Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

This Account reports the synthesis and characterization of dendrimer-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions within dendrimers followed by chemical reduction to yield the corresponding zerovalent metal nanoparticle. The size of such particles depends on the number of metal ions initially loaded into the dendrimer. Intradendrimer hydrogenation and carbon−carbon coupling reactions in water, organic solvents, biphasic fluorous/organic solvents, and supercritical CO2 are also described.

http://rcrooks.cm.utexas.edu/research/resources/Publications/rmc113.pdf

Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis

The applications of copper (Cu) and Cu-based nanoparticles, which are based on the earth-abundant and inexpensive copper metal, have generated a great deal of interest in recent years, especially in the field of catalysis. The possible modification of the chemical and physical properties of these nanoparticles using different synthetic strategies and conditions and/or via postsynthetic chemical treatments has been largely responsible for the rapid growth of interest in these nanomaterials and their applications in catalysis. In addition, the design and development of novel support and/or multimetallic systems (e.g., alloys, etc.) has also made significant contributions to the field. In this comprehensive review, we report different synthetic approaches to Cu and Cu-based nanoparticles (metallic copper, copper oxides, and hybrid copper nanostructures) and copper nanoparticles immobilized into or supported on various support materials (SiO2, magnetic support materials, etc.), along with their applications i...

http://pubs.acs.org/doi/pdfplus/10.1021/acs.chemrev.5b00482

Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis.

Hydrogen peroxide (H2O2) is widely used in almost all industrial areas, particularly in the chemical industry and environmental protection. The only degradation product of its use is water, and thus it has played a large role in environmentally friendly methods in the chemical industry. Hydrogen peroxide is produced on an industrial scale by the anthraquinone oxidation (AO) process. However, this process can hardly be considered a green method. It involves the sequential hydrogenation and oxidation of an alkylanthraquinone precursor dissolved in a mixture of organic solvents followed by liquid–liquid extraction to recover H2O2. The AO process is a multistep method that requires significant energy input and generates waste, which has a negative effect on its sustainability and production costs. The transport, storage, and handling of bulk H2O2 involve hazards and escalating expenses. Thus, novel, cleaner methods for the production of H2O2 are being explored. The direct synthesis of H2O2 from O2 and H2 using a variety of catalysts, and the factors influencing the formation and decomposition of H2O2 are examined in detail in this Review.

http://www.sciencemadness.org/talk/files.php?pid=171164&aid=9838

Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process

Reduced transition metal colloids: a novel family of reusable catalysts

The preparation and catalytic applications of dispersed metal catalysts supported on organic functional polymers are presented. The advantages of these catalysts, such as the easy tailoring with respect to the nature of the used support, the “nanoscale” size control of metal crystallites by the polymer framework, the high accessibility and consequent catalytic activity in a proper liquid or liquid–vapor reaction systems are stressed. Various proposed catalytic processes making use of these materials are presented and evaluated, including multifunctional catalysis, e.g. redox-acid. Interesting peculiar aspects such as the enhancement of the hydrogenation rate by nitrogen containing moieties anchored to the polymer backbone are emphasised. When suitable, a comparison with catalysts based on inorganic supports is given.

Catalysis by metal nanoparticles supported on functional organic polymers

The liquid phase hydrogenation of chloronitrobenzene isomers ( x -CNB, x =2, 3, 4) to corresponding chloroanilines ( x -CAN) at 0.5 MPa and 25 °C was studied. Palladium catalysts containing 1 wt.% of Pd on charcoal and sulphonated poly(styrene- co -divinylbenzene) Pd/D were used. At stoichiometric consumption of hydrogen (3 mol of H 2 /mol of x -CNB) 85 and 90% selectivities were obtained with Pd/C and Pd/D catalysts, respectively. Mathematical treatment of kinetic data revealed the dependence of reaction mechanism on the type of catalyst. A simultaneous hydrogenation of the nitro group and hydrogenolysis of chlorine in x -CNB was observed for the reaction over the Pd/C catalyst. However, over the Pd/D catalyst hydrogenolysis of chlorine is a subsequent reaction. The influence of substituents and sterical hindrance on the substrate reactivity is also discussed.

