Immobilization of ionic liquids: types of materials and applications

Supported liquid phase materials obtained by coating a solid support material with a thin liquid film have been investigated for catalytic applications for a long time. However, a common problem in these systems arises from the evaporation of the loaded liquid, particularly exacerbated when the liquid phase is water – supported aqueous phase systems. Away to overcome this shortcoming is by using liquids that do not evaporate at atmospheric conditions, as is the case of supported ionic liquids (SILs) that are obtained by immobilizing ionic liquids (ILs) in a suitable support material [1, 2]. Figure 1 depicts a schematic representation of SIL-based biocatalysis. The concept of SILs involving covalent anchoring or physisorption of ILs to solid supports was first described by Mehnert et al. [3] and Riisager et al. [4], respectively. Both groups of researchers proposed the use of silica-supported ILs in the hydroformylation of 1-hexene to form n,i-heptanal and in the continuous gasand liquidphase hydroformylation of propene and 1-octene, respectively; since then, distinct applications have been reported, being expected that this number continues to grow (Fig. 2). Immobilized ILs allow to overcome specific problems often observed with the application of bulk ILs in the liquid state, namely, high viscosity, low diffusion coefficients, difficulties in product purification and solvent recycling, and high cost derived from the large quantities of ILs required. From an engineering point of view, SIL materials present many advantages over classical gas-liquid or liquid-liquid systems, namely, a high surface area supplied by the support, a thin film of liquid that circumvents mass transport problems, adjustable solvent properties, application of fixed-bed or fluidized-bed reactor technologies, and efficient


Table of Contents
Chapter -Immobilization of ionic liquids: types of materials and applications

Definition:
SILs -Supported Ionic Liquids are obtained by the immobilization of ionic liquids in a solid support, either by physical confinement or covalent grafting.

Introduction:
Supported liquid phase materials obtained by coating a solid support material with a thin liquid film have been investigated for catalytic applications for a long time. However, a common problem in these systems arises from the evaporation of the loaded liquid, particularly exacerbated when the liquid phase is watersupported aqueous phase systems. A way to overcome this shortcoming is by using liquids that do not evaporate at atmospheric conditions, as is the case of supported ionic liquids (SILs) that are obtained by immobilizing ionic liquids (ILs) in a suitable support material [1,2]. Figure 1 depicts a schematic representation of SIL-based biocatalysis. The concept of SILs involving covalent anchoring or physisorption of ILs to solid supports was first described by Mehnert et al [3] and Riisager et al [4], respectively. Both groups of researchers proposed the use of silica supported ILs in the hydroformylation of 1-hexene to form n,i-heptanal and in the continuous gas-and liquid-phase hydroformylation of propene and 1-octene as main applications of these materials; since then, distinct applications in other scientific domains have been reported, being expected that this number continuous to grow ( Figure 2). Immobilized ILs allow to overcome specific problems often observed with the application of bulk ILs in the liquid state, namely high viscosity, low diffusion coefficients, difficulties in product purification and solvent recycling, and high cost derived from the large IL quantities required. From an engineering point of view, SIL materials present many advantages over classical gas-liquid or liquid-liquid systems, namely a high surface area supplied by the support, a thin film of liquid that circumvents mass transport problems, adjustable solvent properties, application of fixed-bed or fluidized-bed reactor technologies, and efficient catalyst immobilization in a confined environment. Simultaneously, the immobilization of ILs in the surface of "hard" solids can modify and improve the properties of the solid supports, such as wettability, lubricating features and separation efficiency [1,5,6]. Owing to the remarkable advantages of ILs immobilized in solid supports, a plethora of applications in distinct fields, ranging from chemical to biological sciences, have been developed, as summarized in Figure 3.   IL and the support. Adapted from [1,8].
Covalent bonding is established for the chemisorption of an ILusually monolayered -to a support material, as is the case of SILLPs. In these materials, specific properties of the bulk ILs are lost since there is no longer a liquid phase.
