Palladium catalysts in the selective hydrogenation of hexa-1,5-diene and hexa-1,3-diene in the liquid phase. E ect of tin and silver addition

Abstract

Palladium, palladiumÈtin and palladiumÈsilver catalysts, supported on two alumina samples with di erent acidity, have been prepared by di usional impregnation for use in the selective hydrogenation of hexa-1,5- diene and hexa-1,3-diene in the liquid phase. UVÈVISÈNIR (transmission and di use reÑectance) studies of the palladium starting solutions and of the solids obtained during all preparation stages (impregnation, drying, calcination) show that the nature of the supported precursor species depends on the acidity of the alumina. Palladium tetrachloropalladate (strongly bound anionic species) are predominant on the more acidic alumina while the more basic one favours the formation of weakly bound neutral complexes through exchange of chloride ligands with hydroxy groups and water. The nature of these species inÑuences not only the particle size of monometallic Pd samples (the stronger the interaction, the smaller the metal particle size), but also the nature of the Ðnal bimetallic PdÈAg catalysts ; a PdÈAg solid solution is observed on the bimetallic catalyst prepared by interaction of a cationic Ag precursor with supported PdCl whereas interaction with neutral 42~, Pd species leads to separate metal phases. The interaction of Sn precursor solutions with supported PdII species induces the formation of heteronuclear species which decompose upon drying; Pd or phases 2Sn Pd3Sn are detected on the Ðnal catalysts, depending on the initial Pd/Sn ratio and on the nature of the Sn precursor solvent (using ethanol favours the formation of Pd3Sn).