Metal-organic frameworks (MOFs) are an emerging class of porous materials with potential applications in gas storage, separations, catalysis, and chemical sensing. Despite numerous advantages, applications of many MOFs are ultimately limited by their stability under harsh conditions. Herein, the recent advances in the field of stable MOFs, covering the fundamental mechanisms of MOF stability, design, and synthesis of stable MOF architectures, and their latest applications are reviewed. First, key factors that affect MOF stability under certain chemical environments are introduced to guide the design of robust structures. This is followed by a short review of synthetic strategies of stable MOFs including modulated synthesis and postsynthetic modifications. Based on the fundamentals of MOF stability, stable MOFs are classified into two categories: high-valency metal-carboxylate frameworks and low-valency metal-azolate frameworks. Along this line, some representative stable MOFs are introduced, their structures are described, and their properties are briefly discussed. The expanded applications of stable MOFs in Lewis/Brønsted acid catalysis, redox catalysis, photocatalysis, electrocatalysis, gas storage, and sensing are highlighted. Overall, this review is expected to guide the design of stable MOFs by providing insights into existing structures, which could lead to the discovery and development of more advanced functional materials.

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Epitaxial growth of MOF-on-MOF composite is an evolving research topic in the quest for multifunctional materials. In previously reported methods, the core-shell MOFs were synthesized via a stepwise strategy that involved growing the shell-MOFs on top of the preformed core-MOFs with matched lattice parameters. However, the inconvenient stepwise synthesis and the strict lattice-matching requirement have limited the preparation of core-shell MOFs. Herein, we demonstrate that hybrid core-shell MOFs with mismatching lattices can be synthesized under the guidance of nucleation kinetic analysis. A series of MOF composites with mesoporous core and microporous shell were constructed and characterized by optical microscopy, powder X-ray diffraction, gas sorption measurement, and scanning electron microscopy. Isoreticular expansion of microporous shells and orthogonal modification of the core was realized to produce multifunctional MOF composites, which acted as size selective catalysts for olefin epoxidation with high activity and selectivity.

The controlled synthesis of multicomponent metalorganic frameworks (MOFs) allows for the precise placement of multiple cooperative functional groups within a framework, leading to emergent synergistic effects. Herein, we demonstrate that turn-on fluorescence sensors can be assembled by combining a fluorophore and a recognition moiety within a complex cavity of a multicomponent MOF. An anthracenebased fluorescent linker and a hemicyanine-containing CN Àresponsive linker were sequentially installed into the lattice of PCN-700. The selective binding of CN À to hemicyanine inhibited the energy transfer between the two moieties, resulting in a fluorescence turn-on effect. Taking advantage of the high tunability of the MOF platform, the ratio between anthracene and the hemicyanine moiety could be fine-tuned in order to maximize the sensitivity of the overall framework. The optimized MOF-sensor had a CN À -detection limit of 0.05 mm, which is much lower than traditional CN À fluorescent sensors (about 0.2 mm).Metal-organic frameworks (MOFs), a highly crystalline, class of hybrid inorganic-organic materials, have attracted considerable attention in recent years for applications spanning gas storage, gas separation, catalysis, energy harvesting, biomedicine, and chemical sensing. [1] These materials are particularly attractive as they contain high inherent porosity, designable structures, and tunable physicochemical properties. As a result, MOFs stands out as one of the ideal platforms for the design of chemical sensors. [2] For example, luminescent MOFs have been extensively explored as sensors for metal cations, anions, and organic species. [3] Generally, the analytes interact with the MOF sensors through host-guest interactions or through energy transfer. The energy level of the lowest unoccupied molecular orbitals (LUMOs) for most analytes generally fall below the LUMOs or conduction bands (CBs) of most MOF materials. Thus, the electron transfer in luminescent MOFs occurs from the MOFs to the guest molecule. As a result, most MOF-based sensors are turn-off type systems, where the fluorescence of the MOF is quenched by the analyte by photoinduced electron or energy transfer. [4] This turn-off fluorescence sensing mechanism ultimately limits the sensitivity of the MOF for chemical sensing. Moreover, molecules with similar electronic properties can exert comparable quenching effects, making it difficult to have high selectivity for a particular analyte. To improve both the sensitivity and selectivity of MOF materials for chemical-sensing applications, detections based on a luminescence turn-on mechanism are highly desired.Molecular turn-on sensors can be designed by combining a fluorophore with a recognition moiety. [5] Initially, the electron or energy transfer occurs between the fluorophore and the recognition moiety, rendering the molecule nonfluorescent. The analyte triggers an electronic or conformational change of the recognition moiety, altering the intramolecular energy-transfer efficiency and resulting in turn-o...

Zirconium-based metal-organic frameworks (Zr-MOFs) exhibit great structural tunability and outstanding chemical stability, rendering them promising candidates for a wide range of practical applications. In this work, we synthesized a series of isostructural PCN-224 analogues functionalized by ethyl, bromo, chloro, and fluoro groups on the porphyrin unit, which allowed us to explicitly study the effects of electron-donating and electron-withdrawing substituents on catalytic performance in MOFs. Owing to the different electronic properties of ethyl, bromo, chloro, and fluoro substitutes, the molecular-level control over the chemical environment surrounding a catalytic center could be readily achieved in our MOFs. To investigate the effects of these substitutes on catalytic activity and selectivity, the oxidation of 3-methylpentane to corresponding alcohols and ketones was utilized as a model reaction. Within these five analogues of PCN-224, an extremely high turnover number of 7680 and turnover frequency of 10 240 h was achieved by simply altering the substitutes on porphyrin rings. Moreover, a remarkable 99% selectivity of the tertiary alcohol over the five other possible by-products are realized. We demonstrate that this strategy can be used to efficiently screen a suitable peripheral environment around catalytic cores in MOFs for catalysis.