A Uniform Bimetallic Rhodium/Iron Nanoparticle Catalyst for the Hydrogenation of Olefins and Nitroarenes

Synthesis of Saturated N-Heterocycles via a Catalytic Hydrogenation Cascade

Saturated N-heterocycles are prominent motifs found in various natural products and pharmaceuticals. Despite the increasing interest in this class of compounds, the synthesis of saturated bicyclic azacycles requires tedious multi-step syntheses. Herein, we present a one-pot protocol for the synthesis of octahydroindoles, decahydroquinolines, and octahydroindolizines through a cascade reaction.

Alloying at a Subnanoscale Maximizes the Synergistic Effect on the Electrocatalytic Hydrogen Evolution

Bonding dissimilar elements to provide synergistic effects is an effective way to improve the performance of metal catalysts. However, as the properties become more dissimilar, achieving synergistic effects effectively becomes more difficult due to phase separation. Here we describe a comprehensive study on how subnanoscale alloying is always effective for inter-elemental synergy. Thirty-six combinations of both bimetallic subnanoparticles (SNPs) and nanoparticles (NPs) were studied systematically using atomic-resolution imaging and catalyst benchmarking based on the hydrogen evolution reaction (HER). Results revealed that SNPs always produce greater synergistic effects than NPs, the greatest synergistic effect was found for the combination of Pt and Zr. The atomic-scale miscibility and the associated modulation of electronic states at the subnanoscale were much different from those at the nanoscale, which was observed by annular-dark-field scanning transmission electron microscopy (ADF-STEM) and X-ray photoelectron spectroscopy (XPS), respectively.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Heterogeneous Dendrimer-Based Catalysts

The present review compiles the advances in the dendritic catalysis within the last two decades, in particular concerning heterogeneous dendrimer-based catalysts and their and application in various processes, such as hydrogenation, oxidation, cross-coupling reactions, etc. There are considered three main approaches to the synthesis of immobilized heterogeneous dendrimer-based catalysts: (1) impregnation/adsorption on silica or carbon carriers; (2) dendrimer covalent grafting to various supports (silica, polystyrene, carbon nanotubes, porous aromatic frameworks, etc.), which may be performed in a divergent (as a gradual dendron growth on the support) or convergent way (as a grafting of whole dendrimer to the support); and (3) dendrimer cross-linking, using transition metal ions (resulting in coordination polymer networks) or bifunctional organic linkers, whose size, polarity, and rigidity define the properties of the resulted material. Additionally, magnetically separable dendritic catalysts, which can be synthesized using the three above-mentioned approaches, are also considered. Dendritic catalysts, synthesized in such ways, can be stored as powders and be easily separated from the reaction medium by filtration/centrifugation as traditional heterogeneous catalysts, maintaining efficiency as for homogeneous dendritic catalysts.

Controlling the Oxidation of Magnetic and Electrically Conductive Solid-Solution Iron-Rhodium Nanoparticles Synthesized by Laser Ablation in Liquids

This study focuses on the synthesis of FeRh nanoparticles via pulsed laser ablation in liquid and on controlling the oxidation of the synthesized nanoparticles. Formation of monomodal γ-FeRh nanoparticles was confirmed by transmission electron microscopy (TEM) and their composition confirmed by atom probe tomography (APT). For these particles, three major contributors to oxidation were analysed: (1) dissolved oxygen in the organic solvents, (2) the bound oxygen in the solvent and (3) oxygen in the atmosphere above the solvent. The decrease of oxidation for optimized ablation conditions was confirmed through energy-dispersive X-ray (EDX) and Mössbauer spectroscopy. Furthermore, the time dependence of oxidation was monitored for dried FeRh nanoparticles powders using ferromagnetic resonance spectroscopy (FMR). By magnetophoretic separation, B2-FeRh nanoparticles could be extracted from the solution and characteristic differences of nanostrand formation between γ-FeRh and B2-FeRh nanoparticles were observed.

Rhodium Nanoparticles Stabilized by PEG-Tagged Imidazolium Salts as Recyclable Catalysts for the Hydrosilylation of Internal Alkynes and the Reduction of Nitroarenes

PEGylated imidazolium (bromide and tetrafluoroborate) and tris-imidazolium (bromide) salts containing triazole linkers have been used as stabilizers for the preparation of water-soluble rhodium(0) nanoparticles by reduction of rhodium trichloride with sodium borohydride in water at room temperature. The nanomaterials have been characterized (Transmission Electron Microscopy, Electron Diffraction, X-ray Photoelectron Spectroscopy, Inductively Coupled Plasma-Optical Emission Spectroscopy). They proved to be efficient and recyclable catalysts for the stereoselective hydrosilylation of internal alkynes, in the presence or absence of solvent, and in the reduction of nitroarenes to anilines with ammonia-borane as hydrogen donor in aqueous medium (1:4 tetrahydrofuran/water).

