Energy transfer on a two-dimensional antenna enhances the photocatalytic activity of CO2 reduction by metal-organic layers

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.

Photoreactive Carbon Dioxide Capture by a Zirconium-Nanographene Metal-Organic Framework

The mechanism of photochemical CO₂ reduction to formate by PCN-136, a Zr-based metal-organic framework (MOF) that incorporates light-harvesting nanographene ligands, has been investigated using steady-state and time-resolved spectroscopy and density functional theory (DFT) calculations. The catalysis was found to proceed via a "photoreactive capture" mechanism, where Zr-based nodes serve to capture CO₂ in the form of Zr-bicarbonates, while the nanographene ligands have a dual role of absorbing light and storing one-electron equivalents for catalysis. We also find that the process occurs via a "two-for-one" route, where a single photon initiates a cascade of electron/hydrogen atom transfers from the sacrificial donor to the CO₂-bound MOF. The mechanistic findings obtained here illustrate several advantages of MOF-based architectures in molecular photocatalyst engineering and provide insights on ways to achieve high formate selectivity.

Enhanced Carrier Separation in Visible-Light-Responsive Polyoxometalate-Based Metal-Organic Frameworks for Highly Efficient Oxidative Coupling of Amines

Photocatalytic technology is widely studied, while it comes with drawbacks such as low sunlight utilization efficiency and high carrier recombination rates. Herein, for the first time, we present two crystalline polyoxometalate (POM)-based metal-organic frameworks (POMOFs), {[Cd(DMF)₂Ru(bpy)₂(dcbpy)]₂(POMs)(DMF)₂} xDMF (PMo-1, POMs = [PMo^VI₁₁Mo^VO₄₀]^4-, x = 5; SiW-2, POMs = [SiW₁₂O₄₀]^4-, x = 4) through assembling the photosensitizer [Ru(bpy)₂(H₂dcbpy)]Cl₂ and POMs into a single framework. The assembly not only enhances light absorption in the visible light regime but also improves carrier separation efficiency; atop of that, both POMOFs demonstrate activities in the photocatalytic oxidative coupling of amines. Particularly, PMo-1 enables the quantitative completion of oxidative coupling of benzylamine reaction within 30 min (yield = 99.6%) with a high turnover frequency (TOF = 6631.6 h^-1). To our knowledge, the PMo-1 catalyst outperforms any other photocatalysts previously reported in similar use cases where TOF values were usually obtained <2000 h^-1.

A novel Bismuth hydroxide (Bi(OH)3) semiconductor with highly-efficient photocatalytic activity

Herein, a novel Bi(OH)3 photocatalyst was successfully synthesized. Benefiting from the suitable band positions, abundant alkaline groups and oxygen vacancies, the as-prepared semiconductor exhibits efficient activity in both photocatalytic CO2...

Excited State Energy Transfer in Metal-Organic Frameworks

Excited state energy transfer in metal-organic frameworks (MOFs) is of great interest due to potential application of these materials in photocatalysis and fluorescence sensing. In photocatalysis, a light-harvesting antenna of MOFs can collect energy from a much larger area than a single reaction center and efficiently transport the energy to the active site to enhance photocatalytic efficiency, mimicking nature photosynthesis. In fluorescence sensing, excited state traveling on the framework can search for analyte quencher molecules to give amplified fluorescence quenching, so that one quencher turns off multiple excited states to enhance signal. Key to these designer performances is highly efficient energy transfer on these framework materials that are determined by types of excited states, dimension of the materials, and structure of the frameworks. Advancement of MOF synthetic chemistry provides new tools to control the rate and directionality of energy transfer in these materials, opening opportunities in manipulating excited states at an unprecedented level.

