A model study of effect of M = Li+, Na+, Be2+, Mg2+, and Al3+ ion decoration on hydrogen adsorption of metal-organic framework-5

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A model study of effect of M [ LiD, NaD, Be2D, Mg2D, and Al3D ion decoration on hydrogen adsorption of metal-organic framework-5 Tuhina Adit Maark, Sourav Pal* Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India

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abstract

Article history:

The effect of light metal ion decoration of the organic linker in metal-organic framework

Received 20 June 2010

MOF-5 on its hydrogen adsorption with respect to its hydrogen binding energy (DB.E.) and

Received in revised form

gravimetric storage capacity is examined theoretically by employing models of the form

14 August 2010

MC6H6:nH2 where M ¼ Liþ, Naþ, Be2þ, Mg2þ, and Al3þ. A systematic investigation of the

Accepted 16 August 2010

suitability of DFT functionals for studying such systems is also carried out. Our results show that the interaction energy (DE) of the metal ion M with the benzene ring, DB.E., and charge transfer (Qtrans) from the metal to benzene ring exhibit the same increasing order:

Keywords:

Naþ < Liþ < Mg2þ < Be2þ < Al3þ. Organic linker decoration with the above metal ions

Metal-organic frameworks

strengthened H2-MOF-5 interactions relative to its pure state. However, amongst these ions

Hydrogen storage

only Mg2þ ion resulted in DB.E. magnitudes that were optimal for allowing room temper-

Hydrogen adsorption

ature hydrogen storage applications of MOF-5. A much higher gravimetric storage capacity

Binding energy

(6.15 wt.% H2) is also predicted for Mg2þ-decorated MOF-5 as compared to both pure MOF-5 and Liþ-decorated MOF-5. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

A major challenge in application of hydrogen (H2) as an energy carrier in fuel cells is development of a safe, efficient, and practical hydrogen storage method. Physical storage of H2 as a gas or liquid is technically demanding requiring maintenance of high pressures and low temperatures, respectively in order to have a good energy density. Solid state storage of H2 offers an attractive alternative. Microporous metal-organic frameworks (MOFs) with their extremely high surface areas, fast H2 desorption kinetics, and reversible H2 uptake and release are one of the most promising class of hydrogen storage materials [1]. Here the H2 binds as a molecule via van der Waals interactions between the physisorbed H2 molecules and the host MOFs. Thus, the consequent H2 adsorption enthalpies are typically in the range of 2.26e5.2 kJ/

mol [2e4]. As a result for achieving a significant H2 storage capacity, low temperatures such as 77 K are needed [2,5e11]. Based on simple entropic arguments Lochan and HeadGordon [12] have made the following rough estimate of the ideal H2 binding energy (DB.E.) range: at w-20  C DB.E. varies between 21 and 32 kJ mol-1; at 0  C, DB.E. w24e34 kJ mol-1; at room temperature, DB.E. w28e40 kJ mol-1, and at w50  C, DB.E. w30e42 kJ mol-1. The U.S. Department Energy 2010 targets [13,14] for a hydrogen storage system requires amongst other characteristics an ability to operate within the temperature range of -30  C to 50  C. This signifies the need to develop MOFs with drastically enhanced H2 adsorption enthalpies (20e40 kJ mol-1). Clearly, today much research in MOFs is devoted to strengthening the H2 interaction energies and to increasing H2 adsorption and storage capacity to significant levels at or around room temperature.

* Corresponding author. Tel.: þ91 20 25902002; fax: þ91 20 2590 2636. E-mail address: [email protected] (S. Pal). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.054

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Experimentally efforts have been made to enhance the H2 gravimetric weight percentage of MOF-5 by increasing its specific surface area and pore volume. Li et al. [15] found that the type of synthesis method used influenced the porestructure parameters, morphologies and hydrogen storage behavior of the obtained MOF-5. MOF-5 synthesized by the solvothermal approach showed both a higher surface area and larger pore volume than the samples prepared by the direct mixing of triethylamine (TEA) and by the slow diffusion of TEA. In several cases hydrogen storage capacities, at moderate temperature and pressure conditions, of carbon nanostructures have been enhanced by doping them with a metal support such as Pd, V or Ni, owing to the greater interactions between the highly dispersed nickel particles and the carbon support that produce a higher activation of the solid through a spillover effect [16e18]. Similar to the above cases modification techniques based on the hydrogen spillover effect, modified samples of MIL-101 and MIL-53 witnessed a great increase in hydrogen uptakes at 5.0 MPa and 293 K when compared with the pristine samples [19]. In a current study transition metal (Ni, Rd, and Pd) incorporation into the mesoporous materials, namely, MCM-41, MCM-48, HMS and SBA15, also led to the same effect at 303 K [20]. To determine different ways for tuning H2 storage in MOFs it is important to understand the fundamental interactions that lead to its adsorption. The first such theoretical study was } ber et al. [21] in 2004 on the interaction of H2 carried out by Hu with the aromatic systems C6H5X (X ¼ H, F, OH, NH2, CH3, and CN), naphthalene, azulene, anthracene, coronene, terephthalic acid, and dilithium terephthalic acid using secondorder MøllerePlesset (MP2) [22] calculations. The authors showed that the DB.E.s to benzene and naphthalene were 3.91 and 4.28 kJ mol-1, respectively, implying that interaction energy increased with the size of the aromatic system. Sagara et al. [23] calculated H2 binding energies to both the organic linker part and metal oxide part of MOF-5, that is, H2e1,4ebenzenedicarboxylateeH2 and the Zn4O(HCO2)6 cluster, using the MP2 method and exhibited that between the two the zinc oxide cluster would preferentially adsorb H2 molecules. The periodic density functional theory (DFT) [24] based calculations of the MOF-5 as a whole were performed by Mulder et al. [25], and Mueller and Ceder [26]. In agreement with the results of Sagara et al. [23] both works showed that the strongest interactions with hydrogen were prevalent near the Zn4O clusters. Buda and Dunietz [27] have performed ab initio computations to analyze physisorption of H2 on conjugated systems, which are used as models for the organic linker within MOFs. Two different orientations, top and edge, of adsorbed H2 molecule were considered. The calculated adsorption energies illustrated, similar to results predicted from experimental observations, that the top site had a stronger interaction than the edge site. Chemical modification by introduction of an electron donoreacceptor atom pair into the conjugated rings enforced these adsorption sites, there by suggesting this to be crucial for improving the uptake properties of these materials to the goal defined by US DOE for efficient hydrogen transport materials. Such studies prove that ab initio and DFT calculations are useful for determining the H2 binding energies as well as the H2 adsorption sites in MOFs.

