With the tracer-exchange positron emission profiling (TEX-PEP) technique, the reexchange process of radioactively labeled molecules with a steady-state feed stream can be measured inside a zeolite-packed bed reactor. When the experimental tracer-exchange curves are modeled, values for the micropore diffusion and adsorption constant can be obtained. As one can choose which component to label, this technique is ideally suited for studying multicomponent diffusion. In the present study, this technique has been used to measure the diffusive and adsorptive properties of an n-hexane/2-methylpentane mixture in zeolite silicalite. The measurements were performed at different ratios of n-hexane and 2-methylpentane in the gas phase at a constant total hydrocarbon pressure of 6.6 kPa and a temperature of 433 K. A slight preference for the adsorption of n-hexane was found because it is entropically more favorable to adsorb these molecules as they have no preferential siting in the zeolite pores. The diffusivity of the slowly moving 2-methylpentane is not strongly affected by the presence of the fast moving n-hexane. The mobility of the linear alkane however strongly decreases with increasing 2-methylpentane ratio and suddenly drops at a loading of approximately three 2-methylpentane molecules per unit cell. This is caused by the fact that the branched alkanes are preferentially sited in the intersections between the straight and zigzag channels of silicalite and therefore effectively block the zeolite pore network. These results show that the adsorptive properties of the components and the structure of the zeolite network play an important role in the behavior of multicomponent mixtures in zeolites.

Molecular dynamics simulations are used to study the self-diffusion of linear and branched alkanes in the zeolites Mordenite, ZSM-5, Ferrierite, and ZSM-22. A rigid zeolite model and a united atom model for the alkanes was used in these simulations. Even at maximum loading, no single-file diffusion behavior of the time dependency of the mean-square displacement was observed in the one-dimensional zeolites Mordenite and ZSM-22. This might be the result of the low activation barriers for the molecular motion in the channels, causing the molecules not to move in a “hopping-like” motion. Both the diffusion constant and activation energy strongly depend on zeolite loading. At high loadings, the motion is predominantly determined by the alkane−alkane collisions. The chainlength dependency of the diffusion constant and activation energy seems to indicate the occurrence of a resonant diffusion mechanism. These effects are stronger in the medium-pore zeolites, as the molecules are more constrained in these systems. Diffusion of i-butane and 2-methyl-pentane in Mordenite was found to be less than 1 order of magnitude slower than their linear counterparts, and diffusion barriers are only 4.7 and 6.3 kJ/mol, respectively. In ZSM-5 and ZSM-22, where the bulky iso-alkanes fit tightly into the pores, differences are as large as 3 orders of magnitude, and the activation energies are in the order of 30 kJ/mol.

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The intracrystalline diffusivities and component loadings of mixtures of n-hexane and 2-methylpentane within silicalite-1 (crystal size 150 µm) and HZSM-5 (140 µm) 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.