COMPARATIVE-STUDY OF N-HEXANE AROMATIZATION ON PT KL, PT/MG(AL)O, ANDPT/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 contai ning feedstocks. Examination of the product distribution as a function of conversion suggests that the formation of benzene is preceded by t he formation of hexenes. In contrast with previous reports, it has bee n found that the Pt/KL catalyst exhibits much higher aromatization act ivity than the Pt/Mg(Al)O catalyst. On Pt/KL the main product is benze ne, with hexenes and lighter compounds as the principal by-products. B y contrast, on the Pt/Mg(Al)O, the main products were hexenes. Since h exenes are primary products and benzene is a secondary product, the ex ceptional aromatization activity of Pt/KL is explained in terms of its ability to convert hexene into benzene. In the presence of sulfur, th e Pt/KL exhibits a rapid loss in n-hexane conversion and benzene selec tivity. Under these conditions, the sulfided Pt/KL catalyst presents a catalytic behavior typical of Pt/Mg(Al)O and Pt/SiO2, generating larg er amounts of hexenes. The observed results are consistent with the hy pothesis that the most important role of the zeolite is to inhibit bim olecular interactions that lead to coke formation. The formation of co ke 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 c oking inside the channels of the zeolite rapidly become unselective. I n support of this hypothesis, the hydrogenolysis reaction which also r equires a large ensemble of atoms, decreases in parallel with the arom atization reaction. The high sensitivity of Pt/KL, to sulfur may be du e to a combination of effects which may involve growth of metal partic les outside the zeolite which would become unselective and partial poi soning of the particles inside the zeolite, causing a similar selectiv e deactivation. (C) 1998 Academic Press