PFG NMR investigation of hydrocarbon diffusion in large NaX zeolite crystals : Effect of internal field gradients on diffusion data

Self-diffusion coefficients for intracrystalline diffusion of hydrocarbon molecules adsorbed in large crystals of NaX zeolite have been measured by the pulsed field gradient (PFG) NMR technique, at ambient temperature and at different diffusion times (from 6 to 12 ms). Two NMR pulse sequences, stimulated and 13-interval bipolar spin echo, were used to examine the influence of internal field gradients on diffusion data. For both sequences the effective self-diffusion coefficient of the guest molecules was found to decrease with increasing observation times. The extrapolated intracrystalline diffusion coefficient is independent of the NMR sequence. In contrast, the estimated extent of molecular diffusion depends strongly on the pulse program. For the small molecules (butane to hexane), the domain size, R, of restricted diffusion obtained with the stimulated spin-echo sequence is smaller than the crystal dimension whereas R is always comparable to it when the 13-interval pulse sequence is used. This shows the effect of internal field gradients on the diffusion data leading to wrong values of R if the stimulated pulse sequence is used. The light hydrocarbons diffuse freely inside the zeolite particles whereas the crystal boundaries act as reflecting surfaces, as previously observed. On the other hand, even with the 13-interval pulse sequence, the smaller values of R obtained for large molecules as n-heptane and octane shows that their displacement is hindered by restrictions in the NaX macro-crystals.