuum) Porous silica monolith (6.2−19.3 nm, 496−512 m2 g−1), postimpregnation Porous silica monolith (6.2−7.1 nm, 919 m2 g−1), postimpregnation Silica gels (5.5−7.9 nm, 383−444 m2 g−1, 0.72−0.86 cm3 g−1), 5−60 wt % IL, in situ sol−gel method Carbon aerogels (276−308 m2 g−1, 0.43−0.70 cm3 g−1), postimpregnation Porous silica monolith (6.2−19.3 nm, 496−512 m2 g−1), postimpregnation Silica gels (3.7−7.5 nm, 431−546 m2 g−1, 0.63−1.19 cm3 g−1), in situ sol−gel method Silica gels (7.4−7.8 nm, 125−202 m2 g−1, 0.46−0.60 cm3 g−1), in situ sol−gel method MCM-41 (3.4 nm, 764 m2 g−1, 0.84 cm3 g−1), 23−34 wt % IL, post-impregnation

More ordered structure inside CNTs than bulk state Rigid crystalline cationic networks and mobile chloride anions exist in the 2 and 3 nm CNTs Tm increases (ΔTm = 5 or 22 °C), electron transfer from [Br] to [EMIm], the anion−cation distance decreases Tc increases (ΔTc = 4 °C), Tm decreases (ΔTm = 14 °C)

Nanoporous carbons (0.6−2.4 nm, 670−1730 m2 g−1, 0.2−0.9 cm3 g−1), 8−46 wt % IL, post-impregnation BP2000 carbon black (1374 m2 g−1, 2 cm3 g−1), postimpregnation Porous silica monolith (6.2−19.3 nm, 496−512 cm3 g−1), post-impregnation Porous silica monolith (6.2−7.1 nm, 919 m2 g−1), postimpregnation Carbon aerogels (276−308 m2 g−1, 0.43−0.70 cm3 g−1), postimpregnation Porous silica monolith (6.2−19.3 nm, 496−512 m2 g−1), postimpregnation Silica gels (2.6−2.7 nm, 233−526 m2 g−1, 0.15−0.34 cm3 g−1), in situ nonhydrolytic sol−gel method HKUST-1 or Cu-BTC (MOF, 0.5 and 0.9 nm), postimpregnation Silica gels (2.2−12.1 nm, 576−634 m2 g−1, 1.04−2.23 cm3 g−1), in situ sol−gel method ZIF-8 (MOF), 25 wt % IL, post impregnation ZIF-8 (MOF, 1.16 nm, 1947 m2 g−1, 0.636 cm3 g−1), postimpregnation Silica gels (3−12 nm, 300−700 m2 g−1, 0.6−1.1 cm3 g−1), 26.5 wt % IL, in situ sol−gel method Ordered mesoporous silica (3.7 nm), post-impregnation (under high vacuum) MIL-101 (MOF, 2956 m2 g−1, 1.63 cm3 g−1), ship-in-bottle NaY (zeolite), 11.4−22.5 wt % IL, ship-in-bottle

[BMIm][DCA] [BMIm][CH3SO3] [BMIm][AlCl4] [BMIm][NO3] [BMIm][TfO] [BMIm][BF4]

Silica gels (7.7 nm, 389 m2 g−1, 0.8 cm3 g−1), 24.6 wt % IL, in situ sol−gel method Silica gels (4.6 nm,469 m2 g−1, 0.7 cm3 g−1), 24.5 wt % IL, in situ sol−gel method MIL-101 (MOF, 1.3 and 2.0 nm, 3574 m2 g−1, 1.9 cm3 g−1), 11−50 wt % ILs, post-impregnation Porous metallic silver (39 ± 10 nm), precipitation in ILs Controlled-pore glasses (7.5−11.5 nm, 120−140 m2 g−1), post-impregnation Silica gels (5.9−10.4 nm, 350−390 m2 g−1, 0.8−1.1 cm3 g−1), 5.2−40.2 wt % IL, in situ sol−gel method Porous SiO2 membranes (7.5−10.4 nm) with and without silanization, post-impregnation Silica surfaces with variable separation distance from 0 to 15 nm, post-impregnation Methylated silica gels (3.8−7.1 nm, 255−576 m2 g−1) containing radius-various H2 pore or slit-shaped H3 pore, in situ sol−gel method

Tc increases (ΔTc = 5 °C), Tm decreases (ΔTm = 15 °C), phase-transition enthalpies (ΔHc and ΔHm) decrease Enhanced fluorescence emission, NC Raman bands change

ref 297 215 225 203 279 196

Complex change in phase-transition temperatures, phase-transition enthalpies decrease, thermal stability slightly improves Tg slightly increases (ΔTg = 1 °C), Tc and Tm disappear

204

Tg increases by about 5−8 °C, thermal stability improves, vibrational bands change Tc and Tg increase, change in Tm is complex, thermal stability decreases

197

203

189

Tg decreases (ΔTg = 14 °C), Tc and Tm disappear, thermal stability decreases, blue shift in fluorescence spectra, change in vibrational bands related to the imidazolium ring Ion−ion distance decreases, change in vibrational bands

178

Tc and Tm disappear, new Tg peaks appear at −88 and −89 °C

246

Tg disappear, Tc and Tm decrease (ΔTc = 132 °C, ΔTm = 8 °C)

203

Tg increases (ΔTc = 2 °C), Tm decreases (ΔTm = 9 °C), phase-transition enthalpies (ΔHc and ΔHm) decrease Tg and Tc disappear for C-900 and C-700, phase-transition enthalpies decrease, thermal stability slightly improves Tg slightly increases (ΔTg = 1 °C), Tc and Tm disappear

279

Tg increases (ΔTg = ∼20 °C), improved thermal stability, change in vibrational bands and fluorescence Anions interact with metal Cu ions and perturb the symmetry of the MOF; two different types of ion pairs formed inside the MOF Change in Tc and Tm, binding energy, vibrational bands, and crystallization kinetics; conversion of anion from trans to cis conformers occurs Tc and Tm disappear; gradual and continuous narrowing of the 19F NMR line width on heating Tc and Tm disappear (except for EZ125), remain liquid at low temperature; EZ100 shows a higher ionic conductivity than the bulk IL below −23 °C Tc and Tm disappear, change in vibrational bands

287

204 203 188 254 257 212 309 50

Tm increases (ΔTm = 50 °C), crystalline-like phase

310

Slightly improved thermal stability, excellent stability, and efficient reusability for the liquid phase adsorption of benzothiophene Improved thermal stability, change in vibrational bands, high cycling stability for CO2 adsorption Specific heat capacity decreases, enhanced fluorescence emission

180

Tc and Tm disappear, specific heat capacity decreases

50

Remarkable improvement in the adsorption capacity (∼71%) for adsorptive desulfurization Phase-transition behavior changes, improved thermal stability Tm decreases; ΔTm depends on the pore diameter

307

Tg disappears; specific heat capacities decrease; improved CO2 adsorption

50

Diffusion coefficients remain unaltered by silanization but enhanced by more than two decades compared to the bulk IL Oscillatory solvation layer structure; viscosities are 1−3 orders of magnitude higher than those of the bulk ILs Arrangement of IL near the pore wall is random in both H2 and H3 pores, while ordered at the center area of the H3 pore

222

M

52 50

183 47

238 236

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Table 2. continued ILs

[BMIm][PF6]

[BMIm][NTf2]

[(BMIm)2][PtCl4]

Confining conditions

Structure and properties of confined ILs

Graphene multilayers, post-impregnation BP2000 carbon black (1374 m2 g−1, 2 cm3 g−1), postimpregnation Silica gels (1.6−5.4 nm, 338−843 m2 g−1), in situ sol−gel method Silica gels (14.8−23.2 nm), 26.9−65.0 wt % IL, in situ sol−gel method Silica gels (11.4−22.6 nm, 182−299 m2 g−1, 1.3−1.6 cm3 g−1), in situ sol−gel method MCM-41 (2.1 nm, 1070 m2 g−1), SBA-15 (3.7 nm, 700 m2 g−1), post -impregnation Mesoporous silica (2−6 nm), 35.9 wt % IL, post-impregnation (under high vacuum) MWCNTs (internal diameters: 5−10 nm, external diameters: 40−60 nm), post-impregnation (under high vacuum) Graphene multilayers, post-impregnation Silica gels (monolith, 12−15 nm), in situ sol−gel method Silica gels (12 nm, 780 m2 g−1, 1.5−3.5 cm3 g−1), in situ sol− gel method Silica gels (11−15 nm, 780−1200 m2 g−1, 1.50 cm3 g−1), in situ sol−gel method Silica gels, in situ sol−gel method KIT-6 (9.5 nm, 783 m2 g−1, 1.11 cm3 g−1), post-impregnation

Tg increases from −71.4 to 56.1 °C and 42.5 °C; change in vibrational bands Tc decreases (ΔTc = 12 °C)

273 246

Tm decreases (ΔTm = 2 °C); red shift in fluorescence spectra; shift in vibrational bands related to imadazolium ring Thermal stability decreases; two observable dielectric relaxation peaks

255

Tg increases; thermal stability decreases

312

Complete pore filling with IL for SBA-15, while the pore core is empty for MCM-41 Tm greatly increases to 201.6 °C; formation of crystallized structure; thermal stability decreases Formation of a stable, polymorphous crystal having a melting point of above 200 °C Two endothermic peaks near 15.5 and 3.3 °C; change in vibrational bands Liquid-like behavior at temperatures Tm and Tc disappear; high ionic conductivity and thermal stability (Td = 357 ° C in air) close to bulk ILs Tm and Tc disappear while Tg kept constant

45

Controlled-pore glasses (7.5−11.5 nm, 120−140 m2 g−1), post-impregnation Silica surfaces with variable interwall distance from 0 to 15 nm, post-impregnation Vertically aligned MWCNT membrane (internal diameters: 4 nm), post-impregnation Mesoporous carbon (8.8 ± 2.1 nm), post-impregnation ZIF-8 (MOF, 1.12 nm, 1379 m2 g−1, 0.67 cm3 g−1), ionothermal synthesis SnO2 monolith ionogels ( 50 nm.275 The types of pores in these hosts are mainly microporous, mesoporous, or hierarchical microporous/mesoporous; however, the macropores have been less investigated due to their relatively poor confinement effect. Moreover, these porous materials can be structurally ordered with very well-defined pore sizes, or they can be structurally disordered with a wide variety of different pore sizes. Table 1 comprehensively lists the inorganic materials that have been experimentally used as porous supports for the immobilization of ILs. Notably, the choice of the host is of paramount importance for the successful implementation of the concept of nanoconfinement for a given IL as well as their potential applications, because the structure and properties of confined ILs are significantly influenced by the pore structure and the surface chemistry of the hosts, which dictates the nature and extent of the IL−pore wall interactions. For example, the melting points of ILs have been found to decrease when confined in a porous SiO2 matrix;174,175 however, melting points increase when confined in porous conducting matrices such as silver183 and CNTs.48 Therefore, the changes in structures and properties of representative ILs after confinement in various matrices listed in Table 2 have been provided along with detailed confining conditions, such as the type, pore size, surface area, and pore volume of the porous matrix, the infiltration method, and IL loading (wt %), if any. Among all porous materials being investigated, silica gel is one of the most popular matrices to immobilize ILs in confined geometry due to its large surface areas, high thermal and mechanical stability, nontoxicity, and easy synthesis,276 and many studies of nanoconfined ILs in SiO2 gel have been reported so far.50,56,195,196,277,278 Silica hosts are synthesized via the well-established sol−gel technique, and their properties, such as surface area, porosity, and surface chemistry, can be controlled during the production process. Morphological control also grants them versatility in the method of deployment, whether as bulk powders, monoliths, thin films, or embedded in coatings. Silica gels, in general, have a bicontinuous gyroid pore structure with disordered micropores and mesopores. The surface of silica gel bears polar silanol (Si− OH) groups as well as exposed siloxane (Si−O−Si) bonds. The siloxane groups (hydrophobic) are markedly different from completely hydroxylated surfaces (hydrophilic); the chemistry of the silica surface is, therefore, dependent on the content of Si−OH or Si−O−Si groups and, thus, on the degree of dihydroxylation. In general, the Si−OH groups dominate the properties of the silica surface. The reactivity of the Si−OH

group is a drawback as far as the hydrothermal stability is concerned; nonetheless, is an important bonus for the chemical modifications of the silica surface by grafting of organic groups. For example, the inner surfaces of the nanoporous silica can be modified to be hydrophobic by replacing the hydrophilic Si− OH groups with trimethylsilyl molecules via postsurface silanization220,222 or by copolymerization of functionalized silane, such as methyltrimethoxysilane, during the sol−gel process.236,279 The notable difference between the two methods is that postsurface modification would unavoidably influence the pore size and specific surface area of the formed gel network. Other silica-based ordered mesoporous materials, such as SBA-15 and MCM-41, exhibiting large surface areas and well-defined structures, have shown significant potential as porous hosts for the confinement of ILs. For instance, MCM41, which consists of a regular hexagonal arrangement of nonintersecting cylindrical mesopores (space group P6mm) that form a 1D pore system without detectable micropores, is one of the most extensively investigated mesoporous silicas.280 Typically, MCM-41 possesses pores larger than zeolites and the pore size distribution (PSD) can be easily adjusted, with a diameter of 2−6.5 nm. The pore wall thickness is estimated to be ca. 1 nm, and the surface area is generally higher than 1000 m2 g−1. However, MCM-41 is not hydrothermally stable because of the slight wall thickness and the low degree of crosslinking of the silicate units. Similar to the structure of the honeycomb-like MCM-41, SBA-15 developed in 1998 by Zhao et al.259 also consists of a hexagonally arranged channel-type mesostructure with a very uniform and sharp PSD. However, the pore wall thickness of SBA-15 was calculated to be 3.1−4.8 nm, much thicker than that of MCM-41, which results in higher thermal and hydrothermal stabilities.260 Another feature of SBA-15, different from MCM-41, is the existence of the micropores between the silica walls of the ordered mesopores. Evidence for this is obtained by the fact that metal oxides and carbons (nanowires or nanorod arrays) cast from SBA-15 can retain the ordered 2D hexagonal mesoporous structure. The micropores can even contribute up to 30% of the total pore volume in SBA-15. Such a bimodal pore structure consisting of mesoporous channels surrounded by microporous coronas is special in the sense that it provides an opportunity of depositing guest materials in both micropores and mesopores.281,282 Depending on the synthesis conditions, SBA-15 can be obtained in a wide range of pore sizes (3−10 nm), surface areas (500−1000 m2 g−1), and particle morphologies.260,283 Another ordered mesoporous silica, KIT-6, has also been studied for nanoconfinement of ILs.219,221 KIT-6 has a pore topology that can be described as an interpenetrating bicontinuous gyroid network structure with cubic Ia3d symmetry. The mesopore structures of KIT-6 are thus 3D interconnected and built of two continuous ordered channel systems separated by a silica wall. Therefore, KIT-6 is thought to provide a highly open porous host with an easy and direct access for guest ILs, thus facilitating diffusion throughout the pore channels without pore blockage.284 The liquids located in the connecting part of KIT-6 are expected to be less confined than molecules in the cylindrical part. KIT-6 shows high pore O

