Pickering Interfacial Catalysis-Knoevenagel Condensation in Magnesium Oxide-Stabilized Pickering Emulsion.

In the present study, a novel catalytic route for the Knoevenagel condensation reaction has been developed by Pickering interfacial catalysis using magnesium oxide (MgO) as both an emulsion stabilizer and a base catalyst. MgO was prepared by the precipitation method using sodium hydroxide or ammonium hydroxide as the precipitating agent and calcined at different temperatures. The calcined samples were characterized by XRD, SEM, TEM, AFM, BET, and DLS techniques. The catalytic application of the emulsions stabilized by MgO was investigated for the Knoevenagel condensation reaction of benzaldehyde and its derivatives with malononitrile. All of the reactions were carried out at an ambient temperature (30 °C) under static conditions without stirring. Both the emulsion-stabilizing ability and the catalytic activity of MgO were found to be affected by the method of preparation, calcination temperature, and the nature of the oil phase. It was observed that the method of preparation varied the texture and morphology of MgO and thus the stability and droplet size of the emulsion formed. This was further reflected in the catalytic activity. The highest yield (87%) of the condensation product was obtained with MgO prepared by precipitation using a strong base (NaOH) and further calcined at 400 °C. The developed catalytic system offers several green chemistry advantages such as reusable solid-base catalyst and use of a single material as both emulsion stabilizer and catalyst. Room-temperature reaction under static conditions is an additional advantage of the developed catalytic system.