A comparative study of water interaction with alkali-metal cation-exchanged X-type zeolites by pulsed field gradient NMR and temperature-programmed desorption
A. Kirmse et al., A comparative study of water interaction with alkali-metal cation-exchanged X-type zeolites by pulsed field gradient NMR and temperature-programmed desorption, APP CATAL A, 188(1-2), 1999, pp. 241-246
H-1 pulsed field gradient (PFG) NMR is applied to study water self-diffusio
n in LiX, NaX and CsNaX under the regimes of restricted intracrystalline di
ffusion and long-range diffusion. The dependence of the long-range diffusiv
ities on temperature, loading and type of cation may be easily rationalised
on the basis of a simple microkinetic approach. The apparent activation en
ergies, determined from the Arrhenius plots of the long-range diffusivities
, are compared with the effective desorption energies determined in previou
s temperature-programmed desorption (TPD) studies. Though there are coincid
ing trends, the absolute values of the desorption energies are smaller by 1
0...20 kJ mol(-1) than the maximum values of the apparent activation energi
es of long-range diffusion. This difference is most likely to be attributed
to a remaining influence of molecular diffusion in the TPD measurements. (
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