A gravimetric balance and the frequency response (FR) technique have been used to investigate the adsorption and the mass transfer behaviour of the normal alkanes, from methane to n-hexane, and the cyclic hydrocarbons, benzene, toluene, ethylbenzene, p-xylene, cyclohexane and cis-1,4-dimethylcyclohexane, in silicalite-1, ZSM-5 and theta-1 respectively. The range of the experimental conditions covered by the FR measurements has been extended and the frequency response data have been improved considerably compared to earlier studies using the FR technique. Most of the isotherms of the systems studied in this work can be reproduced well by the Langmuir model. Deviations of the isotherms for n-pentane and n-hexane in silicalite-1 from this model at low temperatures, however, have been observed. These deviations have been attributed to sorption site heterogeneity of the sorbent and to the immobilisation of these molecules in the sinusoidal channels of the zeolite. Distinct steps were observed in the isotherms of benzene and toluene in silicalite-1 at loadings > 4 m./u.c., which were reversible, while a hysteresis loop was found in the isotherms of p-xylene at loadings > 4 m./u.c. 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 which are present in the MFI samples and sorbate-sorbate interactions have been invoked to explain these anomalous results. The mass transfer of benzene and cyclohexane molecules in MFI zeolites is controlled by a pure micropore diffusion process, whereas the diffusivities of ethylbenzene and cis-1,4-dimethylcyclohexane in these sorbents may be hindered by the rotation of the methyl groups in these molecules. The effect of the structure defects of the framework on the diffusivities of aromatics inside the pores of siliclite-1 has been 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 are at least one order of magnitude smaller than the values for benzene. The diffusion coefficients of propane in theta-1 are about two orders of magnitudes smaller that those of propane in silicalite-1. Such a result arises from the single-file diffusion process which is occurring in this system. The activation energy for diffusion of propane in theta-1 is close to the heat of adsorption and is more than twice that found in silicalite-1 where three-dimensional diffusion is involved. This finding provides, for the first time, experimental evidence to support the assumption proposed in single-file diffusion theory that the rate of adsorption and desorption of sorbate molecules at pore entrances of one dimensional channel sorbents may be the rate-controlling step. The results obtained in this study reveal the importance of the effect of the pore structure of zeolites and specific features of the sorbate molecules on the dynamic processes involved in the adsorption and diffusion of these molecules in the zeolite networks.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:662275 |
| Date | January 1999 |
| Creators | Song, Lijuan |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14451 |
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