Layered double hydroxides: where should research stress on for massive scaling up?

International audienceFor more than half a century, layered double hydroxides (LDHs) have attracted great attention from the scientific community. These materials belong to a rare anionic clay group and have demonstrated amazing physicochemical properties, leading to disruptive, cost-effective and eco-friendly applications for health, environment and agriculture. However, LDHs are not widely used in daily items, nor are they massively applied in industry despite their very good potential for large-scale development. This study looks at the current shortcomings preventing the industrial implementation of LDH powders while evidencing and discussing the strategies to encourage research to bridge the gaps on this topic

NHC-heterocycle carbene gold catalyst intercalated in Layered Double Hydroxides as reusable hybrid catalyst in acidic medium

International audienceNHC-heterocyclic carbene metal complexes are efficient catalysts used for the oxidation, amination, cycloaddition or hydration of alkynes. Immobilization of such complex is a strategy to avoid its decomposition/deactivation and allow its recycling. One limitation of the immobilization of carbene metal complexes is the acidic pH required to perform the catalysis of hydration of alkynes. The support used must be resistant in such conditions. Layered Double Hydroxides (LDH) is a widely used material as catalyst support due to their tunable chemical composition and flexible open structure. However, the LDH phases usually found as catalyst support in the literature are chemically instable under severe conditions. In particular, when operating in extreme acidic pH. This study describes the development of a pH-resistant support by screening of LDH with various composition and formulation in order to obtain a stable material at pH down to 3.0, conditions required for a model reaction, the hydration of propargyl alcohol. Then, the intercalation of a NHC-heterocyclic carbene gold anionic complex (NHC-Au) was optimized and fully characterized. The newly heterogeneous catalysts were evaluated for the targeted hydration catalysis reaction. The selected support, a LiAl2 phase obtained by urea method and dried at 80 °C, allowed to immobilized up to 75% of the gold complex. The intercalated catalyst was finally used for the hydration of several alkynes with yields between 84% and 100% and was recycled up to 12 cycles without any activity loss

Staging during anion-exchange intercalation into [LiAl2(OH)6]Cl.yH2O: structural and mechanistic insights.

A series of experiments has been performed to seek more insight into the staging process that occurs during anion-exchange intercalation of some organic carboxylates and phosphonates into the layered double hydroxide [LiAl2(OH)6]Cl.yH2O. High resolution transmission electron microscopy has been employed to gain additional insight into the second-stage intermediates, providing strong evidence that the Rüdorff model of staging is applicable. Small-angle X-ray scattering was used to study the very early stages of the intercalation of succinate into [LiAl2(OH)6]Cl.yH2O: it was observed that the only species present during the reaction were the host, a second-stage intermediate and the first-stage product. The influence of temperature and solvent on the reaction mechanism was investigated. Staging was observed only at low temperatures (T<60 degrees C), and found to be confined largely to aqueous systems. Reactions performed in a 95:5 (v/v) mixture of water and a second non-aqueous solvent such as ethanol, acetone, THF or formamide proceeded via a second-stage intermediate, whereas for those undertaken in 50:50 (v/v) mixtures a direct transformation from host to product was usually observed

Étude du mécanisme d'échange et de la structure des matériaux hydroxydes doubles lamellaires (HDL) par diffraction et diffusion des rayons X

La maîtrise des performances des matériaux HDL (Hydroxydes Doubles lamellaires) nécessite une compréhension de plus en plus fine de leur structure ainsi que des relations entre structure, morphologie et propriétés d'emploi. Des mesures en diffraction et diffusion des rayons X (rayonnement synchrotron) haute résolution enregistrées à grand Q permettent d'aborder de manière plus précise la description structurale et la microstructure de ces matériaux mal cristallisés notamment vial'analyse de la fonction de distribution de paires (PDF). D'autre part, des mesures réalisées in situ en solution par diffraction des rayons en dispersion d'énergie (EDXRD) ont permis de révéler la formation de composés HDL bi-intercalés porteurs de nombreuses potentialités dans le domaine des matériaux multifonctionnels. Des mesures EDXRD ont également été appliquées à l'étude du mécanisme de formation de nanocomposites HDL-polymère qui représente un domaine d'application important des matériaux HDL

Insights into the Structure and the Electrochemical Reactivity of Cobalt-Manganese Layered Double Hydroxides: Application to H2O2 Sensing and glucose biosensing.

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Thermooxidative degradation of crosslinked EVA/EPDM copolymers: Impact of Aluminium TriHydrate (ATH) filler incorporation

International audienceThis study focuses on the thermal oxidation of model composites of insulating materials based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) highly loaded (60 wt %) with ATH (Aluminium TriHydrate) filler. A thorough analysis of the material is performed, focusing not only on the polymer but also on the ATH filler. This preliminary study is essential firstly to determine the possible influence of a large amount micrometric filler incorporation on the structure of crosslinked EVA/EPDM materials, and secondly on the thermooxidative degradation mechanisms at different levels and scales (chemical structure, microstructure and architecture, degradation profile, functional properties …). The structure of both the polymer and the filler are modified during the processing step of the composite. It is shown that the reactivity of the ATH fillers leads to the probable intercalation of some segments of the polymer into the interlayer space of the ATH, and to a much less dense polymer network in the composite compared to the unfilled material. Then, the role of the added ATH filler on the thermooxidative degradation and on the resistance of the composite to ageing is studied. After thermooxidative ageing, there is only a very limited accelerator effect of ATH on the oxidation rate of the polymer within the composite compared to the unfilled material, but significant difference between materials with or without ATH filler in terms of oxidation profile or insulating properties. Nevertheless, a noticeable effect on the mechanical properties is highlighted: the mechanical properties of the unfilled materials are retained all through the oxidation process while these properties reduce drastically in the case of filled composites. The significant loss of mechanical properties occurring in the filled composites can be explained by the combination of three factors: only 40 wt % of polymer in the composite, a low density polymer network within the composite, and de-cohesion process between the ATH filler and the polymer matrix upon thermooxidative degradation

Structural Insight into Iodide Uptake by AFm Phases

International audienceThe ability of cement phases carrying positively charged surfaces to retard the mobility of 129I, present as iodide (I−) in groundwater, was investigated in the context of safe disposal of radioactive waste. 125I sorption experiments on ettringite, hydrotalcite, chloride-, carbonate- and sulfatecontaining AFm phases indicated that calcium−monosulfate (AFm−SO4) is the only phase that takes up trace levels of iodide. The structures of AFm phases prepared by coprecipitating iodide with other anions were investigated in order to understand this preferential uptake mechanism. X-ray diffraction (XRD) investigations showed a segregation of monoiodide (AFm−I2) and Friedel's salt (AFm−Cl2) for I−Cl mixtures, whereas interstratifications of AFm−I2 and hemicarboaluminate (AFm−OH−(CO3)0.5) were observed for the I−CO3 systems. In contrast, XRD measurements indicated the formation of a solid solution between AFm−I2 and AFm−SO4 for the I−SO4 mixtures. Extended X-ray absorption fine structure spectroscopy showed a modification of the coordination environment of iodine in I−CO3 and in I−SO4 samples compared to pure AFm−I2. This is assumed to be due to the introduction of stacking faults in I−CO3 samples on one hand and due to the presence of sulfate and associated space-filling water molecules as close neighbors in I−SO4 samples on the other hand. The formation of a solid solution between AFm−I2 and AFm−SO4, with a shortrange mixing of iodide and sulfate, implies that AFm−SO4 bears the potential to retard 129I