The pore size enlargement and structural stability have been recognized as two crucial targets, which are rarely achieved together, in the development of metal–organic frameworks (MOFs). Herein, we ...have developed a versatile modulator‐induced defect‐formation strategy, in the presence of monocarboxylic acid as a modulator and an insufficient amount of organic ligand, successfully realizing the controllable synthesis of hierarchically porous MOFs (HP‐MOFs) with high stability and tailorable pore characters. Remarkably, the integration of high stability and large mesoporous property enables these HP‐MOFs to be important porous platforms for applications involving large molecules, especially in catalysis.
Cause and defect: Pore size enlargement and structural stability are two crucial targets but hardly achieved together in metal–organic frameworks (MOFs). A versatile modulator‐induced defect‐formation strategy has been developed to provide hierarchically porous MOFs (HP‐MOFs) with high stability and tailorable pore characters, which are important porous platforms for large‐molecule applications, especially in catalysis.
Metal-organic frameworks (MOFs) have been recognized as one of the most important classes of porous materials due to their unique attributes and chemical versatility. Unfortunately, some MOFs suffer ...from the drawback of relatively poor stability, which would limit their practical applications. In the recent past, great efforts have been invested in developing strategies to improve the stability of MOFs. In general, stable MOFs possess potential toward a broader range of applications. In this review, we summarize recent advances in the design and synthesis of stable MOFs and MOF-based materials
via de novo
synthesis and/or post-synthetic structural processing. Also, the relationships between the stability and functional applications of MOFs are highlighted, and finally, the subsisting challenges and the directions that future research in this field may take have been indicated.
This review summarizes recent advances in the design and synthesis of stable MOFs and highlights the relationships between the stability and functional applications.
Metal–organic frameworks (MOFs), also called porous coordination polymers, represent a class of crystalline porous materials built from organic linkers and metal ions/clusters. The unique features of ...MOFs, including structural diversity and tailorability as well as high surface area, etc., enable them to be a highly versatile platform for potential applications in many fields. Herein, an overview of recent developments achieved in MOF catalysis, including heterogeneous catalysis, photocatalysis, and eletrocatalysis over MOFs and MOF‐based materials, is provided. The active sites involved in the catalysts are particularly emphasized. The challenges, future trends, and prospects associated with MOFs and their related materials for catalysis are also discussed.
Metal–organic frameworks (MOFs), a class of crystalline porous materials, have allowed great progress in catalysis over the past two decades. An overview of recent developments for MOF catalysis, including heterogeneous organic reactions, photocatalysis, and electrocatalysis over MOFs and MOF‐based materials, is provided. The state‐of‐the‐art and opportunities and challenges regarding MOF‐based catalysis are also discussed.
Photocatalytic water splitting requires separation of the mixed H2 and O2 products and is often hampered by the sluggish O2‐producing half reaction. An approach is now reported to address these ...issues by coupling the H2‐producing half reaction with value‐added benzylamine oxidation reaction using metal–organic framework (MOF) composites. Upon MOF photoexcitation, the electrons rapidly reduce the protons to generate H2 and the holes promote considerable benzylamine oxidation to N‐benzylbenzaldimine with high selectivity. Further experimental characterizations and theoretical calculation reveal that the highly conjugated s‐triazine strut in the MOF structure is crucial to the efficient charge separation and excellent photocatalytic activity.
A metal–organic framework (MOF) composite (Pt/PCN‐777) has been prepared to achieve efficient proton reduction and selective benzylamine oxidation simultaneously under light irradiation. The enlarged π‐conjugation in the MOF ligand has been demonstrated to be crucial for improving the separation of charge carriers and thus greatly enhanced catalytic efficiency.
