TEXTURAL PROPERTIES AND CATALYTIC ACTIVITY OF HYDROTALCITES

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 CO32- anions. The basicity of these solids depend s 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 b asic sites. The structural evolution of the hydrotalcite upon exchange of Cl- for CO32- and calcination temperature shows a transformation w ith simultaneous removal of CO2 and water without phase segregation up to 673 K. The mixed oxide formed later could reversibly lead to the l ayered structure upon rehydration and carbonation. This process was st udied by microcalorimetry and IR spectroscopy. The adsorption isotherm of CO2 shows an increase of the uptake and consequently of the basici ty with initial CO32- content and calcination temperature up to 800 K. IR spectroscopy shows that the carbonation of the mixed oxide is esse ntially superficial and that the hydrotalcite structure may be restore d 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 ar e assigned. The appearance of a bicarbonate species implies that the b asic sites are essentially OH groups. Moreover, a small number of stro ng basic sites, adsorbing CO2 with 140 kJ/mol, probably correspond to O2- centers. Hammett correlation in the condensation reaction of aceto ne and different substituted benzaldehydes shows analogies between hyd rotalcite catalysis and the homogeneous basic reaction. The activities increase with CO32- content, calcination temperature, and Mg/Al ratio of the hydrotalcites. The basic strength of hydrotalcites is comparab le to that of piperidine in homogeneous catalysis. (C) 1995 Academic P ress, Inc.