Intercrystalline molecular transport of ethane in NaCaA zeolites studied by NMR pulsed field gradient technique and by uptake experiments

Abstract

The velocity of molecular diffusional transport over distances v(Δ) much larger than the crystallite diameters is represented by an effective self-diffusion coefficient D eff = ( v 2( Δ))/6 Δ, where Δ denotes the observation time. Values of D eff for ethane in NaCaA zeolites were determined by the NMR pulsed field gradient technique and by traditional sorption experiments in the temperature region 200–400°K for concentrations from 1 up to 6 molecules per supercage. In agreement with the general relation between diffusion and self-diffusion, for small sorbate concentrations both techniques provide the same values. Comparison with the coefficient of intracrystalline self-diffusion shows that D eff is predominantly determined by molecular transport in the intercrystalline space and should be given therefore by the relation D eff = P inter D inter where P inter and D inter denote, respectively, the relative number and the self-diffusion coefficient of the molecules in the intercrystalline space. Both values can be estimated from the adsorption isotherm ( p inter) and from the well-known approximations for Knudsen or for gaseous diffusion ( D inter). With it, the above relation allows a prediction of D eff. It is found that the order of magnitude as well as the temperature and concentration dependence of D eff are in reasonable agreement with the experimental data.