One-pot synthesis of cobalt-rhenium nanoparticles taking the unusual β-Mn type structure

Using a facile one-pot colloidal method, it is now possible to obtain monodisperse Co_1-x Re _x nanoparticles (NPs), with excellent control of Re stoichiometry for x < 0.15. Re-incorporation in terms of a solid solution stabilizes the β-Mn polymorph relative to the hcp/ccp variants of cobalt. The NPs are thermally stable up to 300 °C, which may make them attractive as model catalysts.

Highly uniform Rh nanoparticles supported on boron doped g-C3N4 as a highly efficient and recyclable catalyst for heterogeneous hydroformylation of alkenes

A highly efficient and recyclable Rh/B-g-C₃N₄ catalyst was firstly applied in hydroformylation of alkenes.

Hydrogenation of Functionalized Nitroarenes Catalyzed by Single-Phase Pyrite FeS2 Nanoparticles on N,S-Codoped Porous Carbon

Catalytic hydrogenation of nitroarenes is an industrially very important and environmentally friendly process for the production of anilines; however, highly chemoselective reduction of nitroarenes decorated with one or more reducible groups in a nitroarene molecule remains a challenge. Herein, a novel hybrid non-noble iron-based nanocatalyst (named as FeS₂ /NSC) was developed, which was prepared from biomass as C and N source together with inexpensive Fe(NO₃ )₃ as Fe source through high-temperature pyrolysis in a straightforward and cost-effective procedure. Comprehensive characterization revealed that single-phase pyrite FeS₂ nanoparticles with precisely defined composition and uniform size were homogeneously dispersed on N,S-codoped porous carbon with large specific surface area, hierarchical porous channels, and high pore volume. The resultant catalyst FeS₂ /NSC demonstrated good catalytic activity for hydrogenation of functionalized nitroarenes with good tolerance of various functional groups in water as a sustainable and green solvent. Compared with bulk pyrite FeS₂ and other non-noble metal-based heterogeneous catalysts reported in the literature, a remarkably enhanced activity was observed under mild reaction conditions. More importantly, FeS₂ /NSC displayed exclusive chemoselectivity for the reduction of nitro groups for nitroarenes bearing varying readily reducible groups.

Birch-Type Reduction of Arenes in 2-Propanol Catalyzed by Zero-Valent Iron and Platinum on Carbon

Catalytic arene reduction was effectively realized by heating in 2-propanol/water in the presence of Pt on carbon (Pt/C) and metallic Fe. 2-Propanol acted as a hydrogen source, obviating the need for flammable (and hence, dangerous and hard-to-handle) hydrogen gas, while metallic Fe acted as an essential co-catalyst to promote reduction. The chemical states of Pt and Fe in the reaction mixture were determined by X-ray absorption near-edge structure analysis, and the obtained results were used to suggest a plausible reaction mechanism, implying that catalytic reduction involved Pt- and Fe-mediated single-electron transfer and the dehydrogenation of 2-propanol.

The catalytic evaluation of bimetallic Pd-based nanocatalysts supported on ion exchange resin in nitro and alkyne reduction reactions

A simple protocol to produce bimetallic Pd-based nanocatalysts supported on Amberlite resin is presented. Superior catalytic activity and selectivity in nitro and alkyne reduction reactions was achieved, opening up new possibilities in the development of cheaper and selective catalysts for these important industrial reactions.

