Study of alkaline-doping agents on the performance of reconstructed Mg-Al hydrotalcites in aldol condensations

The influence of alkaline-doping agents (Na, Li, K) on the catalytic activity of gas-phase rehydrated hydrotalcites has been investigated for the aldol condensation of citral and acetone to pseudoionone. Various amounts of alkalines were added (0.5-4.9 wt.%) either deliberately by impregnation or retained in the washing step during the preparation of the hydrotalcite by co-precipitation. The samples were characterised by ICP-OES, XRD, SEM and CO2-TPD. The alkaline-free rehydrated hydrotalcite (<0.05 wt.% Na), which requires large amounts of water during the washing step (10 l per 20 g of solid), displayed very poor catalytic activity. Activity improved when the content of alkaline metals in the solid was increased. These metals act as strong basic centers in the aldol condensation. The catalytic performance of the samples correlates with their strength and number of basic sites. Thorough washing is vital during the synthesis of hydrotalcite-like materials in order to remove all alkalines and properly attribute their catalytic performance to basic hydroxyl groups in the interlayer space. Alkaline-promoted hydrotalcites show an important leaching of the alkaline to the reaction medium upon consecutive runs.