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Tuning the Metal Binding Energy and Hydrogen Storage in Alkali Metal Decorated MOF-5 Through Boron Doping: A Theoretical Investigation K. Srinivasu and Swapan K. Ghosh* Theoretical Chemistry Section, Bhabha Atomic Research Centre and Homi Bhabha National Institute, Mumbai 400 085, India ABSTRACT: Using ab initio quantum chemical calculations, we explore the possibility of improving the light metal binding energy as well as the hydrogen adsorption energy in MOF-5 so that the clustering problem at high concentration can be overcome. To mimic the linking group (C6H4(COO)22 ) in MOF5, we have considered the simple benzene ring. Because the binding energy of light metals (Li, Na, and Mg) to benzene is poor, we have modified the carbon ring by substituting two carbon atoms with two boron atoms. The metal ion (atom) interaction with C4B2H62 (C4B2H6) is found to be very strong, and the interaction energies are extremely high as compared with the metal cohesive energies. Metal sites in these complexes are found to have partial ionic (positive) character and are shown to adsorb molecular hydrogen with reasonably good adsorption enthalpies. We have also studied the extended periodic system of lithium as well as sodium-decorated modified MOF-5 and their hydrogenated counterparts. In the case of lithium-decorated MOF5, each formula unit is found to adsorb 18 hydrogen molecules with three H2 per lithium atom at zero temperature, which corresponds to a gravimetric density of ∼4.3 wt %. However, in the case of Na-decorated MOF-5, the corresponding wt % is found to be 7.4 because each Na can adsorb six molecular hydrogen with 36 hydrogen molecules per formula unit.
1. INTRODUCTION During the past decade, much interest has grown in the design of new porous materials with large surface area.1 4 Among such materials, notable ones are the metal organic frameworks (MOFs), which form a new class of ordered organic inorganic hybrid solids composed of metal or metal oxide clusters as building blocks and organic bridges as linkers.5 7 Metal organic frameworks have attracted a great deal of interest because of their promising applications in a number of areas, including gas storage, heterogeneous catalysis, nonlinear optics, host guest induced separation, drug delivery, and so on.8 17 One of the most important areas of application that holds high promise is the storage of hydrogen gas in molecular form.18 20 In recent times, a large number of materials, such as metal hydrides, clathrates, polymers, alanates, MOFs, covalent organic frameworks (COFs), carbon- and boron-based nanomaterials, and so on, have been tested for hydrogen storage.21 35 Among the many materials investigated, porous materials like MOFs and COFs are found to be promising materials for hydrogen storage because of their porous and robust nature and also exceptionally high specific surface areas and pore volumes. In the case of both MOFs and COFs, hydrogen binds in molecular form through van der Waals interaction between the molecular hydrogen and the host (MOFs and COFs), leading to the hydrogen adsorption enthalpies typically in the range of 1.0 to 1.5 kcal/mol.26 30 Consequently, it requires somewhat low temperatures for achieving a significant hydrogen storage capacity. Lochan and HeadGordon36 have made the rough estimate of the ideal hydrogen r 2011 American Chemical Society
binding energy range given as 5.0 7.6 kcal/mol at 20 °C, 5.7 8.1 kcal/mol at 0 °C, 6.6 9.6 kcal/mol at room temperature, and 7.2 10 kcal/mol at 50 °C. These results show that the hydrogen binding energies in MOFs are far less than the requirement, which indicates the need for developing MOFs with improved hydrogen adsorption enthalpies. Various approaches have been pursued for improving the hydrogen adsorption characteristics in these materials. One such strategy that has been of recent interest is the metal ion decoration approach that had already been followed in the case of fullerenes to improve the hydrogen storage capacity.37 41 Kaye and Long42 experimentally studied MOF-5 with Cr metal decoration to its benzene ring. However, the extent of hydrogen adsorption was found to be very low at 298 K. A possible reason for this is the clustering of the metal atoms. Alkali metal atoms have been found to be good candidates to be used for decoration because of their low cohesive energies, which can avoid the clustering problem. Much of the recent work has thus been focused on Li-doped MOFs and COFs for hydrogen storage.43 46 Using an ab-initio-based GCMC simulation, Han and Goddard47 have predicted gravimetric hydrogen adsorption isotherms for several pure and Li-doped MOFs at 300 K. Interesting results have been reported based on their calculation of a high H2 uptake of 6.47 wt % for MOF-C30 at 300 K and 100 bar pressure. Received: April 15, 2011 Revised: July 21, 2011 Published: July 22, 2011 16984
dx.doi.org/10.1021/jp2035218 | J. Phys. Chem. C 2011, 115, 16984–16991
