V. Rives

Layered double hydroxides (LDH) intercalated with metal coordination compounds and oxometalates

Abstract This paper reviews the synthesis, properties and applications of layered double hydroxides (LDHs), also known as anionic clays or hydrotalcite-like materials, containing intercalated anions constituted by metal complexes or oxometalates. After an introduction describing the main features of these compounds, emphasis is put on the synthesis methods, characterization and applications.

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.

Effect of hydrothermal and thermal treatments on the physicochemical properties of Mg-Al hydrotalcite-like materials

The synthesis and characterization of hydrotalcite-like materials is described. As the time to which the samples have been submitted to a hydrothermal treatment is prolonged, a change in the Mg/Al ratio is observed, together with a more ordered structure of the species existing in the interlayer space. Calcination at increasing temperatures leads to decomposition of this compound, with the final formation of MgO and MgAl2O4. Changes taking place during these processes have been related to the decrease observed in the specific surface area of the samples upon prolonging the hydrothermal treatment, as well as its development and further decrease upon thermal treatment.

Layered double hydroxides with the hydrotalcite-type structure containing Cu2+, Ni2+ and Al3+

Layered double hydroxides (LDHs) with the hydrotalcite-like structure have been prepared containing different amounts of Ni2+, Cu2+ and Al3+ in the brucite-like layers by a coprecipitation method. The samples have been characterized by elemental chemical analysis, powder X-ray diffraction, differential thermal analysis and thermogravimetric analysis, FT-IR and UV–VIS diffuse reflectance spectroscopies, temperature-programmed reduction, and specific surface area and porosity assessment by N2 adsorption at −196 °C. The nature of the phase obtained, crystallinity of the HT-like phase and thermal behaviour of these materials were influenced by MII/Al atomic composition and concentration of bivalent metal ion. Calcination at 500 °C leads to mostly amorphous solids containing NiO, and also CuO (tenorite) for large Cu2+ contents (Cu/Ni molar ratio equal to 4). When the calcination temperature is increased to 850 °C, crystallization of well defined phases (NiO, CuO and NiAl2O4, depending on the relative amounts of Ni, Cu and Al) takes place. Reduction of Cu2+ species in these samples takes place at a lower temperature than that for Ni2+, while the temperature of reduction of nickel is increased with an increase in aluminium and copper content of the samples.

The effect of iron on the crystalline phases formed upon thermal decomposition of Mg-Al-Fe hydrotalcites

Layered double hydroxides (LDH) containing Mg II , Fe III , and Al III in the brucite-like layers and interlayer carbonate (with a constant M II /M III ratio but varying Al III /Fe III ratios) have been prepared and characterised by X-ray diffraction, thermal analysis, FT-IR and UV-VIS/diffuse reflectance spectroscopies, temperature-programmed reduction and specific surface area assessment through low temperature adsorption of N 2 . An Mg,Al-LDH, but with intercalated hexacyanoferrate(III), has been also prepared and characterised, in which simultaneous formation of the carbonate analogue did not occur. Thermal decomposition in air at 450 and 750°C leads to MgO and poorly crystallised MgFe 2 O 4 spinel (crystallinity increasing with the iron content), while for the hexacyano-containing sample, crystallization only is observed after calcination at 900°C. This different behaviour has been related to the initial location of the iron ions.

Layered double hydroxides as drug carriers and for controlled release of non-steroidal antiinflammatory drugs (NSAIDs): A review

Non-steroidal anti-inflammatory drugs constitute one of the groups most widely currently used, but show several problems for administration due to low solubility and delivery control. For this reason, several matrices have been tested to support them in order to overcome these drawbacks. Among them, layered double hydroxides have been used in recent years. The aim of this review is to update the current knowledge and findings on this hybrid system, namely, layered double hydroxides intercalated with different NSAIDs. The basic nature of the matrix introduces an additional advantage, i.e., to decrease ulceration damages. We have focused our review mostly on the preparation procedures, as these control, define and determine the performance of the systems in vitro and also in living organisms.

Bioinorganic Magnetic Core−Shell Nanocomposites Carrying Antiarthritic Agents: Intercalation of Ibuprofen and Glucuronic Acid into Mg−Al−Layered Double Hydroxides Supported on Magnesium Ferrite

This paper describes the synthesis and characterization of a composite constituted by an antiarthritic agent (AA) intercalated into a layered double hydroxide (LDH) supported on magnesium ferrite. Core-shell nanocomposites were prepared by depositing Mg-Al-NO(3)-LDH on a MgFe(2)O(4) core prepared by calcination of a nonstoichiometric Mg-Fe-CO(3)-LDH. Intercalation of ibuprofen and glucuronate anions was performed by ion-exchange with nitrate ions. The structural characteristics of the obtained products were investigated by powder X-ray diffraction, element chemical analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Morphologies of the nanocomposite particles were examined by scanning electron microscopy and transmission electron microscopy. The products were shown to intercalate substantial amounts of AA with enhanced thermal stabilities. Room-temperature magnetic measurements by vibrating sample magnetometry revealed that the products show soft ferromagnetic properties suitable for potential utilization in magnetic arthritis therapy.

