By carrying out the condensation of benzaldehyde with activated methylenic groups with different pK values, in the presence of a MgAl hydrotalcite catalyst, it has been found that this material shows basic sites with pK values up to 16.5. However, most of the basic sites show pK values in the range 10.7–13.3. In the use of zeolites as catalysts, the only reaction observed was Knoevenagel condensation, while on calcined hydrotalcite other reactions, the Michael-type addition and the Claisen condensation, also occur. These reactions require stronger basicities than the Knoevenagel condensation to occur. By carrying out the Knoevenagel condensation, using methylenic groups of different pK values, in the presence of increasing amounts of benzoic acid, it is possible to measure the total amount of basic sites and their strength distribution as determined under reaction conditions. By increasing the Mg/Al ratio in the hydrotalcite, the number of basic sites with 9.0 ≤ pK ≤ 13.3 increases, whereas the amount of basic sites within 13.3 ≤ pK ≤ 16.5 decreases.
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