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 Pdl 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 concentr ation, the activity increased until a Pd/acid site ratio of 0.03 was reache d; further improvement in activity and selectivity was obtained with increa sing acid site concentration. The diffusion coefficients obtained by time-r esolved infrared measurements decreased in the sequence n-heptane approxima te to 2-methylhexane > 3-methylhexane for SAPO-11 and n-heptane approximate to 2-methylhexane > 3-methylhexane > 2,4-dimethylpentane > 2,3-dimethylpen tane > 2,2,3-trimethylbutane for SAPO-5. The difference in the diffusion co efficients was reflected in the different reactivities and in the product d istribution of the heptane isomers. The higher reactivity of the dibranched isomers compensated their lower diffusivity over the wider pore SAPO-5. Th e pores of SAPO-11 were only partially accessible for multibranched isomers under reaction conditions. In the n-heptane conversion over hybrid catalys ts (mixed HSAPO-11 and Pd/SiO2) the same activities and selectivities compa red 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 m etal and the SAPO phase are in direct contact. (C) 2000 Academic Press.