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 CO(2) hydrogenation were performed in liquid triethylamine and at subcritical CO(2) pressures to avoid complications from phase behaviour that are observed under supercritical conditions. Kinetic measurements of the hydrogenation of CO(2) to formic acid, catalyzed by RuCl(O(2)CMe)(PMe(3))(4), support a CO(2) insertion mechanism. The reaction is first-order in both H(2) and CO(2) 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.