Metal-organic frameworks (MOFs) for energy production and gaseous fuel and electrochemical energy storage applications

The increasing energy demands in society and industrial sectors have inspired the search for alternative energy sources that are renewable and sustainable, also driving the development of clean energy storage and delivery systems. Various solid-state materials (e.g., oxides, sulphides, polymer and conductive nanomaterials, activated carbon and their composites) have been developed for energy production (water splitting-H2 production), gaseous fuel (H2 and CH4) storage and electrochemical energy storage (batteries and supercapacitors) applications. Nevertheless, the low surface area, pore volume and conductivity, and poor physical and chemical stability of the reported materials have resulted in higher requirements and challenges in the development of energy production and energy storage technologies. Thus, to overcome these issues, the development of metal-organic frameworks (MOFs) has attracted significant attention. MOFs are a class of porous materials with extremely high porosity and surface area, structural diversity, multifunctionality, and chemical and structural stability, and thus they can be used in a wide range of applications. In the present review, we precisely discuss the interesting properties of MOFs and the various methodologies for their synthesis, and also the future dependence on the valorization of solid waste for the recovery of metals and organic ligands for the synthesis of new classes of MOFs. Subsequently, the utilization of these interesting characteristics for energy production (water splitting), storage of gaseous fuels (H2 and CH4), and electrochemical storage (batteries and supercapacitors) applications are described. However, although MOFs are efficient materials with versatile uses, they still have many challenges, limiting their practical applications. Therefore, finally, we highlight the challenges associated with MOFs and show the way forward in overcoming them for the development of these highly porous materials with large-scale practical utility.

Rare-Earth-Modified Metal-Organic Frameworks and Derivatives for Photo/Electrocatalysis

Metal-organic frameworks (MOFs) and their derivatives have attracted much attention in the field of photo/electrocatalysis owing to their ultrahigh porosity, tunable properties, and superior coordination ability. Regulating the valence electronic structure and coordination environment of MOFs is an effective way to enhance their intrinsic catalytic performance. Rare earth (RE) elements with 4f orbital occupancy provide an opportunity to evoke electron rearrangement, accelerate charged carrier transport, and synergize the surface adsorption of catalysts. Therefore, the integration of RE with MOFs makes it possible to optimize their electronic structure and coordination environment, resulting in enhanced catalytic performance. In this review, progress in current research on the use of RE-modified MOFs and their derivatives for photo/electrocatalysis is summarized and discussed. First, the theoretical advantages of RE in MOF modification are introduced, with a focus on the roles of 4f orbital occupancy and RE ion organic coordination ligands. Then, the application of RE-modified MOFs and their derivatives in photo/electrocatalysis is systematically discussed. Finally, research challenges, future opportunities, and prospects for RE-MOFs are also discussed.

A 2D Dy-based metal-organic framework derived from benzothiadiazole: structure and photocatalytic properties

A 2D Dy(III) metal-organic layer (MOL 1) was synthesized under solvothermal conditions. Structural analysis suggests that the Dy(III) ions in each one-dimensional (1D) arrangement are evenly arranged in the form of broken lines. The 1D chains are linked to one another via ligands to form a 2D layer that generates a 2D surface with elongated apertures. The photocatalytic activity study suggests that MOL 1 exhibits good catalytic activity in flavonoids by the formation of an O₂˙^- radical as an intermediate. This is the first reported method of synthesizing flavonoids using chalcones.

Rare-Earth-Based Metal-Organic Frameworks as Multifunctional Platforms for Catalytic Conversion

The development of catalytic conversion is very important for human society. In the catalytic process, metal-organic frameworks (MOFs) can be utilized to obtain effective catalysts for their porous structures and adjustable properties. In addition, the introduction of rare-earth (RE) elements with unique properties for catalysts can realize good catalytic performances. Thus, the RE-MOF related catalysts for catalytic conversion are summarized. Due to the cooperation of RE elements and porous MOF structures, the RE-based MOFs can be used as promising catalysts or precursors/supports for other catalysts in the areas of energy conversion, environmental governance, and organic synthesis. These aggregated studies highlight the RE-MOFs as promising candidates for catalytic conversion.

Two metal-coordination polymers: Photocatalytic hydrogen production and clinical nursing value on gastric cancer combined with hyperthermic intraperitoneal chemotherapy by inducing induce oxidative stress response

Two new coordination polymers (CPs) of {[Dy₂(abtc)_1.5(H₂O)₃(DMA)]·H₂O·DMA}_n (1; DMA = N,N-dimethylacetamide) and [NH₂(CH₃)₂][Ni_1.5(abtc)]·H₂O (2) were synthesized through utilizing 3,3',5,5'-azobenzenetetracarboxylic acid (H₄abtc) as an organic connector with the solvothermal reaction conditions. The HOMO-LUMO gap for ultra-stable complex 1 is 2.16 eV through the detection of the ultraviolet-visible spectrum with an absorption edge of 570 nm. The complex 1 reveals high catalytic effect for the ultraviolet-visible photocatalytic production of hydrogen, which is due to its light collection of the ligand with dye-like and porous architecture. Furthermore, the promotion effect of compounds 1 and 2 against the gastric cancer treatment was assessed when combined with intraperitoneal hyperthermia. Firstly, the inhibition of compounds 1 and 2 against the invasion and migration of cancer cells was evaluated with transwell assay. Moreover, the genes relative expression levels related with the reactive oxygen species (ROS) production in the cancer cells were also measured with real time reverse transcription-polymerase chain reaction (RT-PCR).

