SELECTIVE HYDROGENATION OF PHENYLACETYLENE ON PUMICE-SUPPORTED PALLADIUM CATALYSTS

The liquid phase, selective hydrogenation of phenylacetylene on pumice -supported palladium catalysts has been studied for a large range of m etallic dispersions (14% less than or equal to D-x less than or equal to 62%). The kinetics were analyzed by a five-parameter mathematical m odel. The mechanism was determined by the contribution of three basic routes involving only surface species in the rate-determining steps. T he hydrogenation of phenylacetylene to styrene is ''structure insensit ive.'' The disappearance rate constant of styrene produced from phenyl acetylene is slightly lower than that of phenylacetylene and does not change in the case of the direct hydrogenation of styrene on the same Pd/pumice catalyst. However, Q(3) (the ratio of adsorption constants K -Eb/K-St, where Eb is ethylbenzene and St is styrene), which is typica l of a zero order reaction (Q(3) --> 0) in the case of the direct hydr ogenation, is practically constant ((Q(3) congruent to 2) in the case of hydrogenation of styrene produced from phenylacetylene. This is exp lained by the formation, in the latter case, of polymeric species or o ther species which are difficult to hydrogenate and by the consequent occupation of active sites so that the adsorption of styrene is inhibi ted. These species are also thought to be responsible for a flattening effect in the catalytic activity. Activity and selectivity data are c ritically analyzed and compared with those reported for other supporte d palladium catalysts. Since Pd/pumice catalysts also show high activi ty and selectivity at high metal dispersions, they could be of interes t for industrial applications. (C) 1995 Academic Press, Inc.