Palladium-polyamide catalysts for the hydrogenation of some unsaturated hydrocarbons in the liquid phase

Abstract

The behavior of palladium-Nylon 66 and briefly, of some other palladium-polyamide fiber catalysts has been studied for the liquid phase hydrogenation of two 1-alkynes (1-pentyne and 1-octyne) and of two alka-1,3-dienes (penta-1,3-diene and isoprene) at 1 atm total pressure and between 50 and 70 °C. The activity of the catalysts as a function of metal concentration of the catalyst or of the treating solution concentration was observed to pass through a maximum, which was not related either to a maximum palladium content of the nylon or of the chloropalladous acid treating solution. In addition, both catalyst activity and selectivity were observed to be strongly reactant and solvent dependent, in contrast to conventional supported palladium catalysts. In one polar solvent, catalyst selectivity was observed to be a function of the palladium content of the nylon. Olefin distributions for the first mole of hydrogen uptake were often significantly different to those observed for conventionally supported palladium, especially for the alka-1,3-dienes. A number of physical and chemical techniques were employed to try to characterize the catalysts, including chemical analysis, infrared studies, and temperature programmed reduction (TPR). From the results of the latter and the selectivity and olefin distribution data, it is concluded that the catalyst may well be a polyamide-palladium-chlorine complex, probably containing Pd(II) in a square coplanar arrangement of ligands. Some suggestions are made concerning (i) the possible nature and arrangements of the ligands and (ii) the possible mechanism of the reaction in the light of the observed product distributions, assuming that a square coplanar complex is involved.