Report No 1951/AP - Instytut Fizyki JÄ drowej PAN

EQUILIBRIUM AND NON-EQUILIBRIUM DIFFUSION MEASYREMENTS
IN ZEOLITES BY NMR TECHNIQUES
Krzysztof Banaś 1 , Federico Brandani 2 , Douglas M. Ruthven 3 ,
Frank Stallmach 2 , and Jörg Kärger 2
1 Instytut Fizyki Jądrowej im. H. Niewodniczańskiego ul. Radzikowskiego 152 31-342 Kraków,
Poland
2 Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnestrasse 5, D-04103,
Germany
3 Department of Chemical Engineering, University of Maine, Orono, Maine 04469-5737, USA
In this paper the Zero Length Column (ZLC) technique extended to the case were the
decay of the adsorbed phase concentration is observed directly by NMR is presented. An
adsorption-desorption apparatus compatible with a 400 MHz NMR spectrometer was
developed. It operates with nitrogen as the inert purge gas. The column of the adsorbent
material was placed in the sensitive region of the superconducting magnet and the rf coil of
the NMR spectrometer.
The diffusion of propane, n-butane and isobutane in silicalite-1 was investigated by
using the novel “ZLC-NMR” technique. This method, together with standard PFG NMR
measurements, seems to be a good solution in measuring transport properties of the adsorbed
gas in zeolites using macroscopic and microscopic approaches at the same conditions.
The time scales of the adsorption and desorption processes depend on concentration,
temperature and crystal shape and were found to be in the range of minutes. From the
desorption branch, the non-equilibrium ZLC NMR measurements yield an intracrystalline
diffusion coefficients in the range of 10 -13 to 10 -11 m 2 /s for different alkanes in silicalite-1.
It was found that diffusion coefficients for isobutane, n-butane and propane in silicalite-
1 measured by the non-equilibrium ZLC-NMR are smaller than the values measured by PFG
NMR under equilibrium condition. The differences may indicate transport resistance at the
external surface of the silicalite-1 zeolite crystals.
12