Highly Efficient and Cost-Effective Solid-Base CaO-MgO Catalyst for Knoevenagel Condensation in Water with a Good...

A cost-effective and eco-compatible 1CaO-1.5MgO binary metal oxide (BMO-1) served as an efficient solid-base catalyst in the Knoevenagel condensation (KC) reaction of a range of aldehydes with active methylene reagents (i) malononitrile and (ii) ethyl cyanoacetate in water at room temperature (RT) to produce α,β-unsaturated compounds in purity with a good E-factor. We also report the ketone-malononitrile KC reaction and salicylaldehyde-malononitrile tandem KC-Michael addition effectively catalyzed by BMO-1. We report the synthesis of 31 α,β-unsaturated compounds that include 9 entirely new compounds under optimized conditions. We compared all catalyzed reactions with the "blank test" due to the high reactivity of active methylene reagents and emphasized the implication of the catalyzed aqueous KC reaction. We deduced the structure-activity relationship (SAR) between the catalyst and substrates, the plausible reaction mechanism, and the turnover frequency (TOF) data of the BMO-1 catalyst. For evaluation, we compared the efficiency of CaO, MgO, and 1.5CaO-1MgO (BMO-2), 1CaO-2MgO (BMO-3), and 1CaO-1MgO (BMO-4) catalysts in a model KC reaction. We prepared the MgO, CaO, BMO-1, and BMO-2 materials via an ultradiluted coprecipitation process and characterized the catalysts by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), CO₂ temperature-programmed desorption (CO₂-TPD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. The superior catalytic activity of BMO-1 is due to its high specific surface area of 97.6 m²/g, surface basicity of 152.4 μmol/g, and smaller particle size of 16.9 nm compared to the other three materials. Catalyst recycling experiments indicate that BMO-1 was stable for up to five cycles for the KC reaction. The BMO-1 spent catalyst analysis indicated the fundamental reason for its deactivation. We also report the optimized conditions for a selected KC reaction for upscaling (100 mmol).