Deintercalation of carbonate ions from carbonate-type layered double hydroxides (LDHs) using acid-alcohol mixed solutions

Because of the high affinity of carbonate ions (CO₃²⁻) for LDHs (layered double hydroxides), their decarbonation—deintercalation of carbonate ions and conversion into LDHs containing other anions—has been considered difficult. So far, dilute acids have been used for decarbonation with additional salt to enhance the reaction. We found that the acid resistance of the LDHs is much higher in alcohols such as methanol and ethanol than in water, and complete decarbonation was attained, yielding LDHs containing the conjugated base anions of the acids used for the decarbonation. The carbonate ions were very rapidly deintercalated (0.5-1 h) from the CO₃²⁻LDHs at room temperature (15-25 °C) without any weight loss or any appreciable selective dissolution of the metal components of the LDH layer. In contrast to decarbonation in water, for which an additional supply of anions from a neutral salt is needed, when using an acid-alcohol mixed solution, decarbonation proceeded almost quantitatively, and any additional salts were not required. Use of other polar organic solvents such as acetone, THF (tetrahydrofuran), led to similar quantitative decarbonation, but acid resistance of LDHs was less in these solvents than in alcohols.     

Second staging of tartrate and carbonate anions in Mg–Al layered double hydroxide

The intercalation of tartrate was performed through ion exchange using carbonate Mg–Al layered double hydroxide (LDH) as precursor. The intermediate crystalline phase (so-called second-staging phase) was observed during the intercalation process. The pure second-staging LDH, in which alternate interlayer regions are occupied by carbonate and tartrate anions, was obtained by the deintercalation of interlayer tartrate with carbonate. It was also found that the interlayer tartrates are so stable that they are difficult to exchange by carbonate at room temperature.     

New sulfur adsorbents derived from layered double hydroxides: I: Synthesis and COS adsorption

Mixed oxides, prepared via the thermal decomposition of layered double hydroxides (LDHs), were screened gravimetrically for their ability to adsorb carbonyl sulfide (COS). Based on promising results obtained for Ni/Mg/Al, Ni/Mg/Fe and Co/Mg/Al mixed oxides, a study was undertaken to optimize the composition of these materials for COS adsorption. To investigate the effect of the M(II):M(III) ratio, LDHs of the type [M_zMg_yAl_x(OH)₂](CO₃)_x/2·0.5H₂O (where M = Ni or Co, and x + y + z = 1) were prepared at values of x corresponding to 0.33 and 0.20. Simultaneously, the elemental ratio of transition metal to magnesium (z/y) was varied. Mixed oxides obtained from the resulting LDHs were tested in fixed bed mode with a feed of 100 ppm COS in N₂ to determine breakthrough capacity. In general Ni/Mg/Al mixed oxides showed the best performance, a composition with Ni/Mg/Al = 0.32/0.48/0.20 showing the best adsorption capacity. Treatment of the spent adsorbent under an atmosphere of 5% H₂ in N₂ at 450 °C was found to provide an effective means of restoring the adsorption capacity over two cycles of adsorption and regeneration, although after three such cycles, adsorption capacity decreased.     

Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides

Changes in cation composition and M²⁺/M³⁺ ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations.Heating Mg–Al–CO₃ hydrotalcites with Mg/Al=2, 3 and 3.7, at 600 °C for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations.The presence of Mg²⁺, Zn²⁺, Ni²⁺, Co²⁺, Cu²⁺ salts or of low soluble hydromagnesite increased the M²⁺/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn²⁺, Ni²⁺, Co²⁺ and Cu²⁺ cations in the newly formed hydrotalcite was detected, while Mg²⁺ remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH)₃, the M²⁺/Al ratio approximated the minimal possible value of 2.The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value.The results obtained support the conception of the dissolution–crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes.     

