Amino-functionalized Zr(IV) metal-organic framework as bifunctional acid-base catalyst for Knoevenagel condensation

The amino-functionalized metal-organic framework of Zr(IV) with 2-aminoterephthalate, UiO-66-NH₂, was studied as a solid catalyst for Knoevenagel condensation. The material can efficiently catalyze the condensation reaction of benzaldehyde with ethyl cyanoacetate or malononitrile in highly polar solvents such as DMF, DMSO and ethanol. The catalytic system has also been tested for various aromatic aldehydes, the conversion easily reaching more than 90% under mild conditions. It was demonstrated that the catalytic process is heterogeneous and shows size effects, characteristic of a porous catalyst. The catalyst can be recycled without losing its framework integrity and catalytic activity. The catalytic activity has been compared with dimethyl 2-aminoterephthalate and the isostructural amino-free MOF (UiO-66). The superior performance of UiO-66-NH₂ has been attributed to the site-isolated acid-base bifunctional character. It has been proposed that the Zr site in close proximity to the amino group activates aldehydes to promote the formation of aldimine intermediates from the aldehydes and the amino group.