Syntheses of MgCO3 and Na2Mg(CO3)2 through solid-gas reactions mediated by alkali nitrates

In this work, we devised a set of molten salt-mediated reactions that can make use of gaseous reagents in forming solid products. In order for gaseous molecules to be reacted, its solubility in the molten salt medium needs to be high, which condition, in turn, requires the molten salt material to have a low melting point. We used alkali nitrate (ANO3, A = Li, Na, K, Rb, and Cs) as the reaction medium, for their low melting points, for the reactions of CO2(g) with various Mg-reagents. Reactions between 4MgCO3·Mg(OH)2·4H2O or Mg(OH)2 with CO2 in the presence of an ANO3 above a certain quantity (30 wt% with respect to the MgO content in the Mg-reagent) produced single phase MgCO3, after washing the ANO3 with water. On the contrary, the same reactions without ANO3 did not show any sign of incorporating gaseous CO2 in the final products. Similarly, the reaction of a mixture between Na2CO3 and 4MgCO3·Mg(OH)2·4H2O or Mg(OH)2 with CO2 in an ANO3 medium produced Na2Mg(CO3)2, a double carbonate of MgCO3 with Na2CO3. The X-ray diffraction patterns and the thermogravimetry data under a CO2 flow of MgCO3 and Na2Mg(CO3)2 indicate their high phase purity. The effects of various synthesis parameters such as the nature of ANO3 and the amounts on the products were systematically investigated.