Mechanism and Kinetics of Dehydroxylation and Decarbonation of Mg-Fe Hydrotalcite.

Thermal behavior of hydrotalcites, which is a calcination process, is critical to prepare the metal oxide catalysts with high performances in the practical applications. In this paper, the MgFe-LDH with Mg/Fe molar ratio of 3.0 was prepared by urea method and the calcined products are obtained by calcining at different temperatures (473 K, 573 K, 673 K, 773 K, 873 K and 973 K) under a N2 atmosphere for 4 h. The structure, morphology, texture, pyrolysis kinetics and mechanism of the MgFe-LDH were studied in detail. On one hand, based on the TG/DSC curves, Starink, Kissinger and Flynn-Wall-Ozawa (FWO) methods were used to calculate the activation energy, on the other hand, the Satava-Sesták, Achar and Málek methods were used to define the most probable reaction mechanisms of pyrolysis behavior. The results suggested that the thermal decomposition of the LDH experienced two steps, i.e., removal of the interlayer water, followed by dehydroxylation and decarbonation. Moreover the Mákel method was used to define the most probable reaction mechanisms of the pyrolysis behavior.

Preparation Behavior of the Mg-Fe Hydrotalcite by Urea Method and Its Cr(VI) Sorption Property.

The hydrotalcite of Mg6Fe2(OH)16CO3 x 4.5H2O were synthesized using urea method by adjusting the initial pH and urea amount in the reaction solution. The results showed that the co-precipitation of Mg2+ with Fe3+ cations formed Mg-Fe LDH occurring at pH 8.48-9.35. The pH played a crucial role in the Mg-Fe LDH precipitation by controlling urea/Fe3+ molar ratio in the reaction solution at 105 degrees C. The optimized urea/Fe3+ molar ratio was 12.0, where the relative yield of the Mg-Fe LDH was 80.0% and the Mg-Fe LDH was highly crystalline with small particle sizes (1-2 µm). The affinity of the Mg-Fe mixed oxide (Mg-Fe LDO) with Cr(VI) was studied as a function of contact time, initial pH, temperature of the solutions and calcined time of Mg-Fe LDH. The adsorption conditions were optimized using response surface methodology. The maximum adsorption capacity of 38.86 mg/g was achieved at 85 min with the conditions of initial pH 5.5, temperature 55 degrees C and calcined time 4 h. It was concluded that the Mg-Fe LDO can be used as an adsorbent to removal Cr(VI) in aqueous solutions.

[Phase transition behavior and thermodynamic analysis of hydrotalcite flame-retardant].

The hydrotalcite with the properties of flame-retardant, eliminating smoke, filling and thermostability is a new kind of inorganic flame retardant. In the work, the MgAl hydrotalcite as flame retardant with Mg/Al molar ratio of 4 (MgAl-LDH) was prepared by using urea as the precipitating agent. The thermolysis behavior of the MgAl-LDH flame retardant was investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR) and thermogravimetry-differential scanning calorimetry (TG-DSC) as well as self deconvolution and curve-fitting analyses. Thermal phase transition of the MgAl-LDH was clarified, especially the characteristics of the hydroxyl groups (-OH) in the brucite-like layers and the changes in coordinate of the carbonate (CO3(2-)) from the interlayers. Based on thermodynamic data, thermal decomposition process was discussed. By.XRD analysis; it was found that the phase change took place when the decomposition temperature increased. The MgAl-LDH was decarbonated basically to MgAl mixed metal oxides (Mg-Al-O) at 500 °C, and impurity MgAl204 phase formed at 600 °C. According to the analyses of FT-IR, TG-DSC and curve-fitting technique, the hydroxyl groups (-OH) in the brucite-like layers possessed three the ligands such as [Al-OH-Al], [Al-OH-Mg] and [Mg-OH-Mg] modes. Dehydroxylation of the brucite-like layers based on the binding forces, where the [Mg-OH-Mg] among the three modes was the most difficult to be re- moved during the pyrolysis process. In the same way, the CO3(2-) ligands also possessed three modes such as H2O-bridged CO3(2-), monodentate and bidentate coordination modes. Based on the thermodynamic analysis, the thermodynamic properties of the hydrotalcite as flame retardant were evaluated, and the expressions of the Gibbs free energy, (ΔrGθT), as a function of temperature, were derived for the Mg8Al2 (OH)20CO3 crystal. Thermodynamic analysis showed that the removal of -OH from the brucite-like layers was spontaneous process, when the Gibbs free energy (ΔrGθT) was under zero at the temperature (T) above 228. 65 °C. The result and datum were close to the experimental result from the TG-DSC analyses, indicating that the relationship between the Gibbs free energy (ΔrGθT) and temperature (T) from thermodynamic analysis was reliable.