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Gaining Insights on the H2 – Sorbent Interactions: Robust soc-MOF Platform as a Case Study Amy J Cairns, Juergen Eckert, Lukasz Wojtas, Matthias Thommes, Dirk Wallacher, Peter A Georgiev, Paul M. Forster, Youssef Belmabkhout, Jacques Ollivier, and Mohamed Eddaoudi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02817 • Publication Date (Web): 18 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016
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Chemistry of Materials
Gaining insights on the H2 – Sorbent Interactions: Robust socMOF Platform as a Case Study Amy J. Cairns,a Juergen Eckert,b Lukasz Wojtas,b Matthias Thommes,c Dirk Wallacher,d Peter A. Georgiev,e Paul M. Forster,f Youssef Belmabkhout,a Jacques Ollivier,g and Mohamed Eddaoudia,b* a
3
Functional Materials Design, Discovery and Development Research Group (FMD ), Advanced Membranes and Porous Materials Center (AMPMC), Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia. E-mail:
[email protected]
b
Department of Chemistry, University of South Florida, 4202 East Fowler Avenue (CHE205), Tampa, Florida, 33620, USA
c
Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, Florida, 33426, USA
d
Sample Environment Department (NP-ASE), Helmholtz-Zentrum Berlin für Materialien und Energie, HahnMeitner-Platz 1, D-14109 Berlin, Germany
e f
Department of Structural Chemistry, University of Milan, 21 Via G. Venzian, I-20133 Milano, Italy
Department of Chemistry and Biochemistry, Box 454330, University of Nevada, Las Vegas, Nevada 89154, USA
g
Institut Laue-Langevin, 38042 Grenoble Cedex, France
ABSTRACT: We report on the synthesis and gas adsorption properties (i.e., Ar and H2) of four robust 3-periodic metalorganic frameworks (MOFs) having the targeted soc topology. These cationic MOFs are isostructural to the parent indium-based MOF, In-soc-MOF-1a (for NO3-), previously reported by us, and likewise are constructed from the assembly of rigid µ3-oxygen-centered trinuclear metal carboxylate clusters, [M3O(O2C–)6], where M = In3+ or Fe3+. Each inorganic trinuclear molecular building block (MBB), generated in situ, is bridged by six 3,3',5,5'-azobenzenetetracarboxylate (ABTC4-) ligands to give the extended (4,6)-connected MOF, soc-MOF. In our previous work, we confirmed that the parent socMOF, i.e. In-soc-MOF-1a, possesses unique structural characteristics (e.g., vacant In binding sites and narrow pores with higher localized charge density), which led to exceptional hydrogen (H2) storage capabilities. Therefore, charged MOFs with soc topology can be viewed collectively as an ideal prototypical platform to examine the impact of specific structural parameters on H2-MOF interactions via systematic gas adsorption studies. We infer that enhanced binding of molecular H2 is primarily governed by the presence and type of vacant metal centers (i.e., Fe was shown to exhibit stronger H2-MOF interactions at low H2 loading compared to the In analogues). These findings are evident from the associated isosteric heat of adsorption (Qst) at low loadings and inelastic neutron scattering (INS) experiments of the rotational transitions of sorbed H2, as well as, temperature programmed desorption (TPD) studies (for a select compound). The importance of localized charge density is also highlighted, where the extra-framework nitrate anions in the Fe-soc-MOF-1a (for NO3-) facilitate enhanced binding affinities as compared to the chloride analogue.
