Bio-inspired fabrication of chitosan/PEO/Ti3C2Tx 2D MXene nanosheets supported palladium composite nanofiber catalysts via electrospinning

In this study, novel chitosan/polyethylene oxide/Ti3C2Tx 2D MXene nanosheets (CS/PEO/Ti3C2Tx) nanofibers were successfully prepared by a continuous electrospinning process. During the electrospinning process, induced by the syringe tip capillary effects and electric field force, the Ti3C2Tx nanosheets were aligned along the direction of the nanofiber formation to occur a highly oriented structure. This well-ordered arrangement of the inorganic Ti3C2Tx nanosheets within the organic polymer matrix nanofiber was similar with nacre-like 'brick-and-motar' structure to some extent, resulting in a marked increase in thermal stability and mechanical properties of the resultant CS/PEO/Ti3C2Tx nanofiber. As 4 wt% of Ti3C2Tx nanosheets loaded, the highest tensile strength of the CS/PEO/Ti3C2Tx nanofiber mats was achieved as 31.7 MPa, about two times that of neat CS/PEO nanofibers. Uniformly dispersed Pd nanoparticles in size of about 1.6 nm have been successfully immobilized on the composite nanofiber with a solution impregnation process. With a loading as low as 0.2 mol% of Pd, the resultant Pd@CS/PEO/Ti3C2Tx composite nanofiber catalysts were highly active for both Heck and Sonogashira coupling reactions with broad reactants application scope, and could be recycled 15 runs without significant loss of activities.

Palladium nanoparticles anchored on NiO particles-modified micro-size chitosan spheres: A promising, active, and retrievable catalyst system for treatment of environmental pollutants

Efficient treatment of toxic organic pollutants in water/wastewater by using innovative, cost efficient, and simple technologies has recently become an important issue worldwide. Remediation of these pollutants with chemical reduction in the presence of a nano-sized catalyst and a reducing agent is one of the most useful methodologies. In the present study, we have designed a promising heterogeneous catalyst system (Pd@CS-NiO) by easy and efficient stabilization of palladium nanoparticles on the surface of microspheres composed of chitosan (CS)-NiO particles (CS-NiO) for the reduction of organic pollutants. The nano-structure of the developed Pd@CS-NiO was successfully validated using FE-SEM, XRD, EDS, TEM, and FTIR/ATR and its particles size was determined as 10 nm. The catalytic power of Pd@CS-NiO was then assessed in the reduction of 4-nitro-o-phenylenediamine (4-NPDA), 4-nitrophenol (4-NP), 4-nitroaniline (4-NA), 2-nitroaniline (2-NA), and some organic dyes, namely methylene blue (MB), methyl orange (MO), and rhodamine B (RhB) in aqueous medium at room temperature. The reductions were thoroughly studied spectro-photometrically. The tests displayed that the synthesized Pd@CS-NiO was a highly active and useful catalyst that reduced these pollutants in 0-145 s. Moreover, the rate constants for 2-NA, 4-NP, 4-NA, 4-NPDA, MO, and RhB were found to be 0.017 s-1, 0.011 s-1, 0.006 s-1, 0.013 s-1, 0.023 s-1, and 0.03 s-1, respectively. Moreover, the recycling test indicated that Pd@CS-NiO may be recovered easily thanks to its micro size nature and could be used up to seven steps, confirming its practical application potential.

Palladium Supported on Bioinspired Materials as Catalysts for C–C Coupling Reactions

In recent years, the immobilization of palladium nanoparticles on solid supports to prepare active and stable catalytic systems has been deeply investigated. Compared to inorganic materials, naturally occurring organic solids are inexpensive, available and abundant. Moreover, the surface of these solids is fully covered by chelating groups which can stabilize the metal nanoparticles. In the present review, we have focused our attention on natural biomaterials-supported metal catalysts applied to the formation of C–C bonds by Mizoroki–Heck, Suzuki–Miyaura and Sonogashira reactions. A systematic approach based on the nature of the organic matrix will be followed: (i) metal catalysts supported on cellulose; (ii) metal catalysts supported on starch; (iii) metal catalysts supported on pectin; (iv) metal catalysts supported on agarose; (v) metal catalysts supported on chitosan; (vi) metal catalysts supported on proteins and enzymes. We will emphasize the effective heterogeneity and recyclability of each catalyst, specifying which studies were carried out to evaluate these aspects.

Green Synthesis of Pd Nanoparticles for Sustainable and Environmentally Benign Processes

Among transition metal nanoparticles, palladium nanoparticles (PdNPs) are recognized for their high catalytic activity in a wide range of organic transformations that are of academic and industrial importance. The increased interest in environmental issues has led to the development of various green approaches for the preparation of efficient, low-cost and environmentally sustainable Pd-nanocatalysts. Environmentally friendly solvents, non-toxic reducing reagents, biodegradable capping and stabilizing agents and energy-efficient synthetic methods are the main aspects that have been taken into account for the production of Pd nanoparticles in a green approach. This review provides an overview of the fundamental approaches used for the green synthesis of PdNPs and their catalytic application in sustainable processes as cross-coupling reactions and reductions with particular attention afforded to the recovery and reuse of the palladium nanocatalyst, from 2015 to the present.

