Adsorption and diffusion of cyclic hydrocarbon in MFI-type zeolites studied by gravimetric and frequency-response techniques

The adsorption and diffusion of benzene, toluene, ethylbenzene, p-xylene, c yclohexane and cis-1,4-dimethylcyclohexane in MFI zeolites have been invest igated using a gravimetric balance and the frequency-response (FR) techniqu e respectively. Distinct steps were observed in the isotherms of benzene an d toluene in silicalite-l at loadings of greater than four molecules per un it cell (m./u.c.), which were reversible, whereas a hysteresis loop was fou nd in the isotherms of p-xylene in silicalite-l but not in the isotherms of p-xylene in ZSM-5. In contrast with some explanations in the literature, t he heterogeneous sorption sites of the MFI topology for these sorbates and sorbate-sorbate interactions have been invoked to explain these results. Th e mass transfer of benzene and cyclohexane molecules in MFI zeolites is con trolled by a pure micropore diffusion process. The diffusivities of ethylbe nzene and cis-l,4-dimethylcyclohexane in the sorbents may be influenced by the rotation of the methyl groups in these molecules. The transport propert ies of toluene and p-xylene inside the pores of MFI are dependent on loadin g. At loadings less than or similar to 1 m./u.c. and greater than or simila r to 4 m./u.c. only a single diffusion process can be detected by the FR me asurements, whereas at intermediate loadings a bimodal FR behaviour is foun d. The diffusivities of the four aromatics decrease in the order of p-xylen e > toluene > benzene > ethylbenzene, and the diffusion coefficients of the two cyclic alkanes are at least one order of magnitude smaller than the va lues for benzene, i.e. <10(-15) mt s(-1). The activation energies of the ar omatics in silicalite-l have also been measured in this study. (C) 2000 Els evier Science B.V. All rights reserved.