Calcined Layered Double Hydroxides Research Articles

Facile Sodium Alginate Assisted Assembly of Ni−Al Layered Double Hydroxide Nanostructures

Self-assembled three-dimensional Ni−Al layered double hydroxide (LDH) nanostructures have been successfully synthesized using a facile biopolymer-assisted approach in the presence of a simple, safe, and inexpensive sodium alginate biopolymer under hydrothermal conditions. The structure and morphology of LDH nanostructures obtained have been investigated by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), and UV−visible diffuse spectroscopy (UV−vis). Three types of pompon-like, marigold-like, and coral-like LDH nanostructures could be obtained by adjusting the synthesis parameters including hydrothermal aging time, concentration of metal ions, and reaction temperature. A possible formation mechanism for the controlled organization of primary LDH nanosheet building units into special three-dimensional nanostructures is proposed on the basis of the interaction between sodium alginate molecules and LDH crystals. Furthermore, after calcination of LDHs at 500 and 700 °C, the resulting NiO-based mixed metal oxides still maintain the three-dimensional morphology of their LDH precursors to some extent, indicative of the high thermal stability of nanostructures. The findings in this work provide a feasible approach for the morphosynthesis of LDH nanomaterials with special superstructures.

Iron-modified hydrotalcite-like materials as highly efficient phosphate sorbents

Highly efficient sorbents for phosphate removal from aqueous solutions based on the calcined forms of Fe(III)-substituted Layered Double Hydroxides (LDH) materials have been developed in this study. Hydrotalcite-like materials with Mg/M 3+ ∼3 (where M = Al 3+, Fe 3+ or combined) have been synthesized following simple co-precipitation method and were subsequently calcined in air at 450 °C. Both as-synthesized and calcined materials were characterized by means of X-ray Diffraction (XRD), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), elemental (C) analysis, N 2 porosimetry, Scanning Electron Microscopy (SEM). All the materials were evaluated for the sorption of phosphates by batch equilibrium sorption experiments and kinetic measurements (effect of contact time). It was shown that chlorides or nitrates, being the charge-balancing anions in the LDH structure, are more easily exchanged by phosphates compared to carbonates. In the Fe(III)-modified LDHs, an increase of the Fe loading led to the decrease of the sorption efficiency. The maximum uptake of phosphates for both the Mg–Al LDH and Mg–Fe LDH samples containing mainly carbonates as charge-balancing anions was relatively low (ca. ⩽25 mg P/g sorbent) while it was higher for the LDH samples containing mainly chlorides (∼80 mg P/g). On the other hand, the maximum sorption capacity for the calcined Mg–Al LDHs and the calcined Fe(III)-substituted sorbents were very high, ca. ∼250 and ∼350 mg P/g, respectively. The sorption data of both the as-synthesized and calcined LDHs was best fitted by the Freundlich model. Both the Mg–Al and Fe-substituted LDH sorbents were regenerated with mixed aqueous solution of NaCl and NaOH and were reused with a small loss of removal efficiency.

Adsorption and photocatalytic degradation of phenol and 2,4 dichlorophenoxiacetic acid by Mg–Zn–Al layered double hydroxides

Mg–Zn–Al layered double hydroxides (LDHs) with varying amounts of zinc were prepared by the coprecipitation method. Solids were analyzed by XRD and N2 physisorption, confirming the formation of pure LDH phase; and the production of mixed oxides with high specific surface areas (182–276m2g−1) after calcination. Band gap energy was also determined, presenting the expected decreasing tendency on increasing zinc amounts. These mixed oxides were tested both for the adsorption of 2,4 dichlorophenoxiacetic acid (2,4-d) and for the photocatalytic degradation of 2,4-d and phenol. Nearly total (97%) degradation of initial 1.45mmolL−1 of 2,4-d, with 1g calcined LDH per liter, was accomplished in 9h, while phenol half-life was as short as 3.5h, with the catalyst with lowest zinc amount (5wt.%). Langmuir adsorption isotherms are presented. Solids were also characterized by XRD and FTIR analysis after photocatalytic and adsorption activity, to determine the presence of 2,4-d. The versatility of LDH decomposition products in the elimination of different contaminants by different mechanisms puts them forward as a viable alternative for environmental remediation.

