Hydroisomerization of heptane isomers over Pd/SAPO molecular sieves: Influence of the acid and metal site concentration and the transport properties on the activity and selectivity

Platinum or palladium supported on microporous silico-aluminum phosphates are highly efficient catalysts for the hydroisomerization of long chain alkanes, while the conversion of linear into monobranched alkanes plays a critical role in dewaxing paraffinic feedstock. The hydroconversion of n-heptane and heptane isomers over Pd/SAPO-5 and Pd/SAPO-11 was studied. The Pd/SAPO-11 catalysts differed in concentration of the acid sites but not in the acid strength. The activity in n-heptane conversion increased until a Pd/acid site ratio of 0.02-0.03 was reached. Increases in the concentration of acid sites led to an enhancement of the activity and the isomerization selectivity, demonstrating that hydroconversion over the different Pd-SAPO-11 samples should be governed by acidity. Th Broensted acid sites in SAPO-11 and in SAPO-5 were accessible at 10^-3 mbar for n-C₇ and 2-MC₆. The influence of the n-C₇ and 2-MC₆ transport inside the pores was negligible at low conversions and temperatures. Severe diffusion limitations for the multibranched isomers and an interaction with the bridged acid sites of 20% with 2,3- and 2,4-DMC and of ~ 5% with 2,2,3-TMC₄ at 1 mbar were evident in SAPO-11. All acid sites in SAPO-5 were accessible for the dibranched isomers, but the diffusion coefficients were smaller compared to those of n-C₇ and 2-MC₆. The Pd-particle size estimated from the metal dispersion was between 15 and 30 nm, suggesting that the particles were too large to fit into the SAPO pores. For hybrid catalysts consisting of Pd/SiO₂ and SAPO-11, almost the same activity and selectivity in n-heptane conversion was obtained as those for impregnated SAPO-11. This indicated that the distance between acid and metal sites had only a minor influence, as long as the supported metal and the SAPO phase were in direct contact.