Uptake of heavy metal ions from aqueous solution using Mg–Al layered double hydroxides intercalated with citrate, malate, and tartrate

Mg–Al layered double hydroxide (Mg–Al LDH) was modified with organic acid anions using a coprecipitation technique, and the uptake of heavy metal ions from aqueous solution by the Mg–Al LDH was studied. Citrate·Mg–Al LDH, malate·Mg–Al LDH, or tartrate·Mg–Al LDH, which had citrate³⁻ (C₆H₅O₇³⁻), malate²⁻ (C₄H₄O₅²⁻), or tartrate²⁻ (C₄H₄O₆²⁻) anions intercalated in the interlayer, was prepared by dropwise addition of a mixed aqueous solution of Mg(NO₃)₂ and Al(NO₃)₃ to a citrate, malate, or tartrate solution at a constant pH of 10.5. These Mg–Al LDHs were found to take up Cu²⁺ and Cd²⁺ rapidly from an aqueous solution at a constant pH of 5.0. This capacity was mainly attributable to the formation of the citrate–metal, malate–metal, and tartrate–metal complexes in the interlayers of the Mg–Al LDHs. The uptake of Cu²⁺ increased in the order malate·Mg–Al LDH < tartrate·Mg–Al LDH < citrate·Mg–Al LDH. The uptake of Cd²⁺ increased in the order malate·Mg–Al LDH < tartrate·Mg–Al LDH = citrate·Mg–Al LDH. These differences in Cu²⁺ and Cd²⁺ uptake were attributable to differences in the stabilities of the citrate–metal, malate–metal, and tartrate–metal complexes. These results indicate that citrate³⁻, malate²⁻, and tartrate²⁻ were adequately active as chelating agents in the interlayers of Mg–Al LDHs.     

Experimental investigation of sheet flexibility of layered double hydroxides: One-pot morphosynthesis of inorganic intercalates

The sheet flexibility of layered double hydroxides (LDHs) has been investigated experimentally using co-precipitation and urea hydrolysis methods in an aqueous solution of long-chain anion surfactant in this work. Using dodecylsulfate (DS) anion as morphology-controlling agent, layer-bended or contorted Mg/Al-LDH is obtained successfully. The morphology of bent layers is retained during either in situ decomposition of interlayer DS to SO₄²⁻ or ion exchange of interlayer DS by CO₃²⁻. The direct synthesis of the layer-distorted LDHs intercalated with small inorganic anions is quite difficult. It has been achieved using layer-bended LDHs pillared with bulky organic anions as precursors in this paper. The morphosynthesis is expanded to Co/Al and Ni/Al-LDHs, indicative of the general flexibility of this kind of anionic clays.     

Alkoxylation reaction catalysed by layered double hydroxides

Layered double hydroxides (LDHs) are extensively studied as precursors for catalysts, following a calcination at high temperatures to yield mixed oxides. However, these materials are less used as layered materials, i.e., without undergoing thermal activation. We have focused in this work on the use of a series of as-synthesised LDHs for the catalytic preparation of glycol ethers, which is a reaction of primary commercial importance. Two main systems are considered, namely the [Cu–Cr] and [Mg–Al] LDHs. The [Cu–Cr–Cl] LDH is obtained by the coprecipitation method, then through the appropriate chimie douce exchange reactions the original chloride anions are replaced by a variety of oxo- and polyoxometallates, (CrO₄)²⁻, (Cr₂O₇)²⁻, (V₂O₇)²⁻, (V₁₀O₂₈)⁶⁻ and (Mo₇O₂₄)⁶⁻. On the other hand, the [Mg–Al] hydrotalcites, intercalated by (V₂O₇)²⁻, (V₁₀O₂₈)⁶⁻ and [Fe^III(CN)₆]³⁻ anions, are obtained by structure regeneration. This was done by rehydration of a commercial calcined material (Kyowa) in aqueous solutions containing the desired anion. The different materials have been fully characterised by conventional analytical techniques to evidence their lamellar properties and chemical nature. They were then tested in the catalytic reaction involving butan-1-ol and one or more units of ethylene oxide to make butyl-monoglycol ether (BMGE), di-glycol ether (BDGE), tri-glycol ether (BTGE), etc. The reactions were carried out between 80 and 120°C, temperature range in which no collapse of the lamellar structure is normally observed. In this paper it is shown that decavanadate exchanged LDHs proved to be very selective catalysts for the preparation of the monoglycol adduct, some samples achieving up to 100% selectivity in the screening tests.     

