Ionic liquid catalyst carriers

A. Characteristics of Ionic Liquid Catalyst Carriers A.l. Inert Catalyst Carriers... 

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. 

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. 

Ruy et al. have performed a similar reaction under microreactor conditions in a multiphase solvent system containing an ionic liquid as the catalyst carrier and reaction promoter [35]. Their system consisted of two T-shaped micromixers (i.d. 1,000 and 400 pm) and a capillary stainless steel tube as an RTU (1,000 pm i.d. and 18 m length, giving a 14.1 ml volume), equipped with pumps and control valves. Under the optimized conditions, Pd-catalysed carbonylation of aromatic iodides in the presence of a secondary amine provided only the double carbonylated product, ot-ketoamide, while the amide obtained by the single carbonylation was observed in high quantities only when the reaction was performed in batch (Scheme 13). 

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

After extensive testing of different carrier materials silica proved to be a very suitable support, since here the highest amount of immobilised ionic liquid could be found. Other metal oxides were suitable for immobilisation as well, but usually the amount of ionic liquid immobilised was far lower (Figure 11). The only exception was AI2O3, where the amount of IL found on the carrier after immobilisation was as high as for silica but the product proved to be inactive as a catalyst. 

Likewise, a thermoregulated phase transfer process within the aqueous/organic two-phase system has been reported by Jin and co-workers (cf. Section 3.1.1.1) [290]. A water-soluble supramolecular Rh catalyst based on functionalized /1-cyclodextrin was also described [291]. In a two-phase system this catalyst may function as a carrier for the transfer of both the starting material and the product between the different phases. As an alternative to polar media for biphasic hydroformylation, Chauvin et al., used ionic liquids based on imidazolium salts which are well known for dimerization reactions (cf. Sections 2.3.1.4 and 3.1.1.2.2) [270, 271, 292]. For introduction into technical processes the currently availability and price of ionic liquids could be a drawback, especially for bulk chemicals such as 0x0 products. 

Chiral catalysts with some form of nonconventional reaction medium as the mobile carrier , such as aqueous phase, fluorous phase, ionic liquid or supercritical carbon dioxide (SCCO2). 

Different methods for the preparation of Novel Lewis-Acid Catalysts (NLACs) consisting of ionic liquids immobilised on mesoporous support materials are presented. The focus will be placed on materials bound to the carrier via the organic cation of the ionic liquid, either by grafting or by the preparation of organically modified HMS. After addition of aluminium(III)chloride the materials were used as catalysts e.g. in Friedel-Crafts alkylations, in which they displayed high activities and selectivities. 

Supported ionic liquid compositions are also a vivid field of research in which many companies tried to make their claims [88-90]. By immobilizing ionic liquids onto silica- or alumina-based carriers it is possible to obtain new Lewis acid catalysts with interesting characteristics. These are presently preferably used for alkylation and acylation reactions of aromatic compounds [91, 92] or isomerizations. Even the co-immobilization of ionic liquids with transition metal complexes [93] or Lewis acids [94] has been described, and it can be anticipated that this particular field offers many options for future catalyst development. 

In a related study, L-proline catalyzed asymmetric aldol condensation between aldehydes and ketones was examined using SILP asymmetric catalysts [83] (Fig. 5.6-6). In this approach, the catalysts were prepared by impregnation of [BMIM][PF6] ionic liquid containing L-proline direcdy onto a silica gel support, or by treatment of an additional amount of the ionic liquid containing L-proline onto a silica gel support pre-modified with a monolayer of ionic liquid attached to the carrier surface via grafting of the l-(trimethoxysilylpropyl)-3-methylimidazolium cation (as shown earlier in Scheme 5.6-1, middle). 

When a substantial amount of an ionic liquid is immobihzed on a support, the formation of a film of free ionic liquid on the carrier may act as an inert reaction phase to dissolve various homogeneous catalysts [87]. This further implies that although the resulting material appears as a sohd, the active species dissolved in the ionic liquid phase on the support still comprises the attractive features of a dissolved homogeneous catalyst, for example, high specifidty and uniform nature of the catalyticaUy active sites. 

A new trend in Mizoroki-Heck reactions is the apphcation of supported palladium catalysts with the aim of easy catalyst recycling and higher selectivity. The application of such catalysts results in a higher regioselectivity, which might be rationalized by the increased steric hindrance of the catalyst at the surface. Immobilization techniques use catalyst on a carrier, catalyst and ionic liquid on a carrier, ionic liquid and ligand on a carrier with and without catalyst, fixation of the base and the starting material. 

Besides, it is worth mentioning that despite ionic liquids having the appropriate composition might be an alternative to the less environment friendly H2SO4 and HF acids, it would be much more interesting from the practical viewpoint to immobilize these ionic liquids on solid supports so as the resulting catalyst contains ionic complexes in which either anion or cation is covalently bonded to the porous carrier (70,71). In this respect, it has been shown that the imidazolium-type ionic liquid immobilized on a high surface area (946 m /g) ordered mesoporous silica (Si-MCM-41) performed relatively well for the alkylation reaction at temperatures around 80° C, with a selectivity to isooctanes of about 60 wt% (72). Nevertheless, as usually occurs in solid acid alkylation, the activity of immobilized IL catalyst starts to fall at a certain time on stream after which butenes dimerization instead of alkylation becomes the predominant reaction (72). 

Carriers other than silica have also been used for the preparation of SILP catalyst for the alkenes hydroformylation. Simple immobilized water-soluble TPPTS-Rh complex dissolved in 1,1,3,3-tetramethylguanidinium lactate ionic liquid on MCM-41 mesoporous silicas exhibited high performance and stability for the hydroformylation of 1-hexene, and the catalyst system could be reused many times without reducing the activity and selectivity. In these cases, considerable amounts of IL and Rh species were essentially located in the inner charmel of MCM-41. The SILP process performed using Si02 as the carrier presented much lower 1-hexene conversion as compared to the SILPC using MCM-41 under the same reaction conditions [47,48]. 

Higher alkenes too, liquid at room temperature, can be hydroformylated over a fixed bed SILP catalyst. In this case the carrier gas is scCO . which is soluble in ILs but does not dissolve ionic compounds. For example, 1-octene, CO and H2 are mixed in SCCO2 and flowed through a tubular reactor containing the catalytic system 37 (Figure 40) dissolved in [omim][Tf2N] which, in turn, is supported on silica gel. 

In this chapter, attention is focused on a number of polymers that are either themselves characterized by special properties or are modified for special uses. These include high-temperature and fire-resistant polymers, electroactive polymers, polymer electrolytes, liquid crystal polymers (LCPs), polymers in photoresist applications, ionic polymers, and polymers as reagent carriers and catalyst supports. 

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