Synthesis of layered double hydroxides through continuous flow processes: A review

International audienceContinuous production processes allow scaling up of layered double hydroxides (LDHs) and avoid the drawbacks induced by conventional coprecipitation. These drawbacks result from variable supersaturation rate due to non constant pH and concentration of the solutions and from long residence times hindering a fine control of the size and morphology of the particles. Continuous flow processes allow reducing the residence time and maintaining almost constant supersaturation producing LDHs in large amounts with constant quality. We report here the different continuous flow methods for the production of LDHs particles with controlled size and morphology or individual nanosheets, and of LDH-based hybrids and nanocomposites. The paper will focus on the design of the reactors showing a decrease of their volume and an improvement of the mixing and heat and mass transfers. Cylindrical tank under steady-state conditions lead to particles with a narrower size distribution than in batch reactor. Then processes with vigorously stirred microreactors in the so-called in-line dispersion-precipitation method were developed. Counter-current flow reactors with particles formed at the interface of solutions flowing up and down were further used to obtain ultra-fine LDH nanoplates and efficient surface modification with surfactants. Hydrothermal continuous or co-flow reactors exhibit great versatility allowing the preparation of exfoliated or functional LDHs, LDH nanoplates on alumina-coated substrates, and reduced graphene oxide/LDH nanocomposite films. The microfluidic technology is very promising for preparing LDHs of different compositions and functionalities. The reaction conditions as well as the structural and morphological properties of the materials are discussed and applications are reported

Highly Stable Layered Double Hydroxide Colloids: A Direct Aqueous Synthesis Route from Hybrid Polyion Complex Micelles

International audienceAqueous suspensions of highly stable Mg/Al layered double hydroxide (LDH) nanoparticles were obtained via a direct and fully colloidal route using asymmetric poly(acrylic acid)-b-poly(acrylamide) (PAA-b-PAM) double hydrophilic block copolymers (DHBCs) as growth and stabilizing agents. We showed that hybrid polyion complex (HPIC) micelles constituted of almost only Al3+ were first formed when mixing solutions of Mg2+ and Al3+ cations and PAA(3000)-b-PAM(10000) due to the preferential complexation of the trivalent cations. Then mineralization performed by progressive hydroxylation with NaOH transformed the simple DHBC/Al3+ HPIC micelles into DHBC/aluminum hydroxide colloids, in which Mg2+ ions were progressively introduced upon further hydroxylation leading to the Mg-Al LDH phase. The whole process of LDH formation occurred then within the confined environment of the aqueous complex colloids. The hydrodynamic diameter of the DHBC/LDH colloids could be controlled: it decreased from 530 nm down to 60 nm when the metal complexing ratio R (R = AA/(Mg + Al)) increased from 0.27 to 1. This was accompanied by a decrease of the average size of individual LDH particles as R increased (for example from 35 nm at R = 0.27 down to 17 nm at R = 0.33), together with a progressive favored intercalation of polyacrylate rather than chloride ions in the interlayer space of the LDH phase. The DHBC/LDH colloids have interesting properties for biomedical applications, that is, high colloidal stability as a function of time, stability in phosphate buffered saline solution, as well as the required size distribution for sterilization by filtration. Therefore, they could be used as colloidal drug delivery systems, especially for hydrosoluble negatively charged drugs

Hybrid polyion complex micelles from poly(vinylphosphonic acid)-based double hydrophilic block copolymers and divalent transition metal ions

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Alkaline-earth-doped mixed oxides obtained from LDH nanocomposites as highly basic catalysts

International audienceThe anionic exchange of the nitrate compensating anions in host Mg/Al LDH by negatively charged [M(Edta)]2 (M = Ca2+, Sr2+, Ba2+) guest entities has been performed. The increase from 0.83 nm to 1.5 nm of the d0 0 3 interlayer distance shows that intercalation has taken place leading to well ordered lamellar materials with 3-9 wt.% of alkaline-earth (AE) cations loading. The basic properties of the M/ Mg(Al)O mixed oxides catalysts obtained by thermal decomposition of the [M(Edta)]-Mg/Al LDH precursors have been investigated by TPD of CO2, FTIR spectroscopy of adsorbed CDCl3 and catalytic test reaction of 2-methyl-3-butyn-2-ol (MBOH). The AE-containing mixed oxides exhibit higher basicity than Mg(Al)O with, particularly, a large increase of the densities of sites of medium and high strength. Remarkably, the nature of the AE cations allows to finely tune the basicity. All the AE-containing mixed oxides are able to perform the isomerization reaction of 2,3-dimethyl-1-butene (DB-1) into 2,3- dimethyl-2-butene (DB-2), the Ca-containing sample being the most active according to its larger content of strong basic sites

Highly basic catalysts obtained by intercalation of La-containing anionic complexes in layered double hydroxides

International audienceNanocomposites have been obtained by intercalation of preformed negatively charged guest La hydroxy citrate colloids into host Mg/Al layered double hydroxides (LDH). These compounds have been used as precursors for La/Mg(Al)O mixed oxides catalysts. The La loading has been controlled by the colloids exchange rate in order to achieve a better dispersion of the lanthanum cations and thus improve the basic properties of the new catalysts. The increase of the d003 interlayer distance from 0.830nm in the host LDH to 1.196nm in the nanocomposites shows that the intercalation has taken place leading to 1.82-11.16 wt% of lanthanum cation loadings. The basic properties of the La/Mg(Al)O(x) mixed oxides were evaluated qualitatively using Hammett indicators and quantitatively by TPD of CO2. Both techniques indicated the presence of strong basic sites. The transesterification of 1-phenylethanol with diethylcarbonate (DEC), which is known to involve strong basic sites, was used to evaluate the catalytic activity of the La/Mg(Al)O(x) mixed oxides. Yields into carbonate product increase with the density of strong basic sites of the catalyst