Adsorption of MCPA pesticide by MgAl-layered double hydroxides

In the present study, the adsorption characteristics of the herbicide MCPA (4-chloro-2-methylphenoxyacetic acid) on layered double hydroxides (LDHs) were evaluated under laboratory conditions with particular attention to the effect of layer charge, original interlayer anion and morphology. The final objective is the use of LDHs and modified LDH materials as recyclable adsorbents and heterogeneous catalysts for the treatment of contaminated waste waters.The anionic clays tested were [Mg_1−xAl_x(OH)₂]^x+[X_x/m^m−·nH₂O] materials. The MCPA adsorption capacity was determined from adsorption isotherms and a kinetic study. We looked at the influence of the pH, the Mg²⁺/Al³⁺ ratio, i.e. the anion exchange capacity, the nature of the intercalated anion X (CO₃²⁻, NO₃⁻, Cl⁻) and the morphology of the adsorbent on the extent of adsorption. The adsorption isotherms, described by Freundlich model, are of S-type with tendency to L-type for high MCPA equilibrium concentration. Furthermore, the adsorption capacity increases with the layer charge density. Hence, MCPA adsorption on LDHs occurs by anion exchange in two steps, an external exchange followed by an interlayer exchange, which explain these changes of type within the same isotherm. Besides, the adsorption capacity depends on the nature of the starting anions, following the affinity order (NO₃⁻<Cl⁻<CO₃²⁻) proposed by Miyata and increases with the specific surface area.     

Adsorption of 2,4-D on Mg/Al–NO3 layered double hydroxides with varying layer charge density

Layered double hydroxides (LDHs) have high surface area and high anion exchange capacity, so they have been proposed to be an effective scavenger for contaminants. In this study, the adsorption of 2,4-dichlorophenoxyacetate (2,4-D) on Mg/Al–NO₃ LDHs with varying layer charge density was investigated with particular attention on the effect of the orientation of the interlayer nitrate. Three Mg/Al LDHs were synthesized with Al³⁺/(Al³⁺ + Mg²⁺) molar ratios of 3.3 (LDH3), 2.6 (LDH4) and 2.1 (LDH5). The results of adsorption experiments showed that LDH5 exhibited an S-type isotherm with a low 2,4-D adsorption capacity due to the low accessibility of 2,4-D to the interlayer space. The accessibility was restricted by the small basal spacing of LDH5 as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheet. Thus, the 2,4-D adsorption occurred mainly on the external surface of the material. On the contrary, LDH3, which has the highest layer charge density among the samples, contains nitrate with an orientation perpendicular to the hydroxide sheet of LDH3. The interlayer nitrate was readily exchanged by 2,4-D. Thus, in addition to the adsorption on the external surface, the replacement of the interlayer nitrate by 2,4-D contributed to a higher adsorbed amount of 2,4-D; the 2,4-D adsorption of LDH3 exhibited an L-type isotherm. For LDH4 that contained interlayer nitrate with both parallel and perpendicular orientations, the adsorption characteristics were between those of LDH3 and LDH5. This work has demonstrated the dependence of 2,4-D adsorption characteristics on the nitrate orientation in LDHs, as a consequence of changing layer charge density.     

