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

Pd/Al2O3 catalysts modified by cerium or lanthanum promoters are tested for hydrocarbon conversion: methylcyclopentane (MCP) hydrogenolysis, 2-methylp entane (2MP) isomerization and 3-methylhexane (3MH) hydrocracking, deshydro cyclization and aromatization. The following parameters are reviewed: (i) p recursor salt of palladium (chloride or nitrate), (ii) rare earth nature (L a or Ce), (iii) rare earth content within the range 0-100% and (iv) impregn ation mode (coimpregnation or successive impregnations). The influence of c hloride coming from the precursor salt of palladium on the catalytic behavi our is strongly underlined. Chlorine anions are trapped by rare earth catio ns at the interface, as evidenced in a subsequent paper dealing with charac terization studies of these same catalysts (K. Kill, L. Hilaire, F. Le Norm and, submitted). Although the reactions readily occur on metallic sites, as evidenced by C-13 labelled experiments, the addition of rare earth increas es the activity and modifies the selectivity, especially for 2MP isomerizat ion. These changes are rationalized in terms of significant modification of the kinetic surface parameters (competitive hydrogen and hydrocarbon cover ages). This is explained by (i) lowering of the hydrogen coverage of the pa lladium sites accompanying surface diffusion on the interface with the supp ort and (ii) creation of new selective sites at the transition metal-rare e arth interface. The other parameters investigated yield only minor changes of the catalytic behaviour. (C) 1999 Elsevier Science B.V. All rights reser ved.