However, the IL chemical structure and specific functional groups are still present, which may contribute to improve the target material properties and application performance. This chemical coupling method presents however some limitations and drawbacks, such as the low number of ions resulting from the assembly of a monolayer of IL onto a support, which may limit the performance of catalysis, and the materials pretreatment requirement when it is necessary to render them with a large number of linking functional groups in the outer surface before the IL attachment [1,6,9].
The second immobilization method in the SILs framework corresponds to the simple physisorption of the IL to the surface of a high surface area support. A disadvantage of this approach is related to the detachment or leaching of the IL from the support since the IL is held by electrostatic interactions, van der Waals forces and/or hydrogen bonding. Nevertheless, this concept presents relevant advantages, such as: 1) the extremely low vapor pressure of ILs allows 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; 2) unlike the monolayer of ILs resulting from the covalent grafting method, the multilayers of ILs achievable by physisorption and the specific bulk properties of ILs can be retained to some extent; 3) the ILs physically entrapped in porous matrices may be removed using adequate solvents, allowing the ILs and materials recycling; 4) there is a larger number of ILs that can be used in the physisorption method, allowing to take advantage of the ILs designer solvents ability to fulfil special requirements of a given application [1,5,6].
According to the type of SILs and supports, distinct preparation methods have been described. Physical confined systems can be prepared using relatively simple impregnation techniques of the IL dissolved in a suitable organic solvent or water, the addition of the support, and finally, solvent evaporation under vacuum. Other methods for introducing ILs into nanoconfined environments are post-impregnation, and the "ship-in-a-bottle" method. Contrarily to physical confined systems, grafting of functionalized IL fragments can be achieved using traditional sol-gel synthesis, and often involves multi-step reactions in which pre-functionalized solid supports react with ILs containing specific functional groups. In this case, the chemisorption of the IL to a solid support can be achieved using two synthetic routes, namely by: 1) preparing ILs first, followed by their direct self-assembly on the support; or by 2) self-assembling of the imidazole moiety on the support surface via covalent bonds, followed by the preparation of ILs directly on substrate surfaces. Covalent grafting of ILs in materials have been achieved by the "thiol-ene" click chemistry, surface radical chain-transfer reaction, nucleophilic substitution reactions, alkylations and condensations [1,2,6,8,9,10]. The main characteristics, advantages and drawbacks of SILP and SILLP systems are summarized in Table 1. Table 1 -Summary of characteristics, advantages and drawbacks of SILs, according the IL-support type of interaction.

Support materials:
SILs have been prepared using distinct types of materials, including silica (silica gel [11], mesoporous silica -MCM-41 [12], or silica nanoparticles [13]), carbon nanotubes (CNTs) [14], polymers (e.g. polyethersulfone -PES) [15], crystalline materials such as porous chromium benzenedicarboxylate - Lavoisier) [16], and inorganic materials such as γ-alumina [17]. These materials and IL chemical modifications introduced are listed in Table 2. The choice of the support material is important since the structure and properties of confined ILs are significantly influenced by the pore structure and the surface chemistry of the support, which dictates the nature and extent of the IL−pore wall interactions; however, after confinement, changes in the structure and properties of the support materials and ILs are additionally observed, as reviewed by Zhang et al [6].
Porous silica gels with a high surface area (ca. 300-500 m 2 g -1 ) present high thermal and mechanical stability, being also low cost materials, and as such, arranged channel-type mesostructure with a pore wall thickness between 3.1 -4.8 nm, but presents higher thermal and hydrothermal stabilities than MCM-41 [2,9,18]. As with silica, active species are commonly grafted onto γ-alumina (γ-Al2O3) through their hydroxylated surface, which can be easily casted as a continuous membrane film [17].
