Nitrogen-doped porous carbon-encapsulated copper composite for efficient reduction of 4-nitrophenol

Zr-Based MOF-Stabilized CO2-Responsive Pickering Emulsions for Efficient Reduction of Nitroarenes

A Pickering emulsion is a natural microreactor for interfacial catalysis in which an emulsifier is critical. Recently, a metal-organic framework (MOF) has attracted attention to emulsify water-organic mixtures for constructing a Pickering emulsion. However, a few stimuli-responsive Pickering emulsions based on MOFs have been reported, and the MOF emulsifiers cannot be regenerated at room temperature. Herein, the Zr-MOF with a rodlike morphology is synthesized using ionic liquid as a modulator and then modified with n-(trimethoxysilylpropyl)imidazole (C3im) to prepare a series of functionalized Zr-MOFs (MOF-C3im). It is found that MOF-C3im is an excellent emulsifier to construct stable and CO2-responsive Pickering emulsions even at low content (>0.20 wt %). Notably, the emulsification and demulsification of the emulsions can be easily and reversibly switched by bubbling of CO2 and N2 alternatively at room temperature because CO2 and imidazole molecules anchored on the Zr-MOF underwent a reversible acid-base reaction, resulting in an obvious change in the wettability of the emulsifier. As a proof of concept, the reduction reactions of nitrobenzene have been successfully carried out in these Pickering emulsions, demonstrating the efficient integration as a microreactor for chemical reaction, product separation, and emulsifier recycling under ambient conditions. This strategy provides an innovative option to develop stimulus-responsive Pickering emulsions for sustainable chemical processes.

Construction of continuous flow catalytic reactor-HPLC system with ultrahigh catalytic activity using 2D nanoflower MOF-derived Cu2O/Cu/PDA/CF catalyst

Currently, metal-organic frameworks (MOFs) derived materials have been widely concerned for the reduction of 4-nitrophenol (4-NP). However, complex recovery of powder catalysts and low utilization ratio of active sites make their application challenging. Herein, a novel Cu2O/Cu/PDA/CF catalyst has been developed for the rapid reduction of 4-NP to 4-aminophenol (4-AP). The catalyst was constructed by compositing a two-dimensional nanoflower MOF-derived nanoporous Cu2O/Cu network on a polydopamine (PDA)-modified porous copper foam by a mild and controllable in-situ reduction synthesis. Notably, an enhanced catalytic performance of Cu2O/Cu/PDA/CF was obtained for 4-NP reduction with a rate constant (k) of 0.8001 min-1, outperforming Cu/PDA/CF-X (X = 400, 500 and 600 ℃ pyrolysis temperature) catalysts (2.3-6.4 folds), and even many reported catalysts (2.3-46.5 folds). The ultrafast degradation of 4-NP was completed in 70 s. Moreover, an ingenious online continuous flow catalytic reactor (CFCR)-high performance liquid chromatography (HPLC) system was constructed for automatic and real-time monitoring of the reduction reaction. System stability experiments over 300 min revealed a surprisingly high reaction k value of 76.68 min-1 at low NaBH4 usage, significant increasing by 2-3 orders of magnitude compared with Cu2O/Cu/PDA/CF batch catalysis, due to the high aspect ratio of 2D nanoflower MOF and convection-accelerated mass transfer. This work offers new insights for the rational design of catalytic reactor and its potential application in wastewater treatment.

Sustainable magnetically recoverable Iridium-coated Fe3O4 nanoparticles for enhanced catalytic reduction of organic pollutants in water

