Photocatalytic CO2 reduction to syngas using metallosalen covalent organic frameworks

In Situ Construction of Amide-Functionalized 2D Conjugated Metal-Organic Frameworks with Multiple Active Sites for High-Performance Potassium-Ion Batteries

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) represent a promising class of electrode materials for potassium-ion batteries (PIBs), attributed to their superior conductivity, large specific surface area, high charge carrier mobility, and tunable active sites. However, most reported 2D c-MOF-based cathode materials for PIBs usually encounter challenges, such as low specific capacity and inadequate cycling stability. In this context, we herein designed and synthesized a new hexahydroxy salicylamide ligand (6OH-HBB) via a straightforward two-step synthesis with a high yield of 93%, which was subsequently utilized to construct a 2D Cu-HBB-MOF with multiple active sites through an in situ metal coordination-induced planarization strategy. Thanks to its abundant active sites and large specific surface area, the Cu-HBB-MOF demonstrated an outstanding high initial capacity of 228.1 mA h g-1 at 0.2 A g-1, surpassing most reported porous material-based PIBs. Furthermore, even at 5.0 A g-1, the Cu-HBB-MOF exhibited a large reversible specific capacity of 103.6 mA h g-1 after 2500 cycles, simultaneously maintaining a low-capacity loss of only 0.011% per cycle and achieving a Coulombic efficiency up to 100%, demonstrating good long-term cycle stability. This work provides fundamental insights into engineering 2D c-MOFs with multisite functionality, charting a new course for developing high-performance MOF-based cathodes in next-generation energy storage systems.

Design and structure-function interplay in covalent organic frameworks for photocatalytic CO2 reduction

The escalating global energy demands and the need to alleviate the rapid rise in greenhouse gases have led to colossal interest in designing efficient catalytic systems for photocatalytic CO2 reduction. While inorganic semiconductors have been the frontrunners for a long time, porous photocatalysts, particularly covalent organic frameworks (COFs), are gaining traction due to their atomically precise structures, enabling tuning their structural and chemical properties. Designed using the principles of reticular chemistry, the building units of COFs can be modulated to incorporate catalytically active sites periodically using robust covalent bonds to endow them with high efficiency, selectivity, and stability. Unlike the non-porous congeners, COFs, with their high porosity and precisely defined pore channels, allow for quicker diffusion of substrates and products, enabling the utilization of deeply buried photocatalytic sites. Our approach is to comprehend the significant roadblocks that must be overcome for designing state-of-the-art catalysts for photocatalytic CO2 reduction. Building upon that, we highlight the key strategies devised to design COF-based CO2RR photocatalysts. A fundamental understanding of the structure-property relationship is quintessential for utilizing the precision of COF chemistry for developing next-generation materials combining activity, selectivity, and efficiency in a single system. Throughout this review, we have taken a closer look at how the critical design aspects and molecular engineering reciprocate towards augmenting the bulk photocatalytic properties of efficiency and selectivity. Understanding molecular engineering and structure-property relationships will be conducive to developing sophisticated systems to solve global crises in this burgeoning area of research.

Hierarchical Co9S8@In2.77S4 Heterojunction for Efficient Photocatalytic Reduction of CO2 to Syngas

Photocatalytic reduction of CO2 to solar fuels is recognized as a promising route to address environmental and energy issues. However, there exist two challenges of insufficient CO2 activation and fast charge carrier recombination, impeding this conversion. Herein, a hierarchical Co9S8@In2.77S4 (CoS@InS) heterojunction is developed by the in situ growth of the In2.77S4 nanosheets on the Co9S8 nanotubes for efficient photocatalytic reduction of CO2 to syngas in an aqueous reaction system with [Ru(bpy)3]Cl2 serving as a photosensitizer and triethanolamine as a sacrificial agent. In addition to the promoted charge separation and transfer, the strong interfacial electric field formed in this heterojunction tunes the p-band center of In active sites toward the Fermi level. Accordingly, the adsorption of the key intermediate *COOH is enhanced, and the energy barrier of *CO desorption is reduced. Besides, the hierarchical hollow structure enhances light utilization and mass transfer, increases the specific surface area, and provides abundant reaction sites. As a result, the hierarchical CoS@InS heterojunction exhibits superior activity. The optimized heterojunction yields CO and H2 production rates as high as 83,648 and 28,635 μmol g-1 h-1, respectively, with an apparent quantum yield of 5.60% at 450 nm.

