COKING, AGING, AND REGENERATION OF ZEOLITES .2. DEACTIVATION OF HY ZEOLITE DURING N-HEPTANE CRACKING

Heavy carbonaceous compounds (coke) are deposited very rapidly during n-heptane cracking on a stabilized HY zeolite at 450 °C, decreasing its activity, modifying its selectivity and its adsorption properties. Deactivation of the zeolite can be explained mainly by partial or total blocking of access to the active centers, even for low coke content. Indeed, while the decrease of the pore volume accessible to n-hexane (hence to the reactant n-heptane) is about 3 times less than the decrease of the activity, calorimetric study of NH3 adsorption shows that the sites which no longer adsorb NH3 are the strongest. Moreover, the diffusivity of n-hexane in the pore volume which remains accessible is definitely smaller than in the coke-free zeolite. Finally, the pore volume which has become inaccessible to n-hexane is, even at low coke content, about three times smaller than the volume really occupied by coke (estimated from its density). This deactivation through pore blockage is due to the fact that, even at low coke content, the coke molecules are sufficiently large to prevent access of the reactant to the supercages of the zeolite. © 1987.