Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology Tony Pham, Katherine A. Forrest, Hiroyasu Furukawa, Juergen Eckert, and Brian Space J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04027 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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The Journal of Physical Chemistry

Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology Tony Pham,†,§ Katherine A. Forrest,†,§ Hiroyasu Furukawa,‡ Juergen Eckert,∗,†,⊥ and Brian Space∗,† † Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, FL 33620-5250, United States ‡ Department of Chemistry, University of California-Berkeley, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States ⊥ Department of Chemistry and Biochemistry, Texas Tech University, 2500 Broadway, Box 41061, Lubbock, TX 79409-1061, United States ABSTRACT: The adsorption of H2 in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn2+ ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H2 adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H2 adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (Qst ) value for H2 in ZIF-76 was determined to be 7.7 kJ mol−1 , which is comparable to that for other ZIFs and is fairly high for a material that does not contain open-metal sites. Simulations of H2 adsorption in one of these structures resulted in Qst values that are in very good agreement with experiment within the loading range considered. Two notable H2 binding sites were discovered from simulations in both structures of ZIF-76; however, the preferential regions of H2 occupancy are reversed for the two structures. The inelastic neutron scattering (INS) spectra for H2 adsorbed in ZIF-76 contain several peaks that arise from transitions of the hindered H2 rotor, with the lowest energy peak occurring in the range of 6.0–7.2 meV. Two-dimensional quantum rotation calculations for H2 adsorbed at the considered sites in both structures yielded rotational transitions that are in good agreement with the peaks that appear in the INS spectra. Despite the large degree of disorder in the ZIF-76 crystal structure, the overall environment in the ZIF still gives rise to interconnected INS features as discerned from our calculations. This study demonstrates how important details of the H2 adsorption mechanism in a ZIF with structural disorder can be obtained from a combination of experimental measurements and theoretical calculations.

I.

INTRODUCTION

Metal–organic frameworks (MOFs) are a rapidly emerging class of solid crystalline materials which have some promise for applications in H2 storage.1–3 These materials are generally porous three-dimensional structures that are synthesized from metal ions and organic ligands (or “linkers”),4 many of which exhibit permanent microporosity after removal of guest molecules from the pores. Zeolitic imidazolate frameworks (ZIFs) represent a subclass of MOFs which are synthesized from metal ions and imidazolate-type linkers.5,6 ZIFs typically exhibit greater chemical and thermal stability than most traditional MOFs, presumably because of the greater basicity of the imidazolate-type linkers.7 While ZIFs are composed soley of imidazolate-type linkers, they can be synthesized to have distinct topologies depending on the position and bulkiness of the functionalities of the linker; these topologies are similar to those found in inorganic zeolites. Representative topologies observed in ZIFs include rhodolite (rho), sodalite (sod), gmelinite (gme), and Linde type A (lta).5

tional materials (e.g., zeolites, activated carbons, metal hydrides) for H2 storage. Several experimental7,9,10 and theoretical studies11–13 have shown that ZIFs could indeed be candidates for H2 storage. In this work, we have chosen a lta type ZIF for the investigation of the relationship between structure and H2 adsorption properties in ZIF materials because of its topological similarity with type A zeolites. Type A zeolites usually have lower densities, more void space, and are more thermally stable than most other types of zeolites,14 thus making them advantageous for many industrial applications. Zeolite A is a technologically important zeolite that is used in hydrocarbon cracking, water purification, and gas separations.15,16 ZIFs with lta topology are interesting because they are essentially expanded analogues of zeolite A whose cage walls can, however, be much more easily tuned and functionalized.

ZIF-76 is a Zn2+ -based ZIF with lta topology wherein each metal ion is coordinated to imidazolate and 5chlorobenzimidazolate linkers in a 3:1 ratio.17 The framework of the lta structure consists of three distinct cages (Figure 1): cube, truncated cuboctahedron (α cage), and truncated octahedron (β cage). A notable feature of ZIF-76 comPorous MOFs are capable of adsorbing a considerable pared with most other MOFs is that the crystal structure amount of H2 molecules within their pores (albeit at of this material contains a large amount of disorder within cryogenic temperatures) and have the ability to release the positions of the imidazolate and 5-chlorobenzimidazolate them freely through minor changes in the thermodynamic linkers. Herein, we demonstrate how the disorder of ZIF-76 conditions.2 In addition, the commonly used building block is approximated such that insights into the H2 adsorption approach makes it possible to create or envision myriad difmechanism in this material can be gained through molecular ferent MOF structures with desired topologies and funcsimulations. We note that treating the disorder in MOFs for tionalities, leading to unique properties.8 These features computational studies is usually ignored by other theoretical give crystalline frameworks significant advantages over tradiACS Paragon Plus Environment


