Mg-Al Hydrotalcite Research Articles

Heterogeneous acylation of phenol with acetic anhydride over hydrotalcite-like compounds

Liquid phase acylation of phenol with acetic anhydride was studied over the mixed oxides obtained from Mg-Al, Ni-Al, and Cu-Al hydrotalcites (HTs). The mixed oxides were characterized by XRD and SEM. The main acylation products of phenol over these oxide catalysts are phenylacetate and o-hydroxyacetophenone.

Effect of Temperature on Microstructure and Cr(VI) Adsorption Capacity of MgAl Metal Oxides

The adsorption properties of Cr(VI) on the double oxides(LDO) from calcined MgAl hydrotalcites(LDH) were investigated. The effect of temperature on the adsorption performances of the LDO was studied,and the adsorption kinetics and thermodynamics were studied at different adsorption temperatures. The samples were characterized by XRD and FT-IR to clarify the adsorption mechanism.The results indicated that CrO4 2– had intercalated into the interlayer structures of Mg-Al hydrotalcites. Moderately increasing temperature was beneficial for the reconstruction of LDH crystalline structure from LDO. In the temperature range of 20℃–60℃,adsorption capacity of the LDO was increased with temperature rising,it reached 91 mg/g in the temperature range from 50℃to 60℃. The experimental data were well fit to the Langmuir equation. The negative values of ΔGo indicated that the adsorption process was spontaneous,and positive values of ΔHo and ΔSo showed the endothermic and entropy-increasing nature of the process. The adsorption was consistent with the pseudo-second order kinetic model. The active energy value was 20.04 kJ/mol,which implying that the internal diffusion was the rate-determining step in the chemical adsorption.

The enhanced activity of Ca/MgAl mixed oxide for transesterification

Catalysts based on CaO/MgAl oxides were prepared by Ca(NO3)2.4H2O impregnation onto MgAl hydrotalcite followed by calcination at 600°C/0.5h. The materials were characterized by means of XRD, N2 physisorption, FEG-SEM equipped with energy-dispersive X-ray spectroscopy (EDS) and TPD of CO2. The oxide reactivities were evaluated using the model transesterification reaction between methyl acetate and ethanol under mild reaction conditions (50°C, ethanol/methyl acetate molar ratio=6/1 and 4wt.% of catalyst). MgAl mixed oxide was inactive for transesterification. On the other hand, the Ca addition onto the oxides considerably increased the base site amounts and strengths of the Ca/MgAl, greatly improving their catalytic performances. Remarkably high conversions were obtained for 40-Ca/MgAl compared to commercial CaO. Thus, the CaO dispersion over MgAl mixed oxide showed to significantly improve the catalytic performance of CaO. Stability tests revealed that the Ca/MgAl can be reused for at least 5 reaction cycles and the reaction occurs through a heterogeneous process. Due to the high activity, this type of catalyst can be pointed as a promising heterogeneous catalyst for biodiesel production although the Ca leaching must be formerly resolved for practical use.

Structure-activity correlations of LiNO3/Mg4AlO5.5 catalysts for glycerol carbonate synthesis from glycerol and dimethyl carbonate

MgAl hydrotalcite synthesized via co-precipitation method from magnesium and aluminum nitrates was calcined to produce Mg4AlO5.5 mixed oxides. The oxides were modified by introducing LiNO3 and applied to the synthesis of glycerol carbonate from glycerol and dimethyl carbonate. The research results indicated that LiNO3/Mg4AlO5.5 was an efficient catalyst for the synthesis of glycerol carbonate. The full conversion of glycerol and 96.28% yield of glycerol carbonate were obtained after reacting at 80°C for 1.5h in the presence of LiNO3/Mg4AlO5.5. The structure and properties of the catalysts were studied by means of XRD, TGA, BET, CO2-TPD and Hammett indicator method. It was found that the basic strength and the basicity of the catalysts were enhanced after addition of LiNO3. Over Mg4AlO5.5 oxides, LiNO3 was easily converted to LiAlO2 which constituted new strong basic sites. Too high basic strength and basicity of catalysts may improve the conversion of glycerol at the cost of GC selectivity reduction.

Transesterification of glycerol with dimethyl carbonate over Mg-Al hydrotalcites

Glycerol carbonate (GC) was synthesized via the catalytic transesterification of glycerol with dimethyl carbonate over a series of Mg-Al hydrotalcites with different Mg/Al ratios (0.5–6) under mild conditions. All catalysts exhibited a high selectivity toward GC. The sample with Mg/Al = 2 exhibited 97.1% of selectivity toward GC and 66.9% of glycerol conversion at 70 °C for 3 h. Characterization results indicated that the activity of calcined Mg-Al hydrotalcites depended mainly on the surface area, pore diameter, and crystallinity.