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decorate, which often requires harsh treatment conditions, such as strong acids or high-temperature oxidation.205,301,302 MOFs, also known as porous coordination polymers, are a novel class of hybrid inorganic/organic porous materials with a large number of uniformly sized micropores consisting of metal centers coordinated to organic ligands to form 1D, 2D, or 3D structures.303−305 These materials have attracted significant interest recently because of their ultrahigh porosity (up to 90% free volume), extra-large surface areas beyond 5000 m2 g−1, regular and accessible pores, and crystalline open structures. Furthermore, the crystal structures of these highly crystalline materials can be characterized extremely well by using diffraction techniques. Distinct from the traditional inorganic nanoporous materials, such as zeolites and activated carbons, the structure and properties of the MOFs, including pore size, surface area, framework topology, and polarity of the inner surface, can be multiply tailored from well-defined molecular building blocks via crystal engineering by appropriate selection of the metal centers and organic ligands. These properties, together with their diverse postmodification options, make MOFs of significant interest for potential applications in gas adsorption, separation, drug delivery, and sensing. Recently, MOFs have also been considered as potential host materials to study and control the dynamics of a range of small guest molecules, such as ILs, via nanoconfinement. The IL confined in a MOF could provide further flexibility in this hybrid material to meet the needs of any specific application because of its tunability by changing cations and anions. Consequently, the resultant hybrid materials open the possibilities for new applications, e.g., CO2 capture and catalysis.306−308 However, the maximum porosity of MOFs is not always accessible in practice, perhaps due to problems associated with the removal of guest molecules from inside the materials, defects in the crystalline structure, and the presence of impurities. Furthermore, most MOFs exhibit rather low thermal stability compared to inorganic materials, and the structures collapse on thermal treatment before the guest molecules are removed, which partially limits their applications. ILs confined in MOFs can be obtained by the in situ ionothermal synthesis, postimpregnation, or “ship-in-bottle” method.

volume and large accessible pores tailored between 4 and 12 nm. In general, ILs can be introduced into the nanopores of silica materials by either the in situ sol−gel method (silica gel) or the post impregnation method (mesoporous silica). Zeolites are crystals of aluminosilicate and aluminophosphate, consisting of microporous cages and channels of molecular dimensions (5 to 12 Å), interconnected by smaller windows. They are also called molecular sieves because operation in the size exclusion regime is possible. Their welldefined pores of subnanometric size allow the desired molecule to pass, while keeping larger molecules outside by selecting among over 200 structures available. Therefore, ILs confined in zeolites are often realized by the “ship-in-bottle” method. Moreover, their adsorption properties can be tuned by adjusting the Si/Al ratio (that determines to a large extent the hydrophobic/hydrophilic property and polar/nonpolar character of the host−guest interactions between the zeolite and the liquid molecules) and the exchange cations of the zeolites.285 Zeolites typically have surface areas of several hundred m2 g−1.206,286 Very different from the surface of porous silica materials (silanol-rich surface), the inner surface of pores in zeolites is lined by siloxane bridges and the only hydroxyl groups correspond to lattice defects or isomorphous substitution of aluminum for silicon. Carbon materials including activated carbons and mesoporous carbons, owing to their hydrophobic surfaces, high surface area, large pore volumes, chemical inertness, and good mechanical and thermal stability, also exhibit significant potential as a type of desirable porous matrices for nanoconfined ILs.204,205,229,233,234,237,246,287−292 For example, ordered mesoporous carbons such as CMK-3 and CMK-5 are attractive hosts because of their well-ordered pore structures and tunable pore diameters in the mesopore range. CMK-3 consists of the amorphous carbon rods arranged in a hexagonal manner; however, CMK-5 is formed by carbon pipes arranged in a hexagonal manner. Each carbon rod and pipe is interconnected to maintain the hexagonal structure. Both CMK-3 and CMK-5 are obtained by the sophisticated nanocasting method from the same original template SBA-15 but by using different loading amounts of the carbon precursor, which results in carbons which are either volume-templated with a rod-like structure (CMK-3, which is therefore the reverse hexagonal structure of ordered SBA-15) or surface-templated with a tubular structure (CMK-5). CNTs, including single- and multiwalled CNTs (S/MWCNTs), are another huge family of carbon materials that have been widely used to encapsulate ILs.48,49,177,215,223,224,293−300 CNTs typically have a well-defined structure with a tube diameter of a few nanometers (i.e., on the order of the size of a few IL molecules) and a large surface areato-volume ratio. The hollow interiors of CNTs are expected to be an ideal 1D confined space for investigation of ILs with more tight confinement than the silica systems and mesoporous carbons. Moreover, CNTs possess uniform frictionless or lowfriction hydrophobic surfaces resulting from the covalent sp2 bonds formed between the individual carbon atoms, which may offer the possibility to largely decouple confinement effects from surface interactions. It is noteworthy that the smooth and inert walls of CNTs may still exert weak π−π or π−cation interaction between their interior surface and ILs. Different from silica systems, ILs confined in carbon materials are generally realized by the post-impregnation method. Moreover, in contrast to the easy pore-surface modification for porous silica, the interior surface of porous carbons is difficult to

7. STRUCTURE AND PROPERTIES OF IONIC LIQUIDS CONFINED IN NANOPOROUS MATRICES 7.1. Porous Silica

7.1.1. Structure. Nanoporous silica is the preferred confining matrix for ILs because of its nontoxicity and easy synthesis. The interaction between the pore wall and IL plays a crucial role in understanding the structure and dynamics of confined ILs. The surface of silica pore may contain surface Si atoms, Si−O groups, and silanol groups (Si−OH). When confined in silica, the IL layer in contact with the wall interacts with one or more of the above moieties, whereas some IL far from the wall exists without interfacial interactions with the surface of SiO2. The interactions between the pore surface of silica and confined ILs containing imidazolium cation have been studied by FTIR spectra and density functional theory (DFT)-based QM calculations.178,188,255,277 Generally, intermolecular hydrogen bonds are formed between the hydroxyl groups of silanols and the anions (e.g., [BF4], [PF6], [EtSO4], and [OcSO4]) of ILs because of the strong electronegativity of the F or O atom. Moreover, depending on the structure of ILs, particularly the constituent anions, several interaction sites were P

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Figure 9. (A) Representative configuration of absorbed [BMIM][PF6] ion pairs on Si(OH)2 (bottom) and SiH2 (top) surfaces. (B) Representative configuration of confined ionic species in the Si(OH)2 (bottom surface) and SiH2 (top surface) interfacial region. (a) [BMIm][BF4]; (b) [BMIm][PF6]; (c) [BMIm][TfO]; (d) [BMIm][NTf2]. Reproduced with permission from ref 313. Copyright 2014 Royal Society of Chemistry.

also significantly affects the surface coverage state of ILs on the inner pore surface, depending on both the type of ILs and the porous support used. When confined in ordered mesoporous silicas such as MCM-41 or SBA-15, a strong anion-dependent effect was observed for ILs [HMIm][NTf2], [HMIm][TfO], and [HMIm][OAc]. More specifically, [TfO]- and [OAc]based ILs exhibited preferential interactions with the pore surface, leading to the formation of homogeneous IL layers. The difference between the two ILs is that the ion deposition on the pore surface appears to be rather random for [HMIm][TfO], whereas [HMIm][OAc] shows a peculiar behavior pointing to a side-by-side deposition of the ions, thus completing the monolayer. Unlike the two ILs, noncoordinating anions such as [NTf2] may lead to aggregate formation rather than the homogeneous distribution of ILs on the surface. By changing the supports from ordered to disordered structure, such as silica gel, the regions of smaller pores are filled first, before the surface is completely covered.214 The confinement of ILs inside porous silica also affects the conformation and even the coordination structure of ions.190,191,200,257,258,314 The [NTf2] anion has a flexible structure and can acquire two different conformations: cis and trans. The trans conformer is more stable than the cis conformer by about 2.2 kJ mol−1, as it minimizes the steric repulsion between the symmetric sulphonyl and trifluoromethyl groups. One of the most characteristic and strongest Raman signatures for [NTf2] at ∼740 cm−1 was reported to be the sum of two components arising from the conformational isomerism of the anion: the cis conformer at 738 cm−1 and the trans conformer at 741 cm−1. Furthermore, the intensity of ion−ion interactions experienced by [NTf2] can be evaluated by the position of this vibrational mode at 740 cm−1. Higher frequencies indicate more strongly bound anions, whereas lower frequencies indicate more loosely bound anions.315 Generally, the cis conformer in the bulk state of ILs is dominant. For example, ∼75% content of trans conformer was determined in the bulk IL [EMIm][NTf2], as confirmed by experimental and computational studies.316−318 However, upon the physical

proposed: (a) The O of SiO2 may interact with the end atoms of the long alkyl chain, (b) the O of SiO2 may interact with the C−H of the imidazolium ring of the cation, and (c) the Si of SiO2 may interact with the anion. In the case of [EMIm][EtSO4], the fourth interaction forms Si−O−S linkages, confirmed experimentally by FTIR spectroscopy.188 This result is interesting because no such linkage has been found for an IL, [BMIm][OcSO4], containing an “almost” similar cation, but a much larger anion of octyl sulfate than ethyl sulfate.277 A possible explanation is that the alkyl chain (tail) of the octyl sulfate anion is too long to reach the pore wall, thus allowing the formation of Si−O−S linkages compared to the small ethyl sulfate anion.188 Owing to the interactions between pore surfaces and ILs, the confined IL molecules near the pore wall usually have a specified orientation to the surface of the pore wall, but those located in the center region are randomly orientated or layered. As evidenced by the atomistic MD simulations of [BMIm]based ILs containing various anions ([BF4], [PF6], [TfO], and [NTf2]) on quartz surfaces,313 the confined ionic groups showed distinct structural and orientational preferences depending on the size and shape of anionic groups and quartz surface charge (surface chemistry), because of the strong electrostatic interactions and hydrogen bonds between the confined ionic species and quartz interfacial groups. The [BMIm] cation attached exclusively onto the negatively charged Si(OH)2 surface, with the imidazolium rings lying preferentially perpendicular to the Si(OH)2 surface, whereas the methyl and butyl chains are oriented toward and elongated along the Si(OH)2 surface, respectively. Four anions in the subsequent anionic layer exhibited random orientations because of the partially screened interactions between the adsorbed anions and the Si(OH)2 surface. When the quartz surface is covered by positively charged silane SiH2 groups, the main adsorbed species are anions, instead of cations, because of strong electrostatic interactions. The main axes of asymmetric [TfO] and [NTf2] anions are perpendicular and parallel to the SiH2 surface, respectively (Figure 9). The pore wall−IL interaction Q

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constraint in silica, both Raman spectroscopy190,191,314 and MD simulations258 indicate that the [NTf2] anions rearrange and preferably adopt the cis conformation in the confined state. The cis/trans ratio increased, and the cis conformer became more dominant with increasing degree of confinement (e.g., achieved by reducing either the pore size (thus increasing the surface-tovolume ratio of the pore)258 or the loading amount of ILs191). The trans-to-cis conformational rearrangement arising from the confinement effect can be attributed to the fact that the anions established a new conformational equilibrium different from that in the bulk, and the cis conformer allows more efficient packing of anions at the silica surface. In this case, the four sulfonyl oxygen atoms of the [NTf2] anions interact with the H atoms of hydroxylated silica surfaces. In fact, when [HMIm][NTf2] was confined in the silica gels by in situ nonhydrolytic sol−gel reaction, a blue-shift of the 740 cm−1 mode was observed for lower IL contents, confirming the strong ion−ion interactions involving the [NTf2] anion.190 The thermodynamic parameters of the trans-to-cis transition were determined by measuring the temperature-dependent Raman spectroscopy of protic IL (1-ethylimidazolium bis(trifluoromethylsulfonyl)imide) confined in silica gels. A slightly lower Gibbs energy change (ΔG) than that of the bulk IL was obtained, indicating a more favorable trans-to-cis transition in the nanoconfined state.314 In contrast to the clear increase in the cis form relative to the trans form for anionic [NTf2] upon confinement, the cation of ILs did not undergo significant conformational change in the confined state. For example, the different conformations of [HMIm] cations differing in the orientation of the hexyl chain around the C5−C6 axis of rotation were still in the same equilibrium in the constrained ionogels as in the bulk IL.191 Because of the surface orientation and/or confinement effect, the ILs entrapped in silica are highly structured into layered ion pairs near the solid surfaces. The IL density near the interface was enhanced relative to bulk, and ions close to a solid surface showed a solid-like (rigid layer-by-layer) structure and slow dynamics. As the distance from the interface increased, the IL molecules at the center of a pore were less affected by pore wall interactions, exhibiting a bulk-like dynamic behavior.238,311 The layering structure of confined ILs was experimentally observed by SFA-based resonance shear measurements together with surface force measurements.238 [BMIm][NTf2] and [BMIm][BF4] confined between two silica surfaces showed obvious oscillatory solvation forces. As shown in Figure 10A, the surface force for [BMIm][NTf2] is almost zero for a surface separation (D) of >10 nm, indicating no interaction between the surfaces. When D was decreased below 10 nm, a repulsive force with some gradually decreased inward jumps was observed; this must be a consequence of the solvation force derived from the IL layering at the surfaces. Similarly, an oscillation-like force profile was also detected for [BMIm][BF4], whereas the threshold D value allowing the appearance of repulsion depressed to 6.9 nm. Given the ion-pair size (0.78 nm for [BMIm][NTf2] and 0.68 nm for [BMIm][BF4]), the presumed numbers of layers at the repulsion onset distance were 13 and 10 for [BMIm][NTf2] and [BMIm][BF4], respectively. The less structured ability of the confined [BMIm][BF4] was probably because of the less crystal-forming ability of the bulk IL.238 The layered structure of confined ILs in porous silica was also calculated using MD simulations.55,258,319−321 As shown in Figure 10B, the density profiles of the cation and anion of [BMIm][NTf2] confined in hydroxylated amorphous silica pores showed a clear structure of layers with a high-density

Figure 10. (A) Normal force scaled with surface curvature radius (F/ R) as a function of surface separation (D) between silica surfaces in [BMIm][NTf2]. Open circles show data on approach, whereas closed triangles indicate data on retraction with D calculated from measured stable jump-out positions and spring constants divided by the radius of surface curvature (k/R). Dotted lines represent van der Waals attractions calculated from Lifshitz theory assuming that the ILs are a continuous medium. Solid and dashed lines indicate stable and unstable regions, respectively, in force profiles. Reproduced with permission from ref 238. Copyright 2010 Royal Society of Chemistry. (B) Atomic density profiles of [BMIm][NTf 2 ] confined in hydroxylated amorphous silica pores of variable width at T = 27 °C and P = 1 atm. The color code is (dark blue) center-of-mass of the ring of the IL cation, (light gray) center-of-mass of the alkyl chain of the IL cation, and (red) center-of-mass of the IL anion. The horizontal gray line indicates the bulk number density for the cations and anions. Reproduced with permission from ref 258. Copyright 2014 Taylor & Francis.

peak close to the walls, followed by weaker oscillations toward the center of the cell, and the number of density layers increased upon increasing the pore width. For example, the silica pore of 2.0 nm accommodated four and three layers of IL cations and anions, respectively, whereas the pore of 4.0 nm accommodated seven and six layers of IL cations and anions, respectively. At each gap, the first layer of cations is closer to the walls than that of the anions, and each ion layer was followed by a second layer of opposite sign; for example, the anions formed layers between the cation layers. A higher density always appeared near the wall surface than in the center region. In the pore center (particularly for large pores), the typical bulk IL density was recovered. The density peaks for the anions for all the pore sizes investigated were sharper and had a greater amplitude than those for the cations. These results R

