Commentary pubs.acs.org/accounts
Future Porous Materials Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Susumu Kitagawa* Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT: Developing science and technology of porous materials provides fuels and useful substances from ubiquitous gaseous substances such as air.
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networks, called porous coordination polymers (PCPs) or metal−organic frameworks (MOFs).1 The advantages of such materials include the formation of a regular porous structure via fundamental building blocks in a bottom-up manner and superior designability. Consequently, MOFs are well-researched metal complexes.2 Until the late-1990s, removing the guest molecules in coordination network compounds caused the porous structure to collapse. These are known as the first generation compounds but were not called MOFs because they lacked permanent porosity. The second generation compounds have stable and robust frameworks, which maintain the original porous structures before and after guest sorption. The secondgeneration compounds can be used as an adsorbent and are regarded as analogous to zeolites. They are so-called MOFs, which were developed in the late 1990s and maintained their porous structures without the presence of guest molecules.3 Since then, studies have focused on three features (topology of the spatial structure, porous surface area, and porosity), and diverse MOF structures have been realized (e.g., diamond and halite crystal structures), including those with a huge porous surface area beyond 7000 m2 g−1 or a high porosity. Due to these outstanding features compared to conventional materials, MOFs were received with surprise. Since then, research insight on MOFs has accumulated at an accelerated speed toward their practical application. The third generation MOFs possess flexible or dynamic porous frameworks,4,5 which reversibly respond to external stimuli, not only chemical but also physical, unlike the previous
he industrial revolution in the 19th century spawned technologies that consume huge amounts of energy. Although this energy was initially supplied by coal (solid), energy in the 20th century was provided by oil (liquid). However, the 21st century society is evolving into one that gets its energy from gases, such as natural gas and biogas. As the oil reservoir becomes more depleted, the importance of gases will increase. Ultimately ubiquitous gases, including ambient air with carbon, oxygen, and nitrogen, will serve as energy and chemical resources. On the other hand, to employ such gases, some hurdles must be overcome. First, the inherent properties of gases (e.g., easily dispersed, readily creates mixtures, low concentration) make it difficult to handle. Second, the transportation, storage, and conversion of gaseous materials are extremely energy-intensive (e.g., at high pressure or very low temperature). To overcome such hurdles, science and technology to manipulate gases under mild conditions (e.g., room temperature, low pressure, and low energy consumption) are demanded. I think that creating fuels and useful substances from ubiquitous gaseous molecules should be the “Holy Grail” in the field of chemistry. I believe that nanosized spaces are suitable for this purpose. Materials with such spaces are referred to as porous materials. Although conventional porous materials (e.g., activated carbon and zeolite) have been indispensable for a long time, they cannot be used to achieve the aforementioned goal. To create a paradigm shift in energy sources, society needs to develop novel materials with porous functions. In the late 1990s, a brand-new synthetic concept realized metal complex materials composed of organic molecules and inorganic metal ions. These materials have infinite coordination © 2017 American Chemical Society
Received: October 1, 2016 Published: March 21, 2017 514
DOI: 10.1021/acs.accounts.6b00500 Acc. Chem. Res. 2017, 50, 514−516
Commentary
Accounts of Chemical Research
material development, this Commentary focuses on porous materials. Here I pose a simple question. Are such advanced characteristics even possible? Are new methodologies for synthesis and fabrication developed for the purposes? This answer is gradually being elucidated. The high designability and diverse synthetic methodologies of MOFs discovered in recent years suggest that such a feat is feasible. Revolutionary approaches2,8−12 to MOFs employ coordination modulation, coordination replication, multivariate/solid solution, post synthesis, phase control among crystals, reversible transformation of crystal to glass and liquid, layer-by-layer epitaxial growth, synthesis at air/liquid or liquid/liquid interface, exfoliation, chemical vapor deposition, mechanochemical synthesis, and microwave synthesis. Below, I would like to discuss the status of development considering some of these methodologies.
generations. They were developed in an effort to realize dynamic porous and collective functionality not found in conventional materials. Their compositions of metal ions and organic molecules have achieved diversity in the electronic states. That is, the spatial and electronic structures can be altered, realizing magnetic and dielectric properties as well as oxidation−reduction functions. Besides normal storage, such MOFs have vast potential for separation with an extremely high selectivity, high-efficiency storage, and catalysis, as well as sensing and actuator functions. For these reasons, many studies investigate these materials. In this Commentary, I discuss porous materials with capabilities that exceed current ones (i.e., the fourth generation MOFs) and the future research direction. It would be fabulous if novel porous materials possessed more features than just the third generation’s excellent characteristics (flexibility, collectivity, and diversity) (Figure 1). These additional features include
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MELTING BROADENS PCP/MOF SCIENCE (DOUBLE-H AND DOUBLE-D)
Because MOFs are a crystal solid in which the basic components are infinitely linked, heating may induce a phase transition but should not cause melting. Recently, a solid− liquid reversible phase transition and an amorphous phase were discovered in MOFs. Utilization of such phenomena to manipulate the crystal or to form a membrane should facilitate determining the orientation and the position of the elements within membranes and crystals as well as altering their shapes.
