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

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.

Nanohybrid materials from amine functionalization of sepiolite: Preparation, characterization and application as electrode modifiers for the electroanalytical detection of heavy metal ions

“…The main diffraction peaks observed are listed in Table 1 that are associated with the clay mineral structure [46]. As main observation, no significant change did not occur on the diffraction peak position of sepiolite after the grafting of APTES and AEPTMS.…”

Anisotropic reinforcement of epoxy-based nanocomposites with aligned magnetite–sepiolite hybrid nanofiller

“…2(C)). The first decomposition step of both samples, between 25 and 150°C, corresponds to the physical desorption of water molecules on the sepiolite surface [21,23]. Pure sepiolite also presents three well defined degradation steps, from 200 to 900°C, attributed to the structural water, the dehydroxylation of silanol and magnesium hydroxide present in the clay mineral.…”

“…2(c)). The to Si-OH, Si near edge, Si center and edge, respectively [22,23]. The peak at 85 ppm disappears in the spectrum of the modified sepiolite, which is an indication to a well condensation of amino-silane on the sepiolite surface.…”

“…Pure sepiolite also presents three well defined degradation steps, from 200 to 900°C, attributed to the structural water, the dehydroxylation of silanol and magnesium hydroxide present in the clay mineral. These three steps are not well defined in the case of modified sepiolite (SEP/DAPTMS) because of the superposition of the amino-silane degradation which occurs during this range of temperature [23]. From the values of mass loss observed for functionalized and non-functionalized sepiolite, it is possible to determine the amount of DAPTMS grafted onto the sepiolite fiber as around 6 wt%.…”

Hybrid pigments from fibrous clays and chromophores inspired by the color of fruits, flowers and wine

“…The sepiolite (SEP) used in this work was SepSp-1 reference clay from the Clay Minerals Society Source Clay Minerals Repository; this material was manually ground to a powder that passed freely through a 40 mesh wire sieve but otherwise used with no additional purification. The chemical composition, characterization and properties of this clay have been described (Gionis et al, 2007;He et al, 1996;Madejová et al, 2017;Vogt et al, 2002;Wang et al, 2009;Weir et al, 2002).…”