Nanostructured Mg-Al hydrotalcite as catalyst for fine chemical synthesis.

This paper reviews the recent research of nanostructured Mg-Al hydrotalcite (Mg-Al HT) and its application as an efficient solid base catalyst for the synthesis of fine chemicals. Mg-Al HT has many beneficial features, such as low cost, selectivity, catalytic properties, and wide range of preparation and modification methods. They hold promise for providing sought-after, environmentally friendly technologies for the 21st century. Replacement of currently used homogeneous alkaline bases for the synthesis of fine chemicals by a solid catalyst can result in catalyst re-use and waste stream reduction. We introduce briefly the structure, properties and characterization of the nanostructured Mg-Al HT. The efficacy and benign applications of Mg-Al HT as an alternative solid base to homogenous catalysts in the synthesis of fine chemicals are then reviewed. The challenges for the future applications of Mg-Al HT in the synthesis of fine chemicals in terms of green protocol processes are discussed.

Composite Gel Polymer Electrolytes Containing Layered Mg-Al Hydrotalcite for Quasi-Solid Dye-Sensitized Solar Cells

A composite quasi-solid electrolyte is prepared by adding a layered Mg-Al hydrotalcite (LMAH) into the iodide/triiodide quasi-solid electrolyte including 4-tert-butylpyridine, which obviously improves the photovoltaic properties of quasi-solid dye-sensitized solar cells (DSSCs). When adding 5 wt% LMAH, the maximum photovoltaic conversion efficiency of DSSC is obtained, which reaches almost three times as large as that without LMAH. This enhancement effect is primarily explained by studying the nyquist spectrums, limiting current density, diffusion coefficient and photovoltaic conversion efficiency.

Aerobic oxidation of benzyl alcohol over Co3O4/rehydrated hydrotalcite catalysts: The promotional effect of hydrotalcite support

Co3O4/rehydrated hydrotalcite catalysts were prepared, characterized, and tested in the aerobic oxidation of benzyl alcohol, with an aim to investigate the effect of hydrotalcite supports. It was found that Mg-Al hydrotalcite support promoted the activity of Co3O4. The activity of Co3O4/rehydrated hydrotalcite was sensitive to its hydrotalcite composition in terms of metal cations. The effect of Zn and Fe cations was negative, whereas the effect of Cu cations was positive. The results demonstrated that the promoting effect of hydrotalcite was due to OH− ions of the crystalline network.

Aldol condensation of furfural and acetone over Mg[sbnd]Al layered double hydroxides and mixed oxides

Liquid phase aldol condensation of furfural and acetone catalyzed by solid base catalysts (MgAl hydrotalcites and MgAl mixed oxides) has been investigated as a method to valorize short chain ketones obtainable from biomass pyrolysis for the production of higher-molecular-weight products usable as fuel components. The Mg/Al molar ratios of the investigated catalysts was varied in the range from 2 to 4 and their catalytic activity was tested at different reaction temperatures (20–100°C) using several activation methods. MgAl hydrotalcites were used either after calcination as mixed oxides or after subsequent contact with liquid water or steam as rehydrated materials.Both furfural-acetone condensation products, i.e. C8 and C13, were the desired products of aldol condensation reaction. Higher reaction temperature facilitated the dehydration step and enhanced the catalyst selectivity to both dehydrated products (C8 and C13). The best results were achieved with calcined catalyst sample having Mg/Al molar ratio equal to 3 at 100°C (>95% furfural conversion and >90% selectivity to the desired products). The selectivity to the main by-product, diacetone alcohol, did not exceed 5% in any experiment. The ex situ rehydration of the calcined samples resulted in catalysts with a significantly lower activity, except the catalyst with Mg/Al molar ratio equal to 2. On the other hand, in situ rehydration caused catalyst activity improvement only in the case of the sample with Mg/Al molar ratio 3.

Catalytic Degradation of Rhodamine B with Cobalt Phthalocyanine Supported Hydrotalcites

Rhodamine B is one of biodegradable azo dyes. In this thesis, the CoPc-LDHs were prepared by applying cobalt phthalocyanine on the Mg-Al hydrotalcites, and catalytic oxidation decolorization of rhodamine B (RhB) was studied. The results shown that compared with H2O2, CoPc-LDHs play a key role in catalytic oxidation of RhB. The optimal conditions about CoPc-LDHs/H2O2 system were H2O2 (30% w/w) 1.50 mL, pH 7 and 62.5°C. The catalytic oxidation decolorization effect of RhB reduced with the increase of initial concentration of RhB.

