Clean synthesis of benzylidenemalononitrile by Knoevenagel condensation of benzaldehyde and malononitrile: effect of combustion fuel on activity and selectivity of Ti-hydrotalcite and Zn-hydrotalcite catalysts

Benzylidene malononitrile find applications in pharmaceutical industries, pharmacology, biotech, specialty chemicals, perfumery, and for fluorescence-based assay to determine methane and is produced by polluting routes. Hydrotalcites (HT) have been very effective as solid bases in different reactions and their properties can be changed by using different synthetic methods. In this work, the effect of additional metal in the synthesis of Al-Mg hydrotalcite was systematically studied to prepare Ti-Al-Mg (Ti modified hydrotalcite) and Zn-Al-Mg HT (Zn modified hydrotalcite) using combustion method with glycine as well as glycerol as a fuel. All synthesized catalysts were evaluated in Knoevenagel condensation of benzaldehyde with malononitrile to give benzylidene malononitrile. The catalysts were completely characterized by SEM, EDXS, N 2 Adsorption, CO 2 -TPD and NH 3 -TPD and XRD techniques. Ti-Al-Mg hydrotalcite using glycine as a fuel was found to be the most active, selective and reusable catalyst. Langmuir-Hinshelwood-Hougen-Watson (LHHW) model was used to establish the reaction mechanism and kinetics. All species were weakly adsorbed leading to the second order power law model. Using mole ratio of 1:3 of benzaldehyde to malononitrile with ethyl acetate as a solvent and 2.5 × 10 - 4 g/ cm 3 catalyst loading, 67.1% conversion of benzaldehyde and 97.6% selectivity to benzylidene malononitrile were achieved in 4 h at 60 ∘ C. The apparent activation energy was 10.01 kcal/mol. The process is green. Graphic abstract Ti-Al-Mg and Zn-Al-Mg hydrotalcites were prepared using combustion method with glycine and glycerol as fuel and used in the Knoevenagel condensation of benzaldehyde with malononitrile to give benzylidene malononitrile. Ti-Al-Mg hydrotalcite (glycine) was the most active, selective and reusable catalyst.