Modification of the catalytic properties of palladium by rare earth (La, Ce) addition: Catalytic activity and selectivity in hydrocarbon conversion

Abstract

Pd/Al 2O 3 catalysts modified by cerium or lanthanum promoters are tested for hydrocarbon conversion: methylcyclopentane (MCP) hydrogenolysis, 2-methylpentane (2MP) isomerization and 3-methylhexane (3MH) hydrocracking, deshydrocyclization and aromatization. The following parameters are reviewed: (i) precursor salt of palladium (chloride or nitrate), (ii) rare earth nature (La or Ce), (iii) rare earth content within the range 0–100% and (iv) impregnation mode (coimpregnation or successive impregnations). The influence of chloride coming from the precursor salt of palladium on the catalytic behaviour is strongly underlined. Chlorine anions are trapped by rare earth cations at the interface, as evidenced in a subsequent paper dealing with characterization studies of these same catalysts (K. Kili, L. Hilaire, F. Le Normand, submitted). Although the reactions readily occur on metallic sites, as evidenced by 13 C labelled experiments, the addition of rare earth increases the activity and modifies the selectivity, especially for 2MP isomerization. These changes are rationalized in terms of significant modification of the kinetic surface parameters (competitive hydrogen and hydrocarbon coverages). This is explained by (i) lowering of the hydrogen coverage of the palladium sites accompanying surface diffusion on the interface with the support and (ii) creation of new selective sites at the transition metal–rare earth interface. The other parameters investigated yield only minor changes of the catalytic behaviour.