Role of Alkaline Earth Metal Cations in Improving the Hydrogen

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Role of Alkaline Earth Metal Cations in Improving Hydrogen Storage Capacity of Polyhydroxy Adamantane: A DFT Study Karuppasamy Gopalsamy, and Venkatesan Subramanian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03419 • Publication Date (Web): 18 Aug 2016 Downloaded from http://pubs.acs.org on August 20, 2016

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The Journal of Physical Chemistry

Role of Alkaline Earth Metal Cations in Improving Hydrogen Storage Capacity of Polyhydroxy Adamantane: A DFT Study K. Gopalsamy1,2 and V. Subramanian1,2,* 1

Chemical Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai-600 020, India. 2 Academy of Scientific and Innovative Research (AcSIR), CSIR-CLRI Campus, Chennai.

Abstract The hydrogen storage (H-Storage) capacities of different polyhydroxy adamantanes or adamantanols coordinated (dressed or doped) with alkaline earth metal cations (Mg2+ and Ca2+) have been studied using the density functional theory (DFT) based method. The complex coordinated with one Mg2+ cation adsorbs 4H2 molecules with a binding energy (BE) of 19.98 kcal/mol and the complex comprising of one Ca2+ cation adsorbs maximum of 6H2 molecules with a BE of 21.59 kcal/mol. The maximum number of metal cations which can be anchored to the polyhydroxy adamantane is four. The complexes dressed with four Mg2+ and Ca2+ cations adsorb maximum of 16H2 and 24H2 molecules, respectively. The gravimetric densities of corresponding complexes are 8.4 and 10.4 wt%, respectively. Results reinforce that creation of a number of open metal sites on the adamantanols enhances the hydrogen storage capacity and also binding energy.

Keywords: Polyhydroxy Adamantane, Open Metal Site, Hydrogen Storage, ADMP. *Corresponding author. Tel: +91 44 24411630. Fax: +91 44 24911589. E-mail: [email protected], [email protected].

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Introduction Hydrogen is considered as an alternative energy carrier to replace existing fossil fuels,

due to its high abundance, environment friendliness and high energy density to overcome the future fuel requirement.1,2 In transport sector, combustion of fossil fuel produces carbon dioxide (CO2) as a byproduct. CO2 is one of the greenhouse gases which is responsible for global warming.3,4 Fortunately, hydrogen based fuel cells produce water as a final product in the energy cycle; hence it can be used as a energy carrier in mobile applications.5,6 However, finding a suitable host material to store hydrogen in molecular form is one of the most important and challenging problems. The U.S. Department of Energy (DOE) has set a target for on-board hydrogen storage (H-storage) materials. According to DOE target, the gravimetric density value for the H-storage material is 5.5 wt % for the year 2015, with the operating temperature ranges from -40 to 60 ºC and maximum pressure of 100 atm.7 In previous studies, a variety of materials has been employed and tested for potential gas storage applications.8-17 The basic requirements for a energy storage material (host) are: (1) the host material must absorb number of hydrogen molecules and it should have considerable hydrogen storage capacity (≥ 5.5 wt%), (2) these materials should adsorb hydrogen molecules with appropriate adsorption and desorption kinetics, (3) these materials should possess high structural stability over temperature and pressure, (4) the experimental synthesis of these materials should involve minimum synthetic routes and the cost of production of these new materials need to be low, and (5) the binding energy of adsorbing hydrogen molecules should be in physical adsorption range.18-21 Initially carbon based nano-materials have been considered as H-storage materials with high gravimetric density. Unfortunately, the binding energy of adsorbing H2 molecules on these 2


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materials ranges from 0.95 to 1.19 kcal/mol.22-24 These types of carbon based materials can be effectively employed at very low temperatures. In previous reports, BE of adsorbed hydrogen molecules has been enhanced by using suitable functional groups and metal dopants.25,26 Most commonly used dopants are low atomic weight boron, silicon and nitrogen atoms. By this approach, BEs of H2 molecules have been improved, however, it is not in the range as stipulated by U.S. DOEs target. H-storage capacities of pristine and functionalized carbon based materials have been increased by decorating (non-covalent interaction) them with alkali and alkaline earth metals like Li, Na, K, Be, Mg and Ca.27-30 The s-block elements are low in weight, having high van der Waals radius, which would increase the adsorption of hydrogen molecules as well as the BE of per hydrogen molecule. Further, these metals will not form clusters at high temperature. Like s-block elements, first row transition metals (such as Sc, Ti, Mn, Fe and Ni) are also employed as excellent metal dopants to increase adsorption of capacity of hydrogen molecules.3133

