Reduction Of Nitroaromatics Research Articles

Plasmon-enhanced and controllable synthesis of azobenzene and hydrazobenzene using Au/TiO2 composite

The selective reduction of nitroaromatics has long been the focus of fundamental and practical interest. The challenging issue lies in how to finely control the product selectivity. Herein, we report a light-assisted thermocatalytical process to convert nitrobenzene into azobenzene and hydrazobenzene with high selectivity and high yields by simply controlling the reaction time. The light-enhanced efficiency of Au/TiO2 catalyst is up to 14.8% at 90 °C due to the localized surface plasmon resonance (LSPR) effect of Au nanoparticles. Moreover, the hydrogen-delivery rate here is much faster than that of current reported systems. The reaction mechanism was also investigated by EPR technique, where Au–H intermediate species is found to account for the high efficiency in this reduction process. The favorable results demonstrated that plasmon-enhanced catalytic reaction might be more attractive and applicable in industrial organic synthesis.

Palladium Nanocatalysts on Hydroxyapatite: Green Oxidation of Alcohols and Reduction of Nitroarenes in Water

A green procedure is described for supporting Pd nanoparticles on hydroxyapatite (HAP), which serves as a highly-stable heterogeneous catalyst displaying excellent activity for the aqueous expeditious reduction of nitroaromatics to the corresponding amines with sodium borohydride, and oxidation of primary and secondary alcohols by hydrogen peroxide with high yields and selectivities. The structural features of the prepared catalyst are confirmed by latest techniques including field emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The reusability of the heterogeneous catalyst was affirmed in the aqueous reduction of nitrobenzene and oxidation of cycloheptanol for six consecutive runs without significant loss of catalytic activity.

Effect of linker and metal on photoreduction and cascade reactions of nitroaromatics by M-UiO-66 metal organic frameworks

The use of metal organic frameworks (MOFs) as photocatalysts is a promising and growing area of research. Given the diverse architectures, linkers, and metals, it is important to understand their effects on catalysis. Herein we compare six MOFs of the UiO-66 family towards photocatalytic reduction of nitro-aromatics to anilines. These MOFs differ in metal identity (Hf, Zr) and linker, and hence this systematic study provides insights to developing next generation MOFs. We found that Hf-based MOFs are superior to the more commonly studied Zr-analogues. Moreover, the linker identity also impact the photocatalysis, with pyridine-based linkers out-performing aniline based linkers and those that lack an embedded basic site. The MOFs studied have unique selectivities for the photoreduction and also allow for the one-pot synthesis of imines from aromatic aldehydes and nitro-aromatics.

Atomic Pd on Graphdiyne/Graphene Heterostructure as Efficient Catalyst for Aromatic Nitroreduction

AbstractWith the maximum atom‐utilization efficiency, single atom catalysts (SACs) have attracted great research interest in catalysis science recently. To address the following key challenges for the further development of SACs: i) how to stabilize and avoid the aggregation of SACs, ii) how to enhance the specific surface area and conductivity of supports, and iii) how to achieve scalable mass production with low cost, a SAC consisting of single Pd atoms anchored on well‐designed graphdiyne/graphene (GDY/G) heterostructure (Pd1/GDY/G) is synthesized. Pd1/GDY/G exhibits high catalytic performance, as demonstrated by the reduction reaction of 4‐nitrophenol. Furthermore, density functional theory calculation indicates that graphene in the GDY/G heterostructure plays a key role in the enhancement of catalytic efficiency owing to the electron transfer process, deriving from the gap between the Fermi level of graphene and the conduction band minimum of GDY. The GDY/G heterostructure is a promising support for the preparation of extremely efficient and stable SACs, which can be used in a broad range of future industrial reactions.

