Recycling Homogeneous Catalysts: Tunable Solvents for the Hydroformylation of p-Methylstyrene

We use Organic-Aqueous Tunable Solvents (OATS) for the rhodium-catalyzed hydroformylation of p-methylstyrene.  OATS combine the benefits of homogeneous and heterogeneous catalysis while overcoming the limitations of both: Reactions are carried out under homogeneous conditions, followed by a CO₂-induced heterogeneous separation.    Then the polar protic component (aqueous-rich phase) containing the catalyst phase separates from the relatively nonpolar aprotic component (organic-rich phase, usually THF or acetonitrile) containing the reactant, facilitating the separation and the recycle of catalyst.  The homogeneous reactions provide fast, highly-selective reactions with excellent yields.  Upon the addition of moderate pressures (around 3 MPa) of CO₂, the hydrophobic product partitions into the organic-rich phase with more than 99% removal efficiency and the hydrophilic catalyst is retained in the aqueous-rich phase with 99.9% retention efficiency. In addition, we were able to recycle the catalyst for five consecutive reactions without significant loss of catalytic activity.  Also, we explored applications of monophasic mixtures of organics and ionic-liquids to combine homogeneous reactions with CO₂-induced heterogeneous separations in these systems.