(58b) Advances in the Understanding of the Isomerization of Mixed Xylene – Ethylbenzene Feeds Over ZSM-5

Xylene isomerization is a commercially important process in the production of para-xylene, a precusror for terephthalic acid and therefor for polyester. The formation of methylethylbenzenes and diethylbenzenes is an undesirable side reaction that occurs during the liquid phase, but especially during the vapor phase in the isomerization of mixed xylene-ethylbenzene feeds.  ZSM-5 crystal size and acid site distribution play a significant role in the selectivity of the  isomerization reaction.

Formation of higher molecular weight alkyl aromatics such as methylethyl- and diethylbenzene can be used a probe to assess the intracrystalline diffusion limitations of ZSM-5.  This paper presents an analysis of the shape seectivity and diffusivity exhibited by a range of ZSM-5 catalysts with controlled crystal size and acid site distribution in the isomerization of mixed xylene –ethylbenzene  feedstocks.  Diffusion coefficients are determined using the classical Thiele Modulus approach.  This work shows the significant impact of zeolite shape selectivity on the production of heavier aromatics.