Carbon Dioxide Capture and Conversion Using Metal-Organic Framework (MOF) Materials: A Comprehensive Review

The escalating threat of anthropogenic climate change has spurred an urgent quest for innovative CO2 capture and utilization (CCU) technologies. Metal-organic frameworks (MOFs) have emerged as prominent candidates in CO2 capture and conversion due to their large specific surface area, well-defined porous structure, and tunable chemical properties. This review unveils the latest advancements in MOF-based materials specifically designed for superior CO2 adsorption, precise separation, advanced photocatalytic and electrocatalytic CO2 reduction, progressive CO2 hydrogenation, and dual functionalities. We explore the strategies that enhance MOF efficiency and examine the challenges of and opportunities afforded by transitioning from laboratory research to industrial application. Looking ahead, this review offers a visionary perspective on harnessing MOFs for the sustainable capture and conversion of CO2.

A Planar-Structured Dinuclear Cobalt(II) Complex with Indirect Synergy for Photocatalytic CO2-to-CO Conversion

Dinuclear metal synergistic catalysis (DMSC) has been proved an effective approach to enhance catalytic efficiency in photocatalytic CO2 reduction reaction, while it remains challenge to design dinuclear metal complexes that can show DMSC effect. The main reason is that the influence of the microenvironment around dinuclear metal centres on catalytic activity has not been well recognized and revealed. Herein, we report a dinuclear cobalt complex featuring a planar structure, which displays outstanding catalytic efficiency for photochemical CO2-to-CO conversion. The turnover number (TON) and turnover frequency (TOF) values reach as high as 14457 and 0.40 s-1 respectively, 8.6 times higher than those of the corresponding mononuclear cobalt complex. Control experiments and theoretical calculations revealed that the enhanced catalytic efficiency of the dinuclear cobalt complex is due to the indirect DMSC effect between two CoII ions, energetically feasible one step two-electron transfer process by Co2 I,I intermediate to afford Co2 II,II(CO2 2-) intermediate and fast mass transfer closely related with the planar structure.

Unveiling Advancements: Trends and Hotspots of Metal-Organic Frameworks in Photocatalytic CO2 Reduction

Metal-organic frameworks (MOFs) are robust, crystalline, and porous materials featured by their superior CO2 adsorption capacity, tunable energy band structure, and enhanced photovoltaic conversion efficiency, making them highly promising for photocatalytic CO2 reduction reaction (PCO2RR). This study presents a comprehensive examination of the advancements in MOFs-based PCO2RR field spanning the period from 2011 to 2023. Employing bibliometric analysis, the paper scrutinizes the widely adopted terminology and citation patterns, elucidating trends in publication, leading research entities, and the thematic evolution within the field. The findings highlight a period of rapid expansion and increasing interdisciplinary integration, with extensive international and institutional collaboration. A notable emphasis on significant research clusters and key terminologies identified through co-occurrence network analysis, highlighting predominant research on MOFs such as UiO, MIL, ZIF, porphyrin-based MOFs, their composites, and the hybridization with photosensitizers and molecular catalysts. Furthermore, prospective design approaches for catalysts are explored, encompassing single-atom catalysts (SACs), interfacial interaction enhancement, novel MOF constructions, biocatalysis, etc. It also delves into potential avenues for scaling these materials from the laboratory to industrial applications, underlining the primary technical challenges that need to be overcome to facilitate the broader application and development of MOFs-based PCO2RR technologies.

Strong Second-Harmonic Generation Induced by a Triphenylamine-Based Bismuth-Organic Framework for Photocatalytic Activity Enhancement

Taking advantage of the well-defined geometry of metal centers and highly directional metal-ligand coordination bonds, metal-organic frameworks (MOFs) have emerged as promising candidates for nonlinear optical (NLO) materials. In this work, taking a photoresponsive carboxylate triphenylamine derivative as an organic ligand, a bismuth-based MOF, Bi-NBC, NBC = 4',4‴,4‴″-nitrilotris(([1,1'-biphenyl]-4-carboxylic acid)) is obtained. Structure determination reveals that it is a potential NLO material derived from its noncentrosymmetric structure, which is finally confirmed by its rarely strong second harmonic generation (SHG) effect. Theoretical calculations reveal that the potential difference around Bi atoms is large; therefore, it leads to a strong local built-in electric field, which greatly facilitates the charge separation and transfer and finally improves the photocatalytic performance. Our results provide a reference for the exploration of MOFs with NLO properties.

Hetero-Motif Molecular Junction Photocatalysts: A New Frontier in Artificial Photosynthesis

