Synthesis and intracrystalline oxidation of nitrite-intercalated layered double hydroxides

Nitrite-intercalated LDHs could be prepared by a two-stage process that involves coprecipitation in the presence of nitrite ions followed by stirring the product with excess of nitrite ions. The nitrite ion lies flat in these LDHs with its c₂-axis lying approximately perpendicular to the crystallographic c-axis. The interlayer nitrite ions in these LDHs could be quantitatively oxidized to nitrate ions using H₂O₂ solution. In the LDHs thus obtained the nitrate ion lies flat with its c₃-axis parallel to the crystallographic c-axis (D_3h symmetry) in the interlayer region resulting in lower basal spacing.     

Synthesis and UV absorption properties of 5-sulfosalicylate-intercalated Zn-Al layered double hydroxides

5-sulfosalicylic acid (SSA) anions have been intercalated into layered double hydroxides (LDHs) by an anion-exchange reaction using ZnAl-NO₃-LDHs as a precursor. The samples were characterized by XRD, FT-IR, TG-DTA/MS and UV-visible spectroscopy. The results show that the NO₃⁻ anions in the precursor have been completely replaced by SSA anions to give ZnAl-SSA-LDHs having a high degree of crystallinity. Detailed studies reveal the existence of a supramolecular structure in ZnAl-SSA-LDHs involving electrostatic attraction between opposite charges, hydrogen bonding and other weak chemical bonding interactions between host layers and SSA anions. The thermal stability of ZnAl-SSA-LDHs is considerably enhanced compared with that of a mixture of ZnAl-NO₃-LDHs and SSA. After addition of 2.0 wt% ZnAl-SSA-LDHs to polypropylene (PP), the resistance of the polymer to UV degradation is significantly improved.     

Synthesis and characterization of layered double hydroxides with a high aspect ratio

A new route for synthesis of Mg/Al layered double hydroxide (Mg₆Al₂(OH)₁₆(CO₃)·4H₂O) has been introduced, which can be considered as a modified calcination-rehydration method. Under the hydrothermal conditions, LDHs with a high aspect ratio were synthesized and characterized by inductively coupled plasma-atom emission spectrometer (ICP-AES), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermal measurement (TG-DTG) and scanning electron microscopy (SEM). XRD patterns display the crystalline enhanced with the increase of hydrothermal temperature and aging time. TG-DTG curves show the more stable LDHs were synthesized at higher temperature. SEM images indicate the lateral size of the synthesized LDHs locates at ca. 1-6 μm and the thickness at ca. 35-60 nm. And the particle size depends strongly on the treatment temperature and aging time. A buffer solution consisted of HCO₃⁻ and CO₃²⁻ keeps the pH of reaction system in a certain range and offers a low supersaturated reaction circumstance. This is of high importance for the formation of LDHs with a high aspect ratio.     

Synthesis pretreatment and characterization of a magnetic layered double hydroxides fluorescent probe

Using magnetic layered double hydroxide （MLDH） as carrier of fluorescein （FLU）, a fluorescent composite of MLDH-FLU was prepared via intercalation reaction of ion change. The crystal properties of MLDH-FLU were investigated through XRD, IR, TEM and TG-DSC characterization. It is shown that the crystal type of MLDH-FLU composite matched well with R-hexagonal crystal system of MLDH, with crystal cell parameters of a, c and channel height h equal to 0.3199, 2.411 and 0.3267 nm respectively. The superabundant pigment adsorbed outside the composite should be cleared before interference with cells, but excessive wash would decrease stability and cause crystal phase transformation of MLDH.     

Photoactive oriented films of layered double hydroxides

The treatment of nano-ordered oriented films of layered double hydroxide (LDH) with dodecyl sulfate increased the interlayer distance from 0.4 to 1.96 nm, which allowed the intercalation of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS). The re-stacking of separated layers and the rebuilding of crystals oriented parallel to the surface of quartz slides was confirmed by X-ray diffraction and atomic force microscopy. The hybrid films contained homogeneously distributed porphyrin molecules with preserved photophysical properties such as fluorescence, triplet state formation, and energy transfer, thus forming singlet oxygen.     