Non-toxic hybrid pigments: Sequestering betanidin chromophores on inorganic matrices

Betalain was extracted from bougainvillea flower. Alumina, layered double hydroxides (LDHs) and zeolites were tested as adsorbents of the chromophore. Gamma alumina stabilized betalains over a long period. The adsorption sites which stabilized the chromophores were elucidated by nuclear magnetic resonance. Although the oxides obtained by calcination of LDHs retain betalains, they were oxidized within 20 h or react with Zn 2+ of the calcined LDH. No betalain was adsorbed in zeolite X because of the too small pores.

Co-based catalysts from Co/Fe/Al layered double hydroxides for preparation of carbon nanotubes

Carbon nanotubes have been prepared via catalytic chemical vapor deposition of acetylene on a series of catalysts derived from Co/Fe/Al layered double hydroxides (LDHs). The catalytically active cobalt particles were obtained by calcination of LDHs containing cobalt (II) ions followed by reduction. The obtained products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, high-resolution electron microscopy and Raman spectroscopy. The content of Co in the precursors had a distinct effect on the growth of carbon nanotubes. Increasing Co content enhanced the carbon yield, due to good dispersion of a large number of active Co species. This indicated that the agglomeration of metallic Co particles did not take place even at high Co content. Higher Co content led to the formation of carbon nanotubes with smaller diameters and less structural disorder.

Biodiesel synthesis using calcined layered double hydroxide catalysts

The catalytic properties of calcined Li–Al, Mg–Al and Mg–Fe layered double hydroxides (LDHs) were examined in two transesterification reactions, namely, the reaction of glyceryl tributyrate with methanol and the reaction of soybean oil with methanol. While the Li–Al catalysts showed high activity in these reactions at the reflux temperature of methanol, the Mg–Fe and Mg–Al catalysts exhibited much lower methyl ester yields. CO2 TPD measurements revealed the presence of sites of weak, medium and strong basicity on both Mg–Al and Li–Al catalysts, the latter showing higher concentrations of medium and strong base sites; by implication, these are the main sites active in transesterification catalyzed by calcined Li–Al LDHs. Maximum activity was observed for the Li–Al catalysts when a calcination temperature of 450–500°C was applied, corresponding to decomposition of the layered double hydroxide to the mixed oxide without formation of crystalline lithium aluminate phases.

Copper-containing mixed metal oxides derived from layered precursors: control of their compositions and catalytic properties

In this article, a series of Cu-containing mixed metal oxides were obtained by calcination of layered double hydroxides (LDHs) precursors with Cu/Zn/Al atomic ratios ranging from 1:3:1 to 3:1:1 in synthesis mixture at 773 K. The materials were characterized by means of powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), nitrogen sorption experiments, and X-ray photoelectron spectroscopy (XPS). The results indicate that Cu/Zn/Al cationic ratios in precursors could control the compositions of surface species on the resultant calcined LDHs. Furthermore, The catalytic activities of phenol oxidation in aqueous solutions by hydrogen peroxide over the calcined LDHs as catalysts were also tested. It is found that the high catalytic activity is relative to the high amount and dispersion of composite metal oxides containing active Cu2+ reaction centers on the surface of calcined samples, and the lattice oxygen species on the surface are beneficial to the deep oxidation of phenol.

Bromide ion removal from contaminated water by calcined and uncalcined MgAl-CO 3 layered double hydroxides

A fundamental investigation on the uptake of bromide ion from contaminated water by calcined and uncalcined MgAl-CO 3 layered double hydroxides (LDHs) were conducted in batch mode. The uptake capacity of calcined LDHs (CLDH) is higher than that of uncalcined LDHs, due to their different mechanisms which are confirmed by powder X-ray diffraction, FT-IR spectroscopy and TG–MS measurements. The former mechanism is based on the reconstruction of CLDH to Br-LDHs, whilst the latter is related to the surface adsorption. It has been found that the LDHs calcined at 500 °C with Mg/Al molar ratio of 4 represents the highest capacity to remove bromide ion from aqueous solution. The equilibrium isotherms of uptake of bromide by CLDH were well fitted by the Langmuir equation, and thermodynamic parameters such as Δ G 0, Δ H 0 and Δ S 0 were calculated from Langmuir constants. The negative value of Δ H 0 confirms the exothermic nature of adsorption. Three kinetics models were used to fit the kinetics experimental data, and it was found that the pseudo-second order kinetics model could be used to describe the uptake process appropriately. The value of E a was calculated to be 79.9 kJ/mol, which suggests that the process of uptake bromide is controlled by the reaction rate of bromide with the CLDH rather than diffusion.

