A new way to perform reactions in core—shell double emulsions is reported herein. The phase boundaries of the threephase droplet flow were used to pressurize the reactants in the shell liquid, enhancing the reaction rate of a cycloaddition greatly in comparison to known methods. As key parameters, solvophobic effects and precise control over the droplet sizes were used to exploit a reaction with a negative volume of activation. The internal pressure of the reaction solution was regulated purely by the thickness of the shell liquid without adding additional reagents. Additionally, the reaction performed better when the core droplet was used to stir the shell droplet, considerably improving the mass transfer inside the otherwise diffusion-limited process.
Authors:Viktor Misuk, Andreas Mai, Konstantinos Giannopoulos, Dominik Karl, Julian Heinrich, Daniel Rauber and Holger Löwe
The method of combining the concept of fluorous biphasic catalysis (FCB) with micro multiple emulsions benefits from the advantages of homogeneous as well as from heterogeneous catalysis in continuous micro flow. In this particular case, three immiscible fluid phases in continuous micro segmented flow were used to perform palladium-catalyzed Heck cross-coupling reactions of styrene with aryl halides. A capillary tube-in-tube coaxial flow setup in combination with a glass micro reactor was used to produce monodisperse aqueous phase/organic phase/perfluorinated phase double emulsions. The resulting emulsions had a core–shell droplet structure composed of a perfluorcarbon fluid in which a palladium catalyst with fluorinated phosphine ligands was dissolved, an organic phase consisting of a solvent and two reagents, and an alkaline aqueous solution. The fluorous and organic phases of the double emulsion form a thermomorphous system which can be converted into one phase by an increase of temperature above 150 °C, and the catalytic reaction is performed temporarily. By decreasing the temperature, a phase separation takes place; after that, the organic phase contains the product and the catalyst is located in the fluorous phase. The separated catalyst solution was reused several times without a noticeable loss of activity. The main advantage of this method is to use temporarily very high catalyst concentrations in each droplet, while employing only small amounts of the catalyst for the overall reaction volume.