Pd Nanoparticles in Ionic Liquid Brush: A Highly Active and Reusable Heterogeneous Catalytic Assembly for Solvent-Free or On-Water Hydrogenation of Nitroarene under Mild Conditions

Tuning the electronic structure of Pd by the surface configuration of Al2O3 for hydrogenation reactions

The electronic interaction between a metal and a support modulates the electronic structures of supported metals and plays an important role in manipulating their catalytic performance. However, this interaction is mainly realized in heterogeneous catalysts composed of reducible oxides. Herein, we demonstrate the electronic interaction between γ-Al2O3 and η-Al2O3 with varying acid-base properties and supported Pd nanoparticles (NPs) of 2 nm in size. The strength and number of acid-base sites on the supports and catalysts were systemically characterized by FT-IR spectroscopy and TPD. The supported Pd NPs exhibit electron-rich surface properties by receiving electrons from the electron-donating basic sites on γ-Al2O3, which are beneficial for catalyzing the hydrogenation of nitrobenzene. In contrast, Pd NPs loaded on η-Al2O3 are electron-deficient because of the rich electron-withdrawing acid sites of η-Al2O3. As a result, Pd/η-Al2O3 exhibits higher catalytic activity in phenylacetylene hydrogenation than Pd/γ-Al2O3. Our results suggest a promising route for designing high-performance catalysts by adjusting the acid-base properties of Al2O3 supports to maneuver the electronic structures of metals.

Comparative Study of Batch and Continuous Flow Reactors in Selective Hydrogenation of Functional Groups in Organic Compounds: What Is More Effective?

Many research papers describe selective hydrogenation of functional groups, such as nitro groups, carbonyl groups, or unsaturated carbon bonds to obtain fine chemicals or precursors of pharmaceuticals. Quite often, the catalyst performance is investigated in batch or continuous flow reactors without finding advantages and disadvantages of this or that regime. At the same time, the transition from batch processes to continuous flow occurs on the industrial scale. However, the batch process can be preferable for some reactions, in spite of its drawbacks. This review article aims to identify all publications that consider selective hydrogenation of functional groups in organic compounds, both in batch and continuous flow reactors, at the same reaction conditions that allow making conclusions about the benefits of one of the regimes in a particular case.

The influence of adsorption geometry on the reduction affinity of nitroaromatics on Au(111)

Chemoselective reduction of nitro groups in multifunctional nitroaromatics is a challenging catalytic process with high interest due to the importance of the resulting anilines for the chemical industry. Molecular-level understanding of the ways by which adsorption geometry of nitroaromatics influence their affinity toward nitro reduction will enable the development of highly selective reactions. Herein, taking advantage of the well-ordered self-assembly of para- and ortho-nitrothiophenol (p-NTP and o-NTP, respectively) monolayers on Au(111), we examined the correlation between adsorption geometry and nitro reduction affinity. The anchoring geometry of NTPs and their nitro reduction affinity were determined by conducting polarized X-ray absorption spectroscopy while the influence of NTPs' adsorption geometry on the interaction with the Au surface was analyzed by density functional theory (DFT) calculations. Exposure of surface anchored p-NTPs to reducing conditions led to their reorientation from a tilt angle of 52° to 25°, which enabled strong interactions between the π system of the molecules and the Au surface. Direct correlation was identified between the surface proximity of the nitro group, its parallel position to the surface and the resulting reduction yield. The asymmetric structure of o-NTP led to a tilted adsorption geometry in which the nitro group was rotated away from the plane of the aromatic ring and therefore was positioned parallel and in high proximity to the Au surface. This positioning led to surface-bonding that involved the oxygen atoms of o-NTP. The higher surface proximity and stronger surface interactions of the nitro group in o-NTP enabled nitro reduction already at 180 °C, while in p-NTP nitro reduction was achieved only at 230 °C, due to the longer distance between the NO₂ group and the Au surface that led to weaker adsorbate-surface interactions. Thus, parallel positioning of the nitro group and high surface proximity were found as essential descriptors for nitro reduction affinity in both p-NTP and o-NTP on the Au surface. These findings provide explicit guidelines for tuning the reactant and surface properties in order to control the reactant's adsorption geometry for selective nitro reduction in multifunctional nitroaromatics.

Metal-Free, Rapid, and Highly Chemoselective Reduction of Aromatic Nitro Compounds at Room Temperature

In this study, we developed a metal-free and highly chemoselective method for the reduction of aromatic nitro compounds. This reduction was performed using tetrahydroxydiboron [B₂(OH)₄] as the reductant and 4,4'-bipyridine as the organocatalyst and could be completed within 5 min at room temperature. Under optimal conditions, nitroarenes with sensitive functional groups, such as vinyl, ethynyl, carbonyl, and halogen, were converted into the corresponding anilines with excellent selectivity while avoiding the undesirable reduction of the sensitive functional groups.

