LIMITATION OF N-HEXANE AND 3-METHYLPENTANE CONVERSION OVER ZEOLITE ZSM-5 BY INTRACRYSTALLINE DIFFUSION

Conversion rates of the title compounds over H-ZSM-5 crystals ranging in size from 5 to 150 μm have been measured. Constraint Indices and Arrhenius plots proved the cracking rates sometimes to be intracrystalline diffusion-limited at 723 K and 811 K. With a simple Thiele theory and by measuring cracking rates on ground crystals, diffusivity of 3-methylpentane to ambient temperatures agree with literature data obtained by measuring the uptake rate of the adsorbate by ZSM-5. This indicates that the measured strong aluminium zoning in the ZSM-5 crystals does not provoke questioning of the use of the simple Thiele theory instead of a modified theory. Probably aluminium located in outer crystal zones of the sample crystals contributes little to cracking activity. In order to verify this tentative conclusion syntheses of ZSM-5 crystals with a homogeneous aluminium distribution are recommended as well as a direct comparison with sorption kinetics. © 1990.