INTRODUCTION 1-15
Porous metal-organic frameworks (MOFs) have been widely investigated and continue to gain momentum as a promising class of fine tunable solid-state crystalline materials that offer potential to address some enduring industrial challenges, such as efficient and cost-effective sorbents for gas separation, capture and storage technologies.9,16-30 Scientific interest has particularly peaked for hydrogen (H2) storage31 for mobile applications due to its high energy density and attractive by-products, and thus many research groups, in academia and industry alike, have studied H2 sorption on a variety of multifunctional MOF adsorbents.32-44 The sorption-based approach to H2 storage in porous MOFs emerges as an ideal solution for the use of adsorption-based storage materials due to asso-
ciated fast adsorption/desorption kinetics, plausible very large capacities, and the likely stability and durability of such a system. Nevertheless, storage capacities at ambient temperatures and relatively modest pressures are too low to address and satisfy the criteria for mobile applications, a limitation associated to the fact that the adsorption of H2 is dominated by weak interactions between H2 and the host internal surface.45 Accordingly, access to highly porous MOFs with uniform and enhanced H2-MOF binding affinities in the range of 15 – 25 kJ mol-1 will pave the way toward practical H2 storage at room temperature. A better understanding of the H2–MOF interactions will permit the systematic enhancement of H2 sorption energetics toward a made-to-order MOF for efficient H2 storage. MOF crystal chemistry (e.g., isoreticular synthesis)
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allows distinctive structural parameters to be systematically modified and fine-tuned; and the individual impact on the H2 sorption energetics can be pinpointed in order to gain a better insight on the structure-property relationship.46-52 In fact, we have previously reported the structure and sorption properties of a porous MOF, an ionic MOF with soc topology and hereby denoted as In-soc-MOF-1a (for NO3-), which elucidated several key requisite parameters for efficient H2 uptake.33 These include (1) importance of a high concentration of accessible unsaturated metal centers with a strong affinity for H2, (2) optimal/favorable pore size (below 1 nm) and geometry, and (3) utilization of charged frameworks having extra-framework ions to enhance electrostatic interactions. The soc-MOF platform [composed of 6-connected metal trinuclear MBBs, a 4-connected rectangular-planar organic ligand, ABTC4-, and a counter-ion] is ideally suited for systematic fine-tuning to discern the relative contribution of each factor towards the overall binding energies of H2 in MOFs. Possible structural modifications include: (1) the metal-ion in the inorganic MBB can be replaced with lighter metal cations (e.g., Al, Cr, Fe, Ga, Mn, Ni, Sc, etc.)10-13,15 to generate MOFs with relatively lower densities and varying degrees of binding affinities for H2; (2) extra-framework counter-ions confined in the nanometer scale carcerand-like cage can be substituted for different ions in order to evaluate the impact on electrostatic field; and (3) contributions from polarizable substituents via decorated inorganic MBBs (i.e., saturate the axial positions of the inorganic MBB with one counter ion and two solvent molecules thereby rendering the cage anion free). Here we report on the crystal structures for four such variants, where either the metal ion or the polarizable substituents were modified. The materials were extensively characterized using a combination of high-resolution low- and high-pressure gas adsorption, inelastic neutron scattering (INS), and temperature programmed desorption (TPD) studies. Our results reveal that Fe-based socMOFs exhibit exceptional H2 uptake capacities and stronger H2-MOF interactions at low loading, as compared to the In analogues. These results were anticipated, given that Fe is known to form the greatest number of organometallic metal dihydrogen complexes among first row transition metals, and hence is expected to interact strongly with the adsorbed H2 molecules. However, the extra-framework counter-ion selection, where nitrate or chloride ions are encapsulated in the cage, was found to have only a negligible effect on the Qst.
