Enhanced Hydrogen Uptake and the Electronic Structure of Lithium

May 29, 2008 - Binding sites and the binding energies of H2 are calculated with and without the presence of a Li dopant. It is found that Li ..... Pos...
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J. Phys. Chem. C 2008, 112, 9278–9284

Enhanced Hydrogen Uptake and the Electronic Structure of Lithium-Doped Metal-Organic Frameworks P. Dalach,†,‡ H. Frost,‡,§ R. Q. Snurr,‡,§ and D. E. Ellis*,†,‡ Department of Physics & Astronomy, Institute for Catalysis in Energy Processes, and Department of Chemical and Biological Engineering, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: February 2, 2008; ReVised Manuscript ReceiVed: April 9, 2008

Metal-organic framework (MOF) materials show potential for gas separation and storage, and as hosts for highly selective catalyst molecules. Density functional theory (DFT) is applied to periodic band structures and to selected clusters representative of the cornerpost and strut environments of two MOFs to characterize the electronic environment. Binding sites and the binding energies of H2 are calculated with and without the presence of a Li dopant. It is found that Li enhances the H2 binding energies, both on the linking strut ring structures and for sites near cornerpost oxygen. MP2 correlation studies of the basic H2-Li-bipyridine interaction are carried out to explore effects of correlation beyond DFT. Contrary to previous model assumptions, we find that Li associates strongly with the cornerposts and less so with aromatic rings. I. Introduction In recent years, so-called metal-organic frameworks (MOFs) have become recognized as a promising class of materials for adsorption, catalysis, and separation.1–4 MOFs are composed of organic molecules or “struts” coordinated to metal or metal oxide corner units.5 Typically the corner units consist of transition metals (e.g., Zn, Cu, Mn) to which the organic molecules coordinate via a specific functional group (e.g., carboxylate, pyridyl). The framework geometry is dictated by the coordination sites of the metal corner and the layout of the functional groups on the organic strut. The innumerable possible combinations lead to the rich variety of MOFs that can be synthesized. MOF organic struts are highly customizable, ranging in length from under 3 Å to over 16 Å, and can be chosen to control the material’s properties, such as surface area and microporosity, that could be useful for gas separation6 through selective adsorption, gas storage7 (e.g., CO2, H2), and catalysis.8,9 MOFs for use in hydrogen storage applications have been studied extensively.10,11 A manganese-based MOF has been reported to achieve 6.9 wt % hydrogen gas uptake through the use of the unsaturated Mn2+ corner unit as the primary binding site at 77 K and 90 bar.12 This may be the first reported MOF that can reach the DOE goal for hydrogen storage weight percent, but at a temperature below the DOE goal.13 A copper-based MOF also has shown strong uptake, again with the corner unit providing the primary binding sites.14 It has recently been reported15 that the addition of a platinum catalyst attached to an activated carbon support bridged to a MOF can allow dissociated hydrogen atoms to “spill over” onto the MOF structure, greatly enhancing hydrogen storage capabilities. The MOFs studied in the present work are defined by a boxlike structure formed by four benzene rings coordinated to the Zn2 cornerpost complexes, making a “paddle-wheel” configuration (Figure 1). The Zn2 cornerpost is further coordinated to two pyridyl groups, forming an “axle” for the paddle-wheel. * Corresponding author. E-mail: [email protected]. † Department of Physics & Astronomy. ‡ Institute for Catalysis in Energy Processes. § Department of Chemical and Biological Engineering.

Figure 1. Paddle-wheel structure with Zn2 cornerpost surrounded by aromatic struts. Color key: silver, carbon; red, oxygen; purple, zinc; blue, nitrogen.

