In the present work, Mg–Al–Fe layered double hydroxides were prepared by coprecipitation reaction with hydrothermal treatment. The characterization of precursors and their corresponding calcinated ...products (mixed oxides) were carried out by X ray diffraction, X-ray photoelectron spectroscopy (XPS), termogravimetric analysis and differential scanning calorimetry, diffuse reflectance UV–vis spectroscopy, specific surface area, Mössbauaer and magnetic properties. The Fe3+ species were observed in tetrahedrally and octahedrally coordination in brucite layered. The XPS analysis shows that the Fe3+ ions can be found in two coordination environments (tetrahedral and octahedral) as mixed oxides, and as spinel-structure. Oxides show a decrease in the specific surface areas when the iron loading is increased. The magnetic and Mössbauaer response show that MgAlFe mixed oxides are different behaviours such as different population ratios of ferromagnetic, weak-ferromagnetic, paramagnetic and superparamagnetic phases. The better crystallization of spinel structure with increased temperature, is correlated with the improved magnetic properties.
•Mg–Al–Fe were successfully prepared by coprecipitation with hydrothermal treatment.•MgO, α-Fe2O3, MgFe2O4 were detected by XRD in the calcined samples.•The Fe3+ is in tetrahedral and octahedral coordination in the brucite layered.•The specific surface area is directly related with the iron content.•The magnetic properties and MgFe2O4 improve with increasing calcination temperature.
In the present work, Mg–Al–Zn–Fe layered double hydroxides (LDH) were prepared by coprecipitation reaction with hydrothermal treatment. Their corresponding calcinated products (mixed oxides) were ...used as catalyst for ethylbenzene dehydrogenation to styrene. The characterization of precursors and the mixed oxides was carried out by X-ray diffraction, XPS, TGA-DSC, UV–vis-DRS, specific surface area, and magnetic properties. The Fe3+ species were detected by XPS; they exist in two chemical states related to oxides and spinel environment. Zn is found in two possible contributions, such as ZnO or spinel. XRD data indicate that the hydrotalcite phase exists in all precursor samples, except for the sample without magnesium (HT100). In the mixed oxides the ZnO phase increases with the rise of the Zn content. Oxides show a decrease of surface areas with the increase of the Zn content. Ethylbenzene dehydrogenation was carried out with the mixed oxides synthesized. The HT25 sample with Zn/(Zn + Fe) = 0.25 molar ratio shows a 31.5% conversion and selectivity to styrene superior to 87%, which is directly related to the quantity of oxide, spinel phase and magnesium, as well as the high surface area and the magnetic response obtained.
In the field of biomass valorization, the use of levulinic acid, a platform molecule to obtain products with higher-added value, has attracted research interest in recent years. Therefore, the ...present work proposes the synthesis of levulinic esters by means of catalytic esterification of levulinic acid with methanol, ethanol, and butanol. For this purpose, metal–organic framework UiO-66-NH2 catalysts have been synthesized, with zirconium as the active metal, 2-aminoterephthalic acid as the binding agent, and dimethylformamide as the solvent. In addition, an alternative method to the traditional synthesis is presented, which involves ultrasonic agitation and variations of autoclave time in 24, 18, 12, and 6 h. The catalysts were analyzed by infrared spectroscopy, X-ray diffraction, thermal analyses, acidity measurement with a probe molecule (CO), temperature-programmed desorption of CO2, N2 adsorption–desorption, scanning electron microscopy, microwave plasma atomic emission spectroscopy, and X-ray photoelectron spectroscopy. The catalytic reaction was carried out in a pressure reactor. The products were analyzed by gas chromatography and mass spectrometry. Pressure was optimized to improve the catalytic performance. All samples showed a high catalytic yield, greater than 80%. The M-UT/6 catalyst showed a significant decrease in the yield values toward methyl levulinate after consecutive recycles. This is attributed to the leached Zr and lost BDC-NH2 ligand, propitiated by the high polarity and small size of methanol molecules. In the case of ethyl and butyl levulinate, it was determined that the M-UT/6 catalyst can be reused for three cycles without a considerable loss of yield (∼85–95%).
The massive use of petroleum and its possible exhaustion are driving the current research trend to study alternative raw materials from biomass for organic reactions. In this context, the present ...article presents a study of the catalytic esterification of levulinic acid, a platform molecule, with ethanol. Metal-organic framework (MOF) type compounds UiO-66-NH2 have been synthesized. Zirconium was incorporated, using zirconium chloride as a metal precursor, together with 2-aminoterephthalic acid as an organic binding agent. An alternative route of synthesis was proposed using more favorable conditions from an economic and environmental point of view, replacing dimethylformamide by 50 and 75% acetone as substitute solvent. The physicochemical properties of the materials were evaluated by X-ray diffraction (XRD), Infrared Spectrometry with Fourier Transform (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), microwave plasma atomic emission spectroscopy (MP-AES) and N2 adsorption to understand their morphology, crystalline, chemical and pore structure. The progress of the reaction was followed by gas chromatography and mass spectroscopy. The catalytic activity result of MOF25% in autoclave reactor, showed 100% of selectivity to ethyl levulinate and a turnover number (TON) of 66.18 moles of product/moles of Zr. This good catalytic performance obtained by partial solvent replacement in the synthetic material provides a more economical and eco-friendly process for ethyl levulinate generation.