A novel composite: layered double hydroxides encapsulated in vesicles

We report a novel composite: layered double hydroxides (LDHs) encapsulated in vesicles. It was found that positively charged Mg3Al-LDH nanoparticles can induce the spontaneous formation of vesicles in a mixture of a zwitterionic surfactant, dodecyl betaine (C12BE), and an anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and importantly, we obtain simultaneously a novel composite of Mg3Al-LDH encapsulated in vesicles. The obtained composite is very stable and expected to be potentially used in drug delivery and gene therapy.     

Spontaneous magnetization in Ni-Al and Ni-Fe layered double hydroxides

Layered double hydroxides containing paramagnetic Ni (II) and diamagnetic/paramagnetic Al (III)/Fe (III) ions have been prepared and characterized. Ni 2Al(OH) 6(NO 3). nH 2O ( 1), Ni 2Fe(OH) 6(NO 3). nH 2O ( 2), Ni 2Fe(OH) 6(C 6H 8O 4) 0.5. nH 2O ( 3), and Ni 2Fe(OH) 6(C 10H 16O 4) 0.5. nH 2O ( 4) were prepared by coprecipitation at controlled pH as polycrystalline materials with the typical brucite-like structure, with alternating layers of hydroxide and the corresponding anions, which determine the interlayer separation. Magnetic studies show the appearance of spontaneous magnetization between 2 and 15 K for these compounds. Interestingly, the onset temperature for spontaneous magnetization follows a direct relationship with interlayer separation, since this is the only magnetic difference between compounds 2, 3, and 4. Magnetic and calorimetric data indicate that long-range magnetic ordering is not occurring in any of these materials, but rather a freezing of the magnetic system in 3D due to the magnetic disorder and competing intra- and interlayer interactions. Thus, these hydrotalcite-like magnetic materials can be regarded as spin glasses.     

Cyclodextrin-intercalated layered double hydroxides for fragrance release

In order to attain the controlled release of fragrance, the intercalation of cyclodextrins (CDs) and fragrance in layered double hydroxides (LDHs) was examined. Carboxymethyl-β-CDs (CMCDs) of various degrees of substitution as well as Mg–Al support were synthesized. CMCDs were intercalated into LDH by the reconstruction method. Powder X-ray diffraction, thermal gravimetric analyses and Fourier transform infrared indicated a successful intercalation of CMCDs into the LDH gallery. The retention capacities of the hybrid materials were investigated in aqueous phase and in gaseous solution by static headspace gas chromatography and multiple headspace extraction. The functionalization of the LDH with CMCD allowed the encapsulation of various organic guests and could prolong the fragrance release time in comparison to that from LDH without CMCD, which can be attributed to the inclusion of the fragrance compound in the CMCD cavity.     

Thermal decomposition of silylated layered double hydroxides

Anionic surfactant and silane modified layered double hydroxides (LDHs) were synthesized through an in situ coprecipitation method. The structure and morphology were characterized by XRD and TEM techniques, and their thermal decomposition processes were investigated using infrared emission spectroscopy (IES) combined with thermogravimetry (TG). The surfactant modified LDHs (H-DS) shows three diffractions located at 1–7° (2θ), while there is only one broad reflection for silane grafted LDHs (H–Si) in this region. The morphologies of the H-DS and H–Si show fibrous exfoliated layers and curved sheets, respectively. The IES spectra and TG curves indicate that alkyl chain combustion and dehydroxylation are overlapped with each other during heating from 373 to 723 K in H-DS and to 873 K in H–Si. Sulfate anion transformation process occurs at 473 K in H-DS and 523 K in H–Si. The derivant of sulfate can exist even above 1073 K. After further decomposition, the metal oxides and the new type of Si–O compounds are formed beginning at around 923 K in silane modified sample.     

