An Advanced Approach for MgZnAl-LDH Catalysts Synthesis Used in Claisen-Schmidt Condensation

Abstract

Using organic-base tetramethylammonium hydroxide (TMAH) is a viable, cheap, and fast option for the synthesis of MgZnAl-LDH-type materials by both co-precipitation and mechano-chemical methods. TMAH presents several advantages, such as the smaller quantity of water required in the washing step compared to the use of inorganic alkalis, the prevention of LDH contamination with alkali cations, and its action as a template molecule in texture tailoring. It also has disadvantages, such as its presence in small quantities in the resulting layered materials. Regardless of the use of organic/inorganic bases and co-precipitation/mechano-chemical methods, zincite stable phase was found in all the synthesized solids. The basicity of catalysts followed the trend: mixed oxides > reconstructed > parent LDH. The memory effect of LDH was supported only by the presence of Mg and Al cations, while Zn remained as a zincite stable phase. The catalytic activities for Claisen-Schmidt condensation of benzaldehyde with cyclohexanone provided values higher than 90% after 2 h, with a total selectivity toward 2,6-dibenzylidenecyclohexanone, while self-condensation of cyclohexanone yielded no more than 7.29% after 5 h. These behaviors depended on catalyst basicity as well as on the planar rigidity of the compound.