New Horizons for the Physical Chemistry of Nanoporous Materials

EDITORIAL pubs.acs.org/JPCL

New Horizons for the Physical Chemistry of Nanoporous Materials

P

orous materials play an important role in many technologies related to energy and sustainability, such as catalysis, gas separation, water purification, and batteries. Traditional materials such as zeolites, clays, and activated carbons are now complemented by explosive growth of new classes of nanoporous materials. In particular, the discovery of metal organic frameworks (MOFs) is creating exciting opportunities to synthesize porous materials with unprecedented control over pore architecture and surface functionality. MOFs are porous, crystalline solids constructed from metal nodes interconnected by organic linker molecules. The nodes are drawn from well-known clusters in inorganic chemistry, such as the copper acetate “paddlewheel.” The organic linkers are rigid molecules having functional groups at the terminal ends that bind to the inorganic nodes through coordination bonds. Pores are in the micropore (less than 20 Å) or mesopore (20 to 500 Å) range. The ability to combine nodes and linkers with different coordination geometries (for example, linkers with 2 versus 3 connection points), along with the ability to incorporate functional groups in the organic components, opens up an almost unlimited number of possible structures. This issue contains three perspectives about MOFs and related materials, highlighting issues that should be of interest to readers of The Journal of Physical Chemistry.1 3 Wang, Zheng, and Lin discuss the use of chiral MOFs in asymmetric catalysis.1 Chiral MOFs are strong candidates for development into practical catalysts for the production of enantiopure molecules. Welldefined, tailorable catalytic sites can be incorporated into either the nodes or the linkers of MOFs, and the crystalline nature of the materials aids in detailed characterization by a variety of tools. High loadings of catalytic sites can be achieved, and the pores leading to the active sites are uniform and can also be tailored through the MOF synthesis. Wang et al. write that, “heterogeneous asymmetric catalysis using chiral MOFs represents one of the research directions where MOFs can outshine traditional materials.” To move the field forward and improve our fundamental understanding of asymmetric catalysis in MOFs, they identify five critical issues where skills and insights from the physical chemistry community are critically needed. For example, there is a need for better understanding and more measurements of diffusion of guest molecules through MOF channels, particularly counter-diffusion of reactants and products in the presence of solvents. A wide variety of techniques have been developed in the past for measuring diffusion in zeolites, and now many of these techniques are being used to investigate MOFs.4,5 A related issue is how to construct MOFs with channels large enough that large organic molecules can pass through the pores. Increasing the length of the organic linkers in the MOF is the obvious strategy, but this often leads to interpenetrated structures, which places a practical limitation on the size of the pores that can be achieved. Adjusting the synthesis conditions can suppress interpenetration, but our understanding of the nucleation and crystallization of MOFs is limited, and searching for the right synthesis conditions to yield noninterpentrated structures is largely done by trial and error at the moment. A better understanding of MOF r 2011 American Chemical Society

