Deactivation and coke formation on palladium and platinum catalysts in vegetable oil hydrogenation

Deactivation of palladium and platinum catalysts due to coke formation was studied during hydrogenation of methyl esters of sunflower oil. The supported metal catalysts were prepared by impregnating gamma -alumina with either palladium or platinum salts, and by impregnating a-alumina with palladium salt. The catalysts were reused for several batch experiments. The Pd/gamma -Al2O3 catalyst lost more than 50% of its initial activity after four batch experiments, while the other catalysts did not deactivate. Samples of used catalysts were cleaned from remaining oil by repeated extractions with methanol, and the amount of coke formed on the catalysts was studied by temperature-programmed oxidation. The deactivation of the catalyst is a function of both the metal and the support. The amount of coke increased on the Pd/gamma -Al2O3 catalyst with repeated use, but the amount of coke remained approximately constant for the Pt/gamma -Al2O3 catalyst. Virtually no coke was detected on the Pd/alpha -Al2O3 catalyst. The formation of coke on Pd/alpha -Al2O3 may be slower than on the Pd/gamma -Al2O3 owing to the carrier's smaller surface area and less acidic character. The absence of deactivation for the Pt/gamma -Al2O3 catalyst may be explained by slower formation of coke precursors on platinum compared to palladium.