Diffusion of linear paraffins in NaCaA studied by the ZLC method

The zero length column (ZLC) technique has been applied to investigate the kinetics of linear alkanes (n-hexane to n-tetradecane) in a sample of zeolite NaCaA. The diffusivity values are found to be monotonically decreasing with chain length from n-hexane to n-undecane, slightly increasing between n-undecane and n-tridecane, and eventually levelling-off at n-tetradecane. The initial monotonic decreasing trend is similar to previous ZLC studies, although the values are somewhat higher for the heavier alkanes. The values found are reasonably close to PFG-NMR results and have similar qualitative trends, but do not show a marked maximum in comparison to recent neutron spin echo (NSE) results. The difference between the infinite dilution ZLC data and the microscopic techniques can not be ascribed to the intrusion of surface resistances since partial loading experiments provide clear evidence that the measurements are controlled by internal diffusion. The activation energy of linear alkanes in NaCaA lies between 20 and 26 kJ/mol.