Supported ionic liquid phase systems

The term Supported Ionic Liquid Phase (SILP) catalysis has recently been introduced into the literature to describe the heterogenisation of a homogeneous catalyst system by confining an ionic liquid solution of catalytically active complexes on a solid support [68], In comparison to the conventional liquid-liquid biphasic catalysis in organic-ionic liquid mixtures, the concept of SILP-catalysis offers very efficient use of the ionic liquid. Figure 7.10 exemplifies the concept for the Rh-catalysed hydroformylation. 

A rather new concept for biphasic reactions with ionic liquids is the supported ionic liquid phase (SILP) concept [115]. The SILP catalyst consists of a dissolved homogeneous catalyst in ionic liquid, which covers a highly porous support material (Fig. 41.13). Based on the surface area of the solid support and the amount of the ionic liquid medium, an average ionic liquid layer thickness of between 2 and 10 A can be estimated. This means that the mass transfer limitations in the fluid/ionic liquid system are greatly reduced. Furthermore, the amount of ionic liquid required in these systems is very small, and the reaction can be carried in classical fixed-bed reactors. 

It must be also noted that supported ionic liquid phase (SILP) catalysis can also be successfully combined with supercritical fluids. Cole-Hamilton et al. [127] have reported recently high activity (rates up to 800 h ), stable performances (>40 h) and minimum rhodium leaching (0.5 ppm) in the hydroformylation of 1-octene using a system that involves flowing the substrate, reacting gases and products dissolved in... 

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. 

In the literature terms such as supported molten salt (SMS) catalysts, supported ionic liquid catalysts (SILC) and supported ionic liquid-phase (SILP) catalysts, have been used somewhat indiscriminately to describe catalyst systems containing a catalytic ionic phase. In this section vye will use the terms molten salt or ionic liquid to indicate the melting point of the fluid phase in the systems. Furthermore, we will distinguish between the terms SILC and SILP. SILP is used when the ionic liquid is performing mainly as an immobihzing solvent for the catalytic components. SILC is used in cases where the ionic hquid itself, ionic hquid ions or ionic liquid-like fragments are behaving as the catalytic species. 

A second and somewhat simpler approach that can be applied to obtain supported ionic liquid catalyst systems involves the treatment of a solid, porous carrier material by a substantial amount of a catalytically active ionic liquid, allowing the reaction to take place in the dispersed phase. In these systems the ionic liquid phase can itself act as the catalytically active component or it may contain other dissolved compounds or reagents, for example, transition metal complexes, which function as the catalytically active species (i.e. generating SILP catalysts). Importantly, the ionic liquid catalyst phase in these SILP catalyst systems are confined to the carrier surface only by weak van der Waals interactions and capillary forces interacting in the pores of the support. In special cases electrostatic attachment of the ionic liquid phase may also be applied. Usually, the catalysts are prepared by traditional impregnation techniques, where a volatile solvent is used initially to reduce viscosity for the impregnation process and is finally removed by evaporation leaving the ionic catalyst solution dispersed on the support. 

This modern method, commonly known supported ionic liquid phase catalysis, is based on the simple chemical attachment of ILs to the support surface or the simple deposition of the catalytically active speciesmain idea for the preparation of these alternative materials is to avoid or at least decrease the deactivation of the catalyst after reactions as well as to minimize the amount of IL used in each process. In addition, the SILP method provides some advantages compared to other catalytic systems. For example, SILP catalytic systems offer the elimination/reduction of mass transfer limitations and give access to more robust/recyclable catalysts with an easy separation after reactions. In other words,... 

These investigations convincingly showed that a combined SILP/SCCO2 system has the advantages of excellent diffusion of the substrate and gases to the catalyst surface and excellent solubility of the substrates and gases within the supported ionic liquid phase. Further, it is capable of extracting heavy products in a very convenient way, which had not been possible in a classical SILP approach. 

