Hydrogen Sorption Efficiency of Titanium-Functionalized Mg–BN

Apr 30, 2014 - The average hydrogen desorption temperature using van't Hoff equation is predicted to be 323 K with hydrogen storage capacity of 7.8 wt...
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Hydrogen Sorption Efficiency of Titanium-Functionalized Mg−BN Framework Madhu Samolia and T. J. Dhilip Kumar* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, India S Supporting Information *

ABSTRACT: Hydrogen is considered as a potential candidate for future energy source in automobile industry. Hydrogen storage is the major problem in achieving this goal. In this study, metal−organic framework (MOF) with organic linker is replaced with BN linker namely borazocine (B4N4H8) and is functionalized with Ti, enhancing the stability and storage capacity of the framework. A first-principles electronic structure calculation using spin polarized generalized gradient approximation with Perdew−Burke−Ernzerhof functional, structural optimization and molecular dynamics (MD) simulations have been performed for hydrogen sorption efficiency of the Ti-functionalized Mg4O−BN framework (MBF). Low adsorption and desorption energies suggest the high hydrogen reversibility of the system. BN ring coordinates strongly with the Ti metal by Dewar interaction while each Ti metal adsorbs 4 H2 molecules by Kubas interaction. In MD simulations, 75% of the physisorbed H2 molecules are desorbed at 300 K while at 373 K chemisorbed hydrogen also began to desorb with the MBF framework remaining structurally stable. The average hydrogen desorption temperature using van’t Hoff equation is predicted to be 323 K with hydrogen storage capacity of 7.8 wt %. For the first time, the H2 sorption efficiency of the Ti-functionalized metal−BN framework has been studied and is found to be better than MOF or metal-functionalized MOF with respect to storage capacity, stability, and reversibility, making it a potential hydrogen storage material. (MOF),32−40 and so on. Over the past several years, many porous materials for various applications, particularly gas storage and CO2 capture, have been explored that include amorphous polymers of intrinsic microporosity (PIMs),41 covalent boron oxide-base frameworks (COFs),42 porous aromatic frameworks (PAFs),43 conjugated microporous polymers (CMPs),44 porous polymer networks (PPNs),45 hyper-cross-linked polymers (HCPs),46 and triazine-based organic frameworks (CTFs).47 But none of those materials exactly meet all the targets for ideal hydrogen storage material set by the DOE. In particular, MOFs have attracted attention for hydrogen storage as they offer high porosity, stability, can be flexibly designed through control of the architecture and functionalization of the building blocks, and materials are stable even when the pores are emptied by heating.48 Isoreticular MOFs are reported to have high thermal stability up to 673 K.49 Hydrogen adsorption in MOFs is improved by the addition of guest molecules in the pores and by optimization of the pore size.50 The binding energy is improved by inclusion of open metal sites51 and introduction of electron donating groups. The main drawback of MOFs is that they require low temperature for a sufficient amount of hydrogen adsorption, which is around

1. INTRODUCTION Today’s transportation sector is totally dependent on petroleum-derived fuel. But nowadays, these fuels are at the edge of depriving and require a better replacement. Hydrogen is considered a potential candidate for future energy as it is clean, producing only water, contains the highest energy density per unit mass, and is highly abundant.1,2 This feasibility depends on the hydrogen economy, which involves the efficient production, storage, and usage of hydrogen. Storage of hydrogen is the main hurdle in making hydrogen technology commercially feasible. Hydrogen can be stored in the form of compressed gas and in cryogenic liquid form. Storing hydrogen in compressed gas form requires a sufficiently large volume and the storage tank must be made of composite material, which is not economically viable. Storing hydrogen in liquid form also has some drawbacks; for example, it requires very low temperature and the use of liquid hydrogen, which can be dangerous. The best alternative is to store hydrogen in solid materials.3 The ideal hydrogen storage system should have high hydrogen reversibility and stability, with hydrogen interaction being in between physisorption and chemisorption. It should meet all the hydrogen storage parameters set by the U.S. Department of Energy (DOE).4 Many materials have already been studied to explore their hydrogen storage properties, like metal hydrides,5−13 clathrates,14−17 covalent organic framework and zeolites,18−25 alanates,26−31 metal organic framework © 2014 American Chemical Society

Received: February 18, 2014 Revised: April 29, 2014 Published: April 30, 2014 10859

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coordination,61 while it will interact with the hydrogen molecule by Kubas bonding,62−64 simultaneously. By applying first-principles electronic structure calculations and molecular dynamics simulations, the structural stability and hydrogen sorption efficiency has been studied on Ti-functionalized MBF. This paper is organized as follows: The details of the computational methodology are given in section 2, followed by a presentation and discussion of the results in section 3. A summary of our findings and conclusions is given in section 4.

