GEt Quote

  • Introduction to MOFs & COFs
    Nov 15, 2017 | ACS MATERIAL LLC

    Porous organic materials display unique qualities that can potentially be used in various applications. New classes of these porous materials, such as metal-organic frameworks and covalent organic frameworks, have gained attention due to its distinctive attributes.  Thousands of new frameworks have since been discovered after its first discovery and further researched to gain more insight on their properties. This article will discuss these materials’ structure and synthesis as well as their promising benefits in different applications.

    Introduction

    Porous materials have played an important role in technology, but two classes of molecular sieves with distinctive qualities have been extensively studied for various applications such as gas storage and catalysts. After its first reports (Yaghi, et. al), metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) has revealed an expansion of porous layers with high thermal stability, permanent porosity and large surface area. By putting together molecular building units using strong bonds and forming them into extended structures, many new porous materials have since been discovered. In fact, a huge array of frameworks was realized and further researched to understand more of its properties and the applications that are possibly suitable.

    Covalent Organic Frameworks (COFs)

    COFs are well-defined 2D and 3D crystalline that are completely made up of light elements (i.e. oxygen, boron, nitrogen) and are connected by strong covalent bonds to form rigid, porous structures with pore sizes ranging between 7-27 Å. This architecture contains thermal stability and low densities with tunable pore sizes and structures that make COFs exceptional porous materials. Common frameworks are thermally stable above 300°C and are impervious to solvents that do not interfere with their linkages or stacking, making them more stable compared to most MOFs.1

    There are many different types of COFs -- one of them being the COF-LZU1 (Lan Zhou University-1), which is a 2D imine-based COF that possess stability with the pore size of 1.8 nm and a hexagonally-shaped channel and eclipsed-layered sheet structure.2 Another new, chemically stable COF-TpPa-1 consists of 1,3,5-triformylphloroglucinol (Tp) and p-pheylenediamine (Pa) and was designed using a combination of reversible and irreversible organic reactions. Its approximate aperture diameter is 15.8 Å but it can be reduced by incorporating functional groups.3 Meanwhile, the DAAQ-TFP COF is a redox-active 2,6-diaminoanthraquinone (DAAQ) formed with 1,3,5-triformylphloroglucinol (TFP) that is contained on each edge of its hexagonal polygons that assemble themselves into large hexagonal rings with single holes in the center.4 A crystal is then formed from stacks of hexagons with tiny pores and a surface area similar to activated carbons.5 

     COF-LZU1

    Figure 1. Synthesis route of ACS Material COF-LZU1.

    The required microscopic reversibility of the crystallization of linked organic molecules into solid form was first believed to be unattainable.6 Additionally, most synthesized COFs needed special care due to their moisture instability and previously attained COFs typically turned out to be irregularly shaped.  However, room-temperature solution-phase synthesis can be used to create a spherical COF for chromatic separation with large surface area and high thermostability.7 Error correction in forming covalent bonds reversibly was used to reticulate molecular building blocks into extended, crystalline COFs.  COFs have also been formed by reticular synthesis where strong bonding allowed for the removal of residual solvent molecules from these organic solids which resulted in highly porous 2D and 3D frameworks, a great advantage for creating these materials.8

    Metal-organic Frameworks (MOFs)

    MOFs are constructed by joining metal-containing units [secondary building units (SBUs), metal ions or clusters] with organic linkers using strong bonds to create open crystalline frameworks with permanent porosity.  Recent advancements have been made regarding its geometric construction by linking the SBUs with rigid shapes (i.e. squares and octahedral). Utilizing SBUs has varied the size and structure without changing its underlying topology. Incorporating organic units and metal-organic complexes through reactions with linkers have also surfaced for changing the reactivity of the pores. Since these structures were made from long organic linkers, they encompass void space and have been viewed to potentially be permanently porous, which is the case for zeolites.9

    Zeolitic imidazolate frameworks (ZIFs) are considered to be a subclass of MOFs that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions connected by imadazolate linkers formed through self-assembly. The structures of ZIFs are similar to aluminosilicate zeolites with the metal-imidazole-metal angle of approximately 145°. Since ZIFs simultaneously have characteristics of both zeolites and MOFs, they display properties that combine advantages from these materials.10

    MOFs and zeolites are mainly produced by hydrothermal or solvothermal synthesis in which reactions take place with temperatures typically above or below the boiling point of a solution; then, a slow formation of a crystalline MOF can be achieved. Various reaction temperatures influence product formation and more condense/dense structures are reported at higher temperatures.11 Similar to COFs, a reticular synthesis has recently been applied to produce MOFs. 

