Ionic Liquids on Support


Traditional biphasic ionic liquid-organic systems require large amounts of ionic liquid which makes them inefficient based on economic considerations since ionic liquids still are expensive solvents, even though being commercially available by now.
In addition, the high viscosity of ionic liquids can induce mass transfer limitations if the chemical reaction is fast, in which case the reaction takes place only within the narrow diffusion layer and not in the bulk of the ionic liquid catalyst solution. Hereby, only a minor part of the ionic liquid and the dissolved precious transition metal catalyst are utilized.
The problem of mass transport limitation can be circumvented by immobilizing a thin film of ionic liquid onto a high surface area support, as schematically depicted in the following Figure. The Supported Ionic Liquid Phase (SILP) catalysts formed allow the application of fixed-bed reactors and make the separation and catalyst recycling obsolete.

Principle of biphasic catalysis using ionic liquids for catalyst immobilization

Immobilization of ionic liquids on porous support material can be achieved in two different ways, either by covalent bonding of ionic liquid fragments to the support or by physisorption via van der Waals and dipole forces. The fixation of ionic liquids by physisorption requires less synthetic effort. However, the ionic liquid film can be more susceptible to be removed from the support in the presence of polar solvents in a liquid-liquid biphasic system. In both cases a solid material is obtained in which the ionic liquid film thickness is in the range of a few nm.

Variety of SILP catalyst materials synthesized at CRT.

The use of SILP systems in catalysis has been reviewed recently covering Friedel-Crafts reactions, hydroformylations (Rh-catalyzed), hydrogenation (Rh-catalyzed), Heck reactions (Pd-catalyzed), and hydroaminations (Rh-, Pd-, and Zn-catalyzed). Since this review emerged, the SILP concept has been extended to other catalytic reactions, other support materials than silica, and to membrane facilitated separation processes.

References

  • Very Stable and Highly Regioselective Supported Ionic-Liquid-Phase (SILP) Catalysis: Continuous-Flow Fixed-Bed Hydroformylation of Propene, Anders Riisager, Rasmus Fehrmann, Stephan Flicker, Roy van Hal, Marco Haumann, Peter Wasserscheid, Angew. Chem. Int. Ed. 2005, 44, 185.
  • Supported Ionic Liquid Phase (SILP) Catalysis: An Innovative Concept for Homogeneous Catalysis in Continuous Fixed-Bed Reactors, Anders Riisager, Rasmus Fehrmann, Marco Haumann, Peter Wasserscheid, Eur. J. Inorg. Chem. 2006, 695.