The reduction of nitroarenes to aromatic amines is one of the potential pathways to remediate the hazardous impact of toxic nitroarenes on the aquatic environment. Aromatic amines obtained from the reduction of nitroaromatics are not only less toxic than nitroaromatics but also act as important intermediates in the synthesis of dyes, drugs, pigments, herbicides, and polymers. There is a huge demand for the development of cost-effective, and eco-friendly catalysts for the efficient reduction of nitroarenes. In the present study, Fe₃O₄@trp@Ir nanoparticles were explored as efficient catalysts for the reduction of nitroarenes. Fe₃O₄@trp@Ir magnetic nanoparticles were fabricated by surface coating of Fe₃O₄ with tryptophan and iridium by co-precipitation method. As-prepared Fe₃O₄@trp@Ir nanoparticles are environmentally benign efficient catalysts for reducing organic pollutants such as 4-nitrophenol (4-NP), 4-nitroaniline (4-NA), and 1-bromo-4-nitrobenzene (1-B-4-NB). The key parameters that affect the catalytic activity like temperature, catalyst loading, and the concentration of reducing agent NaBH₄ were optimized. The obtained results proved that Fe₃O₄@trp@Ir is an efficient catalyst for reducing nitroaromatics at ambient temperature with a minimal catalyst loading of 0.0025%. The complete conversion of 4-nitrophenol to 4-aminophenol took only 20 s with a minimal catalyst loading of 0.0025% and a rate constant of 0.0522 s^-1. The high catalytic activity factor (1.040 s^-1 mg^-1) and high turnover frequency (9 min^-1) obtained for Fe₃O₄@trp@Ir nanocatalyst highlight the possible synergistic effect of the two metals (Fe and Ir). The visible-light photocatalytic degradation of 4-NP was also investigated in the presence of Fe₃O₄@trp@Ir. The photocatalytic degradation of 4-NP by Fe₃O₄@trp@Ir is completed in 20 min with 95.15% efficiency, and the rate of photodegradation of 4-NP (0.1507 min^-1) is about twice the degradation rate of 4-NP in the dark (0.0755 min^-1). The catalyst was recycled and reused for five cycles without significant reduction in the conversion efficiency of the catalyst.

Rapid biosynthesis and characterization of metallic gold nanoparticles by olea europea and their potential application in photoelectrocatalytic reduction of 4-nitrophenol

In recent years, photoelectrocatalysis of gold nanoparticles (Au NPs) has received considerable attention due to their potential to improve catalytic efficiency. Herein, ultra-small Au NPs were successfully synthesized in a single pot using olea europea leaf extract as a green reducing agent for the degradation of 4-nitrophenol. The TEM images showed uniform distribution and spherical shape of Au NPs with an average diameter of 5 nm. Taking advantage of the ability of Au nanoparticles to absorb visible and near-infrared light, 4-nitrophenol can be successfully reduced in the presence of NaBH₄. Additionally, the electrochemical activity of the fabricated Au photocathode was investigated by linear sweep voltammetry in the dark and at VIS-NIR light irradiation. This showed an increased photocurrent density of 27 mA cm^-2 with an onset potential of -0.71 V. This indicates that the Au photocathode is highly active at VIS-NIR light. Interestingly, the Au photocathode showed a higher current density of 37 mA cm^-2 with an onset potential of -0.6 V in the presence of 4-nitrophenol during VIS-NIR irradiation, indicating that 4-nitrophenol was efficiently reduced by the photocathode. The Au photocathode completely reduced 4-nitrophenol in the wastewater within 35 min. Recyclability studies showed that the Au NPs photocathode exhibited higher stability over multiple cycles, confirming the ability of the electrode to treat wastewater over a longer period of time. This study demonstrates the effectiveness of the photoelectrochemical (PEC) process in reducing organic compounds in wastewater.

Opportunities from Metal Organic Frameworks to Develop Porous Carbons Catalysts Involved in Fine Chemical Synthesis

In the last decade, MOFs have been proposed as precursors of functional porous carbons with enhanced catalytic performances by comparison with other traditional carbonaceous catalysts. This area is rapidly growing mainly because of the great structural diversity of MOFs offering almost infinite possibilities. MOFs can be considered as ideal platforms to prepare porous carbons with highly dispersed metallic species or even single-metal atoms under strictly controlled thermal conditions. This review briefly summarizes synthetic strategies to prepare MOFs and MOF-derived porous carbons. The main focus relies on the application of the MOF-derived porous carbons to fine chemical synthesis. Among the most explored reactions, the oxidation and reduction reactions are highlighted, although some examples of coupling and multicomponent reactions are also presented. However, the application of this type of catalyst in the green synthesis of biologically active heterocyclic compounds through cascade reactions is still a challenge.