Porphyrin Grafting on a Mercapto-Equipped Zr(IV)-Carboxylate Framework Enhances Photocatalytic Hydrogen Production

We employ facile aromatic nucleophilic substitution between the mercapto (-SH) and arylfluoro (Ar-F) groups to achieve extensive and robust cross-linking of a coordination host by porphyrin guests that also serve the purpose of versatile postsynthetic functionalization. For this, a tritopic linker with three trident-like thiol-flanked carboxyl units are reacted with ZrOCl₂·8H₂O to afford a two-dimensional (3,6-connected) net. The wide aperture of the porous framework solid, together with its stability in both air and boiling water, facilitates the entry of bulky metalloporphyrin guests and the subsequent property studies. On the porphyrin side, four pentafluorophenyl (C₆F₅-) groups offer multiple fluoro groups to facilitate their replacement by the thiol groups from the host net. The inserted metalloporphyrin bridges impart to the metal-organic framework (MOF) host stable and recyclable activities for photocatalytic hydrogen production. We also disclose an improvement in synthetic methodology, in which BBr₃ is used to simultaneously cleave the ester and benzyl thioether groups to more efficiently access thiol-equipped carboxylic acid building block.

Self-assembly of a cobalt(II)-based metal-organic framework as an effective water-splitting heterogeneous catalyst for light-driven hydrogen production

The solar photocatalysis of water splitting represents a significant branch of enzymatic simulation by efficient chemical conversion and the generation of hydrogen as green energy provides a feasible way for the replacement of fossil fuels to solve energy and environmental issues. We report herein the self-assembly of a Co^II-based metal-organic framework (MOF) constructed from 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetrabenzoic acid [or tetrakis(4-carboxyphenyl)ethylene, H₄TCPE] and 4,4'-bipyridyl (bpy) as four-point- and two-point-connected nodes, respectively. This material, namely, poly[(μ-4,4'-bipyridyl)[μ₈-4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetrabenzoato]cobalt(II)], [Co(C₃₀H₁₆O₈)(C₁₀H₈N₂)]_n, crystallized as dark-red block-shaped crystals with high crystallinity and was fully characterized by single-crystal X-ray diffraction, PXRD, IR, solid-state UV-Vis and cyclic voltammetry (CV) measurements. The redox-active Co^II atoms in the structure could be used as the catalytic sites for hydrogen production via water splitting. The application of this new MOF as a heterogeneous catalyst for light-driven H₂ production has been explored in a three-component system with fluorescein as photosensitizer and trimethylamine as the sacrificial electron donor, and the initial volume of H₂ production is about 360 µmol after 12 h irradiation.

Synthesis, structure and photocatalytic properties of coordination polymers based on pyrazole carboxylic acid ligands

The photocatalytic activities of two novel different 2-D coordination polymers constructed from 5-hydroxy-1H-pyrazole-3-carboxylic acid ligand have been explored.

Effective electron-hole separation over a controllably constructed WP/UiO-66/CdS heterojunction to achieve efficiently improved visible-light-driven photocatalytic hydrogen evolution

The photocatalytic decomposition of water to produce hydrogen is an important strategy to effectively utilize solar energy and solve the energy crisis. In this study, a highly efficient WP-nanoparticle-modified composite catalyst was successfully prepared. WP nanoparticles have been used as an efficient and acid-stable co-catalyst for the HER owing to their specific electronic structure, metalloid characteristics and catalytic activity. On the one hand, the octahedral spatial structure of UiO-66 not only provides attachment space for CdS and WP nanoparticles, but also effectively reduces the particle size and increases the dispersion of CdS and WP nanoparticles. On the other hand, the potential difference and the matching energy band positions of UiO-66 and CdS provide a feasible thermodynamic path for the transmission of photogenerated electrons. The intimate contact between the abovementioned three compounds resulted in a strong synergistic effect, which improved the efficiency of the photocatalytic H2 production. Under visible-light irradiation, the maximum H2 production in 5 h over the [UiO-66@CdS/WP (10 wt%)] photocatalyst was 395 μmol, which was 26.33 times that of pure CdS. The physical and chemical information of the samples could be obtained through XRD, SEM, TEM, XPS, BET and UV-vis DRS characterizations. Furthermore, based on the photoluminescence spectra, photoelectrochemical experiments and Mott-Schottky curves, we could reasonably explain the separation and transfer mechanisms of the photogenerated electrons and holes. The lower recombination rate of charge, enhanced intensity of light absorption, a short fluorescence lifetime (2.11 ns), a faster electron injection rate (KET = 2.32 × 108 s-1), a larger efficiency of electron injection (ηinj = 49.1%), high photocurrent response, and smaller charge transfer resistance accelerate the efficient separation and transfer of spatial charges, finally enhancing the photocatalytic performance.