Ionic liquids enable highly efficient Knoevenagel reaction by dual-responsive emulsion microreactor

Abstract

Knoevenagel condensation reaction is important for generating CC double bonds and providing various valuable intermediates in organic chemistry, which always need harsh conditions including high reaction temperature, difficult catalyst recovery, difficult product separation, etc. In this work, a novel class of dual-responsive emulsion reactor was prepared for the Knoevenagel condensation reaction between malononitrile and aromatic aldehyde, where the temperature and CO2-responsive ionic liquids (ILs) were used as an emulsifier, responder, and catalyst simultaneously. Driven by temperature or CO2, the emulsion reactor undergoes reversible and controllable transformations between emulsification and demulsification, where efficient catalytic reactions, product separation, and reactor recovery are all achieved. The isolated product yield of 2-(4-chlorobenzylidene)-malononitrile remained above 90% after 5 cycles of the emulsion reactor. NMR and molecular dynamics simulations showed that the temperature drives the reversible stretching and curling of the polyether chain on the cation, while CO2 reacts with the –COO- group on the anion to form carbonates and carboxylic acids in water, leading to changes in the surface activity of ILs, thereby reversibly regulating the demulsification and emulsification of the emulsion. Hence, through the dual stimuli-responsive ILs, the efficient reaction can be performed in the homogeneous phase, and the products and catalysts can be easily separated in the heterogeneous phase. These results on the revisable microreactor can not only help to design the smart and efficient “homogeneous reaction-heterogeneous separation” systems but also support the development of green sustainable chemistry.