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.     

Photochemical In Situ Exfoliation of Metal–Organic Frameworks for Enhanced Visible-Light-Driven CO2 Reduction

Two novel two-dimensional metal–organic frameworks (2D MOFs), 2D-M₂TCPE (M=Co or Ni, TCPE=1,1,2,2-tetra(4-carboxylphenyl)ethylene), which are composed of staggered (4,4)-grid layers based on paddlewheel-shaped dimers, serve as heterogeneous photocatalysts for efficient reduction of CO₂ to CO. During the visible-light-driven catalysis, these structures undergo in situ exfoliation to form nanosheets, which exhibit excellent stability and improved catalytic activity. The exfoliated 2D-M₂TCPE nanosheets display a high CO evolution rate of 4174 μmol g⁻¹ h⁻¹ and high selectivity of 97.3 % for M=Co and Ni, and thus are superior to most reported MOFs. The performance differences and photocatalytic mechanisms have been studied with theoretical calculations and photoelectric experiments. This study provides new insight for the controllable synthesis of effective crystalline photocatalysts based on structural and morphological coregulation.     

Metal–Organic Frameworks Based Electrocatalysts for the Oxygen Reduction Reaction

In view of the clean and sustainable energy, metal–organic frameworks (MOFs) based materials, including pristine MOFs, MOF composites, and their derivatives are emerging as unique electrocatalysts for oxygen reduction reaction (ORR). Thanks to their tunable compositions and diverse structures, efficient MOF-based materials provide new opportunities to accelerate the sluggish ORR at the cathode in fuel cells and metal–air batteries. This Minireview first provides some introduction of ORR and MOFs, followed by the classification of MOF-based electrocatalysts towards ORR. Recent breakthroughs in engineering MOF-based ORR electrocatalysts are highlighted with an emphasis on synthesis strategy, component, morphology, structure, electrocatalytic performance, and reaction mechanism. Finally, some current challenges and future perspectives for MOF-based ORR electrocatalysts are also discussed.     

Single-Atom Electrocatalysts from Multivariate Metal–Organic Frameworks for Highly Selective Reduction of CO2 at Low Pressures

Single-atom catalysts (SACs) are of great interest because of their ultrahigh activity and selectivity. However, it is difficult to construct model SACs according to a general synthetic method, and therefore, discerning differences in activity of diverse single-atom catalysts is not straightforward. Herein, a general strategy for synthesis of single-atom metals implanted in N-doped carbon (M₁-N-C; M=Fe, Co, Ni and Cu) has been developed starting from multivariate metal–organic frameworks (MOFs). The M₁-N-C catalysts, featuring identical chemical environments and supports, provided an ideal platform for differentiating the activity of single-atom metal species. When employed in electrocatalytic CO₂ reduction, Ni₁-N-C exhibited a very high CO Faradaic efficiency (FE) up to 96.8 % that far surpassed Fe₁-, Co₁- and Cu₁-N-C. Remarkably, the best-performer, Ni₁-N-C, even demonstrated excellent CO FE at low CO₂ pressures, thereby representing a promising opportunity for the direct use of dilute CO₂ feedstock.     

Designable Guest-Molecule Encapsulation in Metal–Organic Frameworks for Proton Conductivity

Metal–organic frameworks (MOFs), as a porous frame material, exhibit considerable electrical conductivity. In recent decades, research on the proton conductivity of MOFs has made gratifying progress. In this review, the designable guest molecules encapsulated into MOFs are summarized and generalized into four types in terms of promoting proton conductive performance, and then recent progress in the promotion of proton conductivity by MOFs encapsulating guest molecules is discussed. The existing challenges and prospects for the development of this strategy for promoting MOFs’ proton conductivity are also listed.     

Dimensional Reduction of Metal–Organic Frameworks for Enhanced Cryopreservation of Red Blood Cells

To increase the red blood cell (RBC) cryopreservation efficiency by metal–organic frameworks (MOFs), a dimensional reduction approach has been proposed. Namely, 3D MOF nanoparticles are progressively reduced to 2D ultra-thin metal–organic layers (MOLs). We found that 2D MOLs are beneficial for enhanced interactions of the interfacial hydrogen-bonded water network and increased utilization of inner ordered structures, due to the higher surface-to-volume ratio. Specifically, a series of hafnium (Hf)-based 2D MOLs with different thicknesses (monolayer to stacked multilayers) and densities of hydrogen bonding sites have been synthesized. Both ice recrystallization inhibition activity (IRI) and RBCs cryopreservation assay confirm the pronounced better IRI activity and excellent cell recovery efficiency (up to ≈63 % at a very low concentration of 0.7 mg mL⁻¹) of thin-layered Hf-MOLs compared to their 3D counterparts, thereby verifying the dimensional reduction strategy to improved cryoprotectant behaviors.     

Multi-shelled Hollow Metal–Organic Frameworks

Hollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.     