Niu et al. systematically investigated the hydrogen adsorption behavior of Ni+ cation using

first-principle calculation.34 Results obtained from that study emphasized that Ni+ cation adsorbs maximum of 6H2 molecules with 5.99 kcal/mol binding energy per hydrogen molecule. The metal cation polarizes the adsorbing H2 molecules through charge polarization mechanism.34 However, the higher atomic mass of d-block elements reduces the gravimetric densities of these materials, and at high temperature, the d-block elements form clusters over the substrate (due to cohesive energy) which restricts further uptake of hydrogen molecules by reducing the available surface. The synthesis of Metal-Organic Frameworks (MOFs) by Yaghi and coworkers35 has kindled tremendous activities in the development of hydrogen and other gas storage materials. Numerous porous materials have been synthesized by Yaghi et al. which include Covalent3



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Organic Frameworks (COFs),32 Zeolitic-Imidazolate Frameworks (ZIFs)33 and Metal-Organic Polyhedra (MOPs),35 etc. These are three dimensional porous materials made up of linkers (organic acids) and connectors (metal oxides units). These classes of compounds have permanent uniform porosity with tunable pore size, larger surface area and thermal stability. The physicochemical properties of these porous materials are suitable for the adsorption of gas molecules such as H2, CO2, N2, CH4, C2H4 and C2H6, etc. Especially, MOFs, COFs and ZIFs show very high gravimetric density value at 77 K and at maximum pressure (up to 100 atm).36-38 In addition to the Yaghi and coworkers, several other groups have also make seminal contributions to the development of porous materials.39-41 A number of strategies has been developed to enhance the gas storage properties. Recently, the gas storage capacities and BEs of gas molecules on the porous materials have been increased by the introduction of metal dopants and functionalization.42-44 In this context, it is important to mention that Snurr and coworkers have made several significant contributions to the computational modeling of development of gas storage materials.45 They have used experimentally refined crystal structures to create a database for MOFs which can be employed for high-throughput computational screening of gas storage applications.47 Colón et al. have also investigated the hydrogen storage capabilities of 18,383 porous crystalline structures possessing various degrees of Mg functionalization and they have assessed hydrogen capacities of these structures using Grand Canonical Monte Carlo (GCMC) and quantum mechanical approaches.47 It is found from the results that there are relationships between hydrogen uptake and the physical properties of the materials. Furthermore, the incorporation of an optimum amount of magnesium alkoxide increases the isosteric heat of adsorption which in turn improves hydrogen uptake and delivery near ambient temperature.47 In another interesting study, bi-functional group consisting of magnesium or calcium cations have 4


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been incorporated to increase the hydrogen storage capacities of porous aromatic framework materials.48 In similar direction, several studies have been devoted to enhance the uptake of hydrogen in various porous framework materials.49-51 However, a viable class of materials for this application is yet to emerge. Among various classes of these materials, MOFs have several possible potential features due to their adjustable chemical functionality, structural diversity, and ease of functionalization. In the development of MOFs, several molecules with chemical and structural functionalities have been employed. In this regard, adamantane is one of the molecules, which has been exploited to develop MOFs. Ranjbar et al. have determined the hydrogen storage capacity of lithium functionalized adamantane. It is found that the BE of the hydrogen molecule ranges from 3.46 to 5.30 kcal/mol.52 Trujillo et al. have studied the interaction of various anions and cations with cyclohexane and adamantanes.53 In our previous study, we have also investigated the hydrogen storage capacity of adamantane doped with five different metal cations (Li+, Na+, K+, Mg2+ and Ca2+).54 Results show that the doping of light weight metal cations increases the BE of per hydrogen molecules. Recently, Li et al. have investigated the hydrogen storage capacities of adamantane based aromatic framework and covalent organic framework materials using Grand Canonical Monte-Carlo simulations.55,56 It is evident from previous investigations that metal doped adamantane and lithium functionalized adamantanes can be employed as an H-storage material. The alkali earth metals are low atomic weight elements having large van der Waals radii. Hence, it can accommodate more number of H2 molecules. Based on the above-mentioned findings, a systematic investigation on the hydrogen storage capacities of polyhydroxy adamantane coordinated with alkaline earth metal cations have been undertaken using computational chemistry approaches. Furthermore, an attempt has been made to increase the 5



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gravimetric density and uptake capacities of hydrogen molecules (binding energy per hydrogen molecule (BE/H2) for each model system) of polyhydroxy adamantanes by employing dressing of Be2+, Mg2+ and Ca2+ metal cations. 2

