Fe3O4@UiO-66-NH2 core–shell nanohybrid as stable heterogeneous catalyst for Knoevenagel condensation

Magnetic separation is an attractive alternative to filtration or centrifugation for separating solid catalysts from a liquid phase. Here, core-shell Fe3O4@UiO-66-NH2 nanohybrids with well-defined structures were constructed by dispersing magnets in a dimethylformamide (DMF) solution containing two metal–organic framework (MOF) precursors, namely ZrCl4 and 2-aminobenzenetricarboxylic acid. This method is simpler and more efficient than previously reported step-by-step method in which magnets were consecutively dispersed in DMF solutions each containing one MOF precursor, and the obtained Fe3O4@UiO-66-NH2 with three assembly cycles has a higher degree of crystallinity and porosity. The core-shell Fe3O4@UiO-66-NH2 is highly active and selective in Knoevenagel condensations because of the bifunctionality of UiO-66-NH2 and better mass transfer in the nano-sized shells. It also has good recycling stability, and can be recovered magnetically and reused at least four times without significant loss of catalytic activity and framework integrity. The effects of substitution on the reactivity of benzaldehyde and of substrate size were also investigated.