A comparative study of water interaction with alkali–metal cation-exchanged X-type zeolites by pulsed field gradient NMR and temperature-programmed desorption

Abstract

1H pulsed field gradient (PFG) NMR is applied to study water self-diffusion in LiX, NaX and CsNaX under the regimes of restricted intracrystalline diffusion and long-range diffusion. The dependence of the long-range diffusivities on temperature, loading and type of cation may be easily rationalised on the basis of a simple microkinetic approach. The apparent activation energies, determined from the Arrhenius plots of the long-range diffusivities, are compared with the effective desorption energies determined in previous temperature-programmed desorption (TPD) studies. Though there are coinciding trends, the absolute values of the desorption energies are smaller by 10…20 kJ mol −1 than the maximum values of the apparent activation energies of long-range diffusion. This difference is most likely to be attributed to a remaining influence of molecular diffusion in the TPD measurements.