Study on the crystal structure of magnesium ammonium carbonate tetrahydrate and its thermal decomposition to MgO

Magnesium ammonium carbonate tetrahydrate (MAC) is an essential intermediate in the production of active magnesia and CO2 sequestration. It has an unstable structure and decomposes quickly at room temperature to release gases. In this study, the crystal structure of MAC was determined for the first time by single-crystal X-ray diffraction at 200 K. The stability of MAC and the possible mechanism of thermal decomposition to MgO were discussed in the structural analysis of the complex of Mg2+ and crystal of MAC and other magnesium carbonates. MAC was prepared and conversion to MgO was investigated by thermal decomposition and calcination along with XRD and DSC/TGA analysis. Both nesquehonite and hydromagnesite were separately found in the thermal decomposition residue of different batches of MAC. An optimal calcination procedure was suggested to prepare active magnesia based on its thermal decomposition mechanism.