Comparative study of n-hexane aromatization on Pt/KL, Pt/Mg(Al)O, and Pt/SiO2 catalysts:: Clean and sulfur-containing feeds

The n-hexane aromatization has been studied on Pt/KL, Pt/Mg(Al)O, and Pt/SiO2 catalysts at 773 K using sulfur-free and 0.6 ppm sulfur containing feedstocks. Examination of the product distribution as a function of conversion suggests that the formation of benzene is preceded by the formation of hexenes. In contrast with previous reports, it has been found that the Pt/KL catalyst exhibits much higher aromatization activity than the Pt/Mg(Al)O catalyst. On Pt/KL the main product is benzene, with hexenes and lighter compounds as the principal by-products. By contrast, on the Pt/Mg(Al)O, the main products were hexenes. Since hexenes are primary products and benzene is a secondary product, the exceptional aromatization activity of Pt/KL is explained in terms of its ability to convert hexene into benzene. In the presence of sulfur, the Pt/KL exhibits a rapid loss in n-hexane conversion and benzene selectivity. Under these conditions, the sulfided Pt/KL catalyst presents a catalytic behavior typical of Pt/Mg(Al)O and Pt/SiO2, generating larger amounts of hexenes. The observed results are consistent with the hypothesis that the most important role of the zeolite is to inhibit bimolecular interactions that lead to coke formation. The formation of coke has the net effect of selectively deactivating aromatization sites which require a large ensemble of atoms to constitute the active site but not affecting the dehydrogenation activity which is less ensemble-sensitive. Therefore, those particles that are not protected against coking inside the channels of the zeolite rapidly become unselective. In support of this hypothesis, the hydrogenolysis reaction which also requires a large ensemble of atoms, decreases in parallel with the aromatization reaction. The high sensitivity of Pt/KL, to sulfur may be due to a combination of effects which may involve growth of metal particles outside the zeolite which would become unselective and partial poisoning of the particles inside the zeolite, causing a similar selective deactivation. (C) 1998 Academic Press