ConspectusTo cope with the increasingly global greenhouse effect and energy shortage, it is urgent to develop a feasible means to convert anthropogenic excess carbon dioxide (CO2) into energy resources. The photocatalytic CO2 reduction reaction (CO2RR) coupled with the water oxidation reaction (WOR), known as artificial photosynthesis, is a green, clean, and promoting strategy to deal with the above issues. Among the reported photocatalytic systems for CO2 reduction, the main challenge is to achieve WOR simultaneously due to the limited charge separation efficiency and complicated dynamic process. To address the problem, scientists have assembled two nanosemiconductor motifs for CO2RR and WOR into a heterojunction photocatalyst to realize artificial photosynthesis. However, it is difficult to clearly explore the corresponding catalytic mechanism and establish an accurate structure-activity relationship at the molecular level for their aperiodic distribution and complicated structural information. Standing on the shoulders of the heterojunction photocatalysts, a new-generation material, hetero-motif molecular junction (HMMJ) photocatalysts, has been developed and studied by our laboratory. A hetero-motif molecular junction is a class of crystalline materials with a well-defined and periodic structure, adjustable assembly mode, and semiconductor-like properties, which is composed of two predesigned motifs with oxidation and reduction, respectively, by coordination or covalent bonds. The intrinsic properties make these catalysts susceptible to functional modifications to improve light absorption and electrical conductivity. The small size and short distance of the motifs can greatly promote the efficiency of photogenerated electron-hole separation and migration. Based on these advantages, they can be used as potential excellent photocatalysts for artificial photosynthesis. Notably, the explicit structural information determined by single-crystal or powder X-ray diffraction can provide a visual platform to explore the reaction mechanism. More importantly, the connection number, spatial distance, interaction, and arrangement mode of the structural motifs can be well-designed to explore the detailed structure-activity relationship that can be hardly studied in nanoheterojunction photocatalyst systems. In this regard, HMMJ photocatalysts can be a new frontier in artificial photosynthesis and serve as an important bridge between molecular photocatalysts and solid photocatalysts. Thus, it is very important to summarize the state-of-the-art of the HMMJ photocatalysts used for artificial photosynthesis and to give in-depth insight to promote future development.In this Account, we have summarized the recent advances in artificial photosynthesis using HMMJ photocatalysts, mainly focusing on the results in our lab. We present an overview of current knowledge about developed photocatalytic systems for artificial photosynthesis, introduce the design schemes of the HMMJ photocatalysts and their unique advantages as compared to other photocatalysts, summarize the construction strategies of HMMJ photocatalysts and their application in artificial photosynthesis, and explain why hetero-motif molecular junctions can be promising photocatalysts and show that they provide a powerful platform for studying photocatalysis. The structure-activity relationship and charge separation dynamics are illustrated. Finally, we bring our outlook on present challenges and future development of HMMJ photocatalysts and their potential application prospects on other photocatalytic reaction systems. We believe that this Account will afford important insights for the construction of high-efficiency photocatalysts and guidance for the development of more photocatalytic systems in an atom-economic, environmentally friendly, and sustainable way.

Boosting CO2 Photoreduction to Formate or CO with High Selectivity over a Covalent Organic Framework Covalently Anchored on Graphene Oxide

Covalent organic frameworks (COFs) have been widely studied in photocatalytic CO2 reduction reaction (CO2 RR). However, pristine COFs usually exhibit low catalytic efficiency owing to the fast recombination of photogenerated electrons and holes. In this study, we fabricated a stable COF-based composite (GO-COF-366-Co) by covalently anchoring COF-366-Co on the surface of graphene oxide (GO) for the photocatalytic CO2 reduction. Interestingly, in absolute acetonitrile (CH3 CN), GO-COF-366-Co shows a high selectivity of 94.4 % for the photoreduction of CO2 to formate, with a formate yield of 15.8 mmol/g, which is approximately four times higher than that using the pristine COF-366-Co. By contrast, in CH3 CN/H2 O (v : v=4 : 1), the main product for the photocatalytic CO2 reduction over GO-COF-366-Co is CO (96.1 %), with a CO yield as high as 52.2 mmol/g, which is also approximately four times higher than that using the pristine COF-366-Co. Photoelectrochemical experiments demonstrate the covalent bonding of COF-366-Co and GO to form the GO-COF-366-Co composite facilitates charge separation and transfer significantly, thereby accounting for the enhanced catalytic activity. In addition, theoretical calculations and in situ Fourier transform infrared spectroscopy reveal H2 O can stabilize the *COOH intermediate to further form a *CO intermediate via O-H(aq)⋅⋅⋅O(*COOH) hydrogen bonding, thus explaining the regulated photocatalytic performance.

Aptamer-Modified Homogeneous Catalysts, Heterogenous Nanoparticle Catalysts, and Photocatalysts: Functional "Nucleoapzymes", "Aptananozymes", and "Photoaptazymes"

Conjugation of aptamers to homogeneous catalysts ("nucleoapzymes"), heterogeneous nanoparticle catalysts ("aptananozymes"), and photocatalysts ("photoaptazymes") yields superior catalytic/photocatalytic hybrid nanostructures emulating functions of native enzymes and photosystems. The concentration of the substrate in proximity to the catalytic sites ("molarity effect") or spatial concentration of electron-acceptor units in spatial proximity to the photosensitizers, by aptamer-ligand complexes, leads to enhanced catalytic/photocatalytic efficacies of the hybrid nanostructures. This is exemplified by sets of "nucleoapzymes" composed of aptamers conjugated to the hemin/G-quadruplex DNAzymes or metal-ligand complexes as catalysts, catalyzing the oxidation of dopamine to aminochrome, oxygen-insertion into the Ar─H moiety of tyrosinamide and the subsequent oxidation of the catechol product into aminochrome, or the hydrolysis of esters or ATP. Also, aptananozymes consisting of aptamers conjugated to Cu2+ - or Ce4+ -ion-modified C-dots or polyadenine-stabilized Au nanoparticles acting as catalysts oxidizing dopamine or operating bioreactor biocatalytic cascades, are demonstrated. In addition, aptamers conjugated to the Ru(II)-tris-bipyridine photosensitizer or the Zn(II) protoporphyrin IX photosensitizer provide supramolecular photoaptazyme assemblies emulating native photosynthetic reaction centers. Effective photoinduced electron transfer followed by the catalyzed synthesis of NADPH or the evolution of H2 is demonstrated by the photosystems. Structure-function relationships dictate the catalytic and photocatalytic efficacies of the systems.

Manipulating the Spin State of Co Sites in Metal-Organic Frameworks for Boosting CO2 Photoreduction

Photocatalytic CO2 reduction holds great potential for alleviating global energy and environmental issues, where the electronic structure of the catalytic center plays a crucial role. However, the spin state, a key descriptor of electronic properties, is largely overlooked. Herein, we present a simple strategy to regulate the spin states of catalytic Co centers by changing their coordination environment by exchanging the Co species into a stable Zn-based metal-organic framework (MOF) to afford Co-OAc, Co-Br, and Co-CN for CO2 photoreduction. Experimental and DFT calculation results suggest that the distinct spin states of the Co sites give rise to different charge separation abilities and energy barriers for CO2 adsorption/activation in photocatalysis. Consequently, the optimized Co-OAc with the highest spin-state Co sites presents an excellent photocatalytic CO2 activity of 2325.7 μmol·g-1·h-1 and selectivity of 99.1% to CO, which are among the best in all reported MOF photocatalysts, in the absence of a noble metal and additional photosensitizer. This work underlines the potential of MOFs as an ideal platform for spin-state manipulation toward improved photocatalysis.

