Crystallization of synthetic hydrotalcite under hydrothermal conditions

The synthetic Mg-Al hydrotalcite with Mg/Al molar ratio of 2 was hydrothermally treated in autoclaves under autogenous water vapour pressure at 120-200 °C for 2-18 h. The obtained samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements. A well-crystallized hydrotalcite-like phase was present in the coprecipitated product. No other crystalline phases were detected in the powder XRD patterns of both coprecipitated and hydrothermally treated samples. The integral intensity and full width in half maximum (FWHM) of the (003) and (006) hydrotalcite diffraction lines were evaluated in order to compare the crystallinity of samples hydrothermally treated under various conditions. The hydrothermal treatment increased the hydrotalcite content in the samples and improved significantly the hydrotalcite crystallinity. In general, the higher temperature and longer time of hydrothermal treatment, the higher hydrotalcite enrichment, as well as an increasing growth of crystallites, were observed. After the first 2-4 h, the time dependence was less evident, whereas temperature seemed to be a crucial parameter affecting the hydrothermal crystallization. The temperature of 120 °C was too low to increase significantly the sample crystallinity during several hours. According to XRD results, the marked crystallinity improvement was observed at 160 °C and higher temperatures. The hydrothermal treatment resulted in a marked decrease of surface area and a growth of hydrotalcite crystals. SEM micrographs of coprecipitated samples showed the aggregates composed of small thin crystals of ca. 0.1 μm in diameter, which were gradually transformed into thin plates with hexagonal morphology and particle size of several tenths of micrometers. Both a decrease of surface area and the crystallinity improvement of hydrothermally treated samples can be explained by the increase of hydrotalcite crystal size.