synthesis might also be helpful in efforts to grow MOF thin films and membranes. The perspective by Klontzas, Tylianakis, and Froudakis focuses on the use of MOFs and other nanostructured materials for storing hydrogen.2 Hydrogen has many attractive features as a fuel, but it is notoriously difficult to store. Production and delivery of hydrogen provide their own challenges, but hydrogen storage is widely considered the most challenging barrier to wider adoption of hydrogen as an energy carrier, especially for vehicles. A brief review of the literature reveals that key contributors to the hydrogen storage capacity of a host material are its surface area, the pore volume, and the adsorption enthalpy of hydrogen. Many MOFs have sufficient surface area and pore volume but display adsorption enthalpies that are too low for room-temperature hydrogen storage. Several studies indicate that a binding energy in the range of 20 to 40 kJ/mol is needed to meet established hydrogen storage targets. Klontzas et al. provide an analysis of the types of interactions between molecular hydrogen and host materials such as MOFs, grouping them into three main categories: van der Waals interactions, electrostatic interactions, and interactions with empty d orbitals. Several options for enhancing electrostatic and orbital interactions are discussed. Molecular modeling, neutron diffraction, and spectroscopic studies play an important role in assessing strategies for increasing the binding energy of hydrogen,6 9 especially through the introduction of undercoordinated metal cations. The third perspective article addresses ways that MOFs can form framework isomers. Makal, Yakovenko, and Zhou classify three different types of framework isomers, provide examples of each type, and discuss methods for future control over the synthesis of a particular framework isomer.3 One class of framework isomers consists of interpenetrated versus noninterpenetrated structures. As noted also by Lin and co-workers,1 there is a need to control interpenetration through better understanding of MOF synthesis. Farha et al. have also shown that it is possible to separate catenated and noncatenated MOFs by taking advantage of their density differences such that one phase floats in a solvent of appropriate density while the other sinks.10 An example of conformational framework isomers are the “breathing” MOFs that undergo drastic changes in pore size upon adsorption of guest molecules or other triggers. Such effects have been widely studied both experimentally and theoretically.11 13 The third class of framework isomers are termed orientation isomers by Zhou and co-workers.3 These isomers have identical chemical composition, but the orientations of the ligands and/or metal nodes are different in the two crystal structures. Amirjalayer and Schmid have also discussed similar ideas and investigated the energies of such isomers using force field calculations.14 Makal et al. propose that additional computational research on framework isomers is needed, and more generally they highlight the need for improved methods for characterization of MOFs.15,16

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The Journal of Physical Chemistry Letters These three perspective articles and the references below provide a glimpse into active areas of MOF research with an emphasis on aspects related to physical chemistry. The field of metal organic frameworks is inherently interdisciplinary, requiring skills in synthesis, characterization,8,9,13,15,16 modeling,4 7,17,18 and applications ranging from gas separations19 to sensors. New researchers are being drawn to this field in increasing numbers, not only due to the potential practical applications of MOFs, but also due to the elegance of the synthetic strategies, the new opportunities to study molecules in well-defined, tailored nanospaces, and the aesthetic beauty of the structures themselves. Randall Q. Snurr* 2145 Sheridan Road, Department of Chemical & Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ REFERENCES (1) Wang, C.; Zheng, M.; Lin, W. Asymmetric Catalysis with Chiral Porous Metal Organic Frameworks: Critical Issues. J. Phys. Chem. Lett. 2011, 2, 1701–1709. (2) Klontzas, E.; Tylianakis, E.; Froudakis, G. E. On the Enhancement of Molecular Hydrogen Interactions in Nanoporous Solids for Improved Hydrogen Storage. J. Phys. Chem. Lett. 2011, 2, 1824–1830. (3) Makal, T. A.; Yakovenko, A. A.; Zhou, H.-C. Isomerism in Metal Organic Frameworks: “Framework Isomers”. J. Phys. Chem. Lett. 2011, 2, 1682–1689. (4) Wehring, M.; Gascon, J.; Dubbeldam, D.; Kapteijn, F.; Snurr, R. Q.; Stallmach, F. Self-Diffusion Studies in CuBTC by PFG NMR and MD Simulations. J. Phys. Chem. C 2010, 114, 10527–10534. (5) Deroche, I.; Rives, S.; Trung, T. K.; Yang, Q.; Ghoufi, A.; Ramsahye, N. A.; Trens, P.; Fajula, F.; Devic, T.; Serre, C.; Ferey, G.; Jobic, H.; Maurin, G. Exploration of the Long Chain N-Alkanes Adsorption and Diffusion in the MOF-Type MIL-47(V) Material by Combining Experimental and Molecular Simulation Tools. J. Phys. Chem. C, 2011, Just Accepted Manuscript DOI:10.1021/jp2039527. (6) Han, S. S.; Choi, S.-H.; Goddard, W. A., III. Improved H2 Storage in Zeolitic Imidazolate Frameworks Using Li+, Na+, and K+ Dopants, with an Emphasis on Delivery H2 Uptake. J. Phys. Chem. C 2011, 115, 3507–3512. (7) Getman, R. B.; Miller, J. H.; Wang, K.; Snurr, R. Q. Metal Alkoxide Functionalization in Metal Organic Frameworks for Enhanced Ambient-Temperature Hydrogen Storage. J. Phys. Chem. C 2011, 115, 2066–2075. (8) Sumida, K.; Her, J.-H.; Dinca, M.; Murray, L. J.; Schloss, J. M.; Pierce, C. J.; Thompson, B. A.; FitzGerald, S. A.; Brown, C. M.; Long, J. R. Neutron Scattering and Spectroscopic Studies of Hydrogen Adsorption in Cr3(BTC)2;A Metal Organic Framework with Exposed Cr2+ Sites. J. Phys. Chem. C 2011, 115, 8414–8421. (9) Peterson, V. K.; Brown, C. M.; Liu, Y.; Kepert, C. J. Structural Study of D2 within the Trimodal Pore System of a Metal Organic Framework. J. Phys. Chem. C 2011, 115, 8851–8857. (10) Farha, O. K.; Mulfort, K. L.; Thorsness, A. M.; Hupp, J. T. Separating Solids: Purification of Metal Organic Framework Materials. J. Am. Chem. Soc. 2008, 130, 8598–8599. (11) Neimark, A. V.; Coudert, F.-X.; Boutin, A.; Fuchs, A. H. StressBased Model for the Breathing of Metal Organic Frameworks. J. Phys. Chem. Lett. 2010, 1, 445–449. (12) Ghoufi, A.; Maurin, G.; Ferey, G. Physics Behind the GuestAssisted Structural Transitions of a Porous Metal Organic Framework Material. J. Phys. Chem. Lett. 2010, 1, 2810–2815.

