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

The assignment of the Si CP/MAS-NMR spectrum of naturally-occurring sepiolite clay was re-examined using Si COSY and H-Si 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 Q(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 1208C to water vapor or to D2O vapor. The rehydration results support the Si NMR peak assignments that were made on the basis of the COSY and HETCOR experiments. The Si 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 3508C 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 ~3508C exists as a distinct phase to be differentiated from that of the completely dehydrated state. Key Words—Dehydrated Sepiolite, Magic Angle Spinning NMR, Sepiolite, Si CP/MAS-NMR, Si NMR Signals Assignment, 2-Dimensional Solid State NMR.