Modification of Montmorillonite with Polyethylene Oxide and Its Use as Support for Pd0 Nanoparticle Catalysts.

In this study, montmorillonite (MMT) was modified by intercalating polyethylene oxide (PEO) macromolecules between the interlayer spaces in an MMT-water suspension system. X-ray diffraction results revealed that the galleries of MMT were expanded significantly after intercalation of different loading of PEO. MMT/PEO 80/20 composite was chosen as the support platform for immobilization of Pd species in preparing novel heterogeneous catalysts. After immobilization of Pd species, the interlayer spacing of MMT/PEO (80/20) (1.52 nm) was further increased to 1.72 nm (Pd2+@MMT/PEO) and 1.73 nm (Pd0@MMT/PEO), confirming the well-immobilization of the Pd species in the interlayer spaces of PEO-modified MMT. High-resolution transmission electron microscopy (HR-TEM) observation results confirmed that Pd nanoparticles were confined inside the interlayer space of MMT and/or dispersed well on the outer surface of MMT. The conversion of Pd2+ to Pd0 species was evidenced by binding energy characterization with X-ray photo electron spectroscopy (XPS). The microstructure variation caused by the Pd immobilization was sensitively detected by positron annihilation lifetime spectroscopy (PALS) studies. The prepared Pd0@MMT/PEO (0.2/80/20) catalytic composite exhibits good thermal stability up to around 200 °C, and it showed high activities for Heck reactions between aryl iodides and butyl acrylates and could be recycled for five times. The correlations between the microstructure and properties of the Pd@MMT/PEO catalytic composites were discussed.

One-pot carbonization of chitosan/P123/PdCl2 blend hydrogel membranes to N-doped carbon supported Pd catalytic composites for Ullmann reactions.

Novel porous nitrogen-doped carbon supported Pd (Pd@N-C) catalytic composites were prepared by one-pot thermal carbonization of chitosan/poly(ethylene glycol)­block­poly(propylene glycol)­block­poly(ethylene glycol)/PdCl2 (CS/P123/PdCl2) blend hydrogel membranes at different temperature in N2 atmosphere. The porous structure of the Pd@N-C catalytic composite was governed by both the addition of P123 and the carbonization temperature. At highest carbonization temperature of 900 °C, the prepared Pd@N-C catalytic composite from CS/P123/PdCl2 blend membrane showed the highest specific area (SBET) of 293.7 m2/g and total volume of pores (Vtot) of 0.79 cm3/g. The chemical state of the elements of C, N, O, Pd within the Pd@N-C catalytic composites were confirmed with X-ray photoelectron spectroscopy (XPS) measurements. Raman spectrum results showed that the prepared Pd@N-C catalytic composite contained mainly disordered carbon together with some graphite carbon. Pd nanoparticles sized in 5-20 nm dispersed well on the porous matrix of the carbon. The Pd@N-C catalytic composites showed excellent activities for the Ullmann homo-coupling reactions of aromatic halides, and can be recycled for 10 times. In such one-pot carbonization process, the polymer porogen is simultaneously decomposed without further etching and removal steps, which simplifies the preparation process and is beneficial to obtain Pd@N-C catalytic composites with desirable Pd loading.

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 Pd2+ into Pd° 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.