Liquid phase semihydrogenation of phenylacetylene to styrene on Pd/TiO₂ in supercritical carbon dioxide

Abstract

In this thesis, liquid-phase selective hydrogenation of phenylacetylene over Pd/TiO₂ has been studied in supercritical carbon dioxide (scCO₂) and compared to that in organic solvent (ethanol) and solventless condition. It was observed that hydrogenation rate increased in the order: scCO₂ > solventless > ethanol > ethanol+scCO₂. The H₂ solubility in phenylacetylene may be enhanced by dissolution of CO₂ resulting in an increase in hydrogenation activity. On the other hand, the mixture of ethanol and scCO₂ may lead to a biphasic reaction mixture so that lower catalytic activity was obtained. The Pd particle size did not affect specific activity for semihydrogenation of phenylacetylene in scCO2 but in ethanol, the reaction appeared to be structure sensitive since the TOF increase with an increase in Pd particles size. Reduction at high temperature (500℃) caused metal sintering and TiO₂ phase change for micron-size anatase TiO2 and nano-size brookite TiO₂, respectively. As a consequence, lower Pd dispersion and lower hydrogenation activity were obtain over these catalysts. On the other hand, the use of solvothermol-derived TiO₂ supported catalyst exhibited the strong metal-support interaction effect which improved the catalyst performance especially when the conversion of phenylacetylene was nearly completed in scCO₂