Morphology and composition controllable synthesis of Mg–Al–CO3 hydrotalcites by tuning the synthesis pH and the CO2 capture capacity

In order to achieve a controllable synthesis of hydrotalcites (HTs), a systematic investigation on the synthesis of Mg–Al HTs at different pH values was performed. The physical and chemical properties of the synthesized HTs were characterized by X-ray diffraction, scanning electron microscope, thermogravimetric analysis, differential scanning calorimetry, temperature programmed desorption, and BET. The chemical compositions were determined by inductively coupled plasma. The results revealed that the synthesis pH plays a crucial role on the morphology, pore structure and chemical composition of the final products. When the synthesis pH equaled to the isoelectric point (IEP) of Mg–Al HT, “rosette” morphology was formed; while when the pH was higher than the IEP, meso-porous HTs were synthesized. The inter-layered charge compensating anions as well as the Mg/Al ratio also varied with the synthesis pH. Based on our observations, a synthesis mechanism which describes the formation process under various synthesis conditions, was proposed. In addition, we have shown that this mechanism could also be applicable to other synthesis methods. Finally, the CO2 adsorptions on these synthesized HTs were evaluated. It is clear that the synthesis conditions strongly affected the adsorption capacity. The highest capacity was obtained over the Mg3Al1–CO3 (pH 12) sample, showing a CO2 capture capacity of 0.83 mmol/g when pre-calcined for 1 h and 0.58 mmol/g when pre-calcined for 6 h, respectively.