Harnessing Synergistic Cooperation of Neighboring Active Motifs in Heterogeneous Catalysts for Enhanced Catalytic Performance

Understanding the intricate interplay between catalytically active motifs in heterogeneous catalysts has long posed a significant challenge in the design of highly active and selective reactions. Drawing inspiration from biological enzymes and homogeneous catalysts, the synergistic cooperation between neighboring active motifs has emerged as a crucial factor in achieving effective catalysis. This synergistic control is often observed in natural enzymes and homogeneous systems through ligand coordination. The synergistic interaction is especially vital in reactions involving tandem or cascade steps, where distinct active motifs provide different functionalities to enable the co-activation of the reaction substrate(s). Situated within a 3D spatial domain, these catalytically active motifs can shape favorable catalytic landscapes by modulating electronic and geometric characteristics, thereby stabilizing specific intermediate or transition state species in a specific catalytic reaction. In this review, we aim to explore a diverse array of the latest heterogeneous catalytic systems that capitalize on the synergistic cooperativity between neighboring active motifs. We will delve into how such synergistic interactions can be utilized to engineer more favorable catalytic landscapes, ultimately resulting in the modulation of catalytic reactivities.

Nanomaterial ZnO Synthesis and Its Photocatalytic Applications: A Review

Zinc oxide (ZnO), a cheap, abundant, biocompatible, and wide band gap semiconductor material with easy tunable morphologies and properties, makes it one of the mostly studied metal oxides in the area of materials science, physics, chemistry, biochemistry, and solid-state electronics. Its versatility, easy bandgap engineering with transitional and rare earth metals, as well as the diverse nanomorphology empower ZnO as a promising photocatalyst. The use of ZnO as a functional material is attracting increased attention both for academia and industry, especially under the current energy paradigm shift toward clean and renewable sources. Extensive work has been performed in recent years using ZnO as an active component for different photocatalytic applications. Therefore, a thorough and timely review of the process is necessary. The aim of this review is to provide a general summary of the current state of ZnO nanostructures, synthesis strategies, and modification approaches, with the main application focus on varied photocatalysis applications, serving as an introduction, a reference, and an inspiration for future research.

Decoupling Interlayer Interactions Boosts Charge Separation in Covalent Organic Frameworks for High-Efficiency Photocatalytic CO2 Reduction

Covalent organic frameworks (COFs) have emerged as promising photocatalysts owing to their structural diversity, tunable bandgaps, and exceptional light-harvesting capabilities. While previous studies primarily focus on developing narrow-bandgap COFs for broad-spectrum solar energy utilization, the critical role of interlayer coupling in regulating charge transfer dynamics remains unclear. Conventional monolayer-based theoretical models inadequately address interlayer effects that potentially hindering intralayer electron transport to catalytic active sites. This work employs density functional theory (DFT) calculations to investigate the influence of interlayer interactions on intralayer charge transfer in imine-based COFs. Theoretical analyses reveal that bilayer architectures exhibit pronounced interlayer interference in intramolecular charge transfer processes which has not been observed in monolayer models. Based on these mechanistic insights, this work designs two isomeric pyrene-based COFs incorporating identical electron donor (pyrene) and acceptor (nickel bipyridine) units but with distinct interlayer coupling strengths. Strikingly, the optimized COF with weakened interlayer interactions demonstrates exceptional photocatalytic CO2 reduction performance, achieving a CO evolution rate of 553.3 µmol g-1 h-1 with 94% selectivity under visible light irradiation without additional photosensitizers or co-catalysts. These findings establish interlayer engineering as a crucial design principle for developing high-performance COF-based photocatalysts for solar energy conversion applications.

Cyclic Trinuclear Units Based Covalent Metal-Organic Frameworks for Gold Recovery

The development of porous solids for gold recovery is highly desired from both an economical and environmental point of view; however, the design of solid adsorbents with high overall performance is still challenging. Herein, we designed three copper(I) cyclic trinuclear units, Cu(I)-CTUs, based covalent metal-organic frameworks (CMOFs) by networking metal clusters with dynamic covalent linkages. These CMOFs deliver a high gold adsorption capacity of 3687 mg g-1, ultra-fast adsorption kinetics (i.e., removal efficiency > 99% within 20 s), excellent selectivity and reusability. In addition, they can be readily used for gold extraction from e-waste. Owing to the rich redox properties of Cu-CTUs, these CMOFs show three sorption mechanisms including adsorption, chemical reduction and photocatalytic reduction. Interestingly, the gold-loaded CMOFs can be used as catalysts for hydration of alkynes to ketones with high yields (e.g., up to ∼ 95% for 6 examples).

