Thermal decomposition of hydrotalcites with variable cationic ratios

Abstract

Thermal analysis complimented with evolved gas mass spectrometry has been applied to hydrotalcites containing carbonate prepared by coprecipitation and with varying divalent-trivalent cation ratio. The resulting materials were characterized by XRD, and TGA/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg4(Fe,Al)2(OH)16(CO3,Cl).4H2O, Mg6(Fe,Al)2(OH)16(CO3,Cl).4H2O, and Mg8(Fe,Al)2(OH)16(CO3,Cl).4H2O formed by intercalation with the carbonate anion as a function of divalent/trivalent cationic ratio show variation in the d-spacing attributed to the size of the cation. The thermal decomposition of carbonate hydrotalcites consist of two decomposition steps between 300 and 400 ˚C, attributed to the simultaneous dehydroxylation and decarbonation of the hydrotalcite lattice. Water loss ascribed to dehydroxylation occurs in two decomposition steps, where the first step is due to the partial dehydroxylation of the lattice, while the second step is due to the loss of water interacting with the interlayer anions. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal oxides, including MgO, MgAl2O4, and MgFeAlO4.