Low Temperature 1H MAS NMR Spectroscopy Studies of Proton Motion in Zeolite HZSM-5

Low temperature H-1 MAS NMR spectroscopy is used to study protonic motion in zeolite HZSM-5 in both samples that have been dried using procedures that are standard in the literature and samples that have been more carefully dehydrated. A significant enhancement of proton mobility is seen for the standard dehydrated HZSM-5 sample in comparison to that seen for the much drier sample. This is ascribed to a vehicle-hopping mechanism involving the residual water that is present in these zeolites. A gradual change of the framework structure is observed on cooling to approximately 213 K, as monitored via the change in H-1 chemical shift values of the Bronsted acid resonances and by X-ray diffraction. A more sudden change in structure is seen by differential scanning calorimetry and NMR at approximately 220-230 K, which is associated with changes in both the mobility and the modes of binding of the residual water to the Bronsted acid sites and the zeolite framework.