Heterogenization of Salen Metal Molecular Catalysts in Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

A Covalent Organic Nanoribbon: Preparation, Single-Crystal Structure with Chinese Luban Lock Configuration, and Photocatalytic Behavior

The multiple mortise-and-tenon joint parts are the core factors to provide the structural stability and diversity of Chinese Luban locks; however, constructing such structures is very challenging. Herein, single crystals of a covalent organic nanoribbon (named CityU-27) are prepared through the assembly of hexahydroxytriphenylene (HHTP), 4,4'-vinylenedipyridine (BYE), and phenylboronic acid (BA) together through dative boron←nitrogen (B←N) bonds. The single-crystal X-ray diffraction analysis indicates that CityU-27 has a covalent organic nanoribbon structure, where each nanoribbon forms multiple and tight π-π interactions with four neighboring others to generate a Luban lock-like configuration. CityU-27 has been demonstrated to be an efficient photocatalyst in a one-pot tandem reaction of hydrogen evolution reaction (HER) and semi-hydrogenation reaction of alkynes in series to produce olefins without any additional photosensitizers and co-catalysts (metal-free).

Pyrene-based covalent organic frameworks (PyCOFs): a review

Recently, pyrene-based covalent organic frameworks (PyCOFs) have aroused great interest because the large planar structure of the pyrene unit could effectively enhance the interlayer π-π interaction and promote the separation and migration of carriers, significantly improving the crystallinity and photoelectrical properties of PyCOFs. Since the first PyCOF-containing boroxate linkage was reported in 2008 by the Yaghi group, many PyCOFs with different kinds of linkages have been reported, exhibiting great potential applications in different fields such as adsorption/separation, chemical sensing, catalysis, energy storage, etc. However, as far as we know, the reviews related to PyCOFs are rare, although PyCOFs have been widely reported to show promising applications. Thus, it is right time and important for us to systematically summarize the research advance in PyCOFs, including the synthesis with different linkages and applications. Moreover, the prospects and obstacles facing the development of PyCOFs are discussed. We hope that this review will provide new insights into PyCOFs that can be explored for more attractive functions or applications.

Ultrahigh Single Au Atoms Loaded Porous Aromatic Frameworks for Enhanced Photocatalytic Hydrogen Evolution

Supported single-atom catalysts (SACs) are promising in heterogeneous catalysis because of their atom economy, unusual transformations, and mechanistic clarity. The metal SAs loading, however, limits the catalytic efficiency. Herein, an in situ pre-metallated monomer-based preparation strategy is shown to achieve ultrahigh Au SAs loading in catalyst formations. The polymerization of single-atom loaded monomers yield a new porous aromatic framework (PAF-164) with Au SAs loading up to a record high 45.3 wt.%. SACs of Au-PAFs exhibit excellent photocatalytic activity in hydrogen (H2) evolution, and the H2 evolution rate of Au100%-SAs-PAF-164 can reach 4.82 mmol g-1 h-1 with great recyclability.

A scalable approach using a gC3N4-covalent organic framework hybrid catalyst towards sustainable hydrogen production from seawater and wastewater

The photocatalytic generation of H2 using covalent organic frameworks (COFs) is gaining more interest. While numerous reports have focused on the production of H2 from deionized water using COFs, the inability to produce H2 from industrial wastewater or seawater is a common limitation in many reported catalysts. Additionally, many of these reports lack a clear path to scale up the catalyst synthesis. In this study, we explore the prospect of hybridizing a COF with gC3N4 to create a robust photocatalyst for efficient H2 generation. This hybrid exhibits outstanding performance not only in deionized water, but also in wastewater, and simulated seawater. Furthermore, we explore the feasibility of the bulk-scale synthesis and successfully produce a 20 g hybrid catalyst in a single batch, and the synthesis method is scalable to achieve the commercial target. Remarkably, a maximum HER rate of 94 873 μmol g-1 h-1 and 109 125 μmol g-1 h-1 was obtained for the hybrid catalyst from industrial wastewater and simulated seawater, respectively. The performance of bulk-scale batches closely matches that of the small-scale ones. This research paves the way for the utilization of organic photocatalysts on a commercial scale, offering a promising solution for sustainable large-scale H2 production.

