Supported ionic liquid films

Figure 7.13 Asymmetric cyclopropanation using Cu-bis(oxazoline) complex 42a in supported ionic liquid film (SILF).

Figure 1.3 Categorization of materials based on supported ionic liquid films according to the phase behavior of the supported ionic liquid (a) covalently attached monolayer and (b) multilayers of ionic liquid.

Scheme 10 (a-d) Categorization of catalysts based on supported ionic liquid films... 

In an extension of this work, either zinc(II), palladium(II), rhodium(I) or copper(I) salts were immobilised in an ionic liquid film (SILP, vide supra) onto diatomic earth and the catalysts tested for activity in the reaction between phenylacetylene and 4-isopropyl-phenylaminc.1 391401 The supported rhodium, ruthenium and zinc complexes afford higher rates and selectivities relative to their use under homogenous reaction conditions. Lower rates are, however, observed with the copper salt, which is rationalised by strong complexation of the ionic liquid to the Cu(I) centre. 

If the transport limitation is significant, the catalysis occurs predominantly near the surface of the ionic liquid, and the [Rh(CO)2l2] dissolved in the bulk is not fully utilized. One attempt to address these issues was to use a supported ionic liquid phase (SILP) catalyst, as reported by Riisager et al. [Ill], In this system, the ionic liquid (l-butyl-3-methylimidazolium iodide) was supported as a thin film on solid silica (the thin film offers little mass-transport resistance) and used in a fixed-bed continuous reactor with gas-phase methanol. Rates were achieved that were comparable to those in Eastman s bubble column carbonylation reactor with gas-phase reactants [109], but using a much smaller amount of ionic liquid. 

Structured supported ionic liquid-phase (SSILP) catalysis is a new concept that combines the advantages of ionic liquids (ILs) as solvents for homogeneous catalysts with the benefits of structured solid catalysts. In an attempt to prepare a homogeneous IL film on a microstructured support, SMFs were coated by a layer of carbon nanofibers as described above. An IL thin film was then immobilized on the CNF/SMF support. The high interfacial area of the IL film enabled the efficient use of a transition metal catalyst for the selective gas-phase hydrogenation of acetylenic compounds [267,268]. 

The reactants in SILP catalysis are preferentially processed in gaseous form. Processing of solid SILP catalysts in a liquid reaction phase as a slurry requires extremely low solubility of the ionic liquid film in the liquid reaction mixture and affords special constraints upon the mechanical stability of the liquid film. In contrast, for the - by r more attractive - gas-phase applications of SILP catalysts the extremely low volatility of the ionic liquids is the key success fector. It is noteworthy that earlier attempts to apply supported liquid catalysts in continuous gas phase reactions - using organic liquid phases [32] or water [33] as the immobilized liquid phase - resulted in catalyst systems too unstable for technical use due to evaporation of the liquid film with time. In contrast, SILP systems have been... 

Finally, a very interesting approach has been suggested by the same group to apply an ionic liquid to the preparation of Ru nanopartides of defined size on a meso-porous silica support [279]. The authors made use of the ionic liquid [TGA] [lactate] to customize particles obtained by reduction of dissolved RuQs on the silica support. However, prior to catalysis, the ionic liquid film was removed thermally by heating to 220 °C for 3 h. In this way a quite active catalyst for the hydrogenation of benzene to cyclohexane (TOFs up to 83 h at 10 bar H2 pressure) was obtained. Such an approach obviously combines successfully features of SILP catalysis (see Section 5.6 for details) with the use of catalytic nanopartides. 

In Section 5.3 it was demonstrated with many examples that ionic hquids are indeed a very attractive class of solvents for catalysis in liquid-liquid biphasic operation (for some selected reviews see Refs. [16-20]). In this section, we wfll focus on a different way to apply ionic liquids in catalysis, namely the use of an ionic liquid catalyst phase supported on a solid carrier, a technology that has become known as supported ionic liquid phase (SILP) catalysis. In comparison to the conventional liquid-liquid biphasic catalysis in ionic liquid-organic liquid mixtures, the concept of SILP-catalysis combines well-defined catalyst complexes, nonvolatile ionic liquids, and porous solid supports in a manner that offers a very efficient use of the ionic liquid catalyst phase, since it is dispersed as a thin film on the surface of the high-area support. Recently, the initial applications using such supported ionic liquid catalysts have been briefly summarized [21]. In contrast to this report, where the applications were distinguished by the choice of support material, the compilation here will divide the applications using the supported ionic liquid catalysts into sections according to the nature of the interaction between the ionic liquid catalyst phase and the support. 

