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 liquid triethylamine and at subcritical CO2 pressures to avoid complications from phase behaviour that are observed under supercritical conditions. Kinetic measurements of the hydrogenation of CO2 to formic acid, catalyzed by RuCl(O2CMe)(PMe3)4, support a CO2 insertion mechanism. The reaction is first-order in both H2 and CO2 under most conditions. The rate is strongly dependent on the choice of additive, with methanol giving the greatest rates. Because only trace amounts of methanol are needed, the effect of the additive is believed to involve direct interactions with the catalyst rather than changes in the physical properties of the reaction medium. The optimized rates exceed 3500 h–1. Addition of an inert gas affects the rate of the reaction, probably via the phenomenon of gas expansion of the liquid phase.Key words: carbon dioxide, homogeneous catalysis, hydrogenation, formic acid, expanded liquids.