Recent Progress in Pd-Based Nanocatalysts for Selective Hydrogenation

Selective hydrogenation plays an important role in the chemical industry and has a wide range of applications, including the production of fine chemicals and petrochemicals, pharmaceutical synthesis, healthcare product development, and the synthesis of agrochemicals. Pd-based catalysts have been widely applied for selective hydrogenation due to their unique electronic structure and ability to adsorb and activate hydrogen and unsaturated substrates. However, the exclusive and comprehensive summarization of the size, composition, and surface and interface effect of metal Pd on the performance for selective hydrogenation is still lacking. In this perspective, the research progress on selective hydrogenation using Pd-based catalysts is summarized. The strategies for improving the catalytic hydrogenation performance over Pd-based catalysts are investigated. Specifically, the effects of the size, composition, and surface and interfacial structure of Pd-based catalysts, which could influence the dissociation mode of hydrogen, the adsorption, and the reaction mode of the catalytic substrate, on the performance have been systemically reviewed. Then, the progress on Pd-based catalysts for selective hydrogenation of unsaturated alkynes, aldehydes, ketones, and nitroaromatic hydrocarbons is revealed based on the fundamental principles of selective hydrogenation. Finally, perspectives on the further development of strategies for chemical selective hydrogenation are provided. It is hoped that this perspective would provide an instructive guideline for constructing efficient heterogeneous Pd-based catalysts for various selective hydrogenation reactions.

Unsupported Nanoporous Platinum-Iron Bimetallic Catalyst for the Chemoselective Hydrogenation of Halonitrobenzenes to Haloanilines

An unsupported nanoporous platinum-iron bimetallic catalyst (PtFeNPore) was prepared with an electrochemical dealloying technique. Its structure and composition were characterized through various measurement methods, such as X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS). An intermetallic compound and iron oxide species were both found in the PtFeNPore catalyst. The nanoporous structure and Lewis acidity (caused by iron oxide species) of the PtFeNPore catalyst resulted in superior catalytic activity and high selectivity. The PtFeNPore-catalyzed hydrogenation of various halonitrobenzenes proceeded successfully under mild reaction conditions and produced good to excellent yields of the corresponding haloanilines with high selectivity. PtFeNPore can be recycled through magnetic separation easily and reused five times without significant deactivation.

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).

Role of dissolved oxygen in nitroarene reduction by a heterogeneous silver textile catalyst in water

The effects of dissolved oxygen concentration on the rate constant of the 4-nitrophenol reduction reaction with a silver-coated textile as a ‘dip-catalyst’ were studied.

Enhanced Hydrogenation Performance over Hollow Structured Co-CoOx@N-C Capsules

It is desirable to design nonprecious metal nanocatalysts with high stability and catalytic performance for fine chemicals production. Here, a method is reported for the preparation of cobalt metal and cobalt oxide cores confined within nanoporous nitrogen-doped hollow carbon capsules. Core-shell structured Zn/Co-ZIF@polymer materials are fabricated through a facile coating polymer strategy on the surface of zeolitic imidazolate frameworks (ZIF). A series of hollow carbon capsules with cobalt metal and cobalt oxide are derived from a facile confined pyrolysis of Zn/Co-ZIF@polymer. The hollow Co-CoOx@N-C capsules can prevent sintering and agglomeration of the cobalt nanoparticles and the nanoporous shell allows for efficient mass transport. The specific surface area and Co particle size are optimized through finely tuning the original Zn content in ZIF particles, thus enhancing overall catalytic activity. The yolk-shell structured Zn4Co1Ox@carbon hollow capsules are shown to be a highly active and selective catalyst (selectivity >99%) for hydrogenation of nitrobenzene to aniline. Furthermore, Zn4Co1Ox@carbon hollow particles show superior catalytic stability, and no deactivation after 8 cycles of reaction. The hollow Co-CoOx@N-C capsules may shed light on a green and sustainable catalytic process for fine chemicals production.

