The coefficients of intracrystalline self-diffusion of the n-alkanes, from propane to n-hexane, adsorbed in zeolite ZSM-5 were studied (figure 21) by using the pulsed field gradient NMR method with field gradient amplitudes of up to 25T/m at -20 to 380C. The diffusivities were found to decrease monotonically with increasing chain length. They were in satisfactory agreement with the results of quasi-elastic neutron scattering and molecular dynamics simulations, as well as with non-equilibrium measurements obtained using the frequency-response and single-step adsorption techniques. In comparison with zero-length-column desorption and permeation studies, these data exhibited the same trends in chain-length dependence but were some two orders of magnitude larger. The diffusivities were intermediate between those for zeolite NaX and NaCaA, but exhibited the lowest activation energies. This indicated that the differences in potential energies of the adsorbate molecules on their intracrystalline trajectories were generally smaller than in NaCaA and NaX zeolites.

High-Temperature Pulsed Field Gradient Nuclear Magnetic Resonance Self-Diffusion Measurements of n-Alkanes in MFl-Type Zeolites. W.Heink, J.Kärger, H.Pfeifer, K.P.Datema, A.K.Nowak: Journal of the Chemical Society - Faraday Transactions, 1992, 88, 3505-9

Figure 21

Diffusivity of n-Alkanes in ZSM-5 at 298K

as a Function of Chain Length