Magnetically Recoverable Nanoparticulate Catalysts for Cross-Coupling Reactions: The Dendritic Support Influences the Catalytic Performance

Carbon-carbon cross-coupling reactions are among the most important synthetic tools for the preparation of pharmaceuticals and bioactive compounds. However, these reactions are normally carried out using copper, phosphines, and/or amines, which are poisonous for pharmaceuticals. The use of nanocomposite catalysts holds promise for facilitating these reactions and making them more environmentally friendly. In the present work, the PEGylated (PEG stands for poly(ethylene glycol) pyridylphenylene dendrons immobilized on silica loaded with magnetic nanoparticles have been successfully employed for the stabilization of Pd2+ complexes and Pd nanoparticles. The catalyst developed showed excellent catalytic activity in copper-free Sonogashira and Heck cross-coupling reactions. The reactions proceeded smoothly in green solvents at low palladium loading, resulting in high yields of cross-coupling products (from 80% to 97%) within short reaction times. The presence of magnetic nanoparticles allows easy magnetic separation for repeated use without a noticeable decrease of catalytic activity due to the strong stabilization of Pd species by rigid and bulky dendritic ligands. The PEG dendron periphery makes the catalyst hydrophilic and better suited for green solvents. The minor drop in activity upon the catalyst reuse is explained by the formation of Pd nanoparticles from the Pd2+ species during the catalytic reaction. The magnetic separation and reuse of the nanocomposite catalyst reduces the cost of target products as well as energy and material consumption and diminishes residual contamination by the catalyst. These factors as well as the absence of copper in the catalyst makeup pave the way for future applications of such catalysts in cross-coupling reactions.

Bimetallic Nanoparticles Anchored on Core-Shell Support as an Easily Recoverable and Reusable Catalytic System for Efficient Nitroarene Reduction

We report an easily recoverable and reusable versatile magnetic catalyst (Fe3O4@CS_AgNi, where CS = chitosan) for organic reduction reactions. The catalytic system is prepared by dispersing AgNi bimetallic nanoparticles on the magnetite core-shell (Fe3O4@CS). The as-synthesized catalyst has been characterized by spectroscopic techniques, such as IR, UV-vis, and X-ray photoelectron spectroscopy (XPS), and analytical tools, such as thermogravimetric analysis, powder X-ray diffraction, Brunauer-Emmett-Teller adsorption, FEG-scanning electron microscopy, high-resolution transmission electron microscopy (HR-TEM), inductively coupled plasma-atomic emission spectroscopy, and magnetic measurements. HR-TEM studies indicate the core-shell structure of Fe3O4@CS and confirm the presence of AgNi nanoparticles on the surface of Fe3O4@CS spheres. IR spectral and XPS studies lend evidence for the occurrence of a strong chemical interaction between the amino groups of CS and AgNi nanoparticles. The nano-catalyst Fe3O4@CS_AgNi rapidly reduces p-nitrophenol to p-aminophenol using NaBH4 as the reductant within a few minutes under ambient conditions (as monitored by UV-visible spectroscopy). The utility of this catalytic system has also been extended to the reduction of other nitroarenes. A strong interaction between Fe3O4@CS and AgNi nanoparticles impedes the leaching of AgNi nanoparticles from the core-shell support, leading to excellent reusability of the catalyst.

Pt-Functionalized PdO Nanowires for Room Temperature Hydrogen Gas Sensors

In this work, we prepared a well-aligned palladium oxide nanowire (PdO NW) array using the lithographically patterned Pd nanowire electrodeposition (LPNE) method followed by subsequent calcination at 500 °C. Sensitization with platinum (Pt) nanoparticles (NPs), which were functionalized on PdO NWs through a simple reduction process, significantly enhanced the detection capability of the Pt-loaded PdO NWs (Pt-PdO NWs) sensors toward hydrogen gas (H₂) at room temperature. The well-distributed Pt NPs, which are known chemical sensitizers, activated the dissociation of H₂ and oxygen molecules through the spillover effect with subsequent diffusion of these products to the PdO surface, thereby transforming the entire surface of the PdO NWs into reaction sites for H₂. As a result, at a high concentration of H₂ (0.2%), the Pt-PdO NWs showed an enhanced sensitivity of 62% (defined as Δ R/ R_air × 100%) compared to that (6.1%) of pristine PdO NWs. The Pt-PdO NWs exhibited a response time of 166 s, which was 2.68-fold faster than that of pristine PdO NWs (445 s). In addition, the Pt-PdO NWs responded to a very low concentration of H₂ (10 ppm) with a sensitivity of 14%, unlike the pristine PdO NWs, which did not exhibit any response at that concentration. These outstanding results are mainly attributed to a homogeneous decoration of Pt NPs on the surface of well-aligned PdO NWs. In this work, we demonstrated the potential suitability of Pt-PdO NWs as a highly sensitive H₂ sensing layer at room temperature.