A heterogeneous catalytic system was prepared upon grafting a cationic dihydroimidazolium-tagged silane on solid Si02. The resulting supported ionic liquid phase (SILP) has been used to immobilize [y-l,2-H2SiV2Wio04o] and was employed in a mixture of acetonitrile/t-butyl alcohol at 20°C for the oxidation of different substrates terminal olefins (66-82% yield), non-hindered internal olefins (> 70% yield) in 24 h, and sulfides (81-95% yield) in 4—lOh. ... 

Reiche A, Stemich T., Sandner B., Lobitz P. Fleischer G. (1995). Ion transport in gel electrolytes, Electrochimica Acta, International symposium on polymer electrolytes, vol.40, n°13-14, p>p. 2153-2157, (October 1995), ISSN 0013-4686 Riisager A, Wasserscheid P., Van Hal R. Fehrmann R. (2003). Continuous fixed-bed gas-phase hydroformylation using supported ionic liquid phase (SILP) Rh catalysts. /. Catal, vol.219, n°2, pp.452-455, (October 2003), ISSN 00219517 Robert D., Schneider M., Bom M., Mieloszynski J.L. Paquer D. (19%). Influence of heteroatomic systems on anti-wear and extreme pressure properties of organo-sulfur compounds. C, R. Acad, Set, serie Tib, vol.323, p>p.l27-132, (19%), ISSN 0320-8437 Rogers R.D. Seddon K.R. (2003). Ionic Liquids—Solvents of the Future , Scimee, Vol. 302, n°5646, pp. 792-793 (October 2003), ISSN 0036-8075 Seddon K.R. (1997). Ionic Liquids for Clean Technology, J. Chem. Technol. Biotechnol, vol. 68, n°4, pp.351-356, (April 1997), ISSN 0268-2575... 

Supporting ionic liquids in the pores of solid materials offers the advantage of high surface areas between the reactant phase and that containing the supported liquid catalyst. This approach is particularly useful for reactants with less than desired solubility in the bulk liquid phase. Another incentive for using such catalysts is that they can be used in continuous processes with fixed-bed reactors (26S). The use of an ionic liquid in the supported phase in addition to an active catalyst can help to improve product selectivity, with the benefit being similar to what was shown for biphasic systems. However, care has to be taken to avoid leaching the supported liquids, particularly when the reactants are concentrated in a liquid phase. 

The hydroformylation of 1-hexene by supported ionic liquid catalysis (SILC) was recently reported by researchers at ExxonMobil. In this system, the active catalyst HRh(CO)(tppti)3 (tppti = tri(m-sulfonyl)triphenyl phosphine tris(l-butyl-3-methyl-imidazolium)) is contained within the ionic liquid phase while excess tppti ligand is immobilized in the support material. TOP values of 65 min" were obtained with silc while an unsupported biphasic ionic liquid medium gave TOP values of 23 min. ... 

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

Of the inorganic supports, best results were reported for a mesoporous MCM-41 [337]. Support on ionic-liquid phases has been studied by different groups with variable results [338, 339], Of the non-conventional organic polymers, non-covalent immobilization on poly(diallyldimethylammonium) is notable [340], Catalysts 133 (15 mol.%) promoted the aldol reaction of acetone and benzaldehydes to afford the corresponding (i-hydroxyketones in 50-98% yields and 62-72% ee, which are clearly lower than those reported for other polymer-supported systems. Recycling of the catalysts was possible at least six times without loss of efficiency. More recently, proline has been attached to one DNA strand while an aldehyde was tethered to a complementary DNA sequence and made to react with a non-tethered ketone [341], To date, the work has focused more on conceptual development than on the analysis of its practical applications in organic synthesis. 

For all reactions studied, the activity of the supported catalysts was higher than for the similar biphasic ionic liquid system, which was ascribed to improved mass transfer between the substrates and the ionic liquid phase. In addition, the observed product selectivities of 64-87% and enantioselectivity of 97% for the SILP-Ru-(S)-BINAP catalyzed reaction equalled those of the homogeneous reference reactions. No indication of rhodium metal leeching was found by AAS analysis of the reaction filtrate. 

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