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(13) Hamon, L.; Leclerc, H.; Ghoufi, A.; Oliviero, L.; Travert, A.; Lavalley, J.-C.; Devic, T.; Serre, C.; Ferey, G.; De Weireld, G.; Vimont, A.; Maurin, G. Molecular Insight into the Adsorption of H2S in the Flexible MIL-53(Cr) and Rigid MIL-47(V) MOFs: Infrared Spectroscopy Combined to Molecular Simulations. J. Phys. Chem. C 2011, 115, 2047–2056. (14) Amirjalayer, S.; Schmid, R. Conformational Isomerism in the Isoreticular Metal Organic Framework Family: A Force Field Investigation. J. Phys. Chem. C 2008, 112, 14980–14987. (15) Jiang, Y.; Huang, J.; Marx, S.; Kleist, W.; Hunger, M.; Baiker, A. Effect of Dehydration on the Local Structure of Framework Aluminum Atoms in Mixed Linker MIL-53(Al) Materials Studied by Solid-State NMR Spectroscopy. J. Phys. Chem. Lett. 2010, 1, 2886–2890. (16) Jee, B.; Koch, K.; Moschkowitz, L.; Himsl, D.; Hartman, M.; P€oppl, A. Electron Spin Resonance Study of Nitroxide Radical Adsorption at Cupric Ions in the Metal Organic Framework Compound Cu3(btc)2. J. Phys. Chem. Lett. 2011, 2, 357–361. (17) Grajciar, L.; Bludsky, O.; Nachtigall, P. Water Adsorption on Coordinatively Unsaturated Sites in CuBTC MOF. J. Phys. Chem. Lett. 2010, 1, 3354–3359. (18) Watanabe, T.; Manz, T. A.; Sholl, D. S. Accurate Treatment of Electrostatics during Molecular Adsorption in Nanoporous Crystals without Assigning Point Charges to Framework Atoms. J. Phys. Chem. C 2011, 115, 4824–4836. (19) Krishna, R.; Long, J. R. Screening Metal Organic Frameworks by Analysis of Transient Breakthrough of Gas Mixtures in a Fixed Bed Adsorber. J. Phys. Chem. C 201110.1021/jp202203c.

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