Pulsed-field gradient NMR was applied to the study of self-diffusion of the n-alkanes from methane to n-heptane, adsorbed in zeolite NaCaA (figure 2). Using magnetic field gradients up to 10T/m over a temperature range from -100 to 350C, diffusivities between 10-13 and 10-8m2/s could be covered. For methane and ethane in NaCaA the diffusivity was found to increase by up to two orders of magnitude with increasing concentration. In contrast to the case for zeolites NaX and HZSM-5, there was a dramatic jump in the chain-length dependence of the diffusivities between ethane and propane, corresponding to the fact that the critical molecular diameter of propane and the longer n-alkanes was close to the diameter of the free apertures between adjacent super-cages. With further increasing chain length there was a monotonic decrease in the diffusivities. The values obtained were in satisfactory agreement with the results of uptake measurements and neutron scattering experiments.

Self-Diffusion Measurements of n-Alkanes in Zeolite NaCaA by Pulsed-Field Gradient Nuclear Magnetic Resonance. W.Heink, J.Kärger, H.Pfeifer, P.Salverda, K.P.Datema, A.Nowak: Journal of the Chemical Society - Faraday Transactions, 1992, 88[3], 515-9