Selective Oxidative Coupling Research Articles

Donor–Acceptor Mixed-Naphthalene Diimide-Porphyrin MOF for Boosting Photocatalytic Oxidative Coupling of Amines

Integrating electron donor and acceptor chromophores as ligands within one MOF for light harvesting and photocatalysis appears to be a major synthesis challenge. Meanwhile, high-efficiency and selective aerobic oxidative coupling of amines to imines under mild conditions is highly desirable for scientific and industrial fields. Herein, via a facile one-pot synthetic strategy, an electron donor porphyrin ligand 5,15-di(p-benzoato)porphyrin (H2DPBP) was successfully integrated into a naphthalene diimide (electron acceptor)-based Zr-MOF (Zr-NDI) to construct a mixed-ligand MOF (Zr-NDI-H2DPBP) benefiting from the two ligands with similar geometry and connectivity. When utilized as a heterogeneous photocatalyst for aerobic oxidative coupling of benzylamine at room temperature, the prepared mixed-ligand Zr-NDI-H2DPBP exhibited a boosted imine generation rate (136 mmol g–1 h–1) far exceeding those of other noble-metal-free MOF photocatalysts and can be successfully applied to various benzylamine derivatives. Furthermore, from the results of the experiments and density functional theory calculations, the photocatalytic performance of Zr-NDI-H2DPBP is attributed to the photoinduced electron transfer from the donor H2DPBP to the acceptor NDI and the energy transfer facilitated by the photosensitized H2DPBP under irradiation, which effectively improve the electron–hole separation efficiency to generate abundant reactive active species of superoxide radical (O2•–) and singlet oxygen (1O2), respectively. This work provides an avenue to construct electron donor and acceptor chromophore-based light-harvesting systems by utilizing mixed-ligand MOFs as platforms to boost the solar-energy conversion.

Construction of 0D/1D CdS/g-C3N4 Z-scheme heterojunction for highly selective photocatalytic benzylamine oxidative coupling

The efficient solar-to-chemical energy conversion system is a green and cost-effective approach for producing high-value-added chemicals in chemical industry, which is highly demanded to develop. Photocatalytic C–N coupling of benzylamine into economic N-benzylidenebenzylamine is a great potential method for alternative conventional organic synthesis. In this work, a zero-dimensional/one-dimensional CdS/g-C3N4 Z-scheme heterojunction was constructed via in situ growing CdS quantum dot onto g-C3N4 nanorod surface for enhancing the spatial separation of charge carriers and maintaining the strong redox ability of the photogenerated charges and applied simultaneously in photocatalytic C–N coupling of benzylamine. This enables 20 wt% CdS/g-C3N4 to exhibit a remarkable photocatalytic performance enhancement for the production of C–N coupled product of benzylamine with the optimal yield of 199.79 μmol which is approximately 4.3 folds than those of pristine g-C3N4 under 2 h visible irradiation. Moreover, benzylamine conversion of approximately 90% within 100 h can be obtained in a gram scale-up reaction. By means of controlled photoelectric chemistry and in situ electron spin resonance spectroscopy experiments, possible spatial photogenerated charges separation and benzylamine conversion mechanisms on the Z-scheme junction were investigated in detail. This work paves a promising strategy for constructing zero-dimensional/one-dimensional Z-scheme heterojunction for the photocatalytic synthesis of high-value-added products.

Synergy of Ag and AgBr in a Pressurized Flow Reactor for Selective Photocatalytic Oxidative Coupling of Methane.

Oxidation of methane into valuable chemicals, such as C2+ molecules, has been long sought after but the dilemma between high yield and high selectivity of desired products remains. Herein, methane is upgraded through the photocatalytic oxidative coupling of methane (OCM) over a ternary Ag-AgBr/TiO2 catalyst in a pressurized flow reactor. The ethane yield of 35.4 μmol/h with a high C2+ selectivity of 79% has been obtained under 6 bar pressure. These are much better than most of the previous benchmark performance in photocatalytic OCM processes. These results are attributed to the synergy between Ag and AgBr, where Ag serves as an electron acceptor and promotes the charge transfer and AgBr forms a heterostructure with TiO2 not only to facilitate charge separation but also to avoid the overoxidation process. This work thus demonstrates an efficient strategy for photocatalytic methane conversion by both the rational design of the catalyst for the high selectivity and reactor engineering for the high conversion.