Synthesis and Catalytic Activities of Metal Shells (Monolayer, Bilayer, and Alloy Layer)-Coated Gold Octahedra toward Catalytic Reduction of Nitroaromatics

Different types of metal shells such as monolayer, bilayer, and alloy layer of gold (Au) and/or silver (Ag) on gold octahedra (AuOh) nanoparticles were synthesized using N-[3-(trimethoxysilyl)propyl]diethylenetriamine (TPDT) without employing any external reducing agents. TPDT, an alternative to sodium borohydride, was used to prepare 15 different core–shell metal nanostructures. The direct colloidal synthesis and stabilization of silver octahedra nanoparticles is still a challenging task. The proposed methodology is an alternative synthetic route for Ag or Au nanoshell coated onto the AuOh core. UV–vis absorption spectroscopy, high-resolution electron transmission microscopy analyses with selected area electron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, and X-ray diffraction techniques were used to characterize the prepared core–shell octahedra nanostructures. High-angle annular dark field scanning transmission electron microscopy–energy-dispersive X-...

Core-Shell Au@SnO2 Nanostructures Supported on Na2Ti4O9 Nanobelts as a Highly Active and Deactivation-Resistant Catalyst toward Selective Nitroaromatics Reduction.

Catalysis using gold (Au) nanoparticles has become an important field of chemistry. However, activity loss caused by aggregation or leaching of Au nanoparticles greatly limits their application in catalytic reaction. Herein, we report a facile and green synthesis of a core-shell Au@SnO2 nanocomposite, exhibiting excellent activity toward selective nitroaromatics reduction under mild conditions. The core-shell Au@SnO2 nanocomposite (Au size = ∼50 nm; shell thickness = ca. 16 nm) is conceived and validated by a direct redox reaction between HAuCl4 and SnF2. Optimization of the core size, shell thickness, and dispersion of Au@SnO2 has been introduced by an alkaline surface supported by negatively charged metal oxide Na2Ti4O9. The as-obtained Au-Sn-Na2Ti4O9 catalyst with much smaller Au cores (ca. 5 nm) and thinner SnO2 nondensed shells (ca. 4 nm) exhibits highly improved catalytic activities for nitro reduction compared to most of the known Au-based catalysts. Moreover, the core-shell Au@SnO2 structure inhibits the leaching and agglomeration of Au nanoparticles and thus leads to superior catalytic durability.

Novel protocol for synthesis of 1,4-diiminocurcumin stabilized silver nanoparticles and application as heterogenous recyclable catalyst and antibacterial agent

Curcumin [(1E, 6E)-1,7-bis(4-hydroxy-3-methoxy-phenyl)-1,6-heptadiene-3,5-dione] is a low molecular weight yellow-orange polyphenolic pigment extracted from the powdered rhizome of Curcuma longa. Curcumin has wide medicinal applications as an antioxidant, anti-inflammatory, cancer chemopreventive, and potentially chemotherapeutic agents as well as stabilizer/reducing agent in silver nanoparticles (AgNPs) synthesis. However, the low solubility of curcumin in aqueous solutions limits its applications and also, many of AgNP synthetic processes lack a greener synthetic route. In the present work, a Schiff base of curcumin is synthesized condensing curcumin and 1,4-diaminobutane in 2:1 ratio. The resulting product shows improvement in solubility in water and favours the synthesis of AgNPs in aqueous medium at room temperature, acting as a self-reducing/stabilizing agent. This proposed synthetic route is simple, feasible and green. The size and morphology of AgNPs are analyzed by TEM, SEM, EDS and XRD techniques. The recyclable AgNPs as a heterogeneous catalyst in the reduction of nitroaromatics to amino compounds is environmentally benign and can be re-used up to 5th cycle without considerable loss of its catalytic activity. Moreover, both Cur-1,4 and AgNPs show bactericidal properties against bacterial strains (Bacillus subtilis, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa) which find medicinal importance in future. A greener approach has been proposed for the preparation of AgNPs stabilized on curcumin based Schiff base. The AgNPs finds applications as efficient, easily recyclable heterogenous catalyst in the reduction of nitroaromatics to environmentally benign aminoaromatics as well as an antibacterial agent.

Hierarchical porous carbon material restricted Au catalyst for highly catalytic reduction of nitroaromatics.

