Photocatalytic Oxidative Coupling Research Articles

Boosting the photocatalytic benzylamine oxidation and Rhodamine B degradation using Z-scheme heterojunction of NiFe2O4/rGO/Bi2WO6

The construction of heterojunction photocatalysts with powerful interfacial connections is critical for the actual utilization of photocatalytic technology. In this paper, the heterojunction photocatalyst NiFe2O4/rGO/Bi2WO6 (NFO/rGO/BWO) was prepared by in-situ hydrothermal method. Due to its excellent conductivity, rGO promoted the separation of photogenerated carriers, which was beneficial to the formation of Z-scheme heterojunction between NFO and BWO, and the absorption of visible light was improved by the involvement of rGO and NFO. Among a series of synthesized photocatalysts, 3 % NFO/rGO/BWO had the best photocatalytic activity in the photocatalytic oxidative coupling of benzylamine (BA). After 180 min of illumination, high conversion of 96 % for BA was achieved, which was 2.6 times higher than that of BWO. When it was applied to the degradation of Rhodamine B (RhB), complete degradation could be achieved, and the degradation rate was 5.67 times than that of BWO. The important roles of h+ and •O2– in the reaction were confirmed by electron spin resonance (ESR) analysis and radical capture experiments. The density functional theory (DFT) calculation and bandgap structure of the catalysts verified the existence of Z-scheme heterojunction. Potential reaction pathways for photocatalytic BA coupling and RhB degradation were proposed, respectively.

Electronic asymmetry of lattice oxygen sites in ZnO promotes the photocatalytic oxidative coupling of methane

Photocatalytic oxidative coupling of methane with oxygen is promising to obtain valuable muti-carbon products, yet suffering low reactivity. Here, we apply cerium modifications on zinc oxide-supported gold catalysts based on the electronic asymmetry design of lattice oxygen to improve the coupling activity. The methane conversion rate exceeds 16000 μmol g−1 h−1 with muti-carbon selectivity of 94.9% and catalytic durability of 3 days, and it can increase to 34000 μmol g−1 h−1 under more thermal assistance, with a turnover frequency of 507 h−1 for ethane and an apparent quantum efficiency of 33.7% at 350 nm. According to systematic characterizations and theoretical analysis, cerium dopants not only can boost the formation of reactive oxygen species but also intervene in the vivacity of lattice oxygen by manipulating metal-oxygen bond strength, thereby leading to favorable methyl desorption to form ethane and quick water release. This work provides insight into the rational design of efficient photocatalysts for aerobic methane-to-ethane conversion.

Hierarchical porous structured biomass based COFs for photocatalytic oxidative coupling of amines

Covalent Organic Frameworks (COFs) are widely used for photocatalysis but limited by the aggregation of the catalysts, which limited the mass transfer. Herein, corn stalk cores were used as the substrate, and two kinds of biomass based COFs with different electronic structures of thiophene (C@SCOF) and pyridine (C@NCOF) were in-situ fabricated uniformly on the surface of the substrate. In this way, COFs expose more active sites rather than embedding in the agglomerated structure. Biomass based COFs, especially C@SCOF, exhibit a hierarchical porous structure containing micropores and macropores, which offer fast mass transfer channels for the catalytic and adsorbed substrates. Therefore, the biomass based COFs utilized more efficiently to achieve higher photocatalytic performance. The C@SCOF with D-A structure displays stronger photoresponse and lower impedance, which promote the dissociation of a photogenerated exciton and carrier separation efficiency, resulting remarkable photocatalytic performance for oxidation of benzylamine coupling reaction. This work could guide the design and preparation of biomass-based hybrids with promising performance.

BODIPY-Helicene Based Heavy-Atom-Free Photocatalyst for Oxidative Coupling of Amines and Photooxidation of Sulfides.

