Minfeng Zeng

An efficient and recyclable heterogeneous palladium catalyst utilizing naturally abundant pearl shell waste

An efficient and recyclable ligand-free heterogeneous catalyst has been prepared by the immobilization of palladium onto ground pearl shell powders (Pd/shell powders, Pd/SP). The catalytic activity and recyclability of the prepared Pd/SP along with the charcoal and calcium carbonate supported palladium (Pd/C and Pd/CaCO3) catalysts have been evaluated using the reductive homocoupling of aromatic halides. Pd/SP not only has higher catalytic activity, but also exhibits much stronger stability than Pd/C and Pd/CaCO3. The remarkable Pd/SP stability has been attributed to the chelation of palladium species with the surface chitin and protein molecules of the supported pearl shell powders. The X-ray photoelectron spectroscopy (XPS) studies show that the reductive Pd0 species can be regenerated in situ from the oxidative Pd2+ species for the Pd/SP catalyzed reductive homocoupling of aromatic halides in ethanol/DMSO solution, suggesting that the heterogeneous and homogeneous palladium catalysis proceeds through a similar Pd0/Pd2+ cycle catalytic mechanism.

Novel chitosan-based/montmorillonite/palladium hybrid microspheres as heterogeneous catalyst for Sonogashira reactions

The objective of this study was to develop novel chitosan-based/montmorillonite/palladium (CS/MMT/Pd) hybrid microsphere catalysts with improved properties for use in Sonogashira reactions. Interactions between a chitosan matrix and a montmorillonite nanofiller were revealed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermogravimetric (TG) analysis. The results confirmed the formation of the intercalation structure between CS macromolecules and MMT layers. X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HR-TEM) analysis results showed that the Pd species of different valencies were dispersed at the nano-level in both the CS matrix and the interlayers of MMT. CS/MMT/Pd hybrid microspheres were highly active for the Sonogashira reactions of aryl iodides and alkynes at a palladium catalyst loading of 0.3 mol%. They can be recycled 10 times without a significant decrease in coupling yields. It was concluded that introducing MMT into the CS matrix will effectively improve the thermal stability and Pd leaching-resistance of the hybrid microsphere catalysts. The results in this study demonstrated the great potential of such heterogeneous catalysts applied in Sonogashira reactions.

Novel macroporous palladium cation crosslinked chitosan membranes for heterogeneous catalysis application.

A novel palladium supported on chitosan porous membrane heterogeneous catalyst has been prepared by freeze-drying of Pd(2+)-crosslinked chitosan gel solution. The prepared membrane catalyst has three-dimensional porous structure (porosity: >70%). The crosslinking effects of Pd(2+) to chitosan were good for the improvement of the mechanical properties and thermal stabilities. Pd(2+) cations have been shown not only as the crosslinker, but also as the catalytic active sites. The reductive palladium species of the recycled membrane catalysts was found in the nanometer scale (20-40nm). Excellent cross-coupling yields were achieved using as low as 0.12mol% palladium catalyst loading for the Heck-type reaction of aromatic halides with acrylates. The catalyst could be recycled six times without obvious decreased conversion.

Microstructure-stability relations studies of porous chitosan microspheres supported palladium catalysts

In this study, polyethylene glycol (PEG) with different molecular weight, polyvinyl pyrrolidone (PVP), and polyvinyl alcohol (PVA), are chosen as porogens for preparing chitosan base porous microsphere supported palladium catalyst for coupling reactions. The pore structure of the microspheres was controlled by the compatibility of chitosan and counterpart polymers. The prepared porous chitosan microspheres supported palladium heterogeneous catalysts have been evaluated using the well-established Ullmann reductive homocoupling and the Heck cross-coupling reactions. The activities, stabilities and recyclability of the porous chitosan microspheres supported palladium catalysts are not only highly dependent upon the surface areas of the solid supports, but also upon the chemical properties of the water-soluble polymers. The degradation of the prepared heterogeneous palladium catalysts is mainly caused by a combination of the palladium leaching and the morphological transformation of the palladium species from the amorphous into the crystals.

N-doped mesoporous carbons supported palladium catalysts prepared from chitosan/silica/palladium gel beads.

In this study, a heterogeneous catalyst including palladium nanoparticles supported on nitrogen-doped mesoporous carbon (Pd@N-C) is synthesized from palladium salts as palladium precursor, colloidal silica as template, and chitosan as carbon source. N2 sorption isotherm results show that the prepared Pd@N-C had a high BET surface area (640m(2)g(-1)) with large porosity. The prepared Pd@N-C is high nitrogen-rich as characterized with element analysis. X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy characterization of the catalyst shows that the palladium species with different chemical states are well dispersed on the nitrogen-containing mesoporous carbon. The Pd@N-C is high active and shows excellent stability as applied in Heck coupling reactions. This work supplies a successful method to prepare Pd heterogeneous catalysts with high performance from bulk biopolymer/Pd to high porous nitrogen-doped carbon supported palladium catalytic materials.