Complex isomerism influencing the textural properties of organometallic [Cu(salen)] porous polymers: paramagnetic solid-state NMR characterization and heterogeneous catalysis

Although organometallic porous polymer networks are recognized as promising heterogeneous catalysts, the relationship between ligand/monomer geometry and network parameters is usually not well understood due to the lack of atom-resolved characterization methods for the amorphous network matrix. In this work, a series of copper(II) salen-type metal complexes was synthesized, using trans- and cis-1,2-diaminocyclohexane segments, and thoroughly characterized by single-crystal X-ray diffraction and solution- and solid-state NMR spectroscopy. Terminal ethynyl groups of the complexes were then transformed into polyacetylene chains by coordination chain-growth homopolymerization, resulting in highly porous (458-655 m2 g-1) organometallic polymer networks with a copper(II) ion content of about 12 wt%. The presence of paramagnetic copper(II) moieties in these complexes and respective polymer networks required the application of tailored NMR techniques, which together with X-ray crystallography and DFT calculations of the paramagnetic NMR shifts made it possible to investigate the differences in the complex geometry in liquid, powder and crystalline form and compare it with the complex geometry in polymer networks. All prepared organometallic polymer networks were also tested as heterogeneous catalysts for styrene oxidation with uncommonly high substrate conversions and compared with their low-molecular-weight analogues. The high reusability of such heterogeneous polymer-based catalysts was also proven.

Surface-Grafted Single-Atomic Pt-Nx Complex with a Precisely Regulating Coordination Sphere for Efficient Electron Acceptor-Inducing Interfacial Electron Transfer

Based on the electron-withdrawing effect of the Pt(bpy)Cl2 molecule, a simple post-modification amide reaction was firstly used to graft it onto the surface of NH2-MIL-125, which performed as a highly efficient electron acceptor that induced the conversion of the photoinduced charge migration pathway from internal BDC→TiOx migration to external BDC→PtNx migration, significantly improving the efficiency of photoinduced electron transfer and separation. Furthermore, precisely regulating over the first coordination sphere of Pt single atoms was achieved using further post-modification with additional bipyridine to investigate the effect of Pt-Nx coordination numbers on reaction activity. The as-synthesized NML-PtN2 exhibited superior photocatalytic hydrogen evolution activity of 7.608 mmol g-1 h-1, a remarkable improvement of 225 and 2.26 times compared to pristine NH2-MIL-125 and NML-PtN4, respectively. In addition, the superior apparent quantum yield of 4.01 % (390 nm) and turnover frequency of 190.3 h-1 (0.78 wt % Pt SA; 129 times compared to Pt nanoparticles/NML) revealed the high solar utilization efficiency and hydrogen evolution activity of the material. And macroscopic color changes caused by the transition of carrier migration paths was first observed. It holds profound significance for the design of MOF-Molecule catalysts with efficient charge carrier separation and precise regulation of single-atom coordination sphere.

Tris(triazolo)triazine-Based Covalent Organic Frameworks for Efficiently Photocatalytic Hydrogen Peroxide Production

Two-dimensional covalent organic frameworks (2D-COFs) have recently emerged as fascinating scaffolds for solar-to-chemical energy conversion because of their customizable structures and functionalities. Herein, two tris(triazolo)triazine-based COF materials (namely COF-JLU51 and COF-JLU52) featuring large surface area, high crystallinity, excellent stability and photoelectric properties were designed and constructed for the first time. Remarkably, COF-JLU51 gave an outstanding H2O2 production rate of over 4200 μmol g-1 h-1 with excellent reusability in pure water and O2 under one standard sun light, that higher than its isomorphic COF-JLU52 and most of the reported metal-free materials, owing to its superior generation, separation and transport of photogenerated carriers. Experimental and theoretical researches prove that the photocatalytic process undergoes a combination of indirect 2e- O2 reduction reaction (ORR) and 4e- H2O oxidation reaction (WOR). Specifically, an ultrahigh yield of 7624.7 μmol g-1 h-1 with apparent quantum yield of 18.2 % for COF-JLU52 was achieved in a 1 : 1 ratio of benzyl alcohol and water system. This finding contributes novel, nitrogen-rich and high-quality tris(triazolo)triazine-based COF materials, and also designate their bright future in photocatalytic solar transformations.