Computational details Adamantane is a cage like colorless crystalline hydrocarbon consisting of four inter-

connected cyclohexane rings arranged in the armchair configuration. The structure of adamantane is unique, rigid and reactive. In the present study, all electronic structure calculations were performed at M06-2X/6-311++G** level of theory.57 The M06-2X is a highly nonlocal functional with double the amount of nonlocal exchange (2X) and it is based on simultaneously optimized exchange and correlation functionals both including kinetic energy density. Truhlar and co-workers have found that this functional yields very good performance for applications involving thermo chemistry and noncovalent interactions.58-61 The geometrical parameters of adamantane obtained from the DFT calculations are compared with the experimental results62 in Table 1. Table 1 Comparison of Geometrical Parameters of Adamantane obtained from Calculated and Experimental Values. Parameters Calculated Values Experimental Values C-C 1.54a 1.54 C-H 1.09a 1.11 b C-C-C 109.35° 109.08° H-C-H 107.30°b 116.9° a b bond length (Å), bond angle (º) The polyhydroxy adamantanes [Admol], alkaline earth metal cations (Be2+, Mg2+ and Ca2+) coordinated polyhydroxy adamantanes [Admol-(M)m] and hydrogen molecules adsorbed [nH2@Admol-(M)m] complexes were optimized without any symmetry constraints. The vibrational frequency calculations were also performed at the same level of theory to confirm the 6


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minima of the complexes on the corresponding potential energy surfaces. In addition, the charges on the metal cations (before and after adsorption of H2 molecules) were also calculated using Natural Population Analysis (NPA) scheme.63 The BE for nH2 molecules adsorbed complexes were calculated using the following energy expression by adopting supermolecule approach. n   BE = − Etotal − ∑ Ei  1 i =1   where, Etotal is the total energy of the complex, Ei is the energy of the monomer and n denotes the

total number of monomers present in the complex. The calculated BE of all the complexes were further corrected for Basis Set Superposition Error (BSSE) using the counterpoise (CP) procedure.64 To understand the effect of temperature on the desorption of hydrogen molecules, Ab Initio Molecular Dynamics (AIMD) simulations were performed using Atom Centered Density Matrix Propagation (ADMP)65 approach for the complexes having maximum hydrogen storage capacity. ADMP is an extended Lagrangian approach to MD using Gaussian basis functions and propagating the density matrix along the classical nuclear degrees of freedom.66 The ADMP method has been established to be reasonably accurate compared to that of Born-Oppenheimer MD67 at low computational cost. The ADMP calculations were carried out at M06-2X/6-31G* level of theory, at four different temperatures (50, 100, 200, and 300 K) for 1 ps with 0.1 fs time interval. Default fictitious electron mass (0.1 amu) was used in the simulation. Throughout the simulation, temperatures of the systems were maintained by using velocity scaling thermostat.68 The initial value for the Nuclear Kinetic Energy (NKE) was set according to Boltzmann distribution.69 All electronic structure calculations including ADMP simulations were performed using Gaussian 09 suite of program.70

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2.1

Adsorption of Metal Cations on Polyhydroxy Adamantanes The alkaline earth metal cations (Be2+, Mg2+ and Ca2+) coordinated polyhydroxy

adamantane complexes were optimized using M06-2X/6-311++G** level of theory. The calculated total BE and BE/metal cation of the corresponding complexes are given in Tables S1S3 (see supporting information for details). It can be observed from the results that the doped metal cations are chemically bonded with the polyhydroxy adamantane and form stable complexes. 2.2

Adsorption of Hydrogen Molecules on [Admol-(M)m] Complexes Let us first discuss the results on the interaction of hydrogen molecules with metal

cations coordinated [Admol-(M)m] complexes. The findings from calculations reveal that complexes anchored with Mg2+ and Ca2+ cations adsorb relatively more number of H2 molecules than the complex coordinated with Be2+ cation. This is due to the ionic radius of the metal cations. The ionic radius (0.45 Å) of Be2+ is less than the Mg2+ (0.72 Å) and Ca2+ (1.0 Å) cations. The number of H2 molecules adsorbed over metal cation is directly proportional to the ionic radius of the cations. The [Admol-(Be2+)] complex can adsorb maximum of 2H2 molecules. The calculated gravimetric density value of the corresponding complex is 2.2 wt%. It can be found from this study that the H-storage capacity of [Admol-(Be2+)] complex is less than that of 4H2@[Admol-(Mg2+)] (4.1 wt%) and 6H2@[Admol-(Ca2+)] (5.5 wt%) complexes. Since Be2+ doped adamantanols adsorb few number of H2 molecules with less gravimetric density, further calculations were performed on Mg2+ and Ca2+ cations coordinated complexes. 3

Results and Discussion Scheme 1 represents the possible structures of polyhydroxy adamantane and their