Metal–Covalent Organic Frameworks (MCOFs): A Bridge Between Metal–Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal–organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal–covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting-edge applications.     

Metal–Organic Frameworks as Electrocatalysts

Transition metal complexes are well-known homogeneous electrocatalysts. In this regard, metal–organic frameworks (MOFs) can be considered as an ensemble of transition metal complexes ordered in a periodic arrangement. In addition, MOFs have several additional positive structural features that make them suitable for electrocatalysis, including large surface area, high porosity, and high content of accessible transition metal with exchangeable coordination positions. The present review describes the current state in the use of MOFs as electrocatalysts, both as host of electroactive guests and their direct electrocatalytic activity, particularly in the case of bimetallic MOFs. The field of MOF-derived materials is purposely not covered, focusing on the direct use of MOFs or its composites as electrocatalysts. Special attention has been paid to present strategies to overcome their poor electrical conductivity and limited stability.     

Alkali Phosphonate Metal–Organic Frameworks

A new family of porous metal–organic frameworks (MOFs), namely alkali phosphonate MOFs, is reported. [Na₂Cu(H₄TPPA)] ⋅ (NH₂(CH₃)₂)₂ ( GTUB-1 ) was synthesized using the tetratopic 5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin ( H₈-TPPA ) linker with planar X-shaped geometrical core. GTUB-1 is composed of rectangular void channels with BET surface area of 697 m² g⁻¹. GTUB-1 exhibits exceptional thermal stability. The toxicity analysis of the ( H₈-TPPA ) linker indicates that it is well tolerated by an intestinal cell line, suggesting its suitability for creating phosphonate MOFs for biological applications.     

Photoinduced Nonlinear Contraction Behavior in Metal–Organic Frameworks

The photoinduced dynamic behavior of flexible materials has received considerable attention for potential applications, such as in data storage or as smart optical devices and molecular mechanical actuators. Until now, precisely controlling expansion and contraction with light has remained a challenge. Unraveling the detailed mechanisms of photoinduced structural transformations remains a critical step necessary to understand the molecular architecture necessary for the design of sensitive photomechanical actuators. Herein, a two-dimensional flexible metal–organic framework [Zn₂(bdc)₂(3-CH₃-spy)₂]⋅H₂O ( Zn₂-1 ; H₂bdc=1,4-benzenedicaboxylic acid; 3-CH₃-spy=3-methylstyrylpyridine) with a positive volumetric thermal expansion coefficient of +78.78×10⁻⁶ K⁻¹ is reported. Upon light irradiation at different wavelengths, the MOF underwent a [2+2] cycloaddition, which afforded a family of isomeric, three-dimensional MOFs ( Zn₂-2 n , n=a–d) in a single-crystal-to-single-crystal (SCSC) manner. An unprecedented phenomenon, that is, photoinduced nonlinear contraction (PINC), was observed during this conversion. The PINC is caused by conformational changes in the 3-CH₃-spy and bdc²⁻ ligands, the bending of metal–ligand bonds, and the local distortion of the paddle-wheel SBUs. The formation of a “wrinkle morphology” on the crystal surface after the photoreaction was observed by AFM. This PINC behavior can broaden the studies on materials expansion and offer a photodriven approach for the future design of supersensitive photomechanical actuators.     

Facilitated Photocatalytic CO2 Reduction in Aerobic Environment on a Copper-Porphyrin Metal–Organic Framework

Herein, we fabricated a π–π stacking hybrid photocatalyst by combining two two-dimensional (2D) materials: g-C₃N₄ and a Cu-porphyrin metal–organic framework (MOF). After an aerobic photocatalytic pretreatment, this hybrid catalyst exhibited an unprecedented ability to photocatalytically reduce CO₂ to CO and CH₄ under the typical level (20 %) of O₂ in the air. Intriguingly, the presence of O₂ did not suppress CO₂ reduction; instead, a fivefold increase compared with that in the absence of O₂ was observed. Structural analysis indicated that during aerobic pretreatment, the Cu node in the 2D-MOF moiety was hydroxylated by the hydroxyl generated from the reduction of O₂. Then the formed hydroxylated Cu node maintained its structure during aerobic CO₂ reduction, whereas it underwent structural alteration and was reductively devitalized in the absence of O₂. Theoretical calculations further demonstrated that CO₂ reduction, instead of O₂ reduction, occurred preferentially on the hydroxylated Cu node.     

The Significance of Metal Coordination in Imidazole-Functionalized Metal–Organic Frameworks for Carbon Dioxide Utilization

The grafting of imidazole species onto coordinatively unsaturated sites within metal–organic framework MIL-101(Cr) enables enhanced CO₂ capture in close proximity to catalytic sites. The subsequent combination of CO₂ and epoxide binding sites, as shown through theoretical findings, significantly improves the rate of cyclic carbonate formation, producing a highly active CO₂ utilization catalyst. An array of spectroscopic investigations, in combination with theoretical calculations reveal the nature of the active sites and associated catalytic mechanism which validates the careful design of the hybrid MIL-101(Cr).     