A hierarchical flower-like hollow alumina supported bimetallic AuPd nanoparticle catalyst for enhanced solvent-free ethylbenzene oxidation

Currently, oxidation of alkylaromatics is considered as one of the most crucial chemical technologies to produce high added-value alcohols, ketones and carboxylic acids, due to its significant importance both in fine synthetic chemistry and in the academic field. In this work, a novel hierarchical marigold-like hollow alumina supported bimetallic AuPd nanoparticle catalyst was successfully fabricated and employed for highly efficient solvent-free ethylbenzene oxidation to produce acetophenone with the coexistence of both molecular oxygen and tert-butyl hydroperoxide as the oxidant and the initiator. The as-fabricated bimetallic AuPd nanocatalyst conferred a superior catalytic performance to the corresponding monometallic counterparts and commercial Al2O3 or solid Al2O3 microsphere supported AuPd ones, along with a high acetophenone selectivity of 88.2% at a conversion of 50.9% under mild reaction conditions (120 °C and oxygen pressure of 1.0 MPa), as well as an unprecedentedly high turnover frequency value of 46 768 h-1. Such exceptional efficiency of the catalyst was related to both the significant synergy between the Au-Pd atoms and strong metal-support interactions, and the unique hierarchical micro/nanostructure of the support being beneficial to the close contact of reactants with surface adsorption and reaction sites and easy product diffusion. Moreover, the present bimetallic AuPd catalyst was recyclable and stable. The developed approach is expected to offer exciting opportunities for designing other supported monometallic or bimetallic catalysts with various active components applied in heterogeneous catalysis.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Nitrogen-doped graphene-activated metallic nanoparticle-incorporated ordered mesoporous carbon nanocomposites for the hydrogenation of nitroarenes

Herein, nanoscale metallic nanoparticle-incorporated ordered mesoporous carbon catalysts activated by nitrogen-doped graphene (NGr) were fabricated via an efficient multi-component co-assembly of a phenolic resin, nitrate, acetylacetone, the nitrogen-containing compound 1,10-phenanthroline, and Pluronic F127, followed by carbonization. The obtained well-dispersed nitrogen-doped graphene-activated transition metal nanocatalysts possess a 2-D hexagonally arranged pore structure with a high surface area (∼500 m² g⁻¹) and uniform pore size (∼4.0 nm) and show excellent activity for the selective hydrogenation–reduction of substituted nitroarenes to anilines in an environmentally friendly aqueous solution. The high catalytic performance and durability is attributed to the synergistic effects among the components, the unique structure of the nitrogen-doped graphene layer-coated metallic nanoparticles, and electronic activation of the doped nitrogen.

The non-noble M/NGr@OMC-T catalyst exhibits excellent catalytic activity and chemoselectivity for the hydrogenation of nitroarenes to anilines in water with hydrogen.

Finely controlled multimetallic nanocluster catalysts for solvent-free aerobic oxidation of hydrocarbons

The catalytic activity of alloy nanoparticles depends on the particle size and composition ratio of different metals. Alloy nanoparticles composed of Pd, Pt, and Au are widely used as catalysts for oxidation reactions. The catalytic activities of Pt and Au nanoparticles in oxidation reactions are known to increase as the particle size decreases and to increase on the metal-metal interface of alloy nanoparticles. Therefore, multimetallic nanoclusters (MNCs) around 1 nm in diameter have potential as catalysts for oxidation reactions. However, there have been few reports describing the preparation of uniform alloy nanoclusters. We report the synthesis of finely controlled MNCs (around 1 nm) using a macromolecular template with coordination sites arranged in a gradient of basicity. We reveal that Cu-Pt-Au MNCs supported on graphitized mesoporous carbon show catalytic activity that is 24 times greater than that of a commercially available Pt catalyst for aerobic oxidation of hydrocarbons. In addition, solvent-free aerobic oxidation of hydrocarbons to ketones at room temperature, using small amounts of a radical initiator, was achieved as a heterogeneous catalytic reaction for the first time.

Silver and palladium alloy nanoparticle catalysts: reductive coupling of nitrobenzene through light irradiation

Silver and palladium alloy nanoparticle catalysts for reductive coupling of nitrobenzene through light irradiation.

Structure Evolution and Hydrogenation Performance of IrFe Bimetallic Nanomaterials

By a reverse microemulsion method, a series of IrFe bimetallic nanomaterials of variable morphologies and compositions is synthesized and characterized by (57)Fe Mössbauer spectroscopy, XRD, XPS, and TEM. The structure evolution, such as IrFe alloy nanoparticles to Ir nanoparticles on Fe2O3 flakes, can be simply tuned by changing the molar ratio of Ir to Fe precursors. In terms of Fe, the relative content of IrFe alloy decreased with the increase of Fe species doped, while that of Fe2O3 flakes increased until reached 100%. The as-prepared IrFe bimetallic nanomaterials were served as catalysts for the selective hydrogenation of 3-nitrostyrene to 3-aminostyrene, and it is found that the catalytic performance was related to the morphology and composition of these nanomaterials. Ir1Fe4 was subsequently identified to be a highly active and exceedingly selective catalyst with good stability and recyclability for the hydrogenation of 3-nitrostyrene, underscoring a remarkable "synergistic effect" of the two metals appearing as the form of Ir nanoparticles loaded on Fe2O3 flakes. For Ir nanoparticles, they act as an active species for the hydrogenation; for Fe2O3 flakes, they favor the preferential adsorption of nitro groups, which account for the better chemoselectivity to objective product.

