Sulfur-tolerant Pt-supported catalysts for benzene hydrogenation: II. Influence of cation exchange level for Pt/MOR-based catalysts

Two reaction pathways are described for the hydrogenation of benzene over Pt/MOR, i.e., (i) on the metal particles and (ii) on Brønsted acid sites of MOR at the boundary to the metal, with atomic hydrogen being dissociated on the metal. The ratio between the two pathways depends on the zeolite acid site concentration and on the available metal surface sites. The deactivation of fully H⁺ ion exchanged zeolites and the markedly slower intrinsic rate on Na⁺ ion exchanged samples led to a pronounced maximum in the steady-state activity for benzene hydrogenation with partly H⁺ exchanged samples. At high pressure and in the presence of thiophene, the benzene hydrogenation activity of Pt/MOR was higher via the pathway involving Brønsted acid sites than via the pathway involving only Pt. Therefore, in the presence of thiophene the type of alkali cation exchanged did not significantly influence activity, whereas Brønsted acid site concentration did. The relative sulfur tolerance of Pt/MOR during benzene hydrogenation was shown to depend on (i) the acid site concentration, which increases both the benzene conversion and the rate of C-C bond breaking leading to coke formation, and (ii) the partial pressure of hydrogen, which increases the concentration of hydrogen atoms available for hydrogenating benzene molecules activated via both pathways.