Site-Selective Oxidative Coupling Reaction of Diamines toward Aminoazo Compounds

A selective oxidative coupling reaction of a diamine containing both C- and N-NH2 was achieved using acidic potassium permanganate as the coupling reagent. The reaction conditions were optimized, and the reaction selectivity was illustrated by quantum calculations. Furthermore, the azo-coupled product was derivatized to four nitroamine energetic materials with excellent detonation performances.

A porous Anderson-type polyoxometalate-based metal-organic framework as a multifunctional platform for selective oxidative coupling with amines.

Incorporating catalytic units into a crystalline porous matrix represents a facile way to build high-efficiency heterogeneous catalysts, and by rational design of the porous skeleton with appropriate building blocks the catalytic performance can be significantly enhanced for a series of organic transformations owing to the synergistic effect from the multicomponent and confined porous microenvironment around catalytically active sites. Herein, we demonstrate that the design and synthesis of a porous polyoxometalate-based metal-organic framework YL2(H2O)2[CrMo6O18(PET)2]·4H2O (POMOF-1) constructed from Anderson-type [CrMo6O18(PET)2] (PET = pentaerythritol), which can be employed as a multifunctional platform for synthesis of N-containing compounds via selective oxidative coupling with amines. POMOF-1 features microporous 1D channels defined by Y3+ and L, with [CrMo6O18(PET)2] arranged orderly between adjacent Lvia electrostatic interactions. Upon using POMOF-1 as a catalyst and H2O2 as an oxidant, a variety of amines could be effectively converted to value-added amides, imines and azobenzenes via the oxidative cross-coupling with alcohols or homo-coupling. In particular, POMOF-1 showed dramatically improved activity for the N-formylation reaction owing to the synergistic and confinement effect, with the yield of amides up to 95% and 4 times higher than that of homogeneous [CrMo6O18(PET)2]. Meanwhile, the oxidative homo-coupling of arylmethylamines and arylamines can be facilely tuned by adjustment of the amount of oxidant, solvent and additive, affording imines and azobenzenes in high selectivity and yield, respectively. POMOF-1 is robust and can be reused for 5 cycles with little loss of catalytic activity and structural integrity. The work demonstrates that the combination of catalytically active POMs with crystalline porous MOFs holds great potential to build robust and recyclable heterogeneous systems with enhanced activity and selectivity for multifunctional catalysis.

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 Ag0.75Ni0.25 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.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate-Tris(bipyridine)ruthenium Hybrid Compounds.

The development of visible-light photocatalysts for the selective oxidative coupling of amines to imines is an area of great interest. Herein, four hybrid compounds based on polyoxometalate anions and tris(bipyridine)ruthenium cations, Ru(bpy)3[M6O19] (M = Mo, W) 1-2, [Ru(bpy)3]2[Mo8O26] 3, [Ru(bpy)3]2[W10O32] 4, are prepared and characterized by X-ray diffraction (single-crystal and powder), elemental analysis, energy-dispersive X-ray spectroscopy (EDS) analysis, infrared (IR) spectroscopy, and solid diffuse reflective spectroscopy. Single-crystal structural analysis indicates that polyoxometalate anions and tris(bipyridine)ruthenium cations interact with each other through extensive hydrogen bonds in these compounds. These hybrid species with strong visible-light-harvesting abilities and suitable photocatalytic energy potentials show excellent photocatalytic activity and selectivity for the oxidation of amines to imines at room temperature in air as an oxidant. Among them, compound 1 with the [Mo6O19]2- anion has the highest catalytic activity, which can swiftly convert >99.0% of benzylamine into N-benzylidenebenzylamine with a selectivity of 98.0% in 25 min illumination by a 10 W 445 nm light-emitting diode (LED). Its turnover frequency reaches 392 h-1, which is not only better than the homogeneous catalyst [Ru(bpy)3]Cl2 but also much superior to those achieved over most of reported heterogeneous catalysts. Moreover, it shows a wide generality for various aromatic amines, accompanied by the advantages of good recyclability and stability. The photocatalytic oxidation mechanism of amines to the corresponding imines over polyoxometalate-based hybrid compounds was fully investigated.

