Coking and deactivation of zeolites

The modes of coking and of deactivation of zeolites during n-heptane cracking at 723 K were established on the basis of (i) the composition of the carbonaceous compounds responsible for deactivation (coke), (ii) the deactivating effect of the coke molecules and (iii) the reduction by coke of the volume accessible to nitrogen and to n-hexane (kinetic diameter similar to n-heptane). The zeolites [USHY, H Mordenite (HMOR), HZSM5 and H Erionite (HERI)] were chosen to determine the effect of different parameters of the pore structure: (i) pore size, (ii) existence (USHY, HERI) or non-existence of cavities (HMOR, HZSM5), (iii) the possibility for the reactant to diffuse unidirectionally (HMOR) or tridirectionally. The retention of coke molecules is due to trapping in the cavities (or at channel intersections). Their size is intermediate between that of the apertures and that of the cavities (or channel intersections). The coking rate is all the faster when the space available near the acid sites is large and when the coke precursors desorb slowly. On all the zeolites, coke formation occurs through oligomerization of the olefinic cracking products followed by cyclization of the oligomers, transformation through hydrogen transfer into monoaromatics, alkylation of these monoaromatics, then cyclization and hydrogen transfer to give bi-aromatics, tri-aromatics, etc. There is no site poisoning by coke; deactivation occurs through the three following modes: (i) limitation of the access of n-heptane to the active sites, (ii) blockage of the access to the sites of the cavities (or channel intersections) in which the coke molecules are situated and (iii) blockage of the access to the sites of the pores in which there are no coke molecules.