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DOWNSIZING LEADS TO NEW PROPERTIES (DOUBLE-D AND DOUBLE-A)
Current research seems to emphasize matter down to 10 nm (e.g., nanoparticles). On the other hand, mesoscale science is between the nano- and microscales as well as between quantum and classical sciences. It provides a gateway from the “silent” (one nanosized molecule or particle) research area to the “noisy” (a large collective molecular system) one. However, mesoscale science remains terra incognita due to difficulties in synthesis, manipulation, and observation. Thus, it is only natural to ponder what is the current situation of porous materials in mesoscale science. Typically syntheses yield bulk crystals with varying sizes on the micrometer scale. Does downsizing the crystals to several dozen to several hundred nanometers influence the characteristics? A previous study provides an example to this question. The bulk crystal of [Cu2(bdc)2(bpy)] (bdc = 1,4-benzenedicarboxylate, bpy = 4,4′-bipyridine) displays a structural change between an open form with guest molecules in its pores and a closed form without guest molecules. On the other hand, when its crystal size is reduced to around 100 nm, the open form is maintained even when the guest molecules are removed; this crystal exhibits a memory effect.8 Although the mechanism has yet to be elucidated, it is possible that the porous function depends on the disorder and the defects.13 Additionally, an open gate function of a MOF was recently found when a threedimensional structure (pore-closed form) is downsized into a two-dimensional thin film (pore-open form). This phenomenon should be noted as a novel function that appears when the three-dimensional structure is downsized into a two-dimensional thin film.
Figure 1. Third generation MOFs have three attributes of collectivity, diversity, and flexibility. The fourth generation porous materials possessing HAD attributes that exceed current ones and the future research direction.
(1) Hierarchy and Hybrid (double-H), which means to combine different functions and pursue the dynamic development of combined functions, (2) Anisotropy and Asymmetry (double-A), which means to learn from living organisms and then go beyond such organisms’ capabilities, and (3) Disorder and Defect (double-D), which may lead to excellent catalytic reactivities and electronic functions. Hereinafter these three characteristics are referred to collectively as “HAD” characteristics. Double-A is a particularly big challenge. Membrane formation is expressed as a structure formation along two axes, exhibiting anisotropy regarding a residual axis. Increased emphasis should be on the study of formation of membranes of MOF crystals.6 Cell membranes realize anisotropic transport by developing an active transport system against the concentration gradient of target ions and molecules; creating this behavior in synthetic matter is the ultimate challenge of material sciences. On the other hand, molecular recognition utilizing the asymmetric porous environment gives rise to precisive chiral synthesis, a prerequisite for MOF’s catalytic functions.7 The development of synthetic chemistry toward the realization of the HAD characteristics is highly promising. Although the HAD characteristics must be considered not only for porous material development but also for other 515
DOI: 10.1021/acs.accounts.6b00500 Acc. Chem. Res. 2017, 50, 514−516
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HYBRIDIZATION OF MOFS WITH OTHER MATERIALS CREATES NEW WORLDS (DOUBLE-H AND DOUBLE-A) Hybridization of MOF crystals and other materials can enhance or create novel functions. In particular, hybridization with organic polymers14 is highly promising because it provides fundamental insight about MOFs and has yielded a separation membrane that has almost reached the practical-use phase. On the other hand, hybridization with inorganic matter (e.g., Al2O3 and Ti2O3) aiming at the precise positioning of MOFs onto different inorganic platforms allows the hierarchized or asymmetric spatial structures in which micro-, meso-, and macropores coexist8,15 Polymer synthesis in channels of MOFs can control the structure and aggregate state of polymers, resulting in polymer characteristics that have never before been realized.16 For example, when polystyrene and poly(methyl methacrylate) are synthesized within a MOF, the resulting mixture demonstrates a stable compatibility and does not separate into different phases. As discussed above, if wide ranging forms of MOFs (e.g., macro- and mesosized crystals, amorphous state, crystals, and films) can be synthesized and integrated into a form of sequential functions as in Figure 2, targeted gas substances
ACKNOWLEDGMENTS This work is supported by the ACCEL project of the Japan Science and Technology Agency (JST) and KAKENHI Grantin-Aid for Specially Promoted Research (No. 25000007) from the Japan Society of the Promotion of Science (JSPS).
