Betaine–ZnO hybrid catalyst: a sustainable bifunctional system for solvent-free Knoevenagel condensation

A sustainable betaine–ZnO hybrid catalyst was synthesized via a simple mixing approach between betaine hydrochloride and zinc oxide. Structural characterization using Raman, FTIR, and ¹H NMR spectroscopy confirmed the formation of Zn–carboxylate coordination bonds resulting from deprotonation of the carboxylic group in betaine. The hybrid catalyst exhibited exceptional performance in the Knoevenagel condensation of benzaldehyde with malononitrile, outperforming its individual precursors (betaine hydrochloride and ZnO). The enhanced activity originates from the synergistic interaction between Lewis acidic Zn²⁺ and Brønsted basic carboxylate sites. The condensation proceeds efficiently (40 °C, 2.5 wt% catalyst) in a biphasic system, where the upper organic layer contains the product and the lower solid phase holds the catalyst, enabling straightforward separation and reuse. Recycling experiments demonstrated catalyst stability and reusability with minimal loss of activity after multiple runs. This study highlights the potential of bio-derived betaine in the design of multifunctional, recyclable, and environmentally benign hybrid catalysts for sustainable C–C bond formation reactions.