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tational studies. The 5-chlorobenzimidazolate linker occurs in two mixed site positions with an imidazolate linker at equal occupancy for each. There is also disorder in the positions of three of the four linkers that are connected to the Zn2+ ions. Furthermore, the chlorine atom of the 5chlorobenzimidazolate linker is randomly bound to either of the two possible ring carbon atoms. Many reasonable approximations were attempted to remove the disorder and select viable structures of ZIF-76 for simulations of H2 adsorption. In the end, we decided to consider two possible crystal structure configurations of the ZIF (denoted structures 1 and 2) which differ by only the relative positions of one imidazolate linker and one 5-chlorobenzimidazolate linker (Figure 2). These two structures were chosen because they exhibit high symmetry within a unit cell and were the simplest to extract based on the available positions of the atoms within the disordered crystal structure. Other structural models of ZIF-76 are possible, including those that have much lower symmetry, but generating such structures will require creating and optimizing linkers at locations that are not depicted in the experimentally published structure. The 2 × 2 × 2 system cells of the two structures of ZIF76 selected in this work are shown in Figure 3. A version of this figure with the H atoms omitted is displayed in the Supporting Information (Figure S1). In structure 1, the 5chlorobenzimidazolate linkers are connected to the vertices of the cubic cages, which in turn causes them to be located on the face of the octagonal windows of the large β cages (Figure 3(a)). All hexagonal windows that intersect the α and II. METHODS β cages in this structure consist of an alternating arrangement of imidazolate and 5-chlorobenzimidazolate linkers at the sides. In structure 2, the 5-chlorobenzimidazolate linkers A. Experimental Section are located on one of the edges of the cubic cages, which implies that the Cl atoms of the linkers point toward the center ZIF-76 was synthesized and activated according to the proof the β cages (Figure 3(b)). In contrast to structure 1, only cedure reported in reference 17. Permanent porosity of the four of the hexagonal windows of each α and β cage consist material was confirmed by N2 adsorption measurements at of the alternating imidazolate/5-chlorobenzimidazolate ar77 K using the Autosorb-1 (Quantachrome) volumetric anrangement at the sides, while the other four hexagonal winalyzer. The N2 adsorption isotherm collected in this work dows contain all imidazolate linkers at the sides. for ZIF-76 is displayed in the Supporting Information (FigWe note that while the crystal structure configurations ure S6). The Langmuir and BET surface areas and pore volselected in this work contain linkers bonded to the Zn2+ ions ume for this ZIF are summarized in Table 1. The H2 adsorpin a symmetric and periodic manner throughout the unit tion isotherm for ZIF-76 at 77 and 87 K and pressures up cell, the physical crystal structure of the ZIF may actually to about 1.1 atm were measured using the same instrument. contain asymmetric and random positioning of the linkers The H2 Qst values were determined for a range of uptakes by about the Zn2+ ions. However, we believe that simulations applying the virial method23–25 to the corresponding experin the two structures considered herein are sufficient to gain imental isotherms at 77 and 87 K. The INS spectra for ZIFinsights into the types of H2 adsorption sites present and 76 were collected on the inverse geometry Quasi-Elastic Neufor the interpretation of the INS spectra for the material as tron Scattering (QENS) spectrometer at the Intense Pulsed described below (see section III). Neutron Source (IPNS) of Argonne National Laboratory (ANL) using approxmately 1.0 g of the activated sample. All ZIF atoms were assigned Lennard-Jones 12–6 paramAdsorption of predetermined amounts of H2 was carried out eters ( and σ), point partial charges, and scalar point poin situ at 77 K from an external gas handling system into the larizabilities to model repulsion/dispersion, stationary elecevacuated sample cell after first obtaining a spectrum of the trostatic, and many-body polarization interactions, respec“blank” sample. The samples were equilibrated after loading tively. Details for obtaining these parameters are given in the prior to cooling to the data collection temperature of 15 K. Supporting Information. Simulations of H2 adsorption in the two crystal structure configurations of ZIF-76 were carried out using grand canonical Monte Carlo (GCMC) methods26 B. Theoretical Section on a single unit cell of the ZIF. More details of the GCMC simulations are also provided in the Supporting Information. Simulated annealing calculations27 within the canonical All parametrizations and simulations were performed on ensemble were carried out for a single H2 molecule within the the SQUEEZE crystal structure of ZIF-76 reported in refunit cell of both structures of ZIF-76 to identify the global erence 17 (CCDC 671088). Single crystal X-ray diffracminimum as well as possible local minima for the adsorbate tion analysis revealed that the structure of this ZIF is in the material. These simulations started at an initial temhighly disordered,17 which presents a challenge for compuACS Paragon Plus Environment

