NMR Crystallography of Monovalent Cations in Inorganic Matrices: Na+ Siting and the Local Structure of Na+ Sites in Ferrierites

Zeolites are crystalline microporous aluminosilicates of paramount importance. The siting of cations balancing the negative charge of framework Al affects the catalytic and sorption properties of cation-exchanged zeolites. The siting of Na+ cations in dehydrated Si-rich ferrierite zeolites is investigated by Na-23 (ultra)-high-field (MQ)MAS NMR spectroscopy together with DFT to obtain the Na+ siting and the local structure of nine Na+ cationic sites formed by two 6-rings and two 8-rings having one Al atom located in different framework T sites. The occupation of the Na+ cationic sites is not controlled by their relative energies but by the kinetics of the Na+ ion-exchange. Na-23 solid-state NMR spectroscopy alone can determine the ring forming the Na+ site but not which T site is occupied by Al in that ring. The developed methodology represents a highly promising tool for the analysis of the Na+ arrangements in zeolites and other crystalline matrices.

Automated summary

In this study, the siting and local structure of Na+ cations in Si-rich ferrierite zeolites were investigated using NMR spectroscopy and DFT. The results showed that the occupation of Na+ cationic sites is not determined by their relative energies, but by the kinetics of Na+ ion-exchange. Additionally, NMR spectroscopy alone can determine the ring forming the Na+ site, but cannot determine which T site is occupied by Al in that ring.