Mechanistic insights into the Knoevenagel condensation reaction over ZnO catalysts: Direct observation of surface intermediates using in situ FTIR
2019
Li, Jerry P.H. | Kennedy, Eric M. | Adesina, Adesoji A. | Stockenhuber, Michael
Liquid phase Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was studied over ZnO under solvent-free conditions. An induction period of approximately 1 h was observed, the duration of which is dependent on the pre-treatment method adopted for the catalyst and whether the catalyst was exposed to either ethyl cyanoacetate or benzaldehyde prior to reaction. In-situ infra-red experiments show that the adsorption of ethyl cyanoacetate on ZnO results in the formation of an enol intermediate, while the adsorption of benzaldehyde on ZnO results in the formation of an acetal-type intermediate. A proposed reaction mechanism comprises the formation of ethyl trans-α-cyanocinnamate as a reaction intermediate. The key step in the reaction involves interaction between the acetal and the enol species. The induction period arises as a consequence of the formation and relative stability of the surface OH groups from the reactants when the reagents adsorb on the ZnO surface.
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