Adsorption behavior of calcined layered double hydroxides towards removal of iodide contaminants

Abstract Calcined layered double hydroxide (CLDH) as a potential adsorbent to remove iodide contaminants was investigated in batch mode. The uptake capacity of calcined LDH is higher than that of LDHs as precursor, due to their different mechanism which is supported by XRD and TG-MS measurements. The influences of CLDH structure, dosage of CLDH, initial concentration of iodide ion and co-existing anions on the removal of iodide ion have been investigated. The applicability of the Langmuir isotherm shows that the maximum uptake capacity was approximately 376 mg/g, and Gibbs free energy (ΔG 0) for the process is −23.8 kJ/mol. The negative ΔG 0 value shows the spontaneity of the adsorption process.

Effect of High Magnetic Field on Ferrite Materials Obtained by Calcination of Layered Double Hydroxide

In this paper, the effect of high magnetic field on the development of property were investigated by calcination of MgFe-LDH (CO3) as a precursor of ferrite material under applied field of 10 T. Obtained products were identified by X-ray diffraction analysis (XRD) and their microstructures were observed using scanning electron microscopy (SEM). Furthermore, their magnetic properties were evaluated by a superconducting quantum interference device (SQUID). It was observed that the particle size of mixed oxide prepared by calcination under 10 T was homogeneous and small. Saturation magnetization of mixed oxide synthesized under 10 T was higher than that of zero field. Therefore, it was found that calcination due to imposition of high magnetic field affected phase transition to mixed oxide and provided enhancement of its property.

Adsorption behaviour of calcined layered double hydroxides towards removal of fluoride from aqueous solution

Removal of fluoride from aqueous solution by calcined layered double hydroxides (CLDHs) was investigated in batch mode. The influences of calcined temperature, Mg/Al molar ratio of CLDHs, pH of aqueous solution, temperature and co-existing anions on the removal of fluoride have been discussed in detail. It was found that the LDHs with an Mg/Al ratio of 2 calcined at 500°C had the highest capacity to remove fluoride and the maximum percentage removal of fluoride takes place at pH 6.0. The adsorption capacity of CLDHs decreases with a rise in temperature, indicating that the removal process is exothermic in nature. The data from isotherms fitted well to the linearly transformed Langmuir equation. The negative values of Δ G 0 and Δ H o calculated using the Langmuir constant, confirm the spontaneous and exothermic nature of the removal of fluoride.

Adsorption of Arsenic on Layered Double Hydroxides: Effect of the Particle Size

This paper describes a study of arsenic(V) adsorption on appropriately conditioned calcined layered double hydroxide (LDH) adsorbents. Conditioning the adsorbent significantly reduced the dissolution previously observed with uncalcined and calcined LDH. The adsorption rates and isotherms have been investigated in batch experiments. As(V) adsorption is shown to follow a Sips-type adsorption isotherm. The As(V) adsorption rate on conditioned LDH increases with decreasing adsorbent particle size; the adsorption capacity of As(V) on conditioned LDH, on the other hand, is independent of the particle size. When a homogeneous surface diffusion model was used to fit the experimental kinetic data, the estimated diffusivity values were shown to depend on the particle size. On the other hand, when a bidisperse pore model, which considers the adsorbent particles to be agglomerates of microparticles with a porous region present between them, is applied to the same data, it predicts an intracrystalline diffusivity, which is fairly invariable with the particle size.

Preparation and Properties of Nickel and Iron Oxides obtained by Calcination of Layered Double Hydroxides

AbstractA constant pH precipitation method has been applied to obtain solids with Ni/Fe molar ratios of 2/1, 3/2, 1/1, 2/3, and 1/2. In all cases, a phase with the hydrotalcite‐like structure is obtained, containing NiII and FeIII in the brucite‐like layers and carbonate in the interlayer, and, for samples with a Ni/Fe molar ratio lower than 2/1, amorphous hydrated iron oxides, undetected by X‐ray diffraction, are also formed. The solids have been characterized by element chemical analysis, powder X‐ray diffraction, differential thermal analysis, thermogravimetric and differential thermogravimetric analysis, FT‐IR spectroscopy, temperature‐programmed reduction and assessment of specific surface area by nitrogen adsorption at −196 °C. In all cases reduction leads to zero‐valent state for the metals, reduced nickel particles probably favouring reduction of FeIII species; the specific surface area increases with the iron content, probably due to the amorphous nature of the hydrated iron oxides formed. Calcination at 1200 °C in air leads to well crystallized solids, formed by NiFe2O4 spinel and, additionally, rocksalt‐type NiO for Ni/Fe ratios larger than 1/2. In this way, solids with tailored compositions of these two phases can be prepared.