77 K. In addition, Li et al. doped Ni on MOF-5 enhanced larger Langmuir specific surface areas and larger pores than the undoped MOF-5.52 In another study done by Turner et al. it was found that metal insertion on MOF-5 frameworks using a gas phase loading procedure results in an intact structure of MOF-5 crystals.53 Botas et al. found that gas uptake, which includes H2, CH4, and CO2 of MOF-5, systematically increases with the Co content at high pressure.54 In 2012, Stavila et al. found that NaAlH4 confined within the nanopores of a titanium-functionalized MOF template MOF-74 (Mg) can reversibly store hydrogen with minimal loss of capacity.55 Theoretically, Dixit et al. analyzed the effect of metal atom (M = Li, Be, Mg, and Al)56 on hydrogen storage properties of MOF-5 by using the GGA-PBE density functional based plane wave-pseudopotential methodology and it was reported that only Li and Al stabilizes MOF-5 among the four metals studied on the basis of calculated metal-MOF-5 interaction energies. Han et al. carried out GCMC simulations and found that MOF can be improved by increasing the aromatic content of the organic linkers and by replacing Zn with Mg.32 Moreover, H2 adsorption mechanisms of MOFs can be modified by changing the aromatic organic linker. In the search for a potential organic linker, many moieties have been studied separately. Hubner et al. studied the interaction of molecular hydrogen with the aromatic system C6H5X57 (where X = H, F, OH, NH2, CH3, and CN), C 10 H 10 (naphthalene and azulene), C 14 H 10 (anthracene), C24H12 (coronene), p-C6H4(COOH)2 (terephthalic acid), and p-C6H5(COOLi)2 by performing MP2 calculations using TZVPP basis sets. The interaction energy for different substituted benzenes has been analyzed and found to be a high value for aniline. The binding energies between a hydrogen molecule and a metal oxide cluster or Li-terminated 1,4-benzenedicarboxylate (BDC) have been investigated by Sagara et al.58 Yi Gao et al. found that by combining LDAVWN and GGA-PBE calculations provides a cost-effective way to assess the interaction between hydrogen molecules (adsorbent) and the metal organic frameworks (adsorbate) and, thus, offers a guide to experimentally design a new organic framework.59 Also, it was reported that the hydrogen binding energy with Li-terminated benzenedicarboxylate is larger than that with Cu- or Zn-terminated benzenedicarboxylate. Recently, Whitehead et al. reported coordination polymers and rotaxane frameworks in which an organic linker is replaced with functionalized clusters.60 In this study, for the first time, the organic linker in MOF is replaced by eight atoms BN linker, borazocine, with the molecular formula B4N4H8, and explored the physicochemical properties of resulting metal-functionalized Mg4O−BN framework (MBF). The BN ring is chosen in place of an organic entity, as it can be constructed into the BN motif. The borazocine linker will provide symmetric linear structure with 2,6 boron connection on both sides instead of borazine (B3N3H6), which results in the B and N connection with a pluckered chain. In the borazocine B4N4H8 linker, the two B atoms located at the 2,6 position minus the two hydrogen atoms are made to connect directly with the O atoms of the Mg4 moiety, with either side resulting in a straight chain linker. Thus, the linker with the molecular formula, B4N4H6, is expected to show similar behavior as that of benzene due to the presence of a π electron cloud on the top and bottom of the motif. On the π cloud, the Ti metal is doped and the possibility of hydrogen trapping is explored. Metal is expected to bind with the π electron cloud of the BN system by Dewar

