Halloysite Nanotubes as Support for Metal-Based Catalysts

Halloysite nanotubes (HNTs) are a natural, biocompatible, environmental friendly and cheap double-layered aluminosilicate mineral that has a predominantly hollow tubular structure. The general stoichiometry of halloysite is Al2Si2O5·4(H2O). The layer units consist of a tetrahedral SiOH sheet stacked with an edge shared octahedral AlO6 sheet with an internal aluminol group AlOH. A water layer exists between the adjacent two layers. Thanks to their structural features, HNTs are suitable for a potential application as support for catalytic composites. Recently, we reported the synthesis of novel palladium-based catalytic systems using halloysite nanotubes modified with imidazolium or triazolium moieties as supports for PdNPs and we successfully employed these supported catalysts in the Suzuki reaction under microwave irradiation. Herein we report an efficient strategy to prepare HNTs-based catalyst through direct chemical grafting with stimuli-responsive polymer (PNIPAAM) coordinating PdNPs. The HNT-PNIPAAM/PdNPs was tested as catalyst in the Suzuki reaction under microwave irradiation.