The influence of long-term exposure of Mg-Al mixed oxide at ambient conditions on its transition to hydrotalcite

Abstract

The paper is focused on the study of long-term aging of Mg-Al mixed oxides, which causes the rebuilding of the hydrotalcite layered structure in the presence of humidity. The novelty consists in the description of influence during the long-term aging (6 months) on the phase transition of mixed oxides to hydrotalcites at ambient conditions. The structure throughout aging has been determined by several techniques, such as X-ray diffraction, DRIFT and thermogravimetric analysis. The diffraction lines of mixed oxides were observed for 91 days of aging (after that the lines of hydrotalcites started to occur). However, the specific surface area rapidly decreased after only two aging days. The change was caused by an adsorption of water and especially of carbon dioxide from the air (formed carbonates). After 166 days, the hydrotalcite structure was fully recovered. Moreover, all aged material was applied as the heterogeneous catalyst for the aldol condensation with furfural as acid-base reaction. The conversion decreased from 72% (immediately after the thermal treatment) to 27% after 4 aging days. The statistical evaluation of results was also carried out and relation between variables described.