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A critical review of the recent advances in studying MOFs and covalent organic frameworks (COFs) using quantum calculations, Monte-Carlo simulations and molecular dynamics simulations can be found in Ref [28]. This review also provides an overview of the various strategies being pursued for improving hydrogen storage in these materials. One such interesting method involves introduction of open metal coordination sites on the surfaces. A GCMC study had shown that at 77 K and 30 bar, the organic linker in MOF-5 accounted for 74% of its H2 loading [29]. Thus, the above mentioned increase in hydrogen adsorption enthalpies may be brought about by making available the open metal sites within the MOF’s organic linker in place of the metal oxide cornerposts. Another strategy being considered presently is the metal ion decoration approach. This has already been suggested as a means to improve the hydrogen storage capacities of fullerenes and various nanostructures such as nanotubes, nanofibers and nanospheres [30e39]. In the theoretical works done by Zhao et al. [30] and Yildrim and Ciraci [31] it was shown that Sc and Ti coated on carbon fullerenes (C60) and nanotubes can bind H2 molecules with a binding energy of 0.5 eV/H2 and with a high H2 storage capacity of up to 8 wt.%. However, here the metal atoms over C60 were assumed to remain in isolated form. Questions regarding the possibility of metal clustering over fullerenes and its affect there off on the nature and amount of H2 storage were addressed later by Sun et al. [32] A similar study using DFT was carried out for Licoated C60. Interestingly these were not found to suffer from clustering of Li atoms unlike the Ti-coated C60. But the binding energy of H2 molecules was too low to allow room temperature hydrogen storage applications [33]. Chandrakumar and Ghosh [34] have further exhibited through ab initio calculations that doping C60 with Na would have an even stronger effect than Li on its H2 adsorption capacity and that with a high metal coverage of eight Na atoms totally 48H2 molecules can be adsorbed, leading to a gravimetric storage capacity of w9.5 wt.%. In another computational work it was illustrated that Tiþ ion doped on Si6H60 fullerene [35] and on BC4N [36] nanotube can adsorb a maximum of four H2 molecules with successive binding energies lying in the suitable range between the physisorption and chemisorption energies. The effect of Pt doping on BN nanotubes have also been examined [37]. Recently, Li-decorated B doped heterofullerene (Li12C48B12) [38] has been illustrated to possess the following desired properties which can lead to its transportation applications: (i) the Li atoms do not cluster and remain isolated; (ii) Li atoms remain positively charged; (iii) the gravimetric density can achieve the 9 wt.% target, and (iv) the binding energies are in the range of 0.135e0.172 eV/H2, suitable for ambient temperature storage. In a latest tight-binding study of hydrogen adsorption over palladium decorated graphene and carbon nanotubes it was shown that the mechanism of hydrogen storage involved dissociation of H2 molecule on the decoration points and the bonding between resultant atomic hydrogen and the carbon surface [39]. An experimental attempt for synthesizing MOF-5 with Cr metal decorating its benzene was performed by Kaye and Long [40]. However, the H2 adsorption amount was found to be very low at 298 K. A possible reason for this is the clustering of

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the metal element. Metal atoms having a binding energy to the organic linker in MOF higher than their cohesive energies will avoid such clustering. Following these findings Lochan et al. [41] performed a computational study to investigate the nature and characteristics of interaction between H2 with models of exposed metal binding sites using half-sandwich piano-stool shaped complexes of the form (Arene) MeL3en(H2)n where M ¼ Cr, Mo, V-, and Mnþ and n ¼ 1e3. Their results exhibited that from the perspective of improving the H2 sorption characteristics of MOFs at room temperature, incorporation of transition metal groups like Cr(CO)3 onto organic linkers like BDC would give much stronger binding than the weak, dispersion dominant H2 interaction with BDC itself. However, the range 48e84 kJ/mol of the calculated transition metal-dihydrogen binding affinity was found to be too strong for allowing easy desorption under ambient conditions. Very recently the possibility of formation of transition metal (Mnþ)-Bipydc complexes, where n ¼ 0, 1, and 2, M ¼ Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, and Bipydc ¼ 2,20 -bipyridine-5,50 -dicarboxylate to avoid metal atom clustering has been explored by DFT calculations [42]. Furthermore, the interactions between H2 molecules and (Bipydc)M2þ complexes were investigated and the average binding energies for (Bipydc)M2þ(H2)4 systems was found to range from -24.6 kJ mol-1 for Zn2þ to -62.2 kJ mol-1 for V2þ. This suggests that MOFs with such complexes would be promising for practical hydrogen storage. In a current study Zou et al. [43] have explored the feasibility of storing hydrogen in calcium-decorated MOFs by using first-principles electronic structure calculations. It was shown that substitution of boron atoms into the benzene ring of the MOF linker substantially enhances the Ca binding energy to the linker as well as the H2 binding energy to Ca. The Kubas interaction between H2 molecules and Ca added in the MOF gave rise to a large number of bound H20 s (8H20 s per linker) with the binding energy of 20 kJ/mol, making the system suitable for reversible hydrogen storage under ambient conditions. A different work by Li et al. [44] proposes substituting the bridge C2O2B rings in covalent organic frameworks crystals, with different metal-participated rings which can naturally avoid the clustering of metal atoms, as a possible way of promoting the binding of H2 molecules in these systems. First-principles calculations on both crystalline phase and molecular fragments showed that the H2 binding energy can be enhanced by a factor of four with regard to the undoped crystal, i.e. reaching about 10 kJ/mol. Grand canonical Monte-Carlo simulations further confirmed that such substitutional doping would improve the room temperature hydrogen storage capacity by a factor of two to three. Much of the recent papers are focused on studying Lidoped MOFs for their potential for hydrogen storage at ambient conditions. Using an ab initio based GCMC simulation Han and Goddard III [45] predicted gravimetric H2 adsorption isotherms for several pure and Li-doped MOFs at 300 K. An interesting result is their calculation of a high H2 uptake of 6.47 wt.% for MOF-C30 at 300 K and 100 bar pressure. Significant improvement in molecular H2 uptake properties has also been exhibited for Li-decorated MOF-5 by Blomqvist et al. [46] by applying ab initio periodic DFT calculations. The authors