Ethylene‐VInyl acetate/LDH nanocomposites with enhanced thermal stability,flame retardancy,and rheological property

In this contribution, ethylene‐vinyl acetate (EVA)/Zn₂Al‐X layered double hydroxides (LDHs) nanocomposites containing different interlayer anions including CO₃^2–, NO₃^–, Cl^–, SO₄^2– were synthesized using solvent mixing method for the first time. The influence of the interlayer anions of LDHs on the thermal stability, flame retardancy, and rheological behaviors of these nanocomposites were investigated in detail. The results indicated that both the thermal stability and the flame retardancy could be significantly enhanced, and the extent was highly dependent on the type of interlayer anions that were intercalated in LDHs. The influence of different anions on the storage and loss moduli followed the order of SO₄^2– > NO₃^– > CO₃^2– > Cl^–, suggesting that the interlayer anions significantly affect the rheological behaviors of the nanocomposites as well. This work is of great importance to both the academic and industrial fields since it demonstrated that a proper selection of the interlayer anions and an optimization of the chemical composition are crucial for the practical application of such nanocomposites. POLYM. COMPOS., 37:3449–3459, 2016. © 2015 Society of Plastics Engineers     

V-, Mo- and W-containing layered double hydroxides as effective catalysts for mild oxidation of thioethers and thiophenes with H2O2

Catalytic oxidation of thioethers and thiophene derivatives with H₂O₂ was performed at 40 °C and atmospheric pressure, in presence of W-, V- and Mo-containing layered double hydroxides (LDH). The catalysts were prepared by direct ion exchange with metal-oxoanions, i.e. WO₄²⁻, W₇O₂₄⁶⁻, V₂O₇⁴⁻, V₁₀O₂₈⁶⁻, MoO₄²⁻ and Mo₇O₂₄⁶⁻, of the LDH containing aluminum and magnesium atoms in the brucite layer. All the catalysts showed good activity and selectivity in the sulfoxidation reaction, but the catalyst performances strongly depended on the nature of the anion species intercalated in the interlayer gallery. Thus, the W-based LDH was more active and more stable than the V-LDH and Mo-LDH catalysts. The conversion of sulfur-containing compounds is also dependent on the nucleophilicity of substrates and the following order of reactivity was observed benzothiophene < dibenzothiophene < diphenyl-sulfide < benzyl-phenyl-sulfide < methyl-phenyl-sulfide.     

CO adsorption energy on planar Au/TiO2 model catalysts under catalytically relevant conditions

The adsorption of CO on planar Au/TiO₂ model catalysts was studied by polarization-modulation infrared reflection–absorption spectroscopy (PM-IRAS) under catalytically relevant pressure (10–50 mbar) and temperature (30–120 °C) conditions, both in pure CO and in CO/O₂ reaction gas mixtures. The adsorption energy of CO on the Au particles was determined by a quantitative analysis of the temperature dependence of the CO absorption intensity in adsorption isobars. The data reveal considerable effects of the Au particle size when pure CO is used; the initial adsorption energy decreases from 74 kJ mol⁻¹ (2 nm mean Au particle diameter) to 62 kJ mol⁻¹ (4 nm). For CO/O₂ gas mixtures, the initial CO adsorption energy is, irrespective of the Au particle size, constant at 63 kJ mol⁻¹ (i.e., the CO adsorption energy is reduced for smaller Au particles), but this effect vanishes for larger Au particles.     

