Immobilization of a Full Photosystem in the Large-Pore MIL-101 Metal-Organic Framework for CO2 reduction

Unravelling the Molecular Structure and Confining Environment of an Organometallic Catalyst Heterogenized within Amorphous Porous Polymers

The catalytic activity of multifunctional, microporous materials is directly linked to the spatial arrangement of their structural building blocks. Despite great achievements in the design and incorporation of isolated catalytically active metal complexes within such materials, a detailed understanding of their atomic-level structure and the local environment of the active species remains a fundamental challenge, especially when these latter are hosted in non-crystalline organic polymers. Here, we show that by combining computational chemistry with pair distribution function analysis, 129 Xe NMR, and Dynamic Nuclear Polarization enhanced NMR spectroscopy, a very accurate description of the molecular structure and confining surroundings of a catalytically active Rh-based organometallic complex incorporated inside the cavity of amorphous bipyridine-based porous polymers is obtained. Small, but significant, differences in the structural properties of the polymers are highlighted depending on their backbone motifs.

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.

Rhodium-Based MOF-on-MOF Difunctional Core-Shell Nanoreactor for NAD(P)H Regeneration and Enzyme Directed Immobilization

An organometallic complex-catalyzed artificial coenzyme regeneration system has attracted widespread attention. However, the combined use of organometallic complex catalysts and natural enzymes easily results in mutual inactivation. Herein, we establish a rhodium-based metal-organic framework (MOF)-on-MOF difunctional core-shell nanoreactor as an artificial enzymatic NAD(P)H regeneration system. UiO67 as the core is used to capture rhodium molecules for catalyzing NAD(P)H regeneration. UiO66 as the shell is used to specifically immobilize His-tagged lactate dehydrogenase (LDH) and serve as a protection shield for LDH and [Cp*Rh(bpy)Cl]⁺ to prevent mutual inactivation. A variety of results indicate that UiO67@Rh@UiO66 has good activity in realizing NAD(P)H regeneration. Noteworthily, UiO67@Rh@UiO66@LDH maintains a high activity level even after 10 cycles. This work reports a novel NAD(P)H regeneration platform to open up a new avenue for constructing chemoenzyme coupling systems.

Research on photocatalytic CO2 conversion to renewable synthetic fuels based on localized surface plasmon resonance: current progress and future perspectives

Due to its desirable optoelectronic properties, localized surface plasmon resonance (LSPR) can hopefully play a promising role in photocatalytic CO2 reduction reaction (CO2RR). In this review, mechanisms and applications of LSPR effect in this field are introduced in detail.

Visible-Light-Driven Photocatalytic CO2 Reduction to CO/CH4 Using a Metal-Organic "Soft" Coordination Polymer Gel

The self-assembly of a well-defined and astutely designed, low-molecular weight gelator (LMWG) based linker with a suitable metal ion is a promising method for preparing photocatalytically active coordination polymer gels. Here, we report the design, synthesis, and gelation behaviour of a tetrapodal LMWG based on a porphyrin core connected to four terpyridine units (TPY-POR) through amide linkages. The self-assembly of TPY-POR LMWG with Ru^II ions results in a Ru-TPY-POR coordination polymer gel (CPG), with a nanoscroll morphology. Ru-TPY-POR CPG exhibits efficient CO₂ photoreduction to CO (3.5 mmol g^-1  h^-1 ) with >99 % selectivity in the presence of triethylamine (TEA) as a sacrificial electron donor. Interestingly, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) with TEA as the sacrificial electron donor, the 8e^- /8H⁺ photoreduction of CO₂ to CH₄ is realized with >95 % selectivity (6.7 mmol g^-1  h^-1 ). In CPG, porphyrin acts as a photosensitizer and covalently attached [Ru(TPY)₂ ]²⁺ acts as a catalytic center as demonstrated by femtosecond transient absorption (TA) spectroscopy. Further, combining information from the in situ DRIFT spectroscopy and DFT calculation, a possible reaction mechanism for CO₂ reduction to CO and CH₄ was outlined.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Rhodium-Based Metal-Organic Polyhedra Assemblies for Selective CO2 Photoreduction

