Enhancing the Catalytic Performance of MgCuAl-LDH Composites in Claisen-Schmidt Condensation via Organic Alkali Treatment

In the last decade, a distinct trend in the synthesis of Layered Double Hydroxides (LDHs) has emerged: the use of organic alkalis. These compounds serve a dual purpose, acting as both synthesis agents and template molecules. This approach allows for the tailoring of the solid's textural properties while avoiding the drawbacks of inorganic alkalis, such as alkali metal contamination, high energy consumption, and significant waste production. Notably, the Claisen-Schmidt condensation between benzaldehyde and cyclohexanone is a critical pathway for the synthesis of pharmaceutical intermediates and high density fuel precursors.

MgCuAl-LDH catalysts were synthesized using various metal precursors (e.g. nitrates, sulfates and chlorides) in the presence of an organic base via two distinct routes: traditional co-precipitation and non-traditional mechanochemistry. To investigate the influence of the memory effect on the physicochemical properties, the materials were calcined at 460 °C and subsequently hydrated. To assess the impact of these parameters on the physicochemical properties, a comprehensive suite of characterization techniques such as: XRD, DRIFT, ATR, DR-UV-Vis, BET, TEM and surface basicity determination.

Tetramethylammonium hydroxide templating yielded solids with low surface areas and narrow pore distribution. The catalytic activity was correlated linearly with the basicity. The presence of Cu cations introduced acid sites and lowered overall basicity; paradoxically, these factors, often viewed as disadvantages, were crucial for directing the transformation selectively toward the mono-condensed product, i.e. 2-benzylidenecyclohexanone. The highest catalytic performance was reached with the samples obtained from nitrate precursors (conversion of 89% with 97% selectivity in di-condensed compound). In contrast, the sample synthesized from sulfates favored side reactions, converting 46% of the benzaldehyde into benzoic acid. Regarding stability, structural changes occurred after three reaction cycles.

Acknowledgment

This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS-UEFISCDI, project number PN-IV-P1-PCE-2023-1254, within PNCDI IV.