2. COMPUTATIONAL DETAILS The calculations have been performed using the density functional theory (DFT) within the spin-polarized generalized gradient approximation (GGA) method with the parametrization of Perdew, Burke, and Ernzerhof (PBE) exchangecorrelation functional sets, as implemented in the DMOL3 package.65,66 Modeling the interaction between the MBF and H2 is a tedious task due to nonlocal electronic correlation. Ab initio methods that accurately evaluate such an interaction typically scale poorly with system size, making them computationally expensive for such systems. The DFT methods with the local density approximation (LDA) or the GGA scale well with the system size. In general, LDA tends to overpredict, while GGA is reported to underpredict the magnitude of weak interactions. In a recent study by Kelkkanen et al.67 concluded that, among GGAs, PBE underprediction is relatively small and is very successful in studying transition metal complexes over other GGAs.68 The DFT calculations performed are not 3D periodic due to lack of unit cell parameters in the proposed system. Double numeric basis sets augmented with polarization functions (DNP) have been utilized to describe all the electrons. In the generation of the numerical basis sets, a global orbital cutoff of 12 Å is used. Geometries are optimized without symmetry constraints using an energy convergence tolerance of 10−5 hartree and a gradient convergence of 2 × 10−3 hartree/Å. Molecular dynamics (MD) simulations are carried out in the canonical NVT ensemble using Nóse thermostat. Simulations are done at 200, 300, 373, and 573 K using Γ-point sampling with a 1 fs time-step for over 5 ps. Energy attributes, namely, metal binding energy, the H2 adsorption and desorption energies, and frontier orbital energy gaps, and structural attributes, namely, variation in bond lengths, Hirshfeld charges, electrostatic potential maps, and the H2 desorption temperatures, are computed and analyzed. The average hydrogen adsorption energy of Ti-doped MBF system is calculated by 1 Ead = [E MBFTi4 + nE H2 − E MBFTi4H2n] (1) n where EMBFTi4 is the total energy of the Ti metal doped MBF system, EH2 is the energy of the H2 molecule, and EMBFTi4H2n is the total energy of the maximum number of hydrogens adsorbed on the MBF system doped by Ti, where n is the number of hydrogen molecules. The sequential hydrogen desorption energy is calculated using Ed = E H2 + E MBFTi4H2n−2 − E MBFTi4H2n

(2)

where EMBFTi4H2n−2 is the total energy of the preceding hydrogen adsorbed MBF system, MBFTi4H2n. Equilibrium between the gas phase and the condensed phase H2 can be described by the linear form of van’t Hoff equation, 10860

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Figure 1. Optimized structures of (a) MBF, (b) MBFTi4, and (c) MBFTi4H32 systems. Atom colors: Ti, gray; Mg, green; O, red; N, blue; B, peach; H, white.

⎛ ΔH ΔS ⎞⎟ ln Keq = ⎜ − + ⎝ RT R ⎠

Table 1. Metal and Center of the BN Ring Distance (M-Rc), Metal−Hydrogen Distance (M-H), Metal and Chemisorbed Hydrogen Distance (M-Hc), Metal and Physisorbed H Distance (M-Hp), Chemisorbed Hydrogen Distance (Hc-Hc), and Physisorbed Hydrogen Distance Hp-Hp Measured for MBFTi4 System before and after Adsorption of H2 Moleculesa

(3)

where Keq = PH2/P0, PH2 is the equilibrium pressure, P0 is the reference pressure, namely, atmospheric pressure, R is the gas constant, T is the absolute temperature, and ΔH and ΔS are the change in H2 enthalpy and entropy, respectively, from the gas phase to the condensed phase. Substituting KB = R/N and rearranging the above equation, ⎛ ΔH ΔS ⎞ + PH2 = P0 exp⎜ − ⎟ R ⎠ ⎝ kBTN

(4)

From the 0 K adsorption energies, the desorption temperature (TD) can be obtained69,70 from eq 4 as follows: ⎛ E ⎞⎛ ΔS ⎞−1 TD = ⎜ ad ⎟⎜ − ln PH2⎟ ⎠ ⎝ k B ⎠⎝ R

a

system

M-Rc

M-H

M-Hc

M-Hp

Hc-Hc

Hp-Hp

MBFTi4 MBFTi4H8 MBFTi4H16 MBFTi4H24 MBFTi4H32

1.59 1.62 1.62 1.87 2.03

1.71 1.81 1.83 1.86

1.71 1.71 1.71 1.70

1.91 1.90 1.91

1.97 1.98 1.88 1.73

0.81 0.82 0.82

All the distances are average values measured in Å.

