"Model Pumices Supported Metal Catalysts: II. Liquid Phase Selective Hydrogenation of 1,3-Cyclooctadiene

The catalyzed, selective hydrogenation, in liquid phase, of 1,3- cyclooctadiene was studied on a series of Pd catalysts supported on natural pumice, model pumices (with variable content of alkali metal ions), silica, and sodium-doped silica. At constant pressure of H2 (1 atm.) the reaction follows a zero-order kinetic for all the Pd catalysts. At low metal dispersion (Dx<20%), Pd/pumice catalysts exhibit higher activity as compared to Pd/silica catalysts; the turnover frequency is maintained even at high metal dispersion in Pd/pumice but Pd/silica and Pd/model pumices without alkali ions show a decrease in activity. At increasing Dx, the binding energy shift of Pd 3d level is negative in Pd/pumice and positive in Pd/silica with respect to unsupported Pd metal. 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 shift of the binding energy, but the activity is not improved because the number of active sites diminished due to decoration of palladium particles by sodium ions. The Pd catalysts with alkali metal ions in the support are resistant to air oxidation. In Pd catalysts containing alkali metal ions the selectivity to cyclooctene is practically 100% and the constant rates ratio k1/k2 is more than 1000 with a maximum at dispersion 35-40%, whereas the selectivity in Pd catalysts without alkali metal ions decreases continuously at increasing dispersion.