2D Porphyrinic Metal-Organic Frameworks Featuring Rod-Shaped Secondary Building Units

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called 'rod MOFs', which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2L-CuII, where H2 refers to the ligand's carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D MnII- or CoII-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

Recent advances in visible-light-driven carbon dioxide reduction by metal-organic frameworks

Metal-organic frameworks (MOFs) have emerged as promising materials and have attracted researchers due to their unique chemical and physical properties-design flexibility, tuneable pore channels, a high surface-to-volume ratio that allow their distinct application in diverse research fields-gas storage, gas separation, catalysis, adsorption, drug delivery, ion exchange, sensing, etc. The rapidly growing CO₂ in the atmosphere is a global concern due to the excessive use of fossil fuels in the current era. CO₂ is the prime cause of global warming and should be ameliorated either through adsorption or conversion into value-added products to protect the environment and mankind. Nowadays, MOFs are exploited as a photocatalyst for applications of CO₂ reduction. Since the use of semiconductors limits the use of visible light for photocatalytic reduction of CO₂, MOFs are promising options. The current review describes recent development in the application of MOFs as host, composites, and their derivatives in photocatalytic reduction of CO₂ to CO and different organic chemicals (HCOOH, CH₃OH, CH₄). Efficient charge separation and visible light absorption by incorporation of active sites for efficient photocatalysis have been discussed. The selection of material for high CO₂ uptake and potential strategies for the rational design and development of high-performance catalysts are outlined. Major challenges and future perspectives have also been discussed at the last of the review.

Single- and mixed-metal–organic framework photocatalysts for carbon dioxide reduction

This review focuses on the important roles of varied metal types over MOF-based photocatalysts. The basic principles, types of MOF photocatalysts and roles of the reaction system to achieve efficient MOFs for CO₂ photoreduction are discussed.

Metal-Organic Layers for Electrocatalysis and Photocatalysis

Metal-organic layers (MOLs) are two-dimensional analogues of metal-organic frameworks (MOFs) with a high aspect ratio and thickness down to a monolayer. Active sites on MOLs are more accessible than those on MOFs thanks to the two-dimensional feature of MOLs, which allows easier chemical modification around the catalytic center. MOLs can also be assembled with other functional materials through surface anchoring sites that can facilitate charge/energy transport through the hybrid material. MOLs are thus quite suitable for interfacial catalysis like electrocatalysis and photocatalysis. In this outlook, we focus on representative progress of constructing unique interfacial sites on MOLs with designer paths for charge separation and energy transfer, as well as cooperative cavities for superior substrate adsorption and activation. We also discuss challenges and potentials in the future development of MOL catalysts and catalysts beyond MOLs.

Recent Innovation of Metal-Organic Frameworks for Carbon Dioxide Photocatalytic Reduction

The accumulation of carbon dioxide (CO2) pollutants in the atmosphere begets global warming, forcing us to face tangible catastrophes worldwide. Environmental affability, affordability, and efficient CO2 metamorphotic capacity are critical factors for photocatalysts; metal-organic frameworks (MOFs) are one of the best candidates. MOFs, as hybrid organic ligand and inorganic nodal metal with tailorable morphological texture and adaptable electronic structure, are contemporary artificial photocatalysts. The semiconducting nature and porous topology of MOFs, respectively, assists with photogenerated multi-exciton injection and adsorption of substrate proximate to void cavities, thereby converting CO2. The vitality of the employment of MOFs in CO2 photolytic reaction has emerged from the fact that they are not only an inherently eco-friendly weapon for pollutant extermination, but also a potential tool for alleviating foreseeable fuel crises. The excellent synergistic interaction between the central metal and organic linker allows decisive implementation for the design, integration, and application of the catalytic bundle. In this review, we presented recent MOF headway focusing on reports of the last three years, exhaustively categorized based on central metal-type, and novel discussion, from material preparation to photocatalytic, simulated performance recordings of respective as-synthesized materials. The selective CO2 reduction capacities into syngas or formate of standalone or composite MOFs with definite photocatalytic reaction conditions was considered and compared.