Usually, in the presence of base active sites, the Claisen–Schmidt condensation reaction between benzaldehyde and cyclohexanone leads to the formation of a di-condensed compound, e.g. 2,6-dibenzylidene cyclohexanone^[1]. The optimization of layered double hydroxide-type (LDH) catalysts by means of the partial substitution of Mg with Cu(II) cations leads to a shift in the selectivity towards the mono-condensed product (e.g. 2-benzylidene cyclohexanone). Six Mg_0.375Cu_0.375Al_0.125 LDH materials were prepared from metal chlorides, nitrates and sulfate as precursors by applying two methods, co-precipitation and the mechano-chemical method, while using either the usual inorganic alkalis or a non-traditionally organic one for pH adjustment. The influence of the memory effect on the physico-structural properties of the considered materials was also investigated by hydrating them with bi-distilled water of the mixed oxides that were obtained by means of the calcination of the parent LDH at 460 °C for 18 h under an air atmosphere. The characterization of solids with different techniques (e.g. XRD, DRIFT, ATR, DR-UV-Vis, BET and basicity determination using irreversible adsorption organic molecules with different pK_a values) revealed that the pure layered structure was contaminated with different amounts of copper hydroxide depending on the metal salt precursors. The memory effect did not lead to a total reintegration of the Cu(II) in the octahedral positions of the layered structure, since part of it remained as stable CuO, obtained in the calcination step. The basicity and the catalytic activities for Claisen–Schmidt condensation showed similar variation trends, e.g., reconstructed LDH > parent LDH > mixed oxides. The copper's presence in the LDH structure decreased the basicity, leading to a higher selectivity to the mono-condensed product than the one that was obtained with unmodified MgAl-LDH. The copper-containing catalysts also promoted the transformation of benzaldehyde into benzoic acid as a side reaction.

[1] B. Cojocaru, B.C. Jurca, R. Zavoianu, R. Bîrjega, V.I. Parvulescu, O.D. Pavel, Catalysis Communications 170 (2022) 106485