Generation of Cu nanoparticles on novel designed Fe3O4@SiO2/EP.EN.EG as reusable nanocatalyst for the reduction of nitro compounds

Reusable magnetic Cu nanocatalyst was designed and characterized. An efficient and mild method for the reduction of aromatic nitro compounds.

Facile reduction of aromatic nitro compounds to aromatic amines catalysed by support-free nanoporous silver

Reduction of aromatic nitro compounds to aromatic amines was realized in excellent yields by using nanoporous silver as a sustainable, heterogeneous catalyst.

Precision synthesis of subnanoparticles using dendrimers as a superatom synthesizer

Classical metal-based nanomaterials come in two prominent types: a mononuclear or multinuclear complex chemically stabilized by organic ligands or a nanoparticle (also called a nanorod, nanosheet, or nanocrystal) physically stabilized by inorganic or polymer supports. Over the last decade, a class of superatoms that lies between these categories of materials has attracted attention because their properties are dramatically different from those typically ascribed to their component elements. Typically the superatoms include a specific, low number of metallic atoms. Because a one-atom difference can alter the properties of these superatoms, their synthesis must be ultraprecise, requiring one-atom resolution. To date, researchers have only been able to prepare monodisperse superatoms using gas-phase synthesis followed by purification through a flight tube. Though this technique provides monodisperse superatoms, it does not allow researchers to produce them in large quantites. Other researchers have proposed ligand-assisted liquid-phase synthesis as an alternative, but this technique is only useful for a few stable "magic number" clusters. Recently researchers have developed a new approach for the synthesis of superatoms that employs a novel class of molecular templates, which can define the number of metal ions or salts precisely. As a result, researchers can now synthesize nanoparticles or even subnanoparticles successfully. A dendrimer-type template has proven to be especially useful for ultraprecise control of the atomicity of the product, but it works with a full range of metal elements. In this Account, we highlight recent advances in the precise preparation of metal-assembling complexes using the dendrimer as a template. Next we discuss the selective assembly of subnanoparticles that utilize the dendrimer as a superatom synthesizer. The resulting subnanoparticles are almost monodisperse, and as a result, some of them exhibited distinctive characteristics based on their atomicity. For example, because of the quantum-size effect, the reduction in particle size of TiO2 and other metal-oxide subnanoparticles led to a significant shift in the band-gap energy. In addition, a miniaturized platinum particle less than 1 nm in diameter showed unexpectedly high catalytic activity for the oxygen reduction reaction (ORR) and other related reactions. Of particular note, in all these examples, this substantial change in their properties arose out of a single-atom difference in the atomicity. These results suggest that next-generation subnanoparticle design could play an important role in new materials and offer an additional palette of physical and chemical properties for new applications.

A PdAg bimetallic nanocatalyst for selective reductive amination of nitroarenes

Herein we have identified an optimal catalyst, Pd1Ag1.7, for the tandem reductive amination between nitroarenes and aldehydes (selectivity > 93%). Key to the success is the ability to control the compositions of the investigational Pd1-xAgx (x = 0-1) catalysts, as well as the clear composition dependent activity/selectivity trend observed in this study. This catalyst features a wide substrate scope, excellent recyclability, activity and selectivity under ambient conditions.

Synthesis and characterization of TiO2 nanotube supported Rh-nanoparticle catalysts for regioselective hydroformylation of vinyl acetate

Three procedures: the impregnation-borohydride reduction procedure, the impregnation-alcohol reduction procedure and the impregnation-photoreducing procedure, were utilized for preparing TiO₂ nanotube supported rhodium nanoparticle catalysts.

Synergistic effect from Lewis acid and the Ni–W2C/AC catalyst for highly active and selective hydrogenation of aryl nitro to aryl amine

Highly-active and selective hydrogenation of nitroarenes to their corresponding aromatic amines by synergism from Ni–W₂C/AC and Lewis acid is presented.