groups. However, we have shown previously that even slight differences in the orientation and/or positioning of the linkers in MOFs can have a significant effect on the preferred gas adsorption sites.18–21 Here, we report the experimental H2 adsorption isotherms and isosteric heat of adsorption (Qst ) in ZIF-76. In order to obtain details on the H2 adsorption sites, molecular simulations and inelastic neutron scattering (INS) spectroscopic studies were also performed. This paper thereby describes what is only the second INS study of H2 adsorbed in a ZIF, while the first was published just very recently by our group.13 Of importance is the fact that simulations of H2 adsorption were performed in two possible crystal structure configurations of ZIF-76 (see section II.B) to identify the preferential H2 binding sites in this material. Reasonable agreement is obtained between the experimental and simulated H2 adsorption isotherms and Qst values for this ZIF. Moreover, we carried out two-dimensional quantum dynamics calculations for the rotation of H2 adsorbed at the preferred binding sites in both structures of ZIF-76. These predictive calculations are essential for the analysis and interpretation of the INS spectra.22 We will show that our calculated j = 0 to j = 1 transition energies for specific sites in ZIF-76 are in good agreement with those observed experimentally, thus demonstrating the validity of the models used herein, and that certain features in the INS spectra evolve even in the presence of a great deal of structural disorder.

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The Journal of Physical Chemistry 3

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perature of 1000 K and this temperature was scaled by a factor of 0.99999 after every 1000 Monte Carlo steps. The simulations continued until the temperature of the ZIF–H2 system dropped below 2.5 K. The rotational dynamics of H2 adsorbed at the binding sites obtained from simulated annealing calculations in both structures of ZIF-76 were described by two-dimensional quantum rotation calculations. This method has been effectively utilized to calculate the rotational transitions and barriers for H2 adsorbed in different porous materials,22 including zeolites,28 MOFs,21 and covalent organic frameworks.29 More details of implementing the quantum rotation calculations are provided in the Supporting Information. Our simulation results utilize a five–site polarizable H2 potential that was developed previously by Belof et al.30 Simulations using this model have produced adsorption isotherms and Qst values that are in very good agreement with experimental data in various MOFs with distinct topologies.13,31–33 We also simulated H2 adsorption in ZIF-76 using nonpolarizable potentials30,34,35 as a control and these results are shown and discussed in the Supporting Information (Figure S7).

III. A.

RESULTS AND DISCUSSION

Isotherms and Isosteric Heats of Adsorption 1.

Experiment

The experimental H2 adsorption isotherms for ZIF-76 at 77 and 87 K and pressures up to 1.1 atm are shown in Figure 4(a). The atmospheric H2 uptake was found to be approximately 10.6 and 7.5 mg g−1 at 77 and 87 K, respectively. The H2 uptake value for ZIF-76 at 77 K/1 atm is very similar to that for ZIF-20, another ZIF possessing lta topology,16 under the same conditions (Table 1). The experimental Qst values as derived from applying the virial method23–25 to the experimental H2 adsorption isotherms at 77 and 87 K are displayed in Figure 4(d). The experimental initial Qst value for this ZIF was determined to be 7.7 kJ mol−1 , which is comparable to that for other ZIFs that were investigated for H2 adsorption (Table 1). This quantity is also within the range of those for MOFs that include open-metal sites.3,36

2.

The greater calculated value for the uptake of H2 relative to experiment at pressures greater than 0.2 atm at 77 and 87 K may well be a consequence of the structural models used for the ZIF. Specifically, we simulated in crystal structures of ZIF-76 in which the the Zn2+ ions are connected to the four linkers in a symmetric and periodic fashion throughout the unit cell. It is likely that the actual structure of the ZIF contains asymmetric and random positioning of the linkers throughout the entire crystal. Such variations in the positions of the linkers would have an impact on the calculated H2 uptakes at higher pressures. However, we believe that the two symmetric structures that were selected for ZIF-76 were adequate for the elucidation of details of the H2 adsorption mechanism and binding sites in the material. This conclusion is supported by the fact that our calculated rotational transitions for H2 adsorbed at the identified sites in this ZIF are in very good agreement with those that appear in the INS spectra as shown in the next subsection. We may add that the low porosity of the ZIF-76 crystals synthesized herein for the adsorption measurements could also explain the discrepancy between the experimental and simulated uptakes at higher pressures. Using a code developed by D¨ uren et al.,38 the accessible surface area for structures 1 and 2 of ZIF-76 were calculated to be 1682 and 1953 m2 g−1 , respectively. These calculated surface areas are higher than the experimental Langmuir and BET surface areas for the ZIF (1120 and 1000 m2 g−1 , respectively; see Table 1). Simulations in structure 2 predict slightly higher H2 uptakes than in structure 1 for essentially all state points considered (Figures 4(b) and 4(c)). This indicates that the H2 molecules interact more strongly with the framework of structure 2. The calculated H2 Qst values are also greater for structure 2 than those for structure 1 over most of the considered loading range. The initial Qst values for H2 were calculated to be 6.4 and 7.7 kJ mol−1 for structures 1 and 2, respectively, with the latter being in excellent agreement with the corresponding experimental value for ZIF-76. In fact, the theoretical Qst values for structure 2 are close to those for experiment for all H2 uptakes considered. We note that while the experimental Qst values are determined by empirical fitting of the adsorption isotherms, the theoretical quantities are obtained from fluctuations in the partial number and total potential energy of the adsorbent–adsorbate system in GCMC simulation (see Supporting Information).39