Selective Oxidative Coupling of Methane to Ethylene in a Solid Oxide Electrolyser Based on Porous Single‐Crystalline CeO2 Monoliths

AbstractCatalytic conversion of CH4 to C2H4 plays an important role in the light olefin industry. Here, we report the electrochemical conversion of CH4 to C2H4/C2H6 at the anode with the electrolysis of CO2 to CO at the cathode in a solid oxide electrolyser. We constructed well‐defined interfaces that function as three‐phase boundaries by exsolving single‐crystalline Ni nanoparticles in porous single‐crystalline CeO2 monoliths. We engineered the chemical states and flux of active oxygen species for the oxidation of CH4 at the anode by controlling voltage and temperature. We show the unprecedented C2 selectivity (C2H4 and C2H6) of ≥99.5 % at a CH4 conversion of ≈7 %. The electrolyser exhibits excellent durability without performance degradation being observed in a continuous operation of 100 hours. Our work enables a novel path for the selective conversion of CH4/CO2 into useful chemicals, and the technique of building well‐defined interfaces may find potential applications in other fields.

Selective Oxidative Coupling of Methane to Ethylene in a Solid Oxide Electrolyser Based on Porous Single-Crystalline CeO2 Monoliths.

Catalytic conversion of CH4 to C2 H4 plays an important role in the light olefin industry. Here, we report the electrochemical conversion of CH4 to C2 H4 /C2 H6 at the anode with the electrolysis of CO2 to CO at the cathode in a solid oxide electrolyser. We constructed well-defined interfaces that function as three-phase boundaries by exsolving single-crystalline Ni nanoparticles in porous single-crystalline CeO2 monoliths. We engineered the chemical states and flux of active oxygen species for the oxidation of CH4 at the anode by controlling voltage and temperature. We show the unprecedented C2 selectivity (C2 H4 and C2 H6 ) of ≥99.5 % at a CH4 conversion of ≈7 %. The electrolyser exhibits excellent durability without performance degradation being observed in a continuous operation of 100 hours. Our work enables a novel path for the selective conversion of CH4 /CO2 into useful chemicals, and the technique of building well-defined interfaces may find potential applications in other fields.

Enhanced visible-light-driven co-production of H2 and value-added chemicals over AgCl/crystalline carbon nitride with N defects

AgCl nanoparticles are in-situ loaded on crystalline carbon nitride (CCN) with N defects by a molten salt method to develope a novel bifunctional composite (AgCl/CCN) photocatalyst for the coupled reaction of proton reduction for H2 production in water with benzylamine selective oxidative coupling. The coupled strategy can not only effectively improve hydrogen production, but also effectively utilize the photogenerated holes to obtain useful high value-added oxidation product N-benzylidenebenzylamine, which achieves a “win-win” result. The high crystallinity and extended π-conjugated system of CCN and the introduction of N-defects can effectively promote the charge transfer and thus inhibit the radiative charge recombination. Moreover, the energy levels of AgCl and CCN are matched to form a heterostructure, which further promotes the charge transfer and significantly improves the efficiency of coupled photocatalysis.

Divergent oxidative dearomatization coupling reactions to construct polycyclic cyclohexadienones.

Highly selective divergent oxidative dearomatization coupling reactions, in which the chemoselectivity is controlled by catalysts and bases, are reported herein. Three different kinds of polycyclic cyclohexadienones are produced from the same reactants (41 examples, 85-99% yield). Our method marks a novel copper- and palladium-catalyzed C-H oxidative dearomatization of phenolic derivatives.

Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K2WO4/SiO2 Catalyst Revealed by In Situ Characterization

The oxidative coupling of methane (OCM) over a K2WO4/SiO2 catalyst was investigated using a CH4–O2 flow system. The addition of H2O to the reactant stream caused an increase in the CH4 conversion rate and C2 selectivity, consistent with the kinetics associated with the equilibrated OH radical generation, followed by H-abstraction from CH4 with OH radicals. The C2+ (a sum of C2 and higher hydrocarbons) yield reaches a maximum of 24.7% (C2H4 yield 17.0%) at a CH4 conversion of 47.1%, comparable to those over the previously reported Na2WO4/SiO2 catalyst. In situ near ambient pressure X-ray photoelectron spectroscopy uncovered K2O2 and KO2 species at and above 560 °C in the presence of gas-phase O2. These K2O2/KO2 species are considered to catalyze H2O activation for OH radical generation. By in situ high-temperature X-ray diffraction, K2WO4 maintained a crystalline phase in the bulk during the OCM reaction at 850 °C in contrast to the molten state of Na2WO4 previously reported. This indicates that the melting of the supported component and the covering of support are not essential to achieve a high C2 yield in supported alkali metal tungstate catalysts as long as inert support materials are used toward OCM.

