On the regeneration of coked H-ZSM-5 catalysts

Regeneration of H-ZSM-5 zeolite coked during ethylbenzene conversion is studied. Selective removal and transformation of the carbonaceous compounds during reactivation in the presence of air, 0.5% O-2 in N-2 and H-2 at 500 degrees C were examined using the combination of GC, TGA, xenon adsorption, IR, Xe-129, and C-13 CP-MAS NMR spectroscopy and ethylbenzene transformation as a test reaction. A rapid initial rate of regeneration is found while the fouled catalysts were subjected to regeneration in air. The intracrystalline cokes present near the Bronsted acid sites are preferentially removed to those present on the external surface of thee zeolite crystallites. The phenomenon is more obvious when using H-2 as the regenerating gas. During the oxidative removal of the coke with air or 0.5% O-2 in N-2, a portion of the carbonaceous compounds was transformed to more condensed structures before getting completely oxidized. In the presence of H-2, while the internal cokes were selectively removed by hydrocracking, the external cokes were partially cracked from bulky alkyl polyaromatic compounds to less bulky polyaromatic compounds. Moreover, an increase in paradiethylbenzene selectivity was observed. (C) 1998 Academic Press.