COKING, AGING AND REGENERATION OF ZEOLITES .18. COKING AND DEACTIVATION OF HZSM-5 AND GA HZSM-5 CATALYSTS DURING PROPENE AROMATIZATION/

Coking and deactivation of HZSM-5 (Si/Al = 40) and of a 2 wt% impregna ted Ga/HZSM-5 catalyst were investigated during propene aromatization at temperatures between 250 degrees C and 530 degrees C. Whatever the temperature the rate of propene aromatization is greater on the Ga/HZS M-5 catalyst than on HZSM-5 while the rates of coking are practically identical. With both catalysts a minimum is found in the rare of cokin g for temperatures between 400 and 450 degrees C, which can be explain ed by a contrary effect of temperature on the race of formation of cok e precursors and on their retention. Coke causes a pore blockage respo nsible for the catalyst deactivation, No change in the dehydrogenation /cracking rate ratio of methylcyclohexane transformation is observed, which indicates that the dehydrogenating function (Ga species) and the acid one (protonic sites) are affected to the same extent. The coke c omposition does not depend on the catalyst but changes very much with the temperature : alkylmonoaromatics at 250 degrees C, bi- or triaroma tics at 350 degrees C, tetra- to polyaromatics at 400 and 450 degrees C, polyaromatics at 530 degrees C. Coke molecules result either from s uccessive alkylation, cyclization, hydrogen transfer (or dehydrogenati on) steps or through dehydrogenative coupling of aromatics, the signif icance of the second mode increasing with the reaction temperature.