Semihydrogenation of a propargylic alcohol over highly active amorphous Pd81Si19 in supercritical carbon dioxide

The semihydrogenation of a propargylic alcohol (dehydroisophytol) has been studied using glassy Pd81Si19 as a catalyst and supercritical CO2 as a solvent. The continuous fixed-bed reactor experiments were performed in the pressure range 50-250 bar and at temperatures from 42 to 120degreesC. Parallel studies of the phase behavior of the reaction system in a high-pressure view-cell revealed that the number, nature and composition of the mutually saturated phases depended strongly on temperature and pressure. Correlation of the phase behavior with the catalytic studies indicated that a single-phase system is an ideal reaction medium for this catalytic system, In combination with the high activity of the amorphous metal alloy catalyst high conversion and selectivity could be reached at significantly lower temperature than when working in the two-phase region. Comparative catalytic tests revealed that the glassy alloy exhibits a more than 50 times higher turnover frequency than a conventional silica-supported palladium catalyst under similar conditions. Selectivity to isophytol was 100% at low conversion and declined to 77% at around 70% conversion due to overhydrogenation. The combined application of a glassy palladium-silicon alloy together with supercritical CO2 seems to be promising for this type of Lindlar reactions. (C) 2002 Elsevier Science B.V. All rights reserved.