Enhanced corrosion resistance of MgAl hydrotalcite conversion coating on aluminum by chemical conversion treatment

The effects of the chemical conversion treatment in 8-hydroxyquinolate salt on the structure and corrosion resistance of MgAl hydrotalcite conversion coating on aluminum substrate have been investigated by Scanning electron microscopy (SEM), Energy-dispersive X-ray (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and electrochemical measurements. The results show that the microstructure of MgAl hydrotalcite coating is preserved, while the corrosion resistance of Al substrate is effectively enhanced after the chemical conversion treatment. The chemical conversion treatment results in the intercalation of 8-hydroxyquinolate anions into the coating, which induces the further decrease of corrosion current density of not converted sample by one order of magnitude after immersion in 3.5wt.% NaCl solution. It provides a facile way to prepare environmentally friendly conversion coating for corrosion protection of light metal and alloys.

Adsorption Behavior in Dual-solutes of Cr(Ⅵ) and p-Cresol by Mg-Al Hydrotalcite Compounds

The adsorption behavior of Cr(Ⅳ) on Mg-Al hydrotalcite compounds(Mg-Al HTlc) was investigated in the metal-organic pollutant system of Cr(Ⅵ) and p-cresol.The results suggest that in the adsorption of Cr(Ⅵ) on HTlc is in the pseudo-second-order reaction and follows Langmuir isothermal adsorption equation;p-cresol can reduce the adsorption velocity but can increase the adsorption capacity of Mg-Al HTlc to adsorb Cr(Ⅵ) from water.Cr(Ⅵ) and p-cresol exhibit a competitive effect.On the one hand,the competitive effect is dependent to the concentration ratio of p-cresol to Cr(Ⅵ),the adsorption capacity of Mg-Al HTlc toward Cr(Ⅵ) can be increased by decreasing the pH value due to the adsorption of p-cresol on HTlc.The competitive ability of p-cresol decreases relatively with the increase of Cr(Ⅵ) concentration and the removal rate of Cr(Ⅵ) increases significantly when the mass concentration ratio of Cr(Ⅵ) to p-cresol is larger than 3.In addition,the adsorption capacity decreases with increasing temperature.

Aminobenzoate modified Mg[sbnd]Al hydrotalcites as a novel smart additive of reinforced concrete for anticorrosion applications

A carbonate form of MgAl hydrotalcite, Mg(2)Al-CO3 and its p-aminobenzoate (pAB) modified derivative, Mg(2)Al-pAB, were synthesized and characterized by means of XRD, FT-IR and TG/DSC. The anticorrosion behavior of Mg(2)Al-pAB was evaluated based on open circuit potential (OCP) of carbon steel in simulated concrete pore solution and on chloride-exchange experiments. The results reported in this paper demonstrate that ion-exchange indeed occurred between chlorides and intercalated pAB anions, thereby reducing the free chloride concentration in simulated concrete pore solution. The simultaneously released pAB anions were found to exhibit the envisioned inhibiting effect and caused corrosion initiation of the steel to shift to higher chloride concentrations than without the modified hydrotalcites.

Hydrothermal synthesis of corrosion resistant hydrotalcite conversion coating on AZ91D alloy

MgAl hydrotalcite conversion coating was synthesized in situ on AZ91D alloy by hydrothermal treatment in ammonia nitrate aqueous solution. The structure and property of hydrotalcite conversion coating were investigated by Scanning electron microscopy (SEM), Energy-dispersive X-ray (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and electrochemical measurements. The results show that the hydrothermally synthesized hydrotalcite conversion coating effectively confers good corrosion resistance on AZ91D alloy. The corrosion current density of treated AZ91D alloy is decreased by one order of magnitude as compared to untreated one. It provides a facile way to synthesize chromium-free conversion coating for the corrosion protection of magnesium–aluminum alloy.

On the role of the activation procedure of supported hydrotalcites for base catalyzed reactions: Glycerol to glycerol carbonate and self-condensation of acetone

Bulk and carbon nanofiber supported MgAl hydrotalcites have been investigated as solid base catalysts for the synthesis of glycerol carbonate and dicarbonate and for the self-condensation of acetone. The supported materials exhibited a 300 times higher activity compared to bulk activated hydrotalcites for the transesterification. The materials could be reused while maintaining a high yield of glycerol carbonate. The activity of the mixed oxide, i.e., hydrotalcite materials after high temperature activation was considerably higher than both gas-phase and liquid phase rehydrated samples for the glycerol reactions. In contrast in the acetone self-condensation reaction the rehydrated samples were more active. This indicates that the polarity of the catalyst related to the reactant properties has a huge impact on the performance of a solid base catalyst.

