Concentration and temperature dependence of the diffusivity of n-hexane in MFI-zeolites

The concentration dependence of the self-diffusivity of n-hexane in large crystals of silicalite-1 and HZSM-5 zeolites has been investigated with positron emission profiling (PEP) in the temperature range of 393-483 K. A monotonic increase in the diffusivity was observed with increasing alkane loading for both zeolites up to 4 molecules per unit cell. The diffusion coefficient in HZSM-5 was approximately half of that in silicalite-1 due to the interaction of n-hexane with the Brønsted acid sites. The increase in the diffusivities with the loading is assumed to be caused by the repulsive interactions between the molecules adsorbed in the channel intersections and the adjacent straight channel. The apparent activation energy of diffusion was measured to be independent of the partial pressure of alkane, while the jump frequency increased. In HZSM-5 zeolite the apparent activation energy decreased with the partial pressure due to stronger interactions between n-hexane and the acid sites at low pressures.