Metal-free Heterogeneous Photocatalyst Research Articles

Benzothiadiazole-Based Ordered Mesoporous Polymer as a Versatile, Metal-Free Heterogeneous Photocatalyst.

Visible-light active heterogeneous organophotocatalysts have recently gained considerable interest in organic synthetic community. Ordered mesoporous polymers (OMPs) are highly promising as heterogeneous alternative to traditional precious metal/organic dyes-based photocatalysts. Herein, we report the preparation of a benzothiadiazole functionalized OMPs (BT-MPs) through a "bottom-up" strategy. High ordered periodic porosity, large surface area, excellent stability and rational energy-band structures guarantee the high catalytic activity of BT-MPs. As a result, at least six conversions, e.g., the [3+2] cycloaddition of phenols with olefins, the selective oxidation of sulfides, the C-3 thiocyanation of indole and the aminothiocyanation of β-keto ester, could be promoted smoothly by BT-MPs. In addition, BT-MPs was readily recovered with well maintaining its photocatalytic activity and could be reused for at least eight times. This study highlights the potential of exploiting photoactive OMPs as recyclable, robust and metal-free heterogeneous photocatalysts.

Tailoring a decatungstate-incorporated metal-organic framework for light-driven [3+2] cyclization of phenols with olefins

Photocatalytic oxidative [3+2] cycloaddition of phenol-olefin is an economy and green strategy to synthesize 2,3-Dihydrobenzofurans (2,3-DHBs) compared with the thermal catalysis with noble metals under harsh conditions. Thus, it is urgent to design noble metal-free heterogeneous photocatalyst that combined single-electron transfer (SET) and hydrogen-atom transfer (HAT) dual function. Decatungstate [W10O32]4− is considered as an ecofriendly photocatalyst for HAT and SET, but is commonly used in homogeneous catalysis. Herein, we report CuW10-TIPA as a white-light-activated heterogeneous photocatalyst for synthesizing 2,3-dihydrobenzofurans via [3+2] cycloaddition of phenols with olefins, a crucial step in relevant pharmaceutical structural motifs. The coordination bonds between Cu(I) and TIPA, strong H-bonds between TIPA and [W10O32]4− and anion∙∙∙π interactions between [W10O32]4− and TIPA dramatically promote the separation of photoinduced electron-hole and facilitate the charge transfer among each component. Our approach is characterized by its mildness, scalability, and recyclability. We delve into the discussion on [W10O32]4− anions-mediated C-H bond activation via the HAT pathway, with the cooperation of the photosensitizer TIPA and Cu(I), which enhances absorption in the visible region and narrows the energy band.

2-Pyran-4-Ylidene Malononitrile Based Conjugated Microporous Polymers as Metal-Free Heterogeneous Photocatalysts for Organic Synthesis.

Photoactive conjugated microporous polymers (CMPs) as heterogeneous photocatalysts provide a sustainable alternative to classical metal-based semiconductor photosensitizers. However, previously reported CMPs are typically synthesized through metal catalyzed coupling reactions, which bears product separation, but also increases the price of materials. Herein, a new type of sp2 carbon linked DCM-CMPs are successfully designed and synthesized by organic base catalyzed Knoevenagel reaction using 2,6-Dimethyl-4H-pyran-4-ylidene-malononitrile and aromatic polyaldehydes as monomers. The new polymers feature inherent porosity, excellent stability, and fully π-conjugated skeleton with broad visible-light absorption. They effectively induce the synthesis of benzimidazole compounds under light irradiation, and exhibit wide substrate adaptability with outstanding recyclability.