Staggered ABC-Stacking Cobalt-Triptycene Framework for Accelerating CO2 Photoreduction

Metal-organic frameworks (MOFs) are highly efficient photocatalysts due to their highly tunable structures and favorable electronic properties. However, achieving control over framework stacking arrangements, such as the staggered ABC-stacking, presents significant challenges. This difficulty arises from the inherently unfavorable energetics of the ABC arrangement and weaker π-π interactions compared to other stacking modes. Herein, a cobalt-triptycene framework with a staggered ABC-stacking arrangement was successfully synthesized in the aqueous phase, achieving a 90% yield. Experimental evaluations revealed that this framework achieved a CO production rate of 4.43 mmol g-1 h-1, which is comparable to the most reported MOF-based photocatalysts for CO2 reduction. Moreover, density functional theory (DFT) calculations and molecular dynamics (MD) simulations indicated that the ABC-stacking cobalt-triptycene framework exhibits lower activation energy (0.079 eV) for water molecules, reduced Gibbs free energies for key intermediates *COOH (0.76 eV) and *H (0.73 eV), and the highest reaction rate increment (7.488 times). Furthermore, principal component analysis (PCA) reveals a strong correlation between the CO production rate and factors such as the Ik value, optical bandgap, and ΔG*H, revising the previous held notion that ΔG*COOH is the primary determinant of catalytic performance. These results offer valuable insights into the design principles of advanced photocatalysts for CO2 reduction.

Molecular Hybrid Photocatalysts for CO2 Photoreduction by Hybridization of Molecular Catalysts and Photoactive Covalent Organic Frameworks - A Review

Photocatalytic reduction of CO2 to obtain storable fuels is an effective way to fix and utilize greenhouse gas CO2, ultimately achieving a carbon-free energy cycle. The core of this goal lies the development of efficient, sustainable, and economically practical catalysts and light absorbers. Currently, hybrid photocatalytic systems that immobilize molecular catalysts on covalent organic frameworks (COFs) are of intensified interest, as this strategy enables the simultaneous exploitation of the catalytic properties of high-performance molecular catalysts, together with the durability of heterogeneous semiconductors, based on the readily available synthetic tunability of both components. This review focuses on the significant progress and challenges that have been overcome for the photocatalytic CO2 reduction reaction, mediated by COFs hybridized with molecular catalysts, aiming to provide strong guidance for innovative utilization towards sustainable photocatalytic CO2 reduction in the future.

Regulating the Combinations of Donor and Acceptor Units via DFT Calculations for Photocatalysts with Efficient Electron-Hole Separation and Transfer Dynamics

Donor-acceptor (D-A)-type conjugated microporous polymers (CMPs) are considered promising photocatalytic materials due to their easily tunable structures and optical properties. However, the rational combination of D and A units to design D-A-type CMPs with efficient electron-hole separation and transfer dynamics remains an ongoing challenge. Herein, we employed Density Functional Theory (DFT) calculations to evaluate 16 potential D-A pair combinations and their respective electron-hole separation and transfer dynamics. These combinations consisted of M-salens (M = Zn, Cu, Co, and Ni) as bromine-containing monomers and four alkyne-based monomers: 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine (TEPT), 4,4″-diethyl-5'-(4-ethynylphenyl)-1, 1':3',1″-terphenyl (TEPB), tris(4-ethynylphenyl) amine (TEPA), and 3,7-diethyl-10-(4-ethynylphenyl)-10H-phenothiazine (TEPP). Eight D-A pair combinations were obtained via DFT calculation, with their electron-hole separation and transfer dynamics ranking as follows: Zn-salen-TEPA > Zn-salen-TEPP > Zn-salen-TEPT > Cu-salen-TEPP > Cu-salen-TEPA > Cu-salen-TEPT > Ni-salen-TEPT > Co-salen-TEPT. Based on these results, three D-A pairs exhibiting the highest electron-hole separation and transfer dynamics were selected for the synthesis of corresponding CMPs and subsequent photoelectric characterization. Experimental enhancements aligned closely with the DFT predictions. Notably, the photocatalytic aerobic oxidative amidation of diverse aldehydes and amines catalyzed by Zn-salen-TEPA under blue LED irradiation achieved a yield of up to 97%, which surpassed the performance of most reported works. This work offers novel perspectives on the rational design of D-A-type CMPs endowed with highly efficient photocatalytic activity.

