Time-Dependent Non-Steady-State Diffusivities of C6 Paraffins in Silicalite by Zero-Length Column Method

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Abstract
Long-time diffusion experiments of n-hexane, 2-methylpentane and 2,3-dimethylbutane in silicalite according to the zero-length column method showed a significant decrease of the diffusion coefficients for longer times. A new and simple model of "uncoupled dual diffusion" is proposed to fit the experimental data. The calculated fast-diffusion coefficients as well as the calculated mean activation energies of diffusion of n-hexane and 2-methylpentane were almost equal and in reasonable agreement with literature data. A decrease of the adsorption partial pressure from 5.5 to 0.6 Torr did not have any influence on the fast-diffusion coefficient of n-hexane, which implies that the decrease of the diffusion coefficient in this range is not simply due to a non-linear isotherm. The proposed dual diffusion model may represent effects such as the presence of multiple energy states available for adsorbed molecules in the zeolite lattice, and/or diffusion anisotropy due to differences in the channel configurations in the three crystallografic directions.

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