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interwall distance close to the dimensions of the cation−anion pair (e.g., 2 nm), no middle layer with bulk characteristics exists. If the pore width was 4.0 nm, the bulk-like region was 2.0 nm.258 Moreover, by changing the loading fraction (f) of ILs from 0.25 to 1 in a 4.8 nm cylindrical silica pore, the IL layering structure was strongly affected. For f = 0.25, the pore accommodated a layer of cations and anions adsorbed on the surface, whereas the IL was depleted in the pore center. For f = 1, the pore accommodated four cation layers and three anion layers.319 As in the bulk state, the short-chain ILs confined in silica pores showed interesting mesoscopic structural heterogeneities, depending on the pore structure and pore wall−IL interactions. For example, hydrophobic IL [HMIm][PF6] confined in a series of hydrophilic silica matrices exhibited self-organizing nanoscale segregation behavior, strongly dependent on the ILto-TEOS molar ratio (nIL/nTEOS).97,236 As shown in Figure 12B, with increasing IL content, the SAXS measurements and nitrogen adsorption isotherms showed that the pores in silica varied from radius-various type H2 (nIL/nTEOS = 0.5, pore diameter (DBJH) = 8.0 nm), to broad slit-like type H3 (nIL/ nTEOS = 1.5, DBJH = 23.0 nm), and narrow slit-like type H4 (nIL/ nTEOS = 2.0, DBJH = 5.9 nm). Interestingly, the qm peak in the WAXS profile (Figure 12A), corresponding to the low-Q peak resulting from the nanoscale segregation of the alkyl chains into a charged matrix as observed in the bulk ILs,96,323 first disappeared in the ionogel with nIL/nTEOS = 0.5, and then emerged again for nIL/nTEOS ≥ 1.0. Moreover, the peak gradually shifted to a lower q region with increasing nIL/nTEOS, with the corresponding characteristic length dm (dm = 2π/qm) decreasing from 1.56 to 1.71 nm, as clearly shown in the inset of Figure 12A. Finally, the dm of nIL/nTEOS = 2.0 had the same value as the bulk. The increasing trend of dm as a function of the IL-to-TEOS molar ratio reflects the strong effect of the pore morphology in the silica matrix on the self-organizing nanoscale segregation formed by the confined IL. This can be explained

indicate that the exact structure of the confined IL is driven by the subtle interplay between overscreening and crowding effects.322 Both the theoretical258,321 and experimental321 studies clarified that the cationic molecules are molecularly close to the anionic silica surface because of charge-balancing and electrostatic interactions, even though the preferred orientation of the imidazolium ring and the alkyl chain at the surface is still in debate.258,321 In fact, a formation of different layers of ions with significant density oscillations was observed for the confined ILs irrespective of the variations in pore size and pore shape of silica, and structure of ILs.55,219,321 However, the relative ratio between the adsorbed and bulk layers strongly depended on the pore size258 and the loading fraction of ILs.319 As shown in Figure 11 and suggested by Coasne et al., a cation

Figure 11. Representation of the interface and bulk-like regions for the silica pores with width of (A) 2.0 and (B) 4.0 nm. The size of the interface region is ∼1 nm. The yellow, red, and white spheres are the Si, O, and H atoms of the surfaces, respectively. The blue and green spheres are the atoms of the IL anion and cation, respectively. Reproduced with permission from ref 258. Copyright 2014 Taylor & Francis.

or anion can be considered in the vicinity of the silica surface if it is located within ∼1 nm from the silica surfaces (interfacial region), whereas those located at a position >1 nm belong to the bulk-like zone in the pore center. Thus, in the case of an

Figure 12. (A) WAXS patterns of the [HMIm][PF6] in the bulk state (pure IL) and confined in silica gels. The qm in the X-ray pattern corresponding to the low-Q peak is deduced as a the consequence of the nanoscale segregation of the alkyl chains into charged matrix, while qa and qb mean the contributions resulting from the intra-IL interactions. The inset shows the plot of characteristic length dm (dm = 2π/qm, qm corresponds to the low-Q peak) vs nIL/nTEOS ratio, in which the dotted line means the characteristic length (dm = 1.71 nm) of neat IL. (B) Proposed scheme illustrating the formation mechanism of [HMIm][PF6]-based silica ionogels with variable nIL/nTEOS ratios. Reproduced with permission from ref 97. Copyright 2015 American Chemical Society. (C) Proposed scheme illustrating the arrangement of the IL confined within H2 and H3 nanopores. Reproduced from permission from ref 236. Copyright 2014 American Chemical Society. S

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materials can be understood by NMR relaxation measurements for 1H spin−lattice relaxation times (T1) and 1H pulsed-field gradient NMR (PFG NMR),221 respectively. The NMR signals of ILs usually become broad upon confinement, indicating slower dynamics. However, most studies indicate that the IL in ionogels has (at least partly) a near-liquid mobility.191,203,216,238 Unlike the bulk IL, the dynamic properties of confined ILs either increase55,222,320 or decrease191,216,219,221,258,319 with the changing trend and extent depending on the molecular details of the confining space and IL. The loading fraction, affected by the pore volume and loading amount of ILs, also has a significant effect on the dynamic behavior of ILs confined in silica.191,221,258,319,321 As evidenced by the simulated results of the dynamical properties of [BMIm][NTf2] confined in silica containing a cylindrical pore of 4.8 nm (Table 3),319 it was

by the repulsive interactions between the IL and the pore wall. First, the absence of a qm peak in the ionogel with nIL/nTEOS = 0.5 can be attributed to their narrow H2 nanopores with a rough surface in the silica matrix. In this case, the IL molecules tend to arrange along the orientation of the surface topography through favorable interactions and form a close packing structure with different orientations and basically hinder the formation of nanoscale segregation (Figure 12C). Second, the appearance of the qm peak in the narrow slit-like pores (H4-like pore) of ionogel with nIL/nTEOS = 2.0 mainly arises from the contribution of the ordered structural segregation formed by the segregated IL molecules. When an excess amount of IL was embedded within the silica nanopores, segregated IL molecules are believed to coexist with the silica microspheres and confined IL molecules because of the repulsive interactions between the silanol and anionic [PF6] groups. At last, in the ionogels with nIL/nTEOS = 1.0 or 1.5 with much broader pores (H3 type) than the H2 pore, the confined ILs near the pore wall also closely arrange along the rough surface and afford a random arrangement along the direction normal to the wall surface, similar to the case of H2 pores. However, the other IL molecules at the pore center are reoriented along the long-axial direction of the pore via inter-IL interactions, resulting in the formation of an ordered arrangement of IL molecules, as shown in Figure 12C. The measured small dm values indicate that the close packing of the confined IL in the board H3 nanopores is mainly driven by the repulsive interactions between [PF6] and silanol, imposing a force to press the confined [HMIm][PF6] IL.97 Notably, similar repulsive interactions and thus longranged ordered mesoscopic structural heterogeneities of ILs were also achieved when hydrophilic IL [BMIm][BF4] molecules were entrapped within the methylated silica matrix containing hydrophobic H3 nanopores.236 MD simulations were conducted to provide more details on the lateral distributions of confined ILs. This showed that the [EMIm][Br] film constrained in two kaolinite plates can form 2D ordered structures arising from the surface-induced ionic orientational preference and driving force from confinement. Depending on the degree of confinement, more specifically, whether the two confining walls are allowed to be free to move during the simulation, ILs can pack into a 2D ordered structure with an apparent coexistence of liquid−solid phase, or liquidlike structure identified by their structural factors. The lateral structures on the hydroxyl-functionalized kaolinite surface are different from that of ILs adsorbed on one confining wall; therefore, the ILs show an apparent square symmetry. The flexible hydrogen bond formed between the [Br] anion and the surface hydroxyl group is important, breaking down the electrostatic network in ILs at a fixed space and resulting in a highly mobile liquid-like structure.324 7.1.2. Dynamics. Because of the potential applications of confined ILs, ranging from lubrication to electrochemistry, their dynamic properties should be evaluated. Studies have mainly focused on the rotational dynamics,219,221 translational dynamics (self-diffusion),55,191,219−222,319,321 shear dynamics,325 viscosity,258 conductivity,172,175,222,258,319 lubrication behavior,238 and dielectric relaxation.311 These dynamic properties can be evaluated by MD simulations or measured by experimental techniques, such as NMR, BDS, SFA, and dielectric relaxation measurements. For example, diffusion coefficients can be obtained from the slope of MSD for delay times longer than 1 ns,55,258 whereas the effect of confinement on the rotational and translational motions of ILs in porous

Table 3. Dynamical Properties of Bulk and Confined IL [BMIm][NTf2] (silica containing cylindrical pore of 4.8 nm) Obtained by MD Simulationsa Systemb

Dcation (10−8 cm2 s−1)

Danion (10−8 cm2 s−1)

σ (mS cm−1)

f = 0.25 f = 0.50 f = 1.00 Bulk IL

0.3 4.3 8.2 39.7

0.2 3.5 5.0 35.2

4 7 14 15

Data from ref 319. bf, D, and σ denote the filling ratio, the selfdiffusivity, and the ion conductivity, respectively. a

generally concluded that, at lower pore loadings, the rotational and translational motions of ions are very low, because most of the adsorbed ions strongly interact with the silica surface and exhibit restricted dynamics. The ion conductivity of ILs is also very low because ion conduction occurs through the surfaceadsorbed IL layer. Upon increasing the loading fraction, the dynamics of the confined IL became faster because the IL fills the pore center, and a liquid path similar to the bulk IL is formed. Particularly, the ion conductivity at a very high pore loading is close to that of the bulk IL, whereas the selfdiffusivities of both cations and anions are still very low with respect to the bulk. A large increase in the conductivity upon increasing the loading fraction with respect to that in the selfdiffusivity was attributed to the collective dynamical property of conductivity, which is probably a combination of the subtle contribution from the ions close to the pore wall and a bulk-like contribution arising from the ions present in the pore center. Therefore, the ion conductivity of nanoconfined ILs is mainly related to the motion of the ions present in the pore center, whereas the self-diffusivity, which is a single property, is related to the motion of each individual ion. The self-diffusivity is thus quantitatively more affected by the confinement and surface forces. Notably, at all pore loadings, the dynamics of the cation upon confinement is faster than that of the anion despite its bulkier structure, similar to the case of bulk IL.326−328 The faster dynamics of the cation has been attributed to its homogeneous electrostatic field, resulting in a weaker interaction with its environment. The flexible alkyl chain and planar structure of the cation backbone of the imidazolium species also contribute to its faster dynamics. In contrast, the anion, which creates a heterogeneous field with both positive and negative regions, strongly interacts with its environment, thus slowing its dynamics. In addition to the pore loading, the dynamics of confined ILs is also a function of temperature. Generally, with increasing temperature, the rotational correlaT

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tion time decreases, and the diffusion coefficients increase. However, at the same time, the loading fraction still played an important role.219−221,321 For example, when [BMIm][NTf2] was confined in mesoporous silica KIT-6 (9.5 nm, 783 m2 g−1, 1.11 cm3 g−1) at lower loading fractions, the change in the temperature from 7 to 87 °C did not significantly change the diffusion coefficient of the cation, indicating that the IL molecules in direct contact with the pore walls are relatively insensitive to temperature variation. However, the diffusion coefficients of ions located in the pore center are high and exhibit a higher temperature dependence compared to the layer closest to the silica walls.219 The observed two different regimes derived from the diffusivity, i.e., one fraction close to the pore wall and the other fraction in the pore center, were also reflected by the corresponding dielectric relaxation of the confined IL. [BMIM][PF6] confined in a nanoporous silica matrix exhibited two dielectric relaxation peaks at frequencies of fr1 and fr2, whereas the bulk IL exhibited only one relaxation peak.311 The fr2 has been attributed to the ILs in the bound layers formed near the pore wall surface, whereas fr1 has been attributed to the IL molecules in the central zone of the nanopores. Their relative values depend on the pore size of the confining matrix. For smaller pores with a diameter of 14.8 nm, fr2 is smaller than fr1 because of the pore wall “surface interaction effect”, whereas, for larger pores with a diameter of 23.2 nm, fr2 is greater than fr1 because of “steric hindrance caused by confinement effect”. Iacob et al. experimentally determined the self-diffusivity coefficient of the ILs [HMIm][PF6] and [BMIm][BF4] confined in porous silica membranes (pore size = 7.5−10.4 nm) with or without pore-surface modification via postimpregnation.220,222 The diffusion coefficient of the hydrophobic IL [HMIm][PF6] in the hydrophilic and untreated silica membrane decreased by about one decade compared to the bulk IL, because of the hydrogen bonds between the IL and the silanol groups. However, the silanization of the surface of the silica pore reverses this trend. A remarkable increase in the diffusion coefficient of the IL in nanopores with only small deviations from the bulk values was observed upon the silanization of the silica membranes with hexamethyldisilazane. The differences in the diffusivity in the pores compared to the bulk state were explained by the contribution from the layer of reduced mobility at the pore−matrix interface, in addition to the bulk response.220 The slower dynamics for the confined [HMIm][PF6] IL in silica is also consistent with the recent experiments that showed that the translational diffusion of hydrophobic ILs confined in hydrophilic silica mesopores is >10 times lower than that in the bulk.319 Interestingly, when the hydrophobic IL [HMIm][PF6] was replaced with a hydrophilic IL [BMIm][BF4], the translational diffusion exhibited a distinct result. Figure 13 shows the temperaturedependent diffusion coefficients (obtained by BDS and PFG NMR) of [BMIm][BF4] in three silica matrices with different pore sizes of 7.5, 9.5, and 10.4 nm. Under the conditions of geometrical confinement, a change from a Vogel−Fulcher− Tammann (VFT) into an Arrhenius-like thermal activation was observed. At higher temperatures, the measured diffusion coefficients coincided for both the bulk and confined ILs. However, remarkable increases were observed in the lowtemperature regime under nanometric confinement conditions. For nanopores with the smallest mean diameter (7.5 nm), the diffusion coefficients are more than two decades higher than their corresponding bulk values (Figure 13). Moreover, unlike

Figure 13. Diffusion coefficients versus inverse temperature as obtained by PFG NMR (crossed open symbols) and BDS measurements (filled symbols) for bulk and confined [BMIm][BF4] as indicated. The lines denote fits by the empirical Vogel−Fulcher− Tammann (continuous) and Arrhenius (dotted) equations. Inset: the chemical structures of untreated and silanized SiO2 membranes. Reproduced with permission from ref 222. Copyright 2012 Royal Society of Chemistry.

the fact that the decrease in the diffusivity of [HMIm][PF6] in silica mesopores was reversed by silanization,220 the diffusion coefficients in [BMIm][BF4] remained unaltered by silanization ([BMIm][BF4]-7.5 nm vs [BMIm][BF4]-silan. 7.5 nm). The increase in the ionic mobility can be attributed to changes in ion packing, particularly at lower temperatures, because of the reduction in the density of ILs in small pores, also consistent with the results of simulations.320 Similar to the diffusivity of ILs, the corresponding DC conductivity of [BMIm][BF4] in silica mesopores showed a VFT-type thermal activation, with a systematic increase in the conductivity with decreasing pore diameters at lower temperatures.222 In addition to the high conductivity of silica-confined ILs obtained by post-impregnation, ionogels obtained via the in situ sol−gel method were reported to show a conductivity of the same order of magnitude as bulk IL by virtue of interconnected porosity. For example, conductivities of about 3 × 10−2 S cm−1 and 8 × 10−2 S cm−1 at ∼227 °C were observed for [BMIm][NTf2] confined in silica gels (12 nm, 780 m2 g−1, 1.5−3.5 cm3 g−1) and in the bulk state, respectively.175 The viscosity of ILs also increases upon confinement, similar to molecular liquids such as water.329 The viscosities of [BMIm][NTf2] and [BMIm][BF4] confined between two silica surfaces have been quantitatively evaluated by SFA-based resonance shear measurements; they were found to be a function of surface separation distance based on the assumption that the viscosities at larger separations are equal to the viscosity of the bulk IL.238 The viscosities of confined ILs were 1−3 orders of magnitude higher than that of the bulk IL and increased exponentially as the ILs were gradually confined within the range of the IL layering. Note that the highest viscosity detected at the closest separation distance (ca. 102 Pa s) was still much lower than the viscosity estimated for bulk ILs at the calorimetric glass transition (ca. 1012 Pa s).62 The MD simulations further showed the distinct shear dynamics between the two ILs confined between two hydroxylated silica surfaces. The structure of the liquids in nanoconfined spaces determined their dynamical properties at the molecular level. Figure 14B shows the equilibrium arrangement of two nanoconfined ILs ([BMIm][NTf2] and [BMIm][BF4]) and the perturbation U

