Thermal decomposition products of hydrotalcite-like compounds: low-temperature metaphases

Abstract

Synthetic hydrotalcite-like samples with composition [M_{1 − x} ²⁺Al_x ³⁺(OH)₂](CO₃)_x/2 ·nH₂O, where M = Mg, Ni, x = 0.25 and 0.33, n = 2x and 1.5x, are studied by XRD, DTA, TG and IR spectroscopy after heating in the temperature interval 120–260 °C and rehydration in air and water. Structural models of the two metaphases obtained are proposed. Metahydrotalcite-D (HT-D) is formed at 140–180 °C by the reversible dehydration of the interlayer. Metahydrotacite-B (HT-B) is formed at temperatures 240–260 °C as a result of the dehydroxilation of a part of OH groups of the brucite-like layer and inclusion of two oxygenes from the CO₃-group in the same layer. The HT-B has a specific crystal structure. Rehydration does not restore the initial structural state but leads to the formation of a phase (HT-B-r) characterized with increased thickness of the interlayer, high content of water and a low temperature of dehydration. The properties of HT-B depend on the ratio M²⁺: Al of the initial sample. The substitution Mg ↔ Ni does not influence the properties of the metaphases in the low temperature region.