The effects of various hydrothermal treatments on magnesium-aluminium hydrotalcites

Mg/Al hydrotalcites were synthesised by coprecipitation followed by hydrothermal treatment. The materials were characterised by XRD, infrared and Raman spectroscopy, electron microscopy and thermal analysis. The XRD pattern obtained was typical of a hydrotalcite, where the interlayer anion is CO32-, with a basal distance of similar to 23.5 Angstrom. All possible CO32- modes were observed in the infrared and Raman spectra, at 1068 cm(-1) 844 cm(-1), similar to 1380 cm(-1), and similar to 680 cm(-1). XRD, Infrared and Raman spectroscopy complimented each other by showing that with treatment the degree of order increased regardless of the type of treatment. Furthermore, it was shown that aging at increased temperature and pressure increased crystallinity and that treatment in water rather than in the mother liquid resulted in a more crystalline material. TEM showed that crystal size increased with aging, such that growth occurred on the edges resulting in the formation of hexagonal plate shaped hydrotalcite crystals. Thermal analysis showed 3 major weight losses corresponding to the loss of interparticle water, interlayer water, and dehydroxylation of the hydroxide layers and decarbonation of the interlayer region. (C) 2000 Kluwer Academic Publishers.