EXPERIMENTAL Materials and methods. All chemicals and solvents used in the preparation of the compounds described here were of reagent grade and used without further purification. The MOFs were prepared via solvothermal methods using a programmable oven, where the temperature was ramped to the set point at a rate of 1.5oC min-1 and cooled to room temperature at 1.0oC min-1. All compounds were
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determined to be insoluble in H2O and common organic solvents. Synthesis of Fe-soc-MOF-1a. A solution of FeSO4.6H2O (30.0 mg, 0.11 mmol) and 3,3',5,5'azobenzenetetracarboxylic acid, H4-ABTC (12.9 mg, 0.036 mmol), 1 mL N,N'-dimethylformamide (DMF), 1 mL H2O, 0.5 mL chlorobenzene, and 0.30 mL HNO3 (3.5M in DMF) was prepared in a 20 mL scintillation vial and subsequently heated to 85oC for 12h. The as-synthesized sample was purified through repeated washings with DMF to yield small red cube-shaped crystals. Crystals of Fe-soc-MOF-1a were harvested, washed with acetonitrile (CH3CN) and air-dried. (Yield: 10.2 mg, 33.0 % based on Fe). FT-IR Data (range 4000 to 600 cm-1): 3421 (br), 1657 (vs), 1588 (w), 1573 (w) 1496 (w), 1439 (w), 1388 (s), 1255 (s), 1098 (s), 865 (w), 775 (m), 715 (w), 661 (m). Synthesis of Fe-soc-MOF-1b. A solution of FeCl2 (13.7 mg, 0.11 mmol) and H4-ABTC (12.9 mg, 0.034 mmol), 1 mL DMF, 1 mL H2O, 0.5 mL chlorobenzene, and 0.45 mL of HCl (3.5 M in DMF) was prepared in a 20 mL scintillation vial and then heated to 85oC for 12h to give red cubeshaped crystals. Crystals of Fe-soc-MOF-1b were harvested, washed with CH3CN and air-dried. (Yield: 14.2 mg, 52.9 % based on Fe). FT-IR Data (range 4000 to 600 cm-1): 3379 (br), 1623 (m), 1574 (m), 1451 (m) 1372 (s), 1252 (w), 1233 (w), 1109 (w), 983 (w), 927 (w), 776 (s), 716 (s), 620 (m). Synthesis of In-soc-MOF-1b. A solution of InCl3 (30.0 mg, 0.14 mmol) and H4-ABTC (24.3 mg, 0.068 mmol) in 1 mL DMF, 1 mL ethanol (EtOH), and 0.5 mL H2O was prepared in a 20 mL scintillation vial and heated to 85oC for 12h to give orange cube-shaped crystals. Crystals of Insoc-MOF-1b were harvested, washed with CH3CN and airdried (Yield: 26.2 mg, 52.9 % based on In). FT-IR Data (range 4000 to 600 cm-1): 3487 (br), 1669 (vs), 1497 (w), 1439 (w), 1389 (s), 1256 (m), 1096 (m), 777 (m), 715 (m), 661 (m). Synthesis of In-soc-MOF-1c. A solution of InBr3 (49.6 mg, 0.14 mmol) and H4-ABTC (24.1 mg, 0.067 mmol) in 1 mL DMF, 1 mL CH3CN, and 0.5 mL H2O was prepared in a 20 mL scintillation vial and heated to 85oC for 12h to give pure orange cube-shape crystals. Crystals of In-soc-MOF1c were harvested, washed with CH3CN and air-dried. (Yield: 23.4 mg, 44.7 % based on In). FT-IR Data (range 4000 to 600 cm-1): 3508 (br), 1657 (vs), 1578 (w), 1496 (w), 1437 (w), 1388 (vs), 1254 (s), 1092 (s), 1062 (w), 932 (w), 864 (w), 777 (m), 715 (m), 659 (m). Characterization. Single crystal X-ray diffraction (SCXRD) data for compound Fe-soc-MOF-1a was collected using synchrotron radiation (λ = 0.77090 Å) at the Small Molecule Crystallography beamline (11.3.1) at the Advanced Light Source in Lawrence Berkeley National Laboratory Berkeley, California. SCD data for Fe-socMOF-1b, In-soc-MOF-1b and In-soc-MOF-1c was collected on a Bruker-AXS SMART-APEXII CCD diffractometer (CuKα, λ = 1.54178 Å). Powder X-ray Diffraction (PXRD) measurements were collected on a PANalytical X’Pert PRO MD X-ray diffractometer at 45kV, 40mA for Cu Kα
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(λ = 1.5418 Å) with a scan speed of 1o/min. Fouriertransform Infrared (FT-IR) spectra were recorded on a Nicolet Avatar 320 FT-IR spectrometer. High-resolution dynamic thermogravimetric analysis (TGA) experiments were collected on a TA instruments Hi-Res TGA Q500 thermogravimetric analyzer under a nitrogen atmosphere (flow = 25 cm3 min-1). Low-pressure gas sorption measurements were performed on a fully automated Autosorb1C gas adsorption analyzer (Quantachrome Instruments) at cryogenic temperatures of 77 K and 87 K up to a pressure of 1 atm. INS spectra of hydrogen adsorbed on Fesoc-MOF-1a and In-soc-MOF-1b were collected on the IN5 spectrometer at the Institut Laue-Langevin (ILL) in Grenoble, France.
proach due to the narrow window dimensions of the enclosed nano- sized cages (ca. 2.953 Å x 3.965 Å, taking into account van der Waals radii).