Mechanically pulverized covalent organic framework as a metal-free photocatalyst for fenton-like degradation of organic pollutants and hexavalent chromium reduction

Visible light active, metal-free heterogeneous photocatalysts are required to achieve sustainable and green chemical processes. Development of such a catalyst is reported here. Mechanical pulverization of a covalent organic framework (COF) resulted in enhanced visible light photocatalytic activity for environmental remediation. The pulverized COF have achieved the apparent quantum yield (AQY) up to 41% for photo-Fenton-like degradation of organic dyes. Photocatalytic activity of the pulverized COF is one of the highest amongst the metal-free catalysts reported so far. The photocatalyst can operate under a broad range of pH, unlike metal-based photocatalysts. The catalyst does not lose its activities on recycling over several cycles. The exceptional performance of the pulverized COF is due to the high dispersibility, efficient visible light absorption, and excellent charge mobility. Moreover, the photocatalyst have also been used in continuous flow photo-Fenton-like degradation. The photocatalytic space time yield (PSTY) in continuous flow was boosted by twenty-eight times than the conventional batch photocatalysis. Moreover, the pulverized COF showed an outstanding photocatalytic reduction of toxic hexavalent chromium (Cr6+) to non-toxic trivalent (Cr3+).

Unveiling the potential of a covalent triazine framework based on [1]benzothieno[3,2-b][1]benzothiophene (DPhBTBT-CTF) as a metal-free heterogeneous photocatalyst

A novel covalent triazine framework (DPhBTBT-CTF) prepared by microwave cyclotrimerization of BTBT units was proved as very versatile photocatalyst for different reactions: oxidations, bromination, Diels–Alder cycloaddition and hydroxylations.

Metal-free redox-active diphenylpyrenediimides: A comparison between the photocatalytic performance of molecular and polymeric catalysts

Herein we described a donor–acceptor porous conjugated polydiphenylpyreneimide (DPPy-PI) with high thermal stability (530 °C) and surface area of 710 m2.g−1 prepared by condensation of 3,8-diphenylpyrene-1,2,6,7-tetracarboxylic dianhydride (DPPyDA) and tris(4-aminophenyl)benzene and a series of pyrenediimides (DPPy-DIs) as molecular references for comparative purposes. To evaluate the effect of the photoactive pyrene unit the polydiphenylperyleneimide (DPP-PI) having perylene units was also prepared.DPPy-PI has shown to see an efficient metal-free heterogeneous photocatalyst in the selective oxidation of aromatic sulfides and for Diels-Alder cycloadditions reaching almost full conversions with low reaction times and high selectivity that can be reused for at least fifteen cycles without significant loss of catalytic activity. Characterization data reveals that imide radicals are the important active intermediates during the redox processes of these PIs.

Engineering of Phenylpyridine- and Bipyridine-Based Covalent Organic Frameworks for Photocatalytic Tandem Aerobic Oxidation/Povarov Cyclization.

Covalent organic frameworks (COFs) are emerging as a new class of photoactive organic semiconductors, which possess crystalline ordered structures and high surface areas. COFs can be tailor-made toward specific (photocatalytic) applications, and the size and position of their band gaps can be tuned by the choice of building blocks and linkages. However, many types of building blocks are still unexplored as photocatalytic moieties and the scope of reactions photocatalyzed by COFs remains quite limited. In this work, we report the synthesis and application of two bipyridine- or phenylpyridine-based COFs: TpBpyCOF and TpPpyCOF. Due to their good photocatalytic properties, both materials were applied as metal-free photocatalysts for the tandem aerobic oxidation/Povarov cyclization and α-oxidation of N-aryl glycine derivatives, with the bipyridine-based TpBpyCOF exhibiting the highest activity. By expanding the range of reactions that can be photocatalyzed by COFs, this work paves the way toward the more widespread application of COFs as metal-free heterogeneous photocatalysts as a convenient alternative for commonly used homogeneous (metal-based) photocatalysts.

Hyper-Cross-Linked Porous Polymer Featuring B-N Covalent Bonds (HCP-BNs): A Stable and Efficient Metal-Free Heterogeneous Photocatalyst.

