SELECTIVE HYDROGENATION OF CARVONE ON PT AND PT-AU CATALYSTS

The initial selectivity of platinum catalysts in carvone hydrogenation depends on the platinum dispersion. For example carvotanacetone (2-methyl-5-isopropylcyclo 2-ene-1-one) is the major product on catalysts with large platinum particles, whereas carvomenthone (2-methyl-5-isopropylcyclohexanone) is the major product on highly dispersed platinum catalysts. The previous platinum catalysts were modified by gold addition using a surface Redox reaction : Au3+ ions were reduced by hydrogen preadsorbed on the Pt surface area. The addition of gold to platinum on the highly dispersed catalyst modifies the initial selectivity toward that obtained with large platinum particles. In agreement with comparable electronic affinities of Pt and Au and comparable selectivities in O-xylene hydrogenation on Pt and Pt-Au catalysts, such change in selectivity in carvone hydrogenation cannot be explained by a ligand effect. On the other hand, results can be explained by assuming that gold adds selectively on the corners or the edges of the platinum particles. X-ray emission analysis of platinum-gold catalysts bears out that gold decorates the platinum particles rims pointing out that low coordination platinum atoms which are selectively deactivated by gold deposition, are responsible for the direct transformation of carvone into carvomenthone.