Hydrogenation of carbon dioxide catalyzed by ruthenium trimethylphosphine complexes - Effect of gas pressure and additives on rate in the liquid phase

Kinetic and mechanistic studies of CO2 hydrogenation were performed in liqu id triethylamine and at subcritical CO2 pressures to avoid complications fr om phase behaviour that are observed under supercritical conditions. Kineti c measurements of the hydrogenation of CO2 to formic acid, catalyzed by RuC l(O2CMe)(PMe3)(4), support a CO2 insertion mechanism. The reaction is first -order in both H-2 and CO2 under most conditions. The rate is strongly depe ndent on the choice of additive, with methanol giving the greatest rates. B ecause only trace amounts of methanol are needed, the effect of the additiv e is believed to involve direct interactions with the catalyst rather than changes in the physical properties of the reaction medium. The optimized ra tes exceed 3500 h(-1). Addition of an inert gas affects the rate of the rea ction, probably via the phenomenon of gas expansion of the liquid phase.