Reinvestigation of Dehydration and Dehydroxylation of Hydrotalcite-like Compounds through Combined TG-DTA-MS Analyses

Abstract

We reinvestigated dehydration and dehydroxylation of hydrotalcite and four other hydrotalcite-like compounds (HTlcs) with the combined thermogravimetry/differential thermal analysis/mass spectrometry (TG-DTA-MS) technique. The observations indicate that HTlcs undergo dehydration first and then dehydroxylation, followed by or overlapping with decomposition of interlayer anions. The detail deconvolution of MS curves of m / z = 18 reveals that both dehydration and dehydroxylation occurs in two consecutive steps. Dehydration of crystalline water molecules starts on the surface and edge in the first step, and then continues to the interlayer water. Both steps are influenced by the nature of the interlayer anions and layer cations. For the first time, we have assigned two dehydroxylation steps to two types of hydroxyl groups in the HT lattice, i.e. OH−(M(II) 3 ) and OH−(M(II) 2 M(III)). The thermal stability of these two hydroxyl groups is principally determined by the nature of the cations. According to this research, the thermal stability of hydroxyl groups in HTlcs is OH-(Ca 3 ) (∼480 °C) > OH−(Mg 2 Al) (∼410 °C) > OH−(Mg 2 Fe) (∼350 °C) ≈ OH−(Mg 3 ) (300−370 °C) ≈ OH−(Ca 2 Al) (∼330 °C) > OH−(Ca 2 Fe) (∼290 °C) at a ramping rate of 2 °C/min.