The role of rehydration in enhancing the basic properties of Mg–Al hydrotalcites for biodiesel production

An Mg–Al hydrotalcite with a Mg/Al molar ratio of 3 was prepared via co-precipitation. The XRD and FTIR analyses proved the presence of the hydrotalcite structure in the uncalcined solid, its destruction after calcination and its reconstruction after rehydration. TG-DSC analyses revealed a higher thermal stability of the rehydrated solids compared to the uncalcined solid. CO₂-TPD analyses showed the presence of stronger and more abundant basic sites for the rehydrated solids. All prepared solids were then tested in the transesterification of sunflower oil [catalyst to oil ratio (CTOR) of 10 wt%, methanol to oil molar ratio (MOMR) of 12:1, temperature of 60 °C and reaction time of 2 h]. The uncalcined and calcined solids all exhibited low activities as evidenced by very low FAME yields (<1.5%). The catalytic activity was significantly improved for most of the rehydrated solids due to their higher basicity. Calcination of the fresh Mg–Al solid at 450 °C followed by subsequent rehydration (Mg₆Al₂-450RH) allowed the obtainment of a FAME yield of 56%. No biodiesel was formed in the presence of the rehydrated catalysts previously calcined at temperatures greater than 500 °C due to a reduced “memory effect”. The effects of reaction conditions such as the duration of rehydration treatment, reaction temperature, MOMR and CTOR on the activity of Mg₆Al₂-450RH were also studied. The obtained results in this work demonstrate the potential of rehydrated Mg–Al solids for biodiesel production. They could also be useful for optimizing the preparation conditions of reconstructed hydrotalcites as basic catalysts and contribute to the search for ideal conditions for biodiesel production.