Structural Analyses of Layer Structure in Na-Montmorillonite using NMR
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Structural Analyses of Layer Structure in Na-Montmorillonite using NMR Takahiro Ohkubo1, Koji Kanehashi2, Koji Saito2, and Yasuhisa Ikeda1 1 Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan 2 Nippon Steel Corporation, Advanced Technology Research Laboratory, 20-1 Shintomi, Futtsu City, Chiba 293-8511, Japan ABSTRACT The analyses of layer structure in Na-montmorillonite have been performed using 27Al MAS and 27Al Multi Quantum (MQ) MAS NMR techniques. In 27Al 5QMAS NMR spectra measured at higher magnetic field of 16.4 T, two peaks were observed at 74.2 and 62.9 ppm. Hence, it has been suggested that the 4-coordinated Al site in Na-montmorillonite has two different structures. For detail assignment of the two peaks due to such 4-coordinated Al sites in Na-montmorillonite, 27Al MAS NMR measurements have been carried out for Na-montmorillonites with different water contents. The ratio of two 4-coordinated Al sites was found to be affected by water in the interlayer space of Na-montmorillonite, because the area ratio of peaks corresponding to two 4-coordinated Al sites changes with the water content, i.e., the peak area at around 69 ppm is increased sensitively with an increase in water content. Moreover, Cross Polarization/MAS (CP/MAS) NMR measurements have been carried out. As a result, the enhancement of peak at around 69 ppm was observed. Therefore, the two peaks observed by 27Al 5QMAS methods are assigned to AlO4 in the vicinity of water bound to tetrahedral sheets and the remote AlO4 from such water, respectively. INTRODUCTION The solid state NMR experiments for montmorillonite in the past have been almost confined to Si and 27Al MAS NMR [1-3]. The structural analyses using solid state NMR have been applied to aluminosilicate like montmorillonite, since MAS NMR is effectual method for obtaining chemical conformation of layer structure. The 29Si and 27Al MAS NMR spectra have proved that the SiO4 tetrahedron sheets have three bridged oxygen atoms, and made it possible to determine coordination numbers of Al. However, the 27Al MAS NMR spectral data are not sufficient for 29
quantitative discussion, because the central transition (−1/2,1/2) of 27Al MAS NMR spectrum is shifted and broadened by second-order quadrupolar interaction. In this paper, 27Al solid state NMR were applied to the detailed structural analyses of Na-montmorillonite, which can be expected to elucidate accurately layer structure and surrounding of water in the interlayer space. Recently, the multi-quantum (MQ) MAS NMR has been applied to structural analyses of noncrystalline materials, such as glasses and aluminosilicate
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[4][5], since MQMAS NMR can average the second-order quadrupolar interaction. Thus, the MQMAS technique is also expected to be useful for the structural studies on tetrahedral and octahedral Al sheets in Na-montmorillonite. (;3(5,0(17$/ 0$7(5,$/6 Montmorillonite used in the present study is classified as dioctahedral smectite, and was supplied from the
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