RESULTS AND DISCUSSION X-ray Crystal Structure Analysis. Solvothermal reaction of H4-ABTC with FeSO4•6H2O in a mildly acidic solution containing a mixture of N,N΄-dimethylformamide (DMF)/H2O/chlorobenzene yields red crystals with cubelike morphology, characterized and formulated by singlecrystal X-ray diffraction (SCXRD) studies as [Fe3O(C16N2O8H6)1.5(H2O)3](NO3)(H2O)0.67 [(Fe-soc-MOF1a (for NO3-)]. The phase purity of the as-synthesized sample was confirmed by comparison of calculated and experimental powder X-ray diffraction (PXRD) patterns (Figure S3). The crystal structure of the Fe-soc-MOF-1a was indeed determined to be isostructural with the parent material, In-soc-MOF-1a, in which case each Fe atom adopts an octahedral coordination environment (Figure 1). The equatorial plane is occupied by four carboxylate oxygen atoms (dFe–O = 2.005(3) Å to 2.017(3) Å) from four independent ABTC4- ligands, while a terminal aqua ligand (dFe–O = 2.060(3) Å) and the µ3-oxo anion (dFe-O = 1.923(6) Å) are bound in the axial positions and complete the coordination sphere of the Fe(III) cation, [FeO5(H2O)]. The µ3-oxo anion (O2-) is located on a threefold axis and bridges the three Fe octahedra to give a trigonal planar assembly of Fe-( µ3-O)-Fe units at angles of 120o. Each Febased MBB, [Fe3(µ3-O)(H2O)3(O2C–)6], is bridged to neighboring MBBs through six independent ABTC4- linkers (deliberately selected due to the suitable angle between the carboxylate moieties and optimal length of the ligand) to afford a 3-periodic (4,6)-connected MOF with the targeted soc topology. Two types of intersecting channels having different wettability characteristics (i.e., hydrophilic and hydrophobic) (Figure 1) exist in the structure; in addition to a nanometer-scale carcerand-like cage (Figure 2). Each individual Fe atom in the Fe-soc-MOF-1a is trivalent, and hence the overall charge renders a cationic framework (+1 per formula unit). The framework charge is tentatively balanced by disordered [NO3-] anions, presumably located in the corners of the cage in a tetrahedral arrangement, as observed in the parent In-soc-MOF-1a. Anions are disordered about the 3-fold symmetry axes over at least six different positions and therefore could not be modelled reliably. It is to note that the anions cannot be exchanged using the conventional post-synthetic ap-
Figure 1. Select fragments from the X-ray crystal structure of Fe-soc-MOF-1a: (a) Ball-and-stick view of the Fe-based MBB, [Fe3(µ3-O)(H2O)3(O2C–)6], and the deprotonated organic 4linker, ABTC . (b) The 6- and 4-connected inorganic and organic MBBs nodes can be rationalized as having trigonal prismatic and rectangular-planar geometry, respectively. (c) Ball-and-stick and (d) polyhedral representations of the cuboidal cage and (e) framework shown to highlight the two types of intersecting channels. Color code: C = gray, O = red, N = blue, Fe = purple. The estimated cavity size (~10.45 Å), including van der Waals radii, is indicated by the large yellow sphere. Hydrogen atoms, free water molecules, and NO3ions are omitted for clarity.