The main purposes of the present study are (1) to determine the basic electronic and chemical structure of two typical Znbased MOFs and (2) to determine the feasibility of improving the capacity of MOFs for hydrogen storage by cation doping. Specifically, it has been hypothesized that H2 adsorption, especially at nonmetal pore sites such as the struts, can be enhanced by doping the structure with an electron donor such as lithium. In fact, very recently it has been shown that ∼5 mol % Li uptake does in fact enhance hydrogen gas uptake in MOFs.16 The mechanism for this enhancement is believed to

10.1021/jp801008d CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

Lithium-Doped Metal-Organic Frameworks

Figure 2. Composition of struts (top) and primitive cells (bottom) for MOFs I and II, reproduced with permission from ref 16.

be multifaceted: First, it is believed that electron doping onto a strut will enhance the induced dipole-dipole interactions between the strut and H2. Second, in addition to enhanced dipole interactions, there may also occur enhanced strut-H2 and Li-H2 quadrupole interactions. Lastly, as it has been experimentally observed that H2 binds to open metal coordination sites within frameworks, the Li cation effectively opens up an unsaturated metal center to which H2 can bind.10,17 As the focus of our investigation, we used Zn2(BPDC)2(DPNI) and Zn2(NDC)2(BIPY), called I and II, respectively, in the following.18 I has struts composed of biphenyl4,4′-dicarboxylate (BPDC) and N,N′-di(4-pyridyl)-1,4,5,8naphthalenetetracarboxydiimide (DPNI), while II has struts composed of 4,4′-bipyridine (BIPY) and 2,6-naphthalenedicarboxylic acid (NDC), see Figure 2. Structures I and II both form two distinct interpenetrated lattices (Figure 3). These MOFs were chosen because of their well-known redox behavior.19 The present study will examine whether or not doping electrons, via Li, into the structures of I and II does in fact enhance the adsorption energy for H2 in I and II. Han et al. recently proposed a model that inserted several Li atoms above the center of the fused aromatic rings of the struts of several Zn4-based MOFs, predicting that Li does in fact enhance hydrogen uptake.20 Hartree-Fock molecular calculations followed by MP2 correlation corrections were used to find the interaction energies between H-C, H-O, and H-Zn needed to build a force field. The resulting force field was used in a grand canonical Monte-Carlo simulation for the uptake of Lidoped MOFs. Han et al. reported that gravimetric uptake of 6 wt % of H2 could be achieved using a Li-doped MOF at -30 °C and 100 bar. Their model disregarded the possible adsorption of Li onto the MOF cornerposts. Another recent work, by Blomqvist et al., also explored Li doping onto 1,4-benzenedicarboxylate (BDC) struts in a Zn4O MOF.21 They reported from Bader analysis electron doping onto aromatic rings from Li at a level 50% higher than that found in our study using Mulliken population analysis (described below). As with Han et al., their research disregarded a possible strong preference for Li to associate onto the Zn4O cornerposts. In the following, we examine a number of different scenarios for Li siting and H2 adsorption, and evaluate their relative energetic preferences. II. Theoretical Approaches A. Periodic Band Structure Models. The fundamental periodic nature of the MOF structure suggests the use of band

