Synthesis of Mg–Al–Fe–NO3 layered double hydroxides via a mechano-hydrothermal route

Mg–Al–Fe–NO₃ layered double hydroxides (LDHs) with a constant Mg²⁺/(Al³⁺ + Fe³⁺) molar ratio but varying Al³⁺/Fe³⁺ molar ratios were successfully synthesized by a mechano-hydrothermal (MHT) method from Mg(OH)₂, Al(OH)₃ and Fe(NO₃)₃·9H₂O or Mg(NO₃)₂·6H₂O as starting materials. The resulting LDHs (MHT-LDHs) were characterized by XRD, TEM, SEM, FT-IR, and zeta potential, size distribution and specific surface area analyses. It was found that pre-milling played a key role in the LDH formation during subsequent hydrothermal treatment. The MHT route is advantageous in terms of low reaction temperature compared with the conventional hydrothermal method, and the target products are of high crystallinity and good dispersion compared with the conventional mechanochemical (MC) method. The MHT-LDHs had higher specific surface area and zeta potential, and lower hydrodynamic diameter than LDHs obtained by MC method (MC-LDHs). Furthermore, the removal of Cr(VI) from aqueous solutions using the LDHs was examined, showing that the MHT-LDHs are of higher removal efficiency than MC-LDHs for the heavy metal pollutant.     

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.     

Comparative Study of Mg/M(III) (M=Al, Ga, In) Layered Double Hydroxides Obtained by Coprecipitation and the Sol-Gel Method

Various layered double hydroxides (LDHs) consisting of magnesium and a trivalent metal (Al, Ga or In) in an Mg/M(III) ratio of 3 were prepared by precipitation from the corresponding nitrates and also from magnesium ethoxide and the acetylacetonates of the trivalent metals using the sol-gel method. The six LDHs thus obtained were calcined at 500°C. All solids were characterized by XRD and IR spectroscopy prior to and after calcination. Their textural properties were determined from nitrogen adsorption measurements and their surface chemical properties by CO₂ chemisorption.     

Effect of hydrothermal treatment on properties of Ni-Al layered double hydroxides and related mixed oxides

The Ni-Al layered double hydroxides (LDHs) with Ni/Al molar ratio of 2, 3, and 4 were prepared by coprecipitation and treated under hydrothermal conditions at 180 °C for times up to 20 h. Thermal decomposition of the prepared samples was studied using thermal analysis and high-temperature X-ray diffraction. Hydrothermal treatment increased significantly the crystallite size of coprecipitated samples. The characteristic LDH diffraction lines disappeared completely at ca. 350 °C and a gradual crystallization of NiO-like mixed oxide was observed at higher temperatures. Hydrothermal treatment improved thermal stability of the Ni2Al and Ni3Al LDHs but only a slight effect of hydrothermal treatment was observed with the Ni4Al sample. The Rietveld refinement of powder XRD patterns of calcination products obtained at 450 °C showed a formation of Al-containing NiO-like oxide and a presence of a considerable amount of Al-rich amorphous component. Hydrothermal aging of the LDHs resulted in decreasing content of the amorphous component and enhanced substitution of Al cations into NiO-like structure. The hydrothermally treated samples also exhibited a worse reducibility of Ni²⁺ components. The NiAl₂O₄ spinel and NiO still containing a marked part of Al in the cationic sublattice were detected in the samples calcined at 900 °C. The Ni2Al LDHs hydrothermally treated for various times and related mixed oxides obtained at 450 °C showed an increase in pore size with increasing time of hydrothermal aging. The hydrothermal treatment of LDH precursor considerably improved the catalytic activity of Ni2Al mixed oxides in N₂O decomposition, which can be explained by suppressing internal diffusion effect in catalysts grains.     

Green phenol hydroxylation by ultrasonic-assisted synthesized Mg/Cu/Al-LDH catalyst with different molar ratios of Cu2+/Mg2+

