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

Advanced Technology Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu, 293-8511 Japan

^* E-mail address of corresponding author: takahashi.takafumi{at}nsc.co.jp

The configuration of hydroxyl groups around the octahedral cations of 2:1 phyllosilicate minerals has long been an important question in clay science. In the present study, ²⁷Al 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 (^[6]Al_a, ^[6]Al_b, and ^[6]Al_c) are distinguished by ²⁷Al 5QMAS NMR, whereas these sites are not differentiated by ²⁷Al MAS and 3QMAS NMR. The isotropic chemical shift (_cs) and the quadrupolar product (P_Q) were estimated to be 5.8 ppm and 2.6 MHz for ^[6]Al_a, 6.2 ppm and 3.0 MHz for ^[6]Al_b, and 6.7 ppm and 3.7 MHz for ^[6]Al_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, ^[6]Al_a and ^[6]Al_b in the upfield region were assigned to cis sites, and ^[6]Al_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.

Key Words: Cis • Hydroxyl • MQMAS NMR • Na-montmorillonite • Octahedral Al • Trans

This article has been cited by other articles:

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