showed that each Li adsorbed over the benzene ring can cluster up to three H2 molecules with a binding energy of 12.0e18.0 kJ/mol H2. Ab initio molecular dynamics simulations revealed that a hydrogen uptake of 2.9 wt.% and 2.0 wt.% could be achieved even at high temperatures of 200 K and 300 K, respectively. In another work Mavrandonakis et al. [47,48] have carried out a multi-scale theoretical study to investigate the effect of a newly proposed organic linker comprising of a negatively charged sulfonate group and a Li cation on the H2 storage ability of IRMOF-14. It was found that these two charged groups significantly increased the interaction energy between the hydrogen molecules and the new proposed organic linker of the MOF. The substituted group of the linker could host up to six hydrogen molecules with an average interaction energy of 1.5 kcal/mol per H2 molecule which was three times larger than the binding energy over the bare linker. GCMC atomistic simulations verified that the proposed material could be qualified among the highest adsorbing materials for volumetric capture of H2, especially at ambient conditions Recently, Venkataramanan et al. [49] have explored isoreticular MOFs with different metals M ¼ Fe, Cu, Co, Ni and Zn through DFT and predicted that Li doping will be possible only in Zn-based MOFs. In an exhaustive quantum chemical study Srinivasu and co-workers [50] have considered the following model systems: C6H6, NH2eC6H5, CH3eC6H5, COOHeC6H5, CNeC6H5, and NO2eC6H5 and examined the effect of (a) doping with light metal ions (Liþ and Naþ), (b) functional groups in the models, and (c) curvature. It was concluded that ionic surface with a significant degree of curvature will enhance hydrogen adsorption more effectively. In another work the authors [51] have further investigated systems of the form CnHn where n ¼ 4e6 and 8 and their Na doped complexes for H2 adsorption. They found that the interaction energy of H2 for the undoped systems, calculated using both HLYP/-631þþG(2d,2p) and MP2/6-31þþG(2d,2p) methods, to be weak. The strong interaction of all molecular organic systems, except the benzene molecule, with Na was attributed to their peelectron deficiency. The adsorption of hydrogen in C4H4eNa, C5H5eNa, and C8H8-Na was calculated to be 13.8, 12.0, and 5.9 wt.%, respectively. The aromaticity in such systems was suggested to play an important role in stabilizing the alkali metal doped organic complexes. In this work, we explore the feasibility of tuning the hydrogen adsorption properties of MOF-5 by decorating its organic linker with light metal ions M ¼ Naþ, Be2þ, Mg2þ, and Al3þ as opposed to Liþ ion. The focus is on predicting which of these ions will result in enhancing the H2 binding energy (DB.E.) of MOF-5 to the range of 20e40 kJ mol-1 in order to render it suitable for room temperature hydrogen storage. It is agreed that while an intuitive prediction of the effect of M ions is possible, it is important to obtain quantitatively the corresponding DB.E.s Herein we have modeled the interactions between the metal ion (M)-decorated organic linker of MOF-5 with different number (n) of H2 molecules as MC6H6:nH2 complexes. Ab initio and DFT calculations were carried out to examine the corresponding optimized geometries, atomic charges, interaction energy (DE) between metal ion and benzene ring, and the DB.E.s.

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2.

Computational details

In this work we have considered two types of model systems. Through the first model we study the interaction of the bare metal ions with up to two H2 molecules. These are represented as M:nH2 in the paper, where M ¼ Liþ, Naþ, Be2þ, Mg2þ, and Al3þ and n ¼ 1,2. In the second case, we model the interactions of different numbers of H2 molecules with a benzene ring and with a complex of the above metal ions M with the benzene ring, the organic linker in MOF-5. These systems are referred to as C6H6:nH2 and MC6H6:nH2, respectively. The geometries of the M:nH2 molecular systems were optimized using both MP2 [22] and DFT [24] methods. For the MC6H6:nH2 complexes the calculations were performed only at the DFT level. The DFT functionals that were systematically analyzed for their suitability were: B3LYP [52,53], BHHLYP [52,54], PBELYP [52,54,55], and PWLOC [56]. Frequency calculations were carried out for the C6H6:nH2 and MC6H6:nH2 complexes to prove that the resulting stationary points were real energy minima, without imaginary frequencies in their vibration spectra. The effect of inclusion of basis set superposition error (BSSE) corrections computed using Morokuma-Kitaura decomposition [57] and thermodynamic corrections was carried out on certain test cases. The interaction energy DE between the metal ions and organic linker and the hydrogen binding energies DB.E.s were computed using the formulae: DE½MC6 H6  ¼ Etot ½C6 H6  þ Etot ½M  Etot ½MC6 H6 

(1)

DB:E:½M : nH2  ¼ 1=nEtot ½M þ Etot ½H2   1=nEtot ½M : nH2 

(2)

DB:E:½C6 H6 :nH2 ¼1=nEtot ½C6 H6 þEtot ½H2 1=nEtot ½C6 H6 :nH2 

(3)

DB:E:½MC6 H6 : nH2  ¼ 1=nEtot ½MC6 H6  þ Etot ½H2  1=nEtot ½MC6 H6 : nH2 

(4) bsse

The BSSE corrected binding energies DB.E. and H2 binding enthalpies DH(298 K) at 298 K were obtained using the equations: DB:E:bsse ¼ DB:E: þ BSSE=n

(5)

DHð298KÞ ¼ DB:E: þ DHcorr =n

(6)

All the above mentioned calculations were performed using the electronic structure program GAMESS [58] and the

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6-311 þ G* Pople basis set [59,60]. The atomic charges were derived using both Mulliken [61] and Lowdin [62] population analysis available with GAMESS and using the ESP method [63] implemented in MOLDEN [64]. MOLDEN was also used to obtain the structures of the optimized geometries.

3.

Results and discussion

3.1.

Model M:1H2 and M:2H2 systems

In an interesting work Chandrakumar and Ghosh [65] have studied the interaction of hydrogen molecules with cations in M(H2)8 complexes where M ¼ Liþ, Naþ, Kþ, Be2þ, Mg2þ and Ca2þ ions. Through an energy decomposition analysis they found that these complexes are effectively stabilized by electrostatic as well as charge transfer components. The contributions of the polarization components to the binding energy were also detected to be quite substantial. The interaction energy between s-block metal ions and dihydrogen was predicted to be weak, thereby suggesting that alkali metal ions like Naþ and Kþ might be more suitable for the purpose of hydrogen storage than other transition metal cations. In this section we have considered model systems based up on interactions of up to only two H2 molecules with light metal ions M ¼ Liþ, Naþ, Be2þ, Mg2þ, and Al3þ. In order to replicate the likely structural arrangement in MC6H6:nH2 complexes where the H2 molecules will not be able to interact with the metal ion M from all sides, we have deliberately placed the two hydrogens in M:nH2 systems on one side of the M ion instead of putting them diagonally opposite. These structural similarities can be understood from Fig. 1 which shows the BHHLYP/6311 þ G* optimized geometries of Liþ:2H2 and LiþC6H6:2H2. For benchmarking the four DFT functionals under investigation we first compared their performance to MP2 results in regard to calculating bond distances and atomic charges. The corresponding values for the M:nH2 systems are presented in Table 1. The table shows that all DFT functionals yield accurate atomic charges in case of Liþ and Naþ ions. But for other M:2H2 systems PBELYP performs relatively poorly. From Table 1 it is also noticeable that for most cases B3LYP and PWLOC functionals calculate the MeH bond distances more correctly than the other functionals considered in this study. More striking differences in the performance of these functionals were elucidated on computation of the H2 binding

Fig. 1 e The BHHLYP/6-311 D G* optimized geometries of (a) LiD:2H2 and (b) LiDC6H6:2H2.

