2.1 Specific properties of imidazolium ionic liquids

Most works involving sol-gel processing in ionic liquids were performed in imidazolium salts, widely used as solvents in organic chemistry, organometallic catalysis, electrochemistry and separation processes [18]. Many of their typical features arise from the acidic character of the ring proton in C2 position (C2H), located between two electronegative nitrogen atoms [19]. Thus, the interactions between C2H and anions that are disclosed in crystalline structures play a key role in the structure of liquid phases through extended hydrogen bonded networks resulting in supramolecular ions aggregates [20,21]. This contributes to distinguish them from other ionic liquids which generally form ion pairs. With long-chain imidazolium salts, the crystal packing results in alternate polar and non polar layers as in the typical structure of 1-dodecyl-3-methylimidazolium hexafluorophosphate (Fig. 1) [3].

For a longer carbon-chain, this layered structure is maintained in the liquid state as a smectic phase. Thus, in hexadecylmethylimidazolium hexafluorophosphate a liquid crystal domain exists in the range of temperature between about 70 and 120 °C [3]. Surprisingly, there is no more smectic phase when the anion is change to bis(trifluoromethylsulfonyl)imide [NTf₂], which well illustrated the key role of anion-cation interaction [22]. However, even “isotropic” room-temperature ionic liquids are structured on a nanometer-scale [23]. Computer simulation predicted that pure ionic liquids of the 1-alkyl-3-methylimidazolium family show structuring of their liquid phases in a manner that is analogous to microphase separation between polar and nonpolar domains (Fig. 2) [24]. It was observed that the polar domain has the structure of a tridimensional network of ionic channels, whereas the nonpolar domain is arranged as a dispersed microphase for ethylmethylimidazolium ILs and as a continuous one for longer side-chains, such as hexyl, octyl or dodecyl. The butyl side-chain would mark the onset of the transition from one type of structure to the other. As a result, ionic liquids can accommodate both ionic and molecular solutes.

Recent studies evidenced that the dissolution of some surfactants in imidazolium ILs depressed the surface tension in a manner analogous to aqueous solutions [25]. This phenomenon indicates that there are solvatophobic interactions with the hydrocarbon moieties of the surfactants [26]. Accordingly, ILs (both protic and aprotic) support amphiphile self-assembly, in relation to their cohesive energy density (measured by the Gordon parameter) [27]. What is important in sol-gel synthesis is that imidazolium ILs may be regarded both as structured and structuring media, their nanostructural organization acting as “entropic driver” for spontaneous extended ordering of oxide-based structures.

2.2 Gels by impregnation of silica particles

Among a number of studies on nanoparticles, a striking stabilization in ILs has been reported, even in the absence of any stabilizers, as surfactants and polymers [28]. Nevertheless, bare silica colloids were shown to be unstable in ILs, leading to the formation of an interconnected particulate network. In fact, the addition of only 2 to 3 wt % of silica nanoparticles made 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([C₂MIm][NTf₂]) dispersions gelled (Fig. 3) [29].

It has been postulated that charged colloidal particles should easily approach each other, due to the charge screening effect, as far as particles do not have any other repulsive force. Actually, the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which readily explained the qualitative feature of colloidal stability in terms of the interplay between the London-van der Waals attraction and electrostatic repulsion, suggested that in IL-colloid systems the electrostatic repulsive force hardly stabilizes bare silica particles because of the high ionic strength of ILs, even though the particles are assumed to be highly charged [30].

A specific nanostructure of ionic liquids is likely induced at the surface of nanoparticles, which should influence aggregation processes. Thus, the structure of the IL/SiO₂ interface was recently investigated by sum-frequency vibrational spectroscopy (SFVS) for a series of hydrophobic ILs composed of 1-alkyl-3-methylimidazolium (C_nMIm, n = 6, 8 and 10) and bis(trifluoromethylsulfonyl)imide (NTf₂) or bis(trifluoroethylsulfonyl)imide (BETI) anion. SFVS disclosed the orientation of the cation as well as the structure of water at the IL/SiO₂ interface [31,32]. The alkyl chain of the imidazolium cation was determined to be nearly normal to the surface. The water at the surface associated with the IL was determined to be hydrogen-bonded either singly or doubly to the anions at the SiO₂ interface. In addition, the nature of the hydrogen-bonding was found to be dependent on the amount of bulk water contained in the IL. However, the surface modification of silica particles can afford the complete stabilization of dispersions in ILs, as recently shown with polymer-grafted silica particles [29]. Note that emulsions of ionic liquids (both ionic liquid-in-water and ionic liquid-in-oil types) were reported, which were stabilized using fumed silica nanoparticles coated to different extents with hydrophobic methyl groups [33].

