Enhancement of Hydrogen Adsorption in Metal-Organic Frameworks by the Incorporation of the Sulfonate Group and Li Cations. A Multiscale Computational Study Andreas Mavrandonakis, †,§ Emmanouel Klontzas, † Emmanuel Tylianakis, ‡ and George E. Froudakis* ,† Department of Chemistry and Department of Materials Science and Technology, UniVersity of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece Received May 30, 2009; E-mail: frudakis@chemistry.uoc.gr Abstract: By means of ab initio methods, the effect on the H 2 storage ability of a newly proposed organic linker for IRMOF-14 has been studied. The linker comprises a negatively charged sulfonate (-SO 3 -1 ) group in combination with a Li cation. It is found that these two charged groups significantly increase the interaction energy between the hydrogen molecules and the new proposed organic linker of the MOF. The substituted group of the linker may host up to six hydrogen molecules with an average interaction energy of 1.5 kcal/ mol per H 2 molecule. This value is three times larger than the binding energy over the bare linker that has been obtained from DFT calculations. GCMC atomistic simulations verified that the proposed material can be qualified among the highest adsorbing materials for volumetric capture of H 2 , especially at ambient conditions. This functionalization strategy can be applied in many different MOF structures to enhance their storage abilities. A great deal of scientific work has been focused on finding alternative energy resources. Widely used fuels, such as diesel and petroleum, are extinguishing very quickly. Molecular hydrogen is considered as the ideal energy carrier, since it has a large power density in contrast to diesel and is environmental friendly. However, the main problem is finding an appropriate storage material for commercial applications. The 2010 U.S. Department Of Energy (DOE) 1 target of 6.0 wt % capacity for the total storage system has not been accomplished yet. Potential Hydrogen Storage Materials (HSM) must operate under ambient conditions of temperature and pressure. Hydrogen may interact with HSM in two possible ways. The first is dissociative chemisorption. In this case, molecular hydrogen is dissociated and chemically adsorbed as atomic hydrogen. A typical example of such materials are metal hydrides. 2 Although high hydrogen capacities have been achieved, the main disad- vantages of such materials have been the slow kinetics for the hydrogen abstraction from the HSM surface and the irreversible hydrogen adsorption/desorption after a few cycles of loading/ uploading. The second way is the physical or the chemical adsorption of hydrogen in HSM where hydrogen retains its molecular form, which has been the case in carbon-based materials, 3,4 Metal- Organic Frameworks (MOFs), 5 and metal doped polymers. 6,7 This would allow fast kinetics in adsorption and desorption to be observed. However the main disadvantage in the case of MOFs and carbon materials has been the low temperatures that are needed, since small amounts of hydrogen can be kept at room temperature. The main reason for the decreased %wt H 2 adsorption can be attributed to the weak 8 interaction energies (0.5 to 1 kcal per mol, for DFT and MP2 level of theory, respectively) between H 2 and HSM. Although transition metal doped polymers have higher interaction energies with the dihydrogen, the experimental synthesis of the proposed struc- tures has been proved to be very difficult. One of the main problems is that the metal atoms prefer to form clusters 9 instead of being separated. This effect causes a loss of active metal sites for hydrogen adsorption. From our previous work in the field of hydrogen storage in MOFs and carbon nanotubes (CNTs), some useful conclusions have been found for designing promising materials for hydrogen storage applications. 10,11 Among these, it has been found that the decoration of a surface of a porous material with point charges improves the storage capacity since they increase the H 2 binding energy. CNTs doped with alkali metals have shown an increased interaction with H 2 . Li doping in MOFs has been theoretically studied by Mavrandonakis et al., 12 Klontzas et al., 13 and others, 14,15 leading to increased storage performance. Since † Department of Chemistry. ‡ Department of Materials Science and Technology. § Current Address: Institut fu ¨r Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany. (1) See U.S. DOE Web site, http://www.eere.energy.gov. (2) Fichtner, M. AdV. Eng. Mater. 2005, 7, 443–455. (3) Liu, C.; Cheng, H. M. J. Phys. D: Appl. Phys. 2005, 38, R231. (4) Lee, H.; Choi, W. I.; Ihm, J. Phys. ReV. Lett. 2006, 97, 056104-1. (5) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670. (6) Durgun, E.; Ciraci, S.; Zhou, W.; Yildirim, T. Phys. ReV. Lett. 2006, 97, 226102–1. (7) Zhou, Z.; Gao, X.; Yan, J.; Song, D. Carbon 2006, 44, 939. (8) Lochan, R. C.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 1357. (9) Wang, Q.; Jena, P.; Kawazoe, Y. J. Am. Chem. Soc. 2005, 127, 14582. (10) Froudakis, G. E. Nano Lett. 2001, 1, 531. (11) Mpourmpakis, G.; Froudakis, G. E.; Lithoxoos, G. C.; Samios, J. Nano Lett. 2006, 6, 1581. Published on Web 08/31/2009 10.1021/ja9043888 CCC: $40.75  2009 American Chemical Society 13410 9 J. AM. CHEM. SOC. 2009, 131, 13410–13414