In this study, four kinds of porous carbon materials were used as supports to anchor gold nanoparticles (AuNPs) for catalytic reduction of nitroaromatics and 4-nitrophenol (4-NP) was employed as a model material. Results identified that carbon black (CB) restricted-Au catalyst (Au/CB) provided large specific surface area, small AuNPs size, and low cost, which showed highly catalytic activity for 4-NP reduction. Besides, with the increase of Au loadings, the catalytic activity of Au/CB was enhanced and the 1.2 wt% of Au loading exhibited the best catalytic activity with the high rate of 0.8302 min-1 and the turnover frequency of 492.50 h-1. Universality and real water application demonstrated that the as-prepared Au/CB catalyst was promising candidate for other phenols and azo dyes reduction and had great potential for practical application. Furthermore, after ten cycles, Au/CB still retained satisfying stability and activity. These results suggested that the larger specific surface area and smaller particle size attributing to the porosity of CB were conducive to improving the catalytic activity of Au catalysts. This design shows high potential of hierarchical porous carbon materials for highly catalytic reaction in many fields, especially the water purification.

Catalytic Application of Tactically Aligned Cd(II)‐Based Luminescent 3D‐Supramolecular Networks

AbstractA new Cd(II) based coordination polymers {[Cd(Azopy)(H2O)4]⋅(NDC)}n (1) has been designed and synthesized by using 4,4′‐Azopyridine (Azopy), and 2, 6‐Naphthalene dicarboxylic acid (NDC) as a linker at ambient condition. The bulk purity of the 1 was analysed by powder X‐ray diffraction (PXRD) analysis, infrared spectroscopy and further confirmed by single crystal X‐ray studies. The crystal structure of 1 revealed a polythreading feature where each 2, 6‐naphthalene dicarboxylate unit are trapped between two one‐dimensional (1D) chain of [Cd(Azopy)(H2O)4]∞ via strong hydrogen bonding interactions. The hydrogen bonding and π⋅⋅⋅π stacking interactions of 1 play a crucial role in the formation of the infinite 3D‐layered supramolecular network. Furthermore, the reduction of nitro‐aromatics to their corresponding amines was performed using green hydrogen source such as D‐glucose in H2O solvent at room temperature. Additionally, the dye adsorption studies revealed that 1 can adsorb Congo Red (CR) dye in a very short span of time. The kinetic study result reveals that the adsorption of CR for 1 is fitted well (R2 =0.996) with a pseudo‐second‐order kinetic model. Moreover, photoluminescence of 1 shows emission in the blue region with λmax at 395 nm and 408 nm at room temperature in the solid state. 1 shows multi‐facet behaviour such as facile synthesis, luminescent nature in solids‐state, emerged as a heterogeneous dye‐adsorbent and catalyst which makes it easy to scaled‐up.

Stable Pd@Cu Core–Shell Nanocubes with Finely Tuned Sizes for the Reduction of Nitroaromatics

This paper reports a seed-mediated growth of Pd@Cu core–shell nanocubes with controllable edge lengths from 49 to 103 nm in an aqueous system using cetyltrimethylammonium bromide (CTAB) capped Pd n...

Self-Assembled Magnetic Pt Nanocomposites for the Catalytic Reduction of Nitrophenol

Self-assembled nanotechnology has been used widely in various well-defined microstructures fabrication. Herein, this synthetic protocol was successfully utilized to in situ or hierarchically prepare magnetic Pt nanocomposites Fe3O4@Pt via a distinguishing two-step process. The distinction of these two pathways is due to the different synthetic processes. In the case of in situ preparation (Path I), it involves a supramolecular self-assembly of boron clusters with γ-CD on Fe3O4 substrate and then followed by in situ formation of Pt nanoparticles (NPs). But in the case of hierarchical preparation (Path II), the first step is preparing Pt NPs colloids reduced by boron clusters Cs2[closo-B12H12], and subsequent step is immobilizing the as-prepared [closo-B12H12]2–-capped Pt NPs on the Fe3O4@γ-CD. The success of these two fabrication strategies lies in the use of trifunctional boron clusters (reductant, stabilizer, and superchaotropic anchor). Both of the as-prepared Fe3O4@Pt nanocomposites showed good catalytic performance in the selective nitro-group reduction of nitroaromatics. A control test further indicated that the in situ prepared Fe3O4@Pt nanocomposites, with smaller size and higher loading of Pt NPs, exhibited a better catalytic performance and recyclability.