Selective electrochemical oxidative coupling of methane mediated by Sr2Fe1.5Mo0.5O6-\u03b4 and its chemical stability

Efficient conversion of methane to value-added products such as olefins and aromatics has been in pursuit for the past few decades. The demand has increased further due to the recent discoveries of shale gas reserves. Oxidative and non-oxidative coupling of methane (OCM and NOCM) have been actively researched, although catalysts with commercially viable conversion rates are not yet available. Recently, {{{{{{{mathrm{Sr}}}}}}}}_2Fe_{1.5 + 0.075}Mo_{0.5}O_{6 - delta } (SFMO-075Fe) has been reported to activate methane in an electrochemical OCM (EC-OCM) set up with a C2 selectivity of 82.2%1. However, alkaline earth metal-based materials are known to suffer chemical instability in carbon-rich environments. Hence, here we evaluated the chemical stability of SFMO in carbon-rich conditions with varying oxygen concentrations at temperatures relevant for EC-OCM. SFMO-075Fe showed good methane activation properties especially at low overpotentials but suffered poor chemical stability as observed via thermogravimetric, powder XRD, and XPS measurements where SrCO3 was observed to be a major decomposition product along with SrMoO3 and MoC. Nevertheless, our study demonstrates that electrochemical methods could be used to selectively activate methane towards partial oxidation products such as ethylene at low overpotentials while higher applied biases result in the complete oxidation of methane to carbon dioxide and water.

Biowaste-derived hydrochar microspheres: Realizing metal-free visible-light photocatalytic oxidation of amines

Metal-free carbon photocatalysts hold great promise in future visible-light-driven selective photocatalytic organic synthesis. However, designing ideal carbon photocatalysts from sustainable strategies to further improve the photo-efficiency and economic nature is crucial for portable industrial applications. Herein, a clean and economic strategy to fabricate hydrochar microspheres as the first metal-free biochar potocatalysts is reported by a facile hydrothermal post-treatment approach from biowaste. The obtained hydrochars present excellent photocatalytic activity and reusability toward the selective oxidative coupling reactions of various amines under visible-light irradiation. Experimental studies and theoretical calculation revealed that the abundant oxygenated functional groups built-in on the surface could function as photocatalytic active sites. In particular, the diphenolic hydroxyl functional groups and quinone moieties attached to the large π-electronic-system were the main active sites involved in the photocatalytic activity. This research highlights that the utilization of sustainable biowaste-derived hydrochar materials as a low-cost and eco-environmental carbocatalysts, may open up a new avenue for light-driven diverse organic transformations.

Single crystal MnOOH nanotubes for selective oxidative coupling of anilines to aromatic azo compounds

Catalytic synthesis of aromatic azo compounds by oxidative coupling of anilines using molecular oxygen represents a facile, green and valuable process; however, such an economical process suffers from poor catalytic activity and selectivity.

Hollow Dodecahedral Structure of In2O3-In2S3 Heterojunction Encapsulated by N-Doped C as an Excellent Visible-Light-Active Photocatalyst for Organic Transformation.

The low utilization efficiency in the visible region of the sunlight spectrum and the rapid recombination of photogenerated charge carriers are two crucial drawbacks that suppress the practical usage of metal oxide semiconductors as photocatalysts. In this article, we report a rational design of In2O3-In2S3 heterojunctions encapsulated by N-doped carbon with a hollow dodecahedral structure (In2O3-In2S3/N-C HDS), which can effectively handle the two drawbacks of metal oxide semiconductors and behave active for organic transformation under the irradiation of visible light even with long wavelengths. As exemplified by the selective oxidative coupling reaction of amine to imine, the obtained In2O3-In2S3/N-C HDS as the photocatalyst has exhibited excellent activity and stability. Experimental and density functional theory studies have verified that the excellent performance of In2O3-In2S3/N-C HDS can be attributed to the synergistic effect of In2O3-In2S3 heterojunctions, the coating of N-doped carbon, and the hollow porous structure with nanosheets as subunits.

