Direct Oxidative Azo Coupling of Anilines Using a Self-Assembled Flower-like CuCo2O4 Material as a Catalyst under Aerobic Conditions

Visible Light Catalyzed Oxidative Reactions Using Silver-Based Metal-Organic Frameworks Based on Metalloligands

Photocatalysis plays a crucial role in modern organic synthesis by enabling efficient transformations under mild conditions and reducing the need for harmful reagents. Its ability to harness visible light for sustainable chemical processes makes it an attractive alternative to conventional methods. In this study, we have utilized two silver-based MOFs, incorporating two different Co3+-metalloligands, as photocatalysts for oxidation reactions and oxidative coupling processes. The reactions include the oxidation of phenylboronic acids to phenols and the oxidative coupling of amines and anilines to imines and azo compounds. Ag-MOFs demonstrated significant visible light absorption and excellent chemical stability, with superoxide radicals identified as the primary reactive oxygen species. The photocatalytic performance is supported by band gap energy calculation and photophysical studies, including electrochemical impedance, electron paramagnetic resonance, and photoluminescence spectroscopies. The photocatalytic performance of an Ag-MOF was critically influenced by its architecture, including the presence of argentophilic interactions. Such a feature not only enhanced its structural stability but also improved the light absorption, a more significant number of active sites, better charge separation, and a remarkable photocatalytic outcome.

Highly Defined Layered 2D Sulfur Nanosheets from 0D Sulfur Dots and their use in the Photocatalytic Oxidative Coupling of Aryl Amines to Aromatic Azo Compounds

A highly defined multi-layered 2D sulphur nanosheets have been prepared from 0D sulphur dots by hydrothermal based growth process. Slow transformation of sulphur dots to sulphur sheets allow reorganization of elemental sulphur and sulphur containing functional groups, resulting higher crystallinity in sulphur nanosheets with respect to sulphur dots. It is further correlated with the detailed photophysical studies. Low temperature photoluminescence spectra confirm that the exciton binding energy of 2D sulphur sheets are lower than 0D sulphur dots. The exciton-phonon coupling is also higher in sulphur sheet with respect to sulphur dots. Overall, the photoinduced charge separation process improves throughout the highly defined 2D matrix of sulphur sheets. It is further supported by electrochemical impedance spectroscopy and transient photocurrent studies. Finally, both sulphur dots and sulphur sheets have been used for photocatalytic azo bond formation where both photoinduced electrons and holes have been utilized simultaneously. Results suggest almost 3 folds enhancement in catalytic efficiency for sulphur sheets with respect to sulphur dots without any metal co-catalyst. Controlled experiments suggest that the azo bond formation happens through complete photocatalysis process. Detailed mechanism has been proved by identifying the intermediates and free radicals through photo-responsive EPR studies and various chemical scavenging experiments.

Ni-Pd bimetallic nanoparticle stabilized polymer membrane as an efficient dip-catalyst for oxidative coupling of aromatic amines to access symmetrical and unsymmetrical azo compounds

The azo linkage is an important chemical motif with wide applications in the pharmaceutical, agrochemical, textile, paint, and food industries. The development of effective dip-catalysts that would convert aromatic amines to symmetric or asymmetric azobenzenes in the presence of aerobic molecular oxygen under mild conditions is really challenging. Here, we have developed a dip-catalyst by immobilizing poly(vinylpyrrolidone)(PVP) stabilized Ni-Pd bimetallic nanoparticles on a nylon-6 membrane through UV-crosslinking for performing such reactions. The catalyst was highly efficient for the oxidative coupling of aromatic amines to the corresponding aromatic azo compounds under aerobic conditions without employing hazardous substances or ecologically harmful nitrites. Notable reaction features include a broad substrate range (aromatic, heterocyclic, and polyaromatic amines), a high yield (∼98%), low catalyst loading, and a simple work-up method. The dip-catalyst exhibits exceptional reusability for multiple catalytic cycles while retaining its high performance and structural characteristics. The gram-scale synthesis of high-value compounds like azobenzene provided additional evidence of the practical utility of the proposed catalyst.

