pubs.acs.org/NanoLett
Designing 3D COFs with Enhanced Hydrogen Storage Capacity Emmanouel Klontzas,† Emmanuel Tylianakis,‡ and George E. Froudakis†,* †
Department of Chemistry, University of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece, and ‡ Materials Science and Technology Department, University of Crete, P.O. Box 2208, 71409 Heraklion, Crete, Greece ABSTRACT Hydrogen storage properties have been studied on newly designed three-dimensional covalent-organic framework (3DCOF). The design of these materials was based on the ctn network of the ultralow density COF-102. The structures were optimized by multiscale techniques and the optimized structures were checked for their storage capacities by grand canonical Monte Carlo simulations. Our simulations demonstrate that the gravimetric uptake of one of these new COFs can overpass the value of 25 wt % in 77 K and reach the Department of Energy’s target of 6 wt % in room temperature, classifying them between the top hydrogen storage materials. KEYWORDS COF, hydrogen storage, multiscale, ab initio, GCMC
T
he world’s increasing demands for energy and the need for the reduction of the air pollution have led to the quest of new alternative energy sources that would also be environmental friendly. Hydrogen has been recognized as an ideal energy carrier especially for automotive applications, but it has not been available for extensive commercial use yet. One of the major reasons for this has been the difficulty to create efficient storage materials to satisfy the established gravimetric and volumetric storage targets. Nowadays there is an intensive research in this scientific field and has been proposed that the solution of this problem will become from the design, synthesis of new targeted materials, or the development of already existing materials with specific properties. Following both our previous experience and the literature,1-6 some general guidelines can be proposed for new potential materials for hydrogen storage. The main factors that enhance the ability of storing hydrogen in the case of physisorption are surface area, pore volume, and enthalpy of adsorption. These factors can be enhanced by extended aromaticity, unsaturated metal sites, and point charges in the framework of the proposed materials. The design of novel materials for hydrogen storage following the abovementioned rules has been a major task nowadays. A new family of materials that fulfill many of the previous demands was recently synthesized by Yaghi and co-workers, named as three-dimensional covalent-organic frameworks (3D-COFs).7 These three-dimensional periodic covalent structures were synthesized by targeting two nets (ctn and bor) based on the selection of appropriate triangular and tetrahedral nodes. From the first look, COFs present all the
advantages of metal-organic frameworks (MOFs) considering hydrogen storage (surface area, pore volume, rigidity of the structure) while their molecular framework is composed of light elements that lower the relative weight of the structure compared to the MOFs. The main advantage of 3DCOF with respect to other light porous organic materials is the crystalline framework, which reflects high surface area. Our previous theoretical investigations of these materials8 have shown exceptional storage properties where similar results were reported from other theoretical9-11 and experimental studies.12 Taking into account our previous experience in MOFs and the strategies to enhance hydrogen uptake in these materials listed earlier, we have designed four novel 3D-COF in such a way that their storage properties would be superior to the parent framework. Particularly, we substitute the phenylene moieties of COF-102 with other extended aromatic moieties without changing the net topology. We used diphenyl, triphenyl, napthalene, and pyrene molecules (Figure 1) as the new aromatic moieties and produced the so-called COF102-2, COF-102-3, COF-102-4, and COF-102-5, respectively. The construction of these new structures was based on the carbon nitride (C3N4) net topology. This network is the most symmetric for the combination of tetrahedral and triangular building blocks to form a 3D structure, which was proven also experimentally by the formation of three structures based on ctn network against 1 structure that was based on bor network. This was done in two steps. First we substituted every nitrogen atom with a B3O3 ring and afterward we replaced every C-N bond of original net with a new organic moiety. For every new structure, we readjusted the lattice parameters of the C3N4 net to take a more reliable initial structure, and we checked for the right connectivity for the bonds between the boron and tetrahedral carbon atoms with the corresponding carbon atoms of the new organic moieties.
* To whom correspondence should be addressed. Tel: +302810545055. Fax: +302810545001. E-mail:
[email protected]. Received for review: 09/16/2009 Published on Web: 01/05/2010 © 2010 American Chemical Society
452
DOI: 10.1021/nl903068a | Nano Lett. 2010, 10, 452-454
FIGURE 2. Predicted total gravimetric and volumetric adsorption isotherms at 77 and 300 K for COF-102, COF-102-2, COF-102-3, COF102-4, and COF-102-5. FIGURE 1. The different organic moieties that have been used for the construction of the COF materials. By using the same topology as COF-102 and the same connectivity and alternating the organic moieties there have been constructed a series of periodic COF cells ranging from COF-102 to COF-102-5. Carbon, oxygen, boron, and hydrogen atoms are shown as gray, red, pink, and white colors, respectively.