Nitrate-intercalated Mg/Cu/Al-layered double hydroxides (LDHs) were successfully synthesized by the co-precipitation method under ultrasonic irradiation as a fast, simple, and low-cost technique. The LDHs were synthesized with six different molar ratios of Cu²⁺/Mg²⁺, and then they were characterized by FT-IR, XRD, TGA, ICP, BET, TEM, and FE-SEM analyses. The results showed that the nitrate ions were well intercalated between layers without any carbonate ions. According to the XRD and TGA results, increasing of Cu ions in the LDHs lowered the crystallization and improved the thermal stability of samples at the same time. The morphological studies carried out by FE-SEM and TEM analyses showed the morphological structure similar to the lamellar structure and plate-like shape particles. However, the surface property of binary LDHs, including Al and only one of Cu or Mg elements, was better than ternary ones. Furthermore, this catalyst was used for the phenol hydroxylation using H₂O₂ as oxidant and water as a solvent. The results exhibit that the CuMg₃₂Al-NO^3? catalyst had the best catalytic activity, as well as it was found that the catalyst is active under mild reaction conditions such as the temperature of 65 °C, phenol: oxidant ratio of 2:1, phenol/catalyst?=?100, and reaction time of 1 h. Thus, these conditions gave better activity with the conversion of 27%, the selectivity of 92.05% for catechol and hydroquinone, and CAT/HQ ratio of 1.5.

     

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.     

5-磺基水杨酸辅助水热法制备Co-Ni层状双氢氧化物及其超电容性能

采用5-磺基水杨酸(SSA)辅助水热法制备了Co-Ni层状双氢氧化物(CoxNi1-xLDHs),通过X射线衍射(XRD)、傅里叶变换红外(FT-IR)光谱、场发射扫描电镜(FE-SEM)和透射电镜(TEM)研究其结构与形貌.结果表明,通过调控反应体系中Co、Ni的配比,可以得到不同形貌的CoxNi1-xLDHs.在Co的摩尔分数为0.24时,形成了由纳米片组装的花瓣状纳米球.循环伏安和恒流充放电测试表明,这种花瓣状结构的双氢氧化物在1A·g-1电流密度下,比电容值达到1735F·g-1.     

Thermal transformations of Cu–Mg (Zn)–Al(Fe) hydrotalcite-like materials into metal oxide systems and their catalytic activity in selective oxidation of ammonia to dinitrogen

Layered double hydroxides (LDHs) containing Mg²⁺, Cu²⁺ or Zn²⁺ cations in the Me^II positions and Al³⁺ and Fe³⁺ in the Me^III positions were synthesized by co-precipitation method. Detailed studies of thermal transformation of obtained LDHs into metal oxide systems were performed using high temperature X-ray diffraction in oxidising and reducing atmosphere, thermogravimetry coupled with mass spectrometry and temperature-programmed reduction. The LDH samples calcined at 600 and 900 °C were tested in the role of catalysts for selective oxidation of ammonia into nitrogen and water vapour. It was shown that all copper congaing samples presented high catalytic activity and additionally, for the Cu–Mg–Al and Cu–Mg–Fe hydrotalcite samples calcined at 600 °C relatively high stability and selectivity to dinitrogen was obtained. An increase in calcination temperature to 900 °C resulted in a decrease of their catalytic activity, possibly due to formation of well-crystallised metal oxide phases which are less catalytically active in the process of selective oxidation of ammonia.     

Structure and photoluminescence of Mg-Al-Eu ternary hydrotalcite-like layered double hydroxides

A series of Mg-Al-Eu ternary hydrotalcite-like layered double hydroxides (LDHs), with Eu/Al atomic ratios of ∼0.06 and Mg/(Al+Eu) atomic ratios ranging from 1.3 to 4.0, were synthesized by a coprecipitation method. The Mg-Al-Eu ternary LDHs were investigated by various techniques. X-ray diffraction (XRD) results indicated that the crystallinity of the ternary LDHs was gradually improved with the increase of Mg²⁺/(Al³⁺+Eu³⁺) molar ratio from 1.3/1 to 4/1, and all the samples were a single phase corresponding to LDH. The photoluminescent (PL) spectra of the ternary Mg-Al-Eu LDHs were described by the well-known ⁵D₀-⁷F_J transition (J=1, 2, 3, 4) of Eu³⁺ ions with the strongest emission for J=2, suggesting that the host LDH was favorable to the emissions of Eu³⁺ ions. The asymmetry parameter (R) relevant to ⁵D₀-⁷F_J transition (J=1, 2) dependant of the atomic ratios of Mg²⁺/(Al³⁺+Eu³⁺) was discussed, and was consistent with the result of XRD.     

