Hydrotalcites as precursors for Mg,Al-mixed oxides used as catalysts on the aldol condensation of citral with acetone Original Research Article

The use of Mg,Al-mixed oxides derived from hydrotalcites as basic catalysts for the citral/acetone aldol condensation was studied. Three series of hydrotalcite samples with different Al/(Al+Mg) molar ratios (0.2, 0.25, and 0.33), aged at different pH (10 or 13) and temperatures (333 and 473 K), were prepared as precursors. The corresponding Mg,Al-oxides were obtained by calcining the hydrotalcites at 723 K. They were characterized by N2 physisorption, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Their relative basicity was evaluated by using diacetone alcohol (DAA) retroaldolization as a model reaction. The Mg,Al-mixed oxides were efficient catalysts for the aldol condensation of citral with acetone in heterogeneous media. The best catalytic activities and pseudoionone yields were obtained from the samples with Al/(Al + Mg) molar ratio equal to 0.2 derived from hydrotalcites aged at 333 K. On the other hand, neither the aluminum content nor the pH of the synthesis gel influenced significantly the reaction selectivity. For mixed oxides derived from hydrotalcites aged at pH = 10 citral conversion decreased with increasing aluminum content, whereas for those obtained from precursors aged at pH = 13 citral conversion passed through a minimum for the sample with Al/(Al + Mg) = 0.25. Both trends reflect the mesoporous characteristics of the samples and their relative basicity as measured by retroaldolization of DAA. Since the influence of aluminum content on basic properties could not be clearly established, the textural characteristics best explained the observed trends. Catalysts derived from hydrotalcites aged at 473 K presented low catalytic activities, which was attributed to their lower mesoporosity.