Accessibility of lithium cations in high-silica zeolites investigated using the NMR paramagnetic shift effect of adsorbed oxygen

The accessibility of lithium cations in the high-silica zeolites ZSM-5 and beta was investigated using Li-7 MAS NMR spectroscopy. The samples are inve stigated in vacuum and in the presence of adsorbed oxygen molecules. The pa ramagnetic shift effect due to the oxygen molecules was exploited to determ ine the fractions of accessible lithium cations. Approximately 85% of lithi um cations in Al-ZSM-5 (Si/Al= 50) are accessible to oxygen molecules, whil e none of the cation sites in zeolite Al-beta (Si/Al = 18 and 8) and only a small fraction in B-beta (Si/B = 15) were available for adsorption. This d ifference indicates a very distinct coordination environment for Li+ in zeo lite beta as compared to ZSM-5. (C) 2000 Elsevier Science B.V. All rights r eserved.