HOMOGENEOUS CATALYSIS IN SUPERCRITICAL FLUIDS - HYDROGENATION OF SUPERCRITICAL CARBON-DIOXIDE TO FORMIC-ACID, ALKYL FORMATES, AND FORMAMIDES

Rapid, selective, and high-yield hydrogenation of CO2 can be achieved if the CO2 is in the supercritical state (scCO(2)). Dissolving H-2, a tertiary amine, a catalyst precursor such as RuH2[P(CH3)(3)](4) or RuC l2[P(CH3)(3)](4), and a promoting additive such as water, CH3OH, or DM SO in scCO(2) at 50 degrees C leads to the generation of formic acid w ith turnover frequencies up to or exceeding 4000 h(-1). In general, ex periments in which a second phase was formed by one or more reagents o r additives had lower rates of reaction. The high rate of reaction is attributed to rapid diffusion, weak catalyst solvation, and the high m iscibility 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 for mate esters or formamides. With NH(CH3)(2), for example 420 000 mol of dimethylformamide/mol of Ru catalyst was obtained at 100 degrees C. T he demonstrated solubility and catalytic activity of complexes of tert iary phosphines in scCO(2) suggest that scCO(2) could be an excellent medium for homogeneous catalysis and that many phosphine-containing ho mogeneous catalysts could be adopted for use in supercritical media.