Catalysis in Supercritical Fluids

The potential of supercritical fluids as reaction media [4.a] has been in the focus of Baiker since many years. Fundamental aspects investigated embrace mass transfer of dense fluids entrapped in pores and the phase behaviour in multicomponent reaction mixtures involving supercritical fluids. Supercritical carbon dioxide has been explored as an environmentally benign solvent in a variety of reactions, including catalytic chemoselective and enantioselective hydrogenations and more recently aerobic oxidations. Tuning of the properties of supercritical carbon dioxide by applying small amounts of co-solvents allowed to reach reaction rates in the oxidation of alcohols by air that are more than order of magnitude higher than those achieved with conventional solvents. In situ ATR-studies of the catalytic fluid-solid interface combined with transmission IR studies of the bulk liquid phase brought new insight into the crucial role of the phase behaviour and molecular interactions occurring under high pressure conditions. These studies clearly revealed that a profound understanding of the phase behaviour, molecular interactions in the dense liquid phase as well as at the catalytic fluid-solid interface is necessary for proper exploitation of supercritical fluids as reaction media. Particularly interesting is the application where supercritical carbon dioxide acts simultaneously as reactant and reaction medium [4.b]. This approach has been pursued successfully in the Baiker group for more than a decade. Supercritical carbon dioxide has been used for the formylation of various amines with high efficacy. More recently this strategy of utilizing carbon dioxide for chemical synthesis has been successfully extended to the catalytic synthesis of organic carbonates.