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

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. ( C) 1999 Elsevier Science B.V. All rights reserved.