Asymmetrical Interactions between Ni Single Atomic Sites and Ni Clusters in a 3D Porous Organic Framework for Enhanced CO2 Photoreduction

3D porous organic frameworks, which possess the advantages of high surface area and abundant exposed active sites, are considered ideal platforms to accommodate single atoms (SAs) and metal nanoclusters (NCs) in high-performance catalysts; however, very little research has been conducted in this field. In the present work, a 3D porous organic framework containing Ni1 SAs and Nin NCs is prepared through the metal-assisted one-pot polycondensation of tetraaldehyde and hexaaminotriptycene. The single metal sites and metal clusters confined in the 3D space created a favorable micro-environment that facilitated the activation of chemically inert CO2 molecules, thus promoting the overall photoconversion efficiency and selectivity of CO2 reduction. The 3D-NiSAs/NiNCs-POPs, as a CO2 photoreduction catalyst, demonstrated an exceptional CO production rate of 6.24 mmol g-1 h-1, high selectivity of 98%, and excellent stability. The theoretical calculations uncovered that asymmetrical interaction between Ni1 SAs and Nin NCs not only favored the bending of CO2 molecules and reducing the CO2 reduction energy, but also regulated the electronic structure of the catalyst leading to the optimal binding strength of intermediates.

II-Scheme Heterojunction Frameworks Based on Covalent Organic Frameworks and HKUST-1 for Boosting Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) are one type of promising polymer semiconductors in solar-driven hydrogen production, but majority of COFs-based photocatalytic systems show low photocatalytic efficiency owing to lack of metal active sites. Herein, we reported II-Scheme heterojunction frameworks based on COF (TpPa-1) and metal-organic framework (HKUST-1) for highly efficient hydrogen production. The coordination bonding directed self-assembly of HKUST-1 on the surface of TpPa-1 endows the heterojunction frameworks (HKUST-1/TpPa-1) with strong interface interaction, optimized electronic structures and abundant redox active sites, thus remarkably boosting photocatalytic hydrogen evolution. The hydrogen evolution rate for optimal HKUST-1/TpPa-1 is as high as 10.50 mmol g-1 h-1, which is significantly enhanced when compared with that of their physical mixture (4.13 mmol g-1 h-1), TpPa-1 (0.013 mmol g-1 h-1) and Pt-based counterpart (6.70 mmol g-1 h-1). This work offers a facile approach to the construction of noble-metal-free II-Scheme heterojunctions based on framework materials for efficient solar energy conversion.

Photoelectron Migration Boosted by Hollow Double-Shell Dyads Based on Covalent Organic Frameworks for Highly Efficient Photocatalytic Hydrogen Generation

Photocatalytic hydrogen production based on noble metal-free systems is a promising technology for the conversion of solar energy into green hydrogen, it is pivotal and challenging to tailor-make photocatalysts for achieving high photocatalytic efficiency. Herein, we reported a hollow double-shell dyad through uniformly coating covalent organic frameworks (COFs) on the surface of hollow Co9S8. The double shell architecture enhances the scattering and refraction efficiency of incident light, shortens the transmission distance of the photogenerated charge carriers, and exposes more active sites for photocatalytic conversion. The hydrogen evolution rate is as high as 23.15 mmol g-1 h-1, which is significantly enhanced when compared with that of their physical mixture (0.30 mmol g-1 h-1) and Pt-based counterpart (11.84 mmol g-1 h-1). This work provides a rational approach to the construction of noble-metal-free photocatalytic systems based on COFs to enhance hydrogen evolution performance.