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[EMIm][X] ILs (X = [DCA], [EtSO4], [SCN], and [TfO]) had lower melting points when confined in mesoporous silica monoliths. The melting points of [EMIm][DCA] and [EMIm][TfO] were depressed by about 13 and 8 °C, respectively.203 Interestingly, the crystalline−smectic and smectic−isotropic liquid transitions clearly observed for bulk ionic liquid crystal (ILC) [C16MIm][BF4] on both cooling and heating processes disappeared after confining into mesoporous silica gels with a pore size of 3−12 nm, indicating that the thermotropic mesophase of ILC was depressed. Furthermore, the specific heat capacities of small molecule ILs in the same confined system remarkably decreased.50 Studies have shown that the phase behavior of confined ILs was related to the cationic310 and anionic47,203 structure and pore size.174 For example, the anionic size significantly affects the melting point. [EMIm][OsSO4] containing a large anion confined in a nanoporous silica gel matrix demonstrated a much larger depression in melting point (ΔTm = Tm(bulk) − Tm(pore), 52 °C) than those reported earlier for many other ILs with relatively smaller anions.277 The pore size also significantly affects the melting points of confined ILs, and the melting point depression for ILs confined in porous silica glasses showed a linear relationship with the inverse of the mean pore diameter.174 In contrast to the pore size, the surface modification of silica pores by hydrophobic methyl groups without a significant change in the PSD only slightly changed the phase transition.47,174 Although several models and theories, such as the Gibbs−Thomson equation47 and simple mean-field theory,174,255,277 explain the depression of the melting points of ILs confined in porous silica, the variation in the first-order transition for confined ILs is still not fully understood and requires further investigation. At the molecular level, the changes in melting temperatures can be attributed to the combination of the strong interaction of ILs with the silica wall and the confinement effect. At the interface with a solid surface, additional driving forces operate for liquid ordering, such as the screening of surface charges and template effects. Shi et al. also attributed the depression of the melting points of ILs to the total nonbonded energy; for example, this value for the silica-pore-confined [HMIm][NTf2] was about 5−12 kJ mol−1 smaller than that for the bulk IL.320 Instead of the depression or disappearance of melting points, Wu et al. found that the melting points of imidazolium ILs significantly increased when they were entrapped with mesoporous silica (2−6 nm) using ultrahigh vacuum (1 × 10−5 Pa).176,200,225,310 Compared to evaporable water and organic molecules, ILs have extremely low vapor pressure and relatively large molecular weights, enabling the intentional confinement of ILs to the nanopores of silica under vacuum. As shown in Figure 15, the loading of ILs at a high vacuum can remove all the compressed gases from the cavities of porous SiO2, resulting in fully loaded nanopores, as confirmed by HAADF-STEM. The ILs existed mainly with the solid (SiO2)− liquid (IL) interface ensured a close packing of the confined IL, leading to a significantly increased Z value (the number of nearest liquid neighbors according to the simple mean-field theory332) and thus the melting point. In contrast, the filling of IL into SiO2 at atmospheric pressure resulted in the immobilization of ILs mainly onto the surface or partially filled nanopores. For such a complex solid−liquid−gas interphase, the presence of compressed gas inside of the pores leads to a small Z value, which, in turn, leads to a decrease in the melting point. Depending on the pore size and particularly the IL

Figure 14. Schematic representation of the shear dynamics for a fluid ordered in (A) neutral layers as [BMIM][NTf2] and (B) alternating charged layers such as [BMIM][BF4]. Blue and red spheres represent the anions and cations, respectively. The dashed square marks the unit cell of the liquid. Reproduced with permission from ref 325. Copyright 2014 Royal Society of Chemistry.

introduced when shearing the top surface. In both the cases, boundary layers are attached to their respective surface. When [BMIm][NTf2] was sheared, larger molecular fluctuations in the inner layers were required to stabilize the system, resulting in irregular dynamics. In contrast, the alternating charged layers in [BMIm][BF4] stabilized the system through smaller oscillations, and the layers appeared to shear on top of each other in a laminar fashion.325 7.1.3. Thermal Properties. 7.1.3.1. Phase Transition. An understanding of the phase transition of ILs in confined silica is of paramount importance from both the scientific and application perspectives. The typical phase transitions of confined ILs comprise Tg (the compound passed from the glass state to a “‘subcooled liquid”’ phase), Tc (exothermic), and Tm (endothermic), which are generally disclosed by DSC analysis on heating after a fast cooling. When ILs are confined in silica matrices, they show anomalous thermal phase transitions. However, the DSC measurements sometimes may not reveal any thermal events in the investigated temperature range because of the poor sensitivity of the method for nanoconfined ILs if the amount of ILs in the pores is very small. In particular, glass transitions may not be detected because the enthalpy change connected with this transition is spread over a wide temperature range.40,330 In this case, 1H NMR experiments can be considered as alternative methods to obtain information about the phase behavior of the IL confined in the pores of the silica.174,217 The obvious difference between the confined and bulk ILs is the dramatic change in melting point. The effects of nanoconfinement on the melting point, either decrease175 or increase,176 are contradictory among different reports. Many researchers reported that the melting endotherms of nanoconfined ILs were detected at much lower temperatures and sometimes disappeared compared to the bulk state, resulting in liquid-like behavior below the solidification temperature of the neat IL. 47,50,172,174,175,178,203,255,257,277,331 For example, [BMIm][NTf2] confined in silica gels obtained via sol−gel chemistry showed a depression of the melting point of [BF4] > [NTf2], while it was less affected by the alkyl chain length on the cation as well as the pore structure of silica. For example, the thermal decomposition of [BMIm][PF6] confined in either silica gels obtained via the in situ sol−gel method or mesoporous silica obtained via the postimpregnation method starts at a much lower temperature than that of the bulk IL. As shown in Figure 16A, after confining [BMIm][PF6] in mesoporous SiO2 with

Figure 15. Schematic diagram of the change in melting point of ILs entrapped by mesoporous silica under vacuum conditions versus that at atmospheric pressure. Reproduced from ref 176. Copyright 2012 American Chemical Society.

Figure 16. (A) TGA curves of (a) mesoporous SiO 2, (b) [BMIm][PF6]/SiO2, (c) [BMIm][PF6]@SiO2, and (d) pure [BMIm][PF6]. Reproduced from ref 176. Copyright 2012 American Chemical Society. (B) TGA of bulk and confined [EMIm][NTf2] (CIL-1, CIL-2, and CIL-3 having 0.3, 0.5, and 0.7 mol % of IL, respectively) in silica gels (2.2−12.1 nm, 576−634 m2 g−1, 1.04−2.23 cm3 g−1). Reproduced from ref 257. Copyright 2015 American Chemical Society.

structure, a significant increase in the melting point up to 201.6 °C was observed for 35.9 wt % [BMIm][PF6] in mesoporous silica with a pore size of 2−6 nm.176 The high melting point and thus the significant solid-phase property of ILs under confinement may bring new opportunities for future applications, which are usually difficult to obtain under traditional conditions because of the presence of the glass state. The unexpected increase in the melting point under highvacuum conditions appears to be specific to imidazolium ILs, and complex π−π stacking and hydrogen bonding are important, because tributylhexadecylphosphonium bromide ([P44416][Br]) confined in ordered mesoporous silica nanoparticles showed an opposite change in the melting point; that is, Tm was depressed by 8 °C.310 The effect of confinement on the crystallization kinetics of ILs in the silica matrix has been determined by an isothermal technique using DSC.257,333 The crystallization kinetic parameters, such as relative crystallinity, crystallization half time, crystallization rate constant, and Avrami exponents, depend on the amount of IL and isothermal temperatures. A comparison study between the bulk ILs, such as [EMIm][BF4] and [EMIm][NTf2], and ionogels showed that confinement reduces the dimensionality of crystallization from 3D (for bulk IL) to 1D and also slows down the crystallization rate of IL upon confinement. 7.1.3.2. Thermal Stabilities. Compared to the intensive studies related to the structure and dynamic properties of ILs confined in silica, the role of confinement on the thermal stability of IL has been less investigated.172,174−176,178,189,256,257,312,334,335 TGA is often carried out in air or an inert atmosphere of nitrogen to understand such properties. A general conclusion drawn from the previous work is that the thermal decomposition of most commonly used imidazolium ILs confined in silica occurs in a much easier way than the bulk ILs. More specifically, the thermal stabilities of confined ILs are mainly controlled by the type of paired anions. The thermal decomposition temperature decreased in

pore size of 2−6 nm through high-vacuum impregnation ([BMIm][PF6]@SiO2), its onset temperature of thermal decomposition was 258 °C, much lower than 304 °C for the corresponding bulk IL. Interestingly, unlike the one-step decomposition for the bulk IL, [BMIm][PF6]@SiO2 clearly exhibited two main weight-loss steps. The first step was attributed to the breaking of end group alkyl chains of the imidazolium ring, and the second step was attributed to the total decomposition of IL. Without exception, the decreased thermal stability and the two-step weight reduction are related to the interaction of the pore wall with the confined IL because [BMIm][PF6]/SiO2, a sample similar to [BMIm][PF6]@SiO2 but obtained under normal pressure instead of a high vacuum (in this case, the IL was immobilized on the surface of nanoparticles or the entrances of mesopores), also exhibited a similar decomposition process (Figure 16A). The only difference between these two samples was the further depressed onset decomposition temperature of [BMIm][PF6]/SiO2 (221 °C).176 The onset decomposition temperature of [BMIm][PF6] decreased to ∼170 °C when confined in silica gel.312 In the case of a [BF4]-based IL, such as [EMIm][BF4], a decrease in the onset temperature of ca. 30−70 °C accompanied by a single-step decomposition was observed.178,189 In contrast to [PF6]- and [BF4]-based ILs, [NTf2]-based ILs showed no change or even improved thermal stabilities upon confinement (Figure 16B). Very interestingly, when [BMIm][NTf2] was confined into monolithic silica gels by an IL-containing nonhydrolytic sol−gel process, the decomposition of [BMIm][NTf 2 ] within the silica gel underwent a single-step decomposition and started at around 400 °C, about 50 °C higher than the corresponding IL in the open space.335 However, both the loading fraction and the surface W

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Figure 17. Packing arrangement of [Me3NC2H4OH][ZnCl3] inside SWCNTs of different diameters. The observed (left side) and simulated (right side) HRTEM images are comprised of four typical morphologies of ((A) single-chain, (B) double-helix, (C) zigzag tubes, and (D) random tubes). The calculated tube diameters for the single-chain, double-helix, zigzag tubes, and random sizes are 1.2, 1.4, 1.8, and 2.1 nm, respectively. Reproduced from ref 223. Copyright 2009 American Chemical Society.

7.2. Carbon Materials

modification of silica did not significantly affect the thermal stability of nanoconfined ILs.172,174,257,335 7.1.4. Other Properties. Because of the strong interaction between ILs and the pore wall of the silica matrix, and the conformational changes resulting from the confinement effect, nanoconfined ILs in some cases showed obvious changes in the spectroscopic properties, especially the fluorescence50,196 and vibrational spectra (FTIR255,257,277,278 and Raman190,191,196,278,314). For example, when [DCA]-based ILs such as [EMIm][DCA], S-ethyltetrahydrothiophene dicyanamide ([ETh][DCA]), or S-butyltetrahydrothiophene dicyanamide ([BTh][DCA]) were confined in mesoporous silica with pore sizes of 3.5−8 nm, significant changes in the peak positions and intensity of optical fluorescence were observed as compared to the corresponding bulk ILs. The enhancement of fluorescence emissions was largely dependent on the IL loadings. For example, when increasing the IL loading from 5 to 60 wt %, it was the 15 wt % [EMIm][DCA] and 23 wt % [ETh][DCA] confined within mesoporous silica gel that give rise to the strongest emissions, with greatly enhanced emission intensity about 200 and 70 times, respectively, stronger than that of the bulk ILs. The unexpected result was found to be specific to the [DCA] anions, since ILs containing other anions, such as [EMIm][BF4], did not show any enhanced fluorescence emission after being confined into the mesoporous silica gel. In contrast, the fluorescence emissions have been less affected by the cationic structure ([EMIm][DCA] vs [ETh][DCA]), including the alkyl chain length ([ETh][DCA] vs [BTh][DCA]).50,196 In addition to nanoconfinement-induced fluorescence enhancement, vibrational spectra were also found to be often modified upon confinement in a silica matrix. In particular, abnormal FTIR and Raman spectra was observed when ILs ([BMIm][PF6] and [BMIm][BF4]) were confined in nanoporous silica gel, while the phenomena were changed with the pore size of the silica gel matrix. For example, the abnormal result was found for [BMIm][BF4] when the pore size is larger than 11 nm while it was still maintained for [BMIm][PF6] even when the diameter exceeded 30 nm.278 An obvious change in Raman spectra was also found for [NTf2]-containing ILs, probably because the relative ratio between the two flexible conformations of the anion, i.e., cis- and trans-conformers, was affected by the confinement effect,190,191,314 as mentioned above.

