MODEL PUMICES SUPPORTED METAL-CATALYSTS .2. LIQUID-PHASE SELECTIVE HYDROGENATION OF 1,3-CYCLOOCTADIENE

The catalyzed, selective hydrogenation, in liquid phase, of 1,3-cycloo ctadiene was studied on a series of Pd catalysts supported on natural pumice, model pumices (with variable content of alkali metal ions), si lica, and sodium-doped silica. At constant pressure of H-2 (1 atm.) th e reaction follows a zero-order kinetic for all the Pd catalysts. At l ow metal dispersion (D-x<20%), Pd/pumice catalysts exhibit higher acti vity as compared to Pd/silica catalysts; the turnover frequency is mai ntained even at high metal dispersion in Pd/pumice but Pd/silica and P d/model pumices without alkali ions show a decrease in activity. At in creasing D-x, the binding energy shift of Pd 3d level is negative in P d/pumice and positive in Pd/silica with respect to unsupported Pd meta l. The different performances of the Pd/pumice catalysts are explained by the presence of alkali metal ions in the framework of the support. Addition of sodium ions to Pd/silica catalysts produces a negative sh ift of the binding energy, but the activity is not improved because th e number of active sites diminished due to decoration of palladium par ticles by sodium ions. The Pd catalysts with alkali metal ions in the support are resistant to air oxidation. In Pd catalysts containing alk ali metal ions the selectivity to cyclooctene is practically 100% and the constant rates ratio k(1)/k(2) is more than 1000 with a maximum at dispersion 35-40%, whereas the selectivity in Pd catalysts without al kali metal ions decreases continuously at increasing dispersion. (C) 1 997 Academic Press.