Pyrolysis Free Out-of-Plane Co-Single Atomic Sites in Porous Organic Photopolymer Stimulates Solar-Powered CO2 Fixation

Herein, a facile strategy is illustrated to develop pyrolysis-free out-of-plane coordinated single atomic sites-based M-POP via a one-pot Friedel Craft acylation route followed by a post-synthetic metalation. The optimized geometry of the Co@BiPy-POP clearly reveals the presence of out-of-plane Co-single atomic sites in the porous backbone. This novel photopolymer Co@BiPy-POP shows extensive π-conjugations followed by impressive light harvesting ability and is utilized for photochemical CO2 fixation to value-added chemicals. A remarkable conversion of styrene epoxide (STE) to styrene carbonate (STC) (≈98%) is obtained under optimized photocatalytic conditions in the existence of promoter tert-butyl ammonium bromide (TBAB). Synchrotron-based X-ray adsorption spectroscopy (XAS) analysis reveals the single atom coordination sites along with the metal (Co) oxidation number of +2.16 in the porous network. Moreover, in situ diffuse reflectance spectroscopy (DRIFTS) and electron paramagnetic resonance (EPR) investigations provide valuable information on the evolution of key reaction intermediates. Comprehensivecomputational analysis also helps to understand the overall mechanistic pathway along with the interaction between the photocatalyst and reactants. Overall, this study presents a new concept of fabricating porous photopolymers based on a pyrolysis-free out-of-plane-coordination strategy and further explores the role of single atomic sites in carrying out feasible CO2 fixation reactions.

Single-Atom Catalysts on Covalent Organic Frameworks for Energy Applications

Single-atom catalysts (SACs) have been investigated and applied to energy conversion devices. However, issues of metal agglomeration, low metal loading, and substrate stability have hindered realization of the SACs' full potential. Recently, covalent organic framework (COF)-based SACs have emerged as promising materials to enable highly efficient catalytic reactions. Here, we summarize the representative COF-based SACs and their wide application in clean energy devices and conversion reactions, such as hydrogen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, oxygen reduction reaction, and oxygen evolution reaction. Based on their catalysis conditions, these reactions are categorized into photocatalyzed and electrocatalyzed reactions. We also summarize their design strategies, including heteroatom inclusion, donor-acceptor pairs, pore engineering, interface engineering, etc. Although COF-based SACs are promising, more efforts, such as linkage engineering, functional groups, ionization, multifunctional sites for cocatalyzed systems, etc., could improve them to be the ideal SAC materials. At the end, we provide our perspectives on where the field will proceed in the next 5 years.

Fully Conjugated 2D sp2 Carbon-Linked Covalent Organic Frameworks for Photocatalytic Overall Water Splitting

Covalent organic frameworks (COFs) hold great promise for solar-driven hydrogen production. However, metal-free COFs for photocatalytic overall water splitting remain elusive, primarily due to challenges in simultaneously regulating their band structures and catalytic sites to enable concurrent half-reactions. Herein, two types of π-conjugated COFs containing the same donor-acceptor structure are constructed via Knoevenagel condensation and Schiff base reaction to afford cyanovinylene- and imine-bridged COFs, respectively. The difference in the linkage leads to a remarkable difference in their photocatalytic activity toward water splitting. The 2D sp2 carbon-linked COF exhibits notable activity for photocatalytic overall water splitting, which can reach an apparent quantum efficiency of 2.53% at 420 nm. In contrast, the 2D imine-linked COF cannot catalyze the overall water-splitting reaction. Mechanistic investigations reveal that the cyanovinylene linkage is essential in modulating the band structure and promoting charge separation in COFs, thereby enabling overall water splitting. Moreover, it is further shown that crystallinity substantially impacts the photocatalytic performance of COFs. This study represents the first successful example of developing metal-free COFs with high crystallinity for photocatalytic overall water splitting.