Study of iodide sorption to the argillite of Tournemire in alkaline media

Argillaceous rocks are considered potential host rocks for radioactive waste repositories. The concrete matrix that could be used as a barrier could react with the groundwater of the geological site, inducing a drastic change in its chemical composition and its pH (10–13). Consequently, the physicochemical properties of the rock in contact with this alkaline solution may be modified and, in turn, may induce modification on the behaviour of radioelements. This study, applied to the argillite of Tournemire, involves characterizing I⁻ sorption to an argillaceous rock in alkaline media in batch experiments under N₂-controlled conditions. I⁻ was added as a ¹²⁵I radiotracer and measured by γ spectrometry.Preliminary experiments were conducted with different solution/solid ratios (v/m=2.5, 5 and 20 ml g⁻¹) and contact times (1–14 days) in order to determine the optimal experimental conditions. The chosen v/m ratio was 5 ml g⁻¹ as the best compromise between a high K_d value and a low error of the measure. The chosen experiment duration was 1 day because I⁻ sorption was highest and to limit the effects of pyrite oxidation. One of the experiments, performed with a radio-sterilized sample to test possible effects from microorganisms, showed that they could enhance iodide retention, particularly during the first 2 contact days.The influence of pH on I⁻ sorption was tested using solutions between values of 8.3 and 12.8. The K_d values were independent of pH and very low (0.3 ml g⁻¹).Finally, the influence of the chemical composition of concrete fluids was also tested. Three solution compositions corresponding to different steps in the evolution of fluids in contact with altering concrete were used: fluid in contact with fresh concrete (pH 13.2), with moderately degraded concrete (pH 12.1) and with strongly degraded concrete (pH 11.5). Each solution contained variable amounts of sodium, potassium, calcium, silica and sulphate. I⁻ sorption was also very low (K_d0.2 ml g⁻¹). Additional experiments were conducted with alkaline solutions containing different amounts of SO₄²⁻ ions (10⁻³–10⁻² M) to test sulphate–iodide sorption competition. I⁻ retention was independent of the sulphate concentration.     

Dissolution kinetics of dehydroxylated nickeliferous goethite from limonitic lateritic nickel ore

The dissolution kinetics in 2 M H₂SO₄ of variously dehydroxylated nickeliferous goethites was investigated for five oxide-type lateritic nickel deposits. Goethite was the main constituent with minor amounts of quartz, talc, kaolinite and Mn oxides. Dissolution of Fe from heated materials followed the Kabai equation. There was a 9–34-fold increase in the Kabai dissolution rate constant (k) for samples heated at 340–400 °C due to both the increased surface area (1.5–2.6 fold) and higher density of structural defects (5–10 fold) in the variously dehydroxylated products. The presence of structural Al and Cr in goethite appears to reduce dissolution rate possibly through the greater M³⁺–OH, O bond strength relative to Fe³⁺, Ni²⁺–OH, O. Nickel showed congruent dissolution with Fe indicating that Ni was uniformly incorporated in the goethite structure. Pre-heating goethite to 600–800 °C for 30 min resulted in incongruent dissolution of Fe and Ni. It is postulated that some Ni is ejected from the neo-formed hematite structure and resides on the crystal surface or in voids. These results may contribute to the development of more efficient procedures for Ni extraction including heap leaching of lateritic nickel ores.     

Fundamental studies on a new concept of flue gas desulphurization

A new process for removal of sulphur dioxide from waste gases is proposed consisting of both electrochemical and catalytic sulphur dioxide oxidation. In the catalytic step a part of the sulphur dioxide is oxidized by oxygen on copper producing sulphuric acid and copper sulphate. The other part is oxidized electrochemically on graphite. The cathodic reaction of this electrolysis is used for recovering the copper dissolved in the catalytic step. The basic reactions of this process have been studied experimentally in detail. It has been shown that sulphur dioxide can be electrochemically oxidized on carbon electrodes to sulphuric acid with high current efficiency. The reaction rate of the electrochemical copper deposition is increased by dissolved sulphur dioxide in the electrolyte. The catalytic oxidation of sulphur dioxide on copper has been investigated for different sulphur dioxide concentrations and temperatures. The ratio of the reaction products, sulphuric acid and copper sulphate, varies over a wide range depending on the experimental conditions.Nomenclature SO₂ concentration (gas phase) (vol % SO₂) - SO₂ concentration (electrolyte) (g l^–1) - E potential vs saturated calomel electrode (V) - E _s specific energy consumption (W g^–1 SO₂) - F Faraday constant (A s^–1 mol^–1) - i current density (mA cm^–2) - molecular weight (g mol^–1) - T temperature (° C) - U _c cell voltage (V) - v _e number of electrons being transferred - space-time yield of SO₂-oxidation (g SO₂ h^–1 dm^–3) - _cu space-time yield of Cu-corrosion (g Cu h^–1 dm^–3) - ratio - fractional conversion of SO₂ - current efficiency for SO₂ oxidation     

