Selective O2 Sorption at Ambient Temperatures via Node Distortions

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Selective O2 Sorption at Ambient Temperatures via Node Distortions in Sc-MIL-100 Dorina F Sava Gallis, Karena W Chapman, Mark A. Rodriguez, Jeffery A. Greathouse, Marie V Parkes, and Tina M. Nenoff Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00249 • Publication Date (Web): 14 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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Chemistry of Materials

Selective O2 Sorption at Ambient Temperatures via Node Distortions in Sc-MIL-100 Dorina F. Sava Gallis,† Karena W. Chapman,┴ Mark A. Rodriguez,‡ Jeffery A. Greathouse,§ Marie V. Parkes,§ and Tina M. Nenoff†* † Nanoscale Sciences Department, Sandia National Laboratories, Albuquerque, NM 87185, USA. ┴ X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, X-ray Science Division, Argonne, IL 60439 USA. ‡ Materials Characterization and Performance Department, Sandia National Laboratories, Albuquerque, NM 87185, USA. § Geochemistry Department, Sandia National Laboratories, Albuquerque, NM 87185, USA. ABSTRACT: An open pored metal-organic framework (MOF) with oxygen selectivity at exceptionally high temperatures is confirmed by synthesis, sorption and synchrotron structural analyses. The large-pore MIL-100 framework with access to the metal center (e.g. Sc and Fe) resulted in preferential O2 over N2 gas uptake at temperatures ranging from 77 K to ambient temperatures (258 K, 298 K and 313 K). Most notably, Sc-MIL-100 shows exceptional O2 sorption; pair distribution function analyses indicate that this is due to distortions in the framework owing to the size of Sc atoms, in particular in the trimer metal cluster. Experimental studies also correlate very well with GCMC simulations, confirming more favorable O2-framework interactions at pressures up to 1 bar, due to the close proximity of O2 to the high density of metal centers in the small tetrahedral cages. Both materials maintain their crystallinity upon gas adsorption cycling, are regenerable, and show exceptional promise for use in energy efficient oxygen purification processes, such as Pressure Swing Adsorption.

1. INTRODUCTION Oxyfuel combustion is a promising technology with potential to replace less efficient traditional air combustion processes. Combustion of oxyfuels can not only deliver enormous energy savings but can also curtail greenhouse gas emissions.1, 2 Widespread implementation of oxyfuels requires industrial scale quantities of high purity oxygen. Currently, high purity oxygen is derived through air separation methods. These are both costly and energy intensive, with most separations being conducted at cryogenic temperatures.3 Pressure swing adsorption (PSA) is an alternative approach to gas separations that utilizes porous materials and/or membranes to reduce the energy required for separations4 by allowing the process to occur at near ambient temperature and pressure. However, to date, this approach has been limited by the moderate O2 purity achieved. For example, industrial use of Li X zeolite through PSA results in O2 purity of 90-95%.5 In recent years, we6,7 and others8,9 have explored the use of novel adsorbent materials, in particular metal-organic frameworks (MOFs), to increase O2 sorption selectivity and, accordingly, the purity of O2 from air (~ 78% N2, 21% O2, 1% Ar). This is a logical extension of MOF applications in adsorption and separation of other gases.10,11,12,13,14 In a recent comprehensive computational screening study led by our group,7 DFT simulations predicted early transition metals would have much greater O2 over N2 binding energies. Two families of MOFs with unsaturated metal centers (UMCs) were investigated in that study, M2(dobdc) and M3(btc)2. A common observation for both MOF series was found, independent of framework topology: the early first-row transition metal results showed significantly stronger oxygen binding than later transition metals, whereas nitrogen binding

