Heterogeneous Catalytic Composites from Palladium Nanoparticles in Montmorillonite Intercalated with Poly (Vinyl Pyrrolidone) Chains

In this study, poly (vinyl pyrrolidone) (PVP) chains intercalated montmorillonite (MMT) matrices has been demonstrated as an excellent scaffolding material for the immobilization of palladium (Pd) nanoparticles to prepare efficient heterogeneous catalysts for Heck reactions. Multiple layers (up to four) of PVP chains can intercalate the interlayer space of the MMT, resulting in an increase therein from 1.25 to 3.22 nm. MMT/PVP with PVP loading (20%) was selected as the platform for the immobilization of Pd. The in-situ reduction of the chelated Pd(2+) into Pd(0) in the interlayer space of MMT/PVP composite could be easily achieved. For the prepared Pd@MMT/PVP catalytic composite, a unique maze-like microstructure of Pd nanoparticles tightly encaged by PVP chains and by lamellae of layered silica has been detected by high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). Furthermore, the microstructure is well elucidated in molecular level by positron annihilation lifetime analysis of the Pd@MMT/PVP catalytic composite. The prepared Pd@MMT/PVP catalysts were highly active for the Heck coupling reactions between aromatic halides and alkenes, and could be recycled 9 times without significant decreases in coupling yields. The excellent comprehensive catalytic performances of the Pd@MMT/PVP catalytic composites are mainly attributed to their unique maze-like microstructure.