    Applications

    Due to its porous nature and tunable structure, MOFs and COFs are potentially able to work in selective gas storage and adsorption. MOFs can be redesigned to change their structure which have led to materials with ultrahigh porosity and H2 uptake capacities. Meanwhile, 3D COFs are considered to be new candidates in the quest for practical H2 storage materials than MOFs based on its higher surface area and free volume.  It has been reported that MOFs H2 uptakes have been the highest measured thus far at 7.0 wt % whereas COFs H2 uptakes can potentially be 10.0 wt %.12

    MOFs, especially ZIFs, are found to be great potentials as catalysts. MOFs can be used as active centers for organic catalysis and photocatalysis; in fact, homochiral MOFs can be synthesized relatively easily for asymmetric catalysts.13 Since MOFs have a very open architecture, mass transport in the pore system is not delayed and the ordered structure is able to spatially separate active centers which results in a more effective catalytic system.14 

    Conclusion

    MOFs and COFs are unique materials that triggered many new crystalline formations within their respective classes over the past decade. Based on their porous nature, they have great potential to be used in applications such as adsorbents and energy storage. Their unique structure and tunable features allow for thousands of MOFs and COFs to keep producing new properties that will lead to endless future possibilities.

    ACS Materials Products:

    COFs

    MOFs

     

    References

    1. Bisbey, Ryan P., and William R. Dichtel. “Covalent Organic Frameworks as a Platform for Multidimensional Polymerization.” ACS Central Science, vol. 3, no. 6, 2017, pp. 533–543., doi:10.1021/acscentsci.7b00127.

    2. Zhu, Guangshan, and Hao Ren. Porous Organic Frameworks Design, Synthesis and Their Advanced Applications. Springer Berlin Heidelberg, 2015.

    3. Zhang, Kang, et al. “Computational design of 2D functional covalent–organic framework membranes for water desalination.” Environmental Science: Water Research & Technology, The Royal Society of Chemistry, 25 May 2017.

    4. Xu, Qing, et al. “Charge Up in Wired Covalent Organic Frameworks.” ACS Central Science, American Chemical Society, 28 Sept. 2016.

    5. Service, Robert F. “New electric storage material could put more zip in your ...” Science, 21 Aug. 2015.

    6. Côté, Adrien P., et al. “Porous, Crystalline, Covalent Organic Frameworks.” Science, American Association for the Advancement of Science, 18 Nov. 2005. 

    7. Yang, Cheng-Xiong, et al. “Facile room-Temperature solution-Phase synthesis of a spherical covalent organic framework for high-Resolution chromatographic separation.” Chem. Commun., vol. 51, no. 61, 2015, pp. 12254–12257., doi:10.1039/c5cc03413b.

    8. Diercks, Christian S., and Omar M. Yaghi. “The atom, the molecule, and the covalent organic framework.” Science, vol. 355, no. 6328, Feb. 2017, doi:10.1126/science.aal1585.

    9. Furukawa, Hiroyasu, et al. “The Chemistry and Applications of Metal-Organic Frameworks.” Science, vol. 341, no. 6149, 30 Aug. 2013, doi:10.1126/science.1230444.

    10. Chen, Binling, et al. “Zeolitic imidazolate framework materials: recent progress in synthesis and applications.” J. Mater. Chem. A, vol. 2, no. 40, 2014, pp. 16811–16831., doi:10.1039/c4ta02984d.

    11. Stock, Norbert, and Shyam Biswas. “Synthesis of Metal-Organic Frameworks (MOFs): Routes to Various MOF Topologies, Morphologies, and Composites.” ChemInform, vol. 43, no. 16, 2012, doi:10.1002/chin.201216255.

    12. Han, Sang Soo, et al. “Covalent Organic Frameworks as Exceptional Hydrogen Storage Materials.” J. AM.CHEM. SOC., vol. 130, no. 35, 1 May 2008, doi:10.1021/ja803247y.

    13. Huang, Yuan-Biao, et al. “Multifunctional metal–organic framework catalysts: synergistic catalysis and tandem reactions.” Chem. Soc. Rev., vol. 46, no. 1, 2017, pp. 126–157., doi:10.1039/c6cs00250a.

    14. Czaja, Alexander U., et al. “Industrial applications of metal–organic frameworks.” Chemical Society Reviews, The Royal Society of Chemistry, 16 Mar. 2009