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Table 1 e ESP derived atomic charge on M ion and metal ion-hydrogen bond distances in M:2H2 models calculated using MP2 and different DFT functionals with 6e311 D G* basis set. System

MP2

Q(M) (a.u.) Liþ:2H2 Naþ:2H2 Be2þ:2H2 Mg2þ:2H2 Al3þ:2H2

0.83 0.90 1.23 1.60 2.09

M-H (A˚) Liþ:2H2 Naþ:2H2 Be2þ:2H2 Mg2þ:2H2 Al3þ:2H2

(2.06, 2.06) (2.47, 2.47) (1.63, 1.63) (2.08, 2.08) (1.90, 1.90)

BHHLYP 0.82 0.88 1.21 1.57 2.03

(2.05, 2.05) (2.46, 2.47) (1.63, 1.63) (2.08, 2.08) (1.90, 1.90)

(2.01, (2.40, (1.61, (2.06, (1.92,

B3LYP

PBELYP

0.81 0.88 1.20 1.60 2.00

2.01) 2.39) 1.62) 2.03) 1.89)

(1.99, 2.03) (2.38, 2.40) (1.61, 1.62) (2.05, 2.05) (1.92, 1.89)

(2.04, (2.40, (1.63, (2.05, (1.92,

energies of M:H2 and M:2H2. The corresponding uncorrected and BSSE corrected binding energies are listed in Table 2. Amongst all the functionals, DB.E.s calculated using PWLOC were in close agreement with the MP2 energies. Overall the PBELYP functional was found to behave poorly resulting in DB.E. percentage deviations, with respect to MP2 energies, as large as 21.9%, 48.4% and 49.5% for Liþ:2H2, Naþ:H2, and Naþ:2H2 systems. Thus, in the subsequent calculations for LiþC6H6:nH2 complexes the PBELYP functional was not employed. Comparing the DB.E. and DB.E.bsse listed in Table 2 it can be seen that irrespective of the system and method of calculations the difference between the corrected and uncorrected values is small. The maximum BSSE correction evaluated as 0.25 kJ/mol H2 results in a 1.2% and 1.8% decrease in DB.E. for Liþ:nH2 and Naþ:nH2 systems respectively and in only a fractional percentage decrease in DB.E. for all other cases. Thus, the overestimation in uncorrected binding energies of the above systems is not significant and so BSSE can be neglected. In Table 3 we present the H2 binding enthalpies at 298 K obtained using MP2 and the different DFT functionals. Comparing these with the H2 binding energies in Table 2 it can easily be deduced that for most cases DH(298 K) and DB.E. differ by only up to 5 kJ/mol H2. With the exception of Liþ:nH2 and Naþ:nH2 systems, when M ¼ Be2þ, Mg2þ, and Al3þ the effect of including the thermodynamic corrections lead to

PWLOC

0.80 0.87 1.18 1.55 1.95

2.04) 2.39) 1.63) 2.09) 1.95)

(2.01, (2.38, (1.63, (2.05, (2.02,

2.06) 2.40) 1.63) 2.09) 1.87)

0.83 0.90 1.24 1.61 2.11

(2.01, 2.01) (2.38, 2.38) (1.64, 1.63) (2.08, 2.06) (2.00, 1.95)

(1.99, (2.36, (1.64, (2.06, (2.07,

2.03) 2.39) 1.63) 2.08) 1.90)

(2.04, (2.45, (1.61, (2.06, (1.87,

2.04) 2.45) 1.61) 2.06) 1.87)

(2.04, 2.04) (2.45, 2.45) (1.61, 1.61) (2.06, 2.06) (1.87, 1.87)

deviations within 5.0% from the corresponding DB.E. Thus, computing DH(298 K) may not necessarily result in any further insight than that provided by DB.E. It is illustrated from Tables 2 and 3 that whether the calculations are performed using MP2 or DFT the H2 binding energies and H2 binding enthalpies at 298 K follow the order: Naþ < Liþ < Mg2þ << Be2þ << Al3þ. This order is the same as that predicted for MH16 complexes studied by Chandrakumar and Ghosh [65]. The Liþ:Naþ:Mg2þ:Be2þ:Al3þ binding energy or enthalpy ratio is approximately 1:0.5:4:9:15. This suggests that decorating the organic linker of MOF-5 with Naþ as compared to Liþ may not lead to any significant advantage. In comparison employing Mg2þ, Be2þ or Al3þ ions would result in increased H2 adsorption energies for MOF-5. However, in order to determine their practical applicability it is important to study the interaction of H2 molecules with the metal iondecorated benzene ring and precisely determine their corresponding DB.E.s.

3.2.

LiþC6H6:nH2 complexes

In this section we study the interaction of different numbers of H2 molecules with Liþebenzene ring complexes using BHHLYP, B3LYP, and PWLOC functionals. Fig. 2 displays the BHHLYP/6-311 þ G* optimized geometries of the LiþC6H6:nH2 (n ¼ 0e4) systems and distances of Liþ ion from the ring

Table 2 e Uncorrected and BSSE corrected H2 binding energies of M:1H2 and M:2H2 models calculated using MP2 and different DFT functionals with 6e311 D G* basis set.