Nanoparticles have a large surface area that is highly advantageous in many applications. Sidhpuria et al. [13] investigated the catalytic activity of silica nanoparticle catalysts (SILnPs) for the dehydration of fructose to 5hydroxymethylfurfural and found an improved performance over zeolites and strong acid ion exchange resins catalysts. The authors additionally demonstrated that the material can be recycled over seven times without significant losses in fructose conversion and 5-hydroxymethylfurfural yield. In addition to the large surface area provided by nanoparticles, their magnetization may ensure an easy separation of the target product through the application of an external magnetic field. Bouri et al. [19] applied silica-coated Fe3O4 magnetic nanoparticles immobilized with the IL Nmethylimidazolium hexafluorophosphate for the extraction and preconcentration of sulfonylurea herbicides from water, taking advantage of the easy recovery of the analytes through the application of an external magnetic field Although less studied, an alternative approach relies on the use of magnetic ILs supported in chitosan nanoparticles, as exemplified with the preparation of chitosan supported ethylmethylimidazolium tetrachloroferrate (EMImFeCl4) nanoparticles and further application in catalysis [20]. In addition to nanoparticles, graphene sheets doped with Fe3O4 and covalently bound ILs have been prepared, demonstrating to have potential in magnetic dispersive solid-phase extraction (MDSPE). The IL plays a double role, namely by improving the adsorption capacity and by enhancing the dispersion stability of the materials in aqueous solution [21].
Due to their good mechanical strength, high chemical stability and large surface-area-to-volume ratio, CNTs are also promising building blocks for SILs.
Contrarily to porous silica, the interior of porous carbons is difficult to confine, often requiring harsh treatment conditions, such as strong acids or high-temperature oxidation. The functionalization of CNTs is usually achieved by oxidation in aqueous HNO3 to yield carboxylic acid groups on the surface, and IL-NH2 or IL-OH are introduced to the surface via -CONH-or -COO-bonds, forming IL-CNT systems [2,6].
MOFs are a new generation of porous crystalline materials known for their high surface area, large pore volume, and high tunability of their pore size and shape.
MOFs have recently shown high potential as appropriate microporous matrices for the immobilization of ILs [6,16]. The mesoporous structure and high porosity displayed by MIL-101 contributes to their enhanced performance in adsorption processes [16].
In addition to silica-based materials, CNTs, and MOFs, polymers have been also modified with ILs. Envisaging the stabilization of a high amount of paladium nanoparticles, multilayered materials were prepared by the polymerization of bis(vinylimidazolium)bromide on silica and SBA-15 [5]. The advantage of these polymeric materials is that the polymer, or the polymer anchored to a solid support, is highly stable, easy to handle, and can be easily separated from the reaction mixture [9]. In addition to these, the polymers themselves can be directly used as porous hosts for ILs, often resulting in the formation of flexible membranes [6]. For example, hydrophilic polyethersulfone (PES) has been used as a support material of different imidazolium-based ILs, creating supported ionic liquid membranes (SILM) for gas separation [15]. Generally, polymer confined ILs are obtained by in situ polymerization of monomers in ILs or by a solvent casting method [6]. SILM γ-Al2O3

Applications of supported ionic liquids:
The relevant properties displayed by ILs supported in a wide range of materials attracted the interest of researchers to develop novel applications in fields ranging from chemistry to biological and biomedical sciences. Selected applications of SILs are provided in Table 3. the support can be impregnated with the IL containing the dissolved catalyst (usually a metal); 2) the IL may be covalently anchored to the support and the catalyst impregnated on the modified support; 3) the covalently attached or physisorbed IL contains specific functional groups and acts as the catalyst; and 4) the IL is firstly covalently anchored to the support via the cation, and the anion is exchanged by an anionic catalyst species [18]. Taking into account economic features and toxicity/biodegradability concerns, it is highly desirable to minimize the amount of ILs used [3]. Due to an increase in the number of accessible active sites of the catalyst and decrease of mass transfer restrictions, SILPs allow a more efficient use of the catalyst and a significant reduction of the amount of IL required [10].
Due to their advantages, SILPs are particularly relevant in biocatalytic applications. Enzyme-SIL biocatalytic systems have been successfully applied with distinct enzymes, which may be immobilized by simple physisorption (SILP) or through chemical bonding (SILLP) in a wide range of support materials, including ceramics, macroporous membranes and monoliths, celite, magnetic nanoparticles, polymers and resins [22]. separations with low column bleed [23]. In addition to gas chromatography, immobilized ILs on supports like silica and monoliths have been widely applied as stationary phases in liquid chromatography, allowing to work in a multi-mode separation approach based on the establishment of several intermolecular interactions, including anion-exchange, and electrostatic interactions, π-π and hydrophobic interactions, thus allowing different modes of operation [8].