7.2.1. Structure. Confinement of ILs within CNTs is particularly interesting due to their very peculiar, well-defined geometries and surfaces. Despite their hydrophobic character, the initially empty channel of CNTs can spontaneously be filled with surrounding ILs in the reservoir to reach a saturated state. Therefore, experimentally, one can easily obtain the composite material by sinking the opened CNTs into ILs. The spontaneous entrance of ILs into the channel of CNTs has been detailed by MD simulations.49,294,298,299 When confining the hydrophobic IL [BMIm][PF6] into SWCNTs ((9, 9) CNT and (10, 10) CNT), it was found that cations were always faster to enter into the channel of a CNT than anions in the whole filling process. This was mostly due to favorable Lennard-Jones dispersion interactions (which actually represent the electronic attraction of CNT π-electrons).294 The difference in entering speeds of cations and anions can be quantified by free energy changes. In the case of (9, 9) CNT, a single cation, driven by a favorable free energy of −27.3 kJ mol−1, can enter the channel of a CNT from the bulk liquid phase, while the corresponding free energy for a single anion is 32.1 kJ mol−1, which makes it very difficult for the anion to spontaneously enter the channel of this CNT. It seems that the anion prefers to be in the bulk phase. However, instead of a single cation, at the end of the filling process, both cations and anions were indeed located in CNTs in same numbers. Therefore, a more favorable way of understanding the insertion process of ionic pairs may be to consider the scenario where a cation “pulls” an anion to enter the channel of the CNT from the bulk liquid phase along with the anion. The calculated favorable free energy for a pair of ions ([BMIm][PF6]) is negative (−27.6 kJ mol−1), which may also explain the fact that energetically it is more favorable for ions to enter the nanotube as cation−anion pairs. Comparing both CNTs ((9, 9) CNT and (10, 10) CNT), it can be found that the larger diameter CNT (10, 10) can accommodate more ions and also provides faster filling speed for the same number of ions entering the channel.294 Similar to hydrophobic ILs, hydrophilic ILs, such as [EMIm][Cl], can also be easily introduced into CNTs.215,297 The results from both MD simulations299 and experiments223 have shown that the smallest CNT diameter, capable of being filled by commonly used ILs, is 0.95 nm. ILs do not enter spontaneously the nanotube channel below this threshold value under normal conditions. In fact, the threshold value agrees well with the results obtained from X

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theoretical calculations.336 Since, based on the bond lengths and van der Waals radii of the atoms constituting ions, the van der Waals surface of most studied ILs can be approximated by a sphere of 0.7 e nm−2 and even faster than Na+ ions in bulk water when q is more positive than 0.9 e nm−2.361 Comparison of the ionic diffusion coefficients and ionic densities revealed that the fast (or slow) diffusion of ions inside a micropore during charging is correlated strongly with the dense (or loose) ion packing inside the pore. The dynamics of ILs confined within mesoporous or hierarchical microporous−mesoporous carbons have been measured by experimental techniques such as QENS and neutron spin echo (NSE).54,233,234,237 Quantitative information on the dynamics of ILs [BMIm][NTf2] and N,N,N′,N′tetramethylguanidinium bis(perfluoroethylsulfonyl)imide ([H2NC(dma)2][NTf2]) confined within mesoporous carbon having 8.8 nm cylindrical pores was obtained by QENS experiments in the temperature range 290−350 K.234,237 [BMIm][NTf2] in both bulk phase and confined state displays at least two dynamical processes in the time range accessible by AC

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Figure 24. Temperature dependence of the diffusion coefficients in bulk (filled symbols) and confined (open symbols) (A) [BMIm][NTf2] and (B) [H2NC(dma)2][NTf2] in mesoporous carbon consisting of 8.8 ± 2.0 nm cylindrical pores measured using QENS. The inset in (B) shows the molecular structure of [H2NC(dma)2][NTf2]. Reproduced with permission from ref 234. Copyright 2012 IOPscience. Reproduced with permission from ref 237. Copyright 2013 IOPscience.

interaction potentials of IL ions with the mesopore and micropore walls are 154 and 247 kJ mol−1, respectively. These results suggest that although the ions in bulk, mesopore, and micropore remain coordinated with at least their close-contact counterions, the IL−micropore interaction disrupts the bulk IL structure to a greater extent than the IL−mesopore interaction. Strong micropore−IL interactions result in a less-coordinated IL within the micropores than in the bulk fluid, which, in turn, affects the observed dynamics. Unlike the case of a slit graphitic pore wherein the distance between the pore walls is constant and the confinement effects are relatively uniform throughout the pore, the dynamics of ILs confined inside CSAC are more complex. The dynamics of [EMIm][NTf2] confined within CSAC with irregularly connected pores of different sizes (0.75, 0.93, and1.23 nm) have been comparatively studied with respect to the slit graphitic nanopore by MD simulations.345 In analogy to the previous studies,344,351 the ions in bulk systems move significantly faster than those inside the pores of CSAC model material or the slit graphitic pore. Similar to what was observed in slit pores, the dynamics of the confined IL exhibit significant spatial heterogeneities due to the complex pore geometry of the CSAC model material, wherein the ions near the pore walls of the CSAC model material also move more slowly than the ions which are in the center of the pore and farther from the walls. However, the dynamics of the IL inside the CSAC model materials were significantly faster than those observed inside the slit pores of the same average size. In addition, different from the fact that the dynamics of ions become monotonically faster with increasing size of the slit pore, an increase in average pore size in the CSAC model materials results in a nonmonotonic (but small) variation in the dynamics of confined ions. These observations are probably due to the irregularly connected pores, resulting from the carbon sheets of different sizes and shapes in the CSAC model, which, in turn, result in nonuniform and smaller confinement effects than those observed in a slit graphitic pore model of the same nominal size. The surface curvature and hexagonal arrangement of the carbon rods and nanopipes in CMK-3 and CMK-5, respectively, are expected to result in nonuniform confinement effects. The smaller MSDs observed for both confined [MMIm][Cl] and [EMIm][NTf2] at the same temperature suggested that, irrespective of the IL structure, all ions inside CMK-3 and CMK-5 model materials exhibited slower dynamics than those observed for the bulk

the QENS experiment. The slower one of the two processes is the long-range translational diffusion process, whereas the faster process is related to the confined diffusion, which is spatially localized on the length scale of about 0.1 nm. Remarkably, as shown in Figure 24A, the diffusivity values which characterize both processes have been found to be comparatively higher in the confined state, at least at lower temperatures. At higher temperatures, the difference in the diffusion coefficients between the bulk and confined states decreases or disappears.234 The enhanced diffusivity for a confined IL compared to the bulk state appears to be a common behavior irrespective of the specifics of the IL. Changing the model IL from [BMIm][NTf2] to a protic IL [H2NC(dma)2][NTf2] also gives rise to a similar two diffusion process consisting of both the localized and long-range translational diffusive motions of highly mobile cations. However, more interestingly, the faster component exhibits an unusual trend of slowing-down as the temperature is increased, and this trend continues until the localized molecular diffusivity is reduced to the corresponding bulk IL value at the temperature 350 K.237 The rate of decrease in the higher diffusivity is more pronounced within the range 57−77 °C (Figure 24B). This behavior is highly unusual, as diffusivity normally increases with an increase in temperature, and was attributed to be resulting from the temperatureinduced structural changes of the IL within the pores. It was suggested that, at low temperatures, the IL formed a layered structure near the pore walls, whereas, at high temperatures, the layering dissipates and the structure of IL becomes bulk-like. Using the NSE technique and MD simulation, the IL dynamics inside the micropores have experimentally been decoupled from the better understood dynamics within a mesopore. When [BMIm][NTf2] was confined within mesoporous (5.8 nm cylindrical mesopores) and hierarchical microporous−mesoporous carbons (7.8 nm cylindrical mesopores and 0.75 nm slit-shaped micropores), a fraction of the dynamics, corresponding to IL inside 0.75 nm micropores located along the mesopore surfaces, was found to be faster than those in direct contact with the walls of 5.8 and 7.8 nm mesopores.233,362 A comparison of the cohesive energy of [BMIm][NTf2] with the pore wall−IL interaction potentials may help to explain the faster short-range motions inside the micropore. The cation−anion bond strength calculated for a single ion pair of [BMIm][NTf2] is 305 kJ mol−1,363 and the standard molar enthalpy of vaporization for this IL is estimated to be between 136 and 174 kJ mol−1.364 In contrast, the AD

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systems.289,290 The detailed dynamics depend upon the location of ILs inside these ordered mesoporous carbons. Generally, the ions inside the nanopipes of CMK-5 exhibit the slowest dynamics, and the ions closer to the carbon surfaces in CMK-3 and closer to the outer surface in CMK-5 move slower than the ions that are farther away from these surfaces. The simulated results suggested that both the ILs examined here had faster dynamics when they were adsorbed outside the nanopipes in CMK-5 (i.e., with IL inside the nanopipes) than when these were in CMK-3 materials of the same pore size. Furthermore, the presence of IL adsorbed on the outer surface of a nanopipe in CMK-5 was found to affect the dynamics of an IL adsorbed inside the nanopipe, but the effects were found to be IL-specific. [MMIm][Cl] has slower dynamics when it is inside an isolated nanopipe than when inside the nanopipes in CMK-5. On the other hand, [EMIm][NTf2] moves faster when it is inside an isolated nanopipe than when it is inside the nanopipes in CMK-5 (i.e., with IL adsorbed outside the nanopipes). 7.2.3. Thermal Properties. 7.2.3.1. Phase Transition. Different from the depression of the melting point, as observed in most silica-confined ILs, the melting points of ILs often increase after being confined within most carbon materials, such as CNTs. This is largely due to the reason that the ions of disordered arrangement in the bulk phase would fill into the CNT and form an ordered arrangement inside the channel of the CNT, thus reducing the melting point. The structural transformation results in an anomalous phase behavior from liquid to high-melting-point crystallites. Wu et al.48 demonstrated for the first time the dramatic transition of IL [BMIm][PF6] from a room-temperature liquid to a stable, high-melting-point crystal, which possessed melting points of above 200 °C when confined within MWCNTs having average internal and external diameters of 5−10 nm and 40−60 nm, respectively. The presence of a high-melting-point crystal of ILs inside CNTs was evidenced by a comparative TEM study before and after the confinement of ILs within MWCNTs (Figures 25A and 25B). Clearly, the empty cavities of MWCNTs for IL@MWCNTs are filled with IL. The selected area electron diffraction (SAED) pattern (inset of Figure 25B) indicated the formation of polymorphous [BMIm][PF6] crystals. The crystallization behavior of confined IL was further confirmed by XRD and HRTEM. For example, the HRTEM image of a small section of an individual tube of IL@MWCNTs sample showed contrasting darker areas (designated by arrows) inside the MWCNTs (Figure 25C). Correspondingly, the melting point of [BMIm][PF6] greatly increased from 6 °C to 221 and then to 266 °C.48 The two melting transitions indicate the coexistence of two polymorphous crystals, probably due to two rotational isomers of the [BMIm] cation.365 Interestingly, co-confinement of methanol molecules inside the MWCNTs showed only one fusion peak at 222 °C, which was due to the influence of methanol on the conformation of cation. The liquid−solid phase transition of the nanoconfined ILs can also be predicted by monitoring the potential energy of the system (Upot), consisting of cation−cation, cation−anion, and anion− anion intermolecular interactions, and cation−walls and anion− walls interactions through MD simulation.293,300 As shown in Figure 25D, with increasing temperature, Upot of [BMIm][PF6] confined within SWCNTs having a length of 7.0 nm and pore diameter of 1.94 nm first increases slightly, followed by a dramatic jump, and then becomes stable. The temperature at the beginning of this jump is attributed to the starting melting

Figure 25. TEM images of (A) opened MWCNTs and (B) [BMIm][PF6]@MWCNTs. (C) HRTEM image of a small section of an individual tube of [BMIm][PF6]@MWCNTs. The inset shows the SAED of an individual nanotube filled with [BMIm][PF6]. Reproduced from ref 48. Copyright 2007 American Chemical Society. (D) Temperature dependence of the confined ions potential energy for [BMIm][PF6] confined in SWCNTs having a length of 7.0 nm and pore diameter of 1.94 nm. The potential energy consists of cation− cation, cation−anion, and anion−anion intermolecular interactions and cation−walls and anion−walls interactions. The insets show the radial snapshots of the encapsulated ILs in nanopores before melting and after being molten entirely. Red and blue balls represent center-ofmass locations of cation and anion, respectively. Reproduced from ref 300. Copyright 2011 American Chemical Society.

temperature T1, while the other turning point in the Upot−T curve corresponds to the temperature T2 of the entirely molten state, observed to be nearly 227 and 527 °C, respectively.300 The starting melting point of the nanocrystal is in excellent agreement with the experimentally determined melting point of the [BMIm][PF6] crystals in MWCNTs.48 The observed transformation from the disordered arrangement of ions in the bulk phase to orderly arranged ions after confinement within the channel of MWCNTs also holds true for other ILs, such as hydrophilic IL [HMIm][Br], for which a significantly large increase in melting point (from −58.4 °C for the pure IL to 154.8 °C) was observed after confining within the MWCNTs having average internal diameter of 10 nm.177 The results of MD simulations suggest that the structure and interior radius of CNTs also affect the melting temperature of confined ILs.293 Upot of [BMIm][PF6] inside the zigzag ((20, 0) and (45, 0)) and armchair ((11, 11) and (26, 26)) CNTs showed a dramatic jump between 500 and 900 K, corresponding to the melting process. IL inside the smaller CNT has lower energy (and is more stable) than in the bigger CNT, which can be attributed to greater interactions between the IL and smaller CNT wall. Consequently, the melting temperature was found to increase as the radius of the CNT decreased. It is also shown that the ILs inside the zigzag CNTs have higher melting temperature than in the armchair CNTs because of the lower energy of ILs inside the zigzag CNTs.293 AE

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Figure 26. DSC curves of (A) bulk and confined [Pyr13][NTf2] in nanoporous carbon, and (B) confined [Pyr13][NTf2] with and without polarization at +1.3 or 1.2 V vs AC. Scan rate: 10 mV s−1. Reproduced with permission from ref 246. Copyright 2014 Royal Society of Chemistry.

nonstoichiometric local ionic arrangement. In contrast, the crystallization is in a narrower range and seems to be less affected by polarization. 7.2.3.2. Thermal Stabilities. In contrast to silica-confined ILs, the thermal stabilities of ILs confined in carbon materials have been less studied. Interestingly, different from the lowered thermal stability for most silica-confined ILs, the literature reported that carbon-confined ILs showed improved thermal stability as compared to the bulk ILs. For example, Shinohara et al. demonstrated that when IL [Me3NC2H4OH][ZnCl3] was confined into SWCNT, the onset temperature of the thermal decomposition was found to increase by ca. 60 °C. At the same time, the profile of the TGA curve and thus the weight loss steps were less affected by encapsulation.223 However, to get a more general conclusion of the thermal stabilities of the carbon materials-confined ILs, more experimental results are needed.

Similarly, the wall distance plays a crucial role in the phase transition of IL when these were confined between two parallel graphite walls (slit pore). For example, when [MMIm][Cl] was confined between two hydrophobic graphite walls with a distance of 2.5 nm, the result of MD simulations indicated no “hardening” or transition to a solid-like phase structure.55 However, when the same IL is restricted in a more-confining space, that is, parallel graphite walls at wall distances of less than 1.1 nm, a clearly first-order liquid-to-solid phase transition resulted.339,340 In contrast to the tight confinement offered by CNTs, graphene273 and porous carbons204 give a less confined environment to ILs, and thus less affected thermal behaviors are observed. In comparison to bulk ILs, [BMIm]-based ILs with different anions, such as [BF4], [PF6], and [NTf2] confined within graphene multilayers, showed anomalous thermal transition and crystallization behaviors. The detailed thermal behavior depended upon the anions and occurred due to the molecular orientation of the confined system triggered by the complex π−π stacking and hydrogen-bonding interactions.273 Considering that the ILs are an emerging class of advanced electrolytes for electrochemical energy storage systems, such as carbon supercapacitors,352,353 it is essential to understand how anion/cation separation during charging affects the thermal behavior of ILs confined within porous carbon materials, and may lead to different freezing points at the anode and cathode. It was reported that when IL N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr13][NTf2]) was confined within BP2000 carbon black (1374 m2 g−1, 2 cm3 g−1), the nonequilibrium nucleation process of freezing was dramatically affected and shifted from originally −24 °C for the bulk IL to 2 °C for the nanoconfined IL, yet the melting point was depressed only from 13 to 12 °C (Figure 26A).246 This finding is in line with the enhanced ion mobility and the ability to rearrange in nanoconfined RTILs compared to the bulk.234 Interestingly, when the carbon nanopores were electrically charged, ILs in the confinement demonstrated a different thermal behavior in positively and negatively polarized carbon electrodes. For a symmetric supercapacitor cell fully charged to a voltage of 2.5 V, the disassembled electrodes showed greatly suppressed melting points compared to the same measurements at 0 V. As shown in Figure 26B, the small endothermic peaks observed in DSC curves during a heating process indicate melting points at 8 and 10 °C for positive (+1.3 V) and negative (−1.2 V) polarizations, respectively, while both are lower than that for the nonpolarized carbon (12 °C). The downshift of the melting point and the impeded formation of the extended IL crystal could be attributed to the higher concentration of anions or cations, which results in a

