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 P t/MOR, i.e., (i) on the metal particles and (ii) on Bronsted 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 deactivati on of fully H+ ion exchanged zeolites and the markedly slower intrinsic rat e on Na+ ion exchanged samples led to a pronounced maximum in the steady-st ate activity for benzene hydrogenation with partly HI exchanged samples. At high pressure and in the presence of thiophene, the benzene hydrogenation activity of Pt/MOR was higher via the pathway involving Bronsted acid sites than via the pathway involving only Pt. Therefore, in the presence of thio phene the type of alkali cation exchanged did not significantly influence a ctivity, whereas Bronsted 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 a nd the rate of C-C bond breaking leading to coke formation, and (ii) the pa rtial pressure of hydrogen, which increases the concentration of hydrogen a toms available for hydrogenating benzene molecules activated via both pathw ays. (C) 2001 Academic Press.