Supported ionic liquid catalysts SILC

In the simplest methodology using supported ionic liquid catalysts based on a nonbonding ionic liquid attachment to the support, the ionic liquid phase behaves both as reaction medium and catalyst. 

in a closely related study, a similar sol-gd synthetic pathvray was used to prepare silica gel supported ionic liquid deoximation catalysts from carboxylic acid-functionalized ionic liquids [85]. Here various aryl and alkyl oximes were converted into the corresponding 0x0 compounds with high conversions (up to 94%) and excellent selectivity ( 99%) in aqueous acetone at room-temperature by coproduction of 2-propanone oxime. TONs of up to 200 h obtained with the SILC were about four times higher than the TONs obtained using pure ionic liquids as catalysts. 

After impregnation of the polymeric support material with paUadium(II) tri-(o-tolyl)phosphine complexes the resulting catalyst material was tested in the Heck coupling of bromobenzene with butyl acrylate in DMF to yield 75% of -3-phenylacrylic 

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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. 

Silica gels have been used in recent years as supports for the immobihzation of ILs in order to improve their applicability and reusabiHty in industrially important catalytic processes. In particular, there have also been numerous studies in which silica gels are used as inorganic supports for the development of recyclable and efficient catalytic systems based on supported ILs, namely, supported ionic hquid phase (SILP) catalysts [57-59], supported ionic liquid catalysts (SILCs) [60-62], and solid catalysts with ionic liquid layers (SCILL) [63]. 

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. ... 

A simple and efficient method for the preparation of a-diazo- i-hydroxy esters by the condensation of aldehydes with ethyl diazoacetate using chloromethy-lated polystyrene SIL as a heterogeneous catalyst in water was developed [66]. Moderate to excellent yields of the corresponding a-diazo-jl-hydroxy esters were obtained. The catalyst was separated by filtration and reused in five consecutive cycles without any appreciable loss of activity. Later MacMillan s imidazolidinone was noncovalently immobihzed in the pores of siHca gel with the aid of IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (Mac-SILC) (SILC, supported ionic liquid catalyst) [67]. The Mac-SILC as an SIL catalyst was utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and cinnamaldehyde. The Mac-SILC was more active, especially to an electron-rich dienophile, than its homogeneous counterparts, and the reaction could proceed under mild conditions. 

SILC was also used without covalently anchoring the ionic liquid fragment to the silica support. In this case, [bmim][PF6] was simply added to silica in acetone together with the catalyst. [Rh(norbornadiene)(PPh3)2]PF6 and the solvent evaporated to yield the supported catalyst-philic phase. Catalyst evaluation on the hydrogenation of model olefins showed enhanced activity in comparison to homogeneous and biphasic reaction systems, in analogy to Davis s observations. Also... 

It should, however, be pointed out that the SILP and SILC techniques are only straightforward to apply for continuous gas-phase reactions, where the advantage of using a liquid with very low vapor pressure can be fully exploited. In contrast, the application of the technology in liquid-phase reactions may be very restricted, since even a minor solubility of the ionic liquid in the feedstock/product mixture will remove the catalyst from the surface (due to the very small amount of ionic liquid on the support). Even worse, the film of the immobilized catalyst phase can physically be removed from the support by mechanical forces, for example, the convective liquid flow, even in the case of complete immisdbility. 

This section summarizes the recent progress in supported ionic hquid catalysis, demonstrating synthetic apphcations where the ionic liquid can play its role as either iimocent solvent (SILP) or the catalyst itsdf (SILC), depending on the specific cation/anion combination and the reaction under investigation. 

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