Reactions with Supported Ionic Liquid Catalysts

Coupling Reactions with Supported Ionic Liquid Catalysts 

When the coupling reactions are conducted in homogeneous systems in the presence of ligands, separation of the catalysts from the products can often be problematic. However, catalyst recovery and reuse are often highly desirable, especially when precious metal catalysts are used with high loading. It is known that heterogeneous catalysts can be separated from the reaction products more easily, but the catalysts are usually less active. 

Green chemistry has been developed to meet the increasing demand for environmentally benign chemical processes. The use of room temperature ionic hquids (ILs) as either solvents or catalysts has attracted much attention in recent years [1]. ILs consist entirely of ions and have no measurable vapor pressure that makes them attractive as alternative solvents for homogeneous catalysis. Their polar nature allows the stabilization of ionic transition metal complexes and metal nanoparticles (NPs) [2], 

Normally, catalytic systems that include an IL phase require large amounts of these neoteric solvents in most cases, which are often costly and may affect the economic viability of a chemical process. Even though ILs have become commercially available, they are still relatively expensive compared to most of the conventional solvents. Furthermore, I Ls are usually viscous and have low diffusion coefficients for chemical reactions. In this regard, a new concept of a supported ionic liquid (SIL) phase has been adopted for immobihzation of catalysts [3]. SIL phases, which are much easier to separate, are advantageous for chemical reactions and have great potential in catalysis. This strategy helps to immobilize catalysts 

Supported Ionic Liquids Fundamentals and Applications, First Edition. 

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I 7 7 Coupling Reactions with Supported Ionic Liquid Catalysts... 

P. (2011) Challenging the scope of continuous, gas-phase reactions with supported ionic liquid phase (SILP) catalysts-Asymmetric hydrogenation of methyl acetoacetate. Appl. Catal, A Gen., 399, 35-41. 

Hydroformylation Reactions with an Ionic Liquid-Supported Catalyst. .. 58... 

Even though most of the supported ionic liquid catalysts prepared thus far have been based on silica or other oxide supports, a few catalysts have been reported where other support materials have been employed. One example involves a polymer-supported ionic liquid catalyst system prepared by covalent anchoring of an imidazolium compound via a linker chain to a polystyrene support [79]. Using a multi-step synthetic strategy the polymeric support (e.g. Merrifield resin among others) was modified with l-hexyl-3-methylimidazolium cations (Scheme 5.6-4) and investigated for nucleophilic substitution reactions including fluorina-tions with alkali-metal fluorides of haloalkanes and sulfonylalkanes (e.g. mesylates, tosylates and triflates). 

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. 

Bhanage and coworkers [62] investigated the reaction in polyethylene glycol and used the recycled rhodium phosphinite catalyst up to five times. Wasserscheid s group performed HAM in a continuous reactor operating with supported ionic liquid phase (SILP) catalysts [49]. A particular feature was that, by using a SILP catalyst based on neutral oxide and porous carbon supports and ILs of low basicity, aldol condensation could be fully suppressed. Alternatively, the reaction has been run with the assistance of a rhodium catalyst immobilized in a sol-gel matrix [73]. 

The product distribution in the reaction of benzene with dodecene was determined for a number of catalysts (Table 5.1-4). As can be seen, the reaction with the zeolite H-Beta gave predominantly the 2-phenyldodecane, whereas the reaction in the pure ionic liquid gave a mixture of isomers, with selectivity similar to that of aluminium chloride. The two supported ionic liquid reactions (H-Beta / IL and T 350 / IL) again gave product distributions similar to aluminium(III) chloride (T350 is a silica support made by Degussa). 

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. 

Beside SILP experiments with silica as support material, reports have also been made on the use of membranes coated with ionic liquid catalyst solution for the hydrogenation reaction of propene and ethene. The membranes were obtained by supporting various ionic liquids, each containing 16 to 23 mmol Rh(I) complex Rh(nbd)(PPh3)2 (nbd=norbornadiene), in the pores of poly(vinylidene fluoride) filter membranes [118]. 

The catalyst/substrate ratio is 1.5 mol% for the supported ionic liquid phase (SILP) catalyst, 3 mol% for the impregnated catalyst and 2 mol% for the homogeneous reaction aRuns 1 -4 are consecutive experiments with the same catalyst in a stirred batch reactor. bDimeric Cr (salen) catalyst impregnated on silica cHomogeneous reaction at 0-2 OC optimized for product selectivity dHomogeneous reaction at room temperature optimized for product selectivity... 

Citral (Alfa Aesar, 97%) hydrogenation was carried out in a pressurized reactor working in a special mode, in which small amounts of the liquid phase initially introduced in the reactor were constantly pumped out from the reactor, whereas the supported Pd-ionic liquid catalyst remained in the reactor. The initial liquid phase volume was 325 ml. The catalyst bulk density was changed during the reaction via pumping out the liquid phase from the reactor with a rate of 1 g/min. Such procedure allows efficient investigation of consecutive reactions... 

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