Multiple oxygen sites in synthetic phyllosilicates with expandable layers: 17O solid-state NMR study

Abstract

Recent advances in high-resolution solid-state NMR enabled us to resolve the previously unknown details of atomic configurations around quadrupolar nuclides such as O (17O, spin 5/2) and aluminum (27Al, spin 5/2) in phyllosilicates. Here, we report the 17O 1D and 2D NMR (MAS and MQMAS) spectra of synthetic kaolinite-montmorillonite mixtures. Various O sites in the mixtures revealed from O-17 NMR include apical O (VISi-O-2IVAl), multiple basal O atoms (VISi-O-VISi and two VISi-O−VIAl sites) in tetrahedral layers, and hydroxyl group with distinct ranges for NMR parameters (quadrupolar coupling constant, Cq and isotropic chemical shift, 17O δiso). In particular, the structurally relevant NMR parameters (17O δiso, Cq, and asymmetry parameter, η) for two resolved IVSi-O-VIAl sites are about 52 ± 2 ppm, 2.6 ± 0.2 MHz, and 0.5 for IVSi-O-IVAl (I) and about 44.5 ± 2 ppm, and 3.1 ± 0.2 MHz, and 0.5 for IVSi-O-IVAl (II), respectively. These sites may stem from crystallographically distinct IVSi-O-IVAl sites of varying bond length and IVSi-O-VIAl bond angle or IVAl-O-2VIAl site in the montmorillonite.