Dual-Function Precious-Metal-Free Metal-Organic Framework for Photocatalytic Conversion and Chemical Fixation of Carbon Dioxide

Highly efficient transformation of carbon dioxide (CO2) into value-added chemicals is considered a promising route for clean production and future energy sustainability, which is crucial for realizing a carbon-neutral economy. It remains a great challenge to develop highly stable and active catalysts with low-cost, environmentally friendly, and nontoxic materials for catalytic conversion of CO2. Herein, a precious-metal-free and heterogeneous MOF (LTG-FeZr) catalyst, composed of bis(terpyridine)iron(II) complexes and zirconium(IV) ions, was designed and prepared via a metalloligand approach. LTG-FeZr, with a robust framework and regular 1D channels not only can achieve the photocatalytic reduction of CO2 to HCOOH with a high conversion rate (up to 265 μmol·g-1·h-1) under visible-light irradiation but also exhibits exceptional catalytic activities toward the synthesis of cyclic carbonates via cycloaddition reactions of various epoxides and CO2 in the absence of light. Possible mechanisms for two different conversion processes of CO2 catalyzed by LTG-FeZr have been proposed. LTG-FeZr represents an ideal dual-function MOF platform for the catalytic conversion and utilization of CO2 in all weather conditions.

Reticular framework materials for photocatalytic organic reactions

Photocatalytic organic reactions, harvesting solar energy to produce high value-added organic chemicals, have attracted increasing attention as a sustainable approach to address the global energy crisis and environmental issues. Reticular framework materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are widely considered as promising candidates for photocatalysis owing to their high crystallinity, tailorable pore environment and extensive structural diversity. Although the design and synthesis of MOFs and COFs have been intensively developed in the last 20 years, their applications in photocatalytic organic transformations are still in the preliminary stage, making their systematic summary necessary. Thus, this review aims to provide a comprehensive understanding and useful guidelines for the exploration of suitable MOF and COF photocatalysts towards appropriate photocatalytic organic reactions. The commonly used reactions are categorized to facilitate the identification of suitable reaction types. From a practical viewpoint, the fundamentals of experimental design, including active species, performance evaluation and external reaction conditions, are discussed in detail for easy experimentation. Furthermore, the latest advances in photocatalytic organic reactions of MOFs and COFs, including their composites, are comprehensively summarized according to the actual active sites, together with the discussion of their structure-property relationship. We believe that this study will be helpful for researchers to design novel reticular framework photocatalysts for various organic synthetic applications.

Modulating Photoinduced Electron Transfer between Photosensitive MOF and Co(II) Proton Reduction Sites for Boosting Photocatalytic Hydrogen Production

Photocatalytic hydrogen production via water splitting is the subject of intense research. Photoinduced electron transfer (PET) between a photosensitizer (PS) and a proton reduction catalyst is a prerequisite step and crucial to affecting hydrogen production efficiency. Herein, three photoactive metal-organic framework (MOF) systems having two different PET processes where PS and Co(II) centers are either covalently bonded or coexisting to drive photocatalytic H2 production are built. Compared to these two intramolecular PET systems including CoII -Zn-PDTP prepared from the post-synthetic metalation toward uncoordinated pyridine N sites of Zn-PDTP and sole cobalt-based MOF Co-PDTP, the CoII (bpy)3 @Zn-PDTP system impregnated by molecular cocatalyst possessing intermolecular PET process achieves the highest H2 evolution rate of 116.8 mmol g-1 h-1 over a period of 10 h, about 7.5 and 9.3 times compared to CoII -Zn-PDTP and Co-PDTP in visible-light-driven H2 evolution, respectively. Further studies reveal that the enhanced photoactivity in CoII (bpy)3 @Zn-PDTP can be ascribed to the high charge-separation efficiency of Zn-PDTP and the synergistic intermolecular interaction between Zn-PDTP and cobalt complexes. The present work demonstrates that the rational design of PET process between MOFs and catalytic metal sites can be a viable strategy for the development of highly efficient photocatalysts with enhanced photocatalytic activities.

Stepwise construction of multi-component metal-organic frameworks

Multi-component metal-organic frameworks (MC-MOFs) are crystalline porous materials containing multiple organic ligands or mixed metals, which manifest new properties beyond the linear combination of the single component. However, the traditional one-pot synthesis method for MOFs is not always applicable for synthesizing MC-MOFs due to the competitive coordination of multiple ligands and metals. Therefore, the stepwise construction of MC-MOFs has been explored, which enables more precise control of the heterogeneity within the ordered MC-MOFs. This review provides a summary of the synthesis strategies, namely, ligand exchange, coordinative modification, covalent modification, ligand metalation, cluster metalation, and use of mixed-metal precursors, for the stepwise construction of MC-MOFs. Furthermore, we discuss the applications of MC-MOFs with ordered arrangements of multiple functionalities, focusing on gas adsorption and separation, water remediation, heterogeneous catalysis, luminescence, and chemical sensing.

Relative Local Electron Density Tuning in Metal-Covalent Organic Frameworks for Boosting CO2 Photoreduction

The high local electron density and efficient charge carrier separation are two important factors to affect photocatalytic activity, especially for the CO2 photoreduction reaction. However, the systematic studies on the structure-functional relationship regarding the above two factors based on precisely structure model are rarely reported. Herein, as a proof-of-concept, we developed a new strategy on the evaluation of local electron density by controlling the relative electron-deficient (ED) and electron-rich (ER) intensity of monomer at a molecular level based on three rational-designed vinylene-linked sp2 carbon-covalent organic frameworks (COFs). As expected, the as-prepared vinylene-linked sp2 carbon-conjugated metal-covalent organic framework (MCOFs) (VL-MCOF-1) with molecular junction exhibited excellent activities for CO2 -to-HCOOH conversion (283.41 μmol g-1  h-1 ) and high selectivity of 97.1 %, much higher than the VL-MCOF-2 and g-C34 N6 -COF, which is due to the synergistic effect of the multi-electronic metal clusters (Cu3 (PyCA)3 ) (PyCA=pyrazolate-4-carboxaldehyde) as strong ER roles and cyanopyridine units as ED roles and active sites, as well as the boosted photo-induced charge separation efficiency of vinyl connection and increased light utilization ability. These results not only provide a strategy for regulating the electron-density distribution of photocatalysts at the molecular level but also offers profound insights for metal clusters-based COFs to effective CO2 conversion.