for both total gravimetric and volumetric uptake of these new COF are demonstrated in Figure 2 at 77 and 300 K. As it can be seen, all the proposed structures showed enhanced gravimetric capacity with respect to the corresponding gravimetric uptake of the parent COF-102 structure, both at cryogenic and room temperature. COF-102-3 showed the best performance and reached 26.7 and 6.5 wt % at 77 and 300 K, respectively at 100 bar, which overcome the Department of Energy (DOE) target of 6 wt % even at room temperature. On the other hand, the proposed materials showed lower volumetric performance than COF-102. This can be attributed to the larger pore volume that they possess, since volumetrically, very large pores result in empty space inside the material that is dominated by weak fluid-fluid interactions. In addition, the excess uptake of the new COF materials is remarkably lower than the corresponding total, as it can be seen in Figure S3 of Supporting Information. This is expected due to the large pore volume of these novel frameworks. In this communication, we proposed four novel COF structures; their structural characteristics and their H2 storage performance were investigated by multiscale theoretical techniques. The gravimetric hydrogen uptake of these materials is superior to their parent structure and every other known framework material. Our GCMC results demonstrate that the gravimetric uptake of one of these COFs can overpass the value of 25 wt % at 77 K and reach the DOE target of 6 wt % at room temperature.
The construction of the initial crystal structures was followed by an optimization procedure, using molecular mechanics methods as implemented in Tinker program package.13 For the optimization of the structures, we used an MM3 forcefield that was explicitly parametrized for COF102 on the basis of first principles calculations.14 This forcefield was proven to give accurate description of the tetrahedral building unit of the COF-102. Moreover, we implemented some extra parameters15 in our calculations for COF-102-2 and COF-102-3 to have a better description for the torsions between the adjacent phenyl moieties based on ab initio calculations. The systems were minimized in energy in a periodic cell in space group P1 with respect to all degrees of freedom and the cell parameters. The optimized structures for all the proposed COF (COF-102 to COF102-5) are presented in Figure 1, while structural details of the optimized structures can be found in the Supporting Information. After the optimization of our new structures we used classical grand canonical Monte Carlo (GCMC) simulations to obtain the total gravimetric and volumetric hydrogen uptake of these materials at 77 K and room temperature for pressures up to 100 bar. Details about the simulation method can be also found in our previous works for MOF16,17 and COF.8 We believe that our GCMC simulations are valid, since our previous investigation for the hydrogen storage abilities of COF-102 and COF-103 gave results in very good agreement with the latest experimental results that have been reported.12We predicted that COF-102 has 9.95 wt % total gravimetric adsorption at 77 K and 100 bar, where experimental results showed that the total capacity was just above 10 wt % at the same thermodynamic conditions. Our results © 2010 American Chemical Society
Acknowledgment. The present research study has been supported from the Ministry of Development (General Secretariat-GSRT) by the Greek government (ΠENE∆ 200303E∆ 548). Partial funding by the European Commission DG RTD (FP6 Integrated Project NESSHY, Contract SES6-518271) is gratefully acknowledged. Supporting Information Available. Details about the construction of the initial periodic COF cells, the optimization 453
DOI: 10.1021/nl903068a | Nano Lett. 2010, 10, 452-454
procedure of the initial cells, and the lattice parameters of the optimized cells are presented. This material is available free of charge via the Internet at http://pubs.acs.org.
(8)
REFERENCES AND NOTES
(11)
(1) (2) (3) (4) (5) (6) (7)
(9) (10)
Collins, D. J.; Zhou, H.-C. J. Mater. Chem. 2007, 17, 3154. Bhatia, S. K.; Myers, A. L. Langmuir 2006, 22, 1688. Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304. Froudakis, G. E. Nano Lett. 2001, 1, 531. Mpourmpakis, G.; Froudakis, G. E.; Lithoxoos, G. C.; Samios, J. Nano Lett. 2006, 6, 1581. Klontzas, E.; Mavrandonakis, A.; Froudakis, G. E.; Carissan, Y.; Klopper, W. J. Phys. Chem. C 2007, 111, 13635. El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; Cote, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268.
© 2010 American Chemical Society
(12) (13) (14) (15) (16) (17)
454
Klontzas, E.; Tylianakis, E.; Froudakis, G. E. J. Phys. Chem. C 2008, 112, 9095. Garberoglio, G. Langmuir 2007, 23, 12154. Han, S. S.; Furukawa, H.; Yaghi, O. M.; Goddard, W. A., III. J. Am. Chem. Soc. 2008, 130, 11580. Cao, D.; Lan, J.; Wang, W.; Smit, B. Angew. Chem., Int. Ed. 2009, 48, 1. Furukawa, H.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131 (25), 8875–8883. Ponder, J. W.; Richards, F. M. J. Comput. Chem. 1987, 8, 1016; Tinker version 4.2, June 2004; http://dasher.wustl.edu/tinker/. Schmid, R.; Tafipolsky, M. J. Am. Chem. Soc. 2008, 130, 12600. Amirjalayer, S.; Schmid, R. J. Phys. Chem. C 2008, 112, 14980. Klontzas, E.; Mavrandonakis, A.; Tylianakis, E.; Froudakis, G. E. Nano Lett. 2008, 8, 1572. Mavrandonakis, A.; Tylianakis, E.; Stubos, A. K.; Froudakis, G. E. J. Phys. Chem. C 2008, 112, 7290.
DOI: 10.1021/nl903068a | Nano Lett. 2010, 10, 452-454