Simulation

The simulated H2 adsorption isotherms at 77 and 87 K B. Inelastic Neutron Scattering Spectra and Qst values in structures 1 and 2 of ZIF-76 are presented in Figures 4(b), 4(c), and 4(d), respectively. The simulated INS studies of H2 adsorbed in ZIF-76 were carried out uptakes in both structures are found to be in good agreement in order to obtain insights into the location and energetwith the experimental isotherm at pressures lower than 0.2 ics of the H2 binding sites in this ZIF. The experimental atm at 77 and 87 K to within joint uncertainties, with simH2 adsorption isotherms and Qst values do not reveal this ulations in structure 2 producing better agreement with exinformation. Although techniques such as neutron powder periment under these conditions (Figures 4(b) and 4(c)). At diffraction (NPD),40 infrared (IR) spectroscopy,41 and nuhigher pressures, however, the simulations deviate from exclear magnetic resonance (NMR) spectroscopy42 can also be periment, with a different overall shape in the isotherm at used to pinpoint the location of the H2 adsorption sites in a both temperatures. We found a similar discrepancy between porous material, the INS spectra are particularly useful bethe experimental and simulated H2 adsorption isotherms in a cause they contain information on the dynamics and molecmolecular porous material recently.37 Nevertheless, the fact ular excitations of the adsorbed H2 molecules at different that our simulations generated uptakes in good agreement sites. In the INS spectra for H2 bound in a porous material, with experiment at low pressures in both structures of ZIFthe lowest rotational transition is in fact a tunnelling tran76 confirms proper modeling of the initial H2 adsorption sites sition, for which lower energies (frequencies) correspond to in the material. ACS Paragon Plus Environment


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a higher barrier to rotation imposed on the adsorbate, and therefore, a stronger interaction with the host.43 The INS spectra for H2 adsorbed in ZIF-76 at two different loadings exhibit broad bands within neutron energy transfer ranges of approximately 6.0–7.2, 7.5–9.2, 10.0–13.1, and 13.3–15.7 meV (Figure 5). The band ranging from 6.0–7.2 meV can be attributed to H2 adsorbed at the most favorable binding site in ZIF-76, while those spanning from 7.5–9.2 and 10.0–13.1 meV should correspond to the next most favorable adsorption sites, with the former being associated with a slightly stronger binding site. The broad band in the range of 13.3–15.7 meV may then be assigned to H2 bound at the weakest adsorption sites in the material. We note that peaks appearing beyond 15 meV correspond to higher transitions within the j = 1 sublevel for H2 adsorbed in the ZIF since the rigid rotor limit for unhindered (or “free”) H2 is 14.7 meV.

C.

Identification of Adsorption Sites and Assignment of INS Peaks

The above assignments of the bands in the INS spectra for H2 in ZIF-76 were made by way of identifying the favorable adsorption sites from molecular simulations and subsequently performing two-dimensional quantum rotation calculations for H2 adsorbed at these sites. We have successfully employed this technique to make proper assignments of the INS transitions for H2 adsorbed in various porous materials.13,20,21,29,31,44–48 The calculated rotational transitions and barriers for H2 localized at two different sites in the two crystal structure configurations of the ZIF (Figures 6–9) are summarized in Table 2.

2.