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9279 structure models. Although the unit cells are quite large (196 and 132 atoms/cell for I and II, respectively), they can be treated at some reasonable level of precision within the plane wave basis pseudopotential approach. We have carried out Density Functional Theory (DFT) reference state calculations on these two structures, using the VASP program,22 as a way of verifying and refining X-ray diffraction-derived model atomic positions. These calculations will be reported in detail elsewhere. However, high-precision calculations of relaxed atomic coordinates for the variety of doping scenarios considered here were deemed to be too heavy a computational task. Thus, the computationally efficient molecular fragment approach was mostly adopted for doping studies. B. Fragment Models. DFT calculations were run on smaller fragments or clusters from the periodic structure, each fragment selected for a specific environment and doping/adsorption model. This method proves effective as long as sufficient sites surrounding the regions of interest are included within the variational space.23 Results, reported below, show that quantitative models of electronic structure can be extracted in regions of interest by terminating the fragment with appropriate atoms or molecules, i.e., through shortrange embedding. Where long-range Coulomb interactions are of concern, for example, in the calculation of adsorption isotherms, Ewald summation can be used to complete the effective embedding potential. Typical fragments used in our simulations consist of individual struts, the cornerpost, or a combination of the two. A typical full cornerpost fragment contains 80 atoms (Figure 1). The fragment seen in Figure 4a consists of a cornerpost connected to a DPNI strut containing 113 atoms, used in various doping scenarios described below. C. MP2 Study of Basic Interactions. An alternative quantum chemical approach, that of self-consistent Hartree-Fock calculations followed by low-order perturbation theory, was also considered, analogous to the model of Han et al.20 In some sense the second-order Møller-Plesset perturbation theory (MP2) is more rigorous than DFT for treating electronic correlation; at any rate it may be free of the known limitations of DFT in accounting for dispersion interactions inherent in the systems under study.24 The two main limitations of MP2 are (1) lack of prior knowledge of the specific types of correlation which contribute to the interactions under study, coupled to the use of a finite-basis second-order perturbation expansion, and (2) the rapid increase in computational cost (∼N6, where N ) number of orbitals considered) with problem size. Nevertheless, MP2 has been a popular method in studies on the interaction of H2 with organic molecules found in MOFs.25 In the present work, the interaction of H2 and Li with each other, and with bipyridine, the organic strut found in II, was studied by using MP2: geometry optimizations and energy calculations were performed with the program Gaussian03.26 D. Computational Details. The ADF 2006 code27 was used to determine the electronic structures, optimized geometric configurations, and binding energies of selected fragments of I and II and their interactions with H2 and Li. ADF employs Slater-Type Orbitals (STOs) of the type xiyjzke-ζr located on atomic sites, which are characterized by their radial extent (ζ) and angular extent (polarization beyond occupied minimal basis). Currently, the triple-ζ double-polarized (TZ2P) basis set is the largest available in ADF. Generally, one would expect that with a given exchange-correlation functional, the larger basis sets would give more reliable predictions. This expectation, however, is not always fulfilled in practice. In fact, it is known that the basis set superposition error (BSSE) increases as basis set sizes increase,28 and diffuse basis functions can amplify deficiencies of the DFT functionals in the low-density region.

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Figure 3. Interpenetrated lattices (a) MOF I and (b) MOF II.

polarization function on all atoms. This basis set was chosen as a compromise providing reasonably accurate HF results without excessive computational expense. E. Electronic Binding Energies. Electronic binding energies (Ead) were calculated as the difference in energy of adsorbate A and substrate X in two separate systems:(1) A and X near each other in an optimized geometry (Eeq) and (2) A and X separated by a large distance (Einf). Simply, Ead ) Eeq - Einf. Systematic errors in DFT, basis set limitations, and other computational considerations imply considerable errors in absolute energies, of at least 8 kJ/mol, i.e., greater than or equal to energy differences of interest. Fortunately, as our interest lies in the relative energies of different configurations, a certain degree of error cancelation occurs. Thus, we will argue that any systematic increase in electronic binding energy seen due to the addition of electrons to the system will translate into an increased heat of adsorption.

Figure 4. Selected atoms of (a) frag-I and (b) frag-II.

Therefore, some experimentation was necessary to determine the best mix of DFT exchange and correlation functionals, and basis set size for the problem at hand. Though notorious for overbinding in long-range interactions, the simplest Local Density Approximation (LDA) functional does closely replicate the structures for known, long-range weakly interacting organic systems, such as benzene stacks. This is presumably the result of fortuitous cancelation of errors. Although the more sophisticated Generalized Gradient Approximation (GGA) generally gives better bond energies and bond lengths than LDA, it is known to underestimate long-range interactions, to the extent that repulsive forces are predicted for systems with diffuse electron densities where it is known that the interaction is attractive.29 For comparison of electronic structures between the GGA and LDA functionals, the BLYP GGA functional was used on a variety of small organic fragments as well as the target structures. Other functionals such as PBE30 could be considered; however, a consensus view seems to be that fundamental improvements in DFT treatment of long-range van der Waals interactions should be implemented.31 We found that using the relatively small triple-ζ polarized (TZP) basis and the least sophisticated LDA functional was in the end our best available strategy within the molecular fragment approach. All MP2 calculations employed a small Gaussian basis set of type 6-31++G**, which includes one diffuse and one

