Self-diffusion measurements of n-alkanes in zeolite NaCaA by pulsed-field gradient nuclear magnetic resonance

Abstract

Pulsed-field gradient (PFG) NMR is applied to the study of self-diffusion of the n-alkanes from methane to n-heptane, adsorbed in zeolite NaCaA. Using magnetic field gradients up to 10 T m^–1 over a temperature range from – 100 to 350 °C, diffusivities between 10^–13 and 10^–8 m² s^–1 can be covered. For methane and ethane in NaCaA the diffusivity is 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 is 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 is close to the diameter of the free apertures between adjacent supercages. With further increasing chain length there is a monotonic decrease in the diffusivities. The values obtained are in satisfactory agreement with the results of uptake measurements and neutron scattering experiments.