Thermal decomposition of hydrotalcites. An infrared and nuclear magnetic resonance spectroscopic study

Abstract

Calcined hydrotalcites have been used extensively as catalysts for base-catalysed reactions. The calcination procedure in critical in determining the behaviour of the final material. The characteristics of the hydrotalcite, following the different stages of calcination, have been studied by means of thermogravimetric (TG), X-ray diffraction (XRD), IR, ¹H and ²⁷Al NMR techniques. We have shown that dehydroxylation begins within layers and then in a second stage occurs between adjacent layers, causing collapse of the structure. This process is accompanied by a change from octahedral to tetrahedral coordination of the aluminium. Decarbonation leads to the formation of micropores of radius <1.75 nm. Dehydroxylation and decarbonation are reversible processes, and their rates depend on the calcination temperature.