Palladium catalysts in the selective hydrogenation of hexa-1,5-diene and hexa-1,3-diene in the liquid phase. Effect of tin and silver addition - Part1. Preparation and characterization: from the precursor species to the final phases

Palladium, palladium-tin and palladium-silver catalysts, supported on two a lumina samples with different acidity, have been prepared by diffusional im pregnation for use in the selective hydrogenation of hexa-1,5-diene and hex a-1,3-diene in the liquid phase. W-VIS-NIR (transmission and diffuse reflec tance) studies of the palladium starting solutions and of the solids obtain ed during all preparation stages (impregnation, drying, calcination) show t hat the nature of the supported precursor species depends on the acidity of the alumina. Palladium tetrachloropalladate (strongly bound anionic specie s) are predominant on the more acidic alumina while the more basic one favo urs the formation of weakly bound neutral complexes through exchange of chl oride ligands with hydroxy groups and water. The nature of these species in fluences not only the particle size of monometallic Pd samples (the stronge r the interaction, the smaller the metal particle size), but also the natur e of the final bimetallic Pd-Ag catalysts; a Pd-Ag solid solution is observ ed on the bimetallic catalyst prepared by interaction of a cationic Ag prec ursor with supported PdCl42-, whereas interaction with neutral Pd species l eads to separate metal phases. The interaction of Sn precursor solutions wi th supported Pd-II species induces the formation of heteronuclear species w hich decompose upon drying; Pd2Sn or Pd3Sn phases are detected on the final catalysts, depending on the initial Pd/Sn ratio and on the nature of the S n precursor solvent (using ethanol favours the formation of Pd3Sn).