Multiphase Catalysis Using SCFs as Solvents

1 Catalytic Reactions Under Multiphase Conditions Involving SCFs 

In general, the potential benefits of using SCCO2 or other SCFs in homogeneous catalysis can be exploited best under single-phase conditions. For many of the examples discussed in the previous sections, control experiments demonstrate that the presence of a single homogeneous reaction phase is a necessary prerequisite 

A classical example is the hydrogenation of CO2 in the presence of secondary amines to yield formamides (eq. (7)). The formation of carbamates from the amine and CO2 leads to the presence of a liquid phase that cannot be dissolved in CO2 even at temperatures and pressures way beyond the critical data of pure CO2. Nevertheless, the reaction occurs with extraordinarily high turnover numbers and reaction rates [17, 34], even with catalysts that have no solubility in SCCO2 [35,72]. Most likely, the reaction occurs in the liquid phase, but the supercritical CO2 phase ensures rapid mass transfer of the reactants (CO2, H2) and the product (DMF) between the two phases. It has been shown recently that the addition of ionic liquids (vide infra) can help to control the distribution of reactants, intermediates, and products between the two reaction phases. Additional control over the chemoselectivity of the transformation is thus possible by judicious segregation of various components of the reaction mixture [36, 74]. 

2 Catalysis and Extraction Using Supercritical Solutions (CESS) 

Supercritical/solid Silica-bound Xantphos/Rh complex [80] 

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