Construction of Zr-Metal-Organic Frameworks-Based Composite Materials toward Anhydrous Proton Conduction and Photocatalytic CO2 Reduction

Metal-organic frameworks constructed from Zr usually possess excellent chemical and physical stability. Therefore, they have become attractive platforms in various fields. In this work, two families of hybrid materials based on ZrSQU have been designed and synthesized, named Im@ZrSQU and Cu@ZrSQU, respectively. Im@ZrSQU was prepared through the impregnation method and employed for proton conduction. Im@ZrSQU exhibited terrific proton conduction performance in an anhydrous environment, with the highest proton conduction value of 3.6 × 10-2 S cm-1 at 110 °C. In addition, Cu@ZrSQU was synthesized via the photoinduction method for the photoreduction of CO2, which successfully promoted the conversion of CO2 into CO and achieved the CO generation rate of up to 12.4 μmol g-1 h-1. The photocatalytic performance of Cu@ZrSQU is derived from the synergistic effect of Cu NPs and ZrSQU. Based on an in-depth study and discussion toward ZrSQU, we provide a versatile platform with applications in the field of proton conduction and photocatalysis, which will guide researchers in their further studies.

A Fully Conjugated Covalent Organic Framework with Oxidative and Reductive Sites for Photocatalytic Carbon Dioxide Reduction with Water

Constructing a powerful photocatalytic system that can achieve the carbon dioxide (CO2 ) reduction half-reaction and the water (H2 O) oxidation half-reaction simultaneously is a very challenging but meaningful task. Herein, a porous material with a crystalline topological network, named viCOF-bpy-Re, was rationally synthesized by incorporating rhenium complexes as reductive sites and triazine ring structures as oxidative sites via robust -C=C- bond linkages. The charge-separation ability of viCOF-bpy-Re is promoted by low polarized π-bridges between rhenium complexes and triazine ring units, and the efficient charge-separation enables the photogenerated electron-hole pairs, followed by an intramolecular charge-transfer process, to form photogenerated electrons involved in CO2 reduction and photogenerated holes that participate in H2 O oxidation simultaneously. The viCOF-bpy-Re shows the highest catalytic photocatalytic carbon monoxide (CO) production rate (190.6 μmol g-1  h-1 with about 100 % selectivity) and oxygen (O2 ) evolution (90.2 μmol g-1 h-1 ) among all the porous catalysts in CO2 reduction with H2 O as sacrificial agents. Therefore, a powerful photocatalytic system was successfully achieved, and this catalytic system exhibited excellent stability in the catalysis process for 50 hours. The structure-function relationship was confirmed by femtosecond transient absorption spectroscopy and density functional theory calculations.

Water-Etched Approach to Hierarchically Porous Metal-Organic Frameworks with High Stability

The development of hierarchically porous metal-organic frameworks (MOFs) with high stability is desirable to expand their applications but remains challenging. Herein, an anionic sodalite-type microporous MOF (Yb-TTCA; TTCA3- = triphenylene-2,6,10-tricarboxylate) was synthesized, which shows outstanding catalytic activities for the cycloaddition of CO2 into cyclic carbonates. Moreover, the microporous Yb-TTCA can be transformed into a hierarchical micro- and mesoporous Yb-TTCA by water treatment with the mesopore sizes of 2 to 12 nm. The hierarchically porous Yb-TTCA (HP-Yb-TTCA) not only exhibits a high thermal stability up to 500 °C but also shows a high chemical stability in aqueous solutions with pH values ranging from 2 to 12. In addition, the HP-Yb-TTCA displays enhanced performance for the removal of organic dyes in comparison with microporous Yb-TTCA. This work provides a facile way to construct hierarchically porous MOF materials.

In Situ Encapsulation of Graphene Quantum Dots in Highly Stable Porphyrin Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction to valuable hydrocarbon solar fuel is of great significance but still challenging. Strong CO₂ enrichment ability and easily adjustable structures make metal-organic frameworks (MOFs) potential photocatalysts for CO₂ conversion. Even though pure MOFs have the potential for photoreduction of CO₂, the efficiency is still quite low due to rapid photogenerated electron-hole recombination and other drawbacks. In this work, graphene quantum dots (GQDs) were in situ encapsulated into highly stable MOFs via a solvothermal method for this challenging task. The GQDs@PCN-222 with encapsulated GQDs showed similar Powder X-ray Diffraction (PXRD) patterns to PCN-222, indicating the retained structure. The porous structure was also retained with a Brunauer-Emmett-Teller (BET) surface area of 2066 m²/g. After incorporation of GQDs, the shape of GQDs@PCN-222 particles remained, as revealed by the scanning electron microscope (SEM). As most of the GQDs were covered by thick PCN-222, it was hard to observe those GQDs using a transmission electron microscope (TEM) and a high-resolution transmission electron microscope (HRTEM) directly, the treatment of digested GQDs@PCN-222 particles by immersion in a 1 mM aqueous KOH solution can make the incorporated GQDs visible in TEM and HRTEM. The linker, deep purple porphyrins, make MOFs a highly visible light harvester up to 800 nm. The introduction of GQDs inside PCN-222 can effectively promote the spatial separation of the photogenerated electron-hole pairs during the photocatalytic process, which was proved by the transient photocurrent plot and photoluminescence emission spectra. Compared with pure PCN-222, the obtained GQDs@PCN-222 displayed dramatically enhanced CO production derived from CO₂ photoreduction with 147.8 μmol/g/h in a 10 h period under visible light irradiation with triethanolamine (TEOA) as a sacrificial agent. This study demonstrated that the combination of GQDs and high light absorption MOFs provides a new platform for photocatalytic CO₂ reduction.

