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% impregnated 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/HZSM-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 coking for temperatures between 400 and 450 degrees C, which can be explained by a contrary effect of temperature on the race of formation of coke precursors and on their retention. Coke causes a pore blockage responsible 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 composition does not depend on the catalyst but changes very much with the temperature : alkylmonoaromatics at 250 degrees C, bi- or triaromatics at 350 degrees C, tetra- to polyaromatics at 400 and 450 degrees C, polyaromatics at 530 degrees C. Coke molecules result either from successive alkylation, cyclization, hydrogen transfer (or dehydrogenation) steps or through dehydrogenative coupling of aromatics, the significance of the second mode increasing with the reaction temperature.