of the ring (Rc) distance of 1.59 Å, denoted as M-Rc. The H2 and Ti form Kubas type bonding, where the H2 molecule donates electrons to the empty d-orbitals of the metal atom, which back-donates electrons to the antibonding orbital of the H2 molecule, resulting in chemisorption. The physisorption can be explained by the charge polarization mechanism,71,72 where the charge on the Ti atom, created due to charge transfer from the BN ring, polarizes the H2 molecule, resulting in the near molecular bonding of H2. The sequential hydrogen trapping leads to the stretching of the four M-Rc bonds ranging from 1.62 Å for the first hydrogen to 2.03 Å for the fourth hydrogen. H2 adsorption causes Ti atoms to shift a little away from the framework. Average M-H distance elongates from 1.71 to 1.86 Å as the number of hydrogen molecules adsorbed increases. The average Ti metal and chemisorbed hydrogen distances, denoted as M-Hc, and physisorbed hydrogen distances, denoted as M-Hp, remain fairly constant, with values of 1.71 and 1.91 Å, respectively. When the physisorbed and chemisorbed hydrogen molecule distances are considered separately, the physisorbed hydrogen bond length denoted as Hp-Hp remains constant at 0.82 Å, while the chemisorbed hydrogen bond length denoted as Hc-Hc decreases from 1.97 to 1.73 Å as number of hydrogen molecules adsorbed on the system increases. The elongation of the physisorbed H−H distance is found to be very large (∼0.82 Å) as compared to the bare H2 molecule distance (0.74 Å).

(5)

where Ead = ΔH/N and P0 = 1 atm.

3. RESULTS AND DISCUSSION 3.1. Hydrogen Saturation of the MBFTi4 System. Using the first-principles calculations, Mg4O−BN framework, MBF, is optimized, and the resulting structure is shown in Figure 1a. To the optimized framework, titanium is functionalized on the BN ring of the MBF and the geometry is optimized. The optimized structure functionalized with four Ti atoms, MBFTi4, is shown in Figure 1b. The binding energy for Ti functionalized on inorganic BN linker, borazocine, is found to be 3.8 eV, which indicates that the metal atom is strongly bonded to the system. Then, H2 is introduced sequentially on each Ti atom in MBF, and the resulting geometry is optimized. Introduction of H2 is continued until full saturation. Each Ti-functionalized B4N4H6 linker is found to trap a maximum of 4 H2 molecules resulting in MBFTi4H32. The first H2 molecule is chemisorbed, while the remaining H2 molecules are physisorbed. The optimized geometry of MBFTi4H32 is shown in Figure 1c. 3.2. Structural Changes in MBFTi4 before and after Hydrogen Trapping. The structural parameters have been analyzed to find out the variation in the bond distances before and after trapping of hydrogens. The average bond lengths are listed in Table 1. Metal atoms bind to the BN ring through Dewar coordination with an average Ti metal (M) and center 10861

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A high hydrogen uptake with 6.0 wt % is predicted that can be increased further by additional storage in the pore volume. The pore size has been measured to be 20.2 Å diagonally in MBF and remains almost unchanged in MBFTi4 and MBFTi4H32. The distance between the center of the BN rings (Rc-Rc) measured in a parallel fashion, is found to be 14.6 Å in MBF, while Rc-Rc distance increases to 15.0 and 15.2 Å in MBFTi4 and MBFTi4H32, respectively. In order to find the maximum hydrogen wt % in the inorganic framework, the H2 molecules have also been introduced in the void space of the MBFTi4H32 system. Eight H2 molecules could be accommodated in the available space, and the resulting structure is geometry optimized. The average H−H bond distance of free H2 slightly increases to 0.750 Å with the storage capacity increased to 7.8 wt %. The optimized geometry is shown in Figure 2.

Figure 3. Average hydrogen adsorption and sequential desorption energy of MBFTi4H2n system as a function of number of H2 molecules, n.