To develop heavy-atom-free triplet photosensitizers (PSs) based photocatalysts, we designed and synthesized two BODIPY-helicene dyes by fusing the BODIPY core and modified [5]helicene structures. These BODIPY-helicenes structures are twisted and their twisting angles are increased by the developed synthetic method. The BODIPY-helicenes have broad absorption bands over UV-visible region with high triplet conversions and long triplet lifetimes as compared to planar BODIPY dye, PM567. Consequently, these dyes are also highly efficient in generating 1O2 by transferring their triplet energy to 3O2. All these are confirmed by dye-sensitised photooxidation reaction, nanosecond transient absorption spectroscopy study, phosphorescence measurement and DFT calculations. Finally, photocatalytic activity of the highest 1O2 generating BODIPY-helicene (4 b) was checked. 4 b is highly efficient in photocatalytic oxidative coupling of differently substituted amines through aerobatic oxidation using 1O2 generated by its photosensitization. It is also highly efficient photocatalyst for aerobatic oxidation of sulfides to sulfoxides. Importantly, the photocatalyst could be quantitatively recovered and reused for several cycles. All these results confirmed the potential use of the BODIPY-helicenes as PSs for photocatalytic organic reactions and the design strategy will be useful for the future development of heavy-atom-free photocatalyst.

Efficient Photocatalytic Oxidative Coupling of Amines under Visible Light Using a Trioporphyrins-Based Covalent Triazine Framework.

Porphyrins-based porous organic polymers (POP) were widely used in photocatalytic oxidation under visible light owing to their superiority in the activation of oxygen. In contrast, the efficiency is usually limited due to the fast recombination and slow electron transfer. Herein, we report the use of a trioporphyrins-based covalent triazine framework (Por-CTF) as visible-light-active photocatalyst for the coupling oxidative of amines to imines at room temperature. By incorporating the π-conjugated porphyrin building block led to the enhanced electron transport between molecules, and the extended recombination time of excited electrons. The photocatalytic efficiency of Por-CTF is superior to that of polymer in absence of triazine framework (POP-TSP), which was prepared by radical polymerization using tetra-(4-vinylphenyl) porphyrin (TSP) as monomer. Por-CTF catalyst presented excellent efficiency for various primary amines and stability. This work provides a reasonable guidance of catalyst molecular structure design for enhancing efficiency in the photocatalytic oxidation.

A Porphyrin-Based sp2 Carbon-Conjugated covalent organic polymer containing flexible chains for efficient heterogeneous photocatalysis

Porphyrin based covalent organic polymers are excellent catalysts. Herein, a sp2 carbon linked porphyrin based organic polymer H2Pp-PDAN was designed and synthesized by co-condensation of porphyrin monomer containing flexible chains H2Pp with 1, 4-phenylenediacetonitrile PDAN. As photocatalysts, H2Pp-PDAN exhibited excellent photocatalytic performance for photocatalytic oxidative coupling of amines, and was successfully used for 15 consecutive reactions with no detectable reduction of the catalytic activity. H2Pp-PDAN showed good photocatalytic activity and excellent recyclability for photocatalytic oxidation of thioanisole to methyl phenyl sulfoxide. Mechanistic studies revealed that the photocatalytic oxidation of benzylamine coupling by H2Pp-PDAN proceeds via both energy transfer and electron transfer pathways.

Selective photocatalytic oxidative coupling of methane via directional activation of oxygen over Au-loaded ZnSn-LDH with defect structure

The selective photocatalytic oxidative coupling of methane (OCM) holds great potential in converting low-cost feedstock into C2 products. However, the challenge lies in improving the photocatalytic OCM activity through catalyst design and gaining a deeper understanding of reactant activation in the OCM process. Here, we synthesized Au-ZnSn-LDH with oxygen vacancies to achieve efficient selective oxidation of methane using O2. The optimal hybrid exhibits a C2H6 yield of 2666 μmol g−1 h−1 and 76.8 % selectivity. Our findings showed that the oxygen within the surface of ZnSn-LDH (possibly from adsorption at Ov vacancy sites) provides sites for the evolution of O2 to O2− with assistance from Au cocatalyst. More O2− could be generated from O2 provides a desired reactivity to selectively promote selective C−H activation of CH4 under photoexcitation. The Au cocatalyst plays crucial roles in facilitating the methyl transfer and promoting C−C coupling to produce C2H6 as the main product while avoiding undesirable overoxidation.