Hydrogen-Localization Transfer Regulation in 3D COFs Enhances Photocatalytic Acetylene Semi-Hydrogenation to Ethylene

In this work, a series of new crystalline three-dimensional covalent organic frameworks (3D COFs) based on [8+4] construction was designed and successfully realized efficient photocatalytic acetylene (C2H2) hydrogenation to ethylene (C2H4). By regulating the hydrogen-localization transfer effect in these 3D COFs, the Cz-Co-COF-H containing cobalt glyoximate active centers exhibited excellent C2H2-to-C2H4 performance, with an average C2H4 yield of 1755.33 μmol g-1 h-1 in pure C2H2, also showed near 100 % conversion of C2H2 in 1 % C2H2 contained crude C2H4 mixtures (industry-relevant conditions), and finally obtain polymer grade C2H4. In contrast, the Cz-Co-COF-BF2 only showed one fifth activity due to lack of hydrogen-localization transfer. The density functional theory (DFT), projected density of states (PDOS) and molecular dynamics "slow-growth" kinetic calculations based on precise 3D COF structures confirmed that the rapid hydrogen species transfer, enhanced water dissociation and suitable C2H2 adsorption in COFs jointly contributed efficient photocatalytic acetylene hydrogenation (PAH). This work provides new opportunity towards rational design and development of crystalline photocatalysts for C2H2 hydrogenation.

Controlling crystallization in covalent organic frameworks to facilitate photocatalytic hydrogen production

The catalytic performance, depending on the surface nature, is ubiquitous in photocatalysis. However, surface engineering for organic photocatalysts through structural modulation has long been neglected. Here, we propose a zone crystallization strategy for covalent organic frameworks (COFs) that enhances surface ordering through regulator-induced amorphous-to-crystalline transformation. Dynamic simulations show that attaching monofunctional regulators to the surface of spherical amorphous precursor improves surface dynamic reversibility, increasing crystallinity from the inside out. The resulting COF microspheres display surface-enhanced crystallinity and uniform spherical morphology. The visible photocatalytic hydrogen evolution rate reaches 126 mmol g-1 h-1 for the simplest β-ketoenamine-linked COF and 350 mmol gCOF-1 h-1 for SiO2@COF with minimal Pt cocatalysts. Mechanism studies indicate that surface crystalline domains build the surface electrical fields to accumulate photogenerated electrons and diminish electron transfer barriers between the COF and Pt interface. This work bridges the gap between microscopic molecules and macroscopic properties, allowing tailored design of crystalline organic photocatalysts.

In Situ Integration of Metallic Catalytic Sites and Photosensitive Centers within Covalent Organic Frameworks for the Enhanced Photocatalytic Reduction of CO2

Covalent organic frameworks (COFs) are a promising platform for heterogeneous photocatalysis due to their stability and design diversity, but their potential is often restricted by unmanageable targeted excitation and charge transfer. Herein, a bimetallic COF integrating photosensitizers and catalytic sites is designed to facilitate locally ultrafast charge transfer, aiming to improve the photocatalytic reduction of CO2. The strategy uses a "one-pot" method to synthesize the bimetallic COF (termed PBCOFRuRe) through in situ Schiff-base condensation of Pyrene with MBpy (M = Ru, Re) units. In this structure, Ru and Re are anchored within bipyridine as the photosensitive center and catalytic site, respectively. The bimetallic architecture of PBCOFRuRe significantly boosts the photocatalytic efficiency for CO2 reduction, achieving an impressive CO yield of 8306.6 µmol g-1 h-1 with 99.8% selectivity, surpassing most reported COF materials. This improvement is attributed to the localized ultrafast charge transfer (0.23 ps) from Ru to Re, as demonstrated by femtosecond transient absorption spectroscopy (TAS). Further investigations demonstrate its heterogeneous feature, showcasing exceptional long-term stability and recyclability. This study represents a versatile approach for designing bimetallic COFs with ultrafast charge transfer, paving the pathway for advancements in artificial photosynthesis.

