Combined quasi-elastic neutron scattering and NMR study of hydrogen diffusion in zeolites

Results of the first pulsed field gradient (Pfc) NMR studies of hydrogen di ffusion in zeolites are presented and compared with corresponding measureme nts by quasielastic neutron scattering (QENS). The results of the two techn iques were in satisfactory agreement. Similarly as with other probe molecul es, also the hydrogen diffusion is found to depend decisively on the zeolit e type under study. The hydrogen diffusivity is generally found to decrease with decreasing free apertures of the zeolite pore structure. Only in the case of silicalite, where the diffusivities are found to be exceptionally s mall, is this trend violated. This finding is correlated with a partial cap turing of the hydrogen molecules within the pentasil chains. Comparison bet ween the H-1 MAS NMR and QENS data suggests mean trapping times of the orde r of 200 mu s. Copyright (C) 1999 John Wiley & Sons, Ltd.