Influence of the acid sites on the intracrystalline diffusion of hexanes and their mixtures within MFI-zeolites

The intracrystalline diffusivities and component loadings of mixtures of n-hexane and 2-methylpentane within silicalite-1 (crystal size 150 mum) and HZSM-5 (140 mum) have been measured at 433 K as a function of the n-hexane/2-methylpentane ratio in the gas phase (at constant total hydrocarbon pressure of 6.6 kPa) using the Positron Emission Profiling technique. Strong preferential adsorption of the linear alkane over the branched one has been observed in HZSM-5, while in silicalite only a slightly higher adsorption of n-hexane was observed. The diffusivities of both components in the mixtures decrease with increasing 2-methylpentane content and have been found to be approximately two times lower in HZSM-5 than those in silicalite. When the concentration of the branched hexane in the adsorbed phase becomes close to 3 molecules per unit cell, the influence of the acid sites on the diffusion of linear hexane diminishes compared to the influence of the blockage of the zeolite network with the slowly moving 2-methylpentane. The apparent activation energies of the diffusion for n-hexane and 2-methylpentane have been measured at temperatures between 393 K and 533 K under similar conditions. For n-hexane, these values were found to be very close in silicalite and ZSM-5: 18.5 +/- 1.5 kJ/mol and 22 +/- 2 kJ/mol, respectively. For 2-methylpentane the activation energies were found to be very high: 66 +/- 6 kJ/mol and 72 +/- 3 kJ/mol, respectively.