1D Covalent Organic Frameworks with Tunable Dual-Cobalt Synergistic Sites for Efficient CO2 Photoreduction

Diatomic catalysts enhance photocatalytic CO2 reduction through synergistic effects. However, precisely regulating the distance between two catalytic centers to achieve synergistic catalysis poses significant challenges. In this study, a series of one-dimensional (1D) covalent organic frameworks (COFs) are designed with adjustable micropores to facilitate efficient CO2 photoreduction. CO2 molecules are anchored between dual-cobalt centers within micropores, thus effectively reducing their activation energy and initiating the photocatalytic process. Additionally, the formation of *COOH intermediates is significantly influenced by the coordination microenvironment around dual-cobalt sites. Notably, COF-Co-N4 exhibited remarkable CO2 photoreduction activity with a CO evolution rate of 110.3 µmol·g-1·h-1, which surpasses most of previously reported single-atom-site photocatalysts. Comprehensive characterization and density functional theory (DFT) calculations revealed that 1D COFs with dual-cobalt sites possess the ability to anchor CO2 molecules, thereby enhancing the efficacy of synergistic catalysis. Simultaneously, COF-Co-N4 with quadruple nitrogen coordination significantly reduced the energy barrier of crucial *COOH intermediate, facilitating efficient photocatalytic CO2 reduction. This study meticulously modulated the coordination microenvironment surrounding dual-cobalt synergistic sites, providing new insight into the design of high-performance photocatalysts.

Densely populated macrocyclic dicobalt sites in ladder polymers for low-overpotential oxygen reduction catalysis

Dual-atom catalysts featuring synergetic dinuclear active sites, have the potential of breaking the linear scaling relationship of the well-established single-atom catalysts for oxygen reduction reaction; however, the design of dual-atom catalysts with rationalized local microenvironment for high activity and selectivity remains a great challenge. Here we design a bisalphen ladder polymer with well-defined densely populated binuclear cobalt sites on Ketjenblack substrates. The strong electron coupling effect between the fully-conjugated ladder structure and carbon substrates enhances the electron transfer between the cobalt center and oxygen intermediates, inducing the low-to-high spin transition for the 3d electron of Co(II). In situ techniques and theoretical calculations reveal the dynamic evolution of Co2N4O2 active sites and reaction intermediates. In alkaline conditions, the catalyst exhibits impressive oxygen reduction reaction activity featuring an onset potential of 1.10 V and a half-wave potential of 1.00 V, insignificant decay after 30,000 cycles, pushing the overpotential boundaries of ORR electrocatalysis to a low level. This work provides a platform for designing efficient dual-atom catalysts with well-defined coordination and electronic structures in energy conversion technologies.

Mimic metalloenzymes with atomically dispersed Fe sites in covalent organic framework membranes for enhanced CO2 photoreduction

The massive CO2 emissions from continuous increases in fossil fuel consumption have caused disastrous environmental and ecological crises. Covalent organic frameworks (COFs) hold the potential to convert CO2 and water into value-added chemicals and O2 to mitigate this crisis. However, their activity and selectivity are very low under conditions close to natural photosynthesis. In this work, inspired by the photosynthesis process in natural leaves, we successfully anchored atomically dispersed Fe sites into interlayers of the photoactive triazine-based COF (Fe-COF) membrane to serve as a mimic metalloenzyme for the first time. It is found that under gas-solid conditions and no addition of any photosensitizer and sacrificial reagent, the highly crystalline Fe-COF membrane shows a record high CO2 photoreduction performance with a CO production of 3972 μmol g-1 in a 4 h reaction, ∼100% selectivity of CO, and excellent cycling stability (at least 10 cycles). In such a remarkable photocatalytic CO2 conversion, the atomically dispersed Fe sites with high catalytic activity significantly reduce the formation energy barrier of key *CO2 and *COOH intermediates, the high-density triazine moieties supply more electrons to the iron catalytic center to promote CO2 reduction, and the homogeneous COF membrane greatly improves the electron/mass transport. Thus, this work opens a new window for the design of highly efficient photocatalysts and provides new insights into their structure-activity relationship in CO2 photocatalytic reduction.

Dual Metallosalen-Based Covalent Organic Frameworks for Artificial Photosynthetic Diluted CO2 Reduction

Directly converting CO2 in flue gas using artificial photosynthetic technology represents a promising green approach for CO2 resource utilization. However, it remains a great challenge to achieve efficient reduction of CO2 from flue gas due to the decreased activity of photocatalysts in diluted CO2 atmosphere. Herein, we designed and synthesized a series of dual metallosalen-based covalent organic frameworks (MM-Salen-COFs, M: Zn, Ni, Cu) for artificial photosynthetic diluted CO2 reduction and confirmed their advantage in comparison to that of single metal M-Salen-COFs. As a results, the ZnZn-Salen-COF with dual Zn sites exhibits a prominent visible-light-driven CO2-to-CO conversion rate of 150.9 μmol g-1 h-1 under pure CO2 atmosphere, which is ~6 times higher than that of single metal Zn-Salen-COF. Notably, the dual metal ZnZn-Salen-COF still displays efficient CO2 conversion activity of 102.1 μmol g-1 h-1 under diluted CO2 atmosphere from simulated flue gas conditions (15 % CO2), which is a record high activity among COFs- and MOFs-based photocatalysts under the same reaction conditions. Further investigations and theoretical calculations suggest that the synergistic effect between the neighboring dual metal sites in the ZnZn-Salen-COF facilitates low concentration CO2 adsorption and activation, thereby lowering the energy barrier of the rate-determining step.