7.3. Metal−Organic Frameworks (MOFs)

MOFs are a new generation of porous crystalline materials known for their high surface area, large pore volume, and remarkable storage capacity. One of the interesting applications of ILs in connection with MOFs is the ionothermal synthesis of MOFs in ILs.199,366−369 ILs, owing to their unique properties, can influence and even govern the final crystal structure and morphologies of MOFs. In addition to being used as solvents, structure directing agents (templates), and charge-compensating groups in ionothermal synthesis, MOFs themselves have very recently shown great potential as appropriate microporous matrices for direct confinement of ILs, which can control the structure and dynamics of ILs via nanoconfinement of ILs and tunable interactions of MOFs with the guest ILs.308,370 Many MOFs, such as ZIFs,60,212,309,371−373 IRMOF,374−376 MIL101,180,307,377−379 Na-rho-ZMOF,372 Cu-BTC,254,380,381 CuTDPAT,382 and HKUST-1,306,383 have been employed for MOFs-confined ILs composites. Their structural features were summarized in Table 4. ILs are generally introduced into the pores of MOFs by ionothermal synthesis or a post-impregnation method. The obtained MOF-confined ILs composites have found potential applications mainly in catalysis,209,306,378,390 adsorption,180,307 and CO2 capture.60,383 The successful encapsulation of IL units inside the cage of MOFs can be characterized by XRD, wherein the calculated diffraction patterns, based upon the Rietveld refinements, are in good agreement with the experimental patterns.309 Due to the confinement effects, ILs in MOFs, such as IRMOF-1, are packed more orderly than in the bulk phase.372,375 Figure 27A shows the calculated RDFs of the cation and anion for [BMIm][PF6] in the bulk state and in IL/ IRMOF-1 composite at a weight ratio of IL/MOF = 0.4. The AF

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Table 4. Structural Features of Representative MOFs Investigated for Nanoconfined ILs

AG

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Table 4. continued

MOFs possess a strong interaction with the framework and prefer to remain near the metal sites of MOFs. The strong anion−metal atoms interactions can be reflected by blue shifts of the SO2 symmetric stretching band and the SNS asymmetric stretching band when [BMIm][NTf2] is confined within ZIF-8, compared with the corresponding bulk IL as well as the IL coated on the outer surface of the MOFs.60 The intensity of interactions between anions and MOFs is found to be strongly dependent upon the anions.376,382 When ILs [BMIM][X] ([X] = [Cl], [BF4], [PF6], and [NTf2]) were confined within CuTDPAT, the RDFs of anions around the Cu atom of Cu-

much higher peaks of all ionic pairs observed in the composite suggested the presence of a more ordered structure of nanoconfined IL molecules, particularly for the attractive [BMIm]−[PF6] pair.375 Generally, the bulky imidazolium cations of ILs tend to reside in the open cage of MOF near the organic linkers due to the configuration entropy effect.372,375,376 A more detailed analysis of [BMIm][PF6] confined within IRMOF-1 by MD simulation suggested that the [BMIm] cation is proximal to the benzene ring and the carboxylate group in IRMOF-1 rather than the metal cluster.375,376 In contrast, the anions of ILs confined within AH

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Figure 27. (A) Radial distribution functions (RDFs) of [BMIm] and [PF6] in the bulk state (dash lines) and in IRMOF-1 (weight ratio of IL over IRMOF-1 is 0.4, solid lines). Reproduced with permission from ref 375. Copyright 2011 American Chemical Society. RDFs of (B) anions of [BMIM][X] ([X] = [Cl], [BF4], [PF6], and [NTf2]) around the Cu atom of Cu-TDPAT, and (C) anion−cation pairs in the respective IL/CuTDPAT composites. Reproduced with permission from ref 382. Copyright 2015 American Chemical Society.

Figure 28. (A) DSC curves of bulk [EMIm][NTf2] and [EMIm][NTf2] confined in ZIF-8 with different loadings of 25%, 50%, 75%, 100%, and 125% (denoted as EZ25, EZ50, EZ75, EZ100, and EZ125, respectively). The red and blue lines indicate heating and cooling, respectively, at a fixed scan rate of 5 °C min−1. Reproduced with permission from ref 309. Copyright 2015 Royal Society of Chemistry. (B) Solid-state 19F static NMR spectra of bulk [EMIm][NTf2] and EZ25. Reproduced with permission from ref 212. Copyright 2014 John Wiley & Sons, Inc.

TDPAT showed an increase in the distances for the first peak in the order [Cl] < [BF4] < [PF6] < [NTf2], which is in accordance with the changing trend of sizes of the anions (Figure 27B). Also, the maximum height of the first peak offered by the anion [Cl] suggested that this smallest anion with a negative charge interacts most strongly with the positively charged Cu atom. In contrast, [NTf2] has the weakest interaction with this MOF and, thus, exhibits the strongest interaction with [BMIm], as reflected by the lowest position as well as the highest intensity of the first peak observed in the RDFs (Figure 27C).382 Adding more ILs into MOFs will affect the distribution of ILs in the composites. For example, with an increase in the loading amount of [BMIm][Cl] confined within Cu-TDPAT, the IL molecules start to appear in the cuboctahedral cages, followed by the filling of octahedral cages. Meanwhile, the heights of the first peaks in the RDFs of anions around the Cu atoms (shown in Figure 27B) decrease with an increase in the loading of ILs in the composites, indicating a more uniform distribution of ILs at higher loadings.382 A comparative study between MOFs and COFs revealed that MOFs lead to a better dispersion of IL molecules in their pores than the COFs due to the Coulombic interactions originating from the former’s frameworks. Furthermore, as compared to the easy aggregation in 2DCOFs and 1D-MOFs, IL molecules are more dispersive in the support materials with 3D pore structures.391 The mobility of

the IL in the IL@MOF composite is reported to be much lower than that in the bulk phase, and the mobility of both cations and anions reduces with an increase in the IL loading in the composite, mainly due to the enhanced confinement effect.375 As observed in the bulk ILs and ILs confined within porous silica and carbon, the mobility of bulky [BMIm] is greater than that of [PF6] when [BMIm][PF6] was confined within IRMOF1. The observed smaller mobility of [PF6] is mainly due to the combination of less hindered displacement of the imidazolium ring along the direction of the C1 atom and the strong interaction between [PF6] and the framework as discussed above.375 Similar to the ILs confined in porous silica and carbon, the phase transitions of ILs have been shown to be greatly affected when confined inside MOFs.212,309,371 As shown in Figure 28A, the bulk [EMIm][NTf2] showed sharp peaks at −16 °C upon heating and at −42 °C upon cooling, which correspond to heat anomalies caused by melting and freezing, respectively. The freezing temperature is much lower than the melting temperature resulting from the supercooling of the liquid state.147 However, when this IL was confined into ZIF-8 with loadings of 25%, 50%, 75%, 100%, and 125% (denoted as EZ25, EZ50, EZ75, EZ100, and EZ125, respectively), nearly all of the obtained [EMIm][NTf2]@ZIF-8 composites showed no obvious peaks in the DSC measurements between 123 and 473 K. The only exception is EZ125, which exhibited weak AI

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with the corresponding inflection points observed at 264 and 257 K, respectively. This observation arises from the freezing transition of the bulk IL, as evidenced by the DSC results. In contrast, no sharp decrease in conductivity between −45 and 68 °C was observed for other nanoconfined ILs, such as EZ50, EZ75, and EZ100. This was due to the reason that, in these samples, the IL molecules are believed to be exclusively located inside the ZIF-8 micropores and are thought to remain liquid in this temperature region, as evidenced by the above DSC measurements. Therefore, the ionic conductivity of [EMIm][NTf2] is maintained in the temperature range where bulk IL is frozen. It should be noted that the ionic conductivity of EZ100 was even higher than that of the bulk IL below 250 K. The greatly enhanced ionic conductivity of IL@MOF in contrast to the bulk ILs at low temperatures is very interesting, which thus provides a facile route toward developing novel electrolytes for electrochemical devices which could operate at low temperatures. In combination with the measurement by solid-state 19F NMR spectra, the high ionic conductivity of IL@MOF and the strong IL concentration dependence of conductivity for [EMIm][NTf2]@ZIF-8 were attributed to the formation of connected and continuous conductive paths for mobile ions at high concentrations of ILs.309 Doping lithium salts such as lithium bis(trifluoromethanesulfonimide) (LiNTf2) into ZIF-8confined [EMIm][NTf2] was found to be beneficial to modulate the phase transitions and, thus, increase the ionic conductivity. The [NTf2] anions showed a gradual decrease of mobility in the micropores at low temperatures due to the absence of an apparent freezing transition, while the mobility of Li+ cations showed a slightly steeper decrease than that of the [NTf2] anions at low temperatures. The ionic conductivity of lithium ion-doped IL ([(EMIm)0.8Li0.2][NTf2]) confined within the micropores of ZIF-8 is about 2 orders of magnitude lower than that of the bulk [(EMIm)0.8Li0.2][NTf2], while the activation energies for the two samples were comparable.371 These results suggest that the Li+ cations diffuse through the micropores via exchange of solvating [NTf2] anions, similar to the Grotthuss mechanism in proton conductivity.393

anomalies at almost the same melting and freezing temperatures as those of the bulk IL, which probably resulted from the excess IL that was located outside the micropores of ZIF8.212,309 The absence of a marked phase transition in EZ25, EZ50, EZ75, and EZ100 suggests that [EMIm][NTf2] is prevented from freezing by a nanoconfinement effect in the micropores of ZIF-8. Based upon the van der Waals volumes of [EMIm] cations and [NTf2] anions,392 the storage capacity of each micropore of ZIF-8 for [EMIm][NTf2] is calculated to be three or less ionic pairs; such a small number of ions is not enough to construct ordered crystal structures, and thus, no phase transition between the solid and liquid phases is observed. Solid-state NMR measurement was also conducted to study the phase behavior of the IL incorporated within ZIF8. The observed temperature dependences of the 19F NMR spectra ([NTf2] contains fluorine atoms) of bulk and nanoconfined [EMIm][NTf2] are shown in Figure 28B. Upon heating, for the bulk [EMIm][NTf2] from −150 to 0 °C, only a broad line was detected below 123 K, while a sharp line appeared at −30 °C superimposed on the broad signal. Further increasing the temperature to 0 °C resulted in only a sharp line while the broad signal vanished. This line sharpening is related to the melting transition of bulk IL at approximately 243 K, which completely turned [EMIm][NTf2] into the liquid phase, resulting in free rotation and diffusion of the [NTf2] anions. In contrast, the phase-transition behavior of the composite sample EZ25 under the same conditions was observed to be dramatically different from that of the bulk IL. Instead of a sharp peak, gradual and continuous narrowing of the NMR signal occurred, demonstrating no drastic motional change in this temperature range.212 The absence of obvious melting and freezing of ILs confined within MOFs, even down to very low temperature (such as −150 °C for [EMIm][NTf2]@ZIF-8), suggests that IL@MOF is a promising ionic conductor that could work at low temperatures.309,371 Figure 29 shows Arrhenius plots of the ionic conductivity of [EMIm][NTf2] confined within ZIF-8 with different loadings (EZ50, EZ75, EZ100, and EZ125) on heating, together with the results of the bulk IL for comparison. Both bulk IL and EZ125 having excess IL outside of the micropores of ZIF-8 exhibited a sharp decrease in conductivity,

7.4. Porous Metals

Investigation of ILs confined in metallic pores is important because the obtained structure and dynamics may be helpful for the understanding of interfacial structures of ILs on solid metal surfaces, thus providing better insight into the lubrication and electrochemical capacitor mechanisms. ILs confined in various metal matrices, including porous Ag,183,394 a slit-like Au metal nanopore,395,396 and mesoporous Pt,274 have been carefully studied by both experimental techniques and simulation methods.397,398 A [BMIm][NO3]@Ag composite was obtained by in situ precipitation of AgNO3 in [BMIm][NO3] via reduction of the Ag precursor by sodium hydrophosphite.183,394 [BMIm][NO3] was selected because of the following reasons: First, the IL should be water-soluble for the reduction reaction in a homogeneous solution, and [BMIm][NO3] is watersoluble. The second criterion is to prevent the possibility of anion exchange between Ag salt and the IL anion. The average diameter of the open pores of the obtained Ag matrix was 39 ± 10 nm, which resulted in rather low porosity, and thus a very small content of [BMIm][NO3] (1.5 wt %) was confined. Compared to the bulk IL, the first-order transitions were shifted toward lower temperatures; however, the thermal stability of [BMIm][NO3]@Ag was significantly improved by 50 °C. It was also postulated that Ag-confined ILs exhibit a two-phase

Figure 29. Arrhenius plots of the ionic conductivity of bulk [EMIm][NTf2] confined in ZIF-8 with different loadings (EZ50, EZ75, EZ100, and EZ125) on heating. The solid lines are provided as guides for the eye. Reproduced with permission from ref 309. Copyright 2015 Royal Society of Chemistry. AJ

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resulted in a new ABA triblock polymer poly(styrene-b-ethyl acrylate-b-styrene) (SEAS) ion gel system, which demonstrated significantly higher ionic conductivity than the SMS ion gels at the same polymer concentration. The ionic conductivity of SEAS ion gels was doubled compared to that of the SMS ion gels when the content of the SEAS polymer was 20 wt % and was more than 1 order of magnitude higher by adding 50 wt % SEAS polymer. Furthermore, shortening the midblock length was found to increase synergistically both the ionic conductivity and the modulus of the SEAS ion gels at a given polymer concentration. 4 0 6 When a proton conductive IL, imidazolium:bis(trifluoromethylsulfonyl)imide ([Im][NTf2]), having an excess of imidazole, was confined in a diblock copolymer, poly(styrene-b-2-vinylpyridine) (PS-b-P2VP), the IL molecules selectively resided in the P2VP domains of the block copolymer, forming a lamellae structure with protonconducting domains. The confinement could affect the hydrogen-bonding network of imidazole and thus the proton hopping transport mechanism in the nonstoichiometric [Im][NTf2]. The controlled experiment conducted for comparison purpose in neat IL and a mixture of IL with P2VP homopolymer indicated that the lamellae nanostructure and unique ion aggregation behavior of ILs confined in PS-b-P2VP led to a lower activation energy for macroscopic ion transport and significantly increased the amount of proton hopping.402 Such IL-involved lamellae structure can also be formed by confining ILs in a semicrystalline fluorinated copolymer P(VDF-CTFE) via a solvent casting method. Figure 30 shows that neat P(VDF-CTFE) exhibits a nanostructure with two crystalline lamellae (nonpolar α-phase) separated by the rigid amorphous fraction (RAF). The presence of only a tiny amount of phosphonium ILs (octadecyltriphenylphosphonium iodide (IL-C18) and tributyl(methyl)phosphonium methylsulfate (IL-108)) efficiently induced a complete transition of nonpolar α-phase to polar γ- and/or β-phase. Simultaneously, ILs appeared to diffuse and regularly assemble in the RAF of P(VDF-CTFE) attributed to the “template” confinement effect of the 2D crystalline lamellae structure. The detailed structure of the regularly assembled ILs in P(VDF-CTFE) is strongly dependent on the amount of IL. When the amount of ILs is 5 wt %, the confined ILs is likely to form a 1D rod-like assembled structure; however, it evolves to a 2D profile when 10 wt % of ILs is incorporated. The observed versatile nanostructuration effect of ILs is also closely related to the chemical structure of ILs. Compared to IL-C18, IL-108, owing to its smaller steric hindrance and extra dipolar groups, shows a higher extent to be located in the RAF as well as undergoes stronger interaction with polymer matrix, consequently inducing more polar βphase and more homogeneous dispersion morphology.404

regime, where the IL close to the pore wall is solid, and the IL in the center of the pore is liquid-like.183 MD simulations of [BMIm][PF6] confined inside a slit-like Au nanopore with a pore size of 5.0 nm indicated that both the structure and dynamics of metal-confined ILs were obviously different from the bulk ILs. The calculations showed that the solid-like ordered layers with dense structures were formed in the vicinity of the metal surface. The orientational investigation indicated that the imidazolium ring of the [BMIm] cation preferred to form a small tilt angle (about 19°) with the Au pore walls in the first layer. This orientation was found to be much more energetically favorable than any other orientations, probably due to the strong π−π stacking interactions between the imidazolium ring and the pore walls, as well as the preferential self-organization of planar molecules to form more stable and well-regulated orientation layers at Au surfaces. Furthermore, the calculated MSD indicates that the dynamics of confined ILs are significantly slower than those observed in bulk systems.396 Replacing the [PF6] anion of [BMIm][PF6] inside nanoporous Au by [NTf2] was found to decrease the layered structure of the IL, as indicated by the density profile of cations and anions as a function of the distance to the Au interface and the molar fraction of [PF6] in the IL mixture ([BMIm][PF6] + [BMIm][NTf2]).395 7.5. Polymers

