Abstract: By a series of probe molecule experiments, the aromatics reaction network was studied, the differences between the isomerization/disproportionation reaction path were compared，and the diffusion-reaction characteristics of aromatics with different sizes in ZSM-5 zeolite were analyzed. A model of xylene isomerization reaction was established. It is proves that only the aromatics with the size not larger than the benzene ring at least in one direction can diffuse into the ZSM-5 channel; the isomerization of lean PX feed occurs predominantly on the outside surface of the zeolite, the resulting PX diffuses into the channels, which then forms toluene through dealkylation. Based on this theory, a deethylation catalyst SKI-210 using the new molecular sieve with smaller size and larger surface was produced in industrial scale and applied in commercial units with good results.

Key words: xylene, isomerization, deethylation, mechanism, reaction model, SKI-210