Chitosan microspheres supported palladium heterogeneous catalysts modified with pearl shell powders

Significant enhancement of the catalytic stability and activity was obtained for the heterogeneous palladium catalyst supported on the shell powders-reinforced chitosan microspheres. For example, over 90% cross-coupling yields were achieved using as low as 0.05 mol% palladium catalyst loading for the Heck-type reaction of iodobenzene with n-butyl acrylate. Such significant enhancement of the catalytic stability and activity can be attributed to the intermolecular interactions of the surface polar molecules of the incorporated shell powders with the surrounding chitosan molecules as well as the deposited palladium species.

A Novel Chitosan/Polyvinyl Pyrrolidone (CS/PVP) Three-Dimensional Composite and Its Mechanism of Strength Improvement

Novel biodegradable three-dimensional (3D) composites with good mechanical properties have been prepared by coagulation of a chitosan/polyvinyl pyrrolidone (CS/PVP) solution in NaOH. For example, the strength and modulus of CS/PVP (1/1) were 82.5 MPa and 1.86 GPa, increasing 237% and 644% compared with CS, respectively. Scanning electron microscopy and Fourier transform infrared analysis suggest that the PVP component did not dissolve during the preparation process. The nonsolution of the composites is attributed to the extremely strong hydrogen bonding formed between the CS and PVP macromolecules. It was also found that there are synergistic effects between the formation of hydrogen bonding with PVP and cross-linking with glutaraldehyde (GA) for the improvement in the mechanical properties of CS. The mechanism of strength improvement has been discussed thoroughly from the aspects of free volume.

Preparation and Characterization of a Novel Composite of Chitosan/Shell Particles

Nanometer sized (mean size: 433.9 nm) pearl shell particles (SP) were prepared with a ball mill. Thermal analysis and Fourier transform infrared (FTIR) results proved that the SP contained mainly CaCO3 (about 95%) and a small organic phase (about 5%). Novel biodegradable composites based upon chitosan (CS) and SPs were prepared using an in situ precipitation method. The organic components of SP are highly compatible with both CS and glutaraldehyde (GA). The strength and modulus of CS gel rods, prepared by coagulation of a CS/SP/GA solution in NaOH, in both the dry and wet state, were improved remarkably with addition of appropriate amounts of SP together with GA. For example, with a composition of 3% SP and 0.3% GA added, the strength and modulus were 42.9 MPa and 1.41 GPa, increasing 75 and 464% compared with CS alone, respectively. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results showed that GA acts as not only a crosslinking agent for CS, but also a compatibilizer for CS and SP. It was found that the modified rods have smaller free volume size but nearly the same free volume fraction as pure CS rods. The good interfacial adhesion and more compact microstructure can sensitively reflect the changes in the free volume of the composites.

Free-volume properties and toughening behavior of cyanate ester resin/carboxyl-randomized liquid butadiene-acrylonitrile rubber composites

The toughness of cyanate ester resin (CE) matrix was improved significantly with addition of carboxyl-randomized liquid butadiene-acrylonitrile rubber (CRBN). The curing behavior of the system was studied by differential scanning calorimetric (DSC) and Fourier transform infrared spectrum (FTIR). The results showed that carboxyl groups on the CRBN chain had slight activation effect to CE curing reaction at the beginning of the cure process. Phase separation was the main toughening mechanism for CE/CRBN composites. The existence of micro-size pores induced by small amount of the low weight molecular part of CRBN might be another toughening mechanism. The toughening mechanism was proved powerfully from the aspect of free-volume using positron annihilation lifetime spectroscopy (PALS). PALS is qualitatively sensitive to the existence of pores induced by low molecular weight part of CRBN during curing process.

Co-immobilization of Pd and Zn nanoparticles in chitosan/silica membranes for efficient, recyclable catalysts used in ullmann reaction

In this study, an efficient heterogeneous catalyst including palladium (Pd) and zinc (Zn) nanoparticles supported on chitosan/silica (CS/SiO2) composite membrane is synthesized using partially etching of SiO2 technique. N2 sorption isotherm results shows that the prepared Pd-Zn@CS/SiO2 (1/1) porous membrane had a BET surface area of 26.50m2/g. High-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS), characterization of the catalyst shows that the Pd0 nanoparticles (below 5nm), and Zn0 aggregates (about 10-15nm) dispersed well in the CS/SiO2 matrix. Zinc crystal was also detected by X-ray diffraction study and HR-TEM observation. The prepared Pd-Zn@CS/SiO2 membrane catalyst is highly active for Ullmann reductive homocoupling reactions of aryl iodides and aryl bromides, and can be recycled 5 times without significant loss of activities. This work supplies a successful approach to realize Pd catalysts and Zn reducing reagent co-immobilized in the same carrier material with excellent catalytic performances.