Diffusivities of n-alkanes in 5A zeolite measured by neutron spin echo, pulsed-field gradient NMR, and zero length column techniques

Diffusion of linear alkanes in 5A zeolite crystals has been studied by several different experimental techniques. In the range C-3-C-8 the diffusivity decreases regularly with carbon number (with a corresponding increase in activation energy) and there is good agreement between ZLC values and the data provided by the "microscopic" techniques (PFG NMR and Quasi-Elastic Neutron Scattering). At higher carbon numbers the agreement between the different techniques is less satisfactory, partly because of uncertainty in the thermodynamic correction factors. The microscopic techniques both show that the monotonic decrease in diffusivity does not continue beyond C-8. The self-diffusivities determined by PFG NMR remain essentially constant with some indication of a weak maximum at C-10 while the transport diffusivities measured by the neutron spin-echo technique (NSE) show a clear maximum at C-12, reminiscent of the so-called "window effect" (Gorring, 1973).