Directional Shunting of Photogenerated Carriers in POM@MOF for Promoting Nitrogen Adsorption and Oxidation

The efficient catalysis of nitrogen (N2 ) into high-value N-containing products plays a crucial role in the N economic cycle. However, weak N2 adsorption and invalid N2 activation remain two major bottlenecks in rate-determining steps, leading to low N2 fixation performance. Herein, an effective dual active sites photocatalyst of polyoxometalates (POMs)-based metal-organic frameworks (MOFs) is highlighted via altering coordination microenvironment and inducing directional shunting of photogenerated carriers to facilitate N2 /catalyst interaction and enhance oxidation performance. MOFs create more open unsaturated metal cluster sites with unoccupied d orbital possessing Lewis acidity to accept electrons from the 3σg bonding orbital of N2 for storage by combining with POMs to replace bidentate linkers. POMs act as electron sponges donating electrons to MOFs, while the holes directional flow to POMs. The hole-rich POMs with strong oxidation capacity are easily involved in oxidizing adsorbed N2 . Taking UiO-66 (C48 H28 O32 Zr6 ) and Mo72 Fe30 ([Mo72 Fe30 O252 (CH3 COO)12 {Mo2 O7 (H2 O)}2 {H2 Mo2 O8 (H2 O)}(H2 O)91 ]·150H2 O) as an example, Mo72 Fe30 @UiO-66 shows twofold enhanced adsorption of N2 (250.5 cm3  g-1 ) than UiO-66 (122.9 cm3 g-1 ) at P/P0  = 1. And, the HNO3 yield of Mo72 Fe30 @UiO-66 is 702.4 µg g-1  h-1 , ≈7 times and 24 times higher than UiO-66 and Mo72 Fe30 . This work provides reliable value for the storage and relaying artificial N2 fixation.

Highly Selective Electrocatalytic Olefin Hydrogenation in Aqueous Solution

Selective hydrogenation of olefins with water as the hydrogen source at ambient conditions is still a big challenge in the field of catalysis. Herein, the electrocatalytic hydrogenation of purely aliphatic and functionalized olefins was achieved by using graphdiyne based copper oxide quantum dots (Cux O/GDY) as cathodic electrodes and water as the hydrogen source, with high activity and selectivity in aqueous solution at high current density under ambient temperature and pressure. In particular, the sp-/sp2 -hybridized graphdiyne catalyst allows the selective hydrogenation of cis-trans isomeric olefins. The chemical and electronic structure of the GDY results in the incomplete charge transfer between GDY and Cu atoms to optimize the adsorption/desorption of the reaction intermediates and results in high reaction selectivity and activity for hydrogenation reactions.

Linker Engineering for Reactive Oxygen Species Generation Efficiency in Ultra-Stable Nickel-Based Metal-Organic Frameworks

Interfacial charge transfer on the surface of heterogeneous photocatalysts dictates the efficiency of reactive oxygen species (ROS) generation and therefore the efficiency of aerobic oxidation reactions. Reticular chemistry in metal-organic frameworks (MOFs) allows for the rational design of donor-acceptor pairs to optimize interfacial charge-transfer kinetics. Herein, we report a series of isostructural fcu-topology Ni8-MOFs (termed JNU-212, JNU-213, JNU-214, and JNU-215) with linearly bridged bipyrazoles as organic linkers. These crystalline Ni8-MOFs can maintain their structural integrity in 7 M NaOH at 100 °C for 24 h. Experimental studies reveal that linker engineering by tuning the electron-accepting capacity of the pyrazole-bridging units renders these Ni8-MOFs with significantly improved charge separation and transfer efficiency under visible-light irradiation. Among them, the one containing a benzoselenadiazole unit (JNU-214) exhibits the best photocatalytic performance in the aerobic oxidation of benzylamines with a conversion rate of 99% in 24 h. Recycling experiments were carried out to confirm the stability and reusability of JNU-214 as a robust heterogeneous catalyst. Significantly, the systematic modulation of the electron-accepting capacity of the bridging units in donor-acceptor-donor MOFs provides a new pathway to develop viable noble-metal-free heterogeneous photocatalysts for aerobic oxidation reactions.