High-Z Metal–Organic Frameworks for X-ray Radiation-Based Cancer Theranostics

X-ray radiation is commonly employed in clinical practice for diagnostic and therapeutic applications. Over the past decade, developments in nanotechnology have led to the use of high-Z elements as the basis for innovative new treatment platforms that enhance the clinical efficacy of X-ray radiation. Nanoscale metal–frameworks (nMOFs) are coordination networks containing organic ligands that have attracted attention as therapeutic platforms in oncology and other areas of medicine. In cancer therapy, X-ray activated, high-Z nMOFs have demonstrated potential as radiosensitizers that increase local radiation dose deposition and generation of reactive oxygen species (ROS). This minireview summarizes current research on high-Z nMOFs in cancer theranostics and discusses factors that may influence future clinical application.     

Electroactive Metal–Organic Frameworks as Emitters for Self-Enhanced Electrochemiluminescence in Aqueous Medium

Metal–organic frameworks (MOFs) have limited applications in electrochemistry owing to their poor conductivity. Now, an electroactive MOF (E-MOF) is designed as a highly crystallized electrochemiluminescence (ECL) emitter in aqueous medium. The E-MOF contains mixed ligands of hydroquinone and phenanthroline as oxidative and reductive couples, respectively. E-MOFs demonstrate excellent performance with surface state model in both co-reactant and annihilation ECL in aqueous medium. Compared with the individual components, E-MOFs significantly improve the ECL emission due to the framework structure. The self-enhanced ECL emission with high stability is realized by the accumulation of MOF cation radicals via pre-reduction electrolysis. The self-enhanced mechanism is theoretically identified by DFT. The mixed-ligand E-MOFs provide a proof of concept using molecular crystalline materials as new ECL emitters for fundamental mechanism studies.     

Highly Efficient Electrochemical Nitrate Reduction to Ammonia in Strong Acid Conditions with Fe2M-Trinuclear-Cluster Metal–Organic Frameworks

Nitrate-containing industrial wastewater poses a serious threat to the global food security and public health safety. As compared to the traditional microbial denitrification, electrocatalytic nitrate reduction shows better sustainability with ultrahigh energy efficiency and the production of high-value ammonia (NH₃). However, nitrate-containing wastewater from most industrial processes, such as mining, metallurgy, and petrochemical engineering, is generally acidic, which contradicts the typical neutral/alkaline working conditions for both denitrifying bacteria and the state-of-the-art inorganic electrocatalysts, leading to the demand for pre-neutralization and the problematic hydrogen evaluation reaction (HER) competition and catalyst dissolution. Here, we report a series of Fe₂M (M=Fe, Co, Ni, Zn) trinuclear cluster metal–organic frameworks (MOFs) that enable the highly efficient electrocatalytic nitrate reduction to ammonium under strong acidic conditions with excellent stability. In pH=1 electrolyte, the Fe₂Co-MOF demonstrates the NH₃ yield rate of 20653.5 μg h⁻¹ mg⁻¹_site with 90.55 % NH₃-Faradaic efficiency (FE), 98.5 % NH₃-selectivity and up to 75 hr of electrocatalytic stability. Additionally, successful nitrate reduction in high-acidic conditions directly produce the ammonium sulfate as nitrogen fertilizer, avoiding the subsequent aqueous ammonia extraction and preventing the ammonia spillage loss. This series of cluster-based MOF structures provide new insights into the design principles of high-performance nitrate reduction catalysts under environmentally-relevant wastewater conditions.     

Mesoporous Metal–Organic Frameworks for Catalytic RNA Deprotection and Activation

A metal–organic framework (MOF) with mespores (2 to 50 nm) allows the inclusion of large biomolecules, such as nucleic acids. However, chemical reaction on the nucleic acids, to further regulate their bioactivity, is yet to be demonstrated within MOF pores. Here, we report the deprotection of carbonate protected RNA molecules (21 to 102 nt) to restore their original activity using a MOF as a heterogeneous catalyst. Two MOFs, MOF-626 and MOF-636 are designed and synthesized, with mesopores of 2.2 and 2.8 nm, respectively, carrying isolated metal sites (Ni, Co, Cu, Pd, Rh and Ru). The pores favor the entrance of RNA, while the metal sites catalyze C−O bond cleavage at the carbonate group. Complete conversion of RNA is achieved by Pd-MOF-626, 90 times more efficiently than Pd(NO₃)₂. MOF crystals are also removable from the aqueous reaction media, leaving a negligible metal footprint, 3.9 ppb, only 1/55 of that using homogeneous Pd catalysts. These features make MOF potentially suited for bioorthogonal chemistry.