Influence of the type of sepiolite on the modification of the pore-size distribution in γ-Al2O3 supports

γ-Al₂O₃ modified supports with bimodal pore-size distributions were prepared by the addition of different types of natural sepiolites (α or β) into alumina. The supports were characterized by nitrogen physisorption, mercury porosimetry, X-ray diffraction, HRTEM and DTA techniques. A wide range of S_BET (94–238 m² g^− 1), pore volumes (0.3–0.82 cm³ g^− 1), and pore sizes were obtained in the supports depending on the type of sepiolite and its concentration added into alumina. The pore sizes were distributed as follows: mesopores around 1.8 nm in radius, mesopores in the radius range 3.0–25 nm and macropores between 25 and 300 nm in radius. The shape of the pore-size distributions depended on the type of sepiolite: the modal peak for pores larger than 3.0 nm was broad with β-type sepiolites and narrow with α-type sepiolites. The mesopore and macropore sizes can be controlled by the type of sepiolite as well as its concentration added to alumina.     

Intercalation of halloysite from Djebel Debagh (Algeria) and adsorption of copper ions

Halloysite available in Djebel Debagh, Guelma (eastern region of Algeria) was characterised by XRF, SEM, XRD, FTIR and by CEC, specific surface area and electrokinetic measurements. The crude halloysite was modified with Mⁿ⁺(CH₃COO)_n (M = Na⁺, NH₄⁺ or Pb²⁺), and intercalation of NaCH₃COO was monitored with time. XRD showed that the intercalation rate exceeded 90% for NaCH₃COO at long reaction time. X-ray diffraction also showed an expansion of the interlamellar space of 5.5 and 6.3 Å due to the intercalation of Pb(CH₃COO)₂ and NH₄CH₃COO. FTIR proved that Mⁿ⁺(CH₃COO)_n reacted with the inner surface hydroxyl groups of halloysite, on the basis of disappearance of the frequence bands at 3676 and 3652 cm^− 1 and the appearance of new bands around 3699 and 3457 cm^− 1. Intercalation of Pb(CH₃COO)₂ and NH₄CH₃COO shifted the band at 1650 towards 1663 and 1671 cm^− 1, respectively. The untreated and intercalated halloysite samples were used for removing copper(II) ions from aqueous solutions. The adsorption isotherms were of L-type according to the classification of Giles et al. (Giles, C.H., Mac Ewan, T.H., Makhwa, S.N., Smith, D., 1960. Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. J. Colloid Interface Sci. 3, 3973–3993.). The adsorption of copper ions was explained by electrostatic interaction between the copper(II) ions and negatively charged binding sites on halloysite surface and ion-exchange of the cations associated with acetate within the interlayer space.     

Effect of particle size on cesium exchange kinetics by K-depleted phlogopite

Cation exchange mechanism and rate of Cs⁺ exchange were investigated in < 2 μm and 20–2 μm particle size fractions of K-depleted phlogopite (Na-phlogopite). The K-depleted phlogopite was prepared from a natural phlogopite by a potassium removal method using sodium tetraphenylborate (NaTPB) at room temperature. X-ray diffraction (XRD) patterns revealed that interlayer K⁺ ions were completely replaced with sodium ions after the potassium removal treatment. Ion exchange isotherms and kinetics were determined for Na⁺ → Cs⁺ exchange with two particle size fractions. The isotherms indicated that both particle size fractions showed high selectivity for Cs⁺. Based on the isotherm tests, ΔG^o values of < 2 μm and 20–2 μm particle fractions were − 6.83 kJ/mol and − 7.08 kJ/mol, respectively. Kinetics of Cs exchange revealed that the 20–2 μm particle size fraction of the K-depleted phlogopite took up more Cs⁺ ions than the < 2 μm particle size fraction. Various kinetic models were applied to describe Na⁺ → Cs⁺ exchange process. Elovich model described the kinetic data of the < 2 μm particle size fraction well, while the modified first-order model or parabolic diffusion model described the data of the 20–2 μm particle size fraction well.     

