Catalysis of Organic..

Hallett, Pollet, Eckert and Liotta 399Our fluorous silica technology was also tested (1) on the catalytic hydrogenationof styrene. The fluorous silica phase contained a fluorinated version of Wilkinson’scatalyst (Figure 3) deposited onto the surface of the fluorous silica. The organicphase consisted of styrene dissolved in cyclohexane. No fluorous solvent was used.F-(F 2 C) 6 -H 2 C-H 2 CPRhClFigure 3. Fluorinated Wilkinson’s catalyst33H 2 and then CO 2 pressure were applied, forming a GXL. The fluorinatedcatalyst then partitioned off of the fluorinated silica support and into the CO 2 -expanded organic phase. The reaction was assumed to occur in the expanded liquidphase in which reactants (styrene, hydrogen) and catalyst (fluorinated Wilkinson’scatalyst) are homogeneously present. After the reaction was completed, the pressurewas released and the catalyst then partitioned back onto the silica surface.The recyclability of the fluorinated catalyst was investigated. Five consecutiveruns were carried out successfully with the same initial fluorinated catalyst/silica.The styrene hydrogenation activity proved to be relatively consistent (TOF from250-400 h -1 ) for each of the five runs, indicating minimal loss of catalytic activity.Another way to omit the fluorous solvent would be to utilize a catalystimmobilization solvent that is not fluorinated, such as water. We demonstrated theapplication of a phase change after reaction permits facile recycle of hydrophiliccatalysts. This method is called OATS (Organic-Aqueous Tunable Solvent) (15).Changes in composition or temperature give relatively incomplete separations –the addition of CO 2 has a far more profound effect on system phase behavior. CO 2 ismiscible with most organics but virtually immiscible with water. Addition of CO 2can result in a phase separation of a miscible organic/water mixture (16), ordrastically change distribution coefficients in a two-phase organic/water system (17).In addition, it provides for benign recycle of hydrophilic organometallic catalysts(14). For example, with the traditional aqueous biphasic catalysis technique,popularized by the Ruhrchemie/Rhône-Poulenc process (18), catalyst recovery canbe achieved most easily by maintaining an aqueous catalyst-rich phase separate fromthe substrate-containing (nonpolar) organic phase. The catalyst is immobilized in anaqueous phase by modifying the catalyst ligands with one or more polarfunctionalities, such as sulfonate or carboxylate salts (19). This renders the catalystcompletely insoluble in the substrate- (and later product-) containing organic phase,so that decantation of the organic phase results in no loss of catalyst. The aqueouslayer can be recycled many times, yielding high catalyst turnover with little metallic