Mechano-hydrothermal preparation of Li-Al-OH layered double hydroxides

A mechano-hydrothermal (MHT) method was used to synthesize Li-Al-OH layered double hydroxides (LDHs) from LiOH·H₂O, Al(OH)₃ and H₂O as starting materials. A two-step synthesis was conducted, that is, Al(OH)₃ was milled for 1 h, followed by hydrothermal treatment with LiOH·H₂O solution. Effects of the LiOH/Al(OH)₃ molar ratio (R_Li/Al) and hydrothermal temperature (T_ht) on the crystallinity, morphology, and composition of the product were examined. The resulting LDHs were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared, and elemental analyses. The results showed that pre-milling plays a key role in the LDH formation during subsequent hydrothermal treatment. The Li/Al molar ratio of the obtained LDHs keeps constant at 0.5, independent from theR_Li/Al (0.5–5.0) in the starting materials. An increase in the T_ht (20–80 °C) can enhance the crystallinity and morphology regularity of the products. The so-obtained Li-Al-OH LDHs exhibit high crystallinity and well-dispersity, which may have wider applications than the aggregate ones obtained using conventional mechanochemical and Li⁺-imbibition methods.     

Synthesis of layered double hydroxides and their reactivities in 1-butene isomerization after calcination

Layered double hydroxides were synthesized and their catalytic activities tested in 1-butene isomerization after calcination at 723 K. The heat treatment destroyed the hydrotalcite-like structure creating mixed oxides with wide-ranging acid-base properties as revealed by temperature-programmed desorption of carbon dioxide and the double bond isomerization of 1-butene.     

Synthesis and characterization of new Mg(2)Al-paratungstate layered double hydroxides

Layered double hydroxides (LDHs, or hydrotalcites) with Mg(2+) and Al(3+) cations in the mixed metal hydroxide layer and paratungstate anions in the interlayer have been prepared. Different methods have been followed: anion exchange with Mg,Al LDHs originally containing nitrate or adipate, reconstruction of the LDH structure from a mildly calcined Mg(2)Al-CO(3) LDH, and coprecipitation. In all cases, the tungsten precursor salt was (NH(4))(10)H(2)W(12)O(42). The prepared solids have been characterized by elemental chemical analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, thermogravimetric (TG) and differential thermal (DTA) analyses, scanning electron microscopy (SEM) with EDX (energy-dispersive X-ray analysis), and nitrogen adsorption at -196 degrees C for surface area and surface texture. Most of the synthesis methods used, especially anion exchange starting from a Mg(2)Al-NO(3) precursor at low temperature and short reaction times, lead to formation of a hydrotalcite with a gallery height of 9.8 A; increasing the reaction temperature to 70-100 degrees C and maintaining short contact times leads to a solid with a gallery height of 7.8 A. Both phases have been identified as a result of the intercalation of W(7)O(24)(6)(-) species in different orientations in the interlayer space. If the time of synthesis or the temperature is increased, a more stable phase, with a gallery height of 5.2 A corresponding to a solid with intercalated W(7)O(24)(6)(-), is formed, probably with grafting of the interlayer anion on the brucite-like layers. All systems are microporous. Calcination at 300 degrees C leads to amorphous species, and crystallized MgWO(4) is observed at 700 degrees C.     

Synthesis and characterization of Cd-Cr and Zn-Cd-Cr layered double hydroxides intercalated with dodecyl sulfate

Cd-Cr and Zn-Cd-Cr layered double hydroxides (CdCr-LDH and ZnCdCr-LDH) containing alkyl sulfate as the interlamellar anion have been prepared through a coprecipitation technique. The resulting compounds were characterized using X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. Magnetic property measurements indicate that antiferromagnetic interactions occur between the chromium ions in the two compounds at low temperatures. The introduction of zinc influences the ligand field of Cr^III and the Cr^III-Cr^III interactions in the LDH compound. It is found that both CdCr-LDH and ZnCdCr-LDH can be delaminated by dispersion in formamide, leading to translucent and stable colloidal solutions.     

