From fundamentals to applications: a toolbox for robust and multifunctional MOF materials

The thermally induced decarboxylation mechanism of a mixed-oxidation state carboxylate-based iron metal-organic framework

Investigations into a thermally generated decarboxylation mechanism for metal site activation and the generation of mesopores in a carboxylate iron-based MOF, PCN-250, have been conducted. PCN-250 exhibits an interesting oxidation state change during thermal treatment under inert atmospheres or vacuum conditions, transitioning from an Fe(iii)₃ cluster to a Fe(ii)Fe(iii)₂ cluster. To probe this redox event and discern a mechanism of activation, a combination of thermogravimetric analysis, gas sorption, scanning electron microscopy, ⁵⁷Fe Mössbauer spectroscopy, gas chromatography-mass spectrometry, and X-ray diffraction studies were conducted. The results suggest that the iron-site activation occurs due to ligand decarboxylation above 200 °C. This is also consistent with the generation of a missing cluster mesoporous defect in the framework. The resulting mesoporous PCN-250 maintains high thermal stability, preserving crystallinity after multiple consecutive high-temperature regeneration cycles. Additionally, the thermally reduced PCN-250 shows improvements in the total uptake capacity of methane and CO₂.

Computer-aided discovery of connected metal-organic frameworks

Composite metal-organic frameworks (MOFs) tend to possess complex interfaces that prevent facile and rational design. Here we present a joint computational/experimental workflow that screens thousands of MOFs and identifies the optimal MOF pairs that can seamlessly connect to one another by taking advantage of the fact that the metal nodes of one MOF can form coordination bonds with the linkers of the second MOF. Six MOF pairs (HKUST-1@MOF-5, HKUST-1@IRMOF-18, UiO-67@HKUST-1, PCN-68@MOF-5, UiO-66@MIL-88B(Fe) and UiO-67@MIL-88C(Fe)) yielded from our theoretical predictions were successfully synthesized, leading to clean single crystalline MOF@MOF, demonstrating the power of our joint workflow. Our work can serve as a starting point to accelerate the discovery of novel MOF composites that can potentially be used for many different applications.

Prospects for electroactive and conducting framework materials

Electroactive and conducting framework materials, encompassing coordination polymers and metal-organic frameworks, have captured the imagination of the scientific community owing to their highly designable nanoporous structures and their potential applications in electrochromic devices, electrocatalysts, porous conductors, batteries and solar energy harvesting systems, among many others. While they are now considered integral members of the broader field of inorganic materials, it is timely to reflect upon their strengths and challenges compared with 'traditional' solid-state materials such as minerals, pigments and zeolites. Indeed, the latter have been known since ancient times and have been prized for centuries in fields as diverse as art, archaeology and industrial catalysis. This opinion piece considers a brief historical perspective of traditional electroactive and conducting inorganic materials, with a view towards very recent experimental progress and new directions for future progress in the burgeoning area of coordination polymers and metal-organic frameworks. Overall, this article bears testament to the rich history of electroactive solids and looks at the challenges inspiring a new generation of scientists. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'.

Synthesis and Functionalization of Porous Zr-Diaminostilbenedicarboxylate Metal-Organic Framework for Storage and Stable Delivery of Ibuprofen

A stable porous metal-organic framework (MOF), Zr-diaminostilbenedicarboxylate (Zr-DASDCA), was synthesized and modified with oxalyl chloride (OC) or terephthaloyl chloride (TC) to introduce various functional groups onto the Zr-DASDCA. Both pristine and functionalized Zr-DASDCAs, together with activated carbon, were used as a potential carrier for ibuprofen (IBU) storage and delivery. Zr-DASDCAs, especially the modified ones (OC-Zr-DASDCA and TC-Zr-DASDCA), showed competitive results in IBU delivery. Specifically, the release rate in phosphate-buffered saline solution at pH 7.4 was nearly constant (R 2 ≈ 0.98) for up to 10 days, which would be very effective in IBU dosing to the human body. Moreover, the release rate could be controlled by changing the pH of the releasing solution. The rate of IBU release from both pristine and modified Zr-DASDCAs at pH 7.4 and 3.0 was also explained with a few interactions such as H-bonding and electrostatic repulsion, together with the relative pore size of the Zr-DASDCAs. Therefore, the results suggested that functionalization of MOFs via postsynthetic modification, especially with OC and TC, to introduce various functional groups onto MOFs is an effective approach to not only reducing the release rate of IBU but also inducing a constant release of IBU for as long as 10 days.

Zr-Based Metal-Organic Frameworks with Intrinsic Peroxidase-Like Activity for Ultradeep Oxidative Desulfurization: Mechanism of H2O2 Decomposition

