PUMICE AS SUPPORT FOR METAL-CATALYSTS

Pumice is characterised by chemical, microstructural (SAXS, WAXS, SANS, WANS) and surface (XPS and BET) analyses. Pumice is used as support for the preparation of metal catalysts via reaction with diallyl metal complexes and successive reduction with dihydrogen of the anchored metallorganic substrate. Palladium, platinum and Pd-Pt alloys supported on pumice have been tested for selective hydrogenations of alkadienes and alkynes in liquid phase under mild conditions. The results suggest that pumice-supported palladium catalysts, besides having comparable activity and selectivity as other supported palladium catalysts, maintain their efficiency also at high metal dispersions, when other Pd supported catalysts show very low activity. Similar Pt and Pd-Pt catalysts are less active and selective, although better than the analogous metals on conventional supports. The peculiar behaviour of pumice-supported metal catalysts is attributed to the presence, on the pumice surface, of alkali metal ions which, by supplying electron density to the metal (very likely through electron-donating metal alkali ions oxygen composites), counterbalance the loss of metallic character usually found in supported metal catalysts at high metal dispersions.