金属氢氧化物直接制备对苯二甲酸插层水滑石及其应用

以新制备的Mg(OH)_2和Al(OH)_3滤饼与对苯二甲酸通过水热反应制备了对苯二甲酸插层水滑石(TALDHs).使用X射线衍射(XRD)、热重-差热分析、扫描电镜(SEM)等技术对TA-LDHs与碳酸根型水滑石(CO3-LDHs)进行对比研究,结果显示,对苯二甲酸离子成功插入到LDHs层间,产物结构完整、晶相单一,所制得的TA-LDHs为片状.CO3-LDHs和TA-LDHs分别作为纳米填料,以两种不同的添加方式制备聚对苯二甲酸乙二醇酯(PET)/LDHs纳米复合材料.对复合材料进行XRD和SEM研究,结果表明在酯交换反应前添加2%TA-LDHs所制备的PET/LDHs纳米复合材料的层板被部分剥离,分散性最好.     

In situ polymerisation of monomers in layered double hydroxides

Vinyl or amino-benzene sulfonates (VBS and ABS) are polymerised in situ after a soft thermal treatment (T = 473 K) between the sheets of layered double hydroxides (LDH). The reaction of polymerisation is studied either by ¹³C CP- MAS NMR or ESR spectroscopies. The resonance peak associated to the vinyl group disappears from the VBS/LDH hybrid material, and conversely, a narrow signal, characteristic of free radicals, is observed for the ABS/LDH system. To cite this article: E.M. Moujahid, C. R. Chimie 6 (2003).     

Single-step synthesis of layered double hydroxides ultrathin nanosheets

A novel single-step approach was developed to prepare large-scale MgAl-LDHs ultrathin nanosheets. The key point of the successful realization was that we employed a high concentration of H(2)O(2). Oxygen molecules, derived from in situ decomposition of H(2)O(2), were speculated to be the decisive factor leading to complete separation of LDHs layers. The ultrathin nanosheets were characterized by XRD, TEM, AFM, FT-IR, and TG-DSC. The results indicated that the thickness of these nanosheets was about 1.44 nm, which was almost in perfect agreement with the theoretical thickness of two LDHs layers. From the TG-DSC curves, the weight loss of these exfoliated MgAl-LDHs ultrathin nanosheets at 500°C was 18.5%, which was much smaller compared to the 32.3% weight loss of unexfoliated MgAl-LDHs.     

Hybrid and biohybrid layered double hydroxides for electrochemical analysis

Layered double hydroxides (LDH) are lamellar materials that have been extensively used as electrode modifiers. Nanostructured organic–inorganic materials can be designed by intercalation of organic or metallic complexes within the interlayer space of these materials or by the formation of composite materials based on biopolymers (alginate or chitosan) or biomolecules, such as enzymes. These hybrid or biohybrid materials have interesting properties applicable in electroanalytical devices. From an exhaustive review of the literature, the relevance of these hybrid and biohybrid LDH materials as electrode materials for electrochemical detection of species with an environmental or health impact is evaluated. The analytical characteristics (sensitivity and detection limit) of LDH-based amperometric sensors or biosensors are scrutinized.

Incorporation of phosphorus oxyacids into layered double hydroxides

Four phosphonate anions (methyl-, ethyl-, phenyl- and benzylphosphonate) were successfully incorporated into [Cu₂Cr(OH)₆]Cl·yH₂O. It was found that two phases exist for the phenylphosphonate intercalate; one in which the anions are arranged perpendicular to the layers, and one with a tilted orientation. Systematic variation of the reaction conditions allowed the former to be isolated with phase purity, but not the latter. The solid-state ³¹P NMR data suggest that proton transfer may occur between host and guest. Some neutral guest is incorporated in the case of phenylphosphonate and benzylphosphonate, presumably owing to relatively poor solvation of these guests. Heat treatments only resulted in the formation of a covalent bond between host and guest in the case of the methylphosphonate intercalate. The intercalation of the related and redox-active phenylphosphinate into a range of LDHs is also reported. Time-resolved in situ diffraction techniques were used to both monitor and quantify the intercalation of phenylphosphonate into [Cu₂Cr(OH)₆]Cl·yH₂O and phenylphosphinate into the hexagonal form of [LiAl₂(OH)₆]Cl·yH₂O. Kinetic and mechanistic parameters have been determined from the diffraction data.