The restriction of sulfur content in fuels has become increasingly stringent as a result of the growing environmental concerns. Although several MOF-derived materials like POM@MOF composites have shown the ability to catalyze oxidative desulfurization (ODS), their catalytic activities inevitably obstructed by the encapsulated catalytic sites like POM due to the blockage of cavities. Therefore, MOFs with intrinsic and accessible catalytic sites are highly desirable for their applications in ultradeep ODS. Herein, four representative Zr-based MOFs (Zr-MOFs), namely, UiO-66, UiO-67, NU-1000, and MOF-808, were assessed for catalytic ODS. These MOFs were confirmed that they have peroxidase-like activity and can catalyze ODS with H₂O₂ as oxidant. Among them, MOF-808 showed the highest catalytic activity and it can fully desulfurize dibenzothiophene (DBT) in a model gasoline with a S concentration of 1000 ppm under 40 °C within 5 min. An extremely low apparent Arrhenius activation energy (22.0 KJ·mol^-1) and an extraordinarily high TOF value (42.7 h^-1) were obtained, ranking MOF-808 among the best catalysts for the catalytic DBT oxidation. Further studies confirmed that the excellent catalytic activity is mainly responsible for the high concentration of the accessible Zr-OH(H₂O) catalytic sites decorated in MOF-808. The superoxide radicals (^•O₂^-) and hydroxyl radicals (^•OH) were identified and were proved to involve in the DBT oxidation. Besides, the effects of Brönsted and lewis acidity to the catalytic efficiency were also discussed. Based on the experimental results, a plausible mechanism concerning on Zr-OH(H₂O) groups promoting the H₂O₂ decomposion in to both ^•O₂^- and ^•OH was first proposed. Moreover, MOF-808 can be facilely reused for at least eight runs without significant loss of its catalytic activity. By the integration of facile synthesis, high catalytic efficiency, and good stability, MOF-808 thus represents a new benchmark catalyst for catalytic oxidative desulfurization.

Hybrid Catalysts for CO2 Conversion into Cyclic Carbonates

The conversion of carbon dioxide into valuable chemicals such as cyclic carbonates is an appealing topic for the scientific community due to the possibility of valorizing waste into an inexpensive, available, nontoxic, and renewable carbon feedstock. In this regard, last-generation heterogeneous catalysts are of great interest owing to their high catalytic activity, robustness, and easy recovery and recycling. In the present review, recent advances on CO2 cycloaddition to epoxide mediated by hybrid catalysts through organometallic or organo-catalytic species supported onto silica-, nanocarbon-, and metal–organic framework (MOF)-based heterogeneous materials, are highlighted and discussed.

Metal-Organic Framework Nanoparticles with Near-Infrared Dye for Multimodal Imaging and Guided Phototherapy

From the conception of atom economy to develop multifunctional nanomaterials, it is important to construct nanomaterials by maximizing functional units while minimizing unnecessary components. Noteworthy, metal-organic framework (MOF) nanoparticles are excellent examples to meet this idea. Current approaches for multifunctional MOFs are mainly based on encapsulation of functional molecules or multistep modification; however, high risk for leakage and burst release and time-consuming and complicated organic synthesis limit their applications. Here, we report a one-pot approach to build the defect structure of a metal organic framework with near-infrared dye (cypate), which is based on the interaction between Fe³⁺ and carboxyl group of cypate molecules, to construct a multifunctional MOF. Moreover, this system can achieve multimodal imaging guided phototherapy. Subsequently, the precise cancer phototherapy is investigated in vivo, and the tumors are entirely eliminated without obvious side effects, demonstrating the high efficacy and safety of this multifunctional platform. Hence, it is expected that not only this system is simple, safe, and highly effective but also our method of creating defect structures of MOFs will open a new way to develop multifunctional nanoplatforms for bioapplications.

Structural Transformation in Metal-Organic Frameworks for Reversible Binding of Oxygen

Polycyclic aromatic derivatives can trap ¹ O₂ to form endoperoxides (EPOs) for O₂ storage and as sources of reactive oxygen species. However, these materials suffer from structural amorphism, which limit both practical applications and fundamental studies on their structural optimization for O₂ capture and release. Metal-organic frameworks (MOFs) offer advantages in O₂ binding, such as clear structure-performance relationships and precise controllability. Herein, we report the reversible binding of O₂ is realized via the chemical transformation between anthracene-based and the corresponding EPO-based MOF. It is shown that anthracene-based MOF, the framework featuring linkers with polycyclic aromatic structure, can rapidly trap ¹ O₂ to form EPOs and can be restored upon UV irradiation or heating to release O₂ . Furthermore, we confirm that photosensitizer-incorporated anthracene-based MOF are promising candidates for reversible O₂ carriers controlled by switching Vis/UV irradiation.

Highly Selective and Reversible Sulfur Dioxide Adsorption on a Microporous Metal-Organic Framework via Polar Sites

It is very challenging to achieve efficient and deep desulfurization, especially in flue gases with an extremely low SO₂ concentration. Herein, we report a microporous metal-organic framework (ELM-12) with specific polar sites and proper pore size for the highly efficient SO₂ removal from flue gas and other SO₂-containing gases. A high SO₂ capacity of 61.2 cm³·g^-1 combined with exceptionally outstanding selectivity of SO₂/CO₂ (30), SO₂/CH₄ (871), and SO₂/N₂ (4064) under ambient conditions (i.e., 10:90 mixture at 298 K and 1 bar) was achieved. Notably, the SO₂/N₂ selectivity is unprecedented among ever reported values of porous materials. Moreover, the dispersion-corrected density functional theory calculations illustrated the superior SO₂ capture ability and selectivity arise from the high-density SO₂ binding sites of the CF₃SO₃^- group in the pore cavity (S^δ+···O^δ- interactions) and aromatic linkers in the pore walls (H^δ+···O^δ- interactions). Dynamic breakthrough experiments confirm the regeneration stability and excellent separation performance. Furthermore, ELM-12 is also stable after exposure to SO₂, water vapor, and organic solvents.