The effects of synthesis pH and hydrothermal treatment on the formation of zinc aluminum hydrotalcites

Zn/Al hydrotalcites were synthesised by coprecipitation at increasing pH from 6.0 – 14.0, followed by hydrothermal treatment at 150 oC for 7 days. The materials were characterised by XRD, STEM, ICP-AES, thermal analysis, infrared spectroscopy and Raman spectroscopy. The XRD analysis for the samples prepared between pH 9.0 – 12.0 showed a pattern typical of hydrotalcite, with a c-axis distance of ~22.6 A. STEM showed that the pH of preparation affected the stability of the hydrotalcite and that instability, observed at pH 9.0, favoured the formation of mixed phases when treated hydrothermally. It was also shown that treatment of a stable starting material increased the crystallinity and resulted in the formation of hexagonal plate shaped particles. ICP-AES and thermal analysis showed that the Zn/Al ratio and thermal stability increased with pH. Thermal analysis showed three major weight losses corresponding to the loss of interparticle water, interlayer water and dehydroxylation of the hydroxide layers and decarbonisation of the interlayer region.