The first example of a porous polymer containing B-N covalent bonds, prepared from a tetraphene B-N monomer and biphenyl as a comonomer, is reported. It was prepared using the solvent knitting strategy, which allows the connection between the aromatic rings of the two monomers through methylene groups provided by an external cross-linking agent. The new polymer exhibited micromeso porosity with an SBET of 612 m2/g, high thermal stability, and potential properties as a heterogeneous photocatalyst, since it is very active in the aza-Henry coupling reaction (>98% of conversion and selectivity). After the first run, the catalyst improves its photocatalytic activity, shortening the reaction time to only 2 h and maintaining this activity in successive runs. The presence of a radical in this structure that remains stable with successive runs makes it a new type of material with potential applications as a highly stable and efficient photocatalyst.

Metal-Free Highly Stable and Crystalline Covalent Organic Nanosheet for Visible-Light-Driven Selective Solar Fuel Production in Aqueous Medium

Conversion of CO2 into solar fuels via artificial photosynthesis is one of the most promising and sustainable approaches to mitigate global warming and worldwide energy shortage. Covalent organic frameworks (COFs) exhibit well-defined arrangements of building blocks, tunable porosity, and high thermal and chemical stability in harsh conditions. The tunable band gaps of COFs by suitably introducing chromophoric light-harvesting units make them a unique class of metal-free heterogeneous photocatalysts for the successful conversion of CO2 to solar fuel. In this work, we report a simple, efficient, and low-cost 2D COF (TTA-Tz) composed of 1,3,5-tris-(4-aminophenyl)triazine (TTA) and 4,4′-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde (Tz) for photocatalytic CO2 reduction. The 2D-layered COF is exfoliated into ultrathin covalent organic nanosheets (CONs), which shows visible-light-driven photoreduction of CO2 to CO (yield = 2.8 mmol g–1, rate = 82 μmol h–1 g–1, and selectivity >99%) in aqueous medium without an external sacrificial electron donor. Interestingly, for a mixed solvent system, the CO evolution rate (163 μmol g–1 h–1) is found double than the aqueous medium case with 99% selectivity. By introducing both BNAH and TEA as sacrificial electron donors, the significant amount of CH4 (499 μmol g–1) is produced and the rate of CO evolution (310 μmol g–1 h–1) is further enhanced. The mechanistic insight of CO2 reduction is studied by DFT-based theoretical calculation, which is further supported by in situ diffuse reflectance spectroscopy study.

Constructing metal-free heterophotocatalyst using two-dimensional carbon nitride sheets and violet phosphorene for highly efficient visible-light photocatalysis

Two-dimensional (2D) carbon nitride sheets (CNs) with nanoscale thickness have great potential in solar energy conversion owing to their intrinsic bandgap. However, their photocatalytic efficiency falls far below the practical requirements because the generated electron-hole pairs rapidly recombine. We construct a metal-free 2D p–n junction heterophotocatalyst from CNs and violet phosphorene (VP) using a simple electrostatic self-assembly method. The experimental results and density functional theory calculations show that the built-in electric field of the p–n junction promotes photogenerated carrier transport at the heterojunction interface, thereby promoting the separation of photogenerated electron-hole pairs. Under visible-light illumination, the visible-light photocatalytic hydrogen and CO production rates of the CNs/VP heterojunction were increased by 5.85 and 1.51 times, respectively, when compared to that of CNs. This study provides new insights into the design of metal-free heterogeneous photocatalysts with high catalytic activity.

Heterogeneous photocatalytic borylation of aryl iodides mediated by isoreticular 2D covalent organic frameworks

Metal-free heterogeneous photocatalysts provide an environmental-friendly and cost-efficient avenue for green organic synthesis. Covalent organic frameworks (COFs) as heterogeneous photocatalysts showcase promising potential in the field of photocatalytic organic reactions due to their high porosity, insolubility and tailor-made functions. However, thus far, COF-based catalysts only mediated a few types of reactions. Herein, we developed a series of isoreticular nitrogen-rich covalent organic frameworks (N-COFs) with comparable porous structures as photocatalysts which effectively mediated the borylation of aryl iodides with broad substrate scope. Remarkably, 6N-COF exhibits excellent photocatalytic efficiency and superior recyclability. It suggests a new pathway to construct efficient heterogeneous photocatalysts for the borylation of aryl halides.