Benzotrithiophene-Based Covalent Organic Frameworks with Rhenium Modified for Artificial Photosynthetic CO2 Reduction

Although covalent organic framework (COF)-based photocatalysts for CO2 reduction reaction has been widely reported, there are still some problems such as poor visible-light absorption and low activity to realize the overall reaction of CO2 reduction by the artificial photosynthesis strategy. Herein, anchoring the Re carbonyl complex Re(CO)5Cl in a benzotrithiophene-based COF has been synthesized for artificial photosynthetic CO2 reduction. The photocatalytic results demonstrate that BTT-bpy-COF-Re exhibits the highest CO2RR activity, achieving a rate of 110.9 μmol g-1 h-1 for the conversion of CO2 to CO, without the need for any sacrificial agent or photosensitizer. This performance significantly surpasses that of BTT-COF and BTT-bpy-COF. Additionally, BTT-bpy-COF-Re shows an apparent quantum efficiency of 1.17% at 420 nm. Further characterization analyses indicate that the enhanced photocatalytic activity can be attributed to improved visible-light absorption and efficient charge transfer within the Re complex-modified BTT-bpy-COF-Re system.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Engineering the electron localization of metal sites on nanosheets assembled periodic macropores for CO2 photoreduction

Photocatalytic conversion of CO2 into syngas is highly appealing, yet still suffers from the undesirable product yield due to the sluggish carrier transfer and the uncontrollable affinity between catalytic sites and intermediates. Here we report the fabrication of Co sites with tunable electron localization capability on two dimensional (2D) nanosheets assembled three dimensional (3D) ordered macroporous framework (3DOM-NS). The as-prepared Co-based 3DOM-NS catalysts exhibit attractive photocatalytic performances toward CO2 reduction, among which the cobalt sulfide one (3DOM Co-SNS) shows the highest syngas generation rate up to 347.3 μmol h-1 under the irradiation of visible light and delivers a remarkable catalytic activity (1150.7 μmol h-1) in a flow reaction system under natural sunlight. Mechanism studies reveal that the high electron localization of metal sites in 3DOM Co-SNS strengthens the interaction between Co and HCOO* via the orbital interactions of dyz/dxz-p and s-s, thus facilitating the cleaving process of C-O bond. Additionally, the ordered macroporous framework with nanosheet subunits elevates the transfer efficiency of photoexcited electrons, which contributes to its high activity.

Reticular Materials for Photocatalysis

Photocatalysis leverages solar energy to overcome the thermodynamic barrier, enabling efficient chemical reactions under mild conditions. It can greatly reduce reliance on traditional energy sources and has attracted significant research interest. Reticular materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), represent a class of crystalline materials constructed from molecular building blocks linked by coordination and covalent bonds, respectively. Reticular materials function as heterogeneous catalysts, combining well-defined structures and high tailorability akin to homogeneous catalysts. In this review, the regulation of light absorption, charge separation, and surface reactions in the photocatalytic process through precise molecular-level design based on the features of reticular materials is elaborated. Notably, for MOFsmicroenvironment modulation around catalytic sites affects photocatalytic performance is delved, with emphasis on their unique dynamic and flexible microenvironments. For COFs, the inherent excitonic effects due to their fully organic nature is discussed and highlight the strategies to regulate excitonic effects for charge- and/or energy-transfer-mediated photocatalysis. Finally, the current challenges and future directions in this field, aiming to provide a comprehensive understanding of how reticular materials can be optimized for enhanced photocatalysis is discussed.