Molecular Catalysis of Energy Relevance in Metal-Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Photocatalytic CO2 reduction using La-Ni bimetallic sites within a covalent organic framework

The precise construction of photocatalysts with diatomic sites that simultaneously foster light absorption and catalytic activity is a formidable challenge, as both processes follow distinct pathways. Herein, an electrostatically driven self-assembly approach is used, where phenanthroline is used to synthesize bifunctional LaNi sites within covalent organic framework. The La and Ni site acts as optically and catalytically active center for photocarriers generation and highly selective CO₂-to-CO reduction, respectively. Theory calculations and in-situ characterization reveal the directional charge transfer between La-Ni double-atomic sites, leading to decreased reaction energy barriers of *COOH intermediate and enhanced CO₂-to-CO conversion. As a result, without any additional photosensitizers, a 15.2 times enhancement of the CO₂ reduction rate (605.8 μmol·g^-1·h^-1) over that of a benchmark covalent organic framework colloid (39.9 μmol·g^-1·h^-1) and improved CO selectivity (98.2%) are achieved. This work presents a potential strategy for integrating optically and catalytically active centers to enhance photocatalytic CO₂ reduction.

Iron-Complex-Based Supramolecular Framework Catalyst for Visible-Light-Driven CO2 Reduction

Molecule-based heterogeneous photocatalysts without noble metals are one of the most attractive systems for visible-light-driven CO₂ reduction. However, reports on this class of photocatalysts are still limited, and their activities are quite low compared to those containing noble metals. Herein, we report an iron-complex-based heterogeneous photocatalyst for CO₂ reduction with high activity. The key to our success is the use of a supramolecular framework composed of iron porphyrin complexes bearing pyrene moieties at meso positions. The catalyst exhibited high activity for CO₂ reduction under visible-light irradiation (29100 μmol g^-1 h^-1 for CO production, selectivity 99.9%), which is the highest among relevant systems. The performance of this catalyst is also excellent in terms of apparent quantum yield for CO production (0.298% at 400 nm) and stability (up to 96 h). This study provides a facile strategy to create a highly active, selective, and stable photocatalyst for CO₂ reduction without utilizing noble metals.

Solvent-in-Gas System for Promoted Photocatalytic Ammonia Synthesis on Porous Framework Materials

Photocatalytic nitrogen reduction reaction (PNRR) is emerging as a sustainable ammonia synthesis approach to meet global carbon neutrality. Porous framework materials with well-designed structures have great opportunities in PNRR; however, they suffer from unsatisfactory activity in the conventional gas-in-solvent system (GIS), owing to the hindrance of nitrogen utilization and strong competing hydrogen evolution caused by overwhelming solvent. In this study, porous framework materials are combined with a novel "solvent-in-gas" system, which can bring their superiority into full play. This system enables photocatalysts to directly operate in a gas-dominated environment with a limited proton source uniformly suspended in it, achieving the accumulation of high-concentrated nitrogen within porous framework while efficiently restricting the solvent-photocatalyst contact. An over eightfold increase in ammonia production rate (1820.7 µmol g^-1 h^-1 ) compared with the conventional GIS and an apparent quantum efficiency as high as ≈0.5% at 400 nm are achieved. This system-level strategy further finds applicability in photocatalytic CO₂ reduction, featuring it as a staple for photosynthetic methodology.

Mechanism of CO2 photoreduction by selenium-doped carbon nitride with cobalt clusters as cocatalysts

Doping is an efficient strategy for improving the photocatalytic activity and tuning the electronic structure of carbon nitride. Selenium-doped melon carbon nitride (Se-doped melon CN) as a promising photocatalyst for CO₂ reduction is investigated using density functional theory calculations. In addition, considering the special role of a cocatalyst in CO₂ reduction, we have explored the electronic and optical properties of Co₄ clusters loaded on the Se-doped melon CN surface. After loading cobalt clusters, CO₂ activation is significantly improved, with preference for the 8-electron product CH₄, as the 2-electron products have higher desorption energies. Overall, this work provides a microscopic understanding of the CO₂ reduction mechanism on Se-doped melon CN with cobalt as the co-catalyst.

Strategies for improving the photocatalytic performance of metal-organic frameworks for CO2 reduction: A review

Photocatalytic CO₂ reduction is an appealing strategy for mitigating the environmental effects of greenhouse gases while simultaneously producing valuable carbon-neutral fuels. Numerous attempts have been made to produce effective and efficient photocatalysts for CO₂ reduction. In contrast, the selection of competitive catalysts continues to be a substantial hindrance and a considerable difficulty in the development of photocatalytic CO₂ reduction. It is vital to emphasize different techniques for building effective photocatalysts to improve CO₂ reduction performance in order to achieve a long-term sustainability. Metal-organic frameworks (MOFs) are recently emerging as a new type of photocatalysts for CO₂ reduction due to their excellent CO₂ adsorption capability and unique structural characteristics. This review examines the most recent breakthroughs in various techniques for modifying MOFs in order to improve their efficiency of photocatalytic CO₂ reduction. The advantages of MOFs using as photocatalysts are summarized, followed by different methods for enhancing their effectiveness for photocatalytic CO₂ reduction via partial ion exchange of metal clusters, design of bimetal clusters, the modification of organic linkers, and the embedding of metal complexes. For integrating MOFs with semiconductors, metallic nanoparticles (NPs), and other materials, a number of different approaches have been also reviewed. The final section of this review discusses the existing challenges and future prospects of MOFs as photocatalysts for CO₂ reduction. Hopefully, this review can stimulate intensive research on the rational design and development of more effective MOF-based photocatalysts for visible-light driven CO₂ conversion.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

A water-stable dual-responsive Cd-CP for fluorometric recognition of hypochlorite and acetylacetone in aqueous media

