Catalytic performance of Ni-Al layered double hydroxides in CO purification processes

Ni-Al layered double hydroxides with Ni²⁺/Al³⁺ molar ratios of 1.5 and 3.0 have been synthesized by co-precipitation and studied as catalyst precursors for purification of CO-containing gas-mixtures by means of CO oxidation to CO₂ and conversion of CO by water vapor (water-gas shift reaction). The influence of the alkali additives (K⁺ ions) on the water-gas shift activity has been also examined. It was established that the catalytic activity of both reactions increases with the temperature and the nickel content. Hypothetic schemes are proposed about activation of the catalysts in the WGSR and CO oxidation including redox Ni²⁺ ? Ni³⁺ transition on the catalyst surface. The activity in WGSR is positively affected by the presence of potassium promoter, depending on its amount. The sample with higher nickel loading is the most effective catalyst as for CO oxidation as well as for WGSR at intermediate temperatures after potassium promotion.     

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.     

Fluorinated hydrotalcites: the addition of highly electronegative species in layered double hydroxides to tune basicity

Hydrotalcite-like compounds were synthesized by coprecipitation using a constant-pH method. Aluminum was introduced in the form of an octahedral aluminum fluorine complex during the synthesis. Controlling the pH during the synthesis ensured that aluminum fluorine did not decompose to tetrahedral species but remained in the octahedral (AlF(6))(3-) form to be incorporated into brucite-like sheets. The physicochemical, thermal, and spectroscopic characterizations showed significant modifications by fluorine introduction regarding structural, textural, and adsorption properties. The memory effect of fluorinated hydrotalcites differed from the memory effect commonly observed in fluorine-free hydrotalcites. Nitromethane adsorption showed that the basicity of hydrotalcite was greatly modified by the fluorine loading.     

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.     

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.     

Ni-Al layered double hydroxides electrodeposition from a chloride solution

Ni-Al LDHs was electrodeposited from a NiCl₂-AlCl₃ solution. In order to analyze the electrodeposition process, electrolytes with initial Al content range of 0–20% were used. With increasing Al content in the sample, the preferred orientations of (0 0 3) and (0 0 6), increased crystallinity, and decreased interlayer spacing were observed from the XRD results. A dissolution–recrystallization of (0 0 3) plane was detected among the Ni-Al LDHs from the strongly alkaline solution soaking results, which was found to be conducted easily in high Al-containing samples. The pH of the Al-containing electrolyte was much lower than that of pure NiCl₂ solution because lower pH was needed to start a precipitation reaction in the AlCl₃-NiCl₂ solution. The electrodeposition yield and current efficiency were found to decrease obviously in the electrolytes with initial Al content higher than 10%, which was attributed to the increasing Al content in the sample and diffusion of the complex ions. The electrodeposition pattern was in-situ in the electrolyte initially containing 10% Al, then, it developed toward and off in-situ in electrolytes initially containing 0–10% and 10–20% Al.     

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.     

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.

层状复合氢氧化物的药物控释动力学机制

以层状复合氢氧化物(LDH)为载体组装乙酰苯甲酸(ASP)的插层复合物LDH-ASP,以pH2.48~6.89的磷酸盐溶液为介质进行药物释放动力学实验,通过对不同时间药物释放度的测定及释放药物后载体结构变化分析研究了LDH对层间药物的控制释放机制。结果表明粉末状LDH-ASP能在较宽的pH范围表现明显缓控释作用,药物释放度积分方程与Higchi方程、Ritger-Peppas方程、Bhaskar方程及一级动力学方程等经验模型吻合。复合物与溶出介质的反应机制研究表明,药物释放速率及释放度大小取决于交换配比n(ASP)/n[HnPO4(3-n)-]所决定的LDH层间通道畅阻情况。     

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.     

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.     

Intercalation of dodecyl sulfate into layered double hydroxides

The intercalation of sodium dodecyl sulfate and exchange of dodecyl sulfate anion into layered double hydroxides has been examined by means of X-ray diffraction, infrared and thermogravimetric procedures. Three types of derivatives were obtained having mean interlayer spacings of 26 Å, 36 Å and 47 Å, respectively. These interlayer distances did not correlate with the amount of organic incorporated between the layers but, as shown by computer simulations, depended upon the orientation of the chains within the interlamellar space. In several reactions both intercalation of neutral sodium dodecyl sulfate as well as exchange of the dodecyl anion took place. Attempts to remove the alkyl sulfate chains with dilute acid resulted in dissolution of the more basic metals producing non-stoichiometric layered products.     

Ultrathin defective high-entropy layered double hydroxides for electrochemical water oxidation

One of the fundamental driving forces in the materials science community is the hunt for new materials with specific properties that meet the requirements of ra...     

Mg-Al-carbonate layered double hydroxides as a novel catalyst of luminol chemiluminescence

The interlayer carbonate in Mg-Al layered double hydroxides as a novel catalyst of luminol chemiluminescence has been reported for the first time, which has great potential in further applications of luminol chemiluminescence for sensitive sensors, catalyst-conjugated probes.     

Density functional theory study of CO2 capture with transition metal oxides and hydroxides

We have used density functional theory (DFT) employing several different exchange-correlation functionals (PW91, PBE, PBEsol, TPSS, and revTPSS) coupled with lattice dynamics calculations to compute the thermodynamics of CO(2) absorption/desorption reactions for selected transition metal oxides, (TMO), and hydroxides, TM(OH)(2), where TM = Mn, Ni, Zn, and Cd. The van't Hoff plots, which describe the reaction equilibrium as a function of the partial pressures of CO(2) and H(2)O as well as temperature, were computed from DFT total energies, complemented by the free energy contribution of solids and gases from lattice dynamics and statistical mechanics, respectively. We find that the PBEsol functional calculations are generally in better agreement with experimental phase equilibrium data compared with the other functionals we tested. In contrast, the formation enthalpies of the compounds are better computed with the TPSS and revTPSS functionals. The PBEsol functional gives better equilibrium properties due to a partial cancellation of errors in the enthalpies of formation. We have identified all CO(2) capture reactions that lie on the Gibbs free energy convex hull as a function of temperature and the partial pressures of CO(2) and H(2)O for all TMO and TM(OH)(2) systems studied here.     

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).     

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.