Influence of adsorbed molecules on the configuration of framework aluminum atoms in acidic zeolite-beta. A Al-27 MAS NMR study

The formation of octahedral framework aluminum sites in acidic zeolite-beta depends on the adsorption of water. This could be shown by Al-27 MAS NMR spectroscopy under controlled atmosphere conditions. The Al-27 MAS NMR spectrum of the hydrated zeolite shows two lines corresponding to tetrahedral and octahedral Al sites, respectively. These NMR lines disappear on dehydrated samples. The adsorption of methanol or pyridine as probe molecules on the dehydrated zeolite samples restores only the tetrahedral (framework) aluminum sites in the Al-27 MAS NMR spectrum due to the relaxation of the distorted zeolite lattice by the adsorbed molecules. The acid-catalyzed reaction (470 K) of adsorbed methanol molecules on zeolite-beta, which supplies water, brings back the Al-27 MAS NMR signal caused by the octahedral framework aluminum sites.