Fourier-transform pulsed-field-gradient NMR measurements were used to analyze the diffusion of methane, n-butane and n-pentane in zeolite ZSM-5. Methane clearly exhibited bi-exponential spin-echo attenuation. This implied two types of diffusion: intracrystalline diffusion and long-range diffusion; a combination of inter- and intra-crystalline diffusion. In the case of small crystals, the diffusion of methane into the macropores between the crystals dominated the decay. For larger crystals, the contribution of intracrystalline (or micropore) diffusion increased significantly. The coefficient of intracrystalline diffusion of methane in ZSM-5 was determined to be 3.8 x 10-9m2/s at 25C. The NMR methane diffusion data were in good agreement with values obtained by molecular dynamics calculations. Subsequent NMR measurements of n-butane and n-pentane diffusion in ZSM-5 indicated that the diffusion decreased sharply with increasing chain length of the hydrocarbons: 11 x 10-11 and 4.4 x 10-11m2/s, respectively, at 25C and 20kPa loading). To allow comparison with the diffusivities obtained independently using other techniques, the concentration dependence of the NMR self-diffusion coefficient of n-pentane in ZSM-5 was determined and was found to decrease with increasing sorbate concentration. From the temperature dependence of the diffusion rates, the activation energies for n-butane and n-pentane diffusion in ZSM-5 were determined (8.4 and 12.6kJ/mol at 20kPa, respectively). The PFG NMR and MD results for the diffusion of light n-alkanes in ZSM-5 were also compared with relevant published diffusion data obtained using ZLC and MT. The microscopic self-diffusivity (from PFG NMR and MD) differed systematically by some two orders of magnitude from the much slower macroscopic diffusion observed using uptake, ZLC and MT methods. There was satisfactory agreement between the self-diffusivity of n-butane, obtained with PFG NMR, and the transport diffusivity of the same system measured using the frequency response method.

Fourier-Transform Pulsed-Field-Gradient 1H Nuclear Magnetic Resonance Investigation of the Diffusion of Light n-Alkanes in Zeolite ZSM-5. K.P.Datema, C.J.J.Den Ouden, W.D.Ylstra, H.P.C.E.Kuipers, M.F.M.Post, J.Kärger: Journal of the Chemical Society - Faraday Transactions, 1991, 87[12], 1935-43