The gelation of ionic liquid silica mixtures was shown to take place over a range of ionic liquid content, which depends on the particle size of the silica used as the solidifying agent. Thus, with a particle size of 70 nm, powders turned into gels near 30 wt % of IL, before turning to viscous above 40 to 50 wt % of IL. Ionogels containing more than 90 wt % of IL could be obtained with particle size of 7 nm; however with only 80 wt % of IL they became viscous as early as 120 °C [34]. Quasi-solid electrolytes with liquid-like mobilities could be prepared by simple mechanical mixing process, which were successfully used in high-performance dye-sensitized solar cells (DSSCs) [35]. However, the use of ionic liquids adsorbed on silica particles as free flowing powders has given rise to the so-called supported ionic liquid phase catalysis (SILP; vide infra).

2.3 Gels from silicon alkoxides

Sol-gel processes involving silicon alkoxide precursors, in which the interstitial ionic liquid phase, first present between colloidal particles, is then confined as condensation cross-linking extends, should result in more intimate biphasic systems than the simple impregnation of oxide particles. This approach was developed a few years ago by using a non-aqueous method inspired from the pioneering work of S. Dai et al. on the preparation of aerogel-like silicas (see below) [36], from a mixture of tetramethoxysilane (TMOS) and excess formic acid [37–39]. The method was shown to be a general route to monolith silica ionogels from ionic liquids (e.g. alkylmethylimidazolium [CnMIm] or butylpyridinium [C₄Py] cations combined with a wide range of anions). Other authors reported the immobilization of some ionic liquids as [C_nMIm][BF₄] (n = 2, 4, 10, 16) or [C₄MIm][PF₆] by hydrolytic sol-gel processing using HCl aqueous solution as a catalyst [40].

Besides the preparation of ionogels, ionic liquids have been reported to have significant influence in sol-gel processes on the structure of the resulting silica-based materials, whatever used as additives or as (co-)solvents (Table 1). The effect of ionic liquids as drying control chemical additives was ascribed to the formation of a non volatile liquid film on the walls of pores which could protect them from the interface strains associated with the formation of meniscus on evaporation [41]. This permits the pore walls to strengthen on ageing before the solvent extraction or calcination of ILs, thus giving access to highly porous structure.

As mentioned above, the first sol-gel synthesis in an ionic liquid was reported in 2000 by S. Dai et al., from a mixture of tetramethoxysilane (TMOS) and formic acid in [C₄MIm][NTf₂] [36]. All reactants were liquid and miscible at room temperature; gelation occurred overnight, leading to transparent monoliths after curing at room temperature for three weeks. The ionic liquid was used as a template solvent; after extraction by refluxing in acetonitrile, the porous structure was shown to be close to that of aerogels prepared from molecular solvents after supercritical drying. The N₂ sorption isotherms disclosed a mesoporous structure with typical hysteresis loops. It was shown that the porosity could be adjusted by varying the ratio ionic liquid /silica (mean pore diameters in the 7–20 nm range) [42]. The use of acetic acid was reported similarly to prepare porous aminopropylsilsesquioxanes in [C₄MIm][PF₆] [43].

Similarly, Deng et al. reported that [C₄MIm][BF₄] and [C₄MIm][PF₆] ILs could be completely washed out from the silica matrices under acetone refluxing conditions [40]. Interestingly, the porosity was shown to be influenced by the anion. Thus, in the case of [C₄MIm][PF₆], mesoporous silica gels were also obtained when the ionic liquid was removed, but the corresponding average pore diameters were much larger and the pore-size distribution became quite wide relative to that of silica gels in which the same amount of [C₄MIm][BF₄] was initially confined.

Controlled periodic porosity could be generated in silica-derived materials by using long chain imidazolium salts. The use of [C₁₆MIm][Br] as surfactants in the preparation of MCM-41 instead of traditional [C₁₆NMe₃][Br] illustrates this way [44]. Alternatively, highly ordered monolithic supermicroporous lamellar silicas were prepared under acidic conditions in [C_nMIm][Cl] (n = 10, 14, 16, 18) at temperatures above the melting points of the ILs in a so-called nanocasting sol-gel technique [45,46]. The lamellar arrangement was ascribed to the liquid crystal self-organization of the reaction media (Fig. 4).