Synthesis of magnetic nanocomposite MgO/MgFe2O4 from Mg-Fe layered double hydroxides precursors

The present letter reported the synthesis of MgO-containing magnetic nanocomposites by calcinations of tailored hydrotalcite-like layered double hydroxides (LDHs) of the type [Mg1 − x − yFe2+ y Fe3 + x (OH)2]x +(A)x/2·mH2O (y ≥ 0; A = CO32 − or SO42 −) precursors at 900°C for 2 h. The results indicate that calcination of LDHs gives rise to the formation of magnetic nanocomposites of MgO and MgFe2O4 spinel ferrite, where MgO formed could disperse and separate MgFe2O4 particles, and MgO itself was also dispersed or embedded uniformly in the MgFe2O4 spinel matrix. Furthermore, initial studies on the bactericidal properties with staphylococcus aureus show that these as-synthesized nanometer-sized materials have significant bactericidal effect, which increases with the increasing volume fraction of MgO.

Sorption of an anionic dye by uncalcined and calcined layered double hydroxides: a case study

Layered double hydroxides (LDHs) with a Mg/Al molar ratio of 2:1 were synthesized by using a co-precipitation method and their calcined products (CLDHs) were obtained by heating Mg/Al-LDHs at 500 °C. Sorption of a weak acid dye, Brilliant Blue R (BBR), by LDHs and CLDHs indicated equilibrium time required for BBR sorption by CLDHs was less than 12 h, regardless of initial concentration of BBR, whereas BBR sorption by LDHs was longer than 20 h. Sorption capacity of CLDHs was much larger than that of LDHs. Therefore, CLDHs could be used to remove anionic dyes of relatively high concentrations while LDHs may only be used to remove anionic dyes of low concentrations. Isotherms for BBR sorption by CLDHs and LDHs were well described using the Freundlich and Langmuir equations, respectively. When the initial pH of BBR solutions was lower than 8.0, the final pH of the solution after sorption was enhanced and stabilized at 10.6–10.8. The effect of initial pH (<8.0) on BBR removal was negligible, which would be environmentally important for precipitation/co-precipitation of co-existing metal cations. The effects of both Cl − and SO 4 2− on BBR sorption by CLDHs were minimal; but the presence of CO 3 2− markedly reduced BBR removal. Thermal regeneration for re-use of LDHs and CLDHs after BBR sorption was feasible only within the first two cycles, after which the regenerated materials suffered from a large loss in their sorption capacities.

Structure and Basicity of Mesoporous Materials from Mg/Al/In Layered Double Hydroxides Prepared by Separate Nucleation and Aging Steps Method

This paper presents the synthesis of mesoporous materials from hydrotalcite-like layered double hydroxides (LDHs) with the Mg2 +/(Al3 + + In3 +) molar ratio of 3.0 on the brucite-like lattice and the interlayer carbonate anions, using a novel separate nucleation and aging steps method developed in our laboratory. The physicochemical properties of as-synthesized LDHs and resulting calcined products at 500∘C were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), simultaneous thermogravimetric and differential thermal analysis (TG-DTA), 27Al nuclear magnetic resonance (NMR) and low-temperature nitrogen adsorption-desorption experiments. The results indicate calcination of LDHs with a well-crystallized single phase and nanoscale crystallites leads to the formation of the type MgO-like phase, and the pore size distributions of uncalcined and calcined LDHs are mostly in the range of mesopores. Furthermore, calcined materials have higher surface areas, pore volumes, pore diameters and surface basicities than corresponding precursors. When used as solid base catalysts for the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde, the material with higher density of base sites on the surface also exhibited a higher catalytic activity. Particularly, calcined MgAlIn-LDH has highest density of base sites resulting in a highest catalytic activity, as points to an important role of indium as a modifier of the surface properties of solid base catalysts.