Site 2 in Structure 1

Adsorption of H2 onto the face of the cubic cage was found to be a local minimum in structure 1. This is the second most favored binding site for the H2 molecules in this structure of ZIF-76. Close-up views of the most energetically favorable position in this region (denoted site 2 for the structure) and the simulated distribution of occupied sites are shown in Figures 7 and Figure S8(b) (see Supporting Information), respectively. As the H2 molecules are adsorbed at a face of the cube, they can interact with the surrounding linkers in a nearly concurrent fashion. We note that the H2 molecules cannot penetrate into the cube since the square windows are not large enough to allow guest molecules to enter. This is consistent with what was observed previously for other gases in ZIF-76.51 The lowest rotational transition for H2 adsorbed at site 2 in structure 1 was calculated to be 12.23 meV (Table 2). This value falls well within the broad band from 10.0–13.1 meV in the INS spectra, which may therefore be assigned to originate from H2 adsorbed at the face of the cube in structure 1. The associated rotational barrier was estimated to be 37.07 meV (Table 2). Rotational energy levels for the second j = 1 sublevel were calculated to be 18.73 and 17.20 meV for H2 adsorbed at sites 1 and 2, respectively in structure 1 (Table 2). These transitions should therefore correspond to the band in the range from 16.7–20.0 meV in the INS spectra (Figure 5).

3.


Surprisingly, our molecular simulations revealed that the most favorable binding site for the H2 molecule in structure 2 of ZIF-76 is not located within the analogous hexagonal windows connecting the α and β cages as observed in structure 1. Instead, the most preferred adsorption site in this 1. Site 1 in Structure 1 structure corresponds to the face of the cubic cage, which is analogous to site 2 in structure 1. Molecular illustrations of The global minimum for H2 adsorbed in structure 1 of this adsorption site, referred to as site 1 for the structure, ZIF-76 was found to be located within the hexagonal winare shown in Figure 8, while a distribution of the sites of ocdow connecting the α and β cages (denoted as site 1, Figcupancy in this region from the GCMC simulations is preure 6). Here, the H2 molecule can interact simultaneously sented in the Supporting Information (Figure S9(a)). with the aromatic rings of two neighboring imidazolate linkThis site is strongly favored in structure 2, most likely ers, which results in a favorable interaction. A similar type because the adsorbate can directly interact with the 5of adsorption site was found for H2 in ZIF-68 and ZIF-69 chlorobenzimidazolate linker that is located on one of the through theoretical studies.13 While Figure 6 shows the anedges of the cube. Moreover, it is the bulky benzene ring nealed position of the adsorbate in the hexagonal window, of the 5-chlorobenzimidazolate linker that helps confine the a distribution of the occupied sites in this region for strucH2 molecule in this area. In structure 1, however, the ture 1 as obtained from GCMC simulations is displayed in 5-chlorobenzimidazolate linkers are connected to the verthe Supporting Information (Figure S8(a)). tices of the cubic cage and they interact less with those H2 molecules that bind onto the face of the cube. Because Quantum rotation calculations with two degrees of freethe less bulky imidazolate linker takes the place of the 5dom for H2 adsorbed at site 1 in structure 1 of ZIF-76 yielded chlorobenzimidazolate linker at the appropriate edge of the a value of 6.61 meV for the transition between j = 0 and the cube in structure 1, such a confinement effect is not possible lowest j = 1 sublevel (Table 2). This value falls well within in this structure. the range of energies for the lowest energy peak observed in the INS spectra (6.0–7.2 meV). The rotational barrier for H2 H2 molecules adsorbed at site 1 in structure 2 are associated with the theoretical initial Qst value in this structure. adsorbed at this site was calculated to be 48.81 meV (TaThe agreement between the experimental and simulated H2 ble 2), which is relatively high and is greater than that for a Qst for structure 2 at low loadings (Figure 4(d)) also implies number of MOFs that possess open-metal sites.21,49 Accordthat this site should correspond to the experimental zeroing to an empirical phenomenological model based on a simloading value for the ZIF. Calculation of the rotational enple double-minimum potential,50 the rotational barrier asergy levels for H2 adsorbed at site 1 in structure 2 yielded a sociated with the lowest energy peak in the INS spectra for value of 7.01 meV for the lowest transition (Table 2), which, ZIF-76 ranges from 34.0–41.4 meV. ACS Paragon Plus Environment