Chitosan: A Natural Biopolymer with a Wide and Varied Range of Applications

Although chitin is of the most available biopolymers on Earth its uses and applications are limited due to its low solubility. The deacetylation of chitin leads to chitosan. This biopolymer, composed of randomly distributed β-(1-4)-linked D-units, has better physicochemical properties due to the facts that it is possible to dissolve this biopolymer under acidic conditions, it can adopt several conformations or structures and it can be functionalized with a wide range of functional groups to modulate its superficial composition to a specific application. Chitosan is considered a highly biocompatible biopolymer due to its biodegradability, bioadhesivity and bioactivity in such a way this biopolymer displays a wide range of applications. Thus, chitosan is a promising biopolymer for numerous applications in the biomedical field (skin, bone, tissue engineering, artificial kidneys, nerves, livers, wound healing). This biopolymer is also employed to trap both organic compounds and dyes or for the selective separation of binary mixtures. In addition, chitosan can also be used as catalyst or can be used as starting molecule to obtain high added value products. Considering these premises, this review is focused on the structure and modification of chitosan as well as its uses and applications.

Chitosan modified Ti-PILC supported PdOx catalysts for coupling reactions of aryl halides with terminal alkynes

Biopolymer of chitosan (CS) and titanium pillared clays (Ti-PILCs) have been combined in a hybrid as advanced supports for immobilization of PdOx=0,1 species to prepare novel PdOx=0,1@Ti-PILC/CS nano-composite catalysts. The Ti-PILC materials showed high specific surface areas and abundant meso-porous structure with many irregular pore channels caused by collapses of layered structure of clay during Ti pillaring process. Both CS chains and sub-nano sized PdOx particles were successfully incorporated into the pore channels of Ti-PILC, resulting in a decrease in both the specific surface areas and uniform distribution of pore size. Besides conventional methods characterizations, the strong interactions between PdOx species and Ti-PILC/CS support were further evidenced with positron annihilation lifetime spectroscopy studies. The resultant PdOx@Ti-PILC/CS catalyst was highly active for the coupling reactions of aryl halides with phenyl acetylenes. It was recyclable and gave excellent yield up to 13 runs with low leaching of Pd species.

Bis-salophen palladium complex immobilized on Fe3O4@SiO2 nanoparticles as a highly active and durable phosphine-free catalyst for Heck and copper-free Sonogashira coupling reactions

A new Fe₃O₄@SiO₂ superparamagnetic nanoparticles functionalized by a bis-salophen Schiff base Pd(ii)-complex and used as efficient catalyst for Heck and Sonogashira reactions in aqueous media.

Insightful understanding of the correlations of the microstructure and catalytic performances of Pd@chitosan membrane catalysts studied by positron annihilation spectroscopy

In this study, the catalytic performances of palladium supported on chitosan (Pd@CS) membrane heterogeneous catalysts have been studied from the aspects of free volume by positron annihilation lifetime spectroscopy (PALS). The results showed that the variation in free volume hole size of the Pd@CS membrane catalyst was closely associated with microstructure evolutions, such as increase of Pd content, valence transition of Pd by reduction treatment, solvent swelling, physical aging during catalyst recycling, and so on. The PALS results showed that both the mean free volume hole size of the Pd⁰@CS membrane in the dry or swollen state (analyzed by the LT program) and its distribution (analyzed by the MELT program) are smaller than the molecule size of the reactants and products in the catalysis reaction. However, the results showed that the Pd⁰@CS membrane catalyst has excellent catalytic activity for the Heck coupling reaction of all the reactants with different molecule size. It was revealed that the molecule transport channels of the Pd⁰@CS membrane catalyst in the reaction at high temperature was through a number of instantaneously connected free volume holes rather than a single free volume hole. This hypothesis was powerfully supported by the catalytic activity assessment results of the CS layer sealed Pd⁰@CS membrane catalyst. Meanwhile, it was confirmed that the leaching of Pd⁰ nanoparticles of the reused Pd⁰@CS membrane catalyst during the recycling process was also through such instantaneously connected free volume holes.

A number of instantaneously connected free volume holes act as mass transport channels of the Pd⁰@CS membrane catalyst in reactions.

Encaging Palladium Nanoparticles in Chitosan Modified Montmorillonite for Efficient, Recyclable Catalysts

Metal nanoparticles, once supported by a suitable scaffolding material, can be used as highly efficient heterogeneous catalysts for numerous organic reactions. The challenge, though, is to mitigate the continuous loss of metals from the supporting materials as reactions proceed, so that the catalysts can be recycled multiple times. Herein, we combine the excellent chelating property of chitosan (CS) and remarkable stability of montmorillonite (MMT) into a composite material to support metal catalysts such as palladium (Pd). The in situ reduction of Pd²⁺ into Pd⁰ in the interstices of MMT/CS composites effectively encages the Pd⁰ nanoparticles in the porous matrices, while still allowing for reactant and product molecules of relatively small sizes to diffuse in and out the matrices. The prepared Pd⁰@MMT/CS catalysts are highly active for the Heck reactions of aromatic halides and alkenes, and can be recycled 30 times without significant loss of activities. Positron annihilation lifetime analysis and other structural characterization methods are implemented to elucidate the unique compartmentalization of metal catalysts in the composite matrices. As both CS and MMT are economical and abundant materials in nature, this approach may facilitate a versatile platform for developing highly recyclable, heterogeneous catalysts containing metal nanoparticles.

PEPPSI-SONO-SP2: a new highly efficient ligand-free catalyst system for the synthesis of tri-substituted triazine derivatives via Suzuki–Miyaura and Sonogashira coupling reactions under a green approach

Consecutive Suzuki–Miyaura and Sonogashira cross coupling reactions catalyzed by a new competent palladium catalyst PEPPSI-SONO-SP² under mild and green reaction conditions.

Synthesis and characterization of functionalized titania-supported Pd catalyst deriving from new orthopalladated complex of benzophenone imine: catalytic activity in the copper-free Sonogashira cross-coupling reactions at low palladium loadings

Efficient catalytic activity of TiO₂-supported Pd catalyst in the copper, amine and phosphine free Sonogashira cross-coupling reactions at low Pd-loadings.