Effective Photocatalytic Initiation of Reactive Oxygen Species by a Photoactive Covalent Organic Framework for Oxidation Reactions

Reactive oxygen species (ROS) play a vital role in both chemistry and physiology. Developing effective photocatalysts becomes a feasible approach to trigger the generation of ROS. Herein, a photosensitive covalent organic framework (COF) was rationally designed and constructed by incorporating the photoactive triphenylamine moieties into its skeleton, characterizations of which not only showed the success formation of the highly crystalline and stable framework, but also revealed the photoactive property derived from triphenylamine-based building block. Electron paramagnetic resonance measurements displayed that the COF was an effective photocatalyst for generating ROS with one unpaired electron, that is, superoxide radical anion (O2• –). Subsequently, its high efficiency, selectivity, and reusability in photocatalytic aerobic oxidation of sulfides confirmed that the COF is a promising photocatalyst in the applications of ROS-involved reactions, demonstrating that precisely embedding organic photochromic groups into COFs could be a powerful strategy to fabricate metal-free heterogeneous photocatalysts for targeted reactions.

One-step fabrication of mesoporous sulfur-doped carbon nitride for highly selective photocatalytic transformation of native lignin to monophenolic compounds

Photocatalytic selective transform native lignin into valuable chemicals is an attractive but challenging task. Herein, we report a mesoporous sulfur-doped carbon nitride (MSCN-0.5) which is prepared by a facile one-step thermal condensation strategy. It is highly active and selective for the cleavage Cα−Cβ bond in β−O−4 lignin model compound under visible light radiation at room temperature, achieving 99% substrate conversion and 98% Cα−Cβ bond cleavage selectivity. Mechanistic studies revealed that the Cβ−H bond of lignin model compounds activated by holes and generate key Cβ radical intermediates, further induced the Cα−Cβ bond cleavage by superoxide anion radicals (•O2−) to produce aromatic oxygenates. Waste Camellia oleifera shell (WCOS) was taken as a representative to further understand the reaction mechanisms on native lignin. 33.2 mg of monophenolic compounds (Vanillin accounted for 22% and Syringaldehyde for 34%) can be obtained by each gram of WCOS lignin, which is 2.5 times as that of the pristine carbon nitride. The present work offers useful guidance for designing metal-free heterogeneous photocatalysts for Cα−Cβ bond cleavage to harvest monophenolic compounds.

Direct Solar Energy-Mediated Synthesis of Tertiary Benzylic Alcohols Using a Metal-Free Heterogeneous Photocatalyst

Direct Solar Energy-Mediated Synthesis of Tertiary Benzylic Alcohols Using a Metal-Free Heterogeneous Photocatalyst

A Porphyrin-Based Covalent Organic Framework for Metal-Free Photocatalytic Aerobic Oxidative Coupling of Amines.

Imines are important intermediates in drug synthesis. Photocatalytic aerobic oxidative coupling of amines has been considered as a clean and promising way to produce imines and has attracted great attention. Herein, we designed and synthesized a novel two-dimensional porphyrin-based covalent organic framework (Por-BC-COF) which adopts an AA stacking mode with excellent crystallinity, high Brunauer-Emmett-Teller surface areas (1200 m2 g-1 ), wide light absorption range (200-1300 nm) and good stability in a variety of organic solvents. Por-BC-COF can be used as a metal-free heterogeneous photocatalyst for the photocatalytic oxidation of amines to imines under visible light and red light with a high yield (97 %). This work presents a novel and efficient COF photocatalyst in the application of light-driven organic synthesis.