In contrast, the use of [C₄MIm][BF₄] led to monolithic mesoporous silica with worm-like structure [47]. Note that mesoporous organosilica materials were successfully synthesized using [C₁₆MIm][Br] as a template, under basic conditions [48].

The role of the IL anions in hydrolytic sol-gel processing has been recently addressed. Pierre et al. reported the catalytic activity of alkylmethylimidazolium tetrafluoroborate and chloride IL, probably in relation to the nucleophilic character of anions [49]. Note that BF₄ anion has been reported as a powerful promoter for hydrolytic condensation of alkoxysilanes in the synthesis of mesoporous silica [50]. The synthesis of silica xerogels, using HF as a catalyst in the presence of 1-monoethylene glycol monomethylether-3-methylimidazolium cation based ILs, led to highly distinct morphologies depending on the counter anion: [NTf₂] anion induced the formation of a compact lamellar monolith with a flat surface, while [BF₄] anion led to a free flowing powder of aggregated spherical particles and [PF₆] anion to a porcelain-like aggregates with honeycomb shapes [51].

2.4 Gels from metal alkoxides

Parallel to silica-based ionogels, several other oxides were investigated. Among them, titania was the most studied. Indeed, the high photo-activity of TiO₂ currently attracts most attention for applications as photocatalysts for air and water purification, solar cells and opto-electronics. The physical and chemical properties of TiO₂ closely depend on its particle size, morphology and crystalline phase. In 2003, Nakashima and Kimikuza reported the preparation of hollow TiO₂ microspheres by means of an interfacial sol-gel reaction method in which [C₄MIm][PF₆] was added to a solution of toluene containing titanium tetrabutoxide [52]. The same year, Zhou and Antonietti were the first to report the synthesis of very fine crystalline anatase particles and their assembly toward mesoporous spherical aggregates using a mixture of TiCl₄ and [C₄MIm][BF₄] in water at 80 °C [53]. Dionysou systematically studied the synthesis and the characterization of mesoporous TiO₂ by reaction of titanium isoproxide and water immiscible [C₄mim][PF₆] [54,55]. In another work, a peptization process involving [C₄MIm][BF₄] was applied to the preparation of nanostructured anatase TiO₂ monoliths [56]. In all these studies, anatase-containing nanostructured TiO₂ particles with a high surface area and narrow pore size distribution were obtained at low temperature. Moreover, the TiO₂ nanoparticles were shown to be thermally stable, resisting to pore collapse and to anatase-to- rutile conversion on annealing. Using a very small amount of ionic liquid (one-hundredth of the earlier experiment), Choi managed to synthesize stable particles at high temperature (400 °C), which kept anatase structure even after prolonged treatment at 600 °C [57]. Finally, TiO₂ nanocrystals could be easily obtained using a microwave-assisted route [58]. The role of the ionic liquid in the sol-gel process is not so clear; Zhou and Antonietti postulated the reaction to proceed via reaction-limited-aggregation. Dyonisou proposed that [C₄MIm][PF₆] acted as a capping agent due to its water immiscibility and thus prevent direct hydrolysis of titanium tetraisopropoxide resulting in the formation of completely condensed systems. Kim et al. also studied the synthesis of mesoporous TiO₂ and postulated that the essential factor for structuration was the strength of hydrogen bonds between the anion of the ionic liquid and water [59]. More recently, Liu et al. focused on the role of the IL during the crystallization process to anatase in lower temperature treatments. They evidenced higher anatase crystallinity for [C₄MIm][BF₄] resulting from a better ionic liquid-water hydrogen-bonding [60].

Rutile TiO₂ nanostructures could also be obtained at room temperature from TiCl₄ [61–63]. Hu demonstrated that the formation of rutile phase only occurred when [NTf₂] ILs were used. In the presence of [BF₄] and [PF₆] anions, anatase phases were observed [63]. This specific influence is not clear, but it could be attributed to a reversible ligand [NTf₂] exchange on the Ti surface sites in the amorphous state. Increasing the amount of TiCl₄, the whole morphology of rutile particles from nanorods to microcones and microspheres was illustrated by Zheng [61].