Influence of different anions on the behaviour of aluminium in aqueous solutions

The influence of chloride, sulfate and perchlorate anions on the behaviour of native oxide layers on aluminium is investigated using electrochemical techniques. Due to its influence on the open circuit potential and the cathodic side of the polarization curve the oxygen concentration has been carefully controlled. Two kinds of attack on a commercially pure aluminium (99.5 wt %) have been observed. In all the investigated 0.5 M Cl^–, 0.5 M ClO^– ₄ and 0.5 M SO^2– ₄ aqueous solutions the metal is corroded around the iron and silicon containing precipitates, but only in Cl^– and ClO^– ₄ solutions is crystallographic pitting observed. Comparison with high purity aluminium (99.99 wt %) shows that pitting corrosion is not influenced by the presence of impurities in the aluminium alloys, but by the presence of anions in solution. The pH and/or oxygen concentration determine whether or not the pitting potential coincides with the corrosion potential.     

Intrinsic kinetics of Fischer–Tropsch reactions over an industrial Co–Ru/γ-Al2O3 catalyst in slurry phase reactor

The rate of Fischer–Tropsch synthesis over an industrial well-characterized Co–Ru/γ-Al₂O₃ catalyst was studied in a laboratory well mixed, continuous flow, slurry reactor under the conditions relevant to industrial operations as follows: temperature of 200–240 °C, pressure of 20–35 bar, H₂/CO feed ratio of 1.0–2.5, gas hourly space velocity of 500–1500 N cm³ g_cat^− 1 h^− 1 and conversions of 10–84% of carbon monoxide and 13–89% of hydrogen. The ranges of partial pressures of CO and H₂ have been chosen as 5–15 and 10–25 bar respectively. Five kinetic models are considered: one empirical power law model and four variations of the Langmuir–Hinshelwood–Hougen–Watson representation. All models considered incorporate a strong inhibition due to CO adsorption. The data of this study are fitted fairly well by a simple LHHW form − R_H2 + CO = ap_H2^0.988p_CO^0.508 / (1 + bp_CO^0.508)² in comparison to fits of the same data by several other representative LHHW rate forms proposed in other works. The apparent activation energy was 94–103 kJ/mol. Kinetic parameters are determined using the genetic algorithm approach (GA), followed by the Levenberg–Marquardt (LM) method to make refined optimization, and are validated by means of statistical analysis. Also, the performance of the catalyst for Fischer–Tropsch synthesis and the hydrocarbon product distributions were investigated under different reaction conditions.     

Thin-film CoB catalyst templates for the hydrolysis of NaBH4 solution for hydrogen generation

Thin-film CoB alloy catalysts were prepared on Ni-foam substrates using electroless as well as electroplating techniques. Electroless plating was carried out using cobalt (II) sulfate as the source of Co²⁺, sodium succinate as the complexing agent, and dimethyamine borane as the source of boron as well as the reducing agent. Electroplating was carried out using cobalt (II) sulfate and cobalt (II) chloride as the sources of cobalt, and boric acid as the source of boron. The thin-film CoB/Ni-foam templates were characterized using ICP, XRD and SEM techniques. The normalized B content was in the range of 1.0–1.30 and 0.20–0.60 wt.% for electroless and electroplated templates, respectively. The B content is less than that required for stoichiometric alloy formation, which indicates the simultaneous deposition of the Co metal along with CoB alloy. An optimum condition of 0.100 M L⁻¹ each of cobalt (II) sulfate heptahydrate Co(SO₄)·7H₂O, sodium succinate (Na₂C₄H₄O₄) and dimethylamine borane (CH₃)₂NHBH₃, at 60 °C with the pH value of 4–5 and a plating time of 1 h was identified for the preparation of the catalyst templates by electroless plating. Where as, 0.125 M L⁻¹ each of cobalt (II) chloride hexahydrate (CoCl₂·6H₂O), Co(SO₄)·7H₂O, 0.125 M L⁻¹ of boric acid at the current density range of 160–320 mA cm⁻² and a temperature of 60 °C was identified as the optimum condition for the electroplating method. Maximum H₂ generation rates of 1.64 and 0.30 L min⁻¹ g⁻¹ of catalyst were obtained with electroless and electroplated thin-film CoB/Ni-foam templates, respectively. The suitability of the electroless plated CoB/Ni-foam catalyst template for extended duration of hydrogen generation from NaBH₄ was studied up to 60 h. Activation energies of 44.47 and 54.89 kJ mol⁻¹ were calculated for electroless and electroplated CoB/Ni-foam catalyst templates, respectively.     

