Solvent-Assisted Stepwise Redox Approach to Generate Zeolite NaA-Supported K₂O as Strong Base Catalyst for Michael Addition of Ethyl Acrylate with Ethanol

Solid base catalysts featuring green, robustness, and high activity play an important role in the current fine-chemical and petrochemical industry. Normally, the generation of supported K₂O by thermal decomposition of KNO₃ requires high temperature, and this process can sometimes destroy the structure of supporting materials. We herein report a solvent-assisted stepwise redox (SASR) approach to generate zeolite NaA-supported K₂O, which we call K₂O/NaA, that function as the solid base catalyst for Michael addition reaction between ethanol and ethyl acrylate. The solvent-assisted redox decomposition process of KNO₃ at elevated temperature was investigated by thermogravimetry-mass spectrometry. It reveals that after reducing a minor amount of KNO₃ at 400 °C, the organic solvent decomposes to form carbon, which promotes the reduction of KNO₃ to generate strong basicity on the zeolite NaA at 600 °C. The resulting material, K₂O/NaA-S, exhibits improved catalytic activity in Michael addition reaction over other benchmark base catalysts that have been used in this reaction. This catalyst is durable for at least four catalytic cycles without apparent loss in activity. K₂O/NaA-S exhibits larger reaction rate constant yet lower activation energy than K₂O/NaA prepared by thermal decomposition method. The SASR approach described in this paper represents a new blueprint for the generation of the supported alkali oxide as the solid base catalyst.