The Interlayer Collapse during Dehydration of Synthetic Na0.7-Beidellite: A 23Na Solid-State Magic-Angle Spinning NMR Study1 (original) (raw)
Related papers
An NMR study of the dehydration of NaBr.2H2O and Na2SO4.10H2O crystals
2012
Heat storage in water vapour sorption in salt hydrates is one of the most promising techniques for compact, high efficiency and long term storage of solar heat. However the dehydration and hydration processes are not fully understood and current state of development is not sufficient for a large-scale introduction of this promising technique. In this project, moisture profiles of dehydrating grains of Na2SO4·10H2O and NaBr·2H2O crystals (dehydrated from one side only) were measured with a 4.7 T NMR-setup. NMR is an effective way to measure moisture content in porous materials, because it is specific to water and it is non-destructive, in contrast to other moisture measurement techniques. In these moisture profiles, a drying front is observed, progressing with √ t. This is explained by a simple diffusion model. From this model, the water vapour diffusion coefficients of samples with different grain sizes were calculated. For example, for grain diameter 1.0-2.0 mm: for Na2SO4·10H2O Dv...
CHARACTERIZATION OF SYNTHETIC Na-BEIDELLITE
Clays & Clay Minerals, 1990
Ahstract-Na-beidellite, a member of the smectite group, was grown hydrothermally from a gel of composition 0.35Na2O.2.35Al2O3.7.3SiO2 in NaOH solutions at a pH between 7.5 and 13.5, a pressure of 1 kbar, and a temperature of 350"C. The synthetic Na-beidellite was characterized by means of scanning electron microscopy, X-ray powder diffraction, infrared spectroscopy, electron microprobe, inductively coupled plasma-atomic emission spectroscopy, and thermogravimetric analysis. The unit-cell parameters of the orthorhombic cell are: a = 5.18, b = 8.96, and c = 12.54/~. The cation-exchange capacity was determined to be 70 meq/100 g. A maximum of 40 wt. % water was present and reversibly lost by heating to about 55"C. The loss of water caused a decrease of the basal spacing to 9.98/~. At temperatures >600"C, the Na-beidellite started to dehydroxylate, reaching its maximum in the range 600"-630"C. At 1100"C the remaining solid recrystallized to Alhy6Si2013 (mullite) and SiO2 (cristobalite).
Nuclear Magnetic Resonance Study of Anion and Cation Reorientational Dynamics in (NH 4 ) 2 B 12 H 12
J. Phys. Chem. C, 2018
Diammonium dodecahydro-closo-dodecaborate (NH 4) 2 B 12 H 12 is the ionic compound combining NH 4 + cations and [B 12 H 12 ] 2− anions, both of which can exhibit high reorientational mobility. To study the dynamical properties of this material, we measured the proton NMR spectra and spin− lattice relaxation rates in (NH 4) 2 B 12 H 12 over the temperature range of 6−475 K. Two reorientational processes occurring at different frequency scales have been revealed. In the temperature range of 200−475 K, the proton spin− lattice relaxation data are governed by thermally activated reorientations of the icosahedral [B 12 H 12 ] 2− anions. This motional process is characterized by the activation energy of 486(8) meV, and the corresponding reorientational jump rate reaches ∼10 8 s −1 near 410 K. Below 100 K, the relaxation data are governed by the extremely fast process of NH 4 + reorientations which are not "frozen out" at the NMR frequency scale down to 6 K. The experimental results in this range are described in terms of a gradual transition from the regime of low-temperature quantum dynamics (rotational tunneling of NH 4 groups) to the regime of classical jump reorientations of NH 4 groups with an activation energy of 26.5 meV. Our study offers physical insights into the rich dynamical behavior of (NH 4) 2 B 12 H 12 on an atomic level, providing a link between the microscopic and thermodynamic properties of this compound. ■ INTRODUCTION The properties of closo-polyborates containing complex polyhedral [B 12 H 12 ] 2− and [B 10 H 10 ] 2− anions have received significant recent attention. 1−3 The original interest in these compounds was related to the formation of alkali-metal (A) and alkaline-earth (Ae) salts A 2 B 12 H 12 and AeB 12 H 12 as stable intermediate products of dehydrogenation reactions of prospective hydrogen-storage materialsalkali-metal and alkaline earth borohydrides. 4−6 Recently, it has been found that the disordered high-temperature phases of Li and Na closo-polyborates A 2 B 12 H 12 and A 2 B 10 H 10 (A = Li, Na) exhibit remarkable superionic conductivity. 