SI.docxImpact of organic / inorganic alkalis in tailoring of Mg/Al-hydrotalcite used in Claisen–Schmidt condensation

An organic alkali such as tetramethylammonium hydroxide provides a viable alternative to the inorganic alkalis (NaOH / Na2CO3) as a pH-adjustment agent also acting as template molecule for the synthesis of Mg/Al-hydrotalcites with controlled texture. Its utilization overcame the disadvantages presented by the traditionally inorganic alkalis in terms of the solid contamination with alkaline cations, the amount of generated wastewaters and the energy consumption, etc. Moreover, the addition of the organo-alkali as cetyltrimethylammonium bromide (CTAB) in the synthesis mixture increases the control of the textural properties of the resulting solids. Both the basicity and the catalytic activities in the Claisen-Schmidt condensation between benzaldehyde and cyclohexanone followed a linear variation trend, e.g. mixed oxides > reconstructed layered structure > parent dried hydrotalcites. The conversion in the presence of these hydrotalcites synthesized using inorganic alkalis and CTAB reached 77.2%, while in the presence of the ones synthesized in the presence of TMAH of 74.3%. Further, the extraction of CTAB by ethanol, at RT or reflux temperature, led to an increase of the catalytic activity, while the corresponding mixed oxides generated 85.7% conversion of cyclohexanone. Also important, the specific texture of these afforded a total selectivity to 2,6-dibenzylidene-cyclohexanone. The presence of CTAB in the synthesis step seems to enhance the basicity and implicitly the catalytic activity of the hydrotalcites obtained using inorganic alkalis. Also, the resulted pore size favored a selective formation of the di-condensed compound. The derived mixed oxides also showed a good recyclability, since after five successive reaction cycles their catalytic activity decreased by less than 7%.