Hybrid Pd0.1Cu0.9Co2O4 nano-flakes: a novel, efficient and reusable catalyst for the one-pot heck and Suzuki couplings with simultaneous transesterification reactions under microwave irradiation

We report the fabrication of a novel spinel-type Pd₀.₁Cu₀.₉Co₂O₄ nano-flake material designed for Mizoroki-Heck and Suzuki coupling-cum-transesterification reactions. The Pd₀.₁Cu₀.₉Co₂O₄ material was synthesized using a simple co-precipitation method, and its crystalline phase and morphology were characterized through powder XRD, UV-Vis, FESEM, and EDX studies. This material demonstrated excellent catalytic activity in Mizoroki-Heck and Suzuki cross-coupling reactions, performed in the presence of a mild base (K₂CO₃), ethanol as the solvent, and microwave irradiation under ligand-free conditions. Notably, the Heck coupling of acrylic esters proceeded concurrently with transesterification using various alcohols as solvents. The catalyst exhibited remarkable stability under reaction conditions and could be recycled and reused up to ten times while maintaining its catalytic integrity.

Strengthening the Synergy between Oxygen Vacancies in Electrocatalysts for Efficient Glycerol Electrooxidation

Defect-engineered bimetallic oxides exhibit high potential for the electrolysis of small organic molecules. However, the ambiguity in the relationship between the defect density and electrocatalytic performance makes it challenging to control the final products of multi-step multi-electron reactions in such electrocatalytic systems. In this study, controllable kinetics reduction is used to maximize the oxygen vacancy density of a Cu─Co oxide nanosheet (CuCo2O4 NS), which is used to catalyze the glycerol electrooxidation reaction (GOR). The CuCo2O4-x NS with the highest oxygen-vacancy density (CuCo2O4-x-2) oxidizes C3 molecules to C1 molecules with selectivity of almost 100% and a Faradaic efficiency of ≈99%, showing the best oxidation performance among all the modified catalysts. Systems with multiple oxygen vacancies in close proximity to each other synergistically facilitate the cleavage of C─C bonds. Density functional theory calculations confirm the ability of closely spaced oxygen vacancies to facilitate charge transfer between the catalyst and several key glycolic-acid (GCA) intermediates of the GOR process, thereby facilitating the decomposition of C2 intermediates to C1 molecules. This study reveals qualitatively in tuning the density of oxygen vacancies for altering the reaction pathway of GOR by the synergistic effects of spatial proximity of high-density oxygen vacancies.

Palladium Nanoparticles Incorporated Fumed Silica as an Efficient Catalyst for Nitroarenes Reduction via Thermal and Microwave Heating

The reduction of nitroarenes to arylamines is a synthetically important transformation both in the laboratory and in industry. Herein, Palladium (Pd) nanoparticles were synthesized via incorporation with mesoporous fumed silica material by doping technique. Water was used as a solvent and the as-synthetized material was reduced by using NaBH4 to ensure the total transformation of PdO into Pd nanoparticles. The synthesized sample was characterized by using inductively coupled plasma (ICP) elemental analysis, X-ray powder diffraction (XRD), N2 sorption measurement, scanning electron microscope (SEM), energy-dispersive spectroscopy (EDX), and transmission electron microscopy (TEM). Data showed that the Pd nanoparticles were successfully synthesized and supported on the mesoporous silica with an average size in the ranges of 10–20 nm, with an irregular shape. The purity of the synthesized sample was confirmed by EDX analysis which exhibits the presence of Si, O, and Pd. The catalytic activity of the prepared sample was evaluated in the heterogeneous reduction of nitroarenes to aromatic amines. Reduction reaction was monitored by Shimadzu GC-17A gas chromatography (GC, Japan) equipped with flam ionization detector and RTX-5 column, 30 m × 0.25 mm, 1-μm film thickness. Helium was used as carrier gas at flow rate 0.6 mL/min. Interestingly, the green hydrogenation of nitroarenes to primary amine compounds was achieved in an aqueous solution with high efficiency and in a short time; moreover, the reusability of heterogeneous Pd-SiO2 was performed for four repeated cycles with more than 88% of efficiency at the fourth run. Finally, the heterogeneity of catalysis with high reliability and eco-friendly processes is a super new trend of nitroarenes reduction in the industry and economic scales.