The boron removal capacity from an aqueous solution using MgAlFe mixed oxides from layered double hydroxides (LDH) was studied. They were synthesized by the coprecipitation method at 70 °C and were ...characterized as potential filter materials. The Fe3+ analyzed by X-ray photoelectron spectroscopy and UV–visible diffuse reflectance showed their tetrahedral and octahedral coordination. Scanning electron microscopy micrographs and thermogravimetric and differential scanning calorimetry analysis evidenced the presence of clusters and particles aggregates and decreased dehydroxylation temperature when the iron content increased. Mixed oxides and boron solution in a ratio of 20:1 Mg/B were put in a batch reactor at different contact times. The borate removal process was due to the memory effect of the mixed oxides and superficial adsorption by electrostatic attraction. This fact is directly related to the specific surface area, Fe content, and surface charge. The maximum boron removals were achieved with the CS25 and CS50 samples with values higher than 85%.
Despite arsenite can be reduced to As(0) and deposited at the surface of solid electrodes such as gold, platinum, or copper, it cannot form amalgams with mercury, and so the addition of other metal ...ions is required for its deposition. In this manuscript, mechanistic analysis of the cathodic stripping square-wave voltammetric response of the system copper‑arsenic is presented. For the analysis of experimental responses, a mathematical model is used to consider that a surface-active reagent undergoes a charge transfer step followed by a chemical reaction. The dependences of the differential peak current, and the respective peak potential and half-peak width on the square-wave frequency were used to estimate the apparent stability constant of arsenide formation and that the electrode reaction would involve the direct transfer of 2 electrons, while the simulation and fit of forward and backward voltammetric responses have been useful for inferring other parameters such as the formal charge transfer rate constant of the global electron transfer reaction, a pseudo-first order homogeneous rate constant associated with arsenide formation, and the charge transfer coefficient of the global electron transfer reaction. Besides the values estimated for each of those parameters, all simulations indicate that the reduction of copper instead of arsenic would be taking place during the cathodic stripping scan.
Accordingly, the trace-analysis reaction of arsenic in the presence of copper would be based on the surface accumulation of a metal complex, where Cu2+ would be the metal cation and an arsenide species the ligand. The stability of that complex at the surface of a mercury electrode would depend not only on the applied potential, but also on the ratio between copper, arsenic, and protons.
•Mechanistic analysis of the CS-SWV response of the system copper-arsenic is presented.•This is the first time that a mathematical model of such complexity is used for simulating experimental voltammograms.•The simulation and fit of the experimental curves are step-by-step described.•Simulations indicate that the reduction of Cu(II) instead of As(0) takes place during the cathodic stripping scan.•A surface analysis is required when the scan-rate affect the apparent reversibility of more than one kinetic constant.
A theoretical model about the voltammetric response of electrode processes complicated by adsorption of reactant and a following coupled chemical reaction of soluble species (E(ad)C(sol) mechanism) ...is presented. The results are focused on reactions studied by square-wave voltammetry (SWV) where the electrochemical and the chemical kinetics affect the voltammetric response. The quasi-reversible maximum is a well-known characteristic of systems that involve adsorbed species and that are assessed by SWV. This characteristic usually helps researchers to estimate the value of ks. In this manuscript some warnings about the overuse of the quasi-reversible maximum are presented. Special attention should be paid for those systems where the quasi-reversible maximum does not have the shape of theoretical curves or where the frequency of the maximum shifts when the pH or another experimental variable is modified. All these outcomes are pointing out the presence of a reaction mechanism more complicated than a simple reaction scheme of an adsorbed reagent that releases a soluble product. This type of E(ad)C(sol) reaction mechanism would explain several aspects associated with the reduction of metal cations complexed with organic ligands. To the best of our knowledge, systems of this complexity have been not modeled so far. In this regard, this manuscript presents one step forward for the deconvolution of these not that simple reactions.
•A theoretical model for adsorptive stripping SWV of an E(ad)C(sol) mechanism is presented.•The electrochemical and chemical reactions are assumed to be quasi-reversible processes.•The effect of the equilibrium constant of the chemical reaction on the voltammetric response is analyzed.•Several SW voltammograms are provided for comparison purposes.•The presence and position of the quasi-reversible maximum is conditioned by the coupled chemical reaction.