7−9 Since the high A + conductivity in these phases is accompanied by extremely fast reorientational motion of [B 12 H 12 ] 2− and [B 10 H 10 ] 2− anions, 10−13 it is likely that the enhanced cation mobility is facilitated by reorientations of these large anions. 14,15 By combining the [B 12 H 12 ] 2− anion with an H-containing cation, such as [NH 4 ] + , one may expect to form a potentially useful hydrogen-storage material with high H content. Indeed, the diammonium dodecahydro-closo-dodecaborate, (NH 4) 2 B 12 H 12 , with the H content of 11.3 wt % is known to exist as a stable cubic compound (space group Fm3̅), 16 see Figure 1. Furthermore, ab initio DFT calculations 17 predict favorable thermodynamics for hydrogen release from this compound. (NH 4) 2 B 12 H 12 is isostructural with the cubic alkali-metal salts K 2 B 12 H 12 , Rb 2 B 12 H 12 , and Cs 2 B 12 H 12 , 16 and its lattice parameter (a = 10.8781 Å) 16 is close to that for Rb 2 B 12 H 12 (10.8674 Å). Reorientational motion of large anions strongly contributes to the balance of energies determining the thermodynamic stability of complex hydrides. Therefore, information on the reorientational dynamics is important for understanding the fundamental properties of these compounds, including the nature and the mechanisms of phase transitions and hydrogen desorption. Microscopic information on H jump motion can be obtained from nuclear magnetic resonance (NMR) and quasielastic neutron scattering (QENS) measurements. Studies of the 11 B NMR spectra in the isomorphous cubic dodecahydro-closo-dodecaborates 18 have shown that the reorientational jump rate τ −1 of [B 12 H 12 ] 2− anions at a given temperature increases with increasing cation radius. However, such NMR spectral measurements can trace the atomic jump rate variations over rather limited dynamic ranges (usually not exceeding 2 orders of magnitude); this may lead to unreliable values of the motional parameters (activation energies and pre
Hydrothermal synthesis of Na-beidellite
Geologie & Mijnbouw, 1990
Na-beidellite was hydrothermally synthesized using various starting materials at a range of P-T conditions. The best crystallized Na-beidellite was carefully investigated with XRD, SEM, TGA, MAS-NMR and IR-spectroscopy. Cell parameters are: a= 5.18 ± 0.005 A; b = 8.96 ± 0.008 A; c = 12.54 ± 0.011 A; V = 581.9 ± 0.5 A3. Indexing is based on an orthorhombic cell. 29Si MAS-NMR reveal three peak positions: - 92.7 ppm (Si-0AI); - 88.4 ppm (Si-1Al) ; - 82.3 ppm (Si-2Al), indicating an AlIV/Si ratio of 0.106 per unit cell. The presence of small amounts of F in the hydrothermal fluid causes a significant increase in crystallinity. Na-beidellite is the only crystalline product applying a starting gel of composition Nao 7Al47Si730 22 . A Na10Al50Si70 22 gel results in Na-beidellite + paragonite and gels with higher Na content produce only paragonite.
Advanced Powder Technology, 2017
Micro and nanostructured brucite (Mg(OH 2)) particles synthesized by hydrothermal method from solutions with high content of hydrazine (0.14 M) and nitrate (0.24 g) were compared with samples obtained from low hydrazine content (0.0002 M) and nitrate (0.12 g). The samples were heated at 180 °C for 4 h, 6 h and 12 h. XRD, TEM-HRTEM, SAED and image analysis techniques were used for the morphological and structural characterization. The effect of electron beam irradiation on the brucite dehydration was observed in atomic resolution images at 300 kV. Hexagonal crystals show differences in crystallinity, strains and kinetic of reaction. High hydrazine/nitrate samples have slightly larger crystals with better crystallinity, showing a strong preferential orientation. Rietveld refinements show how unit cell parameters are bigger in samples obtained with higher hydrazine/nitrate content, confirming also the preferential orientation along the 0 0 0 1 plane. Differences in the dehydration process show the rapid formation of a porous surface, the amorphised cortex or the presence of highly oriented strains in samples prepared from higher hydrazine/nitrate content. Conversely, crystals slightly smaller with randomly scattered defect surfaces showing the Mg(OH) 2 /MgO interphase in samples prepared with low hydrazine/nitrate content. Significant differences in the kinetic of reaction indicate how the dehydration process is faster in samples prepared with high hydrazine/nitrate content.