Fluorescent Hybrid Porous Polymers as Sustainable Heterogeneous Photocatalysts for Cross-Dehydrogenative Coupling Reactions.

A series of hybrid porous polymers (HPPs) based on polyhedral oligomeric silsesquioxane (POSS) were synthesized, characterized, and successfully used as metal-free heterogeneous photocatalysts for cross-dehydrogenative coupling reactions (CDC), for which the aza-Henry coupling of tetrahydroisoquinolines and nitroalkanes was studied as the model reaction. The reactions run smoothly at room temperature under visible (blue) light irradiation using gaseous oxygen as an oxidant under atmospheric pressure. These novel metal-free heterogeneous photocatalysts can be readily recovered and reused without a significant loss of reactivity. Mechanistic investigations revealed the intermediacy of 1O2, obtained from 3O2 sensitization (energy transfer) by the photoexcited catalyst.

Modulating the Stacking Model of Covalent Organic Framework Isomers with Different Generation Efficiencies of Reactive Oxygen Species.

The development of a facile synthesis and controllable layer stacking approach for covalent organic frameworks (COFs) is an important issue for modulating their properties and realizing their application diversity. Herein, three COF isomers with different stacking models (eclipsed AA, staggered AB, and ABC stacking) were obtained by modulating the reaction temperature and solvent medium. Experimental and theoretical calculations show that the ABC stacking isomer obtained at room temperature is the kinetic product, while the AA stacking isomer prepared by the solvothermal method is a thermodynamic product. Owing to the tautomerism involved in the reaction process, these isomers possess different ratios of enol and keto forms. Thus, they exhibit different generation efficiencies of Type I and Type II reactive oxygen species (ROS). The ABC stacking isomers could be employed as metal-free heterogeneous photocatalysts for visible-light-induced oxidation of amines to imines, owing to the highest generation efficiency of Type I ROS.

Donor–Acceptor Type Conjugated Microporous Polymer as a Metal-Free Photocatalyst for Visible-Light-Driven Aerobic Oxidative Coupling of Amines

Developing cheap, highly efficient, metal-free heterogeneous photocatalysts remain a great challenge in photoredox reactions. Herein, we utilize a typical Suzuki coupling of low-cost triphenylamine derivative and 9,10-dibromoanthracene to synthesis a donor–acceptor type conjugated microporous polymer (denoted as PAA-CMP). As expected, heterogeneous PAA-CMP exhibits excellent photocatalytic performance, good functional group tolerance and satisfying recyclability in metal-free aerobic oxidative coupling of amines to imines driven by visible light, which is due to its absolute energy level positions and good physicochemical stability. More excitingly, PAA-CMP can enable the gram-scale air-oxidized photocatalytic conversion under natural sunlight irradiation, yielding the desired imine product with an isolated yield of 65% for 48 h. The current work provides a great application prospect for CMPs in low-cost and large-scale organic industrial production in the future. The prepared donor-acceptor (D-A) type conjugated microporous polymer (PAA-CMP) as a metal-free and heterogeneous photocatalyst enable the gram-scale air-oxidized photocatalytic conversion of benzylamine into N-benzylidenebenzylamine under natural sunlight irradiation, yielding the desired imine product with an isolated yield of 65%.

β-Diketone boron difluoride dye-functionalized conjugated microporous polymers for efficient aerobic oxidative photocatalysis

Two new conjugated microporous polymers, TPB-B-CMP and TPA-B-CMP, with β-diketone-boron difluoride dye as the key building block were designed and successfully prepared. They were further employed as efficient metal-free heterogeneous photocatalysts.

Bandgap engineering in benzotrithiophene-based conjugated microporous polymers: a strategy for screening metal-free heterogeneous photocatalysts

A metal-free photocatalyst was screened from the same series of CMP materials through the bandgap engineering strategy. It exhibited excellent photocatalytic performances for the synthesis of benzimidazoles with high activity and recyclability.