Effective adsorption and catalytic reduction of nitrophenols by amino-rich Cu(I)-I coordination polymer

Nitrophenols are identified as the priority organic pollutants due to the chemical stability, water solubility, persistence, and toxicity to human health and the environment. Hence, removal of nitrophenols from waste water is vitally essential. In this study, amino-rich coordination polymer Cu₂I₂(MA)₂ (MA = melamine) has been applied for efficient adsorption and catalytic reduction of nitrophenols, like 4-nitrophenol (4-NP), 2, 4-dinitrophenol (DNP) and 2, 4, 6-trinitrophenol (TNP). The effect of various parameters like contact time, initial concentrations, pH, and temperature on adsorption were investigated. The adsorption of nitrophenols fitted the pseudo-second-order kinetic model and Langmuir isotherms model well. The maximum adsorption capacities were 285.71, 232.02, and 131.57 mg g^-1 for 4-NP, DNP, and TNP when initial concentrations were 50 mg L^-1 at 293.15 K, respectively. The adsorption of nitrophenols is a spontaneous, endothermic, and entropy-driven process. The reduction reaction followed the pseudo-first-order kinetics, and the kinetic rate constants were 0.4413, 0.3167, and 0.17538 min^-1 for 4-NP, DNP, and TNP, respectively. The effect of initial nitrophenols concentration, anions, and temperature on reduction process was investigated. The mechanism of adsorption and catalytic reduction of Cu₂I₂(MA)₂ was studied. The results demonstrated that Cu₂I₂(MA)₂ exhibits excellent adsorption and catalytic activity to remove nitrophenols.

In Situ Implantation of Bi2S3 Nanorods into Porous Quasi-Bi-MOF Architectures: Enabling Synergistic Dissociation of Borohydride for an Efficient and Fast Catalytic Reduction of 4-Nitrophenol

Catalytic hydrogenation reduction based on sodium borohydride (NaBH₄) has gained attention as an appealing "one-stone-two-birds" approach for the simultaneous elimination of nitroaromatic pollutants and the production of high-value aminoaromatics under mild conditions. However, the slow kinetics of NaBH₄ dissociation on the surface of catalysts restrict the catalytic hydrogenation reduction efficiency. Herein, we report an intelligent localized sulfidation strategy for an in situ implantation of Bi₂S₃ nanorods within quasi-Bi-MOF architectures (Bi₂S₃@quasi-Bi-MOF) by fine-tuning the pyrolysis temperature. In this novel Bi₂S₃@quasi-Bi-MOF, the porous quasi-Bi-MOF enables efficient adsorption of BH₄^- and 4-nitrophenol (4-NP), while Bi₂S₃ facilitates the BH₄^- dissociation to form H_ads* species adsorbed on the catalyst surface. Benefiting from the synergistic structure, Bi₂S₃@quasi-Bi-MOF exhibits excellent performance for the catalytic reduction of 4-NP, delivering a high turnover frequency (TOF) of 1.67 × 10^-4 mmol mg^-1 min^-1 and an extremely high normalized rate constant (k_nor) of 435298 s^-1 g^-1. The kinetic analysis and electrochemical tests indicate that this catalytic hydrogenation reduction follows the Langmuir-Hinshelwood mechanism. This study enriches the synthetic strategy of MOF-based derivatives and offers a new catalytic platform for hydrogenation reduction reactions.

Pyridinic nitrogen dominated doping on Pd/carbon catalysts for enhanced hydrogenation performance

The hydrogenation of 4-carboxylbenzaldehyde over Pd catalysts is a crucial process during the production of pure terephthalic acid. Herein, ZIF-8 derived carbon materials (NC) with adjustable N types were synthesized and used as the supports of Pd catalysts. Pd supported on NC with 50.5% of pyridinic N exhibited best hydrogenation activity with a TOF value of 4.1 min-1. The microstructures of NC support and electronic structures of Pd in Pd/NC were investigated by techniques such as XRD, N2 physisorption, XPS, H2-O2 titration and TEM. The nitrogen species in CN surface not only can adjust chemical state and dispersion of Pd nanoparticles (NPs), but also promote the adsorption and activation capability of H2 molecular. Besides, the ratio of Pd0/Pd2+ and Pd dispersion were closely correlated with pyridinic nitrogen content. The improvement in hydrogenation activity and stability of Pd/CN catalyst in relative to Pd/C were ascribed to the synergistic effect of pyridinic nitrogen and active site Pd0.

