New Insights into the Molecular Structures, Compositions, and Cation Distributions in Synthetic and Natural Montmorillonite Clays

International audienceWe present a detailed investigation of the molecular structure of montmorillonite, an aluminosilicate clay with important applications in materials sciences, such as for catalysis, drug delivery, or as a waste barrier. Solid-state 29Si, 27Al, 25Mg, and 1H nuclear magnetic resonance (NMR) measurements combined with density functional theory (DFT) calculations provide a comprehensive picture of the local structure and composition of a synthetic clay and its naturally-occurring analogue. A revised composition is proposed based on NMR results that allow the identification and quantification of the signatures of otherwise undetectable non-crystalline impurities, thus largely complementing the traditional elemental analyses. Solid-state 1H NMR at fast magic-angle spinning (MAS) and high magnetic field provide quantitative information on intra- and inter-layer local environments that are crucial for the determination of the amount of Mg/Al substitution within the octahedral layer. In combination with DFT calculations of energies, it suggests that pairs of adjacent Mg atoms are unfavorable, leading to a non-random cationic distribution within the layers

Modelling the adsorption of molecules of high environmental and health impact in nanoporous materials by coupling quantum and classical approaches

L'adsorption de CO dans la faujasite échangée au CuI et au Na+ a été modélisée à l'aide des approches quantiques (DFT) et classiques (Monte Carlo). Grâce à l'approche DFT, la surface d'énergie potentielle de la faujasite a été explorée. Différents types d'interactions de CO avec les cations ont été identifiés, pour chacune les effets induits par l'adsorption de CO aux niveaux structural et énergétique ont été analysés, et le calcul de la fréquence de vibration de CO a été réalisé. Grâce aux valeurs obtenues, une nouvelle attribution des spectres d'adsorption de CO dans CuY et NaY a été établie. D'un autre côté, grâce aux simulations Monte Carlo dans l'ensemble Grand Canonique, les propriétés d'adsorption (isothermes et enthalpies) de la faujasite vis-à-vis de CO ont été modélisées, et le mécanisme microscopique d'adsorption de CO a été établi. La mise en œuvre de ces simulations a nécessité de paramétrer un nouveau champ de force destiné à décrire les interactions CO/faujasite et CO/CO.CO adsorption in CuI and Na+ exchanged faujasite has been modeled by mean of quantum (DFT) and classical (Monte Carlo) approaches. By mean of the DFT calculations, faujasite potential energy surface has been explored. Different types of CO interactions with the cations have been highlighted, for each one of them CO adsorption effects on the structural and energetic parameters have been analyzed, and calculations of the CO stretching frequency have been performed. Thanks to our calculated values, a new attribution of CO adsorption spectra in CuY and NaY has been established. On another side, by mean of Monte Carlo simulations in the Grand Canonical ensemble, faujasite adsorption properties regarding CO (isotherms and enthalpies) have been modeled, and the CO adsorption mechanism has been established at the microscopic level. The implementation of these simulations has required the derivation of a new force field describing the CO/faujasite and CO/CO interactions

Cooperative Cation Migrations upon CO Addition in CuI- and Alkali-Exchanged Faujasite: A DFT Study

International audienceCO adsorption in Al-rich faujasite zeolite containing copper and alkali cations has been investigated using DFT methods in order to determine how CO interacts and may modify the original position of the cations. Whether a cluster or a periodic model is used, addition of CO induces the formation of stable complexes labeled DI(CO) in which CO interacts by both its C-end and its O-end, resulting from a cooperative rearrangement of cations. In addition to a CuI migration from site II to the supercage, a migration of alkali from site III′ to site III may occur. DI(CO) also induces a downshift of the νCO mode in comparison with the complex containing CO interacting with a single cation, SI(CO). These results suggest a new assignment of the IR spectra of CO adsorbed in YCuI and YNa+: for YCuI, the upshifted signal at ca. 2160 cm-1 in comparison with νCOgas at 2143 cm-1 could be assigned to a SI(CO) structure, whereas the downshifted signal at ca. 2140 cm-1 could be assigned to a DI(CO) complex. For YNa+, the upshifted signal at ca. 2170 cm-1 could be assigned to SI(CO) NaSII · · ·CO and/or to DI(CO) NaSII · · ·OC· · · NaSIII′, whereas the downshifted signal at ca. 2122 cm-1 could be assigned to DI(CO) complex NaSII · · ·CO· · ·NaSIII′. This study shows that the DI(CO) interaction is a key ingredient for understanding the metal-exchanged zeolite properties

Modélisation de l'adsorption des molécules à fort impact sur l'environnement et la santé dans des matériaux nanoporeux en couplant des approches quantiques et classiques

