Physically and chemically mixed TiO 2-supported Pd and Au catalysts: unexpected synergistic effects on selective hydrogenation of citral in supercritical CO 2

Abstract

The selective hydrogenation of citral was studied with various TiO 2-supported monometallic and bimetallic Pd and Au catalysts and their physical mixtures in supercritical CO 2 (scCO 2). Significant synergistic effects appeared when active Pd species was chemically or physically mixed with less active Au species. The total rate of conversion was greatly enhanced and the selectivity to citronellal (CAL) was improved. The physical properties of those catalysts were characterized by TEM, HRTEM-EDS, XPS, and UV/Vis and their features of H 2 desorption were examined by TPD. The physical and chemical characterization results were used to discuss the reasons for the unexpected synergistic effects observed. The same selective hydrogenation was also conducted in a conventional non-polar organic solvent of n-hexane to examine the roles of scCO 2. The use of scCO 2 was effective for accelerating the hydrogenation of citral and improving the selectivity to CAL.