Effect of catalyst and substituents on the hydrogenation of chloronitrobenzenes

A short history of the relationships among adsorption, chemisorption, and catalysis with solid catalysts is reviewed. A special focus is on the development of quality and descriptions accuracy using computers, both for the modeling of elementary physical phenomena and adsorption, as well as for the solution of more complex problems like quantum chemical approach to chemisorption, kinetics over solid catalysts, and reactor systems. Modern approaches to the characterization of solid catalysts from the adsorption-desorption data based mainly on n-layer adsorption and non-linear three parameter BET isotherm regarding the volume of micropores as one of the parameters are demonstrated. Instrumentation techniques like infrared spectroscopy or NMR techniques for the analysis of the strength of component chemisorption are mentioned. As for the kinetics, a vague capability of the Langmuir-Hinshelwood-Hougen-Watson models to describe a reaction system in more complicated cases, e.g. bimolecular surface reactions, is discussed. In this context, the simplest model with a minimum number of parameters is advised. To estimate the most realistic values, intrinsic reaction kinetic and mass transport phenomena are taken into account. Usefulness of quantum mechanistic models for better understanding of the catalytic phenomena and more efficient design of catalysts are outlined.

Adsorption, chemisorption, and catalysis

Hydrogenation of nitrobenzene (NB) was carried out in methanol solutions (initial concentration 0.8 mol l−1) over a Pd/C catalyst (3 wt.% of Pd; reaction mixture contained 2.2 mg Pd l−1) over the pressure range of 2–4 MPa and temperature 30–70 °C in a laboratory scale batch reactor. Zero order kinetics was observed at hydrogen pressures above 2 MPa Under applied experimental conditions the apparent activation energy was 35 ± 1 kJ mol−1. A detailed analysis of the reaction mixture inspired the hypothesis that a C6H5–NO(H) moiety is formed on the catalyst surface and it undergoes further condensation to azoxybenzene (AOB) releasing water. However, very low concentrations of azobenzene (AB) and hydrazobenzene (HAB) in the reaction mixture indicate that the reaction route to the formation of aniline by hydrogenation of AOB to AB, hydrazobenzene (HAB) and subsequent hydrogenolysis to AN is of low probability. Hydrogenolysis of AOB to C6H5–NO(H) and C6H5–N(H), where the latter is hydrogenated to AN, is more likely. Based on the experimental observations a new reaction scheme for the heterogeneous catalytic hydrogenation of NB was proposed.

Liquid phase hydrogenation of nitrobenzene

Pd–Cu bimetallic catalysts supported on the cationic resin Dowex 1 × 4 (gel type poly(styrene- co -divinylbenzene) with −N(CH 3 ) 3 + Cl − groups) and basic γ-Al 2 O 3 were prepared by ion exchange and the incipient wetness impregnation method, respectively, and tested in the liquid-phase hydrogenation of nitrates in water. Various methods of the reduction of metal precursors were used. The effect of the preparation method, the temperature, the reduction conditions and the support on the properties of palladium–copper composites was investigated by means of X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and temperature programmed reduction (TPR). The Pd–Cu catalysts prepared by liquid-phase reduction of anionic chloro-complexes of palladium(II) and copper(II) immobilised in a resin, mainly isolated domains of metallic palladium and copper, if any, contained. The reduction using hydrogen joined with calcination of palladium and copper salts supported on γ-alumina led to well formed crystallites of a palladium–copper alloy. The Pd–Cu catalysts supported on γ-Al 2 O 3 exhibited higher activity of nitrate removal under given reaction conditions in a batch reactor. However, with the Pd–Cu resin-based catalysts a better final selectivity (selectivity to products other than nitrites and ammonia) at a comparable conversion of nitrates has been obtained in comparison with alumina based catalysts. The catalysts exhibited different stability with respect to the leaching of metals.

Milan Králik

Papers

Catalysis by metal nanoparticles supported on functional organic polymers

Effect of catalyst and substituents on the hydrogenation of chloronitrobenzenes

Adsorption, chemisorption, and catalysis

Liquid phase hydrogenation of nitrobenzene

Supported Pd–Cu catalysts in the water phase reduction of nitrates: Functional resin versus alumina