In order to perform a systematic investigation to discern the potential impact of the counter-ion on the H2– MOF interactions (particularly at low loading), it was therefore necessary to synthesize the isostructural analogue by utilizing a different Fe salt and/or acid (i.e., counter-ion source). Indeed, reaction between H4-ABTC, FeCl2, and HCl, under similar conditions as in Fe-socMOF-1a, yields red cube-shape crystals, formulated as [Fe3O(C16N2O8H6)1.5(H2O)3](Cl) [(Fe-soc-MOF-1b (for Cl-)] by SCXRD studies. Crystallographic analysis and PXRD patterns (Figure S4) indicate that the framework is isostructural to Fe-soc-MOF-1a; however, as expected, charge balance in this case is provided by disordered extra-framework chloride anions (Cl-), which statistically
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occupy three positions about the three fold axis with equal probability. Reaction of H4-ABTC with InCl3 or InBr3 in a solution or containing DMF/ethanol (EtOH)/H2O DMF/acetonitrile (CH3CN)/H2O, respectively, yields two homogeneous microcrystalline compounds having cubelike morphology, characterized by SCXRD studies and formulated as [In3(C16N2O8H6)1.5(Cl)(H2O)2]•(H2O)5.35 [(Insoc-MOF-1b (for Cl-)] and [In3(C16N2O8H6)1.5(Br)(H2O)2]•(H2O)6.25 [(In-soc-MOF-1c (for Br-)]. Notably, in this case SCXRD analysis revealed a so-called decorated inorganic MBB where the assembly of eight [In3(µ3-O)(X)(H2O)2(O2C–)6] inorganic MBBs (where X = Cl or Br) and twelve ABTC4- ligands defines the anion-
Figure 2. Ball-and-stick representation of (left) the nitrate ions encapsulated in the cage of Fe-soc-MOF-1a and (right) the chloride ions encapsulated in the cages of Fe-soc-MOF1b. The counter-ions are highlighted in space-filling representation for clarity. All hydrogen atoms and solvent molecules are omitted for clarity. Color code: C = gray, O = red, N = blue, Cl = pink; Fe = green.
free cage (Figure S2). To the best of our knowledge, Insoc-MOF-1b and -1c represent the first examples of Inbased trinuclear MBBs comprised of coordinated halide ions. A summary of the compounds described here, including the charge-balancing counter-ion present in each structure is shown in Table 1. Table 1. Summary of the assigned naming system denoting the counter-ion present in each of the socMOF analogues described in this study. Compound Abbreviation
Counter-ion
In-soc-MOF-1a
NO3- (parent soc-MOF)
In-soc-MOF-1b'
Cl-
In-soc-MOF-1c
Br-
Fe-soc-MOF-1a
NO3-
Fe-soc-MOF-1b'
Cl-
Low-Pressure Gas Adsorption Studies. To gain a better understanding of H2-MOF interaction(s) at specific binding site(s) in the soc-MOF platform, we evaluated systematic modifications of key structural parameters: (1) effect of metal (In vs. Fe) and corresponding open-metal binding sites; (2) impact of localized charge density by comparing the effect of different extra-framework counter
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ions (presence of nitrate vs chloride in the Fe analogues) and (3) importance of polarizing substituents via decorated In-MBBs (coordinated chloride vs. bromide). Firstly, we assessed the gas sorption properties of Fesoc-MOF-1a by exchanging the guest molecules with CH3CN after evacuation at 135oC. The aim of this experiment was to determine if the axial aqua ligands on the FeMBB were bound or removed to yield unsaturated metal binding sites. It is to note that In-soc-MOF-1a was also activated at 135oC for sorption studies, but, since it is known that Fe interacts more strongly with water than does In (dFe-O = 2.060(3) Å; dIn-O = 2.173(3) Å), we anticipated that higher temperatures would be required to ensure complete removal of the coordinated water molecules. The H2 sorption capacity for Fe-soc-MOF-1a was found to be 2.17 wt % at 77 K and atmospheric pressures (Figure S9), while the Qst for H2 was determined to reach a maximum of 6.5 kJ mol-1 at low loading. The values for Qst are comparable with those of In-soc-MOF-1a after analogous evacuation procedures with trivial differences observed at higher loading. This observation suggests that some of the coordinated water molecules remain bound to the Fe-MBB. Accordingly, heating Fe-soc-MOF-1a to elevated temperatures (165oC) resulted in a significant color change from red to dark brown. This observation, coupled with the TGA data, further supports our assumption that the framework is fully desolvated to give unsaturated Fe sites. The Ar sorption isotherm collected at 87 K shows a fully reversible Type I isotherm, characteristic of microporous materials, with apparent BET and Langmuir surface areas estimated to be 1700 m2/g and 2060 m2/g, respectively (Figure S10). The corresponding total pore volume and pore size distribution were determined to be 0.69 cm3 g-1 and 6.4 Å, respectively (Table S1). An evaluation of the H2 sorption properties of the Fesoc-MOF-1a showed that it can store even higher amounts of H2, up to 2.62 wt % at 77 K (Figure 3a). A notable increase in the Qst is observed at low loading, i.e., 7.9 kJ mol-1, which is appreciably higher than both the 135oC data for Fe-soc-MOF-1a and the parent In-socMOF-1a (Figure 3b). The ability of the Fe analogue, defined by a lower framework density, to adsorb such a large amount of H2 under these conditions coupled with the enhanced H2-MOF interactions observed at lower loadings is most likely driven by: (1) the presence of highly polarizable open Fe binding sites on the trinuclear MBBs; (2) narrow pores ( Fe-soc-MOF-1b), which can be rationalized by noting the relative size of the counter ions; that is, a nitrate ion is larger than a chloride ion, and, thus, occupies a higher degree of space in the nano-sized cage. Accordingly, it is conceivable to believe the nitrate ions are more accessible to H2 via the windows than the chloride ions in the desolvated soc-MOFs.