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as with site 1 in structure 1, falls within the range of energies for the lowest energy band in the INS spectra (6.0–7.2 meV). We can therefore predict that the lowest energy peak in the INS spectra for ZIF-76 corresponds to H2 adsorbed at site 1 in both structures even if the physical crystal structure of the ZIF is in fact completely disordered. As with site 1 in structure 1, the rotational barrier for H2 adsorbed at site 1 in structure 2 is rather high, with a value of 47.58 meV. We note that a slightly lower j = 0 to j = 1 transition and higher rotational barrier was obtained for H2 adsorbed at site 1 in structure 1 than in structure 2 (Table 2). This could be due to the fact that the hexagonal windows of the α/β cages are more accessible in structure 1 than the faces of the cubic cages in structure 2. The calculated transitions for H2 adsorbed at site 1 in both structures of ZIF76 are in fact lower than those for the lowest energy peak observed in the INS spectra for several MOFs that contain open-metal sites, such as HKUST-1,52 PCN-12,53 and some members of the M-MOF-74 series.54–56 This is likely due to the confinement effect that the H2 molecule experiences at these sites in both structures. This agrees with previous findings that porous materials that contain small pores or confined regions can impose high rotational barriers on the H2 molecules, which in turn results in lower transition energies in the INS spectra.13,45,48,57

4.


The region between two 5-chlorobenzimidazolate linkers within the hexagonal windows containing alternating imidazolate/5-chlorobenzimidazolate linkers at the edges was identified to be the second most favored binding site in structure 2 of ZIF-76 (denoted site 2, Figure 9); this site is analogous to site 1 in structure 1. The distribution of the occupied sites by H2 in this area for structure 2 is shown in the Supporting Information (Figure S9(b)). This region is less favorable in structure 2 compared with structure 1, presumably because the H2 molecule exhibits some repulsion between its H atom and that of the nearby benzene component of the 5-chlorobenzimidazolate linker as the adsorbate localizes here. We note that the H2 binding sites discovered herein for both structures of ZIF-76 are similar to those observed for other sorbates in the ZIF through theoretical studies.51 The lowest j = 0 to j = 1 transition for H2 adsorbed at site 2 in structure 2 was calculated to be 11.11 meV (Table 2, which, as in the case of site 2 in structure 1, falls within the range of the INS band from 10.0–13.1 meV. We have therefore tentatively assigned this peak to H2 binding onto site 2 in both structures of the ZIF. The rotational barrier for site 2 in structure 2 was calculated to be 24.11 meV, which is appreciably lower than than those for the other sites under consideration (Table 2). This could be attributed to some inadequacies in the nature of the potential energy surface about the hexagonal windows encompassing site 2 in this structure.

positions and angular orientations about the hexagonal windows in structure 1 and the cubic cages in structure 2 of the ZIF. We tested this hypothesis by means of two-dimensional quantum rotation calculations for selected H2 molecule positions in these areas that were found in the GCMC simulations. These calculations resulted in transitions within the aforementioned energy range as proposed. We also tested the dynamics of H2 adsorbed at various slightly shifted positions about site 2 in both structures of ZIF-76 that were obtained from the GCMC simulations. Such calculations resulted in j = 0 to j = 1 transitions ranging from 13 to 15 meV. The INS spectra for ZIF-76 contain a distinct band ranging from 13.3–15.7 meV at both loadings. Thus, we anticipate that this peak should correspond to H2 adsorbed at these shifted positions about site 2 in the two structures. Further, we observed that the H2 molecules can localize within the hexagonal windows consisting of all imidazolate linkers at the sides in structure 2. However, this region is less favorable than that for adsorption at site 2 for the structure. This site also contributes to the intensity of the band ranging from 13.3–15.7 meV according to our calculations. Additional contributions from H2 molecules at weaker binding sites that are not considered here for both structures may also contribute to this band. Moreover, it is possible that the adsorbed H2 molecules may create additional binding sites at 15 K. A summary of the proposed assignments of the peaks occurring below the rigid rotor limit for ZIF-76 is presented in Table 3. For H2 localized at sites 1 and 2 in both structures, the calculated rotational energy levels for the third j = 1 sublevel are close to 26 and 21 meV for the individual sites (Table 2). The INS spectra for ZIF-76 contain peaks ranging from 20.5–22.8 and 23.2–24.8 meV (Figure 5), which should correspond to such higher transitions for the adsorbed H2 molecules in both structures.

IV.

CONCLUSION

Adsorption of H2 in ZIF-76, a Zn2+ -based ZIF with lta topology, was characterized by means of a combined experimental and theoretical investigation. The experimental initial value for Qst in ZIF-76 was determined to be 7.7 kJ mol−1 , which is comparable to that reported for other ZIFs7,10,13,16 and some MOFs that contain open-metal sites.3,36 This demonstrates that ZIFs with certain topologies can give rise to highly favorable binding sites for H2 by way of the arrangement of their imidazolate-type linkers. Simulations of H2 adsorption were performed in two distinct crystal configurations of the disordered ZIF which differ by the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. These H2 adsorption isotherms for both structures are in good agreement with the experimental measurements at low pressures (< 0.2 atm). Our theoretical results for the two structures slightly differ from one another, with simulations in structure 2 yielding Qst values that are in better agreement with experiment. 5. Other Sites Based on these results, one may suggest that the physical crystal structure of ZIF-76 contains linker orientations that are similar to that for structure 2 during the adsorption exIt appears that site 2 in both structures is not associated periments. with the second lowest energy peak in the INS spectra, which We note that other potential structural models of ZIF-76 spans from 7.5–9.2 meV. We therefore assume that this peak are possible based on the disorder found in the original crysshould be attributed to H2 molecules with slightly different ACS Paragon Plus Environment