In addition to the intensively studied rigid inorganic matrices, ILs can also be confined into organic polymer matrices. The use of polymers as porous hosts for ILs results in easy and convenient formation of flexible membranes, although such systems generally experience a loss of their mechanical, i.e., selfstanding, properties when the amount of immobilized IL becomes too high. Polymer-confined ILs are, in general, obtained by either in situ polymerization of monomers in ILs399−401 or a solvent casting method.402−408 The later method, which is special to polymers by taking advantage of the soluble behavior in compatible solvents, is usually conducted by first dissolving polymer and ILs in compatible solvents and then removing the solvents with a vacuum system. The reversible physical cross-links of the ion gels obtained by a solvent casting method also make solution processing a viable fabrication pathway. Several polymers, such as poly(methyl methacrylate) (PMMA),403,405 fluorinated copolymer poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDFCTFE)),404 biopolymers,401,409−411 and epoxy resins,400 have been explored for confinement of ILs. Among them, block copolymers have attracted significant attention because advanced assembled nanostructures are likely to form when the block copolymers meet ILs by virtue of the affinity of a selective block to ILs.402,406,412,413 For example, ion gels were prepared by dispersing an ABA triblock copolymer, poly(styrene-b-ethylene oxide-b-styrene) (SOS) or poly(styrene-bmethyl methacrylate-b-styrene) (SMS), in a hydrophobic, highly conductive IL, [EMIm][NTf2]. In this system, the B midblock was soluble in the IL; however, the two A end blocks were insoluble, which led to the fabrication of a self-assembled network with A cross-links and a continuous B/IL phase. The obtained ion gels exhibited high ionic conductivity (ca. 10−2 S cm−1), modest modulus (ca. 103 Pa), high capacitance, and good mechanical integrity, making them applicable as dielectric layers in thin film transistors.412,413 Changing the midblock from poly(ethylene oxide) (PEO) and PMMA polymers to a low Tg and hydrophobic midblock, poly(ethyl acrylate) (PEA),

7.6. Other Matrices

Other than the nanoporous matrices mentioned above, ILs can also be successfully confined into many other porous matrices, including inorganic matrices (zeolites,52,181,414−416 aluminum hydroxide (Al(OH)3),271,417 alumina (Al2O3),272 titanium dioxide (TiO2),334,338,343 mica,239,244 porous chalcogenides,418 tin dioxide (SnO2),173 kaolinite,324 2D materials beyond graphene,419,420 organic matrices (calixarene,421 cyclodextrin422), and hybrid polymer−silica matrices.423−425 Notably, although the synthesis of 2D crystalline inorganic matrix materials, such as layered simple hydroxides, layered double hydroxides, and clay-type substances, is significantly more complicated than the synthesis of silica ionogels, as discussed AK

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into commercially available Al2O3 membranes (60 mm thickness) with well-defined cylindrical pores (about 100 nm diameter) aligned along the membrane normal (Figure 31A), the confined system shows strongly changed phase-transition behaviors as well as anisotropic ion conductivities due to nanoconfinement effects. The DSC chart for nanoconfined [C16MIm][PF6] shows no peaks around the SmA-to-Iso phasetransition temperature (Figure 31B); however, the ordered layer structure still remains. The surface-induced homogeneous or homeotropic alignment of [C16MIm][PF6] confined in the pores definitely makes a difference in the ion conductivity. For the two composite membranes with pore walls treated with C16TAB and with untreated pore walls (Figures 31A and B), the ion conductivities in the SmA phase are higher than those in the bulk samples, and the ion conductivities decrease at the transition from the SmA phase to the Iso phase, unlike the bulk samples. In the temperature region where the bulk [C16MIm][PF6] shows the SmA phase, the ion conductivity of the composite membrane with pore walls treated with PVA is much lower than that of the composite membrane with pore walls treated with C16TAB (Figures 31A and C), because the ion conductivity σ∥ is higher than σ⊥ in the SmA phases.272

8. APPLICATIONS OF NANOCONFINED IONIC LIQUIDS Owing to the distinguishing properties of ILs under confinement compared to those in bulk, nanoconfined ILs inside different kinds of host networks show great potential in a wealth of areas depending critically on the structure and chemistry of both the hosts and the ILs. More specifically, ILs confined in silica matrices have been extensively used as catalysts57,429 and ionogels175 (for optical materials,270 sensors,430 drug delivery,431,432 etc.), while ILs confined in MOFs have been applied as solid sorbents for gas capture and separation.212,308 In contrast, carbon-confined ILs have found less applications, although there are intensive theoretical studies related to the fundamental understanding of their microscopic structural and dynamical properties. In the following section, we will mainly focus on the promising applications of nanoconfined ILs in catalysis, gas capture and separation, ionogels, supercapacitors, carbonization, and lubrication.

Figure 30. Versatile and tunable nanostructuration of two phosphonium ILs on fluorinated copolymer poly(vinylidene fluorideco-chlorotrifluoroethylene) (P(VDF-CTFE). (A) Neat P(VDF-CTFE) with two crystalline lamellae (α crystalline phases) separated by the rigid amorphous fraction (RAF). Regularly assembled (B) 1D (5 wt % IL) and (C) 2D (10 wt % IL) structure of ILs in the RAF of P(VDFCTFE) between crystalline lamellae with increasing the IL content. The inset in (A) shows the structure of the IL. Reproduced with permission from ref 404. Copyright 2015 American Chemical Society.

by Taubert et al.,419 these materials hold promise for the construction of highly advanced IL/inorganic hybrid materials. Moreover, nanoporous Al2O3 can also be used directly as a transparent or white (depending on the pore diameter) membrane for confinement of ILs. Their porous surfaces can be easily modified by the reaction of the free hydroxyl groups with different chemicals, such as silane molecules, one of the most popular surface-modifying agents.426,427 For example, nanoporous Al2O3 can be used as a promising matrix to confine ILC [C16MIm][PF6], resulting in modified phase-transition behaviors and anisotropic ion conductivity.272 When ILC [C16MIm][PF6] was introduced into thin glass cells, consisting of two indium tin oxide (ITO) coated glasses covered with rubbed poly(vinyl alcohol) (PVA) films or hexadecyltrimethylammonium bromide (C16TAB) films to realize a homogeneous or homeotropic surface alignment, the DSC charts of the bulk ILC showed endothermic peaks at 74.6 and 125.2 °C in the heating run, and exothermic peaks at 121.7 and 63.8 °C in the cooling run,272 corresponding to the crystalline (Cr)-to-Smectic A (SmA) and SmA-to-isotropic (Iso) phase transitions, and Isoto-SmA and SmA-to-Cr phase transitions, respectively.428 The anisotropic ion conductivities of bulk [C16MIm][PF6] are found to gradually increase with increase in temperature in the SmA phase in all cases, as shown in Figure 31C. This is attributed to the fact that the viscosity of [C16MIm][PF6] decreases with increase in temperature, and conductivity along the SmA layer (σ∥) is larger than that along the layer normal (σ⊥) to the SmA phase. In contrast, when this ILC is confined

8.1. Catalysis

8.1.1. Introduction of Confined Ionic Liquid Catalysis. Homogeneous or liquid-phase catalysis is an attractive option for many reactions, because all the catalytically active sites are accessible and uniform in this case. However, solvents are essential in such systems for mass-transfer, which makes it difficult to recover and reuse the catalysts, thus limiting the practical application of homogeneous catalysis. Supporting the catalyst on a surface or confining it into porous materials would be an effective approach to combine the advantages of both homogeneous and heterogeneous catalysis (easy separation and reusability). The immobilization operation could also lead to improved catalytic performance in addition to the heterogenization of a homogeneous catalyst. However, the common organic solvents that are used as reaction media for metal complex catalysts are volatile, so it is difficult to obtain a stable supported/confined homogeneous catalyst. On the other hand, ILs are an attractive option as an alternative reaction medium for homogeneous catalysis. Their unique ionic environment makes them highly suitable for biphasic IL/organic liquid transition-metal catalysis. Moreover, their structural diversity AL

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Figure 31. ILC [C16MIm][PF6] confined in aluminum oxide membranes (60 mm thickness) with well-defined cylindrical pores (about 100 nm diameter). (A) The experimental setup of ion conductivity measurements for composite membranes and molecular structure of [C16MIm][PF6]. (B) DSC curves in the heating run for [C16MIm][PF6] confined in pores aligned (a) homogeneously, (b) naturally, and (c) homeotropically, and for (d) bulk ILC. (C) Ion conductivities for ILC samples introduced into the ITO glass cells (a) treated with PVA, (b) untreated, and (c) treated with hexadecyltrimethylammonium bromide (C16TAB). (D) Ion conductivities for ILC samples introduced into the porous membranes (a) treated with C16TAB, (b) untreated, and (c) treated with poly(vinyl alcohol) (PVA). Reproduced with permission from ref 272. Copyright 2015 Royal Society of Chemistry.

provides the ability to easily tailor the properties of ILs to stabilize the catalytic species. Some functionalized ILs such as Lewis acidic or metal-containing ILs can even be used directly as novel liquid catalysts. Nevertheless, the direct application of ILs in the homogeneous system generally requires a large amount of ILs. This is economically undesirable since ILs remain relatively expensive even though they are now commercially available. Additionally, mass transfer limitations resulting from the high viscosity of ILs often cause the reaction to occur only in a thin boundary layer of the bulk IL phase. Thus, only a small part of the IL and the catalyst dissolved therein may take part in the catalyzed reaction. This issue is more serious if the chemical reaction occurs fast. Therefore, an ideal catalytic system would consist of a catalyst phase having the size of the diffusion layer, thus allowing all the available metal complexes and IL to participate in the catalytic reaction Based on the concept of heterogenization of a homogeneous catalyst, ILs can be incorporated into the pore structure of porous matrices to give nanoconfined (or supported) IL catalytic systems (Figure 32). This could help overcome the major drawbacks of ILs, such as high viscosity, slow gas diffusivity, and high cost, making them feasible options as solvents and catalyst-dispersion agents for large-scale applications in the fields of organic synthesis and heterogeneous catalysis.9,164,434−437 For example, the fabrication of wellstabilized thin IL films on the pore surface of highly porous materials may significantly reduce the quantity of IL that is necessary for a certain scale of application and would also make it possible to utilize the entire mass of the incorporated IL, because viscosity and diffusion effects are diminished in such

Figure 32. Schematic principle of the confined IL catalysis. Reproduced with permission from ref 433. Copyright 2005 American Chemical Society.

ultrathin layers.438,439 The favorable properties of ILs, particularly the negligible vapor pressure, are crucial to realizing the immobilization of a nanoliquid layer on a solid support and make it highly attractive for gas-phase reactions compared to the commonly used volatile organic solvents. Through appropriate selection, specific properties of the anion and cation of an IL can be therefore transferred to the pore surface of a solid material. These confined IL catalysts allow for the application of fixed-bed reactor systems for simple continuous processing when used in combination with gaseous reaction mixtures, eliminating the need for tedious separations and catalyst recycling.435,440,441 Moreover, we can envisage that the special porous structure of support may lead to a “microenvironment effect” and “shape-selective catalysis”, which would have a significant effect on the performance of the AM

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expected to affect the activity and selectivity of the solid catalyst in two ways. First, the IL may have a positive influence such as the so-called “cocatalytic effect” on the “chemical” properties of the catalyst. Second, compared to the uncoated catalyst, the IL may change the effective concentrations of adducts and intermediates, provided that the solubility of reactants in the IL is suitably different from that in the liquid organic phase. (C) Supported catalytically active IL phase (SCAILP), which is similar to the SILP strategy but the catalytically active species are covalently attached to the skeleton of ILs (either cations or anions), instead of being dissolved in the IL layer as observed for SILP. In other words, in contrast to both SILP and SCILL, the IL used for SCAILP is not only a liquid layer supported on the surface for mass transfer but also a homogeneous catalyst with intrinsic activity. The most frequently used catalytically active ILs include Brønsted acidic or basic ILs,195,209,306,390,416 Lewis acidic ILs,470−472 redox moiety-containing ILs,450,453 and ILs with metal-containing anions.473 Generally, the introduction of catalytically active functional moieties into ILs always complicates their structure and increases their viscosities and melting points, which may partially limit the mass transfer and the dispersion on the internal surface of porous materials. However, as mentioned in the previous sections, nanoconfinement of ILs in pore matrices sometimes gives rise to liquid-like behavior below the solidification temperature of the neat IL, which may help to overcome the issues of high viscosities and melting points for catalytically active ILs.50,175 In addition to SILP, SCILL, and SCAILP, other confined IL catalytic systems can also be obtained by selectively combining two or more of these basic three classes. For example, Karimi et al. combined the concepts of SCILL and SCAILP by incorporating a hydrophobic and acidic IL 1-methyl-3octylimidazolium hydrogensulfate ([OMIm][HSO4]) into sulfonic acid-functionalized SBA-15. The resultant confined IL catalyst was found to be efficient and chemoselective for solvent-free thioacetalization of carbonyl compounds at room temperature.448 Similarly, a novel catalytic system based on the combination of SILP and SCILL was obtained by confining a task-specific IL (TEMPO-IL) containing a 2,2,6,6-tetramethyl1-piperidinyloxy moiety (TEMPO) and CuCl2 simultaneously in a silica-gel matrix by the established sol−gel technique. The obtained TEMPO-IL/CuCl2/silica-gel catalytic system was effective for the transformation of a series of benzylic, allylic, and heterocyclic alcohols to the respective carbonyl compounds under O2 atmosphere.450 8.1.3. Reactions Catalyzed by Confined Ionic Liquid Catalytic Systems. 8.1.3.1. Catalytic Hydrogenation. Catalytic hydrogenation is an important method in the chemical industry for the synthesis of fine chemicals. The catalytic hydrogenation process with confined IL catalysts is attractive because many hydrogenation products are insoluble in polar ILs, thus facilitating the product separation and catalyst recovery.437,452,457 Jess et al. first proposed the concept of SCILL for hydrogenation of cyclooctadiene. A commercial Ni catalyst coated with the IL [BMIm][OcSO4] was tested as the model system for the sequential hydrogenation of cyclooctadiene to cyclooctene and cyclooctane. At a pore filling ratio of only 10%, this SCILL was able to strongly enhance the maximum yield for the intermediate cyclooctene from 40% for the original catalyst to 70%. The IL layer was very robust, and no leaching into the organic phase was detected.437 A heterogeneous catalyst Pd/IL/MOF consisting of three components, i.e., Pd nanoparticles, IL microphase, and porous

catalyst. The confined IL catalysts would be an ideal platform to explore ‘‘microscopically homogeneous and macroscopically heterogeneous’’ systems and can conceptually bridge the gap between homogeneous and heterogeneous catalysis. In the following sections, we will discuss the classification and applications of the nanoconfined IL catalytic systems, while the supported IL catalysts with ILs only on the outer surface (e.g., carbon nanofibers442) as well as the covalently supported IL phases443,444 will not be covered. 8.1.2. Classification of Confined Ionic Liquid Catalysts. The confined IL catalysts can be defined as nanoliquid layer catalysts. The key concept of this catalyst is the deposition of an IL layer on the internal pore surface while confining the catalytically active species on the surface of porous material or inside the IL layer.433 To date, various confined IL catalysts have been prepared with a combination of different types of ILs and porous matrices, such as silica gels and porous silica,195,445−456 MOFs,209,306,390,457 zeolites,416 and porous carbons.458,459 As shown in Figure 33, according to the

