Supercritical carbon dioxide as reaction medium: synthesis, catalytic properties and recovery of hybrids of amphiphilic macromolecules with metal nanoparticles

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Supercritical fluids exhibit a range of unusual properties which can be exploited for the development of new reactions in such fields as separation, chemical reactivity, or material processing. Beyond the capability of replacing toxic industrial solvents, the main interest of supercritical fluids is the possibility to tune continuously the fluid properties from liquid to gas with small pressure and temperature variations. Soluble transition metal particles with sizes of a few nanometers are recognized today to possess a crucial role in many synthetically important transfomations. However, their controlled preparation, stabilization and recovery after catalytic reaction remain key challenges. In the research proposed metal nanoparticles will be prepared and stabilized in the polar hyperbranched core of amphiphilic macromolecules. A CO2-philic shell provides solubility in supercritical carbon dioxide (scCO2). Particle size and morphology control by synthesis directly in scCO2, and as an alternative approach in aqueous media with subsequent construction of the shell will be investigated. The catalytic properties in hydrogenation of double bonds and in C-C coupling (Heck reaction) will be studied in scCO2, and compared to catalysis in organic solvents. Recovery of the CO2-philic polymer/nanoparticle hybrides from the products of a catalytic reaction by means of changing the properties of the carbon dioxide medium, particularly the density, will be investigated. This is of particular interest with respect to the recovery of catalytically active nanoparticles, as little stress on colloidal stability is imposed.