One novel cadmium(II)-coordination polymer [Cd₃L₂(datrz)(H₂O)₃] (CP 1) is controllably synthesized by surmising the astute combination of semi-rigid tricarboxylate acid 4-(2',3'-dicarboxylphenoxy) benzoic acid (H₃L) and auxiliary ligand 3,5-diamino-1,2,4-triazole (datrz). Structure analysis shows that CP 1 has a two-dimensional (2D) layer structure with a 5-nodal (4³) (4⁴·6²) (4⁵·6⁴·8) (4⁵·6) (4⁷·6⁶·8²) topology. Further investigations reveal that CP 1 shows superordinary water stability and good thermal stability. The fluorescent explorations suggest that the as-synthesized CP 1 could emit blue light centered at 485 nm, attributing to ligand-based emission. In terms of sensing investigations, CP 1 could act as a fluorescent sensor for detecting hypochlorite (ClO^-) and acetylacetone (acac) through fluorescence turn-off process in aqueous solution, and the detection limit could reach 0.18 μM and 0.056 μM, respectively. Further research reveals that it is more likely the N-H···O-Cl hydrogen bonds between -NH₂ groups of the triazole ligands and O atoms of ClO^- plays the key role in the system, which may serve as a bridge for the energy transfer, leading to fluorescence quenching of the chemosensor. While the photoinduced electron transfer (PET) combined with inner filter effect (IFT) should be responsible for the turn-off fluorescence of CP 1 triggered by acac.

CO2 Photoactivation Study of Adenine Nucleobase: Role of Hydrogen-Bonding Traction

The discovery and in-depth study of non-biocatalytic applications of active biomolecules are essential for the development of biomimicry. Here, the effect of intermolecular hydrogen-bonding traction on the CO₂ photoactivation performance of adenine nucleobase by means of an adenine-containing model system (AMOF-1-4) is uncovered. Remarkably, the hydrogen-bonding schemes around adenines are regularly altered with the increase in the alkyl (methyl, ethyl, isopropyl, and tert-butyl) electron-donating capacity of the coordinated aliphatic carboxylic acids, and thus, lead to a stepwise improvement in CO₂ photoreduction activity. Density functional theory calculations demonstrate that strong intermolecular hydrogen-bonding traction surrounding adenine can obviously increase the adenine-CO₂ interaction energy and, therefore, result in a smoother CO₂ activation process. Significantly, this work also provides new inspiration for expanding the application of adenine to more small-molecule catalytic reactions.

Facet-Dependent Photocatalytic Behavior of Fe-soc-MOF for Carbon Dioxide Reduction

Exposing different facets on metal-organic frameworks (MOFs) is an efficient approach to regulate their photocatalytic performance for CO₂ reduction. Herein, Fe-soc-MOFs exposed with different facets were successfully synthesized, and the morphologies of Fe-soc-MOF exposed with eight {111} facets (Fe-soc-O) and that exposed with eight {111} and six {100} crystal facets (Fe-soc-M) are first reported. Fe-soc-MOFs have facet-dependent active sites on their surface and correspondingly different catalytic performance for photocatalytic CO₂ reduction. Fe-soc-O has the highest CO production of 1804 μmol g^-1 h^-1, while the Fe-soc-MOF exposed with six {100} facets (Fe-soc-C) has the best CO selectivity of 94.7%. Density functional theory (DFT) calculations demonstrate that the (111) facet has more favorable thermodynamic potential for CO₂ reduction and H₂ evolution compared with the (100) one, deriving from its facet-dependent active sites. This work shows that utilizing the facet-engineering strategy to regulate the active sites exposed on the surface of MOFs is feasible. The results display the relation between the facet of MOFs and the photocatalytic behavior for CO₂ reduction.

Enhancing the spatial separation of photogenerated charges on Fe-based MOFs via structural regulation for highly-efficient photocatalytic Cr(VI) reduction

Although iron-based metal-organic frameworks (Fe-MOFs) have displayed the photocatalytic activity, there is still abundant room for improving their photocatalytic performance through tuning the structures. In this work, four novel iron-based metal-organic frameworks (Fe-MOFs) were successfully synthesized via ligand modulation for better photocatalytic Cr(VI) reduction, in which MTBDC-TPT-Fe had the highest catalytic activity (MTBDC = 2,5-bis(methylthio)terephthalic acid, TPT = 2,4,6-tri(4-pyridyl)- 1,3,5-triazine). The boosted photocatalytic reduction may be mainly ascribed to the enhanced electron push-pull effect between iron-oxygen clusters and organic ligands. The introduction of -SCH₃ groups can enhance the light absorption and donate electrons to iron center under visible-light irradiation, meanwhile the separation and transfer of photogenerated charge carriers can be enhanced resulting from the electron-pulling effect when introducing TPT. Moreover, enhanced specific surface areas and positive skeleton charge due to the introduction of TPT may improve active sites exposure and Cr(VI) adsorption, thereby enhancing photocatalytic Cr(VI) reduction activity without the presence of any assisted scavengers. In addition, the photocatalytic mechanism (i.e. active species) were also studied and presented. This work confirmed an effective structure-performance regulation strategy on Fe-MOFs for photocatalytic Cr(VI) reduction.