Enhanced catalytic reduction of p-nitrophenol and azo dyes on copper hexacyanoferrate nanospheres decorated copper foams

Catalytic reduction of nitroaromatic compounds using low-cost non-precious metal containing catalyst remains an essential topic in wastewater treatment. Herein, copper hexacyanoferrate nanospheres decorated copper foams (CF) were prepared by a facile method, and it was used as structured catalysts for the reduction of p-nitrophenol (p-NP) and azo dyes. The catalyst obtained by calcination at 200 °C shows the highest catalytic activity, with an almost complete reduction of p-NP within 3 min with a rate of 2.057 min^-1 at room temperature, and it exhibited excellent reusability in successive 6 cycles. The effects of temperature, initial concentration, pH, and flow rate on p-NP reduction were investigated. Moreover, the mechanistic investigation revealed that fast electron transfer ability and enhanced adsorption for p-NP contributed to its enhanced catalytic performances. This work put forward an efficient approach for the construction of structured catalysts with enhanced performance in catalytic reduction applications.

Controllable Synthesis of Cobalt-Containing Nanosheet Array-Like Ternary CuCoAl-LDH/rGO Hybrids To Boost the Catalytic Efficiency for 4-Nitrophenol Reduction

A series of Co-doped ternary Cu_xCo_3-xAl-layered double hydroxide (LDH)/rGO nanosheet array hybrids (x = 0.5, 1.0, 1.5, and 2.0) were successfully prepared using the preconditioned pH value aqueous-phase coprecipitation strategy. The Cu_xCo_3-xAl-LDH/rGO hybrids are featured as hexagonal CuCoAl-LDH nanosheets in situ anchoring onto both sides of the rGO surface in an ab-plane vertically interlaced growth pattern. The Cu_xCo_3-xAl-LDH/rGO hybrids show excellent activity for the complete conversion of 4-nitrophenol to 4-aminophenol, especially Cu_1.5Co_1.5Al-LDH/rGO with the highest k_app value of 49.2 × 10^-3 s^-1 and TOF of 232.8 h^-1, clearly higher than most copper-containing samples in the literature and even some precious ones. Thermodynamic analysis was carried out, and the values of Ea, ΔH^#, ΔS^#, and ΔG^# were estimated. The best activity of Cu_1.5Co_1.5Al-LDH/rGO can be mainly ascribed to the in situ-formed ultrafine Cu₂O NPs (∼4.3 nm) along with a small amount of Cu⁰ species, the electron transfer effect induced by atomically dispersed Co²⁺ species leading to the formation of electron-rich Cu species along with the Co²⁺/Co³⁺ redox couple, the strong Cu₂O-CuCoAl-LDH-rGO synergy upon the nanosheet array morphology with a high surface area and pore volume, and enhanced adsorption of reactants upon π-π stacking via an rGO layer. Meanwhile, the Cu_1.5Co_1.5Al-LDH/rGO exhibits an excellent universality and good cycling stability for 10 continuous runs. The Cu_1.5Co_1.5Al-LDH/rGO also shows superior efficiency in the catalytic reduction of 4-NP solution with a high concentration (20 mM) and displays excellent reduction performance in the fixed-bed test, implying the potential applications of the current Co-doped hierarchical ternary Cu-based LDH/rGO hybrids in the continuous treatment of practical wastewater.

One-pot synthesis of Au-M@SiO2 (M = Rh, Pd, Ir, Pt) core-shell nanoparticles as highly efficient catalysts for the reduction of 4-nitrophenol

The conversion of p-nitrophenol (4-NP) to p-aminophenol (4-AP) is of great significance for pharmaceutical and material manufacturing. In this work, Au-M@SiO₂ (M = Rh, Pd, Ir, Pt) nanoparticles (NPs) with core-shell structures, which are expected to be excellent catalysts for the transformation of 4-NP to 4-AP, were synthesized by a facile one-pot one-step method. The structure and composition of the NPs were characterized through transmission electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. Au-M@SiO₂ (M = Rh, Pd, Ir, Pt) core-shell NPs showed excellent catalytic activity in the reduction of 4-NP, which is superior to most catalysts reported in the previous literature. The enhanced catalytic activity of Au-M@SiO₂ core-shell NPs is presumably related to the bimetallic synergistic effect. This study provides a simple strategy to synthesize core-shell bimetallic NPs for catalytic applications.

