Thermal decomposition products of hydrotalcite-like compounds: low-temperature metaphases

摘要

Synthetic hydrotalcite-like samples withcomposition [M1-x2+Alx3+(OH)2] (CO3)x/2·nH2O, whereM=Mg,Ni,x=0.25 and 0.33,n=2x and 1.5x,are studied by XRD,DTA,TG and IR spectroscopy after heating in the temperatureinterval 120-260℃and rehydration in air and water. Structuralmodels of the two metaphases obtained are proposed.Metahydrotalcite-D(HT-D) is formed at 140-180℃by thereversible dehydration of the interlayer. Metahydrotacite-B(HT-B)is formed at temperatures 240-260℃as a result of thedehydroxilation of a part f OH groups of the brucite-like layer andinclusion of two oxygenes from the CO3-group in the same layer.The HT-B has a specific crystal structure. Rehydration does notrestore the initial structural state but leads to the formation of aphase (HT-B-r) charactrized with increased thickness of theinterlayer, high content of water and a low temperature ofdehydration. The properties of HT-B depend on the ratio M2+:Al ofthe initial sample.The substitution Mg?Ni does not influence theproperties of the metaphases in the low temperature region.