Metal–organic frameworks (MOFs) are a category of porous materials that offer unparalleled control over their surface areas (demonstrated as higher than for any other material), pore characteristics, ...and functionalization. This allows them to be customized for exceptional performance in a wide variety of applications, most commonly including gas storage and separation, drug delivery, luminescence, or heterogeneous catalysis. In order to optimize biomimicry, controlled separations and storage of small molecules, and detailed testing of structure–property relationships, one major goal of MOF research is “rational design” or “pore engineering”, or precise control of the placement of multiple functional groups in pores of chosen sizes and shapes. MOF crystal growth can be controlled through judicious design of stepwise synthetic routes, which can also allow functionalization of MOFs in ways that were previously synthetically inaccessible. Organic chemists have developed a library of powerful techniques over the last century, allowing the total synthesis and detailed customization of complex molecules. Our hypothesis is that total synthesis is also possible for customized porous materials, through the development of similar multistep techniques. This will enable the rational design of MOFs, which is a major goal of many researchers in the field. We have begun developing a library of stepwise synthetic techniques for MOFs, allowing the synthesis of ultrastable MOFs with multiple crystallographically ordered and customizable functional groups at controlled locations within the pores. In order to design MOFs with precise control over pore size and shape, stability, and the placement of multiple different functional groups within the pores at tunable distances from one another, we have concentrated on methods which allow us to circumvent the lack of control inherent to one-pot MOF crystallization. Kinetically tuned dimensional augmentation (KTDA) is an approach using preformed metal clusters as starting materials and monotopic carboxylates as equilibrium shifting agents to make single crystals of ultrastable MOFs. Postsynthetic metathesis and oxidation (PSMO) takes advantage of the fast ligand exchange rate of a metal ion at the low oxidation state as well as the kinetic inertness of the same metal at high oxidation state to make ultrastable and highly crystalline MOFs. Multiple similar strategies have been successful for the metathesis of Fe-based MOFs to Cr3+. Several highly crystalline Ti-MOFs have also been prepared. Kinetically controlled linker installation and cluster metalation methods utilize a stable MOF with inherent coordinatively unsaturated sites as matrix and postsynthetically install linkers or grow clusters on the matrix, so that a robust MOF with precisely placed functionalities is realized. This method has diverse applications especially when specific functional groups or metals having synergistic effects are desired in the proper proximity. Exceptional porosity and stability are required for many potential applications. We have demonstrated several of these, including entrapment of nanoscaled functional moieties such as enzymes. We have developed a series of metal–organic frameworks (PCN-333) with rationally designed ultralarge mesoporous cages as single-molecule traps for enzyme encapsulation. We successfully incorporated metalloporphyrins, well-known biofunctional moieties, into robust MOFs for biomimetic catalytic applications. By rationally tuning the synthetic conditions, we obtained several different porphyrinic Zr-, Fe-, and Ti-MOFs with distinct pore size and concentrated acid or base stability, which offer eligible candidates for different applications. These and other stepwise kinetic tuning and catalyst incorporation methods are small steps toward achieving the grand challenge of detailed control of the placement of matter on an atomic and molecular level.
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One of the most pressing environmental concerns of our age is the escalating level of atmospheric CO2. Intensive efforts have been made to investigate advanced porous materials, especially porous ...organic polymers (POPs), as one type of the most promising candidates for carbon capture due to their extremely high porosity, structural diversity, and physicochemical stability. This review provides a critical and in‐depth analysis of recent POP research as it pertains to carbon capture. The definitions and terminologies commonly used to evaluate the performance of POPs for carbon capture, including CO2 capacity, enthalpy, selectivity, and regeneration strategies, are summarized. A detailed correlation study between the structural and chemical features of POPs and their adsorption capacities is discussed, mainly focusing on the physical interactions and chemical reactions. Finally, a concise outlook for utilizing POPs for carbon capture is discussed, noting areas in which further work is needed to develop the next‐generation POPs for practical applications.
Significant progress has been made in the exploration of porous organic polymers (POPs) as potential porous solid adsorbents for carbon capture. A detailed correlation study between the structural and chemical features of POPs and their adsorption capacities is discussed, mainly focusing on physical interactions and chemical reactions.
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A major goal of metal-organic framework (MOF) research is the expansion of pore size and volume. Although many approaches have been attempted to increase the pore size of MOF materials, it is still a ...challenge to construct MOFs with precisely customized pore apertures for specific applications. Herein, we present a new method, namely linker labilization, to increase the MOF porosity and pore size, giving rise to hierarchical-pore architectures. Microporous MOFs with robust metal nodes and pro-labile linkers were initially synthesized. The mesopores were subsequently created as crystal defects through the splitting of a pro-labile-linker and the removal of the linker fragments by acid treatment. We demonstrate that linker labilization method can create controllable hierarchical porous structures in stable MOFs, which facilitates the diffusion and adsorption process of guest molecules to improve the performances of MOFs in adsorption and catalysis.