corresponding complexes with metal cations (Mg2+ and Ca2+). 8


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Scheme 1 Schematic representations of Mg2+ and Ca2+ cations coordinated polyhydroxy adamantane The electronic properties of (HOMO-LUMO gap) polyhydroxy adamantanes and the corresponding metal cations (Mg2+ and Ca2+) dressed complexes were calculated. The calculated HOMO-LUMO gap values for the complexes are listed in supporting information (Table S4). It can be observed from the results that coordination of metal cations significantly decreases the HOMO-LUMO gaps of the polyhydroxy adamantanes. Specifically, the (Ca2+)4 doped complex has HOMO-LUMO gap of 3.71 eV. It is possible to create a maximum of four open metal sites in polyhydroxy adamantanes. Hence, the metal cations were added in a step by step manner to design complexes dressed with four cations. Totally ten different complexes [Admol-(M), Admol-(M)2(a-f), Admol-(M)3(a,b), and Admol-(M)4 (where M = Mg2+ and Ca2+)] were designed based on structural isomerism. The optimized geometries of [Admol-(Mg2+)m] and [Admol-(Ca2+)m] (where m=1, 2, 3, 4)complexes and their respective geometrical parameters are shown in Figure 1 and in supporting information (Figure S1). In order to assess the H-storage capacities of these complexes, they were further interacted with hydrogen molecules. To unravel the adsorption ability of these complexes, hydrogen molecules were added successively near the cations within 2.5 – 3 Å from the center of cation in an appropriate orientation. The maximum number of hydrogen molecules adsorbed per Mg2+ cation is four and the same for Ca2+ cation is six. In the following sections, the properties

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of nH2 molecules adsorbed [Admol-(M)m] complexes (m = 1 – 4, M = Mg2+ and Ca2+) are discussed.

[Admol-(Mg2+)]

[Admol-(Mg2+)2](a)

[Admol-(Mg2+)2](b)

[Admol-(Mg2+)2](c)

[Admol-(Mg2+)2](d)

[Admol-(Mg2+)2](e)

[Admol-(Mg2+)2](f)

[Admol-(Mg2+)3](a)

[Admol-(Mg2+)3](b)

[Admol-(Mg2+)4] Figure 1 Optimized geometries of [Admol-(Mg2+)m] (where m = 1, 2, 3, 4) complexes at M062X/6-311++G** level of theory

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3.1

Hydrogen Storage Capacity of [Admol-(Mg2+)m] Complexes It is found from the results that [Admol-(Mg2+)] complex can adsorb a maximum of 4H2

molecules. Two of the H2 molecules interact through the plane formed by O‒Mg2+‒O bonds, whereas the other two molecules bind with the cation perpendicular to the plane. The BE of hydrogen molecules interacting along the plane is higher than the perpendicular mode of interaction. The important geometrical parameters of complexes are shown in Figure 2. The calculated total BE, BE/H2 and gravimetric density values for nH2@[Admol(Mg2+)m] complexes are given in Table 2. The total BE for four hydrogen molecules adsorbed complex is 19.98 kcal/mol (BE/H2 ranges from 4.99 to 5.52 kcal/mol). The presence of O···H interaction in the complex marginally increases the BE of the interacting hydrogen molecules. The gravimetric density of the 4H2@[Admol-(Mg2+)] complex is 4.1 wt% which is closer to U.S. DOEs target for 2015 (5.5 wt%). In our earlier investigation, we have calculated binding energy for complexes with non-covalently doped Mg2+ cation. The charge on the metal is 2+ and hence associated BEs are significantly higher than the present values. In the current study, metal cations are coordinated with the adamantanols. Thus, the calculated charge on the complex is zero. Thus, calculated BEs are relatively lower in the present investigation when compared to those of our previous study. However, we found from our previous study that it is not possible to dope more than one metal cation over the adamantane. Interestingly, we can create number of open metal sites in polyhydroxy adamantane which can facilitate adsorption of more number of H2 molecules. The results obtained from [Admol-(Mg2+)] complex reveal that, single Mg2+ cation can adsorb a maximum of four H2 molecules without any steric hindrance. Therefore, creation of open metal site could possibly enhance the gravimetric density of polyhydroxy adamantane. 11



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1H2@[Admol-(Mg2+)]

2H2@[Admol-(Mg2+)]

3H2@[Admol-(Mg2+)]

4H2@[Admol-(Mg2+)]

M-M: 7.281 Å

M-M: 6.334 Å

8H2@[Admol-(Mg2+)2](b)

8H2@[Admol-(Mg2+)2](a)

M-M: 6.017 Å

M-M: 5.352 Å 2+

2+

8H2@[Admol-(Mg )2](d)

8H2@[Admol-(Mg )2](c) 12


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M-M: 5.192 Å

M-M: 5.202 Å 2+

2+

8H2@[Admol-(Mg )2](e)

8H2@[Admol-(Mg )2](f)

M2 M2

M1

M3 M1

M3

M1-M2: 5.223 Å M2-M3: 6.838 Å M3-M1: 6.351 Å

M1-M2: 6.450 Å M2-M3: 6.492 Å M3-M1: 6.470 Å

12H2@[Admol-(Mg2+)3](b)

12H2@[Admol-(Mg2+)3](a) M2

M3

M1

M1-M2: 5.171 Å M2-M3: 6.367 Å M3-M4: 5.172 Å M4-M1: 6.333 Å M1-M3: 6.815 Å M2-M4: 6.121 Å

M4

16H2@[Admol-(Mg2+)4] Figure 2 Optimized geometries of nH2@[Admol-(Mg2+)m] (where n = 1 – 4, 8, 12, 16 and m = 1, 2, 3, 4) complexes at M06-2X/6-311++G** level of theory 13



