Halloysite nanotubes (HNTs)@ZIF-67 composites—a new type of heterogeneous catalyst for the Knoevenagel condensation reaction

Composite materials based on metal-organic frameworks (MOFs) have shown outstanding performance due to their high porosity, molecular-level characterization, and structural and functional tunability. In this article, we develop a new type of composite material-HNTs@ZIF-67-by the in situ growth of ZIF-67 nanoparticles (NPs) on halloysite nanotubes (HNTs), which were characterized by SEM, TEM, PXRD, FT-IR, TGA, XPS and N2 adsorption-desorption isotherms. The results clearly indicate that HNTs were wrapped in the ZIF-67 shell with a thickness of 50 nm which is much smaller than the 500 nm size of the as-synthesized ZIF-67. The nano-sized HNTs@ZIF-67 can effectively catalyze the Knoevenagel condensation reaction of larger conjugated/heterocyclic aromatic formaldehydes with malononitrile. The catalytic activities with >99% yields for the reaction of 4-pyridinecarboxaldehyde with malononitrile were maintained even after three cycles, and the composite still retained the original structure and morphology.