Utilizing Metal-Thiocatecholate Functionalized UiO-66 Framework for Photocatalytic Hydrogen Evolution Reaction

Exploiting clean energy is essential for sustainable development and sunlight-driven photocatalytic water splitting represents one of the most promising approaches toward this goal. Metal-organic frameworks (MOFs) are competent photocatalysts owing to their tailorable functionality, well-defined structure, and high porosity. Yet, the introduction of the unambiguous metal-centered active site into MOFs is still challenging since framework motifs capable of anchoring metal ions firmly are lacking. Herein, the assembly using 1,4-dicarboxylbenzene-2,3-dithiol (H₂ dcbdt) and Zr-Oxo clusters to give a thiol-functionalized UiO-66 type framework, UiO-66-dcbdt, is reported. The thiocatechols on the struts are allowed to capture transition metal (TM) ions to generate UiO-66-dcbdt-M (M  = Fe, Ni, Cu) with unambiguous metal-thiocatecholate moieties for photocatalytic hydrogen evolution reaction (HER). UiO-66-dcbdt-Cu is found the best catalyst exhibiting an HER rate of 4.18 mmol g^-1  h^-1  upon irradiation with photosensitizing Ru-polypyridyl complex. To skip the use of the external sensitizer, UiO-66-dcbdt-Cu is heterojunctioned with titanium dioxide (TiO₂ ) and achieves an HER rate of 12.63 mmol g^-1  h^-1  (32.3 times that of primitive TiO₂ ). This work represents the first example of MOF assembly employing H₂ dcbdt as the mere linker followed by chelation with TM ions and undoubtedly fuels the rational design of MOF photocatalysts bearing well-defined active sites.

Design of a Four-Atom Cluster Embedded in Carbon Nitride for Electrocatalytic Generation of Multi-Carbon Products

The development of efficient and stable catalysts for the electrocatalytic CO₂ and CO reduction reactions (CORR) is under active investigation, but the problems of poor selectivity and low efficiency for C₂ products still exist. We design a two-dimensional carbon nitride material (C₅N₂H₂) that contains an eight N-atom structure capable of coordinating four-metal atom clusters and supporting simultaneously two carbon oxide molecules needed for the C₂ coupling. The designed material has excellent electrical conductivity and stability. After high-throughput screening of catalytic performance of multiple four-metal clusters embedded into the framework, we systematically investigate the CORR process of 11 candidates. We find that Cu₄-C₅N₂H₂ has superior selectivity and low limiting potential for generating ethylene, while Cu₂Zn₂-C₅N₂H₂ is selective and efficient to synthesize ethanol. Further, we discover a novel type of descriptor related to 2D material flexibility to evaluate the potential-determining step for generating ethylene. Our report both broadens the possibilities for few-atom CO reduction and demonstrates a novel substrate flexibility-related descriptor to predict the catalytic performance of materials.

Metallocavitins as Advanced Enzyme Mimics and Promising Chemical Catalysts

The supramolecular approach is becoming increasingly dominant in biomimetics and chemical catalysis due to the expansion of the enzyme active center idea, which now includes binding cavities (hydrophobic pockets), channels and canals for transporting substrates and products. For a long time, the mimetic strategy was mainly focused on the first coordination sphere of the metal ion. Understanding that a highly organized cavity-like enzymatic pocket plays a key role in the sophisticated functionality of enzymes and that the activity and selectivity of natural metalloenzymes are due to the effects of the second coordination sphere, created by the protein framework, opens up new perspectives in biomimetic chemistry and catalysis. There are two main goals of mimicking enzymatic catalysis: (1) scientific curiosity to gain insight into the mysterious nature of enzymes, and (2) practical tasks of mankind: to learn from nature and adopt from its many years of evolutionary experience. Understanding the chemistry within the enzyme nanocavity (confinement effect) requires the use of relatively simple model systems. The performance of the transition metal catalyst increases due to its retention in molecular nanocontainers (cavitins). Given the greater potential of chemical synthesis, it is hoped that these promising bioinspired catalysts will achieve catalytic efficiency and selectivity comparable to and even superior to the creations of nature. Now it is obvious that the cavity structure of molecular nanocontainers and the real possibility of modifying their cavities provide unlimited possibilities for simulating the active centers of metalloenzymes. This review will focus on how chemical reactivity is controlled in a well-defined cavitin nanospace. The author also intends to discuss advanced metal–cavitin catalysts related to the study of the main stages of artificial photosynthesis, including energy transfer and storage, water oxidation and proton reduction, as well as highlight the current challenges of activating small molecules, such as H2O, CO2, N2, O2, H2, and CH4.