The adsorption and diffusion of benzene, toluene, ethylbenzene, p-xylene, cyclohexane and cis-1,4-dimethylcyclohexane in MFI zeolites were investigated using a gravimetric balance and the frequency-response technique. Distinct steps were observed in the isotherms of benzene and toluene in silicalite-1 at loadings of greater than four molecules per unit cell, which were reversible, whereas a hysteresis loop was found in the isotherms of p-xylene in silicalite-1 but not in the isotherms of p-xylene in ZSM-5. In contrast with some explanations in the literature, the heterogeneous sorption sites of the MFI topology for these sorbates and sorbate-sorbate interactions were invoked to explain these results. The mass transfer of benzene and cyclohexane molecules in MFI zeolites was controlled by a pure micropore diffusion process. The diffusivities of ethylbenzene and cis-1,4-dimethylcyclohexane in the sorbents may be influenced by the rotation of the methyl groups in these molecules. The transport properties of toluene and p-xylene inside the pores of MFI were dependent on loading. At loadings ≤1mol/unit-cell and ≥4mol/unit-cell only a single diffusion process could be detected by frequency-response measurements whereas, at intermediate loadings, a bimodal frequency-response behaviour was found. The diffusivities of the four aromatics decrease in the order of p-xylene > toluene > benzene > ethylbenzene, and the diffusion coefficients of the two cyclic alkanes were at least one order of magnitude smaller than the values for benzene, i.e. < 10-15m2/s. The activation energies of the aromatics in silicalite-1 have also been measured in this study.

Adsorption and Diffusion of Cyclic Hydrocarbon in MFI-Type Zeolites Studied by Gravimetric and Frequency-Response Techniques. L.Song, L.V.C.Rees: Microporous and Mesoporous Materials, 2000, 35-36, 301-14

Table 30

Permeability of Silicalite