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Hence, in the next stage, further functionalization has been carried out. Totally, six structural isomers are possible for complexes with two metal sites and their respective optimized geometries are presented in Figure 1. These complexes are referred to as [Admol-(Mg2+)2](a-f). Results obtained from the previous section indicate that the [Admol-(Mg2+)2] complex can accommodate a maximum of 8H2 molecules. The optimized geometries of [Admol-(Mg2+)2](a-f) complexes loaded with 8H2 molecules are given in Figure 2 along with important geometrical parameters. The presence of O···H2···O interaction (highlighted in blue color) in these complexes is shown in Figure 2. The total BEs and BE/H2 of the corresponding complexes are listed in Table 2. The 8H2@[Admol-(Mg2+)2](f) complex has higher BE than other isomers. The calculated gravimetric density of 8H2@[Admol-(Mg2+)2](a-f) complexes is 6.2 wt%. The synergistic effect of both M-H2 and O···H2···O interactions enhances the total binding energy. Table 2 The Calculated BE, BE/H2, and Gravimetric Density of nH2@[Admol-(Mg2+)m] Complexes (where n = 1 – 4, 8, 12, 16 and m = 1, 2, 3, 4) Using M06-2X/6-311++G** Level of Theory. BE BE/H2 Gravimetric S. No Complexes (kcal/mol) (kcal/mol) density (wt%) 1 1H2@[Admol-(Mg2+)] 5.52 5.52 1.0 2+ 2 2H2@[Admol-(Mg )] 10.98 5.49 2.1 2+ 3 3H2@[Admol-(Mg )] 15.85 5.28 3.1 2+ 4 4H2@[Admol-(Mg )] 19.98 4.99 4.1 5 8H2@[Admol-(Mg2+)2](a) 39.71 4.96 6.2 2+ 6 8H2@[Admol-(Mg )2](b) 38.96 4.87 6.2 2+ 7 8H2@[Admol-(Mg )2](c) 39.39 4.92 6.2 2+ 39.60 4.95 6.2 8 8H2@[Admol-(Mg )2](d) 2+ 9 8H2@[Admol-(Mg )2](e) 39.31 4.91 6.2 2+ 10 8H2@[Admol-(Mg )2](f) 40.75 5.09 6.2 2+ 11 12H2@[Admol-(Mg )3](a) 58.77 4.90 7.5 2+ 12 12H2@[Admol-(Mg )3](b) 55.69 4.64 7.5 2+ a b 13 16H2@[Admol-(Mg )4] 77.51(38.77) 4.84(2.42) 8.4 a b 71 BE, BE/H2 of the complexes calculated at MP2/6-311++G** level of theory.

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After quantifying the H-storage capacities of [Admol-(Mg2+)2] complexes, an additional open metal site was introduced in adamantanols moiety to enhance the storage capacity. Two possible geometries were optimized without any symmetry constraints. The geometries are denoted as 12H2@[Admol-(Mg2+)3](a) and 12H2@[Admol-(Mg2+)3](b). The calculated energetics of the first complex is (BE is 58.77 and BE/H2 is 4.90 kcal/mol) higher than the latter (BE is 55.69 and BE/H2 is 4.64 kcal/mol). The calculated bond distances range from 2.262 to 2.539 Å and the O···H2···O bond distance is 2.246 Å. The estimated gravimetric density value of these complexes is 7.5 wt%. Visual inspection reveals that it is possible to create one more open metal site on adamantanols. The [Admol-(Mg2+)4] complex adsorbs a maximum of sixteen hydrogen molecules. The calculated BE and BE/H2 values of this complex are 77.51 and 4.84 kcal/mol, respectively. The calculated Mg‒H distance varies from 2.270 and 2.491 Å. This complex contains six non-covalent O···H bonds (highlighted with red color) and the respective distances are 2.241, 2.243, 2.244 2.297, 2.307 and 2.349 Å. Two O···H2···O interactions can be found with distances of 2.216 and 2.243 Å. This complex shows higher gravimetric density than the other nH2@[Admol-(Mg2+)m] (where m = 1 – 3) complexes. The calculated gravimetric density of [Admol-(Mg2+)4] complex is 8.4 wt%, which is higher than the current DOE target value (5.5 wt%). All these results are summarized in a graphical form in supporting information (Figure S2). 3.2

Hydrogen Storage Capacity of [Admol-(Ca2+)m] Complexes The optimized geometries of [Admol-(Ca2+)m] (m = 1, 2, 3, 4 ) complexes are presented

in supporting information (Figure S1). The ionic radius of Ca2+ is higher than that of Mg2+. Hence, it is expected that the Ca2+ cation can bind with more number of H2 molecules than Mg2+ 15