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REFERENCES
(1) Batten, S. R.; Champness, N. R.; Chen, X.-M.; Garcia-Martinez, J.; Kitagawa, S.; Ö hrström, L.; O’Keeffe, M.; Paik Suh, M.; Reedijk, J. Terminology of metal−organic frameworks and coordination polymers (IUPAC Recommendations 2013). Pure Appl. Chem. 2013, 85, 1715−1724. (2) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The chemistry and applications of metal-organic frameworks. Science 2013, 341, 1230444. (3) Kitagawa, S.; Kondo, M. Functional Micropore Chemistry of Crystalline Metal Complex-Assembled Compounds. Bull. Chem. Soc. Jpn. 1998, 71, 1739−1753. (4) Horike, S.; Shimomura, S.; Kitagawa, S. Soft porous crystals. Nat. Chem. 2009, 1, 695−704. (5) Ferey, G. Giant flexibility of crystallized organic-inorganic porous solids: facts, reasons, effects and applications. New J. Chem. 2016, 40, 3950−3967. (6) Qiu, S.; Xue, M.; Zhu, G. Metal-organic framework membranes: from synthesis to separation application. Chem. Soc. Rev. 2014, 43, 6116−40. (7) Yoon, M.; Srirambalaji, R.; Kim, K. Homochiral Metal−Organic Frameworks for Asymmetric Heterogeneous Catalysis. Chem. Rev. 2012, 112, 1196−1231. (8) Furukawa, S.; Reboul, J.; Diring, S.; Sumida, K.; Kitagawa, S. Structuring of metal-organic frameworks at the mesoscopic/macroscopic scale. Chem. Soc. Rev. 2014, 43, 5700−5734. (9) Lu, W.; Wei, Z.; Gu, Z. Y.; Liu, T. F.; Park, J.; Park, J.; Tian, J.; Zhang, M.; Zhang, Q.; Gentle, T., 3rd; Bosch, M.; Zhou, H. C. Tuning the structure and function of metal-organic frameworks via linker design. Chem. Soc. Rev. 2014, 43, 5561−5593. (10) James, S. L.; Frišcǐ ć, T. Mechanochemistry. Chem. Soc. Rev. 2013, 42, 7494−7496. (11) Bennett, T. D.; Cheetham, A. K. Amorphous metal-organic frameworks. Acc. Chem. Res. 2014, 47, 1555−1562. (12) Zacher, D.; Shekhah, O.; Woell, C.; Fischer, R. A. Thin films of metal−organic frameworks. Chem. Soc. Rev. 2009, 38, 1418−1429. (13) Fang, Z.; Bueken, B.; De Vos, D. E.; Fischer, R. A. DefectEngineered Metal−Organic Frameworks. Angew. Chem., Int. Ed. 2015, 54, 7234−7254. (14) Zhang, Z.; Nguyen, H. T.; Miller, S. A.; Cohen, S. M. polyMOFs: A Class of Interconvertible Polymer-Metal-OrganicFramework Hybrid Materials. Angew. Chem., Int. Ed. 2015, 54, 6152−6157. (15) Falcaro, P.; Ricco, R.; Doherty, C. M.; Liang, K.; Hill, A. J.; Styles, M. J. MOF positioning technology and device fabrication. Chem. Soc. Rev. 2014, 43, 5513−5560. (16) Uemura, T. Supramolecular Approaches towards Ordered Polymer Materials. Chem. - Eur. J. 2014, 20, 1482−1489. (17) Dhakshinamoorthy, A.; Asiri, A. M.; Garcia, H. Metal−Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production. Angew. Chem., Int. Ed. 2016, 55, 5414− 5445.
Figure 2. An anisotropic gas substance stream channel system created by integrated MOFs with (1) capture and separation, (2) condensation and storage, (3) catalytic conversion using renewable energy such as light,17 and (4) efficient transport. Double H attributes are key to realize the system, where any one of the MOFs could be replaced by other porous materials. Particularly, these functions of porous materials are inevitable to the usage of carbon dioxide from the air.
could be selectively captured, condensed, and converted ad arbitrium, which is currently not possible, and therefore science and technology utilizing ubiquitous resources can advance. In particular, fundamental techniques that convert ubiquitous gaseous substances (e.g., carbon dioxide and dinitrogen as source of carbon and nitrogen elements) into fuel or raw materials will greatly contribute to society’s future. I have great expectations for the development of fundamental sciences of porous materials possessing the aforementioned excellent characteristics.
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Commentary
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest. 516
DOI: 10.1021/acs.accounts.6b00500 Acc. Chem. Res. 2017, 50, 514−516
