Synthesis, characterization and release of curcumin-intercalated Mg–Al-layered double hydroxides

A drug-inorganic clay composite involving a pharmaceutically active compound curcumin (Cur) intercalated Mg–Al layered double hydroxides (Cur-LDHs) with Mg/Al = 3 has been assembled by co-precipitation and ion-exchange methods. Powder X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR) analysis indicate a successful intercalation of Cur between the layers with monolayer horizontal (parallel to the layer) and vertical (perpendicular to the layer) orientations of Cur anion. (TG-DTA) analyses shows that the thermal stability of intercalated organic species is largely enhanced due to host–guest interaction involving the hydrogen bond compared to pure form before intercalation. The release studies show that release percentages decrease with increasing pH from 4.0 to 6.5 due to possible change of release mechanism during the release process. The kinetic simulation for the release data indicates that the dissolution mechanism is mainly responsible for the release behavior of Cur-LDHs at pH 4.0, while the ion-exchange one is responsible for that at pH 6.5. It suggests that layered double hydroxides may have application as the basis of a novel tunable drug delivery system.     

Catalytic combustion of toluene over mixed Cu–Mn oxides

The influence of the synthesis pH and of the Zn and/or Al additives on the Cu–Mn precursors, obtained by co-precipitation at a constant pH from aqueous solutions of appropriate nitrates at (Cu + Zn)/(Mn + Al) ratio equal 2, was investigated. The relative content of Mn increased with the pH of precipitation. Depending on the sample composition the identified crystalline phases included layered hydroxy double salt, hydrotalcite-like structure, CuO and ZnO. Some precursors had strongly amorphous character. Calcination of the precursors at 673 K resulted in mixed oxides, in which CuO of various degrees of crystallinity could be identified. The Mn-containing phases remained amorphous. All calcined materials proved extremely active in catalytic combustion of toluene. Some catalysts reached 100% conversion already at 403 K. High conversions observed in the low temperature regime were partially due to the strong sorption of toluene. In the catalysts containing Al additive this effect was suppressed.     

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.     

Thermal shielding performances of nano-structured intumescent coatings containing organo-modified layered double hydroxides

Cone calorimetry tests performed at 50 kW/m² heat flux have been exploited for assessing the fire resistant properties of nano-structured intumescent coatings containing modified layered double hydroxides (hydrotalcites, LDHs) and deposited on steel plates. The effects of different types of modified hydrotalcites (i.e. magnesium–aluminum lactate hydrotalcite, magnesium–aluminum gluconate hydrotalcite, magnesium–aluminum hydrotalcite modified with a fatty acid, magnesium–aluminum hydrotalcite modified with rosin) on the thermal shielding performances of the intumescent coatings and their intumescent degree have been thoroughly discussed and compared with the pristine unfilled counterparts.More specifically, the coatings containing organo-modified LDHs showed better thermal shielding performances with respect to the reference intumescent coating; on the contrary, the use of unmodified hydrotalcite in the intumescent formulations was found detrimental. The thermal shielding performances of the coatings filled with modified LDHs were found to be strictly related to the intumescent degree developed during the cone calorimetry tests. In addition, it was possible to compare the thermal shielding performances of the nanofilled coatings by evaluating the temperatures achieved after 2000 s exposure to the 50 kW/m² heat flux of the cone: the thermal shielding performance sequence was LDH-GL > LDH-RS > LDH-LA > LDH-FA > LDH).Finally, the intumescent degree of the modified coatings was found to decrease with increasing the hydrotalcite content, hence lowering their thermal shielding performances.     

Influence of M/M ratio in surface-charging behavior of Zn–Al layered double hydroxides

Zn–Al layered double hydroxides with different M^II/M^III ratios were synthesized by the coprecipitation method and the obtained solids were characterized by powder X-ray diffraction, infrared spectroscopy, acid–base potentiometric titrations, electrophoretic mobility and modeling of the electrical double layer. As general behavior, increasing hydroxyl adsorption with increasing pH and decreasing supporting electrolyte concentration takes place between pH 7 and 12. The electrophoretic mobility of the samples are in all cases positive and it decreases when the pH becomes higher than 9. A net positive charge of the particles, with a point of zero charge (PZC) around 11.8, is deduced from electrophoretic data. A modified MUSIC model, where OH⁻/Cl⁻ exchange at structural sites and proton desorption from surface hydroxyl groups are taken into account, is used to estimate the deprotonation constants, the number of surface groups, the number of sites with structural charges an the affinity constants for Cl⁻ and OH⁻ of the samples with different M^II/M^III ratios. The modeling of potentiometric titrations and electrophoresis data indicates that anions between the layers and negatively charged groups in the particle surface balance most of the structural charge at the particles surface. It shows an important variation of the number of structural sites with the M^II/M^III ratio and an important role of surface protonation–deprotonation sites to the electrical behavior and exchange properties of LDHs.     

