Multiphase catalysis and its potential in catalytic processes: the story of biphasic homogeneous catalysis†
Homogeneous catalysis in comparison to heterogeneous catalysis is burdened by the use of a solvent, which makes catalyst recycle and product separation costly and difficult. This is probably one of the main reasons that industry prefers heterogeneous catalysis.
Besides heterogenizing homogeneous catalysts, immobilization of the homogeneous catalysts in multiphase operation (e.g. two-liquid phase approach) offers promising opportunities. The two-liquid phase approach rests on the proper choice of distribution coefficients of the products with immiscible solvents. The following cases can be considered:
• The catalyst operates in a polar phase and the products form the second immiscible phase which can be “spooned off”.
• Two immiscible solvents yielding two phases are used in the reactor. The catalyst remains in one phase, the products are extracted into the second phase.
• The homogeneous catalysis is carried out conventionally followed by extraction of the products with a second solvent, which is immiscible with the solvent of the phase containing the catalyst.
Examples are presented for all three cases. For case one, besides polar organic solvents, water, perfluorinated solvents, ionic liquids and supercritical CO2 will be discussed.
Ionic liquids, having no vapour pressure and supercritical CO2 can be used in a continuous operation. The catalyst must be soluble in the ionic liquid phase. The product will be extracted by the scCO2. After separation, the CO2 can easily be recycled. The combination of ionic liquids and compressed CO2 provides a unique new approach of great promise.
In general — in contrast to heterogeneously catalyzed reactions such as the conversion of ethylene to ethylene oxide — homogeneously catalyzed reactions require, for economic reasons, high conversions. This can be overcome by using the “recycle two-phase approach” as will be demonstrated for the conversion of butadiene–ammonia to the primary octadienylamine. Especially processes with consecutive reactions can benefit here.