value of 1.90 eV. The increase in the HOMO−LUMO energy gap symbolizes the enhancement in the stability of the MBFTi4 system with the loading of the H2 molecules. 3.4. MBFTi4H2n Electrostatic Potential Maps. Electrostatic potential (ESP) maps of MBFTi4 framework and hydrogen trapped MBFTi4H2n have been obtained. In Figure 4, top and bottom views of charge evolution of ESP for MBFTi4 and MBFTi4H2n system (n = 4, 8, 12, 16) are shown. In MBFTi4, Mg4O moiety is found to have high electron density (red) in comparison to the Ti-functionalized BN ring (green), the linker, as evident from the figure. In MBFTi4H2n, from the top view, the electron density variation for the hydrogen molecules doped on the Ti atom can be seen while from the bottom view, the electron density variation in the BN rings can be seen. In the top view, due to H2 trapping the color changes from green to red in the ESP maps emphasizing that the H2 molecules get polarized due to charge generated on the metal atom functionalized on the BN linker. In the bottom view, the color changes from light green to dark green, indicating that the electron density decreases on the BN ring, while it increases on the physisorbed H2 molecules and Ti centers as more H2 molecules are adsorbed on the system due to charge transfer, as evident from the figure. This can be quantified from the Hirshfeld charge analysis. The asymmetry in the ESP maps among linkers in each geometry is due to global optimization performed without any symmetry constrain on the structures. 3.5. Population Analysis: Hirshfeld Charges. Hirshfeld charges on the Ti metal atoms, chemisorbed and physisorbed H2 molecules are calculated to understand the mechanism of charge transfer in the system. Their average values are plotted in Figure 5 as a function of the number of H2 molecules. In MBFTi4, Ti atoms are found to have a positive charge of ∼0.4 electron units (eu). As H2 is sequentially trapped on the Ti atoms, the Hirshfeld charge decreases monotonically, reaching ∼0.2 eu for the fully saturated MBFTi4H32 system, indicating charge gained by the Ti atoms. The average Hirshfeld charges on the chemisorbed hydrogen atoms (Hc) increases moderately from −0.17 to −0.12 eu. For the first H2 molecule resulting in chemisorption shows maximum negative charge, while it becomes less negative as the number of H2 molecules increases. Hirshfeld charges for the physisorbed H2 (Hp) change is not appreciable and remains close to zero for sequential trapping of H2 molecules. Nevertheless, it varies from a more positive value of 7 × 10−2 eu to a less positive value of 4 × 10−2 eu, also

Figure 2. The optimized geometry of the MBFTi4H48 system with a hydrogen storage capacity of 7.8 wt %.

3.3. Adsorption and Desorption Energy. The average H2 adsorption energy (Ead) and sequential desorption energies have been obtained using eqs 1 and 2, respectively, and their trend as a function of H2 is shown in Figure 3. It is known that the zero point energy of hydrogen molecule reduces the static adsorption and desorption energy.73 This quantum effect affects the sorption energy by approximately 25%. With the reduced adsorption energy the chemisorbed hydrogens have high Ead of 0.74 eV, while physisorbed H2 Ead decreases gradually from 0.54 to 0.35 eV. The desorption energy (Ed) also decreases from 0.33 to 0.19 eV, as shown in the figure. Due to the low Ed values, the physisorbed H2 molecules can be easily desorbed. Low adsorption and desorption energy signifies the high reversibility of the H2 in Ti-functionalized MBF system. To determine the stability of the MBFTi4H2n system, the HOMO−LUMO energy gap (Eg) is calculated. For MBFTi4 system, Eg is found to be 1.05 eV. As the H2 molecules are trapped on the Ti atop sequentially, the Eg increases indicating the increase in the stability of the system. Eg of MBFTi4H8 and MBFTi4H16 is found to be 1.63 and 1.85 eV, respectively, while both MBFTi4H24 and MBFTi4H32 are found to have the same 10862

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Figure 4. Top and bottom views of charge evolution of electrostatic potential mapped on total density of MBFTi4 and MBFTi4H2n systems (n = 4, 8, 12, 16). The units are in e/Å3.

Figure 5. Hirshfeld charges for Ti; chemisorbed (Hc) and physisorbed (Hp) hydrogens of MBFTi4H2n system (n = 4, 8, 12, 16).

indicating charge gained by the physisorbed hydrogens. It can be inferred from the figure that the Ti metal atoms electron density and the H2 molecules electron density enhanced due to charge transfer from BN π electrons while chemisorbed hydrogen electron density decreased during the H2 saturation. 3.6. Molecular Dynamics Simulations of Hydrogen Saturated MBFTi4 System. Molecular dynamics on hydrogen saturated MBFTi4 system is simulated at the temperatures of 200, 300, 373, and 573 K and the resulting structures are presented as snapshots in Figure 6. The recommended H2 delivery temperature to fuel cell set by the U.S. DOE4 is in the range of −40 °C to +120 °C (233−393 K). In order to check the storage and reversibility of the hydrogen in MBF, four different temperatures have been chosen for MD simulations. A low temperature of 200 K is chosen for storage corresponding to −40 °C. Room temperature of 300 K is chosen to examine the storage and reversibility. An elevated temperature of 373 K is chosen for H2 release corresponding to +120 °C. For studying the MBF framework stability, a very high temperature of 573 K is chosen for simulation. At 200 K, H2 remains bound to the Ti metal in the framework, while at 300 K, 75% of the physisorbed H2 are desorbed with 1 fs time-step for 5 ps time duration. At 373 K, all the physisorbed H2 and a few chemisorbed hydrogens are also desorbed. Clustering effect