NMR Crystallography Reveals Carbonate Induced Al-Ordering in ZnAl Layered Double Hydroxide

Layered double hydroxides (LDHs) serve a score of applications in catalysis, drug delivery, and environmental remediation. Smarter crystallography, combining X-ray diffraction and NMR spectroscopy revealed how interplay between carbonate and pH determines the LDH structure and Al ordering in ZnAl LDH. Carbonate intercalated ZnAl LDHs were synthesized at different pH (pH 8.5, pH 10.0, pH 12.5) with a Zn/Al ratio of 2, without subsequent hydrothermal treatment to avoid extensive recrystallisation. In ideal configuration, all Al cations should be part of the LDH and be coordinated with 6 Zn atoms, but NMR revealed two different Al local environments were present in all samples in a ratio dependent on synthesis pH. NMR-crystallography, integrating NMR spectroscopy and X-ray diffraction, succeeded to identify them as Al residing in the highly ordered crystalline phase, next to Al in disordered material. With increasing synthesis pH, crystallinity increased, and the side phase fraction decreased. Using ¹H−¹³C, ¹³C−²⁷Al HETCOR NMR in combination with ²⁷Al MQMAS, ²⁷Al-DQ-SQ measurements and Rietveld refinement on high-resolution PXRD data, the extreme anion exchange selectivity of these LDHs for CO₃²⁻ over HCO₃⁻ was linked to strict Al and CO₃²⁻ordering in the crystalline LDH. Even upon equilibration of the LDH in pure NaHCO₃ solutions, only CO₃²⁻ was adsorbed by the LDH. This reveals the structure directing role of bivalent cations such as CO₃²⁻ during crystallization of [M²⁺₄M³⁺₂(OH)₂]²⁺[A²⁻]₁⋅yH₂O LDH phases.     

Mg-Ni-Al-EDTA柱撑LDHs层状材料的水热合成、结构及性能

以Mg(NO3)2.6H2O、Ni(NO3)2.6H2O、Al(NO3)3.9H2O和[CH2N(CH2COOH)2]2为原料,采用水热合成法,合成了Mg-Ni-Al三元EDTA柱撑LDHs层状材料。采用ICP、元素分析仪、XRD、FTIR、TG-DSC、SEM等手段对样品进行了表征。探讨了pH值、反应温度、反应时间和原料配比对EDTA柱撑LDHs材料合成的影响。结果表明,在pH=8、反应温度控制在140℃、反应时间为24 h时,可以合成出结构规整、晶形良好、各层间排列紧密有序的含不同比例金属阳离子的EDTA柱撑LDHs材料。Mg-Ni-Al三元EDTA柱撑LDHs层状材料通过层间EDTA对Co2+的螯合作用,可以在较短时间内吸附溶液中的Co2+,去除率在97%以上。pH值、吸附时间、吸附温度、固体投加量及初始Co2+浓度对去除率均有不同程度的影响。     

Calcined Mg-Al Layered Double Hydroxide as a Heterogeneous Catalyst for the Synthesis of Urea Derivatives from Amines and CO2

The calcined Mg‐Al layered double hydroxides (LDHs) with a Mg/Al molar ratio of 3:1 were synthesized and characterized thoroughly by X‐ray diffraction (XRD), temperature‐programmed desorption (TPD) of CO₂, and thermogravimetric analysis (TGA). Thus the calcined Mg‐Al LDHs were used as catalyst for the catalytic synthesis of disubstituted ureas from amines and CO₂. The effects of reaction time, reaction temperature, pressure, solvent and calcined temperature on activity have been investigated. The results indicated that aliphatic amines, cyclohexylamine and benzylamine can be converted to the corresponding ureas selectively over the calcined Mg‐Al LDHs catalysts with N‐methyl‐2‐pyrrolidone (NMP) as solvent without using any dehydrating regent. The catalyst can be recycled several times with only slight loss of activity.     

Layered double hydroxides intercalated with borate anions: Fire and thermal properties in ethylene vinyl acetate copolymer

Fire and thermal properties of ethylene vinyl acetate (EVA) composites prepared by melt blending with layered double hydroxides (LDH) have been studied. Two types of LDHs intercalated with borate anion were prepared using the coprecipitation method and the metals Mg²⁺, Zn²⁺ and Al³⁺. Characterization of the LDHs and the EVA composites was performed using X-ray diffraction, thermogravimetric analysis, and cone calorimetry. Thermal analyses show that the addition of LDHs improves the thermal stability of EVA. Fire properties evaluated using the cone calorimeter were significantly improved in the EVA/LDH composites. The peak heat release rate was reduced by about 40% when only 3% by weight of the LDH was added to the copolymer. Comparison of the fire properties of the LDHs with those of aluminum trihydrate (ATH), magnesium hydroxides (MDH), zinc hydroxide (ZH) and their combinations at 40% loading, reveal that the LDHs were more effective than when MDH and ZH are used alone.     