Silica gel, hierarchical mesoporous silica and silica nanoparticles, as well as single-walled CNTs and MOFs, are usually applied as solid supports and have been used as sorbents for solid-phase extraction, in catalysis (epoxidations, dehydrations), for immobilization of enzymes and in desulfurization [6,11,12,13,14,16].
In particular, significant achievements have been made with SILs in the pursuing of alternative cost-effective strategies for gas sweetening and gas capture and purification. Among the strategies used for carbon dioxide capture and recovery, liquid amine solutions are one the most widely applied, even though they present many disadvantages such as high energy consumption during absorbent regeneration, equipment corrosion, unavoidable loss, and degradation of the amine. However, these drawbacks can be circumvented by using ILs, which due to their negligible volatility, allow to recover a non-contaminated target gas. An additional interesting feature is that the captured CO2 can undergo catalytic conversion into valuable compounds, especially in the case of electrochemical reduction [6]. Furthermore, by knowing the solubilities and diffusivities of gases in ILs it is possible to design absorption processes, namely in the gas sweetening process [24]. Nanoconfinement of ILs in solid supports appears as a solution to the slow sorption and diffusion rates often observed with bulk ILs. The IL confinement may also lead to an increase in CO2 solubility [6]. Selective permeation through SILMs (γ-alumina-, polyethersulfonebased, among others) has shown to be particularly promising for the separation of a target compound from a mixture of gases [15]. Capillaries are coated using the static coating method and coating solutions of monomers or cross-linker ILs are prepared in dichloromethane (the radical initiator is added before dichlorometane); after coating, the ends of the capillary are flame sealed.
Gas chromatography stationary phase for the separation of fatty acid methyl esters (FAMEs) and polycyclic aromatic hydrocarbons (PAHs) [24] Gas separation SILM γ-alumina 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate The IL is impregnated in the top of two γalumina layers of a tubular porous asymmetric membrane support and their properties (e. g. pore size and surface area) can be properly tailored.
Polymers and inorganic supports such as γ-alumina have been investigated as support materials to immobilize ILs, allowing to prepare membrane films. Amongst all support materials, silica-based materials are usually the preferred choice, which is due to their high mechanical stability, low toxicity, low cost and easiness in functionalization. Most of these materials can be magnetic, e.g. by using a Fe3O4 magnetic core, thereby permitting an easier separation of products, while avoiding extensive filtration and centrifugation procedures.
IL-modified supports with well-defined and tunable properties have been obtained, with a high performance in distinct applications. SILs have been mainly used in chemical catalysis and biocatalysis. SIL-based catalysis relies on two main approaches, consisting either in the dissolution of the catalyst in the IL or being the IL the catalyst itself. Due to an increase in the number of accessible active sites of the catalyst and decrease of mass transfer restrictions, SILPs allow a more efficient use of the catalyst and a significant reduction of the amount of IL required. In addition to catalysis, SILs have been largely investigated in separation processes. This interest is due to the multitude of interactions that ILs can establish with the target species, allowing to improve extraction efficiency and selectivity by a proper tailoring of the IL ions chemical structure. SILs have been investigated as sorbent materials in the solid-phase extraction of organic compounds and trace heavy metals, leading to a higher recovery, a lower LOD and a lower RSD in comparison to more common methods. In this field, gas chromatography stationary phases containing immobilized ILs are one of the most relevant findings of the past decade, being currently commercialized. Significant achievements have also been accomplished with SILs in the pursuing of alternative cost-effective strategies for gas sweetening and gas capture and purification. Nanoconfinement of ILs in solid supports appears as a solution to the slow sorption and diffusion rates often observed with bulk ILs.
It was already demonstrated that SILPs allow to improve the performance of several processes. However, given the plethora of ILs chemical structures, types of materials that can be modified, and applications, the SILPs arena is still in its infancy.
Accordingly, more related investigations are expected in the following years, ultimately resulting in materials with commercial potential for catalysis, gas purification, as chromatographic columns, among others.