Metal-organic frameworks (MOFs) are constructed from metal ions/clusters coordinated by organic linkers (or bridging-ligands). The hallmark of MOFs is their permanent porosity, which is frequently ...found in MOFs constructed from metal-clusters. These clusters are often formed
in situ
, whereas the linkers are generally pre-formed. The geometry and connectivity of a linker dictate the structure of the resulting MOF. Adjustments of linker geometry, length, ratio, and functional-group can tune the size, shape, and internal surface property of a MOF for a targeted application. In this critical review, we highlight advances in MOF synthesis focusing on linker design. Examples of building MOFs to reach unique properties, such as unprecedented surface area, pore aperture, molecular recognition, stability, and catalysis, through linker design are described. Further search for application-oriented MOFs through judicious selection of metal clusters and organic linkers is desirable. In this review, linkers are categorized as ditopic (Section 1), tritopic (Section 2), tetratopic (Section 3), hexatopic (Section 4), octatopic (Section 5), mixed (Section 6), desymmetrized (Section 7), metallo (Section 8), and N-heterocyclic linkers (Section 9).
Advances in metal-organic frameworks are highlighted with an emphasis on tuning the structure and function
via
linker design.
A series of mesoporous metalloporphyrin Fe-MOFs, namely PCN-600(M) (M = Mn, Fe, Co, Ni, Cu), have been synthesized using the preassembled Fe3O(OOCCH3)6 building block. PCN-600 exhibits a ...one-dimensional channel as large as 3.1 nm and the highest experimental pore volume of 1.80 cm3g–1 among all the reported porphyrinic MOFs. It also shows very high stability in aqueous solutions with pH values ranging from 2–11 and is to our knowledge the only mesoporous porphyrinic MOF stable under basic aqueous conditions. PCN-600(Fe) has been demonstrated as an effective peroxidase mimic to catalyze the co-oxidation reaction.
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Enzymatic catalytic processes possess great potential in chemical manufacturing, including pharmaceuticals, fuel production and food processing. However, the engineering of enzymes is severely ...hampered due to their low operational stability and difficulty of reuse. Here, we develop a series of stable metal-organic frameworks with rationally designed ultra-large mesoporous cages as single-molecule traps (SMTs) for enzyme encapsulation. With a high concentration of mesoporous cages as SMTs, PCN-333(Al) encapsulates three enzymes with record-high loadings and recyclability. Immobilized enzymes that most likely undergo single-enzyme encapsulation (SEE) show smaller Km than free enzymes while maintaining comparable catalytic efficiency. Under harsh conditions, the enzyme in SEE exhibits better performance than free enzyme, showing the effectiveness of SEE in preventing enzyme aggregation or denaturation. With extraordinarily large pore size and excellent chemical stability, PCN-333 may be of interest not only for enzyme encapsulation, but also for entrapment of other nanoscaled functional moieties.
We successfully assembled the photocatalytic titanium-oxo cluster and photosensitizing porphyrinic linker into a metal-organic framework (MOF), namely PCN-22. A preformed titanium-oxo carboxylate ...cluster is adopted as the starting material to judiciously control the MOF growth process to afford single crystals. This synthetic method is useful to obtain highly crystalline titanium MOFs, which has been a daunting challenge in this field. Moreover, PCN-22 demonstrated permanent porosity and photocatalytic activities toward alcohol oxidation.
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A truly cost‐effective strategy for the synthesis of amine‐tethered porous polymer networks (PPNs) has been developed. A network containing diethylenetriamine (PPN‐125‐DETA) exhibits a high working ...capacity comparable to current state‐of‐art technology (30 % monoethanolamine solutions), yet it requires only one third as much energy for regeneration. It has also been demonstrated to retain over 90 % capacity after 50 adsorption–desorption cycles of CO2 in a temperature‐swing adsorption process. The results suggest that PPN‐125‐DETA is a very promising new material for carbon capture from flue gas streams.
Energy‐efficient carbon capture: A truly cost‐efficient strategy for the synthesis of amine‐tethered porous polymer networks has been developed. A network containing diethylenetriamine exhibits a high working capacity comparable to current state‐of‐art technology (30 % monoethanolamine solution), yet it requires only one third as much energy for regeneration. It has also been demonstrated that no capacity loss occurs after 50 adsorption–desorption cycles of CO2 in a temperature‐swing adsorption process.
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Knowledge about the interactions between gas molecules and adsorption sites is essential to customize metal-organic frameworks (MOFs) as adsorbents. The dynamic interactions occurring during ...adsorption/desorption working cycles with several states are especially complicated. Even so, the gas dynamics based upon experimental observations and the distribution of guest molecules under various conditions in MOFs have not been extensively studied yet. In this work, a direct time-resolved diffraction structure envelope (TRDSE) method using sequential measurements by in situ synchrotron powder X-ray diffraction has been developed to monitor several gas dynamic processes taking place in MOFs: infusion, desorption, and gas redistribution upon temperature change. The electron density maps indicate that gas molecules prefer to redistribute over heterogeneous types of sites rather than to exclusively occupy the primary binding sites. We found that the gas molecules are entropically driven from open metal sites to larger neighboring spaces during the gas infusion period, matching the localized-to-mobile mechanism. In addition, the partitioning ratio of molecules adsorbed at each site varies with different temperatures, as opposed to an invariant distribution mode. Equally important, the gas adsorption in MOFs is intensely influenced by the gas–gas interactions, which might induce more molecules to be accommodated in an orderly compact arrangement. This sequential TRDSE method is generally applicable to most crystalline adsorbents, yielding information on distribution ratios of adsorbates at each type of site.
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