Synthesis of heptamolybdate‐intercalated MgAl LDHs and its application in polyurethane elastomer

Heptamolybdate (Mo₇O₂₄^6?) was intercalated in the interlayer space between MgAl‐layered double hydroxides (Mo‐MgAl LDHs) by the hydrothermal and ion exchange method, and then polyurethane elastomer (PUE) based composites were prepared by the prepolymerization method with different amounts of Mo‐MgAl LDHs. X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, laser Raman spectroscopy (LRS), and scanning electron microscopy (SEM) were employed to characterize the obtained LDHs. The performance of the PUE/LDHs were evaluated by measuring their thermal gravimetric, heat release rate (HRR), and smoke density (Ds). The results show that PUE/LDH composites exhibit a lower peak heat release rate (pk‐HRR), Ds, and a prolonged combustion time, in comparison with neat PUE. Comparison between NO₃‐MgAl LDHs and Mo‐MgAl LDHs containing composites show that the introduction of Mo⁶⁺ is able to facilitate flame retardance and smoke suppression efficiency, which results mainly from the presence of MoO₃ derived from the decomposition of Mo₇O₂₄^6? intercalated LDHs. Mo‐MgAl LDHs reduce the pk‐HRR of composites by 39% with only 1 wt.% content, and the maximum Ds of composites is reduced to a minimal value of 274 with 10 wt.% Mo‐MgAl LDHs. More importantly, LDHs would improve the mechanical properties at a low content. The experimental results reveal the potential of Mo₇O₂₄^6? intercalated LDHs to improve both the flame retardancy and smoke suppression of PUE. Copyright © 2015 John Wiley & Sons, Ltd.     

Solution decomposition of the layered double hydroxide (LDH) of Zn with Al

The layered double hydroxides (LDH) of Zn with Al containing intercalated CO₃²⁻ and NO₃⁻ ions undergo solution decomposition to yield a highly crystalline oxide mixture comprising ZnO and ZnAl₂O₄ at temperatures as low as 150–180 °C under hydrothermal conditions. In contrast solid-state decomposition takes place at a much higher temperature (240–315 °C) in air. Solution decomposition is not only guided by the low octahedral crystal field stabilization energy of Zn²⁺ ions, a factor that also affects solid-state decomposition, but also by solubility considerations. The LDHs of Mg and Ni with Al do not undergo solution decomposition.     

Site Preference of Cations and Structural Variation in MgAl2–xGaxO4 (0 ≤ x ≤ 2) Spinel Solid Solution

The crystal structures of MgAl_2–xGa_xO₄ (0 ≤ x ≤ 2) spinel solid solutions (x = 0.00, 0.38, 0.76, 0.96, 1.52, 2.00) were refined using ²⁷Al MAS NMR measurements and single crystal X‐ray diffraction technique. Site preferences of cations were investigated. The inversion parameter (i) of MgAl₂O₄ (i = 0.206) is slightly larger than given in previous studies. It is considered that the difference of inversion parameter is caused by not only the difference of heat treatment time but also some influence of melting with a flux. The distribution of Ga³⁺ is little affected by a change of the temperature from 1473 K to 973 K. The degree of order‐disorder of Mg²⁺ or Al³⁺ between the fourfold‐ and sixfold‐coordinated sites is almost constant against Ga³⁺ content (x) in the solid solution. A compositional variable of the Ga/(Mg + Ga) ratio in the sixfold‐coordinated site has a constant value through the whole compositional range: the ratio is not influenced by the occupancy of Al³⁺. The occupancy of Al³⁺ is independent of the occupancy of Ga³⁺, though it depends on the occupancy of Mg²⁺ according to thermal history. The local bond lengths were estimated from the refined data of solid solutions. The local bond length between specific cation and oxygen corresponds with that expected from the effective ionic radii except local Al–O bond length in the fourfold‐coordinated site and local Mg–O bond length in the sixfold‐coordinated site. The local Al–O bond length in the fourfold‐coordinated site (1.92 Å) is about 0.15 Å longer than the expected bond length. This difference is induced by a difference in site symmetry of the fourfold‐coordinated site. The nature that Al³⁺ in spinel structure occupies mainly the sixfold‐coordinated site arises from the character of Al³⁺ itself. The local Mg–O bond length in the sixfold‐coordinated site (2.03 Å) is about 0.07 Å shorter than the expected one. Difference Fourier synthesis for MgGa₂O₄ shows a residual electron density peak of about 0.17 e/ų in height on the center of (Ga_0.59 Mg_0.41)–O bond. This peak indicates the covalent bonding nature of Ga–O bond on the sixfold‐coordinated site in the spinel structure.     