III. Results A. H2-Li-BIPY Basic Interactions According to MP2. To examine the isolated effect of the Li cation, a system containing only Li and H2 was first investigated, using MP2 with a basis set of 6-31++G**. When H2 was placed in the proximity (∼ 2.5 Å) of a neutral Li atom, energy minimization separates the two species completely and the interaction energy falls to zero. However, for the cation, energy minimization places the H2 2.2 Å from the Li+, and a binding energy of -18.27 kJ/mol is determined, indicating that the presence of a positively charged Li ion should provide a favorable adsorption location near Li inside the MOF framework. Next, several studies were performed on the interaction of H2 with the bipyridine molecule. Geometry optimization and energy calculations were performed by using HF/MP2 on the bare BIPY molecule, BIPY with a surrounding H2, and again on the two systems with a charge of -1 e superficially added to determine the effect of charge on the H2 binding energy. Further, a system containing BIPY, H2, and Li was studied to examine the donation of charge by the Li atom to BIPY and its effect on H2 binding. The geometry of the neutral bipyridine molecule with H2 was initially optimized by using various DFT methods and then singlepoint HF/MP2 energies were calculated on these optimized geometries. By comparing MP2 energies from geometries determined from different DFT methods and H2 starting positions it was evident that the MP2 energy minimum was not being determined. Thus, geometry reoptimizations were performed within the MP2 framework. Ultimately, two different H2 starting positions

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Figure 5. (a) Bipyridine with H2. (b) Bipyridine with Li and H2.

TABLE 1: Binding Energies (E, kJ/mol) of H2 on Neutral, Anion, and Li-Doped Bipyridine from MP2 Calculations and the Distance of the Closest BIPY Atom (R, Å) BIPY state

E

R

neutral anion Li doped

-4.5 -6.5 -5.3

2.91 2.94 2.98

both minimized to the configuration seen in Figure 5a. The MP2 optimized geometry for the combined BIPY, Li, and H2 system can be seen in Figure 5b; Li is clearly bound to the ring N. A summary of H2 binding energies can be seen in Table 1. Parts a and b of Figure 5 both indicate that a H2 configuration perpendicular to the plane of the aromatic ring is preferred. This result has also been found in previous studies on the interaction of H2 with aromatic systems.24 The neutral system provides a binding energy of -4.5 kJ/mol, in line with expectations. Hu¨bner et al. found H2 binding energies of 3.91 kJ/mol with C6H6 and 4.52 kJ/ mol with C6H5NH2 using MP2; similar energies have been reported by others.25 Table 1 clearly indicates an increase in binding energy upon the addition of an electron to the system or Li doping; a 44% increase in binding energy is found when an electron is superficially added to the bipyridine. The geometry (not shown) is nearly identical with what was found for the neutral system (Figure 5a). Interestingly, the H2 does not lie any closer to BIPY although the interaction energy is enhanced. Figure 5b shows Li directly adjacent to one of the BIPY N atoms; while this Li placement may be optimal with a free BIPY molecule, it is unrealistic in full MOF configurations as the N atom would be coordinated to a metal, thus blocking the site. Further, the hydrogen is in a slightly different position over the ring as compared to Figure 5a. Regardless, the effect of partial donation of an electron by Li is shown as an 18% increase in binding energy. Attempts to carry out larger scale MP2 calculations with fragments representative of MOFs were unsuccessful, due to extremely long computation times. B. MOF Reference States. We explored the electronic structures of MOFs I and II using periodic band structures and fragments with atoms placed at their crystallographic positions.18 The PAW-PBE relaxed band structures gave atomic positions differing by less than 0.1 Å from X-ray values, giving confidence that relaxations calculated in the presence of a dopant would be meaningful. A slight unphysical bowing of the NDC and DPNI planes seen in the X-ray refinement is removed in the relaxed structures; related small changes in interplanar rotation angles are found. Both compounds are found to be semiconductors, with a band gap of 1.95 eV; the yellow color of I indicates a slightly larger experimental gap, consistent with typical DFT band predictions. Spherical-volume integrated atomic charges show expected electronic distributions, e.g., Zn is found to be quite near its nominal divalent d10s0p0 state. Both LDA and GGA cluster calculations were conducted on undoped fragments of I and II; their electronic structures are very similar. A model fragment frag-I, consisting of a DPNI strut and a cornerpost of I, is shown in Figure 4a. Similarly, the model fragment frag-II consists of a BIPY strut and a cornerpost of II, as shown in Figure 4b. The BPDC strut found