Effect of surfactant concentration on the stacking modes of organo-silylated layered double hydroxides

The effects of the surfactant concentration on the structure, morphology and thermal property of silylated hydrotalcites have been investigated. By in-situ coprecipitation, the surfaces of layered double hydroxides (LDHs) have been modified by using 3-aminopropyltriethoxysilane (APTS) and anionic surfactant, Na-dodecylsulfate (SDS). Two different stacking modes in the resultant materials were detected by X-ray diffraction (XRD). One has an identical structure of LDHs, in which the SDS and APTS only bond to the outside surfaces and plate edges of LDH. The other is with enlarged interlayer distance, in which SDS and APTS combined with the inside surfaces of LDH. With the increased loading of SDS and APTS, the surface of the modified LDH appeared rough as observed in the transmission electron microscopy (TEM) images. The attenuated total reflection Fourier-transform infrared (ATR FTIR) spectra of the silylated hydrotalcites showed a series of bands attributed to –NH₂ and Si–O–M (M = Mg and Al), proving that APTS has successfully been grafted onto LDH. The thermogravimetric curves (TG) showed that the silane grafted samples have less –OH concentration and less interlayer water, as a result of the –OH consumption during the condensation reaction between Si–OH and –OH on LDH surface. These nanomaterials are of potential applications including clay-based nanocomposites, adsorbents for removal of organic contaminants from water and flame retardant materials.     

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 (182–276 m² g⁻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 1.45 mmol L⁻¹ of 2,4-d, with 1 g calcined LDH per liter, was accomplished in 9 h, while phenol half-life was as short as 3.5 h, with the catalyst with lowest zinc amount (5 wt.%). 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.     

Layered double hydroxides pillared by macropolyoxometalates

Pillared derivatives of Mg_1−xAl_x layered double hydroxides (LDHs) were prepared by anion exchange reaction of a synthetic meixnerite precursor, [Mg₃Al(OH)₂](OH), with macromolecular polyoxometalate ions. The intercalated polyoxometalates included the lacunary Dawson ion (α-P₂W₁₇O₆₁)¹⁰⁻, the Finke (Zn₄(H₂O)₂(AsW₉O₃₄)₂)¹⁰⁻ and (WZn₃(H₂O)₂(ZnW₉O₃₄)₂)¹²⁻ ions, the doubled Dawson (P₄W₃₀Zn₄(H₂O)₂O₁₁₂)¹⁶⁻ and the polyoxocryptates (NaSb₉W₂₁O₈₆)¹⁸⁻ and (NaP₅W₃₀O₁₁₀)¹⁴⁻. Anion exchange reaction of [Zn₂Al(OH)₂](NO₃⁻) with (NaP₅W₃₀O₁₁₀)¹⁴⁻ also resulted in a crystalline pillared product. The intercalates exhibited gallery heights up to 16.6 Å and thermal stabilities to 200°C. Nitrogen adsorption/desorption studies for the LDH intercalates showed that access to the gallery micropores was achieved upon POM intercalation. All of the intercalates contained a salt-like impurity phase, as indicated by XRD. The Zn₂Al–(NaP₅W₃₀O₁₁₀)¹⁴⁻ LDH was investigated as a catalyst for the peroxide oxidation of cyclohexene. A comparison of the reactivities of three samples containing different fractions of the salt-like impurity suggested that the impurity phase contributes significantly to the observed activity.