Textural Properties and Catalytic Activity of Hydrotalcites

Abstract

Double-layered hydroxides with hydrotalcite structure were synthesized with Mg/Al atomic ratios of 2.5 and 3 and with different contents of exchangeable Cl− and CO2−3anions. The basicity of these solids depends on several structural parameters and on the activation temperature. Microcalorimetric measurements, IR spectroscopy using a probe molecule, and a catalytic test reaction were carried out to characterize the basic sites. The structural evolution of the hydrotalcite upon exchange of Cl− for CO2−3 and calcination temperature shows a transformation with simultaneous removal of CO2 and water without phase segregation up to 673 K. The mixed oxide formed later could reversibly lead to the layered structure upon rehydration and carbonation. This process was studied by microcalorimetry and IR spectroscopy. The adsorption isotherm of CO2 shows an increase of the uptake and consequently of the basicity with initial CO2−3 content and calcination temperature up to 800 K. IR spectroscopy shows that the carbonation of the mixed oxide is essentially superficial and that the hydrotalcite structure may be restored upon H2O addition and 373-K treatment. Two types of Lewis acid sites related to a linear coordination of CO2 with Mg2+ and Al3+ cations are assigned. The appearance of a bicarbonate species implies that the basic sites are essentially OH groups. Moreover, a small number of strong basic sites, adsorbing CO2 with 140 kJ/mol, probably correspond to O2− centers. Hammett correlation in the condensation reaction of acetone and different substituted benzaldehydes shows analogies between hydrotalcite catalysis and the homogeneous basic reaction. The activities increase with CO2−3 content, calcination temperature, and Mg/Al ratio of the hydrotalcites. The basic strength of hydrotalcites is comparable to that of piperidine in homogeneous catalysis.