It remains highly desired but a great challenge to achieve atomically dispersed metals in high loadings for efficient catalysis. Now porphyrinic metal–organic frameworks (MOFs) have been synthesized ...based on a novel mixed‐ligand strategy to afford high‐content (1.76 wt %) single‐atom (SA) iron‐implanted N‐doped porous carbon (FeSA‐N‐C) via pyrolysis. Thanks to the single‐atom Fe sites, hierarchical pores, oriented mesochannels and high conductivity, the optimized FeSA‐N‐C exhibits excellent oxygen reduction activity and stability, surpassing almost all non‐noble‐metal catalysts and state‐of‐the‐art Pt/C, in both alkaline and more challenging acidic media. More far‐reaching, this MOF‐based mixed‐ligand strategy opens a novel avenue to the precise fabrication of efficient single‐atom catalysts.
Iron islands: Based on a mixed‐ligand strategy, a porphyrinic MOF was pyrolyzed to afford high‐content single‐atom iron‐implanted N‐doped porous carbon (FeSA‐N‐C). Thanks to the FeSA sites, hierarchical pores, oriented mesochannels, and high conductivity, FeSA‐N‐C exhibits excellent oxygen reduction activity and stability, surpassing almost all non‐noble‐metal catalysts and Pt/C, in both alkaline and the more challenging acidic media.
Defect engineering is a versatile approach to modulate band and electronic structures as well as materials performance. Herein, metal–organic frameworks (MOFs) featuring controlled structural ...defects, namely UiO‐66‐NH2‐X (X represents the molar equivalents of the modulator, acetic acid, with respect to the linker in synthesis), were synthesized to systematically investigate the effect of structural defects on photocatalytic properties. Remarkably, structural defects in MOFs are able to switch on the photocatalysis. The photocatalytic H2 production rate presents a volcano‐type trend with increasing structural defects, where Pt@UiO‐66‐NH2‐100 exhibits the highest activity. Ultrafast transient absorption spectroscopy unveils that UiO‐66‐NH2‐100 with moderate structural defects possesses the fastest relaxation kinetics and the highest charge separation efficiency, while excessive defects retard the relaxation and reduce charge separation efficiency.
Volcano‐type trend: A series of metal–organic frameworks (MOFs) decorated with Pt nanoparticles, Pt@UiO‐66‐NH2‐X, were fabricated with increasing levels of structural defects in the MOF to investigate how defect levels affect photocatalysis. The catalysts exhibit an impressive volcano‐type trend in H2 production, maximizing at a moderate defect level.
The built‐in electric field can be generated in the piezoelectric materials under mechanical stress. The resulting piezoelectric effect is beneficial to charge separation in photocatalysis. ...Meanwhile, the mechanical stress usually gives rise to accelerated mass transfer and enhanced catalytic activity. Unfortunately, it remains a challenge to differentiate the contribution of these two factors to catalytic performance. Herein, for the first time, isostructural metal–organic frameworks (MOFs), i.e., UiO‐66‐NH2(Zr) and UiO‐66‐NH2(Hf), are adopted for piezo‐photocatalysis. Both MOFs, featuring the same structures except for diverse Zr/Hf‐oxo clusters, possess distinctly different piezoelectric properties. Strikingly, UiO‐66‐NH2(Hf) exhibits ≈2.2 times of activity compared with that of UiO‐66‐NH2(Zr) under simultaneous light and ultrasonic irradiation, though both MOFs display similar activity in the photocatalytic H2 production without ultrasonic irradiation. Given their similar pore features and mass transfer behaviors, the activity difference is unambiguously assignable to the piezoelectric effect. As a result, the contributions of the piezoelectric effect to the piezo‐photocatalysis can be clearly distinguished owing to the stronger piezoelectric property of UiO‐66‐NH2(Hf).
Two isostructural metal–organic frameworks (MOFs) with distinctly different piezoelectric responses are used in piezo‐photocatalysis. Remarkably, the H2 production efficiency of Hf‐MOF is 2.2 times that of Zr‐MOF under simultaneous light and ultrasonic irradiation. The role of the piezoelectric effect can be distinguished owing to their similar pore features and mass transfer behaviors.