Two-Dimensional Covalent Organic Frameworks: Structural Insights across Different Length Scales and Their Impact on Photocatalytic Efficiency

ConspectusCovalent organic frameworks (COFs) are a rapidly emerging class of crystalline porous polymers, characterized by their highly defined, predictable, and tunable structure, porosity, and properties. COFs can form both two-dimensional (2D) and three-dimensional (3D) architectures, each with unique characteristics and potential applications. 2D COFs have attracted particular interest due to their favorable structural and optoelectronic properties. They can be equipped with a range of different functional moieties in their backbone, ranging from acidic to basic, from hydrophilic to hydrophobic, and from metal-coordinating to redox-active functions. In addition, their crystallinity, high specific surface area, and remarkable thermal and chemical stability make them attractive for a variety of applications, including gas separation, catalysis, energy storage, and optoelectronics.This Account provides a detailed overview of our recent efforts to synthesize and apply 2D COFs. First, various synthesis routes are discussed, focusing on methods that involve reversible and irreversible linkage reactions. Reversible reactions, such as imine or boronate ester formation, are advantageous for producing highly crystalline COFs because they allow for error correction during synthesis. In contrast, irreversible reactions, such as carbon-carbon or carbon-nitrogen bond formation, yield COFs with greater chemical stability, although controlling crystallinity can be more challenging. Our group has contributed significantly to refining these methods to balance crystallinity and stability, enhancing the performance of the resulting 2D COFs.In addition to different binding patterns, we have also developed strategies to control the micro- and macromorphologies of COFs, which is crucial for optimizing their properties for specific applications. For example, we have explored the synthesis of hierarchical porous COFs by using templating techniques or by forming composites with other functional materials. These strategies enable us to fine-tune the porosity and surface properties of COFs, thereby improving their performance in applications like catalysis. Hierarchical structures in particular enhance photocatalytic efficiency by providing a larger surface area for light absorption and facilitating the transport of photogenerated charge carriers.We further examine the practical applications of 2D COFs, with a primary focus on photocatalysis. Photocatalysis uses light to enable or accelerate chemical reactions, and 2D COFs are ideal for this purpose due to their tunable band gaps and large surface areas. Our research has shown that 2D COFs are highly versatile photocatalysts that can effectively catalyze reactions such as water splitting, carbon dioxide reduction, hydrogen peroxide formation, and cross-coupling reactions. By exploiting the unique properties of 2D COFs, we have achieved significant improvement in many photocatalytic reactions.With this comprehensive overview, we aim to contribute to the further development and understanding of 2D COFs and encourage further research and innovation in this promising field.

Flexible Hydrazone-Linked Metal-Covalent Organic Frameworks with Copper Clusters for Efficient Electrocatalytic Oxygen Evolution Reaction

Despite the challenges associated with the synthesis of flexible metal-covalent organic frameworks (MCOFs), these offer the unique advantage of maximizing the atomic utilization efficiency. However, the construction of flexible MCOFs with flexible building units or linkages has rarely been reported. In this study, novel flexible MCOFs are constructed using flexible building blocks and copper clusters with hydrazone linkages. The heterometallic frameworks (Cu, Co) are prepared through the hydrazone linkage coordination method and evaluated as catalysts for the oxygen evolution reaction (OER). Owing to the spatial separation and functional cooperation of the heterometallic MCOF catalysts, the as-synthesized MCOFs exhibited outstanding catalytic activities with an overpotential of 268.8 mV at 10 mA cm-2 for the OER in 1 M KOH, which is superior to those of the reported covalent organic frameworks (COFs)-based OER catalysts. Theoretical calculations further elucidated the synergistic effect of heterometallic active sites within the linkages and frameworks, contributing to the enhanced OER activity. This study thus introduces a novel approach to the fundamental design of flexible MCOF catalysts for the OER, emphasizing their enhanced atomic utilization efficiency.

Covalent Organic Frameworks for Photocatalytic Reduction of Carbon Dioxide: A Review

Covalent organic frameworks (COFs) are crystalline networks with extended backbones cross-linked by covalent bonds. Due to the semiconductive properties and variable metal coordinating sites, along with the rapid development in linkage chemistry, the utilization of COFs in photocatalytic CO2RR has attracted many scientists' interests. In this Review, we summarize the latest research progress on variable COFs for photocatalytic CO2 reduction. In the first part, we present the development of COF linkages that have been used in CO2RR, and we discuss four mechanisms including COFs as intrinsic photocatalysts, COFs with photosensitive motifs as photocatalysts, metalated COF photocatalysts, and COFs with semiconductors as heterojunction photocatalysts. Then, we summarize the principles of structural designs including functional building units and stacking mode exchange. Finally, the outlook and challenges have been provided. This Review is intended to give some guidance on the design and synthesis of diverse COFs with different linkages, various structures, and divergent stacking modes for the efficient photoreduction of CO2.