Figure 6. LUMO and HOMO of undoped (a) frag-I and (b) frag-II. Color key: blue (red) represents the positive (negative) wave function.

on I and the NDC strut found on II were not surveyed for Li and H2 uptake in this study, since BPDC’s first excited state (lowest unoccupied molecular orbital, LUMO) lies 325 and 324 kJ/mol above the highest occupied molecular orbital (HOMO) for LDA and GGA, respectively; likewise, NDC’s first excited state falls 296 and 310 kJ/mol above the HOMO for LDA and GGA, respectively. These high excited states make BPDC and NDC poor electron acceptors compared to other MOF components, and so they were not considered further. The LUMO for frag-I was found, as expected, to be delocalized on the DPNI strut, with energy of (41, 44) kJ/mol in (LDA, GGA) above the HOMO (Figure 6a). The LUMO for frag-II was found, also as expected, to be delocalized on the BIPY strut, with energy of (192, 200) kJ/mol in (LDA, GGA) above the HOMO (Figure 6b). Thus, for our systems, LDA and GGA potentials result in remarkably similar one-electron energy differences, despite large differences in binding energy (total energy difference) and atomic force predictions. As was previously discussed, GGA can and does sometimes result in repulsive forces for diffuse and long-range interactions. For this reason, LDA is used as the functional of choice in the following analyses. C. Electron-Doped MOF. We performed calculations in which frags-I and -II were doped with a single “free” electron to examine the donor electron enhancement “best case scenario”, where a full electron is donated onto the structure and its effect can be seen in isolation, with no stray Coulomb fields from the Li donor. It was found that the HOMOs of frags-I and -II closely matched the previously found LUMOs, i.e., the donor electron does not significantly modify the ground-state structure. D. Li-Doped MOF. D.1. Li Binding. A survey of likely equilibrium positions for the doped Li was conducted on MOF I. It was expected that Li+ would settle near terminal oxygen anions, on the cornerposts and on the strut carbonyl groups. Likely Li binding sites were determined by placing (initially neutral) Li near sites of interest and self-consistently minimizing forces to find the optimized position. Table 2 shows the relative

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TABLE 2: Relative Binding Energies (∆E, kJ/mol) of Li on frag-I and frag-II by Location (see Figure 7)a sites

∆E

A

0

B

3

C

18

D E F G H INT

46 64 85 88 150 -18

site description coordinated between cornerpost O atoms, above pyridine ring plane coordinated to carbonyl O, above pyridine ring of DPNI coordinated between cornerpost O atoms, beside pyridine ring plane above the center DPNI aromatic ring coordinated to single cornerpost O atom above the side DPNI aromatic ring above the centermost C-C DPNI bond in plane of DPNI, coordinated to two H atoms intercalated between DPNI and BPDC struts

Figure 8. Three different views of fragment of MOF I used for intercalation. Color key: red, DPNI; green, BPDC; blue, cornerpost.

a Site A is used as the reference energy. All sites are stable Li binding locations.