þ

Li :H2 Liþ:2H2 Naþ:H2 Naþ:2H2 Be2þ:H2 Be2þ:2H2 Mg2þ:H2 Mg2þ:2H2 Al3þ:H2 Al3þ:2H2

DB.E.bsse (kJ/mol)

DB.E. (kJ/mol)

System MP2

BHHeLYP

B3LYP

PBEeLYP

PWeLOC

MP2

BHHeLYP

B3LYP

PBEeLYP

PWeLOC

25.4 25.2 13.6 13.5 224.6 212.3 91.6 88.4 350.9 325.1

28.1 28.1 12.1 14.0 240.1 225.1 103.4 99.6 388.1 352.4

26.8 27.0 15.0 15.2 241.7 225.0 100.4 100.2 402.8 360.2

28.9 30.7 20.2 20.2 255.1 237.8 100.8 96.7

25.2 24.7 13.1 13.0 224.1 209.5 90.5 85.0 341.1 317.7

25.3 25.0 13.5 13.3 224.4 212.1 91.4 88.2 350.6 324.9

27.9 27.9 11.9 14.0 239.9 224.9 103.2 99.4 387.8 352.2

26.6 27.0 14.8 15.0 241.5 251.0 104.3 100.9 402.5 359.9

28.7 30.5 21.3 20.0 254.8 237.5 100.6 96.5

25.0 24.4 13.0 12.8 223.9 209.3 90.4 84.9 340.9 317.4

386.6

386.3

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Table 3 e H2 binding enthalpies of M:1H2 and M:2H2 models at 298 K calculated using MP2 and different DFT functionals with 6e311 D G* basis set. DH (298 K) (kJ/mol)

System

Liþ:H2 Liþ:2H2 Naþ:H2 Naþ:2H2 Be2þ:H2 Be2þ:2H2 Mg2þ:H2 Mg2þ:2H2 Al3þ:H2 Al3þ:2H2

MP2

BHHLYP

B3LYP

PBELYP

PWLOC

23.1 21.0 12.3 10.2 221.5 206.5 89.5 84.2 356.1 325.4

22.6 19.9 11.8 9.8 232.3 215.1 98.2 94.1 393.4 351.9

29.1 20.3 14.2 10.8 240.6 244.3 57.7 94.5 411.0 364.3

25.7 23.6 20.2 13.8 253.5 231.1 93.4 90.8

22.8 21.7 11.8 9.7 220.2 203.1 88.2 81.7 345.4 317.3

388.1

(Liþering) and from H atoms belonging to the adsorbed H2 molecules (LiþeHads). The figure shows that the first H2 molecule is adsorbed on top of LiþC6H6. As n increases from one to three, the arrangement of the H2 molecules changes around the Liþ ion in order to accommodate the additional H2 molecules. These modifications are in turn accompanied with a simultaneous increase in the Liþ-ring and LiþeHads distances indicative of a weakening of interactions with number of adsorbed H2 molecules. The view of LiþC6H6:4H2 taken from the top is also illustrated in Fig. 2. It is clearly visible that as the fourth H2 is added, in an attempt to avoid overcrowding two H2 molecules ˚ while two H2 molecules are brought are pushed away to w3 A closer to Liþ ion. It appears as if only two H2 of four H2 molecules are strongly ads- orbed. Infact the LiþeHads distances of

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the H2 molecules that are pushed closer to Liþ ion are similar in magnitude to those in LiþC6H6:2H2. This structural geometry analysis suggests that LiþC6H6 is capable of strongly adsorbing a maximum of three H2 molecules. Srinivasu et al. [50] have also studied the interaction of H2 molecules with Liþdoped model systems. In contrast to our optimized structure discussed above, the geometry of C6H6Liþe(H2)4 shows three H2 molecules attached to the Liþ ion with the fourth one far away from the metal ion center. However, both pictures confirm that as more than three H2 molecules are added, the extra molecules will only get weakly adsorbed and are therefore, unlikely to remain bound to the Li-decorated MOF-5 at operating conditions for fuel cells. In order to gain further insight into this scenario we next calculated the associated thermodynamics. Adsorption of molecular hydrogen in MOFs is physisorptive. Previously reported studies have attempted to decipher whether it is the London dispersion interactions (LDI) or interactions due to the electrostatic potential of the host materials, that is major contributing factor to this process. Through a combination of IR spectroscopy of adsorbed H2 over MOF-5 performed at 15 K and ab initio calculations Bordiga et al. [3] showed that the adsorptive properties of this material are mainly due to dispersive interactions with the internal wall structure and to weak electrostatic forces associated with O13Zn4 clusters. Kuc et al. [66,67] have examined the role of the non-bonding interactions, London dispersion interactions, and electrostatic interactions in IRMOF-1 by reducing the MOF structure to clusters modeling the MOF connectors and linkers. Their calculations illustrated that the physisorption of H2 in MOFs is mainly due to London dispersion between linkers and connectors with hydrogen. In

˚. Fig. 2 e The BHHLYP/6-311 D G* optimized geometries of LiDC6H6:nH2 systems where n [ 0e4. All the bond distances are in A

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contrast the hosteguest induced electrostatic interactions are unimportant, as the charge separation in the MOF is not large enough to induce significant dipole moments in H2. The authors further conclude that to correctly describe the LDI it is necessary to use correlation methods and large basis sets. Thus, methods based on DFT which are unable to correctly describe dispersion forces and van der Waals interactions face challenges in studying MOFs. We obtained a H2 binding energy of 1.837 kJ/mol for C6H6:1H2 using BHHLYP/6-311 þ G* which clearly lies outside the 2.26.5e5.2 kJ/mol range [2e4] mentioned earlier in Section 1. For arriving at the correct magnitude we therefore carried out single point MP2 calculations using the BHHLYP optimized geometries. This gave a value of 5.248 kJ/mol in close agreement with the expected values. The DB.E.s of LiþC6H6:nH2 (n ¼ 1e3) complexes evaluated using BHHLYP, B3LYP, and PWLOC functionals are presented in Table 4. Irrespective of the method the DB.E.s of Liþ6H6:nH2 systems are much greater than that of C6H6:1H2. Thus, the interaction of H2 molecules with the decorated MOF-5 is larger than the typical van der Waals forces. The magnitudes suggest that the H2 molecules are bound to the metal ions adsorbed on the benzene ring through electrostatic charge-quadrupole, charge induced-dipole and other orbital interactions. Thus, interactions are governed largely by electrostatics and to a less extent by LDI for these particular systems under study. Therefore, DFT methods should prove effective in calculating DB.E.s to a reasonable degree of accuracy. The appropriateness of application of these functionals is first tested by comparing them to previously reported DB.E.s of the Li-decorated MOF-5 systems evaluated using MP2/631þþG(2d,2p) method [50] and a DFT plane wave-pseudopotential based method employing the PW91 functional [46]. An examination of Table 4 exhibits that B3LYP fails to predict correctly the DB.E.s for each of the three systems. As n increases from one to three, the binding energies increasingly deviate particularly from those calculated by MP2. PWLOC yields DB.E.s in reasonable agreement with the periodic DFT results for n ¼ 1, 2 cases but gives a large error in DB.E. of LiþC6H6:3H2. For n ¼ 3 system PWLOC performs even more poorly than B3LYP. In contrast, with BHHLYP we were able to obtain good concurrence in DB.E.s of LiþC6H6:nH2 for all n with the periodic DFT values. As compared to B3LYP and PWLOC results the BHHLYP binding energies show much less deviation from the MP2 values thereby suggesting that it is a better