2.04 wt % and 1.95 wt%, respectively, which is considerably lower than the maximum capacities reported for the soc-MOF analogues discussed above (Figure 3a). The corresponding values of Qst for both compounds were calculated to be relatively constant at, ca. 6.7 kJ mol-1 at low loading (Figures S13, S14). These results clearly demonstrate that the nature of the halide on the in trinuclear inorganic MBB has a negligible effect on enhancing H2MOF interactions, even at low loading. We may therefore conclude that the added polarizability and modest reduced pore size from the halide ions is enough to compensate for the loss of one open In binding site and of the localized charge density associated with the cuboidal cage of the previous analogs. Temperature Programmed Desorption Studies. Figure 4 shows the adsorption behavior of H2 in Fe-socMOF-1b from the boiling temperature (20 K) up to supercritical temperatures. The differences observed in the shape of the variable-temperature isotherms are governed by the liquid-gas equilibrium phase state of the adsorbate. The isotherm at 20 K clearly shows a perfect Type I behavior (IUPAC classification) indicating that the pores in the framework have been saturated with a dense liquidlike adsorbate (H2) at very low pressures. Conversely, the supercritical isotherms in the range of 77 K to 107 K are no longer Type I due to the fact H2 is no longer a condensable gas and thus affords a much lower uptake in the absolute pressure range. Accordingly, at 77 K the same saturation uptake (500 cm3 g-1 STP) at 4.5 wt%, observed at 20K and low pressure, could be achieved only at 30 bar (Figure S16).
Figure 3. Comparison of the (a) H2 adsorption isotherms collected at 77 K and atmospheric pressures for all soc-MOF analogues described herein and (b) Qst for H2 for Fe-socMOF-1a and -1b compared to In-soc-MOF-1a.
The pair of soc-MOFs constructed from decorated Inbased trinuclear MBBs (In-soc-MOF-1b and -1c) whose axial positions are occupied by a disordered chloride ion and bromide ion (one halide per inorganic MBB, [In3(µ3O)(X)(H2O)2(O2C–)6] where X = Cl or Br) can be directly compared to elucidate potential effects from polarizable halide substituents. The H2 uptake for In-soc-MOF-1b and -1c at atmospheric pressures and 77 K was found to be
Figure 4. Variable-temperature H2 adsorption data for Fesoc-MOF-1b collected in the range of 20 to 107 K.
Temperature Programmed Desorption (TPD) data were also collected on the desolvated form of the Fe-soc-MOF1b, which was loaded to varying amounts to a maximum of 4 wt% of H2 attainable at 10 K. The resulting desorption curves for the six different loadings ranging from 14% to 90% correspond to 0.7, 1.5, 2.4, 3.4, 3.7 and 4.5 H2 per Fe (Figure 5). The observed changes in the desorption curves, in terms of the amount of H2 in the system; can be interpreted in a qualitative manner in terms of the types of sites occupied at a particular loading. The four curves which correspond to capacities at or below the maximum at 1 bar and 77 K consist of a single broad peak that grad-
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ually shifts to lower temperatures, from approximately 58 K to 45 K, and also broadens appreciably. The width of the peaks may be taken to reflect the range of H2 sorption sites with somewhat different binding energies that are occupied, and this is, of course, expected to increase with loading. The shift in the peak position with temperature in turn is an indication of the overall decrease of binding strengths as more sites with lower energies are occupied. The curve for the lowest loading of 14 %, or 0.7 H2 per Fe should correspond primarily to the strong interaction of hydrogen molecules with the open Fe sites, and does, accordingly peak at the highest temperature.