Mussel inspired green synthesis of silver nanoparticles-decorated halloysite nanotube using dopamine: characterization and evaluation of its catalytic activity

Naturally occurring ceramic tubular clay, Halloysite nanotubes (HNTs), having a significant amount of surface hydroxyls has been coated by self-polymerized dopamine in this work. The polydopamine-coated HNTs acts as a self-reducing agent for Ag+ ion to Ag0 in nanometer abundance. Herein, nano size Ag0 deposited on solid support catalyst has been used to mitigate water pollution within 10 min. To establish the versatility of the catalyst, nitroaryl (4-nitrophenol) and synthetic dye (methylene blue) have been chosen as model pollutant. The degradation/reduction of the aforementioned pollutants was confirmed after taking UV–visible spectra of the respective compounds. All the study can make sure that the catalyst is green and the rate constant value for catalytic reduction of 4-nitrophenol and methylene blue was calculated to be 4.45 × 10−3 and 1.13 × 10−3 s−1, respectively, which is found to be more efficient in comparison to other nanostructure and commercial Pt/C nanocatalyst (1.00 × 10−3 s−1).

Synergistic Effects of Gold-Palladium Nanoalloys and Reducible Supports on the Catalytic Reduction of 4-Nitrophenol

Herein we report on the catalytic activity of mesoporous nickel, iron, cerium, cobalt, and manganese oxides prepared using KIT-6 as a hard template via evaporation-assisted precipitation. The mesoporous metal oxides (MMOs) were characterized and used as heterogeneous catalysts in the reduction of 4-nitrophenol (4-Nip) by sodium borohydride (BH₄^-). Furthermore, polyamidoamide (PAMAM) dendrimers were used to synthesize gold-palladium nanoalloy particles. The size of AuPd/PAMAM was found to be 3.5 ± 0.8 nm in diameter before being immobilized on the aforementioned mesoporous metal oxides and used as catalysts in the reduction of 4-Nip. Prior to catalytic evaluation, the reduction profiles of the mesoporous metal oxides were investigated by hydrogen-temperature-programmed reduction (H₂-TPR) and showed that mesoporous metal oxides can be easily reduced at lower temperatures and that the immobilization of gold-palladium nanoalloy particles lowers their reduction temperatures. Mesoporous cobalt and manganese oxides showed catalytic activity toward 4-Nip reduction, and the activity was enhanced after immobilization of the gold-palladium nanoalloys. Isolation of nanoparticles activity was achieved by immobilization of the gold-palladium nanoalloys on the inert silica support. From this we postulated an electron relay mechanism for the reduction of 4-nitrophenol. With the use of power rate law we showed that 4-Nip reduction follows pseudo-first-order kinetics.

Synthesis of double-shell hollow magnetic Au-loaded ellipsoids as highly active and recoverable nanoreactors

A schematic for the synthesis of double-shell magnetic Au-loaded ellipsoids (Fe@MO₂–Au@H–SiO₂) and the reaction mechanism for the catalytic reduction of 4-NP.

Direct synthesis of platinum nanodots in ZIF-8/Fe3O4 core–shell hybrid nanoparticles

Platinum nanodots were synthesized inside ZIF-8/Fe₃O₄ core–shell hybrid nanoparticles without additional reducing agents, which showed high catalytic activity for the reduction of 4-nitrophenol.

Solvent-thermal preparation of a CuCo2O4/RGO heterocomposite: an efficient catalyst for the reduction of p-nitrophenol

The catalytic activity of an as-synthesized CuCo₂O₄/RGO composite was researched in the reduction of p-nitrophenol to p-aminophenol by NaBH₄, which exhibited complete conversion in 6 min.