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cation. The mode of binding of hydrogen molecule on these complexes is depicted in Figure 3. The calculated BE, BE/H2 and gravimetric density values of these complexes are presented in Table 3. Due to the larger ionic radius of calcium, the calculated distances between the metal and adsorbing hydrogen molecule are marginally longer than those of Mg2+ complexes. The preferable binding site for the H2 molecules in the complexes is metal oxygen plane. Scrutiny of results reveals that [Admol-(Ca2+)] can bind with a maximum of six H2 molecules. Among these six hydrogen molecules, first five hydrogen molecules preferably adsorb on regions closer to the metal site and the sixth H2 interacts with the cation in a T-shaped fashion. The total BE of the complex with 6H2 molecules is 21.59 kcal/mol (BE/H2 ranges from 3.60 to 3.95 kcal/mol). The calculated distances between Ca2+ and H2 range from 2.508 to 2.736 Å. The gravimetric density of [Admol-(Ca2+)] complex with 6H2 molecules is 5.5 wt%, which is closer to the current DOE target value (5.5 wt%).

1H2@[Admol-(Ca2+)]

2H2@[Admol-(Ca2+)]

3H2@[Admol-(Ca2+)]

4H2@[Admol-(Ca2+)]

5H2@[Admol-(Ca2+)]

6H2@[Admol-(Ca2+)]

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M-M: 7.983 Å

M-M: 6.949 Å

12H2@[Admol-(Ca2+)2](a)

12H2@[Admol-(Ca2+)2](b)

M-M: 6.609 Å

M-M: 5.863 Å

2+

2+

12H2@[Admol-(Ca )2](c)

12H2@[Admol-(Ca )2](d)

M-M: 5.672 Å

M-M: 5.638 Å

2+

2+

12H2@[Admol-(Ca )2](e)

12H2@[Admol-(Ca )2](f)

17



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M2 M2

M1

M3 M1 M1-M2: 5.606 Å M2-M3: 7.419 Å M3-M1: 6.948 Å

M1-M2: 6.903 Å M2-M3: 7.046 Å M3-M1: 6.965 Å

M3

18H2@[Admol-(Ca2+)3](a)

18H2@[Admol-(Ca2+)3](b)

M3 M2

M1-M2: 5.558 Å M2-M3: 6.879 Å M3-M4: 5.566 Å M4-M1: 6.753 Å M1-M3: 7.146 Å M2-M4: 6.714 Å

M4

M1

24H2@[Admol-(Ca2+)4] Figure 3 Optimized geometries of nH2@[Admol-(Ca2+)m] (where n = 1 – 4, 8, 12, 16 and m = 1, 2, 3, 4) complexes at M06-2X/6-311++G** level of theory It can be seen from Figure 3 that, [Admol-(Ca2+)2] complexes adsorb maximum of 12H2 molecules. These complexes are referred to as 12H2@[Admol-(Ca2+)2](a-f). The calculated BE (BE/H2) for these 12H2@[Admol-(Ca2+)2](a-e) complexes are 43.62 (3.64), 43.71 (3.64), 45.36 (3.78), 46.60 (3.88) and 42.38 kcal/mol (3.53 kcal/mol), respectively. Among 12H2@[Admol(Ca2+)2](a-f) complexes, 12H2@[Admol-(Ca2+)2](f) has the highest BE of 46.65 kcal/mol and