energies were moderate across the transition metal series. These findings directed our experimental work here to target early transition metal MOFs for O2 adsorption over N2. Due to the predicted metal-related preference of O2 over N2, we have selected a MOF with large pores (MIL-100)15 to ensure that the metal sites are readily accessible. Two isostructural MIL-100 systems with an early, Sc16, and a middle, Fe,17 transition metal were prepared, and their structure and O2 and N2 gas sorption behaviors characterized. The MIL-100 framework is constructed from oxo-centered metal trimers linked by benzenetricarboxylate (btc) ligands, forming tetrahedral cages, referred to as “supertetrahedra”.15 Corner-sharing of the supertetrahedral building blocks defines a highly porous structure with two types of mesoporous cages of 25 Å and 29 Å in diameter, accessible through pentagonal windows of ~ 5.5 Å and hexagonal windows of ~ 8.6 Å, respectively, Figure 1. Because of the importance of studying both the fundamentals of adsorption, and real world applications, sorption studies were carried out over a wide temperature range including 77 K, 258 K, 298 K and 313 K. In order to gain a fundamental understanding of the structure-property relationship of MIL-100 and gas sorption specifics, including the metal-O2 or metal-N2 bonding, both the framework structure determination and the adsorption properties were targeted. This required a variety of experimental and modeling tools to characterize the materials: Grand Canonical Monte Carlo (GCMC) molecular simulations, Ideal Adsorbed Solution Theory (IAST) selectivity calculations, crystallography (high resolution synchrotron powder X-ray diffraction and pair distribution function analysis), microscopy (Scanning Electron Microscopy with Energy Dispersive Spectroscopy),

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thermogravimetric analyses (TGA) and variable temperature gas sorption studies. Detailed structure determination and comparisons between Sc- and Fe-MIL-100 were completed

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and described to highlight differences in gas sorption behavior. 2. EXPERIMENTAL SECTION All reactant materials were purchased from commercially available sources and used without further purification. Fe-MIL100 powder was purchased under the commercial name of KRICT F100 (Strem Chemicals, Inc.). 2.1. Materials synthesis. Synthesis of Sc-MIL-100: A unique synthesis was employed here, which differs from that previously reported.16 A mixture of Sc(NO3)3·xH2O (0.02 g, 0.087 mmol) and 1,3,5benzenetricarboxylic acid (1,3,5-BTC, 0.018 g, 0.087 mmol) were dissolved in 3 mL of N,N’-dimethylformamide (DMF) and concentrated HCl (37.0 %, 1 drop) was heated in a scintillation vial to 373 K in a convection oven at a rate of 1.5 K/min, held at 373 K for 18 hours, and then cooled to room temperature at a cooling rate of 1 K/minute. The solvent was decanted and the solid was washed with 5x10 mL of DMF. The purity of the as-

synthesized compound was confirmed by powder X-ray diffraction study, Figure S1. 2.2. Powder X-ray Diffraction (PXRD). Measurements were performed on a Siemens Kristalloflex D500 diffractometer, CuKα radiation (λ = 1.54178 Å). 2.3. Thermogravimetric-mass spectrometry analyses (TGAMS). Measurements were conducted on a SDTQ600 TA instrument, equipped with a mass-spectrometer gas analyzer MSThermoStarTM from Pfeiffer Vacuum. The samples were heated to 1073 K at a 5 K/min heating rate, under continuous nitrogen flow.

Figure 1. Ball-and-stick representation of pentagonal and hexagonal windows and two types of mesoporous cages in the MIL-100 structure. Hydrogen atoms and coordinated water molecules have been omitted for clarity; atom color scheme: metal= blue; C=grey, O= red. The yellow ball represents the largest sphere that can fit inside the cage, considering the van der Waals radii of the nearest atoms.