Electronic Modulation of Palladium in Metal Phosphide Nanoparticles for Chemoselective Reduction of Halogenated Nitrobenzenes

Tuning the electronic property of a transition metal plays an important role in the selective catalysis. Herein, the control synthesis of (Pd_x Ni_y )-P nanoparticles is reported. The binding energy of Pd3d_5/2 as a function of x/y ratio is well tunable from 335.3 to 335.9 eV. The composition-induced electronic modulation was correlated with the selective catalysis of (Pd_x Ni_y )-P in the reduction of halogenated nitrobenzenes. The electro-deficiency of Pd helped to improve the selectivity. The amorphous (Pd₃₈ Ni₂₆ )P₃₆ /C performed an exceptional selectivity in comparison with other related (Pd-Ni)-P/C, Pd₃₈ Ni₂₆ /C, and Pd/C. Various halogenated nitrobenzenes (chlorides, bromides, and iodide) were tolerant and the corresponding halogenated anilines were obtained in high yields. This work provides some clues for the rational design of bimetallic phosphides with covalent interactions to boost the catalysis.

Cross-linked chitosan with a dicationic ionic liquid as a recyclable biopolymer-supported catalyst for cycloaddition of carbon dioxide with epoxides into cyclic carbonates

A dicationc ionic liquid was synthesized and immobilized on chitosan as a catalyst for cycloaddition of CO₂ with epoxides for synthesis of cyclic carbonates.

Highly efficient aqueous phase reduction of nitroarenes catalyzed by phosphine-decorated polymer immobilized ionic liquid stabilized PdNPs

Phosphino-decorated polymer immobilised ionic liquid-stabilised PdNPs are highly efficient catalysts for the aqueous phase hydrogenation and transfer hydrogenation of aromatic nitro compounds in batch and continuous flow.

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.

Nitrogen-rich graphitic-carbon stabilized cobalt nanoparticles for chemoselective hydrogenation of nitroarenes at milder conditions

Herein, we report the synthesis of nitrogen-rich graphitic-carbon supported cobalt nanoparticles and its application for the hydrogenation of nitroarenes in the water-THF mixture at 50 °C with maximum TOF of 14 117 h⁻¹.

Microwave-activated Ni/carbon catalysts for highly selective hydrogenation of nitrobenzene to cyclohexylamine

Biocarbon supported Ni catalysts have been prepared by facile impregnation of Ni species by microwave-heating and used for selective hydrogenation of nitrobenzene to cyclohexylamine. These catalysts were characterized by X-ray diffraction, Raman spectra, N2 sorption measurement, X-ray photoelectron spectroscopy, temperature programmed reduction of H2 and H2 temperature-programmed desorption. The morphology and particle size of catalysts were imaged by scanning electron microscope and transmission electron microscope. For the hydrogenation of nitrobenzene to cyclohexylamine, 10%Ni/CSC-II(b) exhibits the best catalytic activity to achieve 100 mol% conversion of nitrobenzene and 96.7% selectivity of cyclohexylamine under reaction conditions of 2.0 MPa H2 and 200 °C, ascribed to high dispersion of Ni species and formation of nanosized Ni particles on the support aided by microwave-heating. Thus-prepared Ni/CSC catalyst is greatly activated, in which the addition of precious metal like Rh is totally avoided.

Single-Site Cobalt Catalysts at New Zr12(μ3-O)8(μ3-OH)8(μ2-OH)6 Metal-Organic Framework Nodes for Highly Active Hydrogenation of Nitroarenes, Nitriles, and Isocyanides

We report here the synthesis of a robust and porous metal-organic framework (MOF), Zr₁₂-TPDC, constructed from triphenyldicarboxylic acid (H₂TPDC) and an unprecedented Zr₁₂ secondary building unit (SBU): Zr₁₂(μ₃-O)₈(μ₃-OH)₈(μ₂-OH)₆. The Zr₁₂-SBU can be viewed as an inorganic node dimerized from two commonly observed Zr₆ clusters via six μ₂-OH groups. The metalation of Zr₁₂-TPDC SBUs with CoCl₂ followed by treatment with NaBEt₃H afforded a highly active and reusable solid Zr₁₂-TPDC-Co catalyst for the hydrogenation of nitroarenes, nitriles, and isocyanides to corresponding amines with excellent activity and selectivity. This work highlights the opportunity in designing novel MOF-supported single-site solid catalysts by tuning the electronic and steric properties of the SBUs.

Cobalt complexes of pyrrolecarboxamide ligands as catalysts in nitro reduction reactions: influence of electronic substituents on catalysis and mechanistic insights

Square-planar Co³⁺ complexes, displaying substrate binding abilities and facile Co^3+/2+ redox potentials, function as efficient catalysts for hydrazine-mediated reduction of aromatic, heterocyclic, as well as aliphatic nitro substrates.

Ionic liquid brush as an efficient and reusable heterogeneous catalytic assembly for the tosylation of phenols and alcohols in neat water

An efficient and reusable heterogeneous catalytic assembly-ionic liquid brush was developed for tosylation of phenols and alcohols in neat water.