Influence of pH on U(VI) Adsorption using a Thermally-Treated Mg-Al Hydrotalcite and a Natural Zeolite in a Batch System

The capacity of a calcined hydrotalcite and a natural zeolite to remove U(VI) chemical species from both acidic and basic aqueous systems has been investigated. The U(VI) uptake by the calcined hydrotalcite and the natural zeolite was determined to be from 5 to 200 µmolmL−1 and from 2 to 10 µmolmL−1, respectively. Uranium was measured using UV-Vis spectroscopy in the remaining solutions. The U(VI) adsorption of both materials was higher in the acidic aqueous system. Independent of the pH of the aqueous media, the [UO2(CO3)3]4− species play an important role in the regeneration of the original crystalline structure of the HT.

Transesterification of tributyrin with methanol over MgAl mixed oxides derived from MgAl hydrotalcites synthesized in the presence of glucose

A series of MgAl hydrotalcites (HTs) was synthesized in the presence of glucose and used as the precursors for the preparation of MgAl mixed oxides. The as-synthesized MgAl HTs and their corresponding MgAl mixed oxides were characterized by XRD, TG, SEM, N2-adsorption/desorption, FTIR, Raman spectroscopy, ICP and CO2 TPD. The results show that MgAl mixed oxides derived from MgAl HTs prepared in the different synthesis systems have different textural characteristics, basic properties, and catalytic performances. The glucose in MgAl HT synthesis system was transformed into amorphous carbon during the process of MgAl HT crystallization, and the in-situ formed highly dispersed carbon in the as-synthesized MgAl HT matrices can act as a mesoporous template. MgAl mixed oxides derived from MgAl HTs prepared in the presence of glucose have higher surface areas, more uniform mesoporous pores, stronger basic sites as well as higher activities in transesterification of tributyrin with methanol than that from MgAl HT prepared in the absence of glucose.

Solvent-free oxidation of aldehydes to acids by TBHP using environmental-friendly ${\rm{MnO}}_{\bf{4}}^{\bf{-1}}$ -exchanged Mg-Al hydrotalcite catalyst

A number of hydrotalcite (Mg–Al, Mn–Al, Co–Al, Ni–Al, Mg–Fe, Mg–Cr and Cu–Al) catalysts, with or without MnO\(_{4}^{-1}\)-exchange, were evaluated for their performance in the solvent-free oxidation of benzaldehyde to benzoic acid by tert-butyl hydroperoxide under reflux in the absence of any solvent. The MnO\(_{4}^{-1}\)-exchanged Mg–Al-hydrotalcite (Mg/Al = 10) showed high activity in the oxidation of different aromatic and aliphatic aldehydes to their corresponding acids and also showed excellent reusability in the oxidation process which is environmental-friendly. A number of hydrotalcite catalysts, with or without MnO\(_{4}^{-1}\)-exchange, were evaluated for their performance in the solvent-free oxidation of benzaldehyde to benzoic acid by tert-butyl hydroperoxide. The catalyst is environmentalfriendly because of its excellent reusability.

Biodiesel production over thermal activated cerium modified Mg-Al hydrotalcites

Abstract Heterogeneous basic catalysts can be used for biodiesel production. The performances of several types of such materials are described in the literature but small attention is given to its deactivation in reaction medium. This paper put in evidence that catalysts are not static identities in the reaction medium, and relates the physical characteristics of fresh and post reaction samples with deactivation mechanisms. Catalysts, tested during the methanolysis of soybean oil, were prepared by thermal activation of Ce modified Mg-Al hydrotalcites (Mg/Al = 3, Ce/Mg 0.03 (atomic ratio) presented FAME yields higher than 90%. Thermogravimetry analysis of post reaction catalysts evidenced that cerium influences the reaction mechanism. The post reaction catalysts evidenced that surface restructuring occurs during reaction with sizeable change of the elements distribution which as beneficial effect on the catalytic performances.

Study of a Continuous Process of Glycerolysis of Rapeseed Oil with the Solid Base Catalysts

Glycerolysis of rapeseed oil over several solid bases using fixed bed flow reactor has been studied. It was found that Mg-Al hydrotalcite and Al2O3 supported CaO- ZnO demonstrate high activity in this reaction. The yield of monoglycerides reaches 70 % with 96 % oil conversion at 513 K. It was shown that pores size of a catalyst is an important factor of intensificati on of the glycerolysis process. It was found that turpentine additive decreases the reaction temperature due to higher solubility of glycerol in oil.