Research
Research topics:1. Metal-Organic Frameworks
2. Metal-Organic Polyhedra (MOPs)
3. MOFs for Selective Gas Adsorption and Separation(Particularly for CO2 Capture)
4. MOF-Based Mesh-Adjustable Molecular Sieves
5. Hydrogen Storage in MOFs
6. Methane Storage in MOFs
7. Mesoporous MOFs
8. From MOPs to MOFs
9. Porous Polymer Networks (PPNs)
10. Metal-Organic Materials-Mesoporous Silica Composites
11. Light responsive MOFs
12. MOFs with Tuneable Magnetic Properties
I. Metal-Organic Frameworks(MOFs)
Also known as porous coordination polymers, are crystalline frameworks consisting of metal ions (or clusters) and organic ligands. In some cases, pores inside an open MOF are stable after removal of guest molecules (often solvents) and the MOF can be used for storage of gases such as hydrogen, methane, and carbon dioxide. Other possible applications of MOFs include gas purification and separation, catalysis, drug-delivery, gas-sensing, optics, and photovoltaics.
Representative articles:
"Syntheses, characterization, and photoluminescence of isostructural Mn, Co, and Zn MOFs having a diamondoid structure with large tetrahedral cages and high thermal stability" Sun, D., et al.. Chem. Commun. 2005, 2663.
"Potential applications of metal-organic frameworks" Kuppler, Ryan J., et al.. Coord. Chem. Rev. 2009, 253, 3042.
"Tuning the topology and functionality of metal-organic frameworks by ligand design" Zhao, D. et al.. Accounts Chem. Res. 2010, 44, 123.
“Isomerism in Metal-Organic Frameworks: “Framework Isomers””, Makal, T. A., et al.. J. Phys. Chem. Lett. 2011, 2, 1682.
“A Route to Metal-Organic Frameworks through Framework-Templating”, Wei, Z.; Lu, W.; Jiang, H.-L; Zhou, H.-C., Inorg. Chem., 2013, 52 (3), 1164 - 1166.
II. Metal-Organic Polyhedra (MOPs)
Metal-organic polyhedra (MOPs)–discrete molecular architectures constructed through the coordination of metal ions and organic linkers–have recently attracted considerable attention due to their intriguing structures, potential for a variety of applications, and relevance to biological self-assembly. Quite a few synthetic routes have been investigated to prepare these complexes, but to date these preparative methods have typically been based on the direct assembly of metal ions and organic linkers. Our recent work demonstrates a synthetic strategy based on the substitution of bridging ligands in soluble MOPs. The introduction of linkers with different properties from those of initial MOPs can thus lead to new MOPs with distinct properties (such as size and shape). Other synthetic approaches are also exploring for the preparation of these complex molecular systems.
Representative articles:
"Metal-Organic Hendecahedra Assembled from Dimetal-Paddlewheel Nodes and Mixtures of Ditopic Linkers with 120 and 90° Bend-Angles", Li, J.-R. and Zhou, H.-C. Angew. Chem. Int. Ed. 2009, 48, 8465.
"Bridging-Ligand-Substitution Strategy for the Preparation of Metal–Organic Polyhedra", Li, J.-R. and Zhou, H.-C., Nature Chem. 2010, 2, 893.
"Ligand Bridging-Angle-Driven Assembly of Molecular Architectures Based on Quadruply Bonded Mo−Mo Dimers", Li, J.-R., et al.. J. Am. Chem. Soc. 2010, 132, 17599.
III. MOFs for Selective Gas Adsorption and Separation(Particularly for CO2 Capture)
Adsorptive separation is very important in industry. Generally, the process uses porous solid materials such as zeolites, activated carbons, and silica gels as adsorbents. With ever increasing need for efficient, energy-saving, and environmentally benign procedure for gas separation, adsorbents with tailored structures and tunable surface properties must be found. Metal-organic frameworks (MOFs) are promising candidates as adsorbents or membrane filters for separations due to their large surface areas, adjustable pore size and controllable properties, and acceptable thermal stability. Our efforts in this topic include 1) the synthesis of new MOFs with designed pore sizes, shapes and surface functionalities for selective adsorption and separation, 2) the screen of reported MOFs for selective adsorption and separation, 3) the exploration of the mechanism of selective adsorption in MOFs, 4) the investigation of related separation process, 5) the exploitation of MOF-based membranes for gas separations.
Representative articles:
"Ultramicroporous Metal-Organic Framework Based on 9,10-Anthracenedicarboxylate for Selective Gas Adsorption" Ma, S., et al.. Inorg. Chem. 2007, 46, 8499.
"Carbon Dioxide Capture-Related Gas Adsorption and Separation in Metal-Organic Frameworks", Li, J.-R., et al.. Coord. Chem. Rev. 2011, 255, 1791.
"Metal-Organic Frameworks for Separations", Li, J.-R. , et al.. Chem. Rev. 2012, doi: 10.1021/cr200190s.