Experimental Validation of Hydrogen Atom Transfer Gibbs Free Energy as a Predictor of Nitroaromatic Reduction Rate Constants.

Nitroaromatic compounds (NACs) are a class of prevalent contaminants. Abiotic reduction is an important fate process that initiates NAC degradation in the environment. Many linear free energy relationship (LFER) models have been developed to predict NAC reduction rates. Almost all LFERs to date utilize experimental aqueous-phase one-electron reduction potential ( EH1) of NAC as a predictor, and thus, their utility is limited by the availability of EH1 data. A promising new approach that utilizes computed hydrogen atom transfer (HAT) Gibbs free energy instead of EH1 as a predictor was recently proposed. In this study, we evaluated the feasibility of HAT energy for predicting NAC reduction rate constants. Using dithionite-reduced quinones, we measured the second-order rate constants for the reduction of seven NACs by three hydroquinones of different protonation states. We computed the gas-phase energies for HAT and electron affinity (EA) of NACs and established HAT- and EA-based LFERs for six hydroquinone species. The results suggest that HAT energy is a reliable predictor of NAC reduction rate constants and is superior to EA. This is the first independent, experimental validation of HAT-based LFER, a new approach that enables rate prediction for a broad range of structurally diverse NACs based solely on molecular structures.

Stimulating Charge Transfer Over Quantum Dots via Ligand-Triggered Layer-by-Layer Assembly toward Multifarious Photoredox Organic Transformation

Semiconductor quantum dots (QDs) have garnered tremendous attention by virtue of their substantial light-harvesting and conversion efficiencies, large number of active sites, unique quantum size confinement, and multiple exciton generation effects. In this regard, recent years have witnessed their widespread applications in photocatalysis. Nonetheless, intrinsic disadvantages of QDs including unfavorable photostability, ultrafast charge recombination rate, and sluggish kinetic of charge carriers retard the construction of high-efficiency QDs-based photocatalysts for solar energy conversion. Thus far, in-depth investigation on the visible-light-driven photoredox organic transformation over QDs has not yet been exhaustively explored, and corresponding photocatalytic mechanisms remain elusive. In this work, selecting cadmium selenide (CdSe) as a quintessential category of semiconductor QDs, we demonstrated a facile, green, easily accessible, and rather efficient electrostatic self-assembly strategy to conspicuously boost the versatile photoredox performances of CdSe QDs toward selective organic transformation under visible light irradiation by intimately integrating with graphene (GR) via judicious surface charge tuning. In this scenario, intrinsically negatively charged CdSe QDs and surface-modified positively charged GRs were utilized as the building blocks for spontaneous electrostatic self-assembly buildup, which gives rise to well-defined CdSe QDs–GR ensembles. More intriguingly, ligands capped on the CdSe QDs surface enable the alternate layer-by-layer (LbL) assembly of CdSe QDs and GR forming three-dimensional spatially multilayered heterostructures. Furthermore, it was significant to unveil that such self-assembled CdSe QDs–GR nanocomposites exhibit remarkably enhanced and multifunctional photoredox performances toward selective oxidation of aromatic alcohols to corresponding aromatic aldehydes and selective reduction of nitroaromatics to amino compounds under visible light irradiation, which far exceeds the pristine CdSe QDs counterpart, which exhibits almost negligible photoactivities. This can be ascribed to the pivotal role of GR for conspicuously capturing and shuttling electrons from band-gap photoexcitation of CdSe QDs, intimate interfacial contact between the building blocks, and enlarged specific surface area stemming from seamless GR encapsulation and intercalation, along with the unique ligand-triggered LbL assembly integration mode between CdSe QDs and GR, hence synergistically reducing the recombination rate and prolonging the lifetime of charge carriers. Furthermore, photoredox mechanisms of the CdSe QDs–GR ensemble were elucidated. It is anticipated that our work would afford an efficacious avenue to finely modulate the charge transport over QDs for solar energy conversion.