Recapitulative Table 2 shows the general trend to form anatase from Ti(OⁱPr)₄ and ionic liquids as [C₄MIm][BF₄] or [C₄MIm][PF₆] and to form rutile from TiCl₄ and [NTf₂] or halide containing ionic liquids.

The use of IL in the synthesis of tin dioxide is very recent. In 2008, Dong and Liu described the synthesis of 2.5 μm SnO₂ microspheres under microwave conditions using IL as templates and starting from SnCl₄ in aqueous conditions [64]. Among the ILs used, only BF₄ containing ionic liquids were shown to be able to produce these microspheres. On the other hand, interactions between SnO₂ and hydrophobic PF₆ anion were supposed to be too weak to promote nucleation and growth, whereas strong interactions with water would minimize the interactions with SnO₂ nuclei in the case of hydrophilic chloride anion. At the same time, using [C₁₃MIm][Br] as template and SnCl₄ as precursor, Yan et al. reported the synthesis of nanocrystalline porous tin oxide SnO₂ materials, which showed a tetragonal cassiterite structure after annealing. These materials showed some potential for gas sensing applications [65]. In a recent work, we explored a quite different approach, using a β-diketonato stabilized tin precursor to produce translucent monolith ionogels. A major advantage over the previous results was the possibility to use a wide range of IL ([C₄MIm][BF₄], [C₄MIm][PF₆], [C₄MIm][Br]), thanks to the chelating ligand on the tin precursor which permitted to control the polycondensation rate [66].

The fabrication of large mesoporous alumina nanostructures was recently reported by Hong et al. using ionothermal synthesis starting from aluminium tri-sec-butoxide [67]. This technique, first described by Morris, relies on the use of an ionic liquid as both the solvent and the structure-directing agent [68]. One major advantage is that most ionic liquids have vanishingly small vapor pressures, which means that no autogenous pressure is produced on heating and that ionothermal synthesis can take place at high temperature while keeping the pressure at ambient levels. Indeed, the synthesis was performed in an open container under ambient pressure at 120 °C. Once again, hydrogen bonding and co-π-π stacking interactions induced the formation of the oxide-based structure, and finally the conversion into boehmite crystallites, as proved by FT-IR and XRD analysis. The same authors also reported the synthesis of alumina ionogels with various shapes, including 1D nanorods with hexagonal tips, straight and curved nanofibers embedded in a porous network, 3D octahedral-, polyhedral- and angular spherical shapes [69]. These hybrids were still obtained via a one-pot ionothermal process by tuning cooperative interactions between the ionic liquid and the building blocks (i.e. by varying the nature of the IL anionic part and the humidity conditions). In the case of [C₄MIm][BF₄], high proton conductivity was obtained as a result of continuous proton conduction channels and confinement effect.

The only sol-gel synthesis of vanadia in ionic liquid media was reported by Endres’ group. These authors used two different classes of IL, pyridinium and imidazolium salts: a higher crystallinity was observed for the samples prepared from pyridinium [NTf₂] in acetone, whereas a higher porosity was observed for those prepared from imidazolium [NTf₂] in isopropanol as cosolvent. The surface area could be enlarged four times using acidic hydrolysis instead of pure water [70].

In some cases, mixed oxides make it possible to overcome some drawbacks encountered with the single oxides. For instance, TiO₂ is a good catalyst but it exhibits low surface area, whereas Al₂O₃ is stable, has acceptable surface area, but is poorly catalytically active. Accordingly, TiO₂-Al₂O₃ mixtures are interesting for catalytic applications. Using a sol-gel synthesis in the presence of [Pyr][NTf₂], amorphous alumina-titania powders were obtained. After calcinations, the surface reached around 500 m².g^-1 whereas the surfaces of boehmite and titania were around 100 m².g^-1. The authors found that the presence of alumina stabilized the anatase phase and elevated the temperature at which the phase transformation to rutile occurred [71]. Another example was given by the synthesis of mesoporous Al-MCM-41 using [C₁₆MIm][Br]. The material obtained was shown to have a high surface area and uniform mesopores, and to keep the tetrahedral environment of aluminum after calcination [72].

3.1 Stability of ionogels

Despite no accurate study of the interfacial tension between silica and IL has been performed, a strong wettability of porous silica networks by ILs was observed. In fact, monolithic silica ionogels did not undergo any expulsion of [BMIm][NTf₂] when submitted to around 10 kHz centrifugation in a MAS NMR rotor probe [73].