Heterogenization of molecular catalysts via their immobilization within extended structures often results in a lowering of their catalytic properties due to a change in their coordination sphere. Metal-organic polyhedra (MOP) are an emerging class of well-defined hybrid compounds with a high number of accessible metal sites organized around an inner cavity, making them appealing candidates for catalytic applications. Here, we demonstrate a design strategy that enhances the catalytic properties of dirhodium paddlewheels heterogenized within MOP (Rh-MOP) and their three-dimensional assembled supramolecular structures, which proved to be very efficient catalysts for the selective photochemical reduction of carbon dioxide to formic acid. Surprisingly, the catalytic activity per Rh atom is higher in the supramolecular structures than in its molecular sub-unit Rh-MOP or in the Rh-metal-organic framework (Rh-MOF) and yields turnover frequencies of up to 60 h^-1 and production rates of approx. 76 mmole formic acid per gram of the catalyst per hour, unprecedented in heterogeneous photocatalysis. The enhanced catalytic activity is investigated by X-ray photoelectron spectroscopy and electrochemical characterization, showing that self-assembly into supramolecular polymers increases the electron density on the active site, making the overall reaction thermodynamically more favorable. The catalyst can be recycled without loss of activity and with no change of its molecular structure as shown by pair distribution function analysis. These results demonstrate the high potential of MOP as catalysts for the photoreduction of CO₂ and open a new perspective for the electronic design of discrete molecular architectures with accessible metal sites for the production of solar fuels.

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity vs. flexibility, and other features and properties. Collectively and individually, these properties help define overall catalyst functional behaviour. This review focuses on how porous, catalyst-containing MOFs capitalize on molecular-scale confinement, containment, isolation, environment modulation, energy delivery, and mobility to accomplish desired chemical transformations with potentially superior selectivity or other efficacy, especially in comparison to catalysts in homogeneous solution environments.

Heterogenization of molecular cobalt catalysts in robust metal–organic frameworks for efficient photocatalytic CO2 reduction

Efficient, selective and recyclable heterogeneous catalysts for photocatalytic CO2 reduction to CO under visible light irradiation are readily prepared by immobilization of cobalt molecular catalysts into Zr(iv)-based MOFs.

Topology- and wavelength-governed CO2 reduction photocatalysis in molecular catalyst-metal–organic framework assemblies

Optimising catalyst materials for visible light-driven fuel production requires understanding complex and intertwined processes including light absorption and catalyst stability, as well as mass, charge, and energy transport. These phenomena can be uniquely combined (and ideally controlled) in porous host–guest systems. Towards this goal we designed model systems consisting of molecular complexes as catalysts and porphyrin metal–organic frameworks (MOFs) as light-harvesting and hosting porous matrices. Two MOF-rhenium molecule hybrids with identical building units but differing topologies (PCN-222 and PCN-224) were prepared including photosensitiser-catalyst dyad-like systems integrated via self-assembled molecular recognition. This allowed us to investigate the impact of MOF topology on solar fuel production, with PCN-222 assemblies yielding a 9-fold turnover number enhancement for solar CO₂-to-CO reduction over PCN-224 hybrids as well as a 10-fold increase compared to the homogeneous catalyst-porphyrin dyad. Catalytic, spectroscopic and computational investigations identified larger pores and efficient exciton hopping as performance boosters, and further unveiled a MOF-specific, wavelength-dependent catalytic behaviour. Accordingly, CO₂ reduction product selectivity is governed by selective activation of two independent, circumscribed or delocalised, energy/electron transfer channels from the porphyrin excited state to either formate-producing MOF nodes or the CO-producing molecular catalysts.

Two MOF molecular catalyst hybrids with differing topologies show mass and light transport governed photocatalysis. MOF-specific, irradiation wavelength-dependent product control is unlocked by switching between two energy/electron transfer channels.

Host-Guest Interactions in a Metal-Organic Framework Isoreticular Series for Molecular Photocatalytic CO2 Reduction

A strategy to improve homogeneous molecular catalyst stability, efficiency, and selectivity is the immobilization on supporting surfaces or within host matrices. Herein, we examine the co‐immobilization of a CO₂ reduction catalyst [ReBr(CO)₃(4,4′‐dcbpy)] and a photosensitizer [Ru(bpy)₂(5,5′‐dcbpy)]Cl₂ using the isoreticular series of metal–organic frameworks (MOFs) UiO‐66, ‐67, and ‐68. Specific host pore size choice enables distinct catalyst and photosensitizer spatial location—either at the outer MOF particle surface or inside the MOF cavities—affecting catalyst stability, electronic communication between reaction center and photosensitizer, and consequently the apparent catalytic rates. These results allow for a rational understanding of an optimized supramolecular layout of catalyst, photosensitizer, and host matrix.