18


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BE/H2 of 3.89 kcal/mol. The calculated H-storage capacity [Admol-(Ca2+)2](a-f) with 12H2 complexes is 8.1 wt%. The maximum number of H2 molecules which can interact with [Admol-(Ca2+)3] is eighteen. These complexes are denoted as 18H2@[Admol-(Ca2+)3](a) and 18H2@[Admol(Ca2+)3](b) are depicted in Figure 3. The calculated total BEs (BE/H2) of both 18H2@[Admol(Ca2+)3](a) and 18H2@[Admol-(Ca2+)3](b) complexes are 68.37 (3.80) and 66.00 kcal/mol (3.67 kcal/mol), respectively. The gravimetric density value of these complexes is 9.5 wt%. Table 3 The Calculated BE, BE/H2, and Gravimetric Density of nH2@[Admol-(Ca2+)m] Complexes (where n = 1 – 6, 12, 18, 24 and m = 1, 2, 3, 4) Using M06-2X/6-311++G** Level of Theory. BE Gravimetric BE/H2 S. No Complexes density (wt%) (kcal/mol) (kcal/mol) 2+ 1 1H2@[Admol-(Ca )] 3.95 3.95 0.9 2+ 2 2H2@[Admol-(Ca )] 7.94 3.97 1.9 3 3H2@[Admol-(Ca2+)] 11.14 3.71 2.8 4 4H2@[Admol-(Ca2+)] 15.58 3.89 3.7 2+ 5 5H2@[Admol-(Ca )] 19.37 3.87 4.7 2+ 6 6H2@[Admol-(Ca )] 21.59 3.60 5.5 2+ 7 12H2@[Admol-(Ca )2](a) 43.62 3.64 8.1 2+ 8 12H2@[Admol-(Ca )2](b) 43.71 3.64 8.1 2+ 9 12H2@[Admol-(Ca )2](c) 45.36 3.78 8.1 2+ 10 12H2@[Admol-(Ca )2](d) 46.60 3.88 8.1 2+ 11 12H2@[Admol-(Ca )2](e) 42.38 3.53 8.1 2+ 12 12H2@[Admol-(Ca )2](f) 46.65 3.89 8.1 2+ 13 18H2@[Admol-(Ca )3](a) 68.37 3.80 9.5 2+ 66.00 3.67 9.5 14 18H2@[Admol-(Ca )3](b) 2+ a b 15 24H2@[Admol-(Ca )4] 91.00(41.67) 3.79(1.74) 10.4 a b BE, BE/H2 of the complexes calculated at MP2/6-311++G** level of theory The last member in the series of [Admol-(Ca2+)m] complex is [Admol-(Ca2+)4] which adsorbs a maximum of 24H2 molecules. The total BE of the complex is 91.00 kcal/mol and the corresponding BE/H2 is 3.79 kcal/mol. The calculated gravimetric density value for this complex is 10.4 wt%. The variation of BE and BE/H2 with respect to number of hydrogen molecules is 19



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plotted in supporting information (Figure S3). Results show that the interaction of H2 molecules with Mg2+ and Ca2+ coordinated complexes adopts T-shaped geometry which is favored by the electrostatic charge-quadrupole interaction and electron donor-acceptor bonding. This preference for this geometrical orientation has also been supported by previous theoretical reports.48 3.3

Natural Population Analysis The NPA was performed for all [Admol-(M2+)m] and nH2@[Admol-(M2+)m] (where M =

Mg2+ and Ca2+) complexes using M06-2X/6-311++G** method. The NPA charges for the [Admol-(Mg2+)m],

nH2@[Admol-(Mg2+)m],

[Admol-(Ca2+)m]

and

nH2@[Admol-(Ca2+)m]

complexes are listed in supporting information (Tables S5 and S6). It can be inferred from the tables that the charge of the metal cations decreases after adsorption of hydrogen molecules. This clearly shows that the anchored metal cation polarizes the adsorbed H2 molecules and charge transfer takes place from H2 molecule to the metal cation. 4

ADMP Simulation Among various [Admol-(Mg2+)m] and [Admol-(Ca2+)m] complexes, 16H2@[Admol-

(Mg2+)4] and 24H2@[Admol-(Ca2+)4] exhibit highest BE/H2 and gravimetric density for the Hstorage applications. Hence, ADMP simulations were carried out to understand the desorption of H2 molecules at various temperatures. The snapshots of these two complexes as obtained from ADMP simulations are given in Figures 4 and 5.

1 ps (50 K)

1 ps (100 K) 20


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1 ps (200 K) 1 ps (300 K) 2+ Figure 4 Snapshots of 16H2@[Admol-(Mg )4] complexes at 1 ps and T = 50, 100, 200 and 300 K

1 ps (50 K)

1 ps (100 K)

1 ps (200 K) 1 ps (300 K) 2+ Figure 5 Snapshots of 24H2@[Admol-(Ca )4] complexes at 1 ps and T = 50, 100, 200 and 300 K The variation of potential energy with respect to time at four different temperatures for 16H2@[Admol-(Mg2+)4] and 24H2@[Admol-(Ca2+)4] complexes are presented in supporting information (Figures S4 and S5). It is evident from the Figure S6 that, the dissociation of hydrogen molecule from the 16H2@[Admol-(Mg2+)4] complex does not take place in the temperature range from 50-200 K. However, the dissociation of H2 from 16H2@[Admol-(Mg2+)4] complex occurs at 300 K after 100 fs onwards. Further simulation reveals that only one hydrogen molecule can be desorbed 21