Figure 6. INS spectra (T = 4.3 K) corresponding to 1 H2 per trinuclear inorganic MBB of Fe-soc-MOF-1a (red) and Insoc-MOF-1b (turquoise) in neutron energy loss over the range from 3 – 17 meV.
Figure 5. TPD data collected Fe-soc-MOF-1b from 10 to 90 K.
We note, however, that the INS spectra described below suggest that some weaker binding sites are also occupied at low loadings, presumably because the binding energies do not differ by a large amount. At the two highest loadings the TPD curves exhibit additional features at very low temperature, i.e. weak interactions, which must arise mainly from H2-H2 interactions from condensed hydrogen on the surfaces of the pores. Inelastic Neutron Scattering (INS) Experiments. Measurements were carried out on two select compounds (Fe-soc-MOF-1a and In-soc-MOF-1b) in order to gain insights into the preferential molecular-level binding sites for H2 in these materials. Shown in Figure 6 are the INS spectra after the initial loading of 1 H2 per trinuclear MBB, i.e. [Fe3(µ3-O)(O2C–)6] (Fe-soc-MOF-1a) and [In3(µ3O)(Cl)(O2C–)6] (In-soc-MOF-1b). This particular loading was chosen because it corresponds to 1/3 H2 per Fe, and 2/3 H2 per open In, respectively, which in turn is in the range of the measured Qst where that for Fe-soc-MOF-1a is appreciably greater, than those for the indium analogs. The spectrum for Fe-soc-MOF-2a shows two-well defined peaks at 7.7 meV and 10 meV, while the former does not have any clearly identifiable spectral signatures in the same region. Since we associate lower rotational transition frequencies with higher barriers (rotational tunneling), this observation rationalizes the stronger interaction of H2 with Fe than with the In derived from the experimental Qst
curves at low loading. The second set of loadings (Figure S16) corresponds to filling of the metal binding sites, i.e. 3 and 4/3 H2 per formula unit, respectively, for Fe-socMOF-1a and In-soc-MOF-1b. The pair of peaks for the Fe binding site shows a commensurate increase in intensity, while a weak and rather broad band near 8 meV appears for In-soc-MOF-1b. The two peaks, 7.7 meV and 10 meV, in Fe-soc-MOF-1a show a parallel increase in intensity in much the same way as H2 adsorbed at open-metal binding sites in the Ni-, Co-, and Mg-CPO-27 analogs of MOF-74. Therefore, these peaks can be assigned as two related rotational transitions for H2 at the Fe site. The broad band at 8 meV in the spectrum of In-soc-MOF-1b can, in turn be assigned to H2 binding sites near the chloride ion on the In-based trinuclear MBB. This is analogous with a very similar band observed in ZIF-68, where one of the hydrogen atoms in the benzeneimidazole linkers is substituted by a chloro group. Accordingly, the INS data presented here explicitly demonstrate that it is the accessible Fe binding sites in Fe-soc-MOF-1a which are responsible for the enhancement of Qst at low loadings relative to that of the In soc-MOF analogs.33 On the basis of our detailed computational studies of H2 in In-soc-MOF-1a we can assign the lower energy part of the broad band near 12 meV to H2 at the In sites, while the strongest binding sites at the nitrate ions in the Fe-soc-MOF-1a again are not accessible at low loadings. At higher loadings, strong albeit broad bands are observed in the spectra for In-socMOF-1b in the region above 10 meV (Figure 7), along with an overall increase in unstructured background. The latter suggests the presence of a wide distribution of binding sites, as is frequently observed in the case of charged frameworks. We must also point out that the modest energy resolution available from this type of instrument when used with a short incident wavelength, 2 Å, will not resolve fine structure in these bands, as was possible on the QENS instrument at IPNS used in many of our previous measurements.33 We have previously assigned peaks