At present, the trend towards partial replacement of petroleum-derived fuels by those from the revaluation of biomass has become of great importance. An effective strategy for processing complex ...biomass feedstocks involves prior conversion to simpler compounds (platform molecules) that are more easily transformed in subsequent reactions. This study analyzes the metal–organic frameworks (MOFs) that contain Zr metal clusters formed by ligands of terephthalic acid (UiO-66) and aminoterephthalic acid (UiO-66-NH2), as active and stable catalysts for the esterification of levulinic acid with methanol. An alternative synthesis is presented by means of ultrasonic stirring at room temperature and 60 °C, in order to improve the structural properties of the catalysts. They were analyzed by X-ray diffraction, scanning electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, microwave plasma atomic emission spectroscopy, acidity measurement, and N2 adsorption. The catalytic reaction was carried out in a batch system and under pressure in an autoclave. Its progress was followed by gas chromatography and mass spectrometry. Parameters such as temperature, catalyst mass, and molar ratio of reactants were optimized to improve the catalytic performance. The MOF that presented the highest activity and selectivity to the desired product was obtained by synthesis with ultrasound and 60 °C with aminoterephthalic acid. The methyl levulinate yield was 67.77% in batch at 5 h and 85.89% in an autoclave at 1 h. An analysis of the kinetic parameters of the reaction is presented. The spent material can be activated by ethanol washing allowing the catalytic activity to be maintained in the recycles.
In this work, the adsorption of As(III) species from aqueous solutions onto mixed oxides (MOs) synthesized from layered double hydroxides (LDHs) has been assessed by cathodic stripping square-wave ...voltammetry (CS-SWV). The optimized electrochemical protocol involves the accumulation of arsenic for 60 s at −0.4 V, in presence of 0.5 mM Cu(II), 0.3 μM pyrrolidine dithiocarbamate (PDTC), and 0.9 M HCl, followed by a reductive scan to −1.0 V. A stable and well-defined peak was observed at −0.780 V, with a linear range that goes from 2 to 110 μg L
−1
of As(III). A value of limit of detection (LOD) = 4 μg L
−1
was calculated as three times the ratio between the standard deviation of the ordinate and slope of the linear regression curve. The presence of Cu(II) increases the signal of current and minimizes the effect of interfering species, while PDTC forms a complex that stabilizes the signal observed during the cathodic scan. Ternary LDHs and their MOs composed by MgAlFe were synthesized and characterized as potential filtering materials. X-ray photoelectron spectroscopy and UV-Vis diffuse reflectance analysis showed that the Fe(III) ions can be found in tetrahedral and octahedral coordination environments, while SEM micrographs evidenced the cluster formation and aggregates of particles when the Fe/(Fe+Al) molar ratio is increased. The capacity of MOs for removing As(III) has been studied at different contact times in a batch reactor, and in all cases, removals of As(III) above 75% were achieved.
A mixture of nanocrystals of cobalt ferrite and magnesium ferrite was obtained from Layered Double Hydroxides (LDH) through a co-precitation method with a theoretical molar ratio M2+:Fe3+=3:1, where ...M2+represents Mg2+ and/or Co2+. The molar ratios between Co2+:Fe3+ were 0.0 (0Co), 0.2 (5Co), and 0.4 (10Co). In order to assess the effect on the properties of the LDH and their oxides, the molar percentages were 0, 5 and 10%. Two different synthesis methods were evaluated; (i) ageing at room temperature (rt), and (ii) hydrothermal ageing at 200°C in autoclave (ht), both methods needed 15h of ageing. Then, these LDH were calcined in air atmosphere at 550°C for 10h. The calcined materials were characterized by X-ray diffraction (XRD), thermogravymetric analysis (TGA), temperature-programmed reduction (TPR), infrared spectroscopy with Fourier transform (FTIR), Diffuse Reflectance UV–visible spectroscopy (UV–vis-DRS), Mössbauer spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP–OES). The magnetic response was analyzed using a vibrating sample magnetometer (VSM). The band gap energy of the iron oxides was determined through the UV–vis-DRS analysis. Through these studies it was possible to identify the presence of a mixture of cobalt ferrite and magnesium ferrite. Samples did not show hematite and cobalt oxides, but the presence of MgO in the periclase phase was determined. This magnesium oxide promoted a good dispersion of the ferrites. Moreover, when a single ferrite phase of Co or Mg was formed, a diminution of the crystal size with consequent enlarged values of band gap energy was observed. Thus, materials synthesized by room temperature ageing promoted the superparamagnetic behaviour of samples, attributed to the content of the cobalt ferrite structure in nanocrystals. In regard to the estimated band gap energy, all samples exhibited low levels. These results indicate that these solids would be suitable for photocatalysts use in all visible light range, and that they could be easily removed from the reaction medium by a simple magnetic separation procedure.
•A mixture of nanocrystals of CoFe2O4 and MgFe2O4 was obtained.•MgO, CoFe2O4 and MgFe2O4 were detected in the calcined samples.•A diminution the crystal size of spinel ferrites, generates a low Eg values.•The superparamagnetic behaviour is attributed to the nanocrystals of ferrite.
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