EPR and NMR Studies of Coke Induced Selectivation over H–ZSM-5 Zeolite during Ethylbenzene Disproportionation Reaction

Abstract

A combination of EPR,13C CP-MAS NMR,129Xe NMR, xenon adsorption, and TGA techniques were employed to examine the preferred location of coke formation in the conversion of ethylbenzene (EB) under various conditions, such as reaction temperature, space velocity, and carrier gas (H2) to EB molar ratio. Effects of coking conditions on the nature of coke as well as on the selectivity ofp-diethylbenzene were studied in parallel so that the correlation between the coke structure and product shape selectivity can be inferred. An increase in reaction temperature resulted in the formation of coke residues consisting of condensed polycyclic aromatic compounds. On the other hand, a change in space velocity and H2/EB molar ratio did not lead to any notable change in the composition of the coke. However, increases in the reaction temperature and space velocity or decreases in the H2/EB ratio tend to enhancepara-selectivity using the H–ZSM-5 zeolite. The induced shape selectivity is ascribed to the preferential deposition of the coke on the external surfaces of the H–ZSM-5 crystallites. Thus, the use of coke as a modifying agent for selectivation of a H–ZSM-5 zeolite in an EB disproportionation reaction is demonstrated.