L'adsorption de CO dans la faujasite échangée au CuI et au Na+ a été modélisée à l'aide des approches quantiques (DFT) et classiques (Monte Carlo). Grâce à l'approche DFT, la surface d'énergie potentielle de la faujasite a été explorée. Différents types d'interactions de CO avec les cations ont été identifiés, pour chacune les effets induits par l'adsorption de CO aux niveaux structural et énergétique ont été analysés, et le calcul de la fréquence de vibration de CO a été réalisé. Grâce aux valeurs obtenues, une nouvelle attribution des spectres d'adsorption de CO dans CuY et NaY a été établie. D'un autre côté, grâce aux simulations Monte Carlo dans l'ensemble Grand Canonique, les propriétés d'adsorption (isothermes et enthalpies) de la faujasite vis-à-vis de CO ont été modélisées, et le mécanisme microscopique d'adsorption de CO a été établi. La mise en œuvre de ces simulations a nécessité de paramétrer un nouveau champ de force destiné à décrire les interactions CO/faujasite et CO/CO.CO adsorption in CuI and Na+ exchanged faujasite has been modeled by mean of quantum (DFT) and classical (Monte Carlo) approaches. By mean of the DFT calculations, faujasite potential energy surface has been explored. Different types of CO interactions with the cations have been highlighted, for each one of them CO adsorption effects on the structural and energetic parameters have been analyzed, and calculations of the CO stretching frequency have been performed. Thanks to our calculated values, a new attribution of CO adsorption spectra in CuY and NaY has been established. On another side, by mean of Monte Carlo simulations in the Grand Canonical ensemble, faujasite adsorption properties regarding CO (isotherms and enthalpies) have been modeled, and the CO adsorption mechanism has been established at the microscopic level. The implementation of these simulations has required the derivation of a new force field describing the CO/faujasite and CO/CO interactions.MONTPELLIER-Ecole Nat.Chimie (341722204) / SudocSudocFranceF

A computational exploration of the CO adsorption in Cation-exchanged Faujasites

International audienc

How Cu I and Na I Interact with Faujasite Zeolite? A Theoretical Investigation

International audienceZeolite materials have complex structures that can be determined by X-ray diffraction (XRD), but characterizing the nonperiodic defects, the distribution of the aluminum atoms, and the position of the exchanged cations remain a challenge. It was shown that quantum chemistry methods (QMs) are well suited to predict the structure, even with low symmetry. Here, QMs were used to determine the location and coordination of NaI and CuI cations in Si-rich faujasites of Y-type (with moderate Si/Al ratio) and Al-rich faujasites of X-type (Si/Al = 1). Focusing on the first shell of the metal site, we used QM analysis tools to study the various distortions induced by the presence of Al in the rings of faujasites. Such microscopic data were not accessible using experimental XRD methods. In contrast, using the present theoretical approach, it was possible to predict the absence of symmetry at the atomic level and that sites I were not occupied by NaI nor by CuI cations, even for Al-rich faujasites of X-type. The infrared CO probe was used to analyze the interaction of both NaI and CuI with the zeolite framework. Single CO adsorption on NaI and CuI via the carbon atom showed that the calculated νCO stretching frequency bands are mainly upshifted in comparison with isolated CO. The νCO stretching frequency range was predicted to be larger for CuI than that for NaI, and the bandwidth would be affected by different Al distributions in the six-membered rings (6MR): the more the Al atoms in the 6MR, the larger the bandwidth. To gain insights into the metal bonding picture with its neighbors, we performed natural bond orbital (NBO) analysis combined with the quantum theory of atoms in molecules and electron localization function topological analyses (QTAIM and ELF methods, respectively). While it is generally reported that Na cations provide electrostatic interactions with zeolite materials, Cu cations are often assumed to favor covalent interactions. The upshifting of the calculated νCO stretching frequency and our topological analyses rather indicated that the interactions of NaI and CuI with the oxygen atoms of the hosted zeolite were mainly ionic with a weak covalent character in the case of CuI. The adsorption of CO on NaI proceeds via an ionic Na···C interaction, while for CuI, the Cu···CO bond was calculated to be dative with a strong polar character. Whatever the Lewis metal cation, CuI or NaI, the present topological analyses predict that their interactions with the O atoms of the zeolite were ionic

Local Environments of Boron Heteroatoms in Non-Crystalline Layered Borosilicates

International audienc

New Insights into the Molecular Structures, Compositions, and Cation Distributions in Synthetic and Natural Montmorillonite Clays

We present a detailed investigation of the molecular structure of montmorillonite, an aluminosilicate clay with important applications in materials sciences, such as for catalysis, drug delivery, or as a waste barrier. Solid-state ²⁹Si, ²⁷Al, ²⁵Mg, and ¹H nuclear magnetic resonance (NMR) measurements combined with density functional theory (DFT) calculations provide a comprehensive picture of the local structure and composition of a synthetic clay and its naturally occurring analogue. A revised composition is proposed based on NMR results that allow the identification and quantification of the signatures of otherwise undetectable noncrystalline impurities, thus largely complementing the traditional elemental analyses. Solid-state ¹H NMR at fast magic-angle spinning (MAS) and high magnetic field provide quantitative information on intra- and interlayer local environments that are crucial for the determination of the amount of Mg/Al substitution within the octahedral layer. In combination with DFT calculations of energies, it suggests that pairs of adjacent Mg atoms are unfavorable, leading to a nonrandom cationic distribution within the layers