Continuous semihydrogenation of phenylacetylene over amorphous Pd81Si19 alloy in "supercritical" carbon dioxide: Relation between catalytic performance and phase behavior

Continuous semihydrogenation of phenylacetylene over an amorphous Pd81Si19 catalyst in supercritical CO2 as a solvent has been studied. The amorphous alloy afforded high selectivity to styrene without catalyst modification, w hich is usually necessary for high selectivity in "Lindlar-type" hydrogenat ions. The application of the highly active glassy metal alloy combined with a supercritical solvent afforded efficient operation in a continuous fixed -bed reactor. A single-phase system was found to be an ideal reaction mediu m for such hydrogenation reactions. Highest conversions and good selectivit ies were achieved near the transition to a single-phase mixture, where mass transport is enhanced by elimination of the gas-liquid interface. In order to reach complete miscibility of the reactants in CO2 at moderate pressure , both temperature and hydrogen excess had to be kept low. The phase behavi or was monitored by online video imaging. Consideration of the phase behavi or was found to be indispensable for assessing single-phase conditions. A b inary fluid-phase diagram is shown to serve as a good guide to understandin g the multicomponent behavior. (C) 2001 Academic Press.