A novel gas-liquid contacting route for the synthesis of layered double hydroxides by decomposition of ammonium carbonate

Layered double hydroxides (LDHs) carbonate with the M²⁺/Al³⁺ (M=Mg or Zn) molar ratio of 2/1 has been synthesized by a gas-liquid contacting route with the decomposition of ammonium carbonate. The key feature of this method is a pH gradient germinated by the diffusion of NH₃ and CO₂ vapors in the metal salts solutions from solid (NH₄)₂CO₃ in a closed environment. The physicochemical properties of these two particles were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, simultaneous thermogravimetric and differential thermal analysis (TG-DTA), scanning electron micrograph (SEM) or transmission electron micrograph (TEM), laser particle size analysis and low-temperature N₂ adsorption-desorption analysis. The results manifested that these two particles obtained by the method present well-crystalline, uniform crystallite size and relatively high surface area. The particle sizes of MgAl-LDH and ZnAl-LDH are around 0.55 and , respectively. Both the BET surface areas of them are about .     

Effect of interlayer anions on chromium removal using Mg–Al layered double hydroxides:Kinetic,equilibrium and thermodynamic studies

The influence of interlayer anions such as NO3-, SO42-and Cl-on Mg–Al hydrotalcites for Cr(VI) removal from aqueous solution was studied. The structure of the prepared LDHs was characterized by XRD, SEM, FTIR, TGA, BET surface area and p Hzpc. The sorbent ability and sorption mechanisms were also investigated. The LDHs exhibit high removal for Cr(VI), and the sorbed amount depends on the nature of interlayer anion, which decreased in the following order: NO3-N Cl-N SO42-. Nitrate-containing LDH reached a Cr(VI) sorption equilibrium within only 30 min. The effects of operating conditions, including initial concentration, solution p H, agitation time and sorbent amount have been studied in batch mode. The optimum conditions were observed at an initial concentration of 100 mg·L-1, p H = 6, agitation time of 60 min and a sorbent dose of 2 g·L-1. The equilibrium data were fitted to the Langmuir, Freundlich and Dubinin–Radushkevich isotherm models. The Langmuir model was found to sufficiently describe the sorption process, offering a maximum sorption capacity of 71.91 mg·g-1. The sorption kinetic follows pseudo-second-order reaction with high accuracy. Thermodynamic parameters suggested that the sorption process is spontaneous and endothermic in nature.     

New sulfur adsorbents derived from layered double hydroxides: II. DRIFTS study of COS and H2S adsorption

H₂S and COS adsorption were studied on two calcined layered double hydroxides (LDHs), Mg_0.75Al_0.25(OH)₂(CO₃)_0.125 and Mg_0.65Al_0.35(OH)₂(CO₃)_0.175, using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and a chemisorption apparatus. Both demonstrated the ability to irreversibly adsorb H₂S, corresponding to uptakes of 1.54 and 1.76 μmol/m², respectively, but Mg_0.75Al_0.25 had a significantly larger capacity for COS, 1.62 μmol/m² compared to 0.80 μmol/m² for Mg_0.65Al_0.35. Analysis of the DRIFT spectra suggests the adsorption of H₂S proceeds via the substitution of lattice oxygen with sulfur, resulting in the formation of H₂O on the surface. COS adsorption is more complicated, although it appears that a similar substitution of lattice oxygen with sulfur occurs. This results in the formation of CO₂ and subsequently bicarbonates and carbonates. The formation of hydrogen thiocarbonate is also involved, although this form is generally only observed in the later stages of adsorption and appears to form at the expense of bicarbonate. The Mg_0.75Al_0.25 LDH retained its ability to adsorb COS in the presence of propene.     

Simultaneous incorporation of palladium and zirconium ions in Mg-Al layered double hydroxides by co-precipitation

Simultaneous incorporation of palladium and zirconium ions in Mg-Al layered double hydroxides (LDHs) was attempted by co-precipitation. Mixed oxides were obtained by calcination at 500 °C. XRD patterns of as-synthesized samples showed the formation of well-crystallized LDHs at lower Zr contents. The CO₃²⁻ content of the solids has been determined to have information on the possible incorporation of Zr⁴⁺ into the brucite layer of the hydrotalcites. The values obtained showed that Zr⁴⁺ can incorporate into brucite sheets at Zr content lower than 10 mol%. SEM and TEM images indicated that the resulted LDHs with lower Zr content exhibited plate-like structure. Thermal calcination at 500 °C results in the formation of mixed oxides containing MgO, PdO, ZrO₂ crystallites and a solid solution formed by some of the Zr⁴⁺ cations dissolving along with Al³⁺ to MgO lattice.     

