The measurement and modelling of the intracrystalline diffusion of cyclohexane in ZSM-5 using zero length column chromatography
Doctoral Thesis
2001
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University of Cape Town
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Abstract
Zeolites are important solid acid catalysts largely because of their microporous nature giving them strong shape selective properties. This shape selectivity may be improved by depositing an inert (e. g. silaceous) layer on the surface of the catalyst crystals, which inertises non-shape selective surface acid sites and increases diffusional transport resistances. This thesis presents an investigation, using the zero length column (ZLC) method, into changes in the diffusional properties of cyclohexane in ZSM-5 that has had tetraethoxysilane deposited on the surface in the liquid phase. Theoretical analyses of the ZLC technique and its use in a detailed study of cyclohexane in unmodified ZSM-5 were performed in order to prove the reliability of the technique and measure baseline behaviour of the selected system. The ZLC method is a chromatographic technique for the measurement of diffusion coefficients in porous sorbents. Originally developed for gaseous hydrocarbon / zeolite powder systems, it has been experimentally extended to the measurement of liquid sorbate systems, macroporous zeolite pellets and resins and the measurement of self-diffusion through tracer exchange. The technique is robust against the intrusion of external heat and mass transfer effects by the use of relatively high flow rates and small catalyst samples, Analysis of the desorption curves is simple: the slope of the linear (on semilogarthmic axes) long time region gives the diffusional time constant. This so-called 'long time' analysis gives an accurate result for the diilusional time constant.
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Bibliography: p. 97-102.
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Duncan, W. 2001. The measurement and modelling of the intracrystalline diffusion of cyclohexane in ZSM-5 using zero length column chromatography. University of Cape Town.