Molecular oxidation-reduction junctions for artificial photosynthetic overall reaction

Constructing redox semiconductor heterojunction photocatalysts is the most effective and important means to complete the artificial photosynthetic overall reaction (i.e., coupling CO₂ photoreduction and water photo-oxidation reactions). However, multiphase hybridization essence and inhomogeneous junction distribution in these catalysts extremely limit the diverse design and regulation of the modes of photogenerated charge separation and transfer pathways, which are crucial factors to improve photocatalytic performance. Here, we develop molecular oxidation-reduction (OR) junctions assembled with oxidative cluster (PMo₁₂, for water oxidation) and reductive cluster (Ni₅, for CO₂ reduction) in a direct (d-OR), alternant (a-OR), or symmetric (s-OR) manner, respectively, for artificial photosynthesis. Significantly, the transfer direction and path of photogenerated charges between traditional junctions are obviously reformed and enriched in these well-defined crystalline catalysts with monophase periodic distribution and thus improve the separation efficiency of the electrons and holes. In particular, the charge migration in s-OR shows a periodically and continuously opposite mode. It can inhibit the photogenerated charge recombination more effectively and enhance the photocatalytic performance largely when compared with the traditional heterojunction models. Structural analysis and density functional theory calculations disclose that, through adjusting the spatial arrangement of oxidation and reduction clusters, the energy level and population of the orbitals of these OR junctions can be regulated synchronously to further optimize photocatalytic performance. The establishment of molecular OR junctions is a pioneering important discovery for extremely improving the utilization efficiency of photogenerated charges in the artificial photosynthesis overall reaction.

Surface, Interface and Structure Optimization of Metal-Organic Frameworks: Towards Efficient Resourceful Conversion of Industrial Waste Gases

Industrial waste gas emissions from fossil fuel over-exploitation have aroused great attention in modern society. Recently, metal-organic frameworks (MOFs) have been developed in the capture and catalytic conversion of industrial exhaust gases such as SO₂ , H₂ S, NO_x , CO₂ , CO, etc. Based on these resourceful conversion applications, in this review, we summarize the crucial role of the surface, interface, and structure optimization of MOFs for performance enhancement. The main points include (1) adsorption enhancement of target molecules by surface functional modification, (2) promotion of catalytic reaction kinetics through enhanced coupling in interfaces, and (3) adaptive matching of guest molecules by structural and pore size modulation. We expect that this review will provide valuable references and illumination for the design and development of MOF and related materials with excellent exhaust gas treatment performance.

Anionic Porous Zn-Metalated Porphyrin Metal-Organic Framework with PtS Topology for Gas-Phase Photocatalytic CO2 Reduction

Presented here are the synthesis and gas-phase photocatalytic CO₂ reduction of an anionic porous Zn-metalated porphyrin metal-organic framework (MOF) induced by an ionic liquid. The desired CO₂ affinity and deep conduction band position of the MOF catalyst provide strong kinetic and thermodynamic advantages for photocatalytic CO₂ to CH₄ conversion with high selectivity (∼70%) in H₂O vapor.

Linking oxidative and reductive clusters to prepare crystalline porous catalysts for photocatalytic CO2 reduction with H2O

Mimicking natural photosynthesis to convert CO₂ with H₂O into value-added fuels achieving overall reaction is a promising way to reduce the atmospheric CO₂ level. Casting the catalyst of two or more catalytic sites with rapid electron transfer and interaction may be an effective strategy for coupling photocatalytic CO₂ reduction and H₂O oxidation. Herein, based on the MOF ∪ COF collaboration, we have carefully designed and synthesized a crystalline hetero-metallic cluster catalyst denoted MCOF-Ti₆Cu₃ with spatial separation and functional cooperation between oxidative and reductive clusters. It utilizes dynamic covalent bonds between clusters to promote photo-induced charge separation and transfer efficiency, to drive both the photocatalytic oxidative and reductive reactions. MCOF-Ti₆Cu₃ exhibits fine activity in the conversion of CO₂ with water into HCOOH (169.8 μmol g⁻¹h⁻¹). Remarkably, experiments and theoretical calculations reveal that photo-excited electrons are transferred from Ti to Cu, indicating that the Cu cluster is the catalytic reduction center.

A crystalline hetero-metallic cluster catalyst based on a covalent organic framework strategy is reported. The catalyst can facilitate both photocatalytic oxidative and reductive reactions leading to efficient production of HCOOH from CO2 and H2O.

Switching Excited State Distribution of Metal-Organic Framework for Dramatically Boosting Photocatalysis

Photosensitization associated with electron/energy transfer represents the central science of natural photosynthesis. Herein, we proposed a protocol to dramatically improve the sensitizing ability of metal-organic frameworks (MOFs) by switching their excited state distribution from ³ MLCT (metal-to-ligand charge transfer) to ³ IL (intraligand). The hierarchical organization of ³ IL MOFs and Co/Cu catalysts facilitates electron transfer for efficient photocatalytic H₂ evolution with a yield of 26 844.6 μmol g^-1 and CO₂ photoreduction with a record HCOOH yield of 4807.6 μmol g^-1 among all the MOF photocatalysts. Systematic investigations demonstrate that strong visible-light-absorption, long-lived excited state and ingenious multi-component synergy in the ³ IL MOFs can facilitate both interface and intra-framework electron transfer to boost photocatalysis. This work opens up an avenue to boost solar-energy conversion by engineering sensitizing centers at a molecular level.

Two 1D Anderson-Type Polyoxometalate-Based Metal-Organic Complexes as Bifunctional Heterogeneous Catalysts for CO2 Photoreduction and Sulfur Oxidation

Sulfur oxides from the combustion of petrol and excessive emissions of carbon dioxide (CO₂) are currently the main causes of environmental pollution. Considerable interest has been paid to solving the challenge, and catalytic reactions seem to be the desired choice. Due to the high density of Lewis acid active sites, polyoxometalates are considered to be the ideal choice for these catalytic reactions. Herein, two captivating polyoxometalate-based metal-organic complexes, formulated as [Co(H₂O)₂DABT]₂[CrMo₆(OH)₅O₁₉] ({Co-CrMo₆}) and [Zn(H₂O)₂DABT]₂[CrMo₆(OH)₅O₁₉] ({Zn-CrMo₆}) (DABT = 3,3'-diamino-5,5'-bis(1H-1,2,4-triazole)) were successfully obtained under hydrothermal conditions. The structural analysis demonstrates that {Co-CrMo₆} and {Zn-CrMo₆} are isostructural with two different transition metal (Co/Zn) ions based on quadridentate Anderson-type [CrMo₆(OH)₅O₁₉]^4- polyanions. A fan-shaped unit of {Co-CrMo₆}/{Zn-CrMo₆} is linked to generate a one-dimensional (1D) ladder-like structure. Intriguingly, benefitting from rich Co centers with a suitable energy band structure, {Co-CrMo₆} displays better photocatalytic activity than {Zn-CrMo₆} for converting CO₂ into CO, endowing the CO formation of 1935.3 μmol g^-1 h^-1 with high selectivity. Meanwhile, {Co-CrMo₆} also exhibits a satisfactory removal rate of 99% for oxidizing dibenzothiophene at 50 °C, which suggests that {Co-CrMo₆} may be utilized as a potential dual functional material with immense prospects in photocatalytic CO₂ reduction and sulfur oxidation for the first time.

