TRANSPORT AND ISOMERIZATION OF XYLENES OVER HZSM-5 ZEOLITES

The diffusion, adsorption, and reaction of xylenes on HSZM-5 was investigated by means of time-resolved in situ FTIR spectroscopy and gas chromatography. The different diffusion coefficients determined for the three sylene isomers (p:o:m (equivalent to) 1000:10:1) were found to influence the isomerization rate above 523 K under the reaction conditions employed. Below that temperature, reactant diffusion played a minor role. For all xylene molecules, the intramolecular mechanism of isomerization suffices to account for all observations. The selectivity of m-xylene isomerization was found to depend primarily upon the transition entropy (transition state selectivity). The selectivity in o- and p-xylene depends upon the surface concentration of the primary product, m-xylene. Its concentration is a function of the activity of the catalyst and the diffusional constraints m-xylene faces. The decrease of the effectiveness of ZSM-5 above 523 K due to reactant diffusional constraints can be seen in lower coverages of m- and o-xylene in comparison to p-xylene. © 1993 by Academic Press, Inc.