Metal‐organic frameworks (MOFs) have been shown to be an excellent platform in photocatalysis. However, to suppress electron–hole recombination, a Pt cocatalyst is usually inevitable, especially in ...photocatalytic H2 production, which greatly limits practical application. Herein, for the first time, monodisperse, small‐size, and noble‐metal‐free transitional‐metal phosphides (TMPs; for example, Ni2P, Ni12P5), are incorporated into a representative MOF, UiO‐66‐NH2, for photocatalytic H2 production. Compared with the parent MOF and their physical mixture, both TMPs@MOF composites display significantly improved H2 production rates. Thermodynamic and kinetic studies reveal that TMPs, behaving similar ability to Pt, greatly accelerate the linker‐to‐cluster charge transfer, promote charge separation, and reduce the activation energy of H2 production. Significantly, the results indicate that Pt is thermodynamically favorable, yet Ni2P is kinetically preferred for H2 production, accounting for the higher activity of Ni2P@UiO‐66‐NH2 than Pt@UiO‐66‐NH2.
Monodisperse, small‐size, and noble‐metal‐free cocatalysts of transition‐metal phosphides, such as Ni2P and Ni12P5, have been incorporated into metal‐organic frameworks. Thermodynamic and kinetic studies demonstrate that Pt as a cocatalyst is thermodynamically favorable, yet Ni2P is kinetically preferred in photocatalytic H2 production. Taking all factors into account, Ni2P exhibits an even better photocatalytic H2‐production activity than Pt.
Metal–organic frameworks (MOFs) have been intensively studied as a class of semiconductor‐like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor ...photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL‐125‐NH2, is integrated with the metal oxides (MoO3 and V2O5) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built‐in electric field of MIL‐125‐NH2, boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H2 production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor‐like nature, which would greatly promote MOF photocatalysis.
A representative metal–organic framework (MOF), MIL‐125‐NH2, was integrated with MoO3 or V2O5. Given their suited work functions and energy levels, band bending of the MOF occurs, giving rise to improved charge separation. Accordingly, the activity of the MOF composites toward photocatalytic H2 production is considerably enhanced. This is an unprecedented report with evidence on semiconductor‐like band bending of MOFs.
Conspectus To meet the ever-increasing global demand for energy, conversion of solar energy to chemical/thermal energy is very promising. Light-mediated catalysis, including photocatalysis (organic ...transformations, water splitting, CO2 reduction, etc.) and photothermal catalysis play key roles in solar to chemical/thermal energy conversion via the light–matter interaction. The major challenges in traditional semiconductor photocatalysts include insufficient sunlight utilization, charge carrier recombination, limited exposure of active sites, and particularly the difficulty of understanding the structure–activity relationship. Metal–organic frameworks (MOFs), featuring semiconductor-like behavior, have recently captured broad interest toward photocatalysis and photothermal catalysis because of their well-defined and tailorable porous structures, high surface areas, etc. These advantages are beneficial for rational structural modulation for improved light harvesting and charge separation as well as other effects, greatly helping to address the aforementioned challenges and especially facilitating the establishment of the structure–activity relationship. Therefore, it is increasingly important to summarize this research field and provide in-depth insight into MOF-based photocatalysis and photothermal catalysis to accelerate the future development. In this Account, we have summarized the recent advances in these two directly relevant applications, photocatalysis and photothermal catalysis, mainly focusing on the results in our lab. Given the unique structural features of MOFs, we have put an emphasis on rational material design to optimize the components and performance and to understand related mechanisms behind the enhanced activity. This Account starts by presenting an overview of solar energy conversion by catalysis. We explain why MOFs can be promising photocatalysts and exemplify the semiconductor-like behavior of MOFs. More importantly, we show that MOFs provide a powerful platform to study photocatalysis, in which the involved three key processes, namely, light harvesting, electron–hole separation, and surface redox reactions, can be rationally improved. Meanwhile, the structure–activity relationship and charge separation dynamics are illustrated in this part. In addition, MOFs for photothermal catalysis have been introduced that are based on the photothermal effect of plasmonic metals and/or MOFs, together with light-driven electronic state optimization of active sites, toward enhanced heterogeneous organic reactions. Finally, our brief outlooks on the current challenges and future development of MOF photocatalysis and photothermal catalysis are provided. It is believed that this Account will afford significant understanding and inspirations toward solar energy conversion over MOF-based catalysts.
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