functional for accounting for electron correlation for the type of systems under investigation. These observations (i) lend confidence to our calculations using BHHLYP which is applied exclusively for all further computations for M ¼ Liþ, Naþ, Be2þ, Mg2þ, and Al3þ ion-decorated MOF-5, and (ii) suggest that up to a rational degree of accuracy it is fitting to consider only the organic linker moiety of MOF-5 instead of the entire unit cell consisting of 106 atoms for studying H2 adsorption in such systems. The BHHLYP/6-311 þ G* calculated BSSE corrected H2 binding energies DB.E.bsse of LiþC6H6:nH2 (n ¼ 1e3) complexes are also listed in Table 4. The BSSE correction was most significant for LiþC6H6:1H2 resulting in a lowering of DB.E. by 1.1 kJ/mol H2. For the other two systems the BSSE corrections were smaller, 0.6 kJ/mol H2 for LiþC6H6:2H2 and 0.4 kJ/mol for LiþC6H6:3H2. The DB.E.bsse’s therefore, do not differ drastically from the periodic DFT values of Table 4. Based on these results the BSSE corrections are henceforth chosen to be neglected in the subsequent calculations of H2 binding energies of all the light metal ion-decorated benzene models being studied herein. As a test case the thermodynamic corrections at 298 K have been evaluated on the basis of rigid rotor and harmonic oscillator approximations. Examination of the H2 binding enthalpies DH(298 K) presented in Table 4 shows that the consequent result is an increase in the exothermicity of the adsorption of the first H2 molecule by LiþC6H6 and a further weakening of the interactions when two additional H2 molecules are added. This scenario is even predicted from the uncorrected DB.E.s of LiþC6H6:2H2 and LiþC6H6:3H2 which lie well outside the H2 binding energy range of interest of 20e40 kJ/mol for practical H2 storage applications also indicate the same. Thus, at fuel cell operating temperature each Liþ ion decorating the MOF-5 may not be able to keep all the three H2 molecules bound to it. In fact Blomqvist et al. [46] have previously shown from ab initio molecular dynamics simulations in a cell containing 18H2 per formula unit of LiþeMOF-5 that only two hydrogen molecules remain coordinated per Liþ ion at 200 K and still less number of H2 molecules at 300 K. Thus, still further increase in the amount and strength of H2 adsorption are needed for MOF-5 to become suitable for onboard vehicular applications. As suggested by the results of this section calculation of simply DB.E.s should prove sufficient for deciding which light metal ion to be used as opposed to Liþ ion for decorating MOF-5 for bringing out the above mentioned improvements.

Table 4 e BHHLYP, B3LYP, and PWLOC calculated H2 binding energies, BHHLYP BSSE corrected H2 binding energies of calculated using BHHLYP and BHHLYP H2 binding enthalpies at 298 K of LiDC6H6:nH2 (n [ 1e3) complexes compared with previously reported DB.E.s of Li-decorated MOF-5 computed from MP2/6-31DDG(2d,2p) (Ref. [50]) and periodic DFT calculations (Ref. [46]) employing PW91 functional. All energies are in kJ/mol H2. n No. of H2 adsorbed 1 2 3

This work DB.E.

This work DB.E.bsse

This work DH(298 K)

Ref. [50]* DB.E.

Periodic DFT DB.E.

BHHLYP

B3LYP

PWLOC

BHHLYP

BHHLYP

MP2

PW91

20.7 15.0 12.5

16.3 12.3 9.6

19.4 13.3 5.3

19.6 14.4 12.1

28.6 14.0 10.0

22.3 19.2 17.3

18 16 12

*Values have been converted from kcal/mol to kJ/mol.

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3.3.

MC6H6:nH2 complexes

In this section we study the effect of decorating benzene ring with M ¼ Naþ, Be2þ, Mg2þ, and Al3þ ions versus Liþ ion on the hydrogen storage attributes of MOF-5. All calculations herein were performed with the BHHLYP/6-311 þ G* method. We begin by analyzing the atomic charges of different C, H and M atoms of C6H6 and MC6H6 systems which are listed in Table 5. In absence of any metal ion on an average the Mulliken, Lowdin and ESP derived atomic charges over C atoms were -0.22 e, -0.21 e, and -0.12 e and H atoms were þ0.22 e, þ0.21 e, and þ0.12 e, respectively in the benzene. Addition of H2 molecules to the pure benzene ring did not lead to any appreciable changes in these atomic charges and so these values are not presented in the table. Charges over the H atoms of the adsorbed H2 molecules, irrespective of the method used, were close to zero thereby indicating that H2 adsorption over the organic linker in MOF-5 does not involve charge transfer. The effect of placing a metal ion on top of benzene can be interpreted in terms of the magnitude of changes in the above atomic charges. It is clearly seen from Table 5 that Liþ and Naþ ions do not decrease the negative charge over the C atoms or increase the positive charge of the H atoms as much as Be2þ, Mg2þ or Al3þ. A possible reason for the greater effect of these ions could be their stronger interaction with the MOF-5 organic linker. The corresponding interaction energies, calculated using Eq. (1), are also presented in Table 5. An assessment of Table 5 shows that the interaction energies exhibit the same trend as mentioned above based on the < Liþ) << effect on atomic charge, i.e.(Naþ (Mg2þ < Be2þ < Al3þ). Interestingly the DE’s of the latter group are much greater than the cohesive energies of the bulk metals implying that Mg2þ, Be2þ, and Al3þ ions will not suffer from the problem of clustering over the MOF-5 surface faced by transition metal ions and therefore, will not lose the advantage of increased H2 storage that they might offer as predicted on the basis of our previously discussed M:nH2 molecular studies. It should be noted from Table 5 that our DEs for LiþC6H6 and þ Na C6H6 match very well with the previously reported DEs [50] obtained using MP2/6-31þþG(2d,2p). This is indicative of the appropriateness of the BHHLYP functional for our calculations.