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tal structure,17 specifically those that contain asymmetric and random positioning of the linkers throughout the unit cell. Thus, one limitation of this work is the ability to extract all different combinations of linker positions about each Zn2+ ion in a unit cell of the material. Indeed, most of these combinations will require creating and optimizing an imidazolate and/or 5-chlorobenzimidazolate linker at a location that is not depicted in the original structure. Future work could involve selecting or generating these structures and evaluating each for their theoretical H2 uptakes and Qst values. It is possible that simulations in one or more of these structures will also yield adsorption isotherms and Qst values that are in good agreement with experiment, perhaps even better than what was obtained for structures 1 and 2 in this work. Nonetheless, we’ve shown that simulations in the two symmetric structures considered herein were sufficient to elucidate important details on the H2 adsorption sites in this ZIF.

The likely assignments of the peaks occurring in the INS spectra for ZIF-76 were made with the aid of twodimensional quantum rotation calculations. The rotational transitions for H2 adsorbed at the binding sites identified from molecular simulations in both structures were found to be in good agreement with the energy transfer values associated with the INS peaks. An interesting finding in this work is that the preferred regions of H2 occupancy in both structures are switched relative to each other. Nevertheless, the hexagonal windows connecting the α and β cages and the faces of the cubic cages are H2 adsorption sites that are common to both structures. We also found that the most favorable H2 binding sites in both structures gave rise to essentially the same peak in the INS spectra even though they are located in distinct regions. The inherent sensitivity of INS is likely compromised by the structural disorder in this system. The theoretical calculations performed herein were therefore critical to obtain significant details of the H2 adsorption mechanism and sites in this ZIF. By simulating H2 in two possible symmetric structures of ZIF-76, we were able to demonstrate how specific features in the INS spectra evolve even in the presence of structural disorder.

ASSOCIATED CONTENT

Supporting Information. Details of electronic structure calculations, GCMC methods, and quantum rotation calculations, pictures of ZIF-76 fragments, tables of properties, and additional simulated H2 sorption results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author ∗ E-mail: [email protected] (B.S.) ∗ E-mail: [email protected] (J.E.) Author Contributions § Authors contributed equally Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

T.P., K.A.F., and B.S. acknowledge the National Science Foundation (Award No. DMR-1607989), including support from the Major Research Instrumentation Program (Award No. CHE-1531590). Computational resources were made available by a XSEDE Grant (No. TG-DMR090028) and by Research Computing at the University of South Florida. B.S. also acknowledges support from an American Chemical Society Petroleum Research Fund grant (ACS PRF 56673ND6). We thank Dr. Rahul Banerjee for preparing a sample of ZIF-76 and Professor Omar M. Yaghi for graciously allowing us to use his adsorption instrument.

The results from this study reinforce the idea that H2 adsorbed in ZIFs is governed by their interactions with the imidazolate-type linkers, which is consistent with findings from other experimental9 and theoretical studies at cryogenic temperatures.11–13,58 This is in contrast to MOFs possessing open-metal sites, as such materials involve the binding of H2 onto (or near) the exposed metal centers for their adsorption mechanism at low loading.40,54,59–62 Despite the elevated adsorption enthalpies associated with the H2 –metal interaction, ZIFs may still have more relevance for environmental and industrial applications because of their much greater chemical and thermal stability than that of most MOFs that contain open-metal sites.7 In addition, MOFs with unsaturated metal centers may have problems with activation and regeneration,63 whereas these issues are not present for MOFs without open-metal sites.64 Improvements of the H2 adsorption performance in ZIFs could perhaps be achieved by design strategies aimed at synthesizing such materials that contain a number of confined accessible regions to allow for increased interactions between the H2 molecules and the surrounding imidazolate-type linkers. ACS Paragon Plus Environment

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Figure 1. Tiling representation of the lta topology observed in ZIF-76. The framework is composed of three distinct cages: cube, truncated cuboctahedron (α), and truncated octahedron (β); these cages are shown in green, red, and orange, respectively.

