You probably already know that molecules are the smallest building blocks of the compounds, what makes up our world around us. They each have their own unique properties, making them our interest in material science. Rather than haphazardly looking for and discovering new materials and utilize their properties, material scientists instead aim to understand materials fundamentally to create new materials with the desired properties. Metal-organic frameworks - this newly arisen area of chemistry - may provide an eternal aspiration for them.
Metal-organic frameworks, or MOFs, have emerged as an extensive class of crystalline materials with ultrahigh porosity (up to 90% free volume) and enormous internal surface areas, extending beyond 6000 m2/g. To give you a better understanding of what this actually means, a typical basketball court has 420 m2, while an ordinary soccer field exhibits 7000 m2, similar to a single gram of most MOFs.
Figure 1. Illustration of MOF’s enormous surface area compared
to a basketball court & a soccer field
These materials are created by joining metal-containing units [secondary binding units (SBUs)] with organic linkers, using strong bonds (reticular synthesis) to construct open crystalline frameworks with permanent porosity.
Figure 2. MOF constructed from SBUs & organic linkers
These properties, together with the extraordinary degree of variability for both the organic and inorganic components of their structures, make MOFs of interest for potential applications in clean energy, most significantly as storage media for gases such as hydrogen and methane, and as high-capacity adsorbents to meet various separation needs. In particular, applications in energy technologies such as fuel cell membranes, supercapacitors, and catalytic conversions have made them objects of extensive study, continuously gaining importance. On a fundamental level, MOFs illustrate the beauty of chemical structures and the power of combining organic and inorganic chemistry, two concepts often regarded as distinct.
Among the many developments made in this field, we will focus on the gas adsorption properties of MOFs. Because of their extraordinarily high surface areas, tunable pore size, and adjustable internal surface properties, they are suited for applications in gas storage and separations stemming from the characteristic adsorptive properties of MOFs.
One of the most popular topics in MOF research has been carbon dioxide capture, which is directly related to clean energy without pollution and environmental protection. A group of researchers at CSIRO have created a "solar sponge" that captures and then releases carbon dioxide when exposed to natural sunlight - as published in the scientific journal Angewandte Chemie. Zn(AzDC)(4,4'-BPE)0.5, the MOF used in this research, incorporates photochromic organic linkers (AzDC, 4,4'-BPE) that can undergo clean and efficient reversible cis-trans photoisomerizability when coordinated to a metal complex.
Figure 3. Dynamic photo-switching in the light-responsive
MOF Zn(AzDC)(4,4’-BPE)0.5 leads to instantly reversible CO2 uptake.
Because of this, light irradiation increases the MOF surface energy, in which intermolecular interactions between CO2 molecules and the surface weakened, and thus triggered instantaneous CO2 release. The following graph shows the effect of light on the MOF's CO2 uptakes.
Figure 4. CO2 adsorption isotherms of Zn(AzDC)(4,4’-BPE)0.5 at 303 K
in the presence of light (red), absence of light (black)
Hydrogen storage is also one of the key applications of MOF. Hydrogen being one of the most promising candidates for the replacement of current carbon-based energy sources, a tremendous amount of research has been done in the hydrogen storage properties in MOF. The H2 adsorption capacities at 77 K and high pressures (up to 100 atm) have a qualitative relationship with the surface areas of MOFs: as surface area increases, the H2 capacity at 77 K also increases. However, in order to apply a MOF as a H2 gas storage material, the MOF should store large amounts of H2 at ambient temperature. However, most MOFs have 5~12 kJ/mol of isosteric heat of H2 adsorption, which is too small for H2 storage at ambient temperature (15~25 kJ/mol). To overcome this, creating open metal sites (OMSs) at the metal cluster nodes or in the organic linkers was tried. The highest isosteric heat of H2 adsorption reported so far is 15 kJ/mol for a Co(II) MOF where every Co(II) site contains OMS with a Co-Co distance appropriate for the side-on interaction with a H2 molecule. However, for MOFs containing OMSs, the exposure to moisture results in coordinating the water molecules from the air, which leads to reduced H2 uptake. Some MOFs are even decomposed in the presence of moisture due to the unstable SBUs, while most reported MOFs are unstable to acids and bases and decompose immediately upon contact. Therefore, to develop MOFs as H2 storage materials that meet the U.S. DOE targets for an onboard hydrogen system, a serious challenge is still present in the design and synthesis of the MOF materials.
Utilizing this gas adsorption property, researchers at MIT created a device called 'solar-powered harvester' and published the research in Science. Only in the prototype phase, this device was able to pull 2.8 liters of water from the air over a 12-hour period. This device's water-harvesting ability comes from MOF-801’s temperature-dependent water adsorption capacities.
Figure 5. Water-adsorption isotherms of MOF-801, measured at 25°C and 65°C
As ambient air diffuses through the MOF crystals, water molecules attach to the interior surfaces. Sunlight then heats the MOF up and pushes the bound water towards the condenser, which is the same temperature as the outside air. This vapor condenses as liquid water and drips into a collector to provide clean drinking water.
Figure 6. The schematic illustrates the vapor adsorption and desorption experiments carried out under isobaric condition / Representative temperature profiles for the MOF-801 layer (experimental, red solid line; predicted, red dashed line), ambient air (gray line), the condenser (blue line), and the ambient dew point (green line), as well as solar flux (purple line)
Right now, the MOF can only absorb 20 percent of its weight in water, but other MOF materials could potentially absorb more than 40 percent.
The chemistry and applications of MOFs have developed substantially since their original inception more than a decade ago. Although it is difficult to rule out the possibility of major advancements arising from MOFs that are constructed from a limited number of building blocks, the future of MOFs lies in the creation of materials whose constituents are many and are systematically varied.
작성자 : 이민석
1. Zhou, H.-C.; Long, J. R.; Yaghi, O. M., Introduction to Metal–Organic Frameworks. Chemical Reviews 2012, 112 (2), 673-674.
2. Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M., The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341 (6149).
3. Lyndon, R.; Konstas, K.; Ladewig, B. P.; Southon, P. D.; Kepert, P. C. J.; Hill, M. R., Dynamic Photo-Switching in Metal–Organic Frameworks as a Route to Low-Energy Carbon Dioxide Capture and Release. Angewandte Chemie 2013, 125 (13), 3783-3786.
4. Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W., Hydrogen Storage in Metal–Organic Frameworks. Chemical Reviews 2012, 112 (2), 782-835.
5. Kim, H.; Yang, S.; Rao, S. R.; Narayanan, S.; Kapustin, E. A.; Furukawa, H.; Umans, A. S.; Yaghi, O. M.; Wang, E. N., Water harvesting from air with metal-organic frameworks powered by natural sunlight. Science 2017, 356 (6336), 430-434.
6. Soaking up CO2 emissions, Science Alert
7. Scientists Have Created a Device That Sucks Water Out of Thin Air, Even in The Desert, Science Alert
3, 4. Lyndon, R.; Konstas, K.; Ladewig, B. P.; Southon, P. D.; Kepert, P. C. J.; Hill, M. R., Dynamic Photo-Switching in Metal–Organic Frameworks as a Route to Low-Energy Carbon Dioxide Capture and Release. Angewandte Chemie 2013, 125 (13), 3783-3786.
5, 6. Kim, H.; Yang, S.; Rao, S. R.; Narayanan, S.; Kapustin, E. A.; Furukawa, H.; Umans, A. S.; Yaghi, O. M.; Wang, E. N., Water harvesting from air with metal-organic frameworks powered by natural sunlight. Science 2017, 356 (6336), 430-434.