The Mulliken, Lowdin and ESP derived charge over Li in LiþC6H6 was found to be þ0.39 e, -0.15 e, and þ0.61 e respectively. Li charge in Liþebenzene structure has been previously reported by Kolmann et al. [68] as þ0.7 e using MP2/6-311 þ G (2df,p) method and as þ0.9 e in Li-decorated MOF-5 unit cell [46,49]. Clearly ESP method is the best route for accurate prediction of these atomic charges. In the MC6H6 complexes the ESP derived atomic charge over the metal ions follows the sequence Li < NaeBe < Mg < Al. At first it appears that this does not conform to the interaction energy order. This is so because it is more correct to compare DE with the amount of charge transferred by the M ion to the benzene ring than simply with the charge available. This charge transfer Qtrans was estimated as the difference between the charge of the bare ion and charge over the ion in the MC6H6 complex. The following order of Qtrans then ensues: Naþ (þ0.21 e) < Liþ (þ0.38 e) < Mg2þ (þ0.75 e) < Be2þ (þ1.21 e) < Al3þ (1.49 e) which reflects the DE variation identically. As in case of C6H6 introduction of H2 molecules did not lead to more differences in charge density picture over the C and H atoms of the benzene ring in MC6H6. However, the charge available over the M ion was found to vary significantly as a function of number of H2 molecules as illustrated by Fig. 3. For all M the charge decreased as the first H2 molecule was added. On adsorption of subsequent hydrogen molecules the charge fell still further, reaching saturation at a certain n. This saturation point was different for different M. This saturation of charge implies that as n increases the H2eH2 intermolecular interactions start playing a more significant role than the H2eMC6H6 interactions in holding the H2 molecules coordinated to the metal ion-decorated MOF-5 organic linker. In Table 6 we present calculated selected bond distances and H2 binding energies of MC6H6:nH2 complexes. The M-ring and M-H (adsorbed) distances are shortest when M ¼ Be2þ and longest in case of Naþ ions. Furthermore, irrespective of the system as number of H2 molecules n increases these bond lengths increase suggestive of a weakening of the involved H2MOF-5 interactions. A comparison from Table 6 of our BHHLYP/6-311 þ G* H2 binding energies for NaþC6H6:1H2 and Naþ C6H6:2H2 systems with those evaluated by Srinivasu et al. [50] using MP2/-631þþG(2d,2p) method shows a very good agreement between the values as opposed to the B3LYP binding energies reported by the authors in the same study, thereby displaying the

Table 5 e Maximum and minimum (represented in paranthesis {.}) Mulliken, Lowdin, and ESP derived atomic charges of C, H and M in MC6H6 complexes and their interaction energies calculated by BHHLYP/6-311 D G* method. System

Q(C) (a.u.) Mulliken

C6H6 LiþC6H6 NaþC6H6 Be2þC6H6 Mg2þC6H6 Al3þC6H6 a

-0.20 -0.17 -0.22 -0.07 -0.13 -0.12

{-0.24} {-0.17} {-0.22} {-0.07} {-0.13} {-0.12}

Q (H) (a.u.)

Lowdin

ESP derived

-0.20 {-0.22} -0.56 {-0.05} -0.14 {-0.13} þ0.07 {þ0.08} -0.06 {-0.04} -0.02 {-0.01}

-0.12 {-0.12} -0.08 {-0.05} -0.11 {-0.04} þ0.02 {þ0.02} -0.04 {-0.01} -0.10 {-0.05}

Mulliken þ0.22 þ0.27 þ0.26 þ0.35 þ0.33 þ0.41

{þ0.22} {þ0.27} {þ0.25} {þ0.35} {þ0.33} {þ0.41}

Lowdin þ0.22 þ0.26 þ0.24 þ0.30 þ0.28 þ0.32

{þ0.20} {þ0.24} {þ0.23} {þ0.28} {þ0.26} {þ0.31}

DE (kJ/mol)

Q (M) (a.u.) ESP derived þ0.12 þ0.13 þ0.13 þ0.18 þ0.15 þ0.17

{þ0.12} {þ0.13} {þ0.11} {þ0.18} {þ0.13} {þ0.18}

Mulliken Lowdin

þ0.39 þ0.79 þ0.30 þ0.83 þ1.24

-0.15 þ0.40 -0.20 þ0.68 þ1.06

DE calculated in Ref. [50] by MP2/6-31þþG(2d,2p) method. Values have been converted from kcal/mol to kJ/mol.

ESP derived þ0.61 þ0.75 þ0.79 þ1.27 þ1.51

172.4 [175.8]a 106.5 [119.8]a 973.5 507.5 1525.4

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Fig. 3 e Variation of BHHLYP/6-311 D G* ESP derived charge over M ion in MC6H6:nH2 complexes with number of H2 molecules adsorbed.

correctness of the choice of the BHHLYP functional in this section. A combined examination of Tables 4 and 6 exhibit the following trends: (i) DB.E.s of MC6H6:nH2 systems are much greater than that of C6H6:1H2, (ii) for all M as n increases DB.E. decreases as reflected by the abovementioned bond length increase and (iii) for the same n DB.E. increases according to the order Naþ < Liþ < Mg2þ < Be2þ < Al3þ which is indeed the same order of DB.E.s exhibited by the bare light metal ions. Thus, it can be generalized that in our choice for M as one goes down a group in the periodic table DB.E. decreases while as we go across a row DB.E. increases. Not only is the H2 binding energy for NaþC6H6:1H2 smaller than for LiþC6H6:1H2 it also falls short of the 20e40 kJ/mol range of interest. The binding energies become still poor with increasing H2 adsorption and therefore decoration with Naþ ions versus Liþ ions will not yield improved H2 storage attributes at room temperature and atmospheric pressure conditions for MOF-5. In comparison the first H2 binding energies of Be2þC6H6 and Al3þC6H6 are approximately six times and nine times, respectively, of the first H2 binding energies of LiþC6H6. Even when five H2 molecules are adsorbed over Al3þC6H6 the DB.E value is about 30 kJ/mol greater than the upper bound of the DB.E range of interest. H2 adsorption in these complexes is therefore likely to be associated with high activation energy barriers and hence, Be2þ and Al3þ-decorated MOF-5 may suffer from slow hydrogen uptake/release. For Mg2þC6H6 systems DB.E. decreases from 62.20 kJ/mol at n ¼ 1 to 32.28 kJ/mol at n ¼ 5. These energies are much higher than the H2 binding energies of Naþ C6H6 and LiþC6H6 but much less than those of Be2þC6H6 and Al3þC6H6. Furthermore, except for the n ¼ 1 and 2 case all DB.E.s lie close to or within the optimal binding energy range for room temperature H2 storage. Eventhough the validity of application of rigid rotor

