Effect of extraframework species on 17O NMR chemical shifts in zeolite A

Zeolite A provides a unique opportunity to explore the relationship between structural parameters and 17O nuclear magnetic resonance (NMR) isotropic chemical shifts (δISO) because differences in site multiplicity for framework oxygens often permit unambiguous site assignments. Strontium exchanged zeolite A and hydrated and dehydrated K- and Na-A were investigated using 17O triple quantum and five quantum magic angle spinning (3QMAS and 5QMAS) NMR. The results concur with previous observations that δISO is influenced by T–O–T bond angle, but also suggest that interactions between framework oxygens and extraframework species (water molecules and cations) affect δISO as well. Apparent chemical shifts for zeolitic water sites also exhibit relative chemical shifts as a function of extraframework cation contents consistent with shifts observed for water sites and non-bridging oxygens in glasses. Abundances of Si–O–Al sites in dehydrated forms indicate that oxygen isotope exchange rates can vary between crystallographic sites, as well as between Si–O–Si and Si–O–Al sites.