IV. MOF-Based Mesh-Adjustable Molecular Sieves
Inorganic zeolite molecular sieves are currently the most commonly used adsorbents in industry for gas separation. The rigidity of the bonds in zeolites affords them with fixed mesh sizes, which is advantageous when the mesh size precisely fits the separation needs. However, when the size disparity of the two gases is very small, a zeolite molecular sieve with the precise mesh size is not always readily available. In such cases, mesh-adjustable molecular sieves (MAMSs) that can always meet the separation needs are highly desirable. We have developed the MOF-based mesh-adjustable molecular sieves. The mesh size is linearly related to temperature. Therefore, by precisely controlling the temperature, one can achieve an accurate pore-size, which is essential for a clean separation of a gas mixture.
Representative article:
"A mesh-adjustable molecular sieve for general use in gas separation." Ma, S., et al.. Angew. Chem. Int. Ed. 2007, 46, 2458.
"Preparation and Gas Adsorption Studies of Three Mesh-Adjustable Molecular Sieves with a Common Structure" Ma, S., et al.. J. Am. Chem. Soc. 2009, 131, 6445.
V. Hydrogen Storage in MOFs
For any potential hydrogen-storage system, raw uptake capacity must be balanced with the kinetics and thermodynamics of uptake and release. MOFs provide unique systems with large overall pore volumes and surface areas, adjustable pore sizes, and tunable framework–adsorbate interaction by ligand functionalization and metal choice. These remarkable materials can potentially fill the niche between other physisorbents such as activated carbon, which have similar uptake at low temperatures but low affinity for hydrogen at ambient temperature, and chemical sorbents such as hydrides, which have high hydrogen uptakes but undesirable release kinetics and thermodynamics.
Representative articles:
"A metal-organic framework with entatic metal centers exhibiting high gas-adsorption affinity." Ma, S. and Zhou, H.-C. J. Am. Chem. Soc. 2006, 128, 11734.
"An interweaving MOF with high hydrogen uptake." Sun, D., et al.. J. Am. Chem. Soc. 2006, 128, 3896.
“The current status of hydrogen storage in metal-organic frameworks-updated”, Sculley, J., et al.. Energy Environ. Sci. 2011, 4, 2721.
VI. Methane Storage in MOFs
In the search for alternative fuels to decrease the dependence on fossil fuels hydrogen and methane lead the pack. However, the difficulty in storing such gases has led to an inability to fully utilize these fuels. The incorporation of metal-organic frameworks (MOFs), with their high porosities and surface areas, in standard gas cylinders can significantly enhance the storage density of these gases, compared to an empty tank at the same temperature and pressure. Currently, methane exhibits much higher packing density and heat of adsorption in MOFs than hydrogen, making it a much more feasible fuel for incorporation into vehicular fuel systems in the near-future. Several MOFs have shown very promising sorption properties at operating conditions, 25-30 °C and 30 bar, with current efforts focused on further optimizing the materials by tuning pore size and enhancing stability to impurities, such as water.
Representative articles:
"Metal-organic framework from an anthracene derivative containing nanoscopic cages exhibiting high methane uptake." Ma, S., et al.. J. Am. Chem. Soc. 2008, 130, 1012.
“An Isoreticular Series of Metal-Organic Frameworks with Dendritic Hexa-Carboxylate Ligands and Exceptionally High Gas Uptake Capacity” Yuan, D., et al.. Angew. Chem. Int. Ed. 2010, 49, 5357.
"Metal-Organic Frameworks with Exceptionally High Methane Uptake: Where and How is Methane Stored?" Wu, H., et al.. Chem.-Eur. J. 2010, 16, 5205.
VII. Mesoporous MOFs
According to the IUPAC definition, a mesoporous material contains pores between 2 and 50 nm in diameter and often exhibits type IV or V sorption isotherm with notable gas-sorption hysteresis. Most of the reported open MOFs are microporous (with pore sizes <2 nm). The preparation of mesoporous MOFs poses a great challenge because crystals of a mesoporous MOF tend to disintegrate once removed from their mother liquor. Recently in our laboratory, by using a newly designed ligand containing hierarchical functional groups, a non-interpenetrated mesoporous MOF was prepared and stabilized by acid treatment. Mesoporous MOF may find its way in applications such as gas storage, separation, gas-sensing, photovoltaics, and catalysis.
Representative article:
"A mesoporous metal-organic framework with permanent porosity." Wang, X.-S., et al.. J. Am. Chem. Soc. 2006, 128, 16474.
"Functional Mesoporous Metal-Organic Frameworks for the Capture of Heavy Metal Ions and Size-Selective Catalysis", Fang, Q.-R., et al.. Inorg. Chem. 2010, 49, 11637.
“Recent Advances in the Study of Mesoporous Metal-Organic Frameworks” Fang, Q.-R., et al.. Comments on Inorg. Chem. 2010, 31, 165.
VIII. From MOPs to MOFs
Supramolecular chemistry has developed into one of the most popular research areas in the last two decades. Most of the discrete or infinite architectures have been constructed from molecular or ionic entities by coordination bonds, hydrogen bonds, or other interactions. Among them, biological assemblies are usually imitated, especially in H-bonded systems, for which it is possible to design step-by-step reactions leading to complex architectures of discrete molecular components. In contrast, the stepwise synthesis of analogous metal-organic frameworks is difficult because the intermediates, such as MOPs, are normally insoluble. Our efforts in this endeavor are focusing on the design and synthesis of soluble and extendable MOPs, along with the rational construction of metal-organic frameworks by using these molecules as building blocks.