Concurrent Oxidation-Reduction Reactions in a Single System Using a Low-Plasma Phenomenon: Excellent Catalytic Performance and Stability in the Hydrogenation Reaction

The catalytic activity and stability of metal nanocatalysts toward agglomeration and detachment during their preparation on a support surface are major challenges in practical applications. Herein, we report a novel, one-step, synchronized electro-oxidation-reduction "bottom-up" approach for the preparation of small and highly stable Cu nanoparticles (NPs) supported on a porous inorganic (TiO₂@SiO₂) coating with significant catalytic activity and stability. This unique embedded structure restrains the sintering of CuNPs on a porous TiO₂@SiO₂ surface at a high temperature and exhibits a high reduction ratio (100% in 60 s) and no decay in activity even after 30 cycles (>98% conversion in 3 min). This occurs in a model reaction of 4-nitrophenol (4-NP) hydrogenation, far exceeding the performance of most common catalysts observed to date. More importantly, nitroarene, ketone/aldehydes, and organic dyes were reduced to the corresponding compounds with 100% conversion. Density functional theory (DFT) calculations of experimental model systems with six Cu, two Fe, and four Ag clusters anchored on the TiO₂ surface were conducted to verify the experimental observations. The experimental results and DFT calculations revealed that CuNPs not only favor the adsorption on the TiO₂ surface over those of Fe and AgNPs but also boost the adsorption energy and activity of 4-NP. This strategy has also been extended to the preparation of other single-atom catalysts (e.g., FeNPs-TiO₂@SiO₂ and AgNPs-TiO₂@SiO₂), which exhibit excellent catalytic performance.

Two-dimensional MXene enabled carbon quantum dots@Ag with enhanced catalytic activity towards the reduction of p-nitrophenol

A composite of cuttlefish ink-based carbon quantum dots@Ag/MXene (CQD@Ag/MXene) was firstly synthesized by solvothermal method as a catalyst for reduction of p-nitrophenol (PNP) to p-aminophenol (PAP). CQD@Ag/MXene was characterized by scanning electron microscopy (SEM), field emission transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman. The results show that loading on 2D material MXene can prevent the aggregation of CQD@Ag and expose more active sites, which contributes to a superior catalytic activity with a pseudo-first-order rate constant k (2.28 × 10-2 s-1) and mass-normalized rate constant k m (5700 s-1 g-1), nearly 2 times higher than CQD@Ag without MXene (k = 1.09 × 10-2 s-1 and k m = 2725 s-1 g-1). Besides, CQD@Ag/MXene showed excellent reusability which even retained about 65% activity in successive 10 cycles. The high adsorption rate to PNP and the promotion of forming H radicals may be the reason for the outstanding catalytic activity of CQD@Ag/MXene. CQD@Ag/MXene can be a potential candidate in the removal of environmental pollutants due to its facile synthesis and high catalytic efficiency.

Novel bimetallic MOF derived N-doped carbon supported Ru nanoparticles for efficient reduction of nitro aromatic compounds and rhodamine B

N-doped carbon enables Ru-NC-15 to exhibit extremely high catalytic activity towards 4-nitrophenol and rhodamine B reduction.

A study on the high efficiency reduction of p-nitrophenol (4-NP) by a Fe(OH)3/Fe2O3@Au composite catalyst

Precious metal nanometric catalysts are widely used in the removal of harmful substances. In the process of synthesis and catalytic reaction, it is particularly important to study green and simple synthesis methods and high catalytic efficiency. In this paper, a green one-step method was used to synthesize the Fe(OH)₃/Fe₂O₃@Au composite catalyst, in which Au was single atom-dispersed. The removal of 4-nitrophenol (4-NP), a typical dangerous chemical widely existing in factory waste gas, waste water and automobile exhaust gas, was catalysed by Fe(OH)₃/Fe₂O₃@Au. The catalytic performance of Fe(OH)₃/Fe₂O₃@Au with different synthesis conditions (different amounts of MES, NaBH₄, FeSO₄, Au and Pt) on the 4-NP reduction reaction were systematically studied. Finally, the stability and recyclability of Fe(OH)₃/Fe₂O₃@Au composite nanocatalyst were investigated thoroughly.