Observation of Two 4-Coordinated Al Sites in Montmorillonite using High Magnetic Field Strength ²⁷Al MQMAS NMR

Analyses of the layer structure of Na-montmorillonite have been performed using 27 Al MAS and 27 Al MQMAS NMR techniques. Results of 27 Al MAS NMR measurements at higher magnetic field strength (16.4 T) suggest that the 4-coordinated Al site in Na-montmorillonite has two different structures. This was confirmed by the fact that two peaks corresponding to 4-coordinated Al are observed in the 27 Al MQMAS NMR at high magnetic field strength. The ratio of two 4-coordinated Al sites was found to be affected by water in the interlayer space because the area ratio of cross peaks corresponding to two 4-coordinated Al sites changes with the water content.