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in this region to weaker binding sites between the organic components of the framework, i.e. carboxylate, azo, and phenyl moieties. One such site was identified in our computational study on In-soc-MOF-1a as being adjacent to the azobenzene nitrogen centers.53 The INS spectra at low loadings appear fairly similar but one may discern three peaks at 13.3, 14.4, and 15.6 meV for Fe-soc-MOF-1a and 12.6, 14.5, and 15.6 meV for In-soc-MOF-1b. The two highest frequency transitions in each system are likely associated to two different transitions for H2 at the same type of organic linkers site, It is of interest to note that the band at 13.3 meV in the Fe-soc-MOF-1a shifts dramatically to lower frequency (12.1 meV) when the loading is increased to the equivalent of 2 H2 per Fe (18 mmol of H2). We observed a similar, though less pronounced, effect in the Insoc-MOF-1a compound.33,53 A plausible explanation to account for this observation is by noting that Fe-soc-MOF-1a and In-soc-MOF-1a have NO3- anions encapsulated in cage; while no charged
species reside in the cage in compound In-soc-MOF-1b. It is thereby apparent, that H2 can be forced into the ionic cavity at high loadings, whereas this cage simply fills gradually in latter case (peak shift: 12.6 to 12.0 meV at high loadings). A similar effect may be discerned from the loading dependence of the TPD curves described above, although it must be pointed out that those data were collected on Fe-soc-MOF-1b, as opposed to Fe-soc-MOF-1a, in the INS experiment. It may be noted that the incremental shift of the peak position in the TPD curves decreases for the two highest loadings relative to that for the two lowest loadings. This observation would support the idea that at higher pressures, i.e. loadings above 2 H2 per Fe, some higher energy binding sites become occupied so that the average decrease in binding energy is not as much as that for the first two loadings. These differences are, however, rather small, and not reflected in an obvious way in the values of Qst at higher loadings since the binding energies for all the sites in this region are very similar in magnitude. Both the INS spectra and TPD curves are appreciably more sensitive to such details than the adsorption isotherms and the resulting values of Qst.
CONCLUSION In summary, the robust isostructural soc-MOFs presented here further highlighted the power of the MBB approach towards targeting made-to-order materials and enabled us to conduct a systematic benchmark study to evaluate the effects of key structural parameters on the enhancement of H2-MOF interactions in this versatile class of adsorbents. Low pressure gas sorption data was complemented by INS data (for Fe-soc-MOF-1a and Insoc-MOF-1b) and TPD data (for Fe-soc-MOF-1b) and thus provided more detailed information on the H2 binding sites. Of the factors important in H2 binding, we demonstrated that in addition to narrow pores with localized charged density, a high concentration of accessible Fe metal binding sites is more effective at improving the H2MOF interactions at low loading, as compared to unsaturated In sites in the analogous soc-MOF materials.
ASSOCIATED CONTENT Supporting Information. Electronic Supplementary Information (ESI) available: PXRD, TGA, FT-IR, INS, TPD, lowpressure gas sorption data, supporting figures, and single crystal X-ray crystallographic data. CCDC reference numbers 1408397-1408400. See DOI: 10.1039/x0xx00000x.
AUTHOR INFORMATION Corresponding Author *M.E., email:
[email protected]
NOTES Figure 7. INS spectra (T = 4.3 K) to emphasize the differences at higher loading in neutron energy loss over the range from 3 to 17 meV: (top) Fe-soc-MOF-1a shown after loadings of 2, 6, 18, and 30 mmol which are represented in red, blue, black and green respectively; (bottom) In-soc-MOF-1b at loadings corresponding 1.3, 2.7, 12, and 20 mmol of H2 represented in blue, pink, yellow and purple, respectively.
The authors declare no competing financial interest.
Author Contributions The manuscript was written through contributions of A.J.C, J.E., M.T., D.W., and M.E. / All authors have given approval to the final version of the manuscript.
ACKNOWLEDGMENT
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We gratefully acknowledge the financial support of KAUST funds. We also acknowledge the Advanced Light Source which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
ABBREVIATIONS MOF, metal-organic framework; TPD, temperature programmed desorption; INS, inelastic neutron scattering; MBB, molecular building block; Qst, isosteric heat of adsorption; SCXRD, single crystal X-ray diffraction; PXRD, power X-ray diffraction; H4-ABTC, 3,3',5,5'-azobenzenetetracarboxylic acid.
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