Microwave-hydrothermally aged Zn,Al hydrotalcite-like compounds: Influence of the composition and the irradiation conditions

Zn,Al–CO₃ compounds with the hydrotalcite-like (layered double hydroxide, LDH) structure were prepared by a co-precipitation method followed by hydrothermal treatment under microwave irradiation. The influence of the ageing treatment was studied in two series of samples with different Zn²⁺/Al³⁺ ratios, namely, 3/1 and 2/1. Moreover, the effects of the heating temperature and of the irradiation time were studied in order to select the optimum preparation conditions. The solids were fully characterised by powder X-ray diffraction (PXRD), ²⁷Al MAS-NMR, FT-IR spectroscopy, thermal analyses (DTA and TG), N₂ adsorption/desorption at ?196 °C and TEM. The results showed that whatever the chemical composition of the starting mixture, HTlcs with Zn²⁺/Al³⁺ ratio equal to 2 were obtained, and it was impossible to overcome the ZnO segregation when the molar ratio was >2. These compounds were stable and their crystallinity could be quickly enhanced when the temperature treatment was 100 °C, whereas at 125 °C the ZnO segregation was not prevented.