A new method for preparation of nanoplates of Zn-Al layered double hydroxides

Methods for preparation of nanoplates of layered double hydroxides (LDHs) have been intensively investigated for the purpose of making nanocomposites. The present study provides a new simple method of nanoplate preparation for Zn-Al LDHs. Zn-Al LDHs containing lactate or combinations of lactate and simple inorganic anions were synthesized by a coprecipitation method, and water-washed and undried LDHs were dispersed in water. Among them, suspensions of Zn-Al LDHs containing both lactate and bicarbonate were unique in that they rapidly became translucent colloidal dispersions when the molar ratio of added bicarbonate to added Al was ≤ 0.5. The resulting LDH nanoplates in the dispersions were delaminated single layers or nanoplates with a thickness of several layers, depending on the amount of bicarbonate.     

Adsorption of fluoride ions by Zn–Al layered double hydroxides

Zn–Al layered double hydroxides (LDHs) with different molar ratios Zn/Al (0, 0.17, 0.34, 0.97, 3.47, ∝) were prepared by the co-precipitation of chlorides, characterized and evaluated for their fluoride adsorption at room temperature from aqueous solutions. The fluoride adsorption of the as-synthesized LDHs was influenced by the chemical composition of the LDHs and ZA-11 (Zn/Al = 0.97) had the highest capacity for fluoride adsorption (1.14–4.16 mg/g). The adsorption increased after calcination of the LDH up to 500 °C. The equilibrium data were fitted to the Freundlich, Langmuir, and Temkin equations. The kinetics of fluoride adsorption followed the pseudo-second order model.     

Intercalation of layered double hydroxides by poly(oxyalkylene)-amidocarboxylates: tailoring layered basal spacing

Mg-Al layered double hydroxides (LDHs) were intercalated with various molecular-weight poly(oxypropylene)-bis-amidoacid salts (POP-acid), synthesized from polyoxyalkylene-diamines and maleic anhydride. The intercalation involves an ionic exchange reaction of LDHs at 120 °C and under N₂ atmosphere in an autoclave to afford a series of organoclays with a maximal basal spacing of 92 Å, revealed by X-ray diffraction and transmission electron microscopy analyses. The unusually wide interlayer spacing was ascribed to the self-alignment of the hydrophobic POP backbone in the layer confinement. In contrast, the intercalation of poly(oxyethylene)-bis-amidoacids (POE-acid) afforded a low basal spacing (7.8 Å) due to their oxyethylene-backbone interaction with the layered surface. The resultant POP- and POE-acid intercalated LDHs also exhibited different dispersing properties in toluene or water. Particularly, the organically-modified and space-enlarged hybrids with a proper amount of the embedded POPs possessed an amphiphilic property of lowering the toluene/water interfacial tension.     

Design,synthesis, and nanoengineered modification of spherical graphene surface by layered double hydroxide (LDH) for removal of As(III) from aqueous solutions

In this study, new nano spherical graphene modified with LDH (Layered Double Hydroxide) was prepared and used to remove As(III) ion from aqueous solutions. At first, graphene oxide was synthesized from graphite using a well-known Hammer method. The obtained graphene oxide solution was sprayed in octanol solution under different temperatures and sprayed speed as influenced variables. The structure and physical characterization of synthesized spherical graphene oxide were determined by various techniques, including FT-IR, N₂ adsorption–desorption, SEM, TEM, and EDX. In the next step, the hydrothermal method was applied to deposition LDH on the spherical graphene oxide. The synthesized spherical graphene modified by LDH was used to remove As(III) as a toxic heavy metal ion. The effect of influenced variables including pH, contact time, amount of sorbent, and type eluent studied and the optimum values were as 8, 30, 50, and HCl (0.5 mol·L^-1), respectively. After optimization, the studied sorbent was shown a high adsorption capacity (149.3 mg·g^-1). The adsorption mechanism and kinetic models exhibited good agreement with the Langmuir isotherm and pseudo-second-order trends, respectively. Besides, the synthesized product was tested for seven times without significant loss in its sorption efficiency.     