Recent Advances and Challenges in Efficient Selective Photocatalytic CO2 Methanation

Solar-driven carbon dioxide (CO2) methanation holds significant research value in the context of carbon emission reduction and energy crisis. However, this eight-electron catalytic reaction presents substantial challenges in catalytic activity and selectivity. In this regard, researchers have conducted extensive exploration and achieved significant developments. This review provides an overview of the recent advances and challenges in efficient selective photocatalytic CO2 methanation. It begins by discussing the fundamental principles and challenges in detail, analyzing strategies for improving the efficiency of photocatalytic CO2 conversion to CH4 comprehensively. Subsequently, it outlines the recent applications and advanced characterization methods for photocatalytic CO2 methanation. Finally, this review highlights the prospects and opportunities in this area, aiming to inspire CO2 conversion into high-value CH4 and shed light on the research of catalytic mechanisms.

Covalent Bonding of Salen Metal Complexes with Pyrene Chromophores to Porous Polymers for Photocatalytic Hydrogen Evolution

The development of low-cost and efficient photocatalysts to achieve water splitting to hydrogen (H2) is highly desirable but remains challenging. Herein, we design and synthesize two porous polymers (Co-Salen-P and Fe-Salen-P) by covalent bonding of salen metal complexes and pyrene chromophores for photocatalytic H2 evolution. The catalytic results demonstrate that the two polymers exhibit excellent catalytic performance for H2 generation in the absence of additional noble-metal photosensitizers and cocatalysts. Particularly, the H2 generation rate of Co-Salen-P reaches as high as 542.5 μmol g-1 h-1, which is not only 6 times higher than that of Fe-Salen-P but also higher than a large amount of reported Pt-assisted photocatalytic systems. Systematic studies show that Co-Salen-P displays faster charge separation and transfer efficiencies, thereby accounting for the significantly improved photocatalytic activity. This study provides a facile and efficient way to fabricate high-performance photocatalysts for H2 production.

Tailored Cis-Trans Isomeric Metal-Covalent Organic Frameworks for Coordination Configuration-Dependent Electrochemiluminescence

Precise manipulation of the coordination configuration within substances can modulate the band structure and catalytic properties of the target material. Metal-covalent organic frameworks (MCOFs), a crystal material amalgamating the benefits of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), can integrate a predetermined coordination environment into the frameworks for amplifying the catalytic effect. In this study, we delicately synthesize isomeric MCOFs using bis(glycinato)copper as the aminoligand via kinetically and thermodynamically favorable pathways to yield cis-MCOF and trans-MCOF products, respectively, thereby introducing a cis-trans isomeric coordination field into the framework. Moreover, the twisted skeleton derived from the flexibility of amino acid and β-ketoenamine linkages endows trans-MCOF with surprising water dispersibility. Compared to cis-MCOF, the trans isomerism displays a significant enhancement in cathodic electrochemiluminescence via the catalysis of Cu nodes toward K2S2O8. The density of states analysis shows that the d-band center of trans-MCOF is closer to the Fermi level, leading to more stable adsorption binding to promote the catalysis. This study is the first report on constructing predesign coordination configuration MCOFs via an easy-handling method, which gives the guidelines for the design of amino acid-based MCOF materials.

Bipolaronic Motifs Induced Spatially Separated Catalytic Sites for Tunable Syngas Photosynthesis From CO2

Photocatalytic reduction of CO2 into syngas is a promising way to tackle the energy and environmental challenges; however, it remains a challenge to achieve reaction decoupling of CO2 reduction and water splitting. Therefore, efficient production of syngas with a suitable CO/H2 ratio for Fischer-Tropsch synthesis can hardly be achieved. Herein, bipolaronic motifs including Co(II)-pyridine N motifs and Co(II)-imine N motifs are rationally designed into a crystalline imine-linked 1,10-phenanthroline-5,6-dione-based covalent organic framework (bp-Co-COF) with a triazine core. These featured structures with spatially separated active sites exhibit efficient photocatalytic performance toward CO2-to-syngas conversion with a suitable CO/H2 ratio (1:1-1:3). The bipolaronic motifs enable a highly separated electron-hole state, whereby the Co(II)-pyridine N motifs tend to be the active sites for CO2 activation and accelerate the hydrogenation to form *COOH intermediates; whilst, the Co(II)-imine N motifs increase surface hydrophilicity for H2 evolution. The photocatalytic reductions of CO2 and H2O thus decouple and proceed via a concerted way on the bipolaronic motifs of bp-Co-COF. The optimal bp-Co-COF photocatalyst achieves a high syngas evolution rate of 15.8 mmol g-1 h-1 with CO/H2 ratio of 1:2, outperforming previously reported COF-based photocatalysts.

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.