New synthesis route of hydrocalumite-type materials and their application as basic catalysts for aldol condensation

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Design of 3D multi-layered electrospun membranes embedding iron-based layered double hydroxide for drug storage and control of sustained release

International audienceNowadays polymer dressings are expected to possess multiply functions. Besides acting as physical barriers, dressings may provide for the injured tissue species able to turn wound healing process faster and painless. In this way, dressings can be designed aiming to enable the release of drugs. The possibility to modulate drug release kinetics is a desired characteristic to be achieved in order to turn drug delivery systems adequate to specific treatments. However, hydrophilic drugs and hydrophobic polymers incompatibility hinders such modulation and a long-term release cannot be achieved efficiently. Here we present the design of poly(lactic acid) (PLA) membranes containing iron-based Layered Double Hydroxide (LDH) particles able to storage a hydrophilic anionic drug (derived from the non-steroidal anti-inflammatory naproxen). LDH particles are excellent candidates to compose multifunctional composites. They may present diverse biocompatible compositions, possess an elevated encapsulation capacity and tends to promote drugs sustained release by its own, besides assisting tissues regeneration process. Nanofibrous membranes were prepared by the combination of electrospun PLA and electrosprayed LDH as alternated layers (approach A) and also by both technics performed at the same time (approach B). In approach A, by varying the thickness of the PLA fibrous layers, it was possible to easily modulate the drug release rate. Half of drug content was released after 1, 4 and 17 days for the membranes containing the thinnest, the intermediate and the thicker PLA layers, respectively, and after 56 h for the membrane prepared by the approach B. Naproxen release was kept for 18 days for the thinnest membrane, 59 days for the membrane prepared by the approach B and 66 days for the thicker membranes. We believe that this work can inspire the development of new functional membranes with tunable drug release profile thanks to the versatile electrospinning and electrospraying techniques

] Investigation of retention mechanisms of dyes by lamellar materials through vibrational spectroscopy

International audienceSome countries with rich textile industries are nowadays facing water quality issues as a consequence of impacting dye pollution. In addition to their inherent toxicity, dyes induce modification in water acidity or salinity which has harmful consequences on the photosynthesis of aquatic plants. As dye are hardly biodegradable, water treatment is necessary to get ride of them [1], and adsorption treatment appears as a good candidate with efficient low-cost adsorbents, such as clays [2]. As a matter of fact, lamellar materials are widely used for sorbtion of ions and molecules harmful for health and environment. In order to gain in efficiency in the use of such materials, it is essential to get a deeper understanding of the retention mechanisms at the interface solid/liquid. In this work, anionic clays, namely layered double hydroxides (LDH) have been chosen as model compounds in order to study sorption mechanism. The presence of charged layer induce strong anionic exchange capacity in these materials, moreover their composition and structual parameters are easily controled through mastered synthesis process [3]. This study work has been focused on the the mechanisms that can lead to the capture of polluting molecules. Several model dyes have been chosen (MethylOrange - MO, Carmen Indigo - CI et Orange G - OG) for their charge, symmetry and hydrophobicity. Parameters such as the solution concentration and the activity of the solvant, playing with the nature of ions, have also been explored.The work presented here rely on vibrational spectroscopies (Infrared and Raman spectroscopies) sensitive to the intermolecular interaction of the dye with its environment particularly at the interface solid/dye. In parallel, X-Ray Diffraction probes the structural modification in the clay during sorption.The experimental results interpreted though the help of simulations (Monte-Carlo, DFT), lead a fine description of ions sorption at the interfaces with these models lamellar materials

Multiscale Mechanistic Study of the Adsorption of Methyl Orange on the External Surface of Layered Double Hydroxide

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Aqueous inks for ecofriendly processing of organic solar cells: investigation of morphological changes

Aqueous dispersions of organic semiconducting nanoparticles (NPs) are particularly attractive as inks for the environmentally friendly preparation of organic solar cells. The internal morphology of the NPs, which depends on their elaboration process, is a key parameter, which has a significant influence on the final morphology of the active layer and therefore its effectiveness. In the present study, core-shell (PF2:PC71BM) NPs were prepared by miniemulsion. Their internal morphology including the composition of the two phases was characterized by scanning transmission X-ray microscopy (STXM) showing a core composed of 77% PC71BM and a shell composed of 75% PF2. It was found that thermal annealing promotes PC71BM diffusion from the core to the shell, increasing its proportion in the shell from 25% to 42%. This annealing, when applied after NPs deposition by spincoating, allows partial coalescence of the NPs, reducing the roughness of the active layer, and increases electron mobility, thus demonstrating the formation of PC71BM percolation paths for electron transport. A PCE of 1.6% could thus be obtained after 10 min of thermal annealing at 100°C. At higher temperature, Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS) analyses demonstrate the modification of the PF2 structuration from randomly oriented lamella after deposition to edge-on orientation after annealing, leading to an unfavorable decrease of the hole mobility in the direction perpendicular to the substrate, while increasing the hole mobility in the substrate plane. This study demonstrates the need to systematically characterize the internal morphology of NPs in order to rationalize the morphology of the active layer and optimize its properties