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