As mentioned above, ionogels are quite stable in non polar organic solvents such as toluene or alkane, whereas the ionic liquid can generally be extracted by polar solvents such as acetonitrile, dichloromethane or ethanol. In some cases, monolith gels could be obtained after washing by extraction with a polar solvent and then refilled with a new ionic liquid. This method made it possible to incorporate highly sensitive metal complexes (which otherwise would have decomposed under the conditions of sol-gel synthesis) within the ionogel. In the case of immersion in water, ionogels were found to be quite stable as far as the ionic liquid was non water-soluble and enough hydrophobic methyl groups were present in the silica network (i.e. some methyltrialkoxysilane was added in the starting precursor solution [42].

As expected, the ionogels exhibited the same thermal stability as the ionic liquids alone. It is noteworthy that the organic modification of silica, by incorporation of a large amount of methyl groups, had no significant effect on the thermal stability [42].

3.2 Effect of confinement on the physicochemical properties of ionic liquids

Ionogels stemming from silicon alkoxides could be pictured as two interpenetrating continuous networks of silica and ionic liquid intermingled at the nanometer scale. Accordingly, the nanoconfined ionic liquid phase experiences a large interface with pore walls which is expected to modify its microscopic structure and dynamics with respect to the bulk phase.

¹H MAS NMR experiments carried out on [BMIm][NTf₂] containing ionogels exhibited quite unexpected high resolution with respect to that usually observed in the solid state [42,73]. This showed liquid-like dynamics of confined ILs in monolithic ionogels (15 nm pore diameter silica host networks containing about 80 wt % [C₄MIm][NTf₂]), since a 400 Hz MAS was sufficient to average the interactions usually broadening the signals in the solid state. T₁ relaxation-time measurements, which gave dynamic information for longer spatial range correlations, confirmed that dynamics of confined ILs was only slightly slowed down [73]. Moreover, ¹H MAS NMR spectra disclosed a magnetic susceptibility gradient, as evidenced by the lack of narrowing of signals when comparing FWHM from 400 to 9000 Hz spinning rates; the resulting distribution of chemical shifts was ascribed to the fact that IL species could be localised either at the centre of the pores (where magnetic susceptibility only arose from surrounding parent anions and cations), or at the wall neighbourhood (where magnetic susceptibility was also influenced by anisotropic constraints on close IL species). Effects of wall neighbourhood on structuration were also reflected by differential scanning calorimetry (DSC) [42,74]. In fact, even though confinement did not affect the glass transition, DSC disclosed the dramatic effect of confinement on both cold crystallization and melting, which even could be no longer observed on increasing the confinement, i.e. on decreasing the mean pore size (Fig. 5). It is worth noting that when cold crystallisation and melting were observed, the melting temperature (T’_m) was lowered with respect to the pristine IL (T_m), whereas the cold-crystallization temperature (T_cc) was shifted to a higher value (T’_cc). Actually, considering the pore/solid and pore/liquid interface energies γ_p/s and γ_p/l, respectively, the effect of confinement is to be related to the chemical features of pore walls, through a Gibbs-Thomson-like equation (T’_cc–T_cc)/T_cc) = -2(γ_p/s-γ_p/l)V_mol/Dλ_f, where V_mol is the molar volume of the IL, λ_f the latent heat of melting, and D the pore diameter [75].

3.3 Use of ionogels as electrolytes

Most sol-gel derived materials are electronic insulators with low charge-transfer efficiency and substrate diffusion. Besides that, ionic liquids are now well-known for their use in dye sensitized solar cells, lithium batteries, supercapacitors and fuels cells, owing to some of their properties such as non flammability, easy solvation of the selected salt (charge carrier), possible non miscibility with water. [76–79] Immobilizing ionic liquids in oxide gels offers a way to endow these oxides with ionic liquid properties, apparently antinomic. Roughly, confined ILs showed the same order of magnitude of conductivity as pristine ILs, which is consistent with their liquid-like dynamics [42]. Their ionic conductivity followed an Arrhenius-like behaviour with temperature, which, of course, could be correlated to viscosity through the Walden product if sufficient data were available. However, besides temperature, water content also influences viscosity: only 2 wt % (20 mol %) of water in [C₄MIm][BF₄] reduces its viscosity by more than 50% [80]. Although this trend strongly depends on the affinity of the IL with water ([C₄MIm][BF₄] is water soluble), any IL shows a decrease in viscosity when the amount of dissolved water increases. This point is to be taken into account in ionogels, since sol-gel methods release water, alcohol and other by-products, making it difficult to ascertain the purity of the confined IL, even after a prolonged heat treatment under vacuum.

