On the limits of the application of the n.m.r. pulsed-field gradient technique for self-diffusion measurements in zeolites

• Published in: Zeolites 1989, Vol.9 (4), p.299-302
• Main Authors: Förste, Chr, Heink, W., Kärger, J., Pfeifer, H., Feokistova, N.N., Zhdanov, S.P.
• Format: Article
• Language: English
• Subjects: benzene; n.m.r; pulsed field gradient technique; self-diffusion; ZSM-5
• ISSN: 0144-2449; 1873-3115
• DOI: 10.1016/0144-2449(89)90075-4

The main limitations for an application of the n.m.r. pulsed-field gradient technique to self-diffusion studies in zeolites are reviewed. It is shown that small amounts of highly mobile molecules adsorbed on the outer surface or within intracrystalline cracks will, in general, lead to an enhanced damping of the n.m.r. signal that may be interpreted erroneously as a high overall mobility of the adsorbed molecules if the two-phase character of the signal is not taken into consideration. As an example, for benzene adsorbed on ZSM-5-type zeolites, the n.m.r. pulsed-field gradient technique is shown to provide only an upper limit of the intracrystalline diffusivity of the order of 10 −10 m 2 s −1. This value does not conflict with the previously reported uptake diffusivities of the order of 10 −13 m 2 s −1.