Molecular Dye-Sensitized Photocatalysis with Metal-Organic Framework and Metal Oxide Colloids for Fuel Production

Colloidal dye-sensitized photocatalysis is a promising route toward efficient solar fuel production by merging properties of catalysis, support, light absorption, and electron mediation in one. Metal-organic frameworks (MOFs) are host materials with modular building principles allowing scaffold property tailoring. Herein, we combine these two fields and compare porous Zr-based MOFs UiO-66-NH2(Zr) and UiO-66(Zr) to monoclinic ZrO2 as model colloid hosts with co-immobilized molecular carbon dioxide reduction photocatalyst fac-ReBr(CO)3(4,4′-dcbpy) (dcbpy = dicarboxy-2,2′-bipyridine) and photosensitizer Ru(bpy)2(5,5′-dcbpy)Cl2 (bpy = 2,2′-bipyridine). These host-guest systems demonstrate selective CO2-to-CO reduction in acetonitrile in presence of an electron donor under visible light irradiation, with turnover numbers (TONs) increasing from ZrO2, to UiO-66, and to UiO-66-NH2 in turn. This is attributed to MOF hosts facilitating electron hopping and enhanced CO2 uptake due to their innate porosity. Both of these phenomena are pronounced for UiO-66-NH2(Zr), yielding TONs of 450 which are 2.5 times higher than under MOF-free homogeneous conditions, highlighting synergistic effects between supramolecular photosystem components in dye-sensitized MOFs.

Understanding entrapped molecular photosystem and metal-organic framework synergy for improved solar fuel production

Artificial photosystems assembled from molecular complexes, such as the photocatalyst fac-ReBr(CO)3(4,4'-dcbpy) (dcbpy = dicarboxy-2,2'-bipyridine) and the photosensitiser Ru(bpy)2(5,5'-dcbpy)Cl2 (bpy = 2,2'-bipyridine), are a wide-spread approach for solar fuel production. Recently metal-organic framework (MOF) entrapping of such complexes was demonstrated as a promising concept for catalyst stabilisation and reaction environment optimisation in colloidal-based CO2 reduction. Building on this strategy, here we examined the influence of MIL-101-NH2(Al) MOF particle size, the electron donor source, and the presence of an organic base on the photocatalytic CO2-to-CO reduction performance, and the differences to homogeneous systems. A linear relation between smaller scaffold particle size and higher photocatalytic activity, longer system lifetimes for benign electron donors, and increased turnover numbers (TONs) with certain additive organic bases, were determined. This enabled understanding of key molecular catalysis phenomena and synergies in the nanoreactor-like host-guest assembly, and yielded TONs of ∼4300 over 96 h of photocatalysis under optimised conditions, surpassing homogeneous TON values and lifetimes.

Cobalt-based metal-organic frameworks for the photocatalytic reduction of carbon dioxide

Metal-organic frameworks (MOFs) are porous materials composed of metal centers and organic connectors. They are formed by complexation reactions and exhibit characteristics of both polymers and coordination compounds. They exhibit numerous advantageous features, including a large specific surface area, adjustable pore size/shape, and modifiable pore wall functional groups. Consequently, MOFs have been extensively applied in the photocatalytic reduction of carbon dioxide (CO2). Despite considerable research on cobalt-based MOFs, the photocatalytic reduction of CO2 in the presence of these materials remains challenging. The present review summarizes the current studies concerning the utilization of cobalt-based MOFs in the photocatalytic reduction of CO2. Additionally, approaches used to enhance the catalytic reduction performance are evaluated and the challenges associated with Co-based MOFs are discussed.

Heterogenisation of polyoxometalates and other metal-based complexes in metal–organic frameworks: from synthesis to characterisation and applications in catalysis

This review provides a thorough overview of composites with molecular catalysts (polyoxometalates, or organometallic or coordination complexes) immobilised into MOFs via non-covalent interactions.

Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction

The Keggin-type polyoxometalate (POM) PW₁₂O₄₀^3- and the catalytic complex Cp*Rh(bpydc)Cl₂ (bpydc = 2,2'-bipyridine-5,5'-dicarboxylic acid) were coimmobilized in the Zr(IV) based metal organic framework UiO-67. The POM is encapsulated within the cavities of the MOF by in situ synthesis, and then, the Rh catalytic complex is introduced by postsynthetic linker exchange. Infrared and Raman spectroscopies, ³¹P and ¹³C MAS NMR, N₂ adsorption isotherms, and X-ray diffraction indicate the structural integrity of all components (POM, Rh-complex and MOF) within the composite of interest (PW₁₂,Cp*Rh)@UiO-67. DFT calculations identified two possible locations of the POM in the octahedral cavities of the MOF: one at the center of a UiO-67 pore with the Cp*Rh complex pointing toward an empty pore and one off-centered with the Cp*Rh pointing toward the POM. ³¹P-¹H heteronuclear (HETCOR) experiments ascertained the two environments of the POM, equally distributed, with the POM in interaction either with the Cp* fragment or with the organic linker. In addition, Pair Distribution Function (PDF) data were collected on the POM@MOF composite and provided key evidence of the structural integrity of the POM once immobilized into the MOF. The photocatalytic activity of the (PW₁₂,Cp*Rh)@UiO-67 composite for CO₂ reduction into formate and hydrogen were evaluated. The formate production was doubled when compared with that observed with the POM-free Cp*Rh@UiO-67 catalyst and reached TONs as high as 175 when prepared as thin films, showing the beneficial influence of the POM. Finally, the stability of the composite was assessed by means of recyclability tests. The combination of XRD, IR, ICP, and PDF experiments was essential in confirming the integrity of the POM, the catalyst, and the MOF after catalysis.