Synthesis, characterization and catalytic hydroxylation of phenol over CuCoAl ternary hydrotalcites

Copper–cobalt–aluminium hydrotalcites with (Cu + Co)/Al atomic ratio of 3.0 and Cu/Co atomic ratios 80:20, 75:25, 50:50, 33:67, 25:75 and 20:80 were synthesized by coprecipitation under low supersaturation and characterized by various physicochemical techniques. Powder X-ray diffraction (PXRD) showed a single phase with a resemblance to hydrotalcite, whose crystallinity increased with increasing copper concentration. A continuous blue shift of the νOH band with a concomitant red shift in the ν2 vibration of the carbonate was observed with increasing cobalt content. Thermal analysis of these samples showed a significant variation depending on the nature of the atmosphere, more marked at higher temperatures, owing to the possibility of oxidation of cobalt. UV–vis spectroscopy showed absorption maxima close to 750 and 530 nm with the former becoming stronger and the latter weaker as the copper content is increased. TPR of fresh samples substantiated the lack of oxidation of cobalt, as evidenced by an M/H2 (M = Cu + Co) ratio for these samples close to 1.0. N2 adsorption measurements showed type II isotherms with a hysteresis loop closing at P/P0 = 0.45. The total pore volume, the specific surface area and the area of the hysteresis loop increased with cobalt content. Thermal calcination of these samples at 500°C showed a diffuse pattern exhibiting both a tenorite and a spinel phase, whose peak intensities varied with Cu/Co atomic ratio. TPR for these samples showed higher M/H2 values, suggesting partial oxidation of Co2+. Liquid phase hydroxylation of phenol was carried out over these catalysts using H2O2 as oxidant. Among the catalysts studied, that with a Cu:Co atomic ratio of 3:1 showed the maximum activity with a catechol: hydroquinone ratio close to 3.4. An increase in substrate:catalyst ratio enhanced the conversion of phenol. Oxidants other than H2O2 and solvents other than water did not show measurable conversion of phenol. Time-on-stream studies indicated that 80% conversion of phenol was achieved in 10 min. The normalized activity of these catalysts indicated the influence of cobalt, probably together with surface properties, on the intrinsic activity of copper in controlling the course of reaction. A reaction pathway involving hydroxyl radical is proposed.

Influence of tungsten oxide on structural and surface properties of sol–gel prepared TiO2 employed for 4-nitrophenol photodegradation

A set of tungsten oxide/TiO2 polycrystalline samples (W/Ti) prepared by the sol–gel method has been characterised using several techniques, namely thermogravimetric analysis, gas-chromatography, mass spectrometry, thermogravimetric analysis combined with mass spectrometry or with gas chromatography/mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, determination of specific surface area, scanning electron microscopy, Fourier-transform infrared spectroscopy and monitorisation of pyridine adsorption for surface acidity. Moreover the samples have been empolyed as catalysts for 4-nitrophenol photodegradation in aqueous suspensions used as a ‘probe’ reaction. Characterisation results indicate that the surface of the W/Ti particles is enriched with homogeneously dispersed tungsten as well as microcrystalline or amorphous species. Maximum photoactivity for 4-nitrophenol photodegradation was achieved for a sample containing 1.7 mol W per 100 mol Ti.

Microwave-assisted homogeneous precipitation of hydrotalcites by urea hydrolysis.

The use of urea as a precipitating agent in the synthesis of Ni-Al and Zn-Al layered double hydroxides having a hydrotalcite-like structure via a microwave-hydrothermal method is reported. For comparison purposes, the samples were also prepared by a conventional hydrothermal method. Ni-Al compounds with the hydrotalcite-like structure were obtained in shorter periods of time by the microwave method than by the conventional method, whereas when zinc cations were involved, no successful synthesis was achieved regardless of the method used. In order to find the best synthesis conditions for the Ni-Al solids, samples were submitted to microwave-hydrothermal treatment at different temperatures for increasing periods of time, and the structural, thermal, and textural properties of the synthesized materials were evaluated. All of the solids were fully characterized by chemical elemental analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, and transmission and scanning electron microscopy as well as by N 2 adsorption/desorption at -196 degrees C for assessment of specific surface area and porosity. The PXRD patterns showed that the layered structure appeared after merely 10 min when the synthesis was carried out at 125 degrees C; however, the FT-IR spectra showed the presence of some cyanate groups that were formed during urea hydrolysis and were quite difficult to remove completely. When the conventional hydrothermal treatment was used, longer periods of time were required in order to develop the hydrotalcite-like structure, but increasing the aging time improved the crystallinity of the compounds and yielded large particles.