2.4. High-energy Synchrotron Scattering and Pair Distribution Function (PDF) Analysis. X-ray scattering data suitable for diffraction and PDF analysis were collected at beamlines 17-BM and 11-ID-B at the Advanced Photon Source at Argonne National Laboratory for Sc-MIL-100, Fe-MIL-100 and the 1,3,5-BTC ligand. Guest solvent molecules were removed from the MIL-100 by gently heating under vacuum, and the samples sealed in glass capillaries within a glove box. For PDF analysis, high energy X-rays (11-ID-B, 58 keV, λ = 0.2114 Å) were used, in combination with a large amorphous silicon-based area detector, to collect data to high values of momentum transfer, Qmax = 22 Å−1.18,19 The two-dimensional images were reduced to one-dimensional scattering data within fit2d. The PDFs, G(r), were extracted within PDFgetX2,20 subtracting contributions from the background, Compton scattering, fluorescence, to the total scattering data as described previously.18 To separate the features in the PDF associated with the metal-coordination, differential PDFs (dPDFs) were calculated, subtracting the PDF measured for the bulk ligand from that of the MOF. The position and area of features of interest within the dPDF were quantified by fitting Gaussian functions within fityk.21 For diffraction analysis, data were collected at 17-BM using moderate energy X-rays (17 keV, λ = 0.72768 Å) and a large amorphous silicon-based area detector placed a large distance (100 cm) from the sample to maximize angular resolution. The two-dimensional images were reduced to one-dimensional scattering data within GSAS-II. To establish an adequate model for fitting of the diffraction data, framework atom positions were taken from previously reported work.17 The fractional coordinates for the framework Fe, Sc, O, and C atoms were fixed and only the unit cell parameter was allowed to refine to convergence in initial refinements. Other pattern dependent parameters such as scale


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factor, peak shape and background were optimized. Structure refinement was executed using GSAS.22 2.5. Scanning Electron Microscopy (SEM) - Energy Dispersive Spectroscopy (EDS). SEM analyses were captured on a FEI NovaNano SEM 230, at various accelerating voltages between 1 and 20 kV. EDS analyses were collected on an EDAX Genesis Apex 2 with an Apollo SDD detector. 2.6. Sample activation and gas adsorption measurements. Prior to measuring the gas adsorption isotherms, the Sc-MIL-100 sample was soaked in 15 mL of methanol for 3 days, with the solvent replenished every 24 hours. The Fe sample was used as received. Both samples were activated under vacuum on a Micromeritics ASAP 2020 surface area and porosity analyzer, at 453K for 12 hours for the Sc-MIL-100 and 423 K for 6 hours for the Fe-MIL-100 sample. Gas adsorption isotherms were measured using a Micromeritics ASAP 2020 surface area and porosity analyzer. Nitrogen and oxygen gas of ultra-high purity (99.999%, obtained from Matheson Tri-Gas) were used in these experiments. Gas adsorption isotherms at 77 K were measured in a liquid nitrogen dewar. N2 and O2 adsorption isotherms at 258 K, 298 K and 313 K were measured in a dewar connected to a Polyscience circulating isothermal bath with an advanced digital temperature controller. O2 adsorption isotherms at 77 K were measured up to ~ 0.2 P/P0, since the saturation vapor pressure of O2 at 77 K is 147.8 mmHg. 2.7. Molecular modeling 2.7.1. Grand Canonical Monte Carlo (GCMC) simulated isotherms. GCMC simulations were performed using the Towhee code23 to obtain single component adsorption isotherms for N2 and O2 at temperatures of 258 K, 298 K, and 313 K. In these simulations, the volume, temperature, and guest chemical potential are held fixed while the guest loading reaches an equilibrium value. Framework atomic coordinates were taken from the crystal structure for MIL-100(Cr),15 and framework atoms were kept at their crystallographic coordinates throughout all simulations. Guest N2 and O2 molecules were treated as rigid bodies using three-site models with atomic charges to accurately represent the quadrupole moment for each molecule.24,25 MOFguest interaction energies included van der Waals and electrostatic interactions. Short-range van der Waals potential parameters for framework atoms were taken from the Universal Force Field,26 while parameters for N2 and O2 reproduce the bulk properties of those pure gases.24,25 Geometric mixing rules were used to obtain MOF-guest van der Waals parameters. Atomic charges for framework atoms were taken from Bernini et al.27 Simulations were performed on a single unit cell using periodic boundary conditions with a short-range cutoff of 12.5 Å, while long-range electrostatic interactions were evaluated using the Ewald summation with a precision of 1.0 × 10−4. GCMC moves were assigned the following probabilities: 40% configurationbiased insertion or deletion, 15% configuration-biased intrabox molecular transfer, 15% molecule regrowth, 15% molecule translation, and 15% molecule rotation. The gas loading pressure at each chemical potential was obtained from separate GCMC simulations of the pure gas at the temperature of interest. Each GCMC simulation consisted of 5.0 × 107 steps with average loadings calculated from the final 2.5 × 107 steps. Equilibrium was established by confirming that the acceptance and rejection ratios for each type of move were equal. 2.7.2. Calculation of Mixture Loadings and Selectivities. Adsorption isotherms from both experimental and GCMC simulations were analyzed using Ideal Adsorbed Solution Theory (IAST),28 using the pyIAST code.29 Each isotherm was fit with a