Anyway, ionogels that combine the chemical versatility of ILs with the shaping versatility of sol-gel, offer a new way to design conducting materials for electrochemical devices, such as batteries, fuel cells, electrochromic windows or photovoltaic cells. Thus, Stathatos et al. fabricated TiO₂ dye-sensitized quasi-solid-state solar cells by using an electrolyte prepared by sol-gel, which was based on silica and a mixture of a surfactant, propylene carbonate, iodine, and 1-methyl-3-propylimidazolium iodide [81]. Interestingly, an IL based on an imidazolium iodide in which the dialkylimidazolium cation was derivatized by covalent attachment of a trimethoxysilane group (namely 1-methyl-3-[3-(trimethoxy-λ⁴-silyl)propyl]-1H-imidazolium iodide) was synthesized and used as a sol–gel precursor to prepare quasi-solid-state electrolytes. Acid catalysed hydrolysis led to positively charged polyhedral cube-like silsesquioxane species, which still contained a small amount of silanol end groups that were removed after heating at 200 °C [82]. Alternatively formic acid solvolysis at 120 °C (in the absence of water) was used as well [83]. The resulting material was a tough, yellowish and transparent solid, consisting of ladder-like polysilsesquioxane species bearing imidazolium end-groups with iodide counter-ions [84]. The addition of iodine (and some imidazolium IL) resulted in quasi-solid-state redox electrolytes for dye-sensitized photoelectrochemical cells of the Graetzel type [83,85,86].

3.4 Use of ionogels in optics

Ionic liquids are very interesting as optical solvents, not only for their high stability and non-volatility, but also due to their transparency through almost the whole visible and near-infrared spectral regions. Moreover, their ionic natures permit the solubilisation of ionic coordination compounds, including lanthanides salts and complexes. Binnemans has thus demonstrated a highly photostability of lanthanides complexes in ionic liquids [87]. The development of ILs for optical applications is however limited by their liquid state. Ionogels recently demonstrated high efficiency to immobilize lanthanides complexes [88,89]. Actually, most silica-based ionogels appear transparent to the naked eye, opening potential applications as luminescent devices. Thus, europium(III)-doped ionogels showed a very intense red photoluminescence, with a very high monochromatic purity, under ultraviolet irradiation [89]. Note that the average decay times were as long in confined IL liquid as in bulk IL (mono-exponential decays of 0.52 ms and 0.55 ms, respectively, versus 0.35 ms in solid state). Thus, it could be concluded that the coordination sphere of the complex (europium[III] tetrakis β-diketonate) was not altered upon confinement and that it experienced a liquid-like environment. Similarly Sm(III)- and Tb(III)-doped ionogels showed the same emission performance as IL solutions (Fig. 6) [88].

Ahmad took advantage of the ionic conductivity of ionogels, which have liquid electrolyte performances despite their solid state, to synthesize an electrochromic device based on tungsten oxide and Prussian blue electrodes. This device exhibited extremely fast switching kinetics (transmission variation ΔT around 90% in 2 s) [90].

3.5 Use of ionogels in catalysis and biocatalysis

Ionic liquids are extensively used in metal catalyzed reactions. However, they still are expensive, which makes it necessary to implement recovering and reusing processes. Other restrictions to their development as industrial solvents arise from their high viscosity, which implies highly energy-consuming stirring and from mass transfer limitations in biphasic systems, which makes that the sole catalyst involved in the process is that contained in the narrow diffusion layer at the liquid-liquid (or gas-liquid) interface. Over the last years, the immobilization of ionic liquids as adsorbed thin films on high surface area supports has been proposed to circumvent these restrictions. This approach is known as supported ionic liquid catalysis phase (SILP) catalysis (Fig. 7) [91–93]. Typically, silica support beads with 25 wt % of ionic liquid loading are easily obtained as free flowing powders. This concept combines the advantages of ionic liquids with those of heterogeneous support materials and could be particularly suitable for continuous fixed-bed reactors. Actually, the layer of free ionic liquid on the carrier (the thickness of which is close to the diffusion layer) was shown to act as a reaction phase able to dissolve various homogeneous catalysts. SILP catalysis has been successfully used in miscellaneous reactions such as Friedel-Crafts reactions [94], Rh-catalyzed hydroformylations [95] and hydrogenations [96], Pd-catalyzed Heck [97,98] and Suzuki [99] reactions, and Rh-, Pd-, and Zn-catalyzed hydroaminations [100,101].