(a)

(b)

Figure 2. The Zn2+ tetrahedral node of ZIF-76, which is assembled by three imidazolate linkers and one 5-chlorobenzimidazolate linker. The two possible crystal structure configurations considered herein differ by the relative positions for one imidazolate and one 5-chlorobenzimidazolate linker. In (a), the atoms shown in normal coloring corresponding to structure 1 and those shown in yellow corresponding to structure 2. Atom colors: C = cyan, H = white, N = blue, Cl = green, Zn = silver. In (b), the atoms shown in violet are common to both structures, while those shown in red are unique to structure 1 and those shown in blue are unique to structure 2.

Table 1. Summary of the experimental properties and H2 adsorption data for different ZIFs. SL = Langmuir surface area. SBET = BET surface area. Vp = Pore volume. a At 77 K/1 atm. b At the lowest loading evaluated. ZIF

Topology SL (m2 g−1 ) SBET (m2 g−1 ) Vp (cm3 g−1 ) H2 Uptake (mg g−1 )a H2 Qst (kJ mol−1 )b Reference

DMA-rho-ZMOF

rho

-

-

-

9.5

8.0

10

Mg-rho-ZMOF

rho

-

-

-

9.1

9.0

10

Li-rho-ZMOF

rho

-

-

-

9.1

9.1

10

ZIF-8

sod

1810

1630

0.636

12.9

-

7

ZIF-11

rho

-

-

-

13.7

-

7

ZIF-20

lta

800

-

0.27

10.4

8.5

16

ZIF-68

gme

1220

1070

-

13.1

8.1

13

ZIF-69

gme

1090

950

-

12.3

8.1

13

ZIF-76

lta

1120

1000

0.397

10.6

7.7

This work


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(a) Structure 1

(b) Structure 2

Figure 3. Orthographic a/b/c-axis view of the 2 × 2 × 2 system cell of the two different crystal structure configurations of ZIF-76 considered in this work: (a) structure 1 and (b) structure 2. Atom colors: C = cyan, H = white, N = blue, Cl = green, Zn = silver.

Figure 4. (a) Experimental low pressure absolute H2 adsorption isotherms in ZIF-76 at 77 K (red) and 87 K (blue); adsorption = filled circles, desorption = open circles. (b) and (c) shows a comparison of the experimental (black) and simulated isotherms (red) for two different structures (structure 1 = solid, structure 2 = dashed) of ZIF-76 at 77 and 87 K, respectively. (d) Isosteric heats of adsorption (Qst ) for H2 plotted as a function of loading for experiment and simulation. The simulated results were obtained using a polarizable H2 potential.30



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Figure 5. Inelastic neutron scattering (INS) spectra for H2 in ZIF-76 at two different loadings: 1 H2 /formula unit (red) and 2 H2 /formula unit (violet). The spectra were collected on the QENS spectrometer at IPNS/ANL at a temperature of 15 K.

(a)

(b)

Figure 6. Molecular illustration of a H2 molecule adsorbed at site 1 in structure 1 of ZIF-76 as determined from simulated annealing: (a) down view; (b) side view. The sorbate molecule is shown in orange. Atom colors: C = cyan, H = white, N = blue, Cl = green, Zn = silver.

(a)

(b)



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(a)

(b)


(a)

(b)


Table 2. Calculated rotational transitions and barriers (in meV) for H2 adsorbed at two different sites in two structures of ZIF-76. Sites 1 and 2 in structure 1 are depicted in Figures 6 and 7, respectively, while sites 1 and 2 in structure 2 are illustrated in Figures 8 and 9, respectively. n is the principal quantum number and j is the rotational angular momentum quantum number. Structure 1 Structure 2 n j Site 1 ∆E (meV) Site 2 ∆E (meV) Site 1 ∆E (meV) Site 2 ∆E (meV) 1 0 0.00 0.00 0.00 0.00 2 6.61 12.23 7.01 11.11 3 1 18.73 17.20 17.83 14.06 4 25.78 21.13 25.70 20.53 5 40.57 42.55 40.27 40.97 6 41.31 43.61 41.13 41.01 7 2 45.72 47.27 45.74 45.57 8 54.52 50.17 53.88 47.70 9 56.17 51.12 55.71 49.48 V 48.81 37.07 47.58 24.11

Table 3. Possible assignments of the peaks observed at certain energy ranges in the INS spectra for ZIF-76 (Figure 5). INS Peak Range (meV) Assignment 6.0–7.2 Site 1 in both structures. 7.5–9.2 Slightly shifted positions about site 1 for both structures. 10.0–13.1 Site 2 in both structures. 13.3–15.7 Slightly shifted positions about site 2 for both structures and/or weaker adsorption sites.



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TOC Graphic


Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta

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