Magnetically Separable Visible Light-Active Ag0.75Ni0.25 Binary Alloy Nanoparticles as a Highly Efficient Photocatalyst for the Selective Oxidative Coupling of Aniline to Azobenzene

Aniline wastes can be converted to useful pharmaceutical and industrial compounds like azobenzene. For this purpose, a bimetallic Ag_0.75Ni_0.25 alloy is designed in the nanoscale range resembling a fivefold twinned morphology using water as the solvent. These newly developed alloy nanoparticles (NPs) are employed for the first time as an efficient visible light-active photocatalyst for the oxidative homocoupling of aniline to azobenzene. Our catalytic protocol is highly sustainable for a large number of aniline substrates with a high yield of the product (up to 95%), which might be attributed to the combinational and superior properties achieved on alloy formation in comparison to the monometallic counterparts. High-electron density amines (p-anisidine) display greater photocatalytic proficiency than that of low-electron density amines (4-fluoroaniline). The developed photocatalyst is magnetically well-separable and can be reused for at least five catalytic cycles without appreciable loss in its activity.

Synthesis, characterization and application of a novel polyazo dye as a universal acid-base indicator

A novel organic polyazo dye is synthesized by the diazotization of aromatic aniline, followed by coupling it with sulfanilic acid and N,N-dimethylaniline. Characterization was done by ¹H-NMR, ¹³C-NMR, and FTIR spectroscopy. Differential scanning calorimetry (DSC) reveals that phase transition for this molecule is exothermic. The optical band gap is estimated from the absorption cutoff point using UV-Visible spectroscopy. Thermal gravimetric analysis (TGA) addresses the thermal stability of the molecule and is found to be at ∼250 °C. The structure of the synthesized molecule is analogous to that of methyl orange and contains three azo groups. These three azo groups help accept more than two protons and provide two pK _a values when diprotic acid or a mixture of acids is used in different titrations. Specifically, when a polybasic acid is in strong base titration, the pK _a values were found to be 3.5 and 9.1. Moreover, for strong base and (strong + weak) acid mixture titration, the pK _a values are found to be 9.2 and 3.3. Furthermore, the pK _a values are found to be 8.6 and 2.8 for (strong and weak) base mixture and (strong and weak) acid mixture titration, respectively. Owing to its increased proton accepting capacity, it can be found in the two pH ranges of 2.1-3.8 for orange color and 8.2-9.8 for yellow color, thus indicating a unique property as a universal indicator for acid-base titration. The dissociation constant of this dye is found to be 3.4 × 10^-6, determined in a mixed aqueous solution of 10 wt% ethanol, and a linear relationship between pK _a and pH is observed in this solvent system.

Rice husk biochar modified-CuCo2O4 as an efficient peroxymonosulfate activator for non-radical degradation of organic pollutants from aqueous environment

A series of rice husk biochar (RHBC) modified bimetallic oxides were prepared using a simple pyrolysis method to activate peroxymonosulfate (PMS) for the degradation of acid orange G (OG). The results demonstrated that 50 mg L⁻¹ OG was completely decomposed by 1 mM PMS activated with 100 mg L⁻¹ RHBC–CuCo₂O₄ within 15 min at initial pH 3.4. The OG degradation rate constant k of RHBC–CuCo₂O₄/PMS (0.95 × 10⁻¹ min⁻¹) was five times greater than that of CuCo₂O₄/PMS (0.19 × 10⁻¹ min⁻¹), suggesting that the introduction of RHBC significantly improved the activity of bimetallic oxides. The effects of the initial pH, catalyst dosage, PMS concentration and reaction temperature on OG removal were also studied. The degradation products of OG were analysed using a gas chromatography-mass spectrometer (GC-MS). Electron paramagnetic resonance (EPR) and quenching experiments showed that singlet oxygen (¹O₂) was the main active species. The RHBC–CuCo₂O₄/PMS oxidation system is not only unaffected by inorganic anions (Cl⁻, NO₃⁻, HCO₃⁻) and humic acid (HA), but also could remove other typical pollutants of acetaminophen (ACT), sulfathiazole (STZ), rhodamine B (RhB), and bisphenol A (BPA). These findings show that RHBC–CuCo₂O₄ has great potential for practical applications in the removal of typical organic pollutants.

A series of rice husk biochar (RHBC) modified bimetallic oxides were prepared using a simple pyrolysis method to activate peroxymonosulfate (PMS) for the degradation of acid orange G (OG).