Homogeneous Catalysis in Supercritical Fluids:  Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides

Rapid, selective, and high-yield hydrogenation of CO 2 can be achieved if the CO 2 is in the supercritical state (scCO 2 ). Dissolving H 2, a tertiary amine, a catalyst precursor such as RuH 2 P(CH 3 ) 3 4 or RuCl 2 P(CH 3 ) 3 4, and a promoting additive such as water, CH 3 OH, or DMSO in scCO 2 at 50 °C leads to the generation of formic acid with turnover frequencies up to or exceeding 4000 h - 1 . In general, experiments in which a second phase was formed by one or more reagents or additives had lower rates of reaction. The high rate of reaction is attributed to rapid diffusion, weak catalyst solvation, and the high miscibility of H 2 in scCO 2 . The formic acid synthesis can be coupled with subsequent reactions of formic acid, for example, with alcohols or primary or secondary amines, to give highly efficient routes to formate esters or formamides. With NH(CH 3 ) 2, for example 420 000 mol of dimethylformamide/mol of Ru catalyst was obtained at 100 °C. The demonstrated solubility and catalytic activity of complexes of tertiary phosphines in scCO 2 suggest that scCO 2 could be an excellent medium for homogeneous catalysis and that many phosphine-containing homogeneous catalysts could be adopted for use in supercritical media.