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from the complex up to 400 fs. The second H2 molecule dissociates from 500 to 900 fs at 300 K. Overall, a maximum of three H2 molecules are released in 1 ps time scale at 300 K by 16H2@[Admol-(Mg2+)4] complex. The simulation at 200 K shows that, the dissociation of H2 molecules from the 24H2@[Admol-(Ca2+)4] complex starts at 600 fs. The snapshots shown in supporting information (Figure S7) elicit that one H2 molecule is released from the complex within 1 ps at 200 K. The trajectory of simulation at 300 K of 24H2@[Admol-(Ca2+)4] complexes indicate that desorption of five H2 molecules occurs from 100 fs. The release of the sixth H2 takes place during the time interval 200-700 fs. Complete analysis of trajectory collected at 300 K (Figure S8) points out that detachment of seven hydrogen molecules occurs in the time scale which ranges from 800-1000 fs. Results highlight that functionalization of adamantanols with open metal sites (Mg2+ and Ca2+) significantly enhances the hydrogen storage capacities of the complexes. Findings from ADMP calculations reveal that, when the temperature is increased from 50 to 300 K, the escaping tendency of H2 molecules is reduced due to the presence of interaction between oxygen atom and adsorbed hydrogen molecules. Hence, this oxygen rich substrate acts as an effective secondary adsorption site for H2 molecules, which significantly stabilizes the H2 molecules. It is found from previous studies that adamantane based linkers have been used to build MOFs.72,73 It was found that MOFs with adamantane exhibited good thermal stability (350-400 °C). Thus, incorporation of these functionalized adamantane as a linker in the designing of MOF may have appreciable hydrogen storage capacity along with thermal stability. We have also investigated the effect of cluster formation (Admol-M-Adoml; M= Ca2+) and hydrogen storage capacities of the corresponding complexes. Our model system has been arrived on the basis of combining stoichiometric amounts (1:1) of M2+ (M = Ca2+) cation and 22


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hydroxy adamantane. Hence, the possibility of formation of two/three hydroxy adamantanes with one M2+ is rare. The geometry of two hydroxyl adamantanes with one Ca2+ ion dressed complex [2Admol-(Ca2+)]2- and their NPA charges were calculated using M062X/6-311++G** method. Result reveals that the calculated NPA charges for Ca2+ is 1.621 a.u. in [2Admol-(Ca2+)]2complex. This value is very much closure to the NPA charge on Ca2+ (1.667 a.u.) in [Admol(Ca2+)] complex. The charge on the metal cation is not significantly decreased even after complex formation. Hence, there may not be any significant difference in the hydrogen binding capacity. We have also calculated the hydrogen storage capacity of the [2Admol-(Ca2+)]2- anion complexes. The complex adsorb maximum of 9H2 molecules with total BE of 27.39 kcal/mol with BE/H2 is 3.04 kcal/mol. The optimized geometries of [2Admol-(Ca2+)]2- anion and 9H2 molecules adsorbed complexes are shown in Figure 6.

Figure 6 Optimized geometries of 9H2@[2Admol-(Ca2+)]2- complex at M06-2X/6-311++G** level of theory The NPA charge for 9H2@[2Admol-(Ca2+)]2- complex is 1.602 a.u. The calculated gravimetric density value of the corresponding complex is 4.6 wt%. Therefore, even when Ca2+ ion binds with more than one hydroxy adamantane, the resultant complex can be still used for H2 storage application. To investigate the stability of the [2Admol-(Ca2+)]2- anion complex we performed ADMP simulations at four different temperature (50, 100, 200 and 300 K) for 1 ps. 23



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The variation of potential energy with respect to time at four different temperatures for [2Admol(Ca2+)]2- anion complexes are shown in Figure 7.

Figure 7 Potential energy trajectories at different temperatures for [2Admol-(Ca2+)]2-complex 6

Conclusions In this investigation, the hydrogen storage capacities of various polyhydroxy

adamantanes anchored with alkaline earth metal cations (Mg2+ and Ca2+) have been calculated using M06-2X/6-311++G** method. Results reveal that the introduction of open metal sites on adamantanols enhances the storage capacity of the complexes. It is evident from the calculations that among Mg2+ incorporated complexes, 16H2@[Admol-(Mg2+)4] exhibits higher total BE (BE/H2) of 77.51 (4.84) kcal/mol and the gravimetric density of 8.4 wt%. These values are akin to that value envisaged for 2015 (5.5 wt%). The comparison of all Ca2+ dressed complexes reveals that 24H2@[Admol-(Ca2+)4] has maximum BE of 91.00 kcal/mol with BE/H2 of 3.79 kcal/mol and concomitant gravimetric density of 10.4 wt%. MD simulations reveal that, the open metal site motifs effectively adsorb hydrogen in molecular form at low and room temperature. The dissociation of H2 molecules from the complexes ensures the applicability of these models

24


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for hydrogen storage materials. Further, GCMC calculations are in progress to assess their desired thermodynamic properties. Supporting Information i) Calculated metal ion binding energy, ii) HOMO-LUMO gap, iii) NPA charges, iv) Total BE and BE/nH2 graph, v) ADMP potential energy graph and vi) ADMP simulation snapshots. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgement This work is supported by Multi-Scale Simulation and Modeling project-MSM (CSC0129) funded by Council of Scientific and Industrial Research (CSIR), New Delhi, India. Authors thank Mr. E. R. Azhagiya Singam and Mr. J. Vijaya Sundar for their help in the preparation of the manuscript.

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Role of Alkaline Earth Metal Cations in Improving the Hydrogen

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