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

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. At a given acid site concentration, the activity increased until a Pd/acid site ratio of 0.03 was reached; further improvement in activity and selectivity was obtained with increasing acid site concentration. The diffusion coefficients obtained by time-resolved infrared measurements decreased in the sequence n-heptane≈2-methylhexane>3-methylhexane for SAPO-11 and n-heptane≈2-methylhexane>3-methylhexane>2,4-dimethylpentane>2,3-dimethylpentane>2,2,3-trimethylbutane for SAPO-5. The difference in the diffusion coefficients was reflected in the different reactivities and in the product distribution of the heptane isomers. The higher reactivity of the dibranched isomers compensated their lower diffusivity over the wider pore SAPO-5. The pores of SAPO-11 were only partially accessible for multibranched isomers under reaction conditions. In the n-heptane conversion over hybrid catalysts (mixed HSAPO-11 and Pd/SiO2) the same activities and selectivities compared to those of Pd/SAPO-11 were found, indicating that the distance between acid and metal sites has only a minor influence as long as the supported metal and the SAPO phase are in direct contact.