Immobilization of Catalysts in Ionic Liquids

1) the active species is known to be ionic in conventional organic solvents, 

Not only cationic, but also anionic, species can be retained without addition of specially designed ligands. The anionic active [H Pt(SnCl3)4] complex has been isolated from the [NEt4][SnCl3] solvent after hydrogenation of ethylene [27]. The PtCl2 precursor used in this reaction is stabiHzed by the ionic salt (liquid at the reaction temperature) since no metal deposition occurs at 160 °C and 100 bar. The catalytic solution can be used repeatedly without apparent loss of catalytic activity. 

To date, these funcHonalized Hgands have been invesHgated on the laboratory scale, in batch operahons to immobilize rhodium catalyst in hydroformylahon. 

Good rhodium retention results were obtained after several recycles. However, optimized ligand/metal ratios and leaching and decomposition rates, which can result in the formation of inactive catalyst, are not known for these ligands and require testing in continuous mode. As a reference, in the Ruhrchemie-Rhone-Poulenc process, the losses of rhodium are 10 g Rh per kg n-butyraldehyde. 

Certain amines, when linked to TPPTS, form ionic solvents liquid at quite low temperatures. Bahrman [33] used these ionic liquids as both ligands and solvents for the Rh catalyst for the hydroformylation of alkenes. In this otherwise interesting 

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Figure 4.5 Ligands used to improve immobilization of metal catalysts in ionic liquids...

In-situ IR-spectroscopic characterization of the Friedel-Crafts acylation of benzene in ionic liquids derived from AICI3 and FeCl3 showed that the mechanism of the reaction in ionic liquids was the same as that in 1,2-dichloroethane (128). The immobilization of ferric chloride-containing ionic liquid onto solid supports (e.g., silica and carbon) however failed to catalyze the acylation reaction, because leaching was a serious problem. When the reaction was carried out with gas-phase reactants, catalyst deactivation was observed. 

Herein, we report on a novel process for the synthesis of organomodlfied polydimethylsiloxanes employing ionic liquids for the heterogenization and/or immobilization of the precious metal catalyst [13]. The advantage of this novel hydrosilylation process is that standard hydrosilylation catalysts can be used without the need for prior modification to prevent catalyst leaching. To the best of our knowledge, this is the first example of a hydrosilylation of olefmic compounds using ionic liquids (Scheme 1). However, a method for the transition metal-catalyzed hydroboration and hydrosilylation of alkynes in ionic liquids has recently been described [14]. 

Attempts to improve the solubility and immobilization of chiral hydrogenation catalysts in ionic liquids were presented by Lee and coworkers [112]. They synthesized a chiral Rh-complex carrying the dicationic bisphosphine ligand depicted in Fig. 5.3-7. Immobilization of the tricationic complex in [BMIM][SbFe] showed better immobilization results in contact with iPrOH compared to the non-modified complex Me-BDPMI in the Rh-catalyzed asymmetric hydrogenation of N-acetylphenylethenamine (Scheme 5.3-11). The ionic catalyst solution was reused three times without loss of activity. In the fourth mn conversion decreased but high conversions could still be realized by increasing the reaction time. 

More recently, the Cole-Hamilton group has been able to develop a continuous-flow process on the basis of a similar concept, using phosphine ligands modified with ionic side groups to disperse and immobilize the catalyst in the liquid product phase [5]. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

The range of ligands developed for ionic liquid catalysis is much smaller than that for other immobilization solvents such as water and fluorous phases as off the shelf ligands and catalysts can often be used in ionic liquids. For example, a number of catalysts that were developed to operate in organic solvents under homogeneous conditions are salts themselves and do not need to be modified for use in ionic liquids [25],... 

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