Inorganic layered double hydroxides as a drug delivery system—intercalation and in vitro release of fenbufen

Layered double hydroxides (LDHs), or so-called anionic clays, consist of cationic brucite-like layers and exchangeable interlayer anions. Because of their biocompatibility, some LDHs, such as Mg/Al, Zn/Al, Fe/Al and Li/Al-LDH, can be used as host materials for drug-LDH host–guest supramolecular structures. The anti-inflammatory drug fenbufen has been intercalated into layered double hydroxides for the first time by co-precipitation under a nitrogen atmosphere. The product has been characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, elemental analysis and thermogravimetry (TG) and shows an expanded LDH structure, indicating that the drug has been successfully intercalated into LDH. In addition, the dependence of the nature of the fenbufen intercalation process on conditions such as pH value and chemical composition of the host has been systematically investigated. The interlayer distance in the intercalated materials increases with increasing pH value, resulting from a change in the arrangement of interlayer anions from monolayer to interdigitated bilayer. Drug release characteristics of the pillared LDH materials were investigated by a dissolution test in a simulated intestinal fluid (buffer at pH 7.8). The results show that the drug release of supramolecular LDH materials was a slow process, especially in the case of Mg/Al intercalated materials, suggesting that these drug-inorganic hybrid materials can be used as an effective drug delivery system.     

The influence of the cationic ratio on the incorporation of Ti in the brucite-like sheets of layered double hydroxides

The incorporation of the tetravalent Ti⁴⁺ cations into layered double hydroxide (LDH) type materials was studied as a function of the cationic ratio in the brucite-like sheets. Therefore, Zn–Ti-LDHs with different cationic ratio were prepared by the coprecipitation method at constant pH. The X-ray diffraction patterns revealed that only for the cationic ratio of 3, the characteristic layered structure was formed. The presence and the symmetry of the interlayer carbonate anions were investigated with FTIR–PAS spectroscopy. The UV–DR shows the octahedral environment of the Ti⁴⁺ cations in the brucite-like sheets. Following the same synthesis procedure, a reference sample containing Zn²⁺ and Al³⁺ (cationic ratio of 3) was synthesized. The Zn–Al- and Zn–Ti-LDHs textural features were investigated with N₂ adsorption at 77 K and scanning electron microscopy. Incorporation of Ti⁴⁺ cations in the brucite-like sheets of LDHs lead to a material with much smaller particles, therefore more interparticle porosity and higher volume of N₂ adsorbed at high relative pressures was observed.     

Electrochemical impedance spectroscopy of electrodes modified with thin films of MgMnCO3 layered double hydroxides

The electrochemical impedance spectra of MgMnCO₃ LDH films oxidized at different dc potentials were recorded. The results were fitted to a Randles type cell by replacing the Warburg impedance with a mass transfer resistance in parallel with a constant phase element. The films charge transfer resistances decreased dramatically at the onset of manganese oxidation. In thin films, R_ct decreased from 10⁴ Ω for an un-oxidized film to a minimum of 40 Ω in a film oxidized at 0.32 V, before increasing back to 10⁴ Ω in a film oxidized at 0.5 V. Iodometry measurements show these changes correspond to increases in the manganese average valence in the films from 3.09+ prior to oxidation, to 3.80+ at 0.32 V and 3.95+ at 0.5 V. In thicker films, however, a much higher dc potential, 1.0 V, was required to return R_ct to 10⁴ Ω. There was also less change in the manganese average valence in the thicker films. Oxidation at 1.0 V only increased the manganese valence to 3.33+. For the partially oxidized films, the Nyquist plots consisted of depressed semicircles at high frequency, followed by linear regions at lower frequency where the impedance was controlled by mass transport. The effective diffusion coefficient estimated from the low frequency impedance was 1 × 10⁻⁹ cm² s⁻¹, consistent with proton diffusion in solid electrodes. The impedance spectrum of a partially oxidized film reduced at −0.2 V was similar to that of the un-oxidized film.     

Ni-based supported catalysts from layered double hydroxides: Tunable microstructure and controlled property for the synthesis of carbon nanotubes

The carbon nanotubes (CNTs) with straight and helical nanostructures have been synthesized by catalytic chemical vapor deposition of acetylene over a series of Ni-based supported catalysts, which were formed from Ni–Mg–Al layered double hydroxide precursors (LDHs) synthesized through homogenous decomposition of urea under hydrothermal conditions. The materials were characterized by power X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), temperature-programmed reduction experiments (TPR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results showed that the introduction of Mg into Ni-based supported catalysts could effectively improve the catalytic activities for the growth of CNTs, mainly proceeding from the inhibition effect of spinel phases formed in calcined LDHs on the agglomeration of metallic Ni particles. Furthermore, it is found interestingly that the addition of Mg also could induce the formation of helical structured CNTs with outer diameters of 20 nm and that the higher Mg content gave rise to the more helical nanotubes. The present work provides a simple and facile way to prepare metal-supported catalysts with a good dispersion of catalytically active metal particles for the growth of straight and helical CNTs.     