Representative articles:
"Interconversion between Molecular Polyhedra and Metal-Organic Frameworks" Li, J.-R., et al.. J. Am. Chem. Soc. 2009, 131, 6368.
IX. Porous Polymer Networks (PPNs)
Compared with MOFs, porous organic polymers offer superior stability by replacing the susceptible coordination bond with robust covalent bond. By judiciously selecting monomers and reactions, our Group synthesized a series of PPNs with high surface areas with different pore size distribution. Among them, PPN-4 shows record-high porosity and exceptionally high gas-storage capacity. Most strikingly, PPN-4 retains its porosity even after exposed to air for a month, largely due to the covalent bonding in the network scaffold as opposed to coordination bonding for other high-surface-area porous materials. Future research will focus on tuning their porosity via judiciously choosing monomers and reaction conditions that cater to different application requirements.
Representative articles:
“Highly Stable Porous Polymer Networks with Exceptionally High Gas-Uptake Capacities”, Yuan, D., et al.. Adv. Mater. 2011, 23, 3723.
"Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation", Lu, W., et al.. Chem. Mater. 2010, 22, 5964.
X. Metal-Organic Materials-Mesoporous Silica Composites
In addition to MOFs and MOPs, another group of porous materials that receives considerable interest are mesoporous silicas. To date an incredible degree of control has been achieved on silica with various pore symmetries. These silica materials contain ordered pores on a nano scale, which makes them valuable as supports to disperse a variety of guests ranged from metal and oxide nanoparticles to peptide and drug macromolecules. When MOFs or MOPs are introduced into silica nanopores, they are highly dispersed, and active sites become more accessible. Moreover, owing to the special microenvironment in silica nanopores, such confinement can enhance the stability of metal-organic systems, and in the meanwhile, modify the mesopores of silica to give unique properties and functions desirable for applications in guest adsorption and catalysis.
Representative articles:
"Confinement of Metal-Organic Polyhedra in Silica Nanopores", Sun, L.-B.; Li, J.-R.; Lu, W.; Gu, Z.-Y.; Luo, Z.; Zhou, H.-C., J. Am. Chem. Soc., 2012, 134, 15923−15928.
“Cooperative Template-Directed Assembly of Mesoporous Metal-Organic Frameworks” Sun, L.-B.; Li, J.-R.; Park, J.; Zhou, H.-C. J. Am. Chem. Soc. 2012, 134 (1), 126–129.
XI. Light responsive MOFs
One of the most popular light-responsive functional groups applied is azobenzene. Triggered by UV irradiation or heat, an azobenzene group can undergo trans to cis isomerization reversibly. Based on this conformational change, introduction of an optically or thermally switchable functional group such as azobenzene to the internal surface of an overall rigid MOF can tune both the selectivity and regeneration efficiency upon light irradiation or with thermal treatment. As an experimental proof-of-concept, a photoswitchable MOF adsorbent with adjustable carbon dioxide uptakes has been demonstrated experimentally. The MOFs adsorbs a significant amount of CO2, but negligible amount of N2. Upon light irradiation, CO2 molecules are released readily due to the change of conformation of the azobenzene groups inside the pores of the MOF. The adsorbent returns to its original state when allowed staying at ambient conditions for a prolonged period of time or with gentle heating, which can be obtained from solar energy as well.
Representative articles:
“Reversible Alteration of CO2 Adsorption upon Photochemical or Thermal Treatment in a Metal-Organic Framework” Park, J.; Yuan, D.; Pham, K.
T.; Li , J.-R.; Yakovenko, A.; Zhou, H. –C. J. Am. Chem. Soc. 2012, 134, 99–102.
XII. MOFs with Tuneable Magnetic Properties
Recently, interest in investigations on the synthesis of magnetic open-framework structures has increased because one of the most important advantages of such materials is the fact that the deintercalation, reintercalation, or exchange of the guest molecules in the host structure offers the opportunity to switch or even to modulate their magnetic properties in a desired way. Consequently, a few compounds were reported that show guest-sensitive spin-crossover, guest-modulated magnetic ordering temperatures, and guest-induced switching between different magnetic states. Among them, magnetic coordination materials have been prepared, in which paramagnetic transition metal ions are bridged by appropriate small-sized ligands, whose nature is one of the most important factors influencing their magnetic properties. In view of these considerations we started systematic investigation on magneto-structural correlations of a series of new metal-organic framework materials with guest-responsive magnetic properties.
Representative articles:
"Reversible Switching from Antiferro- to Ferromagnetic Behavior by Solvent-Mediated, Thermally-Induced Phase Transitions in a Trimorphic MOF-based Magnetic Sponge System", Mario Wriedt, Andrey A. Yakovenko, Gregory J. Halder, Andrey V. Prosvirin, Kim R. Dunbar & Hong-Cai Zhou, Journal of the American Chemical Society, 2013, accepted.
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