Figure 7. Li binding locations on frag-I.

Li binding energies on sites of frag-I shown in Figure 7. Site A, coordinated to the cornerpost O, and site B, coordinated to carbonyl O, are the most stable nonintercalated binding sites, having almost identical energies. Site A binding energy was taken as the reference level (zero point) for comparison with additional sites. Li binding at site A was also calculated with a reduced set of termination groups, i.e., cornerpost benzene terminal groups were replaced with hydrogen and the terminal pyridine was replaced by ammonia. Upon reoptimization of the Li location, it was found that no atom moved by more than 0.004 Å; thus the structural data are very robust. A mechanism for lattice displacement within MOF I accompanying Li absorption has been hypothesized;16 we therefore explored a scenario where Li intercalated between the DPNI-BPDC layer,32 finding that this site is favored by more than 18 kJ/mol over corner site A. It was also observed that both the Li-doped DPNI strut and the intercalated Li induced small framework displacements, similar to those described by Mulfort et al.,16 on the order of 0.1 Å. A band structure study of the Li-intercalated site revealed a half-occupied impurity band located in the middle of the band gap. This prediction suggests the possibility of generating very interesting mixed ionic/electronic (semi)conductors, whose porosity could be exploited to increase ionic conductivity beyond that found in typical compact crystalline materials. This possibility will be explored in detail elsewhere. D.2. MOF Structural Integrity. After finding stable Li positions, two further calculations were performed to confirm that the integrity of frag-I’s structure was not compromised. First, a full reoptimization of frag-I was done; second, a full optimization of a new fragment that contained additional features of the interpenetrated I was made (Figure 8). No great structural differences were found in either case when Li is present. Similar

Figure 9. HOMO of Li-doped fragments at site A: (a) frag-I and (b) frag-II.

calculations were completed for frag-II and similar results were attained, i.e., all significant structural features were maintained under Li doping. A band structure study of Li-intercalalated MOF II was carried out to test the hypothesis of Li-induced sublattice separation. The only significant structural change observed was a slight “bulge” or local expansion of the ring-ring distances bracketing the Li. No tendency toward sublattice separation was observed for the low doping level implied by the model. D.3. Li-Doped HOMOs and Charge Distribution. The HOMOs calculated for a system with Li doped at site A of frag I and the analogue of site A on frag-II revealed that, as in the case of free-electron doping, the additional Li-donated electron predominantly occupied the LUMO of the undoped reference state (Figure 9). Mulliken populations (Table 3) were determined for specific atom types (defined by their chemical equivalence, or local bonding structure) in both I and II . Tabulated results were found by averaging over all atoms of the given chemical type. It was determined that upon doping sites A (frag-I and -II) and B (fragI) with Li, an overall 0.62 e was donated independent of Li placement. There is thus a heightened negative charge on atoms immediately adjacent to the doped Li, shown in Table 3, and the screening charge is seen to be of relatively short range. E. H2 Adsorption. E.1. DFT Method. We consider three different scenarios for H2 binding sites for fragment I: the electron-rich oxygen anions of the cornerpost, the electron-rich

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TABLE 3: Selected Mulliken Atomic Orbital Populations and Net Charge (Q, e) of Undoped frag-I and frag-II (Figure 8) and Changes in Net Charge (∆Q, e) upon Free-Electron Doping and Li Dopinga ∆Q atom

s

p

d

C O.1 N.1 N.2 O.2 Zn

2.90 3.83 3.50 3.44 3.80 6.26

2.34 4.61 3.87 3.93 4.77 12.71

0.16 0.06 0.08 0.08 0.05 10.09

N O Zn

3.50 3.80 6.26

3.88 4.78 12.71

0.08 0.05 10.09

Q

1e

Li@A

Li@B

frag-I 0.61 -0.50 -0.45 -0.45 -0.62 0.94

-0.03 -0.06 -0.01 0.00 0.00 0.00

-0.03 -0.05 -0.06 0.00 -0.00 -0.01

-0.01 -0.06 -0.01 0.00 -0.00 -0.00

frag-II -0.45 -0.62 0.94

-0.02 -0.04 -0.10

-0.05 -0.06 -0.11

TABLE 5: Adsorption Energy (E, kJ/mol) of H on Several Sites of frag-II (H2@A, H2@B)a E dopant