Magnetic methylene-based mesoporous organosilica composite-supported IL/Pd: a powerful and highly recoverable catalyst for oxidative coupling of phenols and naphthols

A novel magnetic methylene-based mesoporous organosilica composite-supported IL/Pd complex (Fe3O4@MePMO-IL/Pd) was synthesized and characterized, and its catalytic performance was investigated. The preparation of the Fe3O4@MePMO composite was achieved through coating of Fe3O4 nanoparticles with a mixture of tetramethoxysilane, bis(triethoxysilyl)methane, and (3-chloropropyl)-trimethoxysilane in the presence of cetyltrimethylammonium bromide surfactant. The Fe3O4@MePMO was then modified with alkyl imidazolium ionic liquid and palladium species to deliver the Fe3O4@MePMO-IL/Pd nanocatalyst. This catalyst was characterized using Fourier transform infrared, thermal gravimetric, wide-angle powder X-ray diffraction, low-angle powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometer, energy-dispersive X-ray, and nitrogen adsorption–desorption analyses. The Fe3O4@MePMO-IL/Pd was effectively used as a highly recoverable and durable catalyst for the selective oxidative coupling of phenols and 2-naphthols under aerobic conditions.

Selective Mesitylene Oxidative Coupling Reaction by Metastructured Electrocatalyst Comprised of Carbonaceous Scaffold Coated with Pd Derived from Zeolitic Imidazole Framework

Electrocatalysis plays a vital role in industrial processes like electrohydrodimerization of acrylonitrile, anodic substitution and addition, electrolytic oxidative/reductive coupling and electrolysis of heterocyclic compounds. Electrocatalysts with suitable size and shape selectivity for reactants, reactant intermediates and products increase their selectivity. Zeolites are the most common vehicles for size-/shape-selective heterogeneous catalysts, but their lack of electronic conductivity vitiates their use in electrocatalysis. In this context, we explore the effectiveness of carbon derived from the zeolitic imidazole framework (ZIF), which possesses both electronic conductivity and structural selectivity for electrocatalytic oxidative coupling of mesitylene to bimesityl. Co-ZIF synthesized by facile solvothermal method was heated in an inert atmosphere and acid-leached to remove Co in order to generate C-based structured material. Further, Pd acting as a catalyst was deposited inside the pore walls of the obtained C scaffold by electroless deposition to derive pore sizes ranging between the molecular size of the particular product. Controlling the pore size of metastructured electrocatalyst at ~ 16 A allows only bimesityl formation, and restricts any larger sized by-product. Metastructured electrocatalyst showed ~ 18 times higher selective conversion for the desired bimesityl production than the planar Pd film electrocatalyst, demonstrating the potential of the proposed approach.

Tungstate-Catalyzed Biomimetic Oxidative Halogenation of (Hetero)Arene under Mild Condition.

SummaryAryl halide (Br, Cl, I) is among the most important compounds in pharmaceutical industry, material science, and agrochemistry, broadly utilized in diverse transformations. Tremendous approaches have been established to prepare this scaffold; however, many of them suffer from atom economy, harsh condition, inability to be scaled up, or cost-unfriendly reagents and catalysts. Inspired by vanadium haloperoxidases herein we presented a biomimetic approach for halogenation (Br, Cl, I) of (hetero)arene catalyzed by tungstate under mild pH in a cost-efficient and environment- and operation-friendly manner. Broad substrates, diverse functional group tolerance, and good chemo- and regioselectivities were observed, even in late-stage halogenation of complex molecules. Moreover, this approach can be scaled up to over 100 g without time-consuming and costly column purification. Several drugs and key precursors for drugs bearing aryl halides (Br, Cl, I) have been conveniently prepared based on our approach.

A dual biomimetic process for the selective aerobic oxidative coupling of primary amines using pyrogallol as a precatalyst. Isolation of the [5 + 2] cycloaddition redox intermediates

Low-cost pyrogallol precatalyst undergoes an oxidative self-processing step for delivering the active organocatalystin situthrough a dual biomimetic process.