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 Part 1. Preparation and characterization: from the precursor species to the final phases

Abstract

Palladium, palladium–tin and palladium–silver catalysts, supported on two alumina samples with different acidity, have been prepared by diffusional impregnation for use in the selective hydrogenation of hexa-1,5-diene and hexa-1,3-diene in the liquid phase. UV–VIS–NIR (transmission and diffuse reflectance) 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 influences 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 final bimetallic Pd–Ag catalysts; a Pd–Ag solid solution is observed on the bimetallic catalyst prepared by interaction of a cationic Ag precursor withsupported PdCl₄^2-, whereas interaction with neutral Pd species leads to separate metal phases. The interaction of Sn precursor solutions with supported Pd^II species induces the formation of heteronuclear species which 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 Sn precursor solvent (using ethanol favours the formation of Pd₃Sn).