Thermal decomposition of hydrotalcite with molybdate and vanadate anions in the interlayer

Abstract

Hydrotalcites containing carbonate, vanadate and molybdate were prepared by coprecipitation. The resulting materials were characterized by XRD, and TGA/DTA to determine the stability of the hydrotalcites synthesised. The thermal decomposition of carbonate hydrotalcites consist of two decomposition steps between 300 and 400 degrees Celsius, attributed to the simultaneous dehydroxylation and de-carbonation 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, Al2O3, MgAl2O4, NaMg4(VO4)3 and Na2Mg4(MoO4)5. The presence of oxy-anions proved to be beneficial in the stability of the hydrotalcite structure, shown by the delay in dehydroxylation of oxy-anion containing hydrotalcites compared to the carbonate hydrotalcite. This is due to the substantial amount of hydroxyl groups involved in a network of hydrogen bonds involving the intercalated anions. Therefore, the stability of the hydrotalcite structure appears to be dependent on the type of anion present in the interlayer. The order of thermal stability for the synthesized hydrotalcites in this study is Syn-HT-V > Syn-HT-Mo > Syn-HT-CO3-V > Syn-HT-CO3-Mo > Syn-HT-CO3. Carbonate containing hydrotalcites prove to be less stable then oxy-anion only hydrotalcites.