Preparation of Co–Al layered double hydroxides by the hydrothermal urea method for controlled particle size

In order to control particle size and the distribution, Co–Al layered double hydroxides were prepared from aqueous glycerol solutions of cobalt chloride and aluminum chloride by urea hydrolysis under hydrothermal conditions. Effects of the synthetic conditions including heating temperature and time, and glycerol/water ratio on the particle size and its distributions of Co–Al layered double hydroxide were discussed. Particle diameter was controlled from 1.0 (glycerol/water ratio = 10 g/40 mL) to 38.3 μm (glycerol/water ratio = 0 g/40 mL, heating at 50 °C for 150 days). Narrow particle diameter distribution as low as 20% coefficient of variation was achieved under optimized synthetic conditions (glycerol/water ratio = 10 g/40 mL, heating temperature < 80 °C).     

Studies on the oxidation reaction of l-cysteine in a confined matrix of layered double hydroxides

The amino acid l-cysteine (l-Cys) was intercalated into a MgAl layered double hydroxide (LDH), and its oxidation reaction by hexacyanoferrate (III) (Fe(CN)₆³⁻) in the confined region between sheets of LDH has been studied in detail. Based on the measurement results of XRD, Raman and FT-IR, it was found that the interlayer l-Cys was oxidized to cystine by Fe(CN)₆³⁻. Furthermore, the kinetics of this reaction was investigated in batch mode. The influences of initial Fe(CN)₆³⁻concentration, l-Cys-LDH quantity and reaction temperature on the interlayer oxidation reaction have been studied, respectively. The reaction follows a diffusion-controlled mechanism represented by Crank-Ginstling and Brounshtein kinetic model with the apparent activation energy of 29.93 kJ/mol. Therefore, this layered material may have prospective application as a novel “molecular reactor” for confined chemical reactions.     

Preparation and properties of layered double hydroxide/poly(ethylene terephthalate) nanocomposites by direct melt compounding

Thermal, rheological and mechanical properties of layered double hydroxide (LDHs)/PET nanocomposites were investigated. To enhance the compatibility between PET matrix and LDHs, organic modification of parent LDH having carbonate anion was carried out using various anionic surfactants such as dodecylsulfate (DS), dodecylbenzenesulfonate (DBS), and octylsulfate(OS) by rehydration process. Then, PET nanocomposites with LDH content of 0, 1.0, and 2.0 wt% were prepared by direct melt-compounding. The dispersion morphologies were observed by transmission electron microscopy and X-ray diffraction, indicating that LDH-DS were exfoliated in PET matrix. From the rheology study, there are some network structures owing to filler-filler and/or filler-matrix interactions in nanocomposite systems. Consequently, DS intercalated LDH provided good compatibility with PET molecules, resulting in exfoliated LDH-DS/PET nanocomposites having enhanced thermal and mechanical properties as compared to other nanocomposites as well as homo PET.     

Study on preparation of layered double hydroxides (LDHs) and properties of EPDM/LDHs composites

Abstract

The aim of this work was to study preparation and characterisation of layered double hydroxides (LDHs) and their effects on the mechanical and flame-retardant properties of ethylene propylene diene (EPDM) polymer. A series of Mg–Al, Ni–Al, and Cu–Al hydrotalcite-like compounds were prepared. The surface morphology, interlayer space, interlamellar structure, and thermal properties of these LDHs were investigated. Results showed that a best thermal stability could be obtained with Cu–Al–LDHs as the most effective components. EPDM/Cu–Al–LDHs composites were prepared by conventional compounding with EPDM and Cu–Al type of LDHs. The cure characteristics, tensile strength, wear resistant, and flame-retardant properties were investigated. The best properties were observed for 10 phr of Cu–Al–LDHs filled composite, which resulted in no obvious changes of tensile strength, increased thermal stability, 45% decrease in abrasion loss, and 53% increase in vertical burning time, respectively, compared to that of pure EPDM matrix.