甘氨酸存在条件下水热合成可层离双氢氧化物

Exfoliative Mg/Al layered double hydroxide (Mg/Al-LDHGly) was obtained via hydrothermal synthesis in the presence of glycine. The product prepared by hydrothermal reaction for 10 h at 120 ℃ possesses high thermal stability and maximal crystallite size in a, c directions. TEM and SEM analyses show that Mg/AlLDHGly was of well-crystallized hexagonal product with stacks of slightly curved layers. Benefiting from mechanism investigation on its gradual delamination in formamide, rapid delamination of Mg/Al-LDHGly at room temperature was realized, which provided fundamental for preparation of (Mg/Al-LDHGly)/polymer nanocomposites by using exfoliation-adsorption method.     

Hydrothermal Synthesis of Glutamic Acid/Layered Double Hydroxide Compounds

In this article, glutamic acid (Glu) was intercalated into the Zn-Al layered double hydroxides (Zn-Al LDHs) by hydrothermal method. The influence of the molar ratio of Zn/Al/Glu (R values for short), the aging time, and the pH value of the washing solution was systematically studied. The resulting Glu-LDH compounds were characterized by x-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), and scanning electron micrograph (SEM). The XRD results showed that the relative diffraction intensity first increased then decreased as the R value and the pH value of the washing solution increased. Moreover, the crystallinity degree decreased gradually with the increase of the aging time. The absorption peaks attributed to the stretch of (R-COO^?) were observed in the FTIR spectra. SEM graphs indicated that the morphology of the particles was largely influenced by the synthesis conditions.     

Synthesis of NaX and NaY Zeolites from Tunisian Kaolinite as Base Catalysts: An Investigation of Knoevenagel Condensation

The synthesis of Faujasite‐type zeolites with high purity has been successfully performed from Tunisian kaolinite and the effects of different crystallization parameters on the final products were widely investigated. The alkaline fusion of kaolinite followed by hydrothermal treatment lead to zeolite NaX synthesis whereas the classic hydrothermal transformation of metakaolinite produces NaY zeolite. The results show that an increase in the synthesis temperature and time has improved the crystallization process of the zeolite NaX whereas the SiO₂/Al₂O₃ and the Na₂O/SiO₂ molar ratios were the key parameters to obtain a pure zeolite NaY. The highest specific surface areas obtained with the optimal crystallization conditions were 554 m2 g^?1 and 592 m2 g^?1 for respectively NaX and NaY zeolites. The basic properties of NaX and NaY zeolites were explored in the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate at 140 °C as a test reaction in the absence of solvent. The influence of ion exchange with cesium cation on the catalytic activity of prepared catalysts was also investigated. It was found that the NaX provided higher activity than that of NaY catalyst due to its lower Si/Al ratio whereas a cesium exchange conferred higher basicity to the prepared Na‐faujasite.     

The Affinity of Gallium(III) and Indium(III) for Nitrogen Donor Ligands

Abstract

Dedicated to Professor Arthur Martell on the occasion of his seventy fifth birthday.

The complexes of In(III) and Ga(III) with a variety of nitrogen donor ligands were studied in aqueous solution by glass electrode potentiometry at 25°C in 0.1 M NaNO₃. The ligands were 2-aminomethylpyri-dine (AMPY), ethylenediamine (EN), N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine (THPED), and N,N-bis(2-hydroxyethyl)glycine (BICIN). A variety of mixed ligand complexes of the MLOH type were detected with many of the above ligands as L. The logK₁ values obtained were with Ga(III) 8.40 (AMPY), 7.94 (THPED) 12.72 (EN), and In(III) 7.6 (AMPY), 8.20 (THPED), and 7.06 (BICIN). These formation constants are discussed in relation to previous predictions that In(III) and Ga(III) would have a substantial chemistry with nitrogen donor ligands. Of particular interest is the Ga(III) system with EN, where a very stable Ga(EN)³⁺ complex is formed, but no higher complexes except for hydrolyzed species such as Ga(EN)OH²⁺ and Ga(EN)(OH)₂ ⁺.