Recent Advances in Metal-Organic Frameworks for Photo-/Electrocatalytic CO2 Reduction

Considerable attention has been paid to the utilization of CO₂ , an abundant carbon source in nature. In this regard, porous catalysts have been eagerly explored with excellent performance for photo-/electrocatalytic reduction of CO₂ to high valued products. Metal-organic frameworks (MOFs), featuring large surface area, high porosity, tunable composition and unique structural characteristics, have been widely exploited in catalytic CO₂ reduction. This Minireview first reports the current progress of MOFs in CO₂ reduction. Then, a specific interest is focused on MOFs in photo-/electrocatalytic reduction of CO₂ by modifying their metal centers, organic linkers, and pores. Finally, the future directions of study are also highlighted to satisfy the requirement of practical applications.

Bismuth-Based Photocatalysts for Solar Photocatalytic Carbon Dioxide Conversion

Photocatalytic CO₂ conversion into solar fuels is an effective means for simultaneously solving both the greenhouse effect and energy crisis. In the past ten years, bismuth-based photocatalysts for environmental remediation have experienced a golden period of development. However, solar photocatalytic CO₂ conversion has only been developed over the past five years and, until now, no reviews have been published on bismuth-based photocatalysts for the photocatalytic conversion of CO₂ . For the first time, solar photocatalytic CO₂ conversion systems are reviewed herein. Synthetic methods and photocatalytic CO₂ performances of bismuth-based photocatalysts, including Sillén-structured BiOX (X=Cl, Br, I); Aurivillius-structured Bi₂ MO₆ (M=Mo, W); and Scheelite-structured BiVO₄ , Bi₂ S₃ , BiYO₃ , and BiOIO₃ , are summarized. In addition, activity-enhancing strategies for this photocatalyst family, including oxygen vacancies, bismuth-rich strategy, facet control, conventional type II heterojunction, Z-scheme heterojunction, and cocatalyst deposition, are reviewed. Finally, the main mechanistic research methods, such as in situ FTIR spectroscopy and theoretical calculations, are presented. Challenges and research trends reported in studies of bismuth-based photocatalysts for photocatalytic CO₂ conversion are discussed and summarized.

Hydrophobic Polyoxometalate-Based Metal-Organic Framework for Efficient CO2 Photoconversion

A novel polyoxometalate (POM)-based metal-organic framework, TBA₅[P₂Mo₁₆^VMo₈^VIO₇₁(OH)₉Zn₈(L)₄] (NNU-29), was in situ synthesized and applied into CO₂ photoreduction. The selection of porous material containing a reductive POM cluster is considered to be helpful for CO₂ reduction; meanwhile, a hydrophobic-group-modified organic ligand enables NNU-29 to exhibit good chemical stability and restrains hydrogen generation to some extent. In the photocatalytic CO₂ reduction, the yield of HCOO^- reached 35.2 μmol in the aqueous solution with selectivity of 97.9% after 16 h.

Controlling Hydrogen Evolution during Photoreduction of CO2 to Formic Acid Using [Rh(R-bpy)(Cp*)Cl]+ Catalysts: A Structure-Activity Study

The photochemical reduction of CO₂ to formic acid catalyzed by a series of [Rh(4,4'-R-bpy)(Cp*)Cl]⁺ and [Rh(5,5'-COOH-bpy)(Cp*)Cl]⁺ complexes (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, and R = OCH₃, CH₃, H, COOC₂H₅, CF₃, NH₂, or COOH) was studied to assess how modifications in the electronic structure of the catalyst affect its selectivity, defined as the HCOOH:H₂ product ratio. A direct molecular-level influence of the functional group on the initial reaction rate for CO₂ versus proton reduction reactions was established. Density functional theory computations elucidated for the first time the respective role of the [RhH] and [Cp*H] tautomers, recognizing rhodium hydride as the key player for both reactions. In particular, our calculations explain the observed tendency of electron-donating substituents to favor CO₂ reduction by means of decreasing the hydricity of the Rh-H bond, resulting in a lower hydride transfer barrier toward formic acid production as compared to substituents with an electron-withdrawing nature that favor more strongly the reduction of protons to hydrogen.