During catalytic reactions using supported ionic liquid-type catalysts gaseous or vapor-phase reactants diffuse through the residual pore space of the catalyst, dissolve in the liquid catalyst phase, and react at catalyst sites within the thin liquid catalyst film dispersed on the walls of the pores in the support material, as illustrated in Fig. 5.6-1. The products then diffuse back out of the catalyst phase into the void pore space and further out of the catalyst particle. 

Figure 1.2 Schematic representation of an ionic liquid film supported on a porous material.

Figure 4.1 Major types of supported ionic liquids multilayer ionic liquid film (supported ionic liquid phase SILP) type A, coated heterogeneous catalyst (solid catalyst with ionic liquid layer SCILL) type B, covalently bound monolayer (supported ionic liquid SIL) type C.

The properties of ILs can also be modified by supporting them on different sohd substrates (e.g., graphite, mica, silica, oxidized silicon, etc.), a concept known as supported ionic liquid phases (SILP) and introduced in the early 2000s by the groups of Mehnert [73], Fehrmann and Wasserscheid [74]. Research done in this area has shown that the SILP strategy can lead to drastically different behavior of the supported ILs, mainly due to the interactions between the anions and/or cations of the I Ls and the solid matrix. For example, Bovio et al. showed that, by supporting the IL l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIMKNTfj]) on a solid matrix (e.g., amorphous silica, oxidized Si(llO), and mica), hquid-solid phase transitions are induced when thin films of IL rearrange to a sohd-like phase [75]. 

One of the most interesting applications of XPS is the study of interfaces since the chemical and electronic properties of elements at the interfaces between thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, membranes, metal protection and solar cells. Concerning the study of membranes, XPS is now a very common characterization technique in combination with other such as AFM or SEM. Different types of membrane systems have been successfully studied such as polyamide membranes [24], modified polysulfone/polyamide membranes [20], or supported ionic liquid membranes [25] where both the porous support and the ionic liquids are characterized. 

Haumann, M., Schonweiz, A., Breitzke, H., Buntkowsky, G., Werner, S. and Szesni, N., Solid-state NMR investigations of supported ionic liquid phase water-gas shift catalysts Ionic liquid film distribution vs. catalyst performance, Chem. Eng. Technol. 35,1421-1426 (2012). 

Virtanen P., Karhu H., Kordas K Mikkola J.P. (2007). The effect of ionic liquid in supported ionic liquid catalysts (sdca) in the hydrogenation of a, 3-unsaturated aldehydes. Chem. Eng. Set, vol.62, n°14, pp.3660-3671, (July 2007), ISSN 0009-2509 Washiro S., Yoshizawa M., Nakajima H. and Ohno H. (2004). Highly ion conductive flexible films composed of network polymers based on polymerizable ionic liquids. Polymer vol.45, n°5, pp. 1577-1582 (March 2004), ISSN 0032-3861 Wasserscheid P. Keim W.A. (2000). Ionic Liquids—New "Solutions" for Transition Metal Catalysis, Ang. Chem. Int. Ed., vol.39, pp.3772-3789, (November 2000), Online ISSN 1521-3773... 

Although this technique has not been used extensively, it does allow structures of adsorbed layers on solid substrates to be studied. Liquid reflectivity may also be performed with a similar set-up, which relies on a liquid-liquid interface acting as the reflective surface and measures the reflectivity of a thin supported liquid film. This technique has recently been used to investigate water-alkane interfaces [55] and is potentially useful in understanding the interaction of ionic liquids with molecular solvents in which they are immiscible. 

The present chapter targets multiphasic catalytic systems that can be represented in general as L-L-S and L-L-L-S systems (Figure 6.3). The liquid phases are two or three, and separate at ambient conditions. One of the Ls is a catalyst-phihc liquid phase that can be either ionic or hydrophilic, the equivalent to the supported liquid film described in the previous section. Figure 6.3 shows the two different arrangements of the multiphasic systems that is considered here. 

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