Utilizing the structural memory effect of layered double hydroxides for sensing water uptake in organic coatings

In this paper, we report results demonstrating the structural memory effect of synthetic calcined layered double hydroxide (LDH) Li 2[Al 2(OH) 6] 2CO 3· nH 2O powder immersed in a bulk electrolyte, exposed to humid air, and embedded in a water-permeable epoxy matrix. Reconstruction of calcined LDH by the structural memory effect can be detected by X-ray diffraction (XRD) leading to a novel approach for remotely and non-destructively detecting water uptake in optically opaque organic coatings. The LDH Li 2[Al 2(OH) 6] 2CO 3· nH 2O was synthesized by aqueous co-precipitation then calcined in air at temperatures in excess of 220 °C to form a Li–Al mixed hydrated oxide powder. Reconstruction in a matter of days was observed when the calcined mixed oxide was immersed in 0.5M NaCl solution. During exposure to humid air, LDH reconstruction was slower occurring over a matter of weeks, perhaps in a deliquescent electrolyte. Paint-like coatings were made and applied to aluminum alloy 2024-T3 (Al–4.4Cu–1.5Mg–0.6Mn) substrates by adding the calcined LDH at a rate of 10 wt.% to a commercial epoxy. Coated substrates were then exposed to 0.5M NaCl solution and LDH reconstruction progressed over tens of days as the coating absorbed water. During these exposure experiments, XRD and electrochemical impedance spectroscopy measurements were made periodically to track LDH reconstruction and measure uptake of water in the coating via capacitance measurements. LDH reconstruction was tracked using the ratio of the {0 0 3} LDH diffraction peak to the {1 1 1} Al diffraction peak. Using the Brasher–Kingsbury equation, the volume fraction of water in the coating was estimated from capacitance data. Up to the point of apparent coating saturation (about 10 vol. %), the XRD peak height ratio varied linearly with the estimated coating water content. This result suggests that additions of calcined LDH to organic coating may lead to methods for sensing early-stage coating degradation due to water uptake and may give an advance warning of substrate corrosion.

Structure and surface chemistry of manganese-doped copper-based mixed metal oxides derived from layered double hydroxides

The paper reports data relative to the preparation of a series of manganese-doped copper-based mixed metal oxides under identical conditions by calcination of layered double hydroxides (LDHs) at 773 K for 5 h. LDHs with Cu 2+/Zn 2+/Mn 2+/Fe 3+/Al 3+ molar ratios in synthesis mixture from 1:1:0:0.3:0.7 to 1:0:1:0.3:0.7 have been synthesized by co-precipitation method. The samples of both the as-synthesized LDHs and the corresponding calcined LDHs have been characterized for phase composition by powder X-ray diffraction (XRD), FT-IR spectroscopy and chemical analysis. The calcined LDHs were submitted to X-ray photoelectron spectroscopy (XPS) investigation with the aim of assessing the actual elemental speciation on the surface of the materials from the analysis of the M 2p (M = Cu, Mn, Fe and Al), Cu 3p and O1s spectral regions. Calculating the relative intensities of spectral components to determine the distributions of different chemical states has been performed by the fitting procedure of the experimental XPS peaks, and significant changes were noted when the partial substitution of manganese for zinc is increased.

Competition between three organic anions during regeneration process of calcined LDH

The calcination of Layered Double Hydroxides (LDH) produces mixed oxides which can undergo rehydration when in contact with aqueous solution containing anions, regenerating the layered structure of LDH, with these anions in the interlayer space. This process is known as ‘ memory effect’. In this work we studied the regeneration of LDH, by sorption of organic anions as terephthalate (TA), benzoate (BA) and 4-methyl-benzoate (m-BA), from the same aqueous solution, starting from the product of calcination of carbonate-containing LDH. The selectivity of the sorbent to each organic anion during reconstruction of the layer structure was also investigated. Results showed a higher extraction rate for TA species and a lower one for BA species. From the results, an order was proposed for the removal of anions as TA≫m-BA>BA, which is related to the capacity of each anion to stabilize the LDH layer. PXRD patterns showed peaks with basal spacing characteristic of intercalated TA anions. The recycling capacity of the sorbent during five cycles of calcination/sorption was also investigated. This capacity decreased after each calcination/sorption cycle, probably due to the formation of magnesium oxide and spinel phases in the material.

Characterisation of layered double hydroxides and their decomposition products

A review on recent results on characterisation of layered double hydroxides (LDHs) prepared by conventional coprecipitation methods is reported. Emphasis is made on the application of few experimental methods for characterising structural properties of these compounds. The use of powder X-ray diffraction, X-ray absorption, MAS-NMR, and FT-IR spectroscopies to characterise particular properties of these solids is described. Thermal analyses (thermogravimetric, differential thermal analysis and temperature-programmed reduction) are used, together with other techniques, to characterise LDHs and mixed oxides produced by thermal decomposition of LDHs. Partial or total reconstruction of the layered structure from medium-temperature calcined LDHs is achieved depending on the calcination temperature and the time of exposure to water vapour.