DEACTIVATION OF ZEOLITES IN N-HEXANE CRACKING

The deactivation process of the H-type zeolites (H-mordenite, H-Y zeolite, and H-ZSM-5) in n-hexane cracking was studied using a pulse method in which the emphasis was put on the clarification of the initial stage of the deactivation of these H-type zeolites. The adsorption behaviors of n-hexane and ammonia on fresh and used zeolites were compared in order to elucidate the relationship between the micropore structure of the zeolites and the deactivation by the carbonaceous deposits (coke). The main deactivation of the H-mordenite (HM) zeolite was found to be caused by pore blockage of the deposited coke. The H-Y zeolite (HY) was suggested to be deactivated by the proton coverage of the coke. No deactivation of the H-ZSM-5 (HZ) was observed under the present pulse conditions. These deactivation behaviors of the H-type zeolites were found to be consistent with the results obtained by a gravimetric flow method. The initial activity, defined as the conversion at the first pulse, could be correlated with the strong acidity, irrespective of the micropore structure of the zeolites used. The correlation of both the initial aromatic and coke yields with the strong acidity was discussed in connection with the micropore structure of the H-type zeolites.