The Journal of Physical Chemistry C Blomqvist et al.48 have carried out ab initio periodic DFT calculations on molecular hydrogen adsorption properties of Li-decorated MOF-5, which showed significant improvement. Their results showed that each Li over the benzene ring can adsorb up to three H2 molecules with a binding energy of 2.8 to 4.3 kcal/mol per H2, and ab initio molecular dynamics simulations revealed that a hydrogen uptake of 2.9 and 2.0 wt % could be achieved even at high temperatures of 200 and 300 K, respectively. Lan et al.49 studied hydrogen adsorption in Lidoped and nondoped covalent organic borosilicate frameworks using GCMC simulations and have predicted that the total gravimetric and volumetric uptakes of hydrogen in the Li-doped COF-202 reach 4.39 wt % and 25.86 g/L at T = 298 K and P = 100 bar, respectively. Cao et al.50 have carried out a GCMC simulation study and shown that COF materials are superior to MOF materials for hydrogen storage, and the gravimetric adsorption capacities for hydrogen in Li-doped COF-105 and COF-108 can reach up to 6.84 and 6.73 wt % at T = 298 K and P = 100 bar, respectively. Mulfort et al.51 showed experimentally that chemical reductive doping of MOF with alkali metal ions can substantially enhance both nitrogen and hydrogen gas uptake. In most of the Li-doped framework materials studied above, the Li atom is positioned on top of the six-membered carbon rings (benzene), which is aromatic in nature. Because of the aromatic nature of the ring, the binding energy of the metal atom is poor, leading to aggregation of adsorbed metal atoms to form clusters, and is therefore not suitable for reversible hydrogen adsorption. Very recently, Wu et al.52 have studied hydrogen adsorption in lithium and calcium-doped COF-10 using firstprinciples calculations and shown that the metal atoms prefer to sit on the benzene ring with adsorption energy of 23.0 kcal/mol. Venkataramanan et al.53 have studied hydrogen adsorption in lithium-decorated MOF-5 and reported that the lithium binding energy is ∼35.9 kcal/mol. In both of these studies, however, the lithium binding energy is found to be less than the cohesive energy of metallic lithium (39 kcal/mol), which shows that at higher concentrations lithium atoms tend to form clusters, resulting in inefficient hydrogen storage capacity. In the present study, our objective is to explore the possibility of improving the adsorption energy of the lithium to the MOF-5. We propose to improve this binding strength of Li by substituting two of the carbons of the carbon ring with two boron atoms, which contain one fewer electron as compared with carbon. These types of systems are also feasible experimentally because the synthesis of boron-substituted heterocycles has been already reported.54 Very recently, Zou et al.55 showed that boron doping in transitionmetal-doped COF can prevent the clustering problem through ab initio calculations. The substituted linker of MOF-5 is decorated with light metal atoms viz. Li, Na, and Mg, and the interaction energy of metal with this modified linker has been calculated. We have also carried out a systematic study on the interaction of molecular hydrogen with this metal-decorated organic linker of MOF-5 with a different number of hydrogen molecules, which lead to C4B2H6M2(H2)2n complexes (M = Li and Na).
2. COMPUTATIONAL DETAILS All energy calculations and geometry optimization for the molecular systems have been performed by using the electronic structure theory-based GAMESS software.56 We have employed the density functional theory (DFT) with pure Hartree Fock
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exchange and Lee Yang Parr (HLYP)57 correlation energy functionals as well as the Perdew Wang (PW91) exchange correlation energy functionals.58 Because the molecular hydrogen interaction is weak in nature, proper inclusion of the electron correlation effect is important. Hence, we have also employed the second-order Moller Plesset (MP2) perturbation method for all systems. We have used the extensive split-valence basis sets with diffuse and polarization functions, 6-31++G(2d,2p). All nucleus-independent chemical shift (NICS) parameter calculations are carried out using the Gaussian 03 software package at HLYP/6-31++G(d, p) level of theory.59 The initial geometries and all reported structures have been obtained using the graphical software GABEDIT60 and MOLDEN.61 The error in the calculated interaction energies arising due to the basis set superposition error (BSSE) was systematically corrected using the counterpoise (CP) method of Boys and Bernardi.62 Three-dimensional structures of MOF, lithium-decorated MOF, and their modified counterparts as well as the corresponding hydrogen adsorbed systems were optimized using the periodic boundary condition (PBC) within DFT calculations implemented in the Vienna ab initio simulation package (VASP)63 66 using a plane wave basis set. All-electron projector augmented wave (PAW)67,68 potentials with an energy cutoff of 800 eV were employed for the elemental constituents. The exchange correlation part εxc of the density functional was treated using the generalized gradient approximation (GGA) of Perdew Wang (PW91).58 The energy of the H2 molecule has been calculated within a cubic box large enough to ensure isolation. In all of these calculations, the convergence threshold was set with the energy change per atom
