The liquid-phase hydrogenation of hexa-1,5-diene and hexa-1,3-diene on alumina-supported palladium catalysts is investigated with special attention paid to the effects of tin or silver addition. All catalysts show a global selectivity near 100%; these high values persist at total conversion when the reactant is hexa-1,3-diene but decrease to about 70% in the case of hexa-1,5-diene. In the hexa-1,5-diene hydrogenation, monometallic palladium catalysts give mainly 1-hexene at conversions lower than 80% and E-hex-2-ene at higher conversions. The fractional selectivity to 1-hexene is significantly improved by tin or silver addition; however, significant yields are observed only on Pd-Sn catalysts with a low Sn/Pd atomic ratio (approximate to 0.1) and when the intermetallic compound Pd3Sn is present. On the other hand, in the hexa-1,3-diene hydrogenation, hex-1-ene is preferentially formed on monometallic catalysts with low dispersion; on bimetallic Pd-Sn or Pd-Ag catalysts, the selectivity to E-hex-3-ene is enhanced and this isomer is even predominant up to 100% conversion on the Pd-Sn catalysts containing the Pd3Sn intermetallic compound. These results are explained by the geometric effect of dilution of Pd atoms, which delays the palladium double-bond isomerization ability. The proposed mechanism suggests that strongly adsorbed alkadienes react with dissolved hydrogen, following a zero-order kinetics. (C) 2000 Academic Press.195196105