Figure 33. Illustration of the concept of confined IL catalytic systems: (A) supported IL phase (SILP), (B) solid catalyst with an IL layer (SCILL), and (C) supported catalytically active IL phase (SCAILP).

microscopic surface structure of the porous materials as well as the composition of the supported IL layer, confined IL catalytic systems can be basically classified into the following three types. (A) Supported IL phase (SILP), which is achieved by simply depositing the IL phase containing catalytically active species on the surface of the support. In the SILP concept, the IL itself does not undergo reactions with the support and acts as an inert reaction phase to dissolve various homogeneous catalysts. In fact, the strong solvation power as well as the easily tailored structure and properties of ILs ensure the dissolution or dispersion of not only homogeneous organocatalysts460,461 and metal complexes,56,179,440,441,451,455,458,462−469 but also heterogeneous metal nanoparticles.452,457 As a result, although the SILP is a solid composite, the active species dissolved in the IL microphase would behave as a homogeneous catalyst.56,57,434,435,462 (B) Solid catalyst with an IL layer (SCILL), which is a heterogeneous porous catalyst coated with an inert IL layer. Unlike SILP, the catalytically active species in SCILL are not dissolved in the IL layer, but instead is covalently grafted to the pore surface of the solid matrix.437,445,454 In the SCILL concept, the coated IL layer is AN

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8.1.3.3. Catalytic Carbonylation. 8.1.3.3.1. Hydroformylation Reactions of Olef ins. Olefin hydroformylation with an IL catalyst system is a milestone in IL-based catalysis. Representative results are the biphasic Rh-catalyzed reaction processes conducted in imidazolium ILs.477 Confined IL catalysts, especially the SILP catalytic systems, can significantly promote the hydroformylation reaction. In 2002, Mehnert et al. introduced a SILP catalyst for homogeneous hydroformylation catalysis.462 The IL ([BMIm][PF6] or [BMIm][BF4]) containing the active catalyst HRh(CO)(tppti)3 and excess of the free ligand (tppti) was confined into a surface-modified silica gel to form a microphase. When using 1-hexene as the model substrate for hydroformylation reaction, these SILP catalysts with both ILs gave the desired product n,i-heptanal with similar turnover frequency (TOF) (60−65 min−1) and ratio of linear aldehyde to branched aldehyde (2.0−2.4). In contrast, the IL− organic biphasic system showed a TOF as low as 23 min−1. The improved activity might be attributed to the higher concentration of active rhodium species at the interface and the larger interface area of the solid support compared to the biphasic system. However, the leaching of the IL [BMIm][BF4] as well as the expensive rhodium complex into the organic phase was unavoidable, especially at high aldehyde concentrations.462 Using the SILP catalytic system in fixed-bed reactors might partially solve this problem. For example, when Rh-monophosphine complexes of bis(mphenylguanidinium)phenylphosphine and NORBOS ligands in [BMIm][PF6] were immobilized on a porous silica support, the formation of aldehyde via hydroformylation of 1-octene exhibited a TOF about 40 (mol Rh)−1 h−1 at a liquid hourly space velocity (LHSV = reactant liquid flow rate/reactor volume) of 16 h−1, and also, the ratio of linear to branched aldehyde was >2. Notably, inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis of outlet samples taken at steady-state conversion demonstrated rhodium metal leaching to be negligible (≤0.7%, detection limit).478 Immobilization of a water-soluble TPPTS-Rh complex in the ILs TMGL (1,1,3,3-tetramethylguanidinium lactate), [BMIm][PF6], and [BMIm][BF4] on mesoporous silica MCM-41 resulted in SILP catalytic systems that showed better hydroformylation performance for 1-hexene than the corresponding IL-organic biphasic catalyst system and the system with SiO2 as carrier, while the catalytic performance was almost independent of the type of IL.463 The combination of large surface area and uniform mesoporous structure of MCM-41 was likely responsible for the enhancement. No obvious deactivation was observed after six runs, and the rhodium concentration in the reaction solution was beyond the detection limit of atomic absorption spectroscopy (AAS). In another study, SILP catalytic systems were combined with supercritical CO2 (scCO2) for the hydroformylation reaction of olefins.479 1-Octene, CO, H2, and scCO2 were mixed and flowed upward through a tubular reactor containing a catalyst composed of [PMIm][Ph2P(3-C6H4SO3)] (PMIm = 1-propyl3-methylimidazolium), [Rh(acac)(CO)2], and [OMIm][NTf2] supported on silica gel. Since scCO2 has high solubility in IL while IL has low solubility in scCO2, scCO2 can dissolve reactants and bring them into the IL phase for reaction, and then, scCO2 can take the product away without dissolving the IL phase. Therefore, the aldehyde product could be easily separated from scCO2 and the scCO2 could be recycled. The catalysts with different IL loadings were stable for at least 40 h.

MOF, was found to show excellent catalytic activity and high selectivity for the hydrogenation of terminal alkynes under mild conditions. The IL microphase offers an excellent environment for stabilizing metal nanoparticles so that highly dispersed metal nanoparticles can be immobilized on MOF, resulting in their enhanced catalytic activity.457 The concept of SILP was also successfully applied to catalytic hydrogenation by dissolution of the catalytically active component, palladium acetate (Pd(OAc)2), in the IL layer coated on active carbon support.474−476 8.1.3.2. Selective Oxidation. The selective oxidation of alcohols into carbonyl compounds is also one of the most important reactions in the industrial synthesis of fine chemicals. Karimi et al. recently designed a SCILL for the selective aerobic oxidation of alcohols at normal oxygen pressures. In this SCILL, the nitroxyl radical TEMPO was first covalently attached to the walls of the mesopores in SBA-15, while IL [BMIm][Br] was then physically confined in the mesoporous channels. In comparison with TEMPO-functionalized SBA-15 without IL, this heterogeneous catalyst system showed remarkably higher selectivities and efficiencies for the transition-metal-free aerobic oxidation of a wide range of structurally diverse alcohols. Moreover, this SCILL is highly recyclable with no significant loss of activity and selectivity, and leaching of confined IL was observed only after 11 reaction runs.450 As shown in Figure 34, this catalyst was prepared

Figure 34. TEMPO-IL/CuCl2/silica-gel catalytic system for oxidation of alcohol with molecular oxygen. (A) Illustration of the synthesis of silica-gel-confined TEMPO-IL with CuCl2. (B) Aerobic oxidation of alcohols catalyzed by TEMPO-IL/CuCl2/silica-gel. Reproduced with permission from ref 450. Copyright 2011 Elsevier.

through confinement of a task-specific IL (TEMPO-IL) with an inexpensive transition-metal salt CuCl2 in a silica-gel matrix by a sol−gel technique, wherein TEMPO-IL and CuCl2 serve as the catalytically active species and cocatalyst, respectively. The obtained TEMPO-IL/CuCl2/silica-gel catalytic system was shown to be effective for the transformations of a wide range of alcohols, including benzylic, allylic, and heterocyclic alcohols, to the corresponding aldehydes in high conversions and selectivities under mild conditions. Notably, this catalytic system is eco-friendly and cost-effective; it can be easily separated from the oxidation system and reused. AO

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Figure 35. Illustration of the synthesis of silica-gel-confined ILs with and without the metal complex (Mc). Reproduced with permission from ref 57. Copyright 2005 John Wiley & Sons, Inc.

formylation of ethylene484 and selective hydroformylation of butenes into n-pentanal.485 8.1.3.3.2. Hydroformylation Reactions of Unsaturated Alcohols and Esters. In addition to their excellent performance in the hydroformylation reactions of olefins, the SILP catalytic systems also showed high catalytic activity in the hydroformylation reaction of unsaturated alcohols and esters.486,487 For example, the sequential hydroformylation and hydrogenation of allyl alcohol was performed using conventional Rh/ PPh3 catalyst in an IL [BMIm][n-C8H17OSO3] film supported on porous silica (Rh/PPh3/silica), with water as the reaction medium. This catalyst system has several advantages over the existing methods, such as the use of inexpensive and commercially available PPh3 ligand, catalyst reusability, high yield of products, simple workup procedure, and mild reaction conditions. Allyl alcohols with different structures could be smoothly transformed to the corresponding aldehydes with about 90% yields. The ratio of linear aldehyde to branched aldehyde was about 22.486 The Rh/PPh3/silica system is also active for the hydroformylation reaction of unsaturated esters. For example, using this SILP catalyst, methyl acrylates could be hydroformylated with high regioselectivity and excellent yield to the corresponding branched aldehydes, with water as the reaction medium.487 8.1.3.3.3. Catalytic Carbonylation with Carbon Monoxide (CO). The oxidative carbonylation of amines to yield N-alkyl carbamate or N,N′-disubstituted urea is a potentially useful method for the nonphosgene-based synthesis of isocyanate. Deng et al. provided an approach to prepare supported nano-IL SILP catalysts for the oxidative carbonylation of amines.57 These SILP catalysts were obtained by confining Rh(PPh3)3Cl and IL [CnMIm][BF4] in silica gel through in situ hydrolysis of TEOS. Different from most confined IL catalysts, the catalytically active species and ILs were confined in a porous silica gel supercage, as illustrated in Figure 35. In this SILP, the solid matrix serves as the nanoscale reactor connected with inlets and outlets to confine the IL and metal complex, and to allow reactants and products to be transported in and out. In addition, the supercage environment and the restrictive dimensions of the silica gel interior may force unusual compound symmetry, coordination geometry, and coordinative unsaturation upon the entrapped IL and metal complex. As a result, these SILP catalysts exhibited high activity in the oxidative carbonylation of amines. The conversions of aniline

Fixed-bed gas-phase hydroformylation of propene was conducted using novel SILP catalysts containing immobilized Rh complexes (Rh-1) of the xanthene-based biphosphine ligand sulfoxantphos in [BMIm][PF6] and halogen-free IL [BMIm][n-C8H17OSO3] on silica support.441 These SILP catalysts were found to be more regioselective than catalysts without ligand and the analogous IL-free catalysts. The selectivity for linear aldehyde reached 96%. However, deactivation of the catalyst system occurred when the flow reaction was prolonged to 24 h. A long-term active catalyst could be obtained by confining the [BMIm][n-C8H17OSO3] microphase containing Rh-1 on a partly dehydroxylated silica support.439 The catalysis in the supported IL layer was demonstrated to be homogeneous. This catalyst was stable and performed well for at least 180 h with 95% n-butanal selectivity in the continuous, gas-phase hydroformylation of propene. Moreover, decreasing the number of surface silanol groups on the support material by thermal treatment was found to be essential to obtain a long-term stable hydroformylation catalyst. Furthermore, a kinetic study of this SILP catalyst in the continuous gas-phase hydroformylation of propene was performed by variation of temperature, pressure, syngas composition, substrate concentration, and residence time.433 The activation energy was determined to be 63.3 ± 2.1 kJ mol−1, which indicated that the catalyst was indeed a homogeneous complex dissolved in an IL film on a support. The Rh-SILP catalyst also performed very similarly to a homogeneous catalyst with regard to variation in syngas composition. These results demonstrated the excellent technical potential of SILP catalysis, which combines molecular defined homogeneous catalysis with heterogeneous, fixed-bed technology. The SILP catalysts for gas-phase propene hydroformylation were further explored in detail.480,481 The active complex was found to be stabilized by the interaction of sulfonate groups of the ligand with silanol groups on the silica support. However, when a large amount of IL was loaded, the interactions between silanol groups and sulfonate groups were partially inhibited and inactive species were formed, because of the interaction of IL with the silanol groups instead. The above SILP catalyst (Rh-1/IL/silica) was also found to be active for the hydroformylation of 1-butene in a Berty-type or fix-bed reactor.482,483 Furthermore, by partially modifying or replacing the components, such as the ligand or IL, the modified SILP catalysts could be used as active catalysts for the hydroAP

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Chemical Reviews

Review

% as a model catalyst, the relationship between the IL film distribution and the catalyst performance of the WGS SILP catalysts was further studied by solid-state NMR. The obtained results indicated an exchange process between the surface silanol groups of the support and the protons of the IL. During the deposition of the IL phase, the IL seemed to first fill the micropores of silica gel and form islands when the IL loadings were below 10 vol %. However, at higher loadings above 10 vol %, complete coverage of the support was achieved and the surface silanol signals almost disappeared. The NMR signals of the bulk IL became dominant when 20−40 vol % IL was loaded, indicating the formation of multilayer ILs on the pore surface of the silica gel. The NMR-based film model was confirmed by gas phase WGS experiments using SILP catalysts with different IL loadings. The maximum activity was obtained when the IL loading was 14 vol %, while a very high IL loading may depress the activity because of pore blocking.489 Instead of Ru metal complexes, Ru nanoparticles can also be dispersed into the IL microphase to form SILP catalysts for WGS reaction. Such kind of SILPs were obtained in two steps, starting with the preparation of Ru nanoparticles in IL [BMIm][BF4], followed by the hydrolysis of TEOS under acidic or basic conditions in the presence of the Ru/IL mixture. The ionogel catalyst prepared under acidic conditions showed much higher catalytic activity because of the large surface area, pore volume, and higher stability to confine the active component in the silica matrix, which resulted in a CO conversion of 37.8% at 140 °C, significantly higher than the 10% achieved with the ionogel catalyst prepared under basic conditions.449 8.1.3.5. Isomerization and Oligomerization. Isomerization and oligomerization of olefins using acidic SCAILP catalysts have been extensively studied.472,490,491 The first attempt was demonstrated by Lin et al. through fabrication of a SCAILP catalyst from chloroaluminate-based Lewis acidic ILs. The support material, sodium montmorillonite, was modified by intercalating halide salts such as [HMIm][Cl] into its layer structure through an ion exchange reaction. This step could expand the spacing of the silicate layers from 1.2 nm to 3.7−4.1 nm. A pyridine hydrochloride/AlCl 3 mixture was then introduced into the composite by impregnation. Under optimized reaction conditions, nearly quantitative conversion of endotetrahydrodicyclopentadiene to the desired product (exotetrahydrodicyclopentadiene) was achieved with quantitative selectivity for the newly developed clay-supported IL catalysts.490 The Lewis acidic ILs in this SCAILP catalytic system can be replaced by Brønsted acidic ILs. A silica gelconfined sulfonic acid-functionalized IL paired with either [TfO] or [HSO4] anion has been used in the oligomerization reaction of isobutene.491 The entrapment of the Brønsted acidic IL was conducted by impregnating the as-prepared silica gel in a methanol solution of IL followed by drying in vacuo. Although the oligomerization reaction carried out at 100 °C for 5 h normally gave 90−100% conversions no matter which catalyst was used, the anion of the IL has a strong influence on the product distribution. The selectivity for C8 product was 82% when the 1-(4-sulfobutyl)-3-methylimidazolium-based IL containing [HSO4] was entrapped. In contrast, only 16% selectivity for C8 was obtained when the anion was changed to [TfO]. 8.1.3.6. C−C Bond-Forming Reactions. C−C bond-forming reactions such as Heck and Suzuki reactions are among the most important transformations in organic chemistry from both an academic and industrial point of view. Such reactions are

and nitrobenzene were up to 93% and 92% when using Rh(PPh3)3Cl/IL/silica gel based on IL [DMIm][BF4] as the catalyst. The corresponding TOF number was as high as 11548 h−1. In contrast, the conversions of aniline and nitrobenzene were 2 mol(H2) mol(Ru)−1 h−1. In contrast, the TOF number of the commercial ShiftMax 240 catalyst was