Hydrogenation of olefins in aqueous phase, catalyzed by polymer-protected rhodium colloids: kinetic study

Highly water-soluble polymer such as PVP was used to stabilize colloidal su spensions of active rhodium particles and to perform catalytic hydrogenatio n of oct-l-ene in a two liquid phase system. The effects of various paramet ers on the stability and activity of these nanoparticles under more or less severe conditions have been investigated. Experiments show that PVP-Rh col loids can be reused twice or more at 50 degreesC and 0.3 MPa without losing activity. Much higher turnover than with usual biphasic catalysis were per formed. The MET micrographs of the catalytic phase of the PVP-Rh system: be fore and after hydrogenation shows two nanometer particles containing rhodi um oxide and metallic rhodium. IR and NMR spectroscopy analysis of the coll oids shows geminal and terminal Rh-CO species with a preponderance of the g eminal ones. The kinetics of biphasic hydrogenation and simultaneous isomer isation of oct-l-ene have been studied in the presence of ethanol cosolvent , using low concentrations of PVP-Rh colloids. The rate was found to be fir st order with respect to catalyst and olefin concentrations and hydrogen pr essure. A kinetic model was fitted to the observed data, and was found to p redict the rates with a good agreement. The activation energy was found to be as low as 3.4 kcal/mol suggesting the reaction to be limited by diffusio n of oct-l-ene through the protective polymer layer. (C) 2001 Published by Elsevier Science B.V.