On the nature and accessibility of the Brønsted-base sites in activated hydrotalcite catalysts.
Base catalysis is of importance for organic synthesis in general and fine chemicals manufacture in particular. Activated hydrotalcites have recently received a great deal of attention as solid base catalysts; however, no systematic work on the nature of their active sites has been published up till now. In this work two different methods have been applied to activate Mg-Al hydrotalcites to obtain Brønsted-base catalysts for liquid-phase condensation reactions. Activation via thermal treatment followed by rehydration (HT-reh) resulted in irregularly stacked platelets ( approximately 60 nm), whereas the sample activated via aqueous ion-exchange (HT-exc) preserved its original hexagonal hydrotalcite platelets ( approximately 100 nm). The specific activity for the self-condensation of acetone of HT-reh was over 10 times that of HT-exc. The enthalpy of CO2 adsorption on the activated hydrotalcites determined with calorimetry to gain insight into the strength of the basic sites showed very similar values. IR spectra of adsorbed CDCl3 as probe molecule on the differently activated samples revealed large differences in adsorbed amounts, but again the strength of the basic sites appeared to be the same. These results point to steric hindrance for the substrate molecules as the main factor determining differences in catalytic activity. The high accessibility of Brønsted-base sites in HT-reh is proposed to involve a distorted edge structure of the platelets. The edge structure of exchanged samples could be distorted too, either by exchange under reflux conditions or under ultrasonic treatment. In line with the proposed model, the distorted exchanged samples displayed a much higher catalytic activity than HT-exc.