Ionogels provide alternative SILP systems [93], as the interconnected 3-D pore structure allows free transport of reactants and products, while pore diameters are small enough to prevent IL leaching. However, the processing of metal catalyzed reactions is quite different in ionogels and supported IL films. In the latter, reactants and products diffuse through the residual voids in pores, the walls of which are only coated with the liquid catalyst film. On the other hand, in ionogels, reactants and products diffuse in the 3D continuous liquid phase which fills all the pores. Actually, Deng et al. reported the enhanced catalytic performance of the carbonylation of aniline into ureas and carbamates in the presence of ionogels loaded with Rh(PPh₃)₃Cl and Pd(PPh₃)₂Cl₂ [40]. The reactions were carried out without additional organic solvent in autoclave at 135–180 °C under 5 MPa CO pressure (Fig. 7).

Recent investigations involving ionogels prepared in the presence of Pd salts have shown an effective catalytic activity for the Heck coupling reaction without leaching [102]. The ability of ionogels to be used as nanoreactors could induce some confinement effects, which have been already reported in others SILP systems. Thus, the coating of organomodified acid silica support with hydrophobic ionic liquid permitted highly selective transformations in formalin and pure water [103]. In another work [104], the immobilization of chiral hydrogenation catalysts led to high enantioselectivities (up to 74%) in the hydrogenation of acetophenone (whereas enantioselectivity of the homogeneous reaction was null in methanol). This enhanced enantioselectivity compared to the corresponding homogeneous catalysis was attributed to confinement effects, which enhanced substrate-catalyst interactions. Actually, the formation of solvent cages around metal complexes has been postulated in ionic liquids; transition states would be modified in these cages in confined ionic liquid, promoting selectively alternative reaction pathways [105,106].

ILs have recently demonstrated attractive performances as a reaction media in biocatalysis [107]. The activity of biomolecules in ILs has been found to be comparable or higher than in conventional solvents and enhanced enzyme stability has been reported. The first reports of applications of ionogels to biocatalysis seem promising. Thus, the activity of horseradish peroxidase (HRP) immobilized in ionogels was shown to be about 30-fold greater than that in silica gel without IL, with an excellent thermal stability [108]. HRP was immobilized in ionogels prepared by acidic hydrolysis of TEOS in [C₄MIm][BF₄]. The enzyme was thought to be protected by the IL from the detrimental effect of ethanol during the synthesis, while the interconnected pore structure of ionogels would facilitate the internal diffusion of the substrate.

3.6 Use of ionogels in sensing and biosensing

The unique combination of ionic conduction and optical properties with the possible diffusion of analytes into the immobilized IL phase makes ionogels good candidate materials for integrated electrochemical and colorimetric approaches in sensing and biosensing. Moreover, the use of IL that decreases the capillary stresses makes it easier to prepare crack-free films (for coating electrodes for instance).

Thus, sulfonaphtalein pH indicators were trapped in TEOS/VTOS ionogels (IL: [C₄MIm][Cl]). The resulting IL modified films showed increased wettability, reduced gel shrinkage, faster response times and better reproducibility compared to classic sol-gel films [109]. Recently, films of ionogels doped with Ru(bpy)₃²⁺ displayed a fast and reversible response to gaseous oxygen. In comparison to traditional sol-gel derived thin films, the microenvironment of the complex was supposed to be not so rigid, leading to more important Stocke's shift values [110].

Enzyme immobilisation in IL confined onto electrodes surface comes under the same concept as SILP catalysis: no covalent grafting is needed and the nature of the catalyst is preserved. Some examples are based on the immobilization of enzyme-IL systems in organic hydrogels [111]. New hydrid organic-inorganic siloxane-based hydrogels entrapping IL have been recently developed for similar uses [112]. Interestingly, an amperometric biosensor was obtained by coating the surface of a glassy carbon electrode with a silica ionogel film doped with HRP and ferrocene (IL:[C₄mim][BF₄]) [113]. The resulting electrode had faster response in sensing hydrogen peroxide than with classic sol-gel film, with a detection limit for H₂O₂ of 1.1 μM.