Photocatalytic methane oxidation over a TiO2/SiNWs p-n junction catalyst at room temperature.

Rapid recombination of charge carriers in semiconductors is a main drawback for photocatalytic oxidative coupling of methane (OCM) reactions. Herein, we propose a novel catalyst by developing a p-n junction titania-silicon nanowires (TiO2/SiNWs) heterostructure. The structure is fabricated by atomic layer deposition of TiO2 on p-type SiNWs. The TiO2/SiNWs heterostructure exhibited an outstanding OCM performance under simulated solar light irradiation compared to the single components. This enhanced efficiency was attributed to the intrinsic electrical field formed between n-type TiO2 and p-type SiNWs, which forces generated charge carriers to move in opposite directions and suppresses charge recombination. Besides, surface morphology and optical properties of the the p-n TiO2/SiNWs catalyst are also beneficial for the photocatalytic activity. It is expected that the results of this study will provide massive guidance in synthesizing an efficient photocatalyst for CH4 conversion under mild conditions.

Graphitic carbon nitride-PTD hybrid catalyst for efficient photocatalytic oxidative coupling of arylamine for the production of azoaromatic under visible light

Oxidative coupling of amine derivatives to selectively synthesize pharmaceutical materials has been realized via photocatalysis. Herein we report on the fabrication and development of a graphitic carbon nitride (G) based visible light active photocatalyst (G-PTD), which is melamine converted graphitic carbon nitride(G) covalently integrated to a light harvesting perylene tetracarboxylic dianhydride (PTD) molecule. The synthesized photocatalyst has various functions in a highly active manner leading to high yield of newly generated azoaromatic compound. The current research endeavour highlights on the fabrication and utilization of a graphitic carbon nitride based photocatalyst for direct oxidative coupling of arylamines.

Fabrication of Z-scheme heterojunction of UCN/BWO for selective photocatalytic benzylamine oxidation

Nanosheet materials have larger specific surface area and more active sites than bulk materials. In this paper, ultrathin graphite carbon nitride (UCN) nanosheets were first prepared by a multi-step calcination method, and then Bi2WO6 nanoflowers were grown in-situ on the surface of UCN by a simple hydrothermal method to synthesize ultrathin graphite carbon nitride and bismuth tungstate composite (UCN/BWO). The synthesized catalyst was applied to the photocatalytic oxidative coupling of benzylamine (BA). After reacted 5 h under light in air atmosphere, BA was almost completely converted into N-benzylidenebenzylamine, which was 3.8 times higher than that of bulk graphite carbon nitride (BCN). The enhanced photocatalytic activity was not only attributed to the thin nanosheet structure of UCN which shortened the migration distance of photogenerated electrons, but also because the construction of Z-scheme heterojunction that inhibited the recombination of photogenerated carriers and improved the oxidation ability of the catalyst. Based on density functional theory (DFT) calculation and photoelectric performance test, the existence of Z-scheme heterojunction was proved. Combined with the active species capture experiments, the pathway of electron transfer in the reaction process and the possible reaction mechanism were proposed.

