Influence of thermal treatment conditions on the activity of hydrotalcite-derived Mg-Al oxides in the aldol condensation of acetone

Magnesium-aluminum hydrotalcite-like compounds (with molar ratios of Mg/Al ranging from 2.1 to 3.6) containing both carbonate and nitrate anions in the interlayer space were synthesized by the co-precipitation method. The obtained materials were characterized with respect to structure (XRD and FTIR) and chemical composition (elemental analysis and XRF). The stability of interlayer anions was studied by means of evolved gas analysis by mass spectrometry (EGA-MS). NO3- ions appeared to be more stable than CO32- anions. Decomposition of the interlayer nitrates began at about 450degreesC, when the carbonates almost completely had left the sample. The synthesized precursors were precalcined at a temperature within the range of 450-600degreesC for 8 h. The structural transformations of hydrotalcite-like compounds caused by calcination were described on the basis of high-temperature XRD and FTIR experiments. The effect of calcination temperature on textural (BET), acid-base (NH3- and CO2-TPD) and catalytic (process of acetone aldolization) properties of the materials was investigated. It was shown that the hydrotalcites calcined at 450degreesC contain non-decomposed nitrate species, which block surface active centers. The removal of NO3- ions from the materials structure, by raising the calcination temperature, results in a significant increase of basicity and catalytic activity of the samples in self-condensation of acetone. The process of acetone aldolization occurs mainly on Bronsted basic sites, but an activation of acetone molecules on Bronsted and/or Lewis acidic sites cannot be also excluded. (C) 2004 Elsevier Inc. All rights reserved.