Ag Nanoparticle-Modified Polyoxometalate-Based Metal-Organic Framework for Enhanced CO2 Photoreduction

The photoreduction deposition method is employed to fabricate a family of silver nanoparticle (Ag NP)-modified polyoxometalate-based metal-organic framework (NENU-5) photocatalysts, named Ag/NENU-5. The title photocatalysts, Ag/NENU-5, can be used for the photocatalytic reduction of CO₂ and are observed to efficiently reduce CO₂ into CO, in which the highest reduction rate is 22.28 μmol g^-1 h^-1, 3 times greater than that of NENU-5. Photocatalytic reduction performances of CO₂ have been extremely improved after the incorporation of Ag NPs as the cocatalyst. The enhancement of the photocatalytic reduction of CO₂ has been attributed to the synergistic effects of Ag NPs and NENU-5, inhibiting the charge recombination during the photocatalytic process and increasing the reaction active sites. Furthermore, the influence of Ag NPs on the photocatalytic activity has also been investigated. The experimental results clearly reveal that the size of Ag NPs could exert a main effect on the photocatalytic activity, and the reasonable size of Ag NPs is able to enhance the photocatalytic reduction activity toward CO₂ significantly.

TiO2-Modulated tetra(4-carboxyphenyl)porphyrin/perylene diimide organic Z-scheme nano-heterojunctions for efficient visible-light catalytic CO2 reduction

Developing efficient Z-scheme heterojunctions with wide visible-light responsive perylene diimide (PDI) is highly desired for CO₂ conversion, while the effective charge transfer and separation are crucial. Herein, TiO₂-modulated tetra(4-carboxyphenyl)porphyrin/perylene diimide (T-TP/PDI) organic nano-heterojunctions have been fabricated for CO₂ reduction, in which TP and PDI are first assembled via π-π interactions between their similar 2D conjugate structures, and then the TiO₂ nanoparticles (ca. 10 nm) are anchored as an energy platform through the carboxyl groups on TP. The optimal one exhibits a ∼10-fold enhancement in photocatalytic activity compared with the pristine PDI. Based on the time-resolved surface photovoltage responses, electron paramagnetic resonance signals, in situ diffuse reflectance infrared Fourier transform spectra and the amount evaluation of H₂O₂ as the water-oxidation intermediate, it is suggested that the exceptional photoactivity be ascribed to the accelerated charge transfer and separation resulting from the constructed Z-scheme nano-heterojunctions with intimate interfacial interactions and the introduced energy platform TiO₂ oriented towards largely inhibiting the type-II charge transfer pathway. This work diversifies the strategies for constructing efficient organic Z-scheme heterojunctions, and provides insight into interface correlation among components.

Coupling Ruthenium Bipyridyl and Cobalt Imidazolate Units in a Metal-Organic Framework for an Efficient Photosynthetic Overall Reaction in Diluted CO2

Artificial photocatalytic CO₂ reduction, using water as the reductant, is challenging mainly because it is difficult for multiple functional units to cooperate efficiently. Here, we show that the classic photosensitive and H₂O-oxidizing ruthenium bipyridyl units and CO₂-reducing cobalt imidazolate units can be incorporated into a metal-organic framework using a classic organic ligand, imidazo[4,5-f][1,10]phenanthroline. Under visible light without additional sacrificial agents and photosensitizers, the overall conversion of CO₂ and H₂O to CO and O₂ was achieved by the multifunctional photocatalyst in the CH₃CN/H₂O mixed solvent with a high CO production rate of 11.2 μmol g^-1 h^-1 and CO selectivity of ca. 100%. Thanks to its ultramicroporous structure with moderately strong CO₂ adsorption ability, the photocatalyst also exhibited high performances with CO/CH₄ production rates of 5.15/0.62 and 4.26/0.20 μmol g^-1 h^-1 in the gas phase with pure and even diluted CO₂, respectively. Photoluminescence emission spectroscopy and photoelectrochemical tests confirmed that the photosensitive and catalytic units cooperated well to give suitable photocatalytic redox potentials and fast electron-hole separation.

Engineering Hierarchical Architecture of Metal-Organic Frameworks for Highly Efficient Overall CO2 Photoreduction

Previous studies on syntheses of metal-organic frameworks (MOFs) for photocatalytic CO₂ reduction are mainly focused on the exquisite control over the net topology and the functionality of metal clusters/organic building blocks. This contribution demonstrates that the rational design of MOF-based photocatalyst can be further extended to the hierarchical structure at micrometer scales well beyond the conventional MOF design at the molecular level. By taking advantage of the disparity of two selective MOFs in nucleation kinetics, a hierarchical core-shell MOF@MOF structure is successfully constructed through a simple one-pot synthesis. Besides inheriting the high porosity, crystallinity, and robustness of parent MOFs, the obtained heterojunction exhibits extended photoresponse, optimized band alignment with large overpotential, and greatly enhanced photogenerated charge separation, which would be hardly realized by the merely molecular-level assembly. As a result, the challenging overall CO₂ photoreduction is achieved, which generates a record high HCOOH production (146.0 µmol/g/h) without using any sacrificial reagents. Moreover, the core-shell structure exhibits a more effective use of photogenerated electrons than the individual MOFs. This work shows that harnessing the hierarchical architecture of MOFs present a new and effective alternative to tuning the photocatalytic performance at a mesoscopic level.