Synthesis of Zn0.2Cd0.8S/MoS2/rGO photocatalyst for efficient solar-driven selective organic conversion

ZnxCd(1-x)S solid solution was constructed by introducing zinc ions in the process of hydrothermal synthesis of CdS. To further improve the catalyst activity, Zn0.2Cd0.8S was combined with graphene and MoS2, and then the Zn0.2Cd0.8S/MoS2/rGO ternary composites were obtained. Its photocatalytic reactivity was evaluated by the catalytic oxidative coupling of benzylamine and the photocatalytic semi-dehydrogenation of 1,2,3,4-tetrahydroisoquinoline (THIQ). When Zn0.2Cd0.8S/MoS2/rGO was used in photocatalytic benzylamine oxidative coupling reaction, the conversion reached 95 % after 7 h and the selectivity of N-benzyl-1-phenylmethylimide (BPMI) was 99 %. It was further applied to the photocatalytic semi-dehydrogenation of THIQ, the conversion reached 96 % after 7 h reaction in O2 atmosphere, and the selectivity of 3,4-dihydroisoquinoline (DHIQ) reached 99 %. The heterojunction between Zn0.2Cd0.8S and MoS2 can effectively reduce the recombination efficiency of carrier. The introduction of graphene can increase the active site and promote the transfer of photogenerated electrons. The synergistic effect of MoS2 and graphene makes Zn0.2Cd0.8S have outstanding photocatalytic performance. Cycling experiments show that the ternary composite photocatalyst of Zn0.2Cd0.8S/MoS2/rGO have high stability and recyclability. The possible reaction mechanism of photocatalytic oxidative coupling of benzylamine and semi-dehydrogenation of THIQ was proposed by theoretical calculations and the active species capturing experiments.

Donor‐Acceptor Pyrene‐Based Covalent Organic Framework for Blue Light Photocatalytic Oxidative Coupling of Amines

AbstractCovalent organic frameworks (COFs) with a donor‐acceptor (D−A) architecture have been investigated as prospective semiconducting materials, particularly for photocatalytic processes. The incorporation of electron donating and accepting moieties into the COF backbone creates nano heterojunctions, enhancing efficient light absorption, charge separation, and charge transport. Herein, three pyrene‐based COFs were well designed for the aerobic coupling of amines under air atmosphere with blue‐LED irradiation. Among the three COFs, the Py‐Por‐COF with D−A structure exhibits the highest photocatalytic performance and demonstrates a remarkable apparent quantum efficiency of 11.3 % at 420 nm. In addition, the Py‐Por‐COF maintains a good level of crystallinity and high yield even undergone four cycles, showcasing the exceptional potential of D−A structure COFs in the field of photocatalytic organic transformation.

Inside Cover Picture

Triphenylamine and its derivatives (TPAs) are promising for the photocatalytic oxidative coupling of thiols to disulfides, but suffer from the easy polymerization and difficult separation. In this paper, by immobilizing the TPAs into MOF with photoredox inert Sr nodes, the photo‐generated electrons were successfully controlled to transfer from the excited TPAs to O2, and finally realize the heterogeneous catalytic oxidation of thiols to disulfides. More details are discussed in the article by Zhang et al. on page 1093—1099.image

Confined synthesis of conjugated microporous polymers for selective photocatalytic oxidation of amines

Photocatalytic oxidative coupling of amines is considered a mild, efficient, and sustainable strategy for the synthesis of imines. As a versatile organic semiconductor, conjugated microporous polymers (CMPs) are attractive in photocatalysis areas due to the diversity of their polymeric monomers. Herein, we report that in addition to the design of monomers, size-confined polymerization is also a feasible strategy to modulate the structure and photocatalysis properties of CMPs. We adopted dibromopyrazine as polymeric units to prepare pyrazine-involved hollow spherical CMPs (H-PyB) using a template method and successfully performed size-confined polymerization of hollow samples by resizing the templates. Interestingly, the small confinement space induced the formation of CMPs with better conjugate extensibility, resulting in enhanced conductivity, narrowed bandgaps, improved photoelectric performance, etc. As a result, small-sized H-PyB CMPs had superior activity for the photocatalytic oxidation of amines. Particularly, the smallest H-PyB CMPs that we designed in the present work exhibited excellent performance for the photocatalytic coupling oxidation of amines. When using benzylamine as a model substrate, the yield of the corresponding imine reached ∼ 113 mmol·g−1·h−1, accompanied by almost 100 % selectivity. Furthermore, the as-designed confined samples exhibited stable photocatalytic activity as well as good applicability for oxidative coupling of different amines. This work not merely reports a kind of CMP photocatalysts with excellent performance for the imine coupling oxidation but also proposes an alternative strategy for constructing high-performance organic photocatalysts by size-confined synthesis.

