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.

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“…The coding of the catalysts based on their preparation conditions is presented in table 1. As previously reported [32], the X-ray diffractogram of the bare LDH shows, at low angles, narrow and intense diffraction lines and, at high diffraction angles, wide and less intense lines characteristic to the LDH materials [33]. In addition to the lines, the additional ones corresponding to the zincite phase (ZnO) appeared, as minor impurities, in the 2 theta range 31-38 o [32].…”

“…As previously reported [32], the X-ray diffractogram of the bare LDH shows, at low angles, narrow and intense diffraction lines and, at high diffraction angles, wide and less intense lines characteristic to the LDH materials [33]. In addition to the lines, the additional ones corresponding to the zincite phase (ZnO) appeared, as minor impurities, in the 2 theta range 31-38 o [32]. The reconstruction by memory effect occurred with the preservation of the lines of the stable ZnO phase (ICDD 005-0664).…”

“…Bare LDH shows a wide absorption band in the range 240 -380 nm with maxima at 348 and 359 nm, respectively, associated to the presence of zincite in the layered structure [32]. The absorption band at 212 nm corresponds to Mg(OH)2 / MgO [32,38].…”

“…The synthesis of LDH Mg0.375Zn0.375Al0.25 has been carried out following a reported procedure [32]. Therefore, two solutions (A and B), were mixed at a inlet flow of 1 cm 3 •min -1 , at RT under stirring (600 rpm).…”

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“…Specifically, Claisen–Schmidt condensation [ 37 , 38 ] is dedicated to the formation of new C–C bonds. The condensation of cyclohexanone and benzaldehyde to mono -, 2-benzylidene-cyclohexanone, or di -condensation, 2,6-dibenzylidene-cyclohexanone, is also an important subject [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ]. However, the Claisen–Schmidt condensation has some limitations due to the possibility of the appearance of side reactions, such as self-condensation, as an effect of the additional presence of the acid sites.…”