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Solid-state Nuclear Magnetic Resonance Study of Sepiolite and Partially Dehydrated Sepiolite

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Abstract:

The assignment of the 29Si CP/MAS-NMR spectrum of naturally-occurring sepiolite clay was re-examined using 29Si COSY and 1H-29Si HETCOR pulse sequences. Each of the three main resonances at ­92.1, ­94.6 and ­98.4 ppm has been attributed to one of the three pairs of equivalent Si nuclei in the basal plane, and the resonance at ­85 ppm to Q2(Si-OH) Si nuclei. On the basis of the COSY experiment, the resonance at ­92.1 ppm is unambiguously assigned to the intermediate, near-edge Si sites. The HETCOR experiment revealed that the resonance at ­94.6 ppm cross-polarizes almost entirely from the Mg ­OH protons, and therefore is assigned to the central Si position. The remaining resonance at ­98.4 ppm correlates strongly to the protons of the structural water molecules and therefore is assigned to the edge Si sites. Nearly complete rehydration was achieved at room temperature by exposing sepiolite samples that had been partially dehydrated at 120°C to water vapor or to D2O vapor. The rehydration results support the 29Si NMR peak assignments that were made on the basis of the COSY and HETCOR experiments. The 29Si CP/MAS-NMR spectrum corresponding to the folded sepiolite structure in which approximately one half of the structural water has been removed by heating to 350°C is reported for the first time. The chemical shift values and relative intensities are significantly different compared to the resonances that are observed in the corresponding spectrum of the true sepiolite anhydride. These observations support the earlier claim that sepiolite heated to ~350°C exists as a distinct phase to be differentiated from that of the completely dehydrated state.

Keywords: -DEHYDRATED SEPIOLITE; 2-DIMENSIONAL SOLID STATE NMR.; 29SI; 29SI CP MAS-NMR; MAGIC ANGLE SPINNING NMR; NMR SIGNALS ASSIGNMENT; SEPIOLITE

Document Type: Research Article

DOI: http://dx.doi.org/10.1346/000986002760832838

Publication date: April 1, 2002

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  • The JOURNAL publishes articles of interest to the international community of clay scientists, including but not limited to areas in mineralogy, crystallography, geology, geochemistry, sedimentology, soil science, agronomy, physical chemistry, colloid chemistry, ceramics, petroleum engineering, foundry engineering, and soil mechanics. Clays and Clay Minerals exists to disseminate to its worldwide readership the most recent developments in all of these aspects of clay materials. Manuscripts are welcome from all countries.

    Clays and Clay Minerals is the official publication of The Clay Minerals Society.

    The Editor-in-Chief is Professor Joseph W. Stucki jstucki@illinois.edu

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