Directional Activation of Oxygen by the Au-Loaded ZnAl-LDH with Defect Structure for Highly Efficient Photocatalytic Oxidative Coupling of Methane.

Photocatalytic oxidative coupling of CH4 (OCM) is a promising CH4 conversion process that can achieve efficient methane conversion with the assistance of O2. It remains to be highly challenging to improve the photocatalytic OCM activity from catalyst design and to deepen the understanding of the reactant activation in the OCM process. In this work, the Au-loaded ZnAl-layered double hydroxides (LDHs) with and without oxygen vacancy are constructed (denoted as Au/ZnAl and Au/ZnAl-v), respectively. When applied for photocatalytic OCM, the Au/ZnAl-v shows a CH4 conversion rate of 8.5 mmol g-1 h-1 with 92% selectivity of C2H6 at 40°C, outperforming most reported photocatalytic OCM systems at low temperature reported in the literature. Furthermore, the catalytic performance of Au/ZnAl-v can be stable for 100h. In contrast, the An/ZnAl exhibits a CH4 conversion rate of 0.8 mmol g-1 h-1 with 46% selectivity of C2H6. Detailed characterizations and DFT calculation studies reveal that the introduced Ov sites on Au/ZnAl-v are able to activate O2, and the resulting superoxide radical O2·- greatly promotes the activation of CH4. The coupling of CH3· groups with the assistance of Au cocatalyst leads to the formation of C2H6 with high photocatalytic activity.

Immobilizing Triphenylamine with Photoredox Inert Sr2+ Forming Sr‐MOF with Controlled Electron Migration for Photocatalytic Oxidation of Thiols to Disulfides

Comprehensive SummaryThe photocatalytic oxidative coupling of thiols is one of the most popular methods to synthesize the disulfides. Triphenylamine and its derivatives (TPAs) are promising for the above reaction, but suffer from the easy polymerization and difficult separation. To overcome these obstacles while controlling the photogenerated electrons transfer directly to target substrates, herein, we constructed one TPA‐based metal‐organic framework (MOF), (Me2NH2)[Sr(TCBPA)]·DMA·3H2O (1), by direct self‐assembly of tris(4′‐carboxybiphenyl)amine (H3TCBPA) and photoredox inert strontium ion (Sr2+). DFT calculations revealed that the valence band maximum (VBM) and the conduction band minimum (CBM) are mainly located on TCBPA3–, successfully inhibiting the undesirable electron migration to metal nodes. Experimental results indicated that 1 displays superior performance than homogeneous H3TCBPA, which may result from the abundant π···π and C—H···π interactions between the well‐arranged TCBPA3– and the build‐in electric field between the anionic framework and the Me2NH2+. This work highlights that immobilizing TPAs into MOFs is one promising approach to designing heterogeneous photocatalysts for the synthesis of disulfides by oxidative coupling of thiols.

