FIRST EVIDENCE OF MULTIPLE OCTAHEDRAL Al SITES IN Na-MONTMORILLONITE By 27Al MULTIPLE QUANTUM MAS NMR

The configuration of hydroxyl groups around the octahedral cations of 2:1 phyllosilicate minerals has long been all important question in clay science. In the present study, Al-27 multiple quantum (MQ) magic angle spinning nuclear magnetic resonance (MAS NMR) was applied to the local structural analysis of octahedral Al positions in a purified Na-montmorillonite. Three octahedral Al Sites (Al-[6](a), Al-[6](b), and Al-[6](c)) are distinguished by Al-27 5QMAS NMR, whereas these sites are not differentiated by Al-27 MAS and 3QMAS NMR. The isotropic chemical shift (delta(cs)) and the quadrupolar product (P-O) were estimated to be 5.8 ppm and 2.6 MHz for Al-[6](a), 6.2 ppm and 3.0 MHz for Al-[6](b), and 6.7 ppm and 3.7 MHz for Al-[6](c), respectively. The three Al sites originated from geometric isomers with cis and trans structures, which have mutually different configurations of the OH groups around the central Al3+ ions. From the view point of symmetry for the OH groups, Al-[6](a) and Al-[6](b) in the upfield region were assigned to cis sites, and Al-[6](c) in the downfield region was assigned to a trans site. The occurrence of multiple Al sites implies that Na-montmorillonite used in the present study has cis-vacant structure in the octahedral sheet, This is a reasonable insight, supported by the chemical composition and the differential thermal analysis data of the Na-montmorillonite.