Pore occupancy

Around a value of the gas-phase fraction of 2-methylpentane of about 0.83, the influence of the acid sites on the n-hexane diffusivity is not dominant anymore in comparison to the pore occupation of slow-diffusing 2-methyl-pentane. Figure 14 shows the dependence of the diffusivities of both components versus the concentration of adsorbed 2-methylpentane in terms of molecules per unit cell. The diffusivities of n-hexane in silicalite-1 and H-ZSM-5 become nearly equal when the concentration of 2-methylpentane reaches approximately 2.75 molecules per unit cell. For 2-methylpentane we And that the self-diffusivity in silicalite-1 becomes very close to the value in H-ZSM-5 at the same loading. 

Earlier, we have reported that the apparent activation energy for diffusion of 3-methylpentane in silicalite-1 increases with the partial pressure. This was explained by the pore occupancy effect influencing the pre-exponential fac-... 

At first sight, these results are somewhat different from those obtained by Masuda et al. [55] for -heptane/n-octane mixtures in sihcahte-l. A decrease in the diffusivity of the faster-diffusing n-heptane was observed as the loading of slower -octane increased, while the octane diffusivity did not change in the presence of -heptane. This difference may be attributed to different experimental conditions, since the temperature was higher (448-498 K) and the total pressure significantly lower (13 Pa). In this case, a relatively small pore occupancy is expected and repulsive interactions should be minimal. This leads to similarly low mobility of the faster hydrocarbon as of the slow one. 

If molecules can pass one another in the pore, then an increased pore occupation may increase the diffusion rates since molecule-molecule interactions may be weaker than moleculi zeolite channel interactions. When molecules cannot pass one another, the phenomenon known as single file diffusion may arise. In single file diffusion, which tends to occur in one dimensional porous systems, the large reduction in diffusion rate is represented by an effective diffusion constant proportional to the center of mass of the file of molecules occluded in the zeolite channel. 

This reverse behaviour with respect to cation content at high pore occupancy is not a rule (Figure 7). For the adsorption of alkenes, the influence of the Si Al-ratio is in line with what could be expected from observations at low coverage. Alkenes are found to be more selectively adsorbed on NaX than on NaY. As for the practical consequence of these observed selectivity patterns, despite it s lower cation content, NaY is proven to be a better separation agent for alkane/alkene/aromatic mixtures compared to NaX. 

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