Selective oxidative coupling of amines through light‐activated bismuth halide perovskites

AbstractBismuth halide perovskites have emerged as promising alternatives for sustainable and efficient photocatalysts for light‐induced organic transformations. Caesium bismuth halide perovskites (Cs3Bi2(ClxBr1‐x)9) with different ratios of chloride and bromide were synthesised using a one‐step solution process. These perovskites were then employed as photocatalysts for the photocatalytic oxidative coupling of benzylamine (BZA) to produce N‐benzylidenebenzylamine (BZI) under ultraviolet‐visible light radiation at ambient conditions. The incorporation of bromide ions into the chloride‐based perovskite resulted in a noticeable reduction in the bandgap, leading to an enhancement in the photocatalytic activity. Conversely, the addition of chloride ions within the bromide‐based perovskite was found to be beneficial for the material's photostability while simultaneously maintaining its high photocatalytic activity. The optimal compromise between the stability and photocatalytic activity for the BZI production was achieved using Cs3Bi2(Cl0.8Br0.2)9, being attained a total BZA conversion and the maximum BZI production with 100 % of selectivity after 6 hours of irradiation. The efficiency of this photocatalyst was ascribed to the highest current density observed during the photocurrent experiments, which improves the charge carrier mobility of the resulting material.

Continuous Flow System for Highly Efficient and Durable Photocatalytic Oxidative Coupling of Methane.

Photocatalytic oxidative coupling of methane (OCM) into value-added industrial chemicals offers an appealing green technique for achieving sustainable development, whereas it encounters double bottlenecks in relatively low methane conversion rate and severe overoxidation. Herein, we engineer a continuous gas flow system to achieve efficient photocatalytic OCM while suppressing overoxidation by synergizing the moderate active oxygen species, surface plasmon-mediated polarization, and multipoint gas intake flow reactor. Particularly, a remarkable CH4 conversion rate of 218.2 μmol h-1 with an excellent selectivity of ∼90% toward C2+ hydrocarbons and a remarkable stability over 240 h is achieved over a designed Au/TiO2 photocatalyst in our continuous gas flow system with a homemade three-dimensional (3D) printed flow reactor. This work provides an informative concept to engineer a high-performance flow system for photocatalytic OCM by synergizing the design of the reactor and photocatalyst to synchronously regulate the mass transfer, activation of reactants, and inhibition of overoxidation.

Linker engineering of benzothiadiazole and its derivative-based covalent organic frameworks for efficient photocatalytic oxidative amine coupling and hydrogen peroxide generation

Benzothiadiazole and its derivative-based COFs were prepared, in which benzothiadiazole-based COF exhibits superior photocatalytic aerobic oxidation of amine coupling, while the naphthothiadiazole-based COF shows the highest H2O2 production rate.

Accurate assembly of thiophene-bridged titanium-oxo clusters with photocatalytic amine oxidation activity.

Designing and synthesizing well-defined crystalline catalysts for the photocatalytic oxidative coupling of amines to imines remains a great challenge. In this work, a crystalline dumbbell-shaped titanium oxo cluster, [Ti10O6(Thdc)(Dmg)2(iPrO)22] (Ti10, Thdc = 2,5-thiophenedicarboxylic acid, Dmg = dimethylglyoxime, iPrOH = isopropanol), was constructed through a facile one-pot solvothermal strategy and treated as a catalyst for the photocatalytic oxidative coupling of amines. In this structure, Thdc serves as the horizontal bar, while the {Ti5Dmg} layers on each side act as the weight plates. The molecular structure, light absorption, and photoelectrochemical properties of Ti10 were systematically investigated. Remarkably, the inclusion of the Thdc ligand, with the assistance of the Dmg ligand, broadens the light absorption spectrum of Ti10, extending it into the visible range. Furthermore, the effective enhancement of charge transfer within the Ti10 was achieved with the successful incorporation of the Thdc ligand, as opposed to PTC-211, where terephthalic acid replaces the Thdc ligand, while maintaining consistency in other aspects of Ti10. Building on this foundation, Ti10 was employed as a heterogeneous molecular photocatalyst for the catalytic oxidative coupling reaction of benzylamine (BA), demonstrating very high conversion activity and selectivity. Our study illustrates that the inclusion of ligands derived from Thdc enhances the efficiency of charge transfer in functionalized photocatalysts, significantly influencing the performance of photocatalytic organic conversion.