PUMICE-SUPPORTED PALLADIUM CATALYSTS .2. SELECTIVE HYDROGENATION OF 1,3-CYCTOOCTADIENE

Two series of pumice-supported palladium catalysts (W and U) have been tested in the liquid-phase selective hydrogenation of 1,3-cyclooctadi ene (1,3-COD) to cyclooctene (COE). The two series of catalysts, obtai ned via organometallic precursors, differ in the preparation procedure . In the W series the reduced metal derives only from Pd intermediates anchored to pumice; in the U series the metal originates also from un reacted Pd(allyl)(2) species in solution. The U catalysts present aggl omerated metal particles. The hydrogenations, free of any diffusion pr oblems, were performed at constant pressure of hydrogen (1 at-m). Anal ysis of the data suggests that the rate-determining step is a surface reaction involving activated 1,3-COD. Selectivity is very high since c yclooctane (COA) is detected only when all 1,3-COD is consumed. The tu rnover frequencies (TOF1) of the first semihydrogenation do not change with palladium dispersion, determined from the Pored diameter D-p up to D-x < 35%, but thereafter they slowly decrease. Experimental eviden ce, such as the absence of oxidation when exposed to air and the negat ive shift of binding energy of the Pd 3d level in WS measurements, ind icate a different behavior of the present catalysts in comparison with other supported palladium catalysts. These differences are attributed to the presence of alkali metal ions (Na+, K+) on the pumice surface and are explained on the basis of the change in electron density and/o r in the ensemble size of the supported palladium. Several interesting features and differences in activity and selectivity between the two series of catalysts can be accounted for by the presence of agglomerat ed palladium particles in the U series and are discussed in terms of t he evolution of parameters with dispersion. (C) 1994 academic Press, I nc.