Calixarene-Assisted Pd Nanoparticles in Organic Transformations: Synthesis, Characterization, and Catalytic Applications in Water for C-C Coupling and for the Reduction of Nitroaromatics and Organic Dyes.

A new type of ternary hybrid, Pd@MCM-Calixox, based on mesoporous silica, calixarene conjugate, and Pd(0) nanoparticles (NPs) was synthesized by sacrificial oxidation of allylic calixarene conjugate eventually functionalized with mesoporous silica without using any external reducing agent. The role of the calix conjugate in the formation of Pd@MCM-Calixox has been established. The hybrid, Pd@MCM-Calixox, was characterized by different techniques to support the formation of well-dispersed Pd(0) NPs of 12 ± 2 nm size. The catalyst, Pd@MCM-Calixox, has been proven to be a resourceful one in water in three different types of reactions, namely, Suzuki C–C cross coupling, reduction of both hydrophilic and hydrophobic nitroaromatic compounds, and reduction and degradation of cationic, anionic, and neutral organic dyes. The catalyst exhibited higher turnover frequencies for all these transformations even when a very low concentration of Pd-based catalyst was used. The Pd@MCM-Calixox hybrid catalyst can be recycled several times without experiencing any significant loss in the activity. Also, the regenerated catalyst showed retention of well-spread Pd(0) species even after several catalytic cycles. The tetraallyl calixarene, allylCalix, conjugate acts as a reducing agent, also controls the size, and yields the well-dispersed Pd(0) NPs. The calix conjugate further provides a hydrophobic core in assisting the diffusion of hydrophobic substrates toward catalytic sites.

Copper oxide\u2013graphene oxide nanocomposite: efficient catalyst for hydrogenation of nitroaromatics in water

A low-cost nanocomposite catalyst containing copper oxide (CuO) nanoparticles (NPs) on graphene oxide (GO) was fabricated by a facile hydrothermal self-assembly process. The segregated CuO NPs and GO exhibited negligible catalytic activities for the reduction of nitroaromatics. However, their hybrid composite accomplished facile reduction with high conversions for several substituted nitroaromatics in aqueous NaBH4 solution; synergetic coupling effect of CuO NPs with GO in the nanocomposite catalyst provided excellent catalytic activity. The nanocomposite catalyst could be separated from the reaction mixture and recycled consecutively.

Phase and Shape Dependent Photoactivity of Titania for Nitroaromatics Reduction Under UV Light Irradiation.

This work describes importance of TiO₂ nanostructures of different shapes (nanospheres and nanorods) and crystal phases (anatase and rutile) for the photoreduction of nitroaromatics into corresponding aminoaromatics under UV light irradiation. Anatase nanoparticles were most active of all the catalysts due to the superior stability of this phase and comparatively the rutile nanorods (L×W = 28-30 nm×3.5-3.8 nm) showed superior photoactivity (~3 times) as compared to rutile nanospheres of size 122 nm for the photoreduction due to electron transport along longitudinal length, larger surface area (69 m²g-1), quenched PL emission, increased lifetime of charge carriers (1.8 ns) of rutile nanorods as compare to rutile nanospheres having lower surface area (18 m²g-1) and charge carrier lifetime (1.1 ns). The overall rate of reduction/hour for m-NBA and m-NT photoreduction with all of these catalysts has been found to vary in the following order ANP > P25-TiO₂ > RNR > ANR > RNP.

Recent Advances in the Nanocatalysts-assisted NaBH4 Reduction of Nitroaromatics in water.

