Preparation and thermal decomposition studies Of L-tyrosine intercalated MgAl, NiAl and ZnAl layered double hydroxides

L-Tyrosine (represented as L-Tyr) intercalated MgAl, NiAl and ZnAl layered double hydroxides (LDHs) have been obtained by the method of coprecipitation. In situ FT-IR, in situ HT-XRD and TG-DTA measurements allow a detailed understanding of the thermal decomposition process for the three intercalated composites. In situ HT-XRD reveals that the layered structure of L-Tyr/MgAl-LDH collapses completely at 450 degrees C with the first appearance of reflections of a cubic MgO phase, while the corresponding temperature for L-Tyr/NiAl-LDH is some 50 degrees C lower. In contrast, there is a major structural change in L-Tyr/ZnAl-LDH at 250 degrees C as shown by the disappearance of its (006) and (009) reflections at this temperature accompanied by the appearance of reflections of ZnO. In situ FT-IR experiments give information about the decomposition of the interlayer -Tyr ions. The decomposition temperature of L-Tyr in the ZnAl host is about 50 degrees C lower than the corresponding values for the MgAl and NiAl hosts. TG-DTA curves show a significant weight loss step (170-260 degrees C) in L-Tyr/ZnAl-LDH which is due to the dehydroxylation of the host layers, with a corresponding weak endothermic peak at 252 degrees C. This temperature range is much lower than that observed for MgAl and NiAl hosts, indicating that the ZnAl-LDH layers are relatively unstable. The data indicate that the order of thermal stability of the three intercalates is: L-Tyr/MgAl-LDH > L-Tyr/NiAl-LDH > L-Tyr/ZnAl-LDH. (c) 2006 Elsevier Ltd. All rights reserved.