Figure 6. Snap-shots of MD simulation of H2 loaded on MBFTi4 at the temperatures of (a) 200, (b) 300, (c) 373, and (d) 573 K.

of Ti atoms has not been observed, even at 373 K, which is common in the Ti-functionalized MOF or carbon nanotubes. The Mg4O−BN framework is found to be structurally stable, even at 373 K. At higher temperature, the structure collapsed only at 573 K simulated for longer time duration of 7 ps. Hydrogen desorption temperature (TD) for MBFTi4H32 has been calculated using van’t Hoff equation (eq 5) within zero temperature ground state calculations. The computed adsorption energy is used to predict the desorption temperature. Pressure is considered as 1 atm and ΔS value is taken from the literature74 has the value of 130 J K−1 mol−1. With the reduced adsorption energy the maximum and minimum hydrogen desorption temperature for physisorbed H2 molecules are 396 and 265 K, respectively, for the MBFTi4H32 system. Also, the average desorption temperature is found to be 323 K. The desorption temperatures calculated using van’t Hoff equation and the information gleaned from MD simulations are correlating very well. Therefore, metal-functionalized inorganic 10863

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framework, with B4N4H8 linker, exhibits promising hydrogen storage properties.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. SUMMARY AND CONCLUSIONS In this study, the Mg4O−BN framework is built with BN linker, borazocine, having the molecular formula B4N4H8. The resulting structure is functionalized with Ti on the borazocine in order to enhance the stability and storage capacity of the framework. By applying first-principles electronic structure calculations and molecular dynamics simulations using GGAPBE functional with DNP basis the hydrogen sorption efficiency of the Ti-functionalized Mg4O−BN framework stability has been studied. Ti metal is strongly bonded to the BN ring through Dewar coordination with high binding energy. On saturation with the H2 each Ti-functionalized BN linker traps 4 H2 molecules with 6.0 wt %. The first H2 molecule is chemisorbed while the remaining H2 molecules are physisorbed. The physisorbed H2 and Ti form Kubas coordination with elongated average H−H bond lengths to ∼0.82 Å. H2 adsorption causes Ti atoms to shift little away from the framework by nearly 9%. The average H2 adsorption and desorption energies for the physisorbed H2 is found to be low and decreases gradually with hydrogen saturation. Low adsorption energy signifies the enhanced reversibility of the Ti-functionalized MBF system. Sequentially adsorbed H2 molecules on Ti metal enhance the stability of the system on the basis of increased HOMO−LUMO energy gap. Hirshfeld charges and ESP plots emphasize that the physisorbed H2 molecules get more polarized. MD simulation has been carried out in canonical NVT ensemble using Nóse thermostat. Simulations are done at 200, 300, 373, and 573 K using Γ-point sampling with 1 fs time-step for over 5 ps. H2 molecules are trapped to the Ti-functionalized BN linkers at low temperatures, and while at higher temperature at 373 K, all the physisorbed H2 and a few chemisorbed hydrogens are desorbed. The framework is stable at 373 K and found to collapse only at 573 K. Clustering effect of Ti atoms is not observed even at high temperature and longer duration of simulation. Average hydrogen desorption temperature computed based on van’t Hoff equation for the MBFTi4H32 system is found to be 323 K with maximum desorption temperature being 396 K. The hydrogen wt % for MBFTi4 is found to be 6.0 wt % and with additional H2 storage in the void space increases the wt % to 7.8. Extension of MBFTi4 into a 3-D structure and storage in the pore volume will increase the wt % further, making it a potential storage material. Thus, this system is better than MOF or metaldecorated MOF materials with respect to higher wt %, stability, and reversibility.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Department of Science and Technology, New Delhi (DST Grant SR/FT/CS85/2010) and the authors thank IIT Ropar and C-DAC (Pune) for National Param Yuva-II Supercomputing Facility.

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ASSOCIATED CONTENT

S Supporting Information *

The internal coordinates of optimized geometries of MBF, MBFTi4, and MBFTi4H2n (n = 4, 8, 12, and 16) are provided. Also, the results of H2 sorption efficiency of Ti-functionalized borazocine (B4N4H8), the linker, are given as additional detail. In Figures S1−S3, the optimized structures of strands of (a) B4N4H8, (b) B4N4H8Ti, and (c) B4N4H8TiH10 systems, their adsorption and desorption energies, and electrostatic potential maps, respectively, are shown. In Table S1, bond length parameters are given. This material is available free of charge via the Internet at http://pubs.acs.org. 10864

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