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.