Coking and deactivation of H-ZSM-5 zeolites during ethylbenzene disproportionation .1. Formation and location of coke

The mode of deactivation and coke formation in a H-ZSM-5 zeolite during ethylbenzene disproportionation has been investigated by TGA, adsorption, IR, and Xe-129 NMR techniques. The initial rate of deactivation and the total coke content are found to increase with increasing conversion level. Within the range of conversion (54-63 %) covered in the present study, the location and extent of coke deposition can be divided into three distinct regions. At low coke content, coke is found to deposit on the Bronsted acid sites and resulted in a notable decrease in catalytic activity. As the amount of coke increases (less than or equal to 7 wt%), while the catalytic activity decreases slightly, most of coke still tends to deposit within the zeolite channels. The slight decrease in catalytic activity is ascribed to steric hindrance by coke residues. When the coke content exceeds 7 wt%, while the activity remains almost constant, the deposition of coke on the external surface of the zeolite crystallites is evident. It is found that neither the pore opening nor the acid site were completely blocked off by coke residues even at the highest coke loading.