H2@A

H2@B

none free electron Li@A Li@A′ Li@A′′ Li@B

-15 -15 -20 -13 -16 -38

-10 -11

a Energies of direct H2-Li interaction are in bold. Note that the very strongly bound site Li@B is unusual; H2 is coordinated directly to both the Li on one side, above the BIPY plane, and the cornerpost on the other, forming a triangular shape.

a Populations are averages over chemically equivalent atoms. Columns labeled 1-e, Li @A, and Li@B refer to values of ∆Q upon doping with a free electron and with Li at sites A and B, respectively.

TABLE 4: Adsorption Energy (E, kJ/mol) of H2 on Several Sites of frag-I (H2@A, H2@B, H2@D)a E dopant

H2@A

H2@B

H2@D

none free electron Li@A Li@A′′ Li@B

-16 -16 -25 -14 -16

-11 -15 -15 -14 -27

-8 -9 -9 -9

a

Position A′′ is the configuration where H2 binds to position A and Li binds on the opposite side of the corner post. Energies of direct H2-Li interaction are in bold.

Figure 11. Li sites for frag-II H2 adsorption study.

Table 5 shows energies for H2 adsorption onto frag-II; as there are fewer binding sites on the struts, a more complete energy study of the placement of H2 on the cornerpost was carried out. H2 was placed in two locations: near the cornerpost, in analogy to position A on frag-I, and above the pyridine plane nearer to the cornerpost, in analogy to fragment B on cluster I. Li was placed on sites as shown in Figure 11. Geometric optimization of H2 relative to frag-II showed that in all cases where Li was doped onto the cornerpost, i.e., sites A, A′, and A′′, it was observed that H2 moved away from the struts and bound to the cornerpost. The H2-Li interaction is thus stronger than that of the H2-strut. IV. Discussion and Conclusions Figure 10. Li at site B of frag-I: (a) H2 at site D and (b) H2 near site B.

carbonyl groups of the struts, and the π-orbitals of the strut aromatic rings. These three locations are closely associated with sites A, B, and D seen in Figure 7. Table 4 shows the H2 adsorption energy onto model frag-I for undoped, free-electron doped, and Li doped geometries. It was found that when H2 is in position D, above the aromatic rings of the DPNI strut (Figure 10 location a), the energies were similar regardless of whether Li was placed at A or B. Additionally, calculations were done where both H2 and Li were initially placed near each other in the same general location, as in Figure 10 location b, as well as the situation where H2 and Li are both on the cornerpost on location-type A but on opposite sides denoted A′. As is shown in Tables 4 and 5, H2 adsorption energy is very high (favorable) when H2 interacts directly with Li+.

HF/MP2 calculations on H2-Li-BIPY interactions reveal the magnitude of basic interactions at work within the Li-doped MOF structures. The Li+ cation interacts strongly with H2, in contrast to the neutral atom, and in the presence of Li+, an 18% H2-BIPY binding enhancement was found. More extensive MOF fragment studies based on DFT methodologies show that doping Li into the MOF structure increases the hydrogen binding energy on both cornerpost and struts; the direct correlation between gas binding energy and lattice gas uptake has been demonstrated elsewhere.11 It is also clear that the Li cation will provide an additional favorable adsorption site within the material. A typical energy of adsorption for H2 onto nonmetal pore sites was calculated to be