Functionalized Core-Shell Polystyrene Sphere-Supported Alkaline Imidazolium Ionic Liquid: An Efficient and Recyclable Catalyst for Knoevenagel Condensation

A supported alkaline imidazolium ionic liquid catalyst with an ionic liquid shell and polystyrene core was synthesized. The core-shell structure was achieved by swelling polymerization of 4-vinylbenzyl chloride and divinylbenzene around polystyrene particles followed by quaternization reaction with 1-propyl-1H-imidazole and ion exchange. The results obtained from FTIR, SEM, TEM, TGA, DLS, and elemental analysis indicated that the ionic liquid shell was successfully coated on polystyrene spheres. The core-shell catalyst displayed excellent activity and solvent tolerance for Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate because of its good dispersion, low mass-transfer resistance, and good swelling property. Meanwhile, it exhibited superb stability and recyclability, benefitting from the active species being anchored to the highly cross-linked polymer carrier. The catalyst was reused 10 times in water or methanol without significant change in catalytic activity, composition, and microstructure. Moreover, the catalytic performance of the prepared catalyst was weakened by the increased mass-transfer resistance with the movement of active sites from the surface to the interior of the carrier.