Preparation and characterization of Mg–Al layered double hydroxides intercalated with benzenesulfonate and benzenedisulfonate

Mg–Al layered double hydroxides (Mg–Al LDHs) intercalated with benzenesulfonate (BS^–) and benzenedisulfonate (BDS^2–) ions were prepared by coprecipitation and characterized by X-ray diffraction, FT-IR spectroscopy, and chemical analyses. The intercalated BS^– and BDS^2– maintained their intrinsic molecular structures within the Mg–Al LDH interlayers. At low intercalation levels, the benzene ring of BS^– in BS · Mg–Al LDH was inclined at 30° relative to the plane of the brucite-like layers of Mg–Al LDH. With increasing BS^– content, the benzene ring adopted an additional configuration perpendicular to the Mg–Al LDH layers. In BDS-intercalated Mg–Al LDH, the benzene ring of BDS^2– was tilted at 26° relative to the plane of the Mg–Al LDH layers. Intercalation levels of BDS^2– were smaller than those of BS^– despite the greater charge density of BDS^2–, which was likely attributable to a greater degree of electrostatic repulsion between intercalated anions.     

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.     

Nanocomposites of polymer gel electrolyte based on poly(ethylene glycol diacrylate) and Mg–Al layered double hydroxides

The present paper describes the synthesis and characterization of nanocomposite materials composed of the polymer gel electrolyte and a layered double hydroxide (LDH) inorganic filler. Mg–Al LDHs were prepared with various ratios of Mg/Al. It was observed that the layered structure of Mg–Al LDH was totally exfoliated by the PEGDA. The ionic conductivity and mechanical strength were highly improved in the nanocomposite systems. In the case of the composite films having 4.5 wt% of the Mg–Al LDH, the ionic conductivity reached 1.6 × 10^?3 S cm^?1 at room temperature. The incorporation of nanoparticles into the gel resulted in a two‐ or threefold increase in the tensile modulus. Copyright © 2004 Society of Chemical Industry     

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.     

Polymer nanocomposites using zinc aluminum and magnesium aluminum oleate layered double hydroxides: Effects of LDH divalent metals on dispersion, thermal, mechanical and fire performance in various polymers

Oleate-containing layered double hydroxides of zinc aluminum (ZnAl) and magnesium aluminum (MgAl) were used to prepare nanocomposites of polyethylene, poly(ethylene-co-butyl acrylate) and poly(methyl methacrylate). The additives and/or their polymer composites were characterized by X-ray diffraction, FTIR, elemental analysis, thermogravimetric analysis, mechanical testing, and cone calorimetry. The unusual packing of the monounsaturated oleate anions in the gallery of these LDHs facilitates the dispersion of these nanomaterials. The inorganic LDH protects the polymer from thermal oxidation, shown by enhancement of the thermal and fire properties of the corresponding polymer nanocomposites. There is a qualitative difference in the morphology of the two LDHs in PE and PMMA. ZnAl is better dispersed in PE while MgAl is better dispersed in PMMA. The zinc-containing material led to a large reduction in the peak heat release rate in polyethylene, while the magnesium-containing material led to enhancement of the fire properties of the more polar poly(methyl methacrylate). These fire properties are consistent with the morphological differences. Neither of these LDHs shows efficacy with poly(ethylene-co-butyl acrylate), which indicates a selective interaction between the LDH and the various polymers.     

Synthesis of metatungstate pillared layered double hydroxides with variable layer composition. Effect of the Mg:Al ratio on the microporous structure

A series of layered double hydroxides (LDHs) pillared with the Keggin ion H₂W₁₂O₄₀⁶⁻ has been synthesized with final Mg:Al ratios of 1.75:1, 2.31:1 and 3.51:1 by ion exchange reaction of the LDH-hydroxide and LDH-adipate precursors. In each case, the pillared product was characterized by specific BET N₂ surface areas of more than 110 m² g⁻¹ and micropore volumes in excess of 0.026 ml g⁻¹. The micropore size distribution plots obtained from the argon adsorption isotherms indicate that the micropore diameters become smaller as the surface charge density of the LDH increases. A study was also performed on each of the LDH precursors in order to determine any problematic steps in the overall synthesis. Significant differences were noticed in the POXRD patterns of the LDH-adipate precursor; these are believed to be due to differences in the orientation and degree of order of the adipate anions in the gallery space of the LDH. It was found that adipate anions orientated with the long axis perpendicular to the inorganic layers were easier to exchange than those where the long axis was aligned parallel to the inorganic layers.     