and harmonic oscillator approximations for systems such as Mg2þC6H6:nH2 is questionable, as a matter of interest we have evaluated the corresponding H2 binding enthalpies at 298 K. The consequent DH(298 K) thereby obtained were 51.23, 45.79, 38.23, 32.04, and 27.47 kJ/mol respectively for n ¼ 1e5H2 molecules adsorbed over Mg2þC6H6. The effect of inclusion of thermodynamic corrections is a reduction in DB.E.s. such that the values of even Mg2þC6H6:1H2 and Mg2þC6H6:2H2 are now much closer to our preferred range thereby rendering H2 storage at ambient conditions in Mg2þ-decorated MOF-5 thermodynamically attractive and most suitable amongst all the possibilities studies herein. The BHHLYP/6-311 þ G* optimized geometries of the Mg2þC6H6:nH2 (n ¼ 1e5) model complexes are depicted in Fig. 4 along with selected bond distances. As n increases from one to four, the H2 molecules rearranged themselves around the Mg2þ ion to accommodate each other. However, when the fifth H2 molecule was added, it was thrown out in order to minimize the resulting overcrowding. Its distance from Mg2þ ion ˚ . This implies that per Mg2þ ion was calculated to be 4.232 A a maximum of four H2 molecules will get directly adsorbed to the metal ion. More H2 molecules may be adsorbed due their interaction with other H2 molecules. The reasonably high H2 binding energy and binding enthalpy at 298 K of Mg2þC6H6:5H2 suggests that unlike the case of Li-decorated MOF-5 these additional H2 molecules may still remain coordinated at room temperature298 K. Assuming each side of the benzene ring in MOF-5 to be decorated by one Mg2þ ion and adsorption of five H2 molecules per Mg2þ ion would lead to a formula unit of Zn4O[BDC(Mg(H2)5)2]3. This is equivalent to a H2 storage capacity of 6.15 wt.%. This is much greater than the gravimetric storage capacity reported for the Li-decorated MOF-5 (4.3 wt.% H2) [37] assuming an adsorption of three H2 molecules per Liþ ion. Thus, both from the perspective of enhanced

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Table 6 e BHHLYP/6-311 D G* calculated M ion-benzene ring and M ion-H atom (adsorbed) bond distances and H2 binding energies in C6H6:1H2 and MC6H6:nH2 models where M [ NaD, Be2D, Mg2D, and Al3D. System C6H6:1H2 NaþC6H6 NaþC6H6:1H2 NaþC6H6:2H2 Be2þC6H6 Be2þC6H6:1H2 Be2þC6H6:2H2 Mg2þC6H6 Mg2þC6H6:1H2 Mg2þC6H6:2H2 Mg2þC6H6:3H2 Mg2þC6H6:4H2 Mg2þC6H6:5H2 Al3þC6H6 Al3þC6H6:1H2 Al3þC6H6:2H2 Al3þC6H6:3H2 Al3þC6H6:4H2 Al3þC6H6:5H2

˚) M-ring (A

˚) M-H (A

DB.E. (kJ/mol) 1.837 {5.248}a

2.373 2.377 2.391 1.295 1.347 1.351 1.924 1.949 1.982 2.018 2.047 2.047 1.642 1.669 1.743 1.754 1.808 1.782

(2.404, 2.434) (2.458, 2.456) (2.465, 2.443)

14.70 [12.7]b 12.33 [12.3]b

(1.602, 1.606) (1.613, 1.569) (3.423, 3.412)

113.32 62.33

(2.077, 2.078) (2.140, 2.136) (2.191, 2.161) (2.266, 2.246) (2.204, 2.215)

(2.140, (2.180, (2.260, (2.282,

2.116) 2.159) (2.191, 2.166) 2.229) (2.275, 2.240) (2.253, 2.236) 2.258) (2.261, 2.218) (2.282, 2.258) (4.104, 4.168)

62.20 55.11 46.01 38.45 32.28

(1.900, 1.883) (1.967, 1.961) (2.044, 2.003) (2.152, 2.101) (2.059, 2.027)

(1.966, (2.038, (2.110, (2.031,

1.966) 2.012) (1.996, 2.051) 2.068) (2.060, 2.088) (2.161, 2.091) 2.011) (1.860, 2.046) (3.190, 3.251) (3.759, 3.599)

178.97 131.61 106.81 81.94 70.64

a DB.E. calculated in the present work by MP2/6-311 þ G* method using geometries optimized by BHHLYP/6-311 þ G*. b DB.E. calculated in Ref. [50] by MP2/6-31þþG(2d,2p) method.

gravimetric hydrogen storage capacities and suitable hydrogen adsorption energy magnitudes, Mg2þ ion decoration of MOF-5 is predicted to render its room temperature H2 storage applications possible.

4.

Conclusion

Herein we have compared the effect of light metal ion (M ¼ Naþ, Be2þ, Mg2þ, and Al3þ) decoration of the organic

linker of MOF-5 on its hydrogen storage ability versus M ¼ Liþ ion. In order to examine the structures, atomic charges, and H2 binding energies (DB.E.) we chose to employ model complexes of the form MC6H6:nH2. It should be stressed that while these are preliminary molecular calculations our results provide a simplistic approach for predicting the trends in actual light metal ion-decorated MOF-5 systems. Our systematic investigations revealed BHHLYP as the most suitable DFT functional for studying these systems. A correlation between the charge transfer (from the M ions to the benzene

˚. Fig. 4 e The BHHLYP/6-311 D G* optimized geometries of Mg2DC6H6:nH2 where n [ 0e4. All the bond distances are in A

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ring) and the interaction of metal ions with the benzene rings was illustrated. All metal ions strengthened the interactions of MOF-5 with H2 molecules as opposed to the bare organic linker. But the calculated H2 binding energies of LiþC6H6:nH2 and NaþC6H6:nH2 systems were still sufficiently low and those of Be2þC6H6:nH2 and Al3þC6H6:nH2 models were predicted to be too large for these modified MOFs to prove useful as good hydrogen adsorbents at room temperature. In comparison, for Mg2þ ion-decorated MOF-5 a high gravimetric storage capacity of 6.15 wt.%. H2 was evaluated along with optimally suited DB.E.s for practical H2 storage applications.

Acknowledgements The authors acknowledge the computational facilities of the Center of Excellence in Scientific Computing at National Chemical Laboratory, Pune. The authors also thank the FP7NMP-EU-India-2 collaborative project HYPOMAP on “New materials for hydrogen powered mobile applications” for providing financial support. One of the authors (T.A.M.) thanks CSIR, India for Senior Research Fellowship. S.P. acknowledges the J. C. Bose Fellowship grant of DST, India towards completion of this work.

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