Influence of Mg/Al Ratio on the Thermokinetic Rehydration of Calcined Mg−Al Layered Double Hydroxides

Abstract

The rehydration process of calcined MgAl-layered double hydroxides (LDHs), with different Mg/Al molar ratios (2.0, 2.5, 3.0, 3.5, and 4.0), was analyzed at different temperatures and relative humidities. Kinetic results clearly showed that LDHs with Mg/Al molar ratios of 2.5 and 3.0 have the fastest H2O absorption. This behavior was attributed to the cations’ structural ordering and to thermodynamic factors. In addition, results allowed obtaining the activation enthalpies for the LDH regeneration of the different samples as a function of the relative humidity. The different activation enthalpies, for each LDH and independently of the Mg/Al molar ratio, showed that the H2O absorption process is more dependent on temperature if the relative humidity is low. Finally, when we analyzed the activation enthalpies as a function of the Mg/Al molar ratio at a defined temperature, their values did not behave linearly. The water absorption process proved to be more dependent on temperature for the LDHs with molar ratios equal to 2.5 and 3.0.

Dehydration-rehydration induced structural changes in pristine and composite layered double hydroxides

“…The activation enthalpies (ΔH) were also obtained as the function of the relative humidity. These results proved that H2O absorption was more dependent on the temperature, when the relative humidity was low in general, and for the samples with 2.5 and 3 Mg to Al ratios, in particular [47].…”

“…Pfeiffer et al [46][47][48] published a thorough study on the rehydration process of MgAl-LDH via taking the adsorption-desorption isotherms and following the steps by thermogravimetric, 27 Al MAS NMR and SAXS (small-angle X-ray diffraction) measurements. The activation enthalpies (ΔH) for the steps of the reconstruction process were determined at various relative humidities.…”

“…The absorption process was found to be more dependent on the temperature, when the relative humidity was lower [46]. The rehydration processes of the samples with varying Mg/Al ratios between 2 and 4 were analysed under isothermal conditions using various humidities and temperatures [47]. Three different processes were identified during the rehydration procedure: (i) H2O adsorption, (ii) H2O absorption and (iii) H2O desorption.…”

Selective hydrogenation of cinnamaldehyde using Pd catalysts supported on Mg/Al mixed oxides: Influence of the Pd incorporation method

“…On the other hand, d 003 decreases as the Mg/Al ratio increases. This is caused not only for the nature and amount of compensation anions, resulting from the M II /M III ratio, but also by the degree of hydration of the samples, while the values of the specific surface areas were not greatly modified with respect to the variation of the Mg/Al ratio. Specific surface area, Mg/Al molar ratio, and mass% of Pd determined by ICP and EDS of final catalysts are presented in Table .…”

Structural Characterization and Thermal Properties of Chiral Poly(amide-imide)/Modified MgAl Layered Double Hydroxide Nanocomposites Prepared via Solution Intercalation

“…The second is the direct ion exchange method, in which LDHs are stirred in a solution of desired anions of a suitable concentration [27][28][29] . The third is the rehydration method in which calcined LDH is added to solutions of desired anions [30][31][32] .…”