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.     

Catalytic properties of Ni–B and Ni–P ultrafine materials

The ultrafine Ni–B and Ni–P amorphous alloy catalysts were prepared by the chemical reduction method. The catalysts were characterized with respect to ICP‐AES, XRD, nitrogen sorption, DSC, SEM, TEM and XPS. Nitrobenzene hydrogenation was used to compare their hydrogenation abilities. The different metalloids of boron and phosphorus bound to the nickel metal for the Ni–B and Ni–P catalysts result in the distinct different surface area, amorphous structure and hydrogenation activity of the catalysts. Ni–B had a larger surface area than Ni–P. The specific activity per surface area of Ni–P was greater than that of Ni–B. The different activities between the Ni–P and Ni–B can be attributed to the difference of the electron density on the nickel metal; boron donates electrons to the nickel metal and phosphorus accepts electrons from the nickel metal. The catalysts were easier to oxidize when they were exposed to air. This would result in the lower activity. However, the activity could be recovered in the reaction process due to the presence of hydrogen in the reaction system. © 2000 Society of Chemical Industry     

Fabrication and properties of Acid Yellow 49 dye-intercalated layered double hydroxides film on an alumina-coated aluminum substrate

The Acid Yellow 49(4-[2-(5-amino-3-methyl-1-phenyl-1H-pyrazol-4-yl)diazenyl]-2,5-dichloro benzenesulfonic acid) (denoted as PPDB) anion intercalated layered double hydroxides (LDH) film was fabricated through an ion-exchange method using a ZnAl-NO₃-LDH/alumina/aluminum film as precursor. The prepared film was investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Thermogravimetric-differential thermal analysis (TG-DTA), UV-visible spectroscopy and the CIE 1976 L*a*b* color difference method. XRD patterns and FT-IR spectra confirm the successful incorporation of PPDB anions into the interlayer galleries of ZnAl-LDH with an expansion of d-spacing from 0.88 nm to 2.51 nm and the disappearance of characteristic absorption band of NO₃⁻ anions at 1384 cm⁻¹. The SEM morphologies show that the LDH films are mainly oriented with c axis of the platelet crystallites parallel to the substrate surface. Additionally, the obtained results suggest that the intercalation of PPDB into ZnAl-LDH host markedly improve the thermal stability and light fastness of PPDB.     

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.     

Towards solving the crystal structure of polytype 3R2 Mg–Al layered double hydroxides

An analysis of the crystal structure of polytype 3R₂ layered double hydroxides has been carried out based on XRPD diffractograms. Measurement of the transmission-XRPD pattern carried out in a spinning and tilting capillary using synchrotron radiation has been shown to diminish the preferred orientation effect of the sample. In order to approach the structure of polytype 3R₂, structural models, based on possible interlayer anion arrangement as revealed by FT-IR, 27-Al solid state NMR and XRPD, were constructed. A structural model is proposed that is consistent with the by XRPD measured interlayer distance, in which tetrahedral aluminate ions are grafted with their apical oxygen ions onto the octahedral sheet. The presence of a measured extra reflection at 2θ value of 6.3, which is not predicted by this structure, can be obtained however by enlargement of the unit cell to a′ = a√3 = 0.5279 nm and c′ = c = 2.1985 nm, which simulates the aluminate anions ordering in the interlayer.     

Adsorption mechanism of humic and fulvic acid onto Mg/Al layered double hydroxides

The adsorption of humic and fulvic acids onto nitrate, chloride and carbonate intercalated layered double hydroxides with Mg/Al ratios ranging from 85/15 to 60/40 was studied by adsorption isotherms at different ionic strengths and the characterization of the preferentially adsorbed humic substance size fractions. The adsorption of humic and fulvic acids onto LDHs occurred by ion exchange with both the intercalated and surface anions of the LDH and ligand exchange reactions with surface groups. The contribution of both mechanisms to the total HA and FA adsorption was estimated. Lower molecular weight humic and fulvic acids were preferentially adsorbed because these fractions can more easily enter the mesoporous LDHs and contain more carboxylic groups, which are known to be involved in ligand exchange reactions with e.g. surface Al-OH groups. Intercalation of the entire HA or FA molecules in-between the LDH sheets is unlikely to occur.     

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.     

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.