In view of the increasing applications of nanocatalysis in chemical transformations, this article illustrates recent advances on the use of nanocatalysts for an important reduction reaction, the hydrogenation of nitroaromatics to significant aminoaromatics with aqueous NaBH4 solution; the utility of mono- and multimetal nanocatalysts with special emphasis on heterogeneous nanocatalysts is included. A progressive trend on the applicability of nanocatalysts is also incorporated with large-scale application and their sustainable recyclization and reuse utilizing supported and magnetic nanocatalysts; representative methods for the synthesis of such reusable nanocatalysts are featured.

A visible-light-responsive metal–organic framework for highly efficient and selective photocatalytic oxidation of amines and reduction of nitroaromatics

A novel visible-light-responsive metal–organic framework for highly efficient and selective transformation of amines and nitroaromatics to produce imines and anilines.

Atomic layer deposition of palladium nanoparticles on a functional electrospun poly-cyclodextrin nanoweb as a flexible and reusable heterogeneous nanocatalyst for the reduction of nitroaromatic compounds.

We here show a rational approach for the fabrication of a flexible, insoluble catalytic electrospun nanoweb of cross-linked cyclodextrin (CD) for the reduction of nitroaromatics. CD nanofibers were produced by electrospinning an aqueous HP-β-CD solution containing a multifunctional cross-linker (i.e., 1,2,3,4-butanetetracarboxylic acid, BTCA) and were subsequently cross-linked by heat treatment, which led to an insoluble electrospun poly-CD nanoweb. The poly-CD nanoweb was decorated with Pd nanoparticles (Pd-NPs) by atomic layer deposition (ALD) technique over 20 cycles to give rise to a catalytic electrospun nanoweb (i.e., Pd@poly-CD). The formation of the Pd-NPs on the poly-CD nanofiber surface was clearly evidenced by TEM and STEM imaging, which displayed the homogeneously distributed Pd-NPs with a mean size of 4.34 nm. ICP-MS analysis revealed that the Pd content on the Pd@poly-CD nanoweb was 0.039 mg per mg of nanoweb. The catalytic performance of the Pd@poly-CD nanoweb was tested for the reduction of a nitroaromatic compound (i.e., 4-nitrophenol (4-NP)), and high catalytic performance of the Pd@poly-CD nanoweb was observed with a corresponding TOF value of 0.0316 min−1. XPS was used to explore the oxidation state of Pd atoms before and after the catalytic reduction of 4-NP, and no significant change was observed after catalytic reactions. In brief, the Pd@poly-CD nanoweb having handy, flexible, structural stability and reusability can be effectively used in environmental applications as a heterogeneous nanocatalyst for the reduction of toxic nitroaromatics.

Reductive transformation of nitroaromatic compounds by Pd nanoparticles on nitrogen-doped carbon (Pd@NC) biosynthesized using Pantoea sp. IMH.

Reductive transformation of nitroaromatic compounds is a central step in its remediation in wastewater, and therefore has invoked extensive catalytical research with rare metals such as palladium (Pd). Herein, we report Pantoea sp. IMH assisted biosynthesis for Pd@NC as an efficient catalyst for the reduction of nitroaromatics. Multiple complementary characterization results for Pd@NC evidenced the evenly dispersed Pd NPs on an N-doped carbon support. Pd@NC exhibited the superior catalytic activity in the reduction of nitroaromatic compounds (4-nitrophenol, 2-nitroaniline, 4-nitroaniline, and 2,6-dichloro-4-nitroaniline). The origin of the catalytic activity was explained by its unique electronic structure, as explored with X-ray absorption near-edge structure (XANES) spectroscopy and density functional theory (DFT) calculations. XANES analysis revealed an increase of 25.6% in the d-hole count in Pd@NC compared with Pd°, as the result of pd hybridization. In agreement with our experimental observations, DFT calculations suggested the formation of Pd-C bonds and charge re-distribution between Pd and the carbon layer, which contributed to the superior catalytic activity of Pd@NC.