quadratic isotherm model, and loadings from a representative 20:80 mixture of O2:N2 were calculated for mixture feed pressures of 0.1 bar – 1.0 bar. The selectivity S(O2/N2) of O2 with respect to N2 in each mixture was calculated from: ‫ݔ‬ ቀ Oଶ ቁ Oଶ ‫ݔ‬Nଶ ܵ൬ ൰ ൌ ‫ݕ‬ Nଶ ቀ Oଶ ቁ ‫ݕ‬Nଶ where the xi represent the mole fraction of gas in the adsorbed phase, and yi represents the mole fraction in the bulk gas phase.

3. RESULTS 3.1.a Materials characterization The Sc-MIL-100 sample was synthesized under new experimental conditions, that differ from those previously reported.16 More detailed insights into the average and local structure of Sc and Fe-MIL-100 were derived from high energy synchrotron X-ray scattering data and PDF data collected at the Advanced Photon Source at Argonne National Laboratory. The diffraction data confirm that the Sc-MIL-100 and Fe-MIL-100 phases are iso-structural with similar diffraction peaks, Figure 2. However, for the Sc analogue, the framework crystallinity is severely reduced.

Figure 2. Synchrotron X-ray diffraction data (17 keV, λ = 0.72768 Å) for Sc-MIL-100 and Fe-MIL-100 samples.

Thermogravimetric analysis of the methanol-exchanged Sc-MIL-100 sample, Figure S2, shows the expected loss of solvent up to 373 K (water and methanol). The sample is thermally stable between 373 K and 573 K. Gradual weight loss above this temperature is accompanied by small amounts of CO2 released above 573 K, and presumably from slow framework decomposition. This finding allowed us to identify the appropriate desolvation temperature for gas sorption studies.



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3.1.b PDF structural analysis

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PDF data were collected on desolvated frameworks to simplify the analysis. The PDF measured for the 1,3,5benzenetricarboxylic acid ligand was subtracted from the PDF of the MIL-100 samples, to isolate the contributions associated with the M-O and M-ligand atom-atom distances that define the metal trimers within the framework. These differential PDFs (dPDF) analysis, shown in Figure 3, provide insight into the different bond length distributions within the Sc and Fe trimeric clusters.

Figure 3. Differential pair distribution functions for Fe-MIL-100 shown in blue, and Sc-MIL-100 shown in red.

The differential PDFs show well defined peaks to long distances as is consistent with the long range ordering of the framework structures, Figure 3. The low r region of the dPDFs are dominated by features associated with the M−O coordination bonds (~2 Å), M···C (carboxylate) distance (~3 Å) and the M···M distance (3.4-3.5 Å). For Sc-MIL-100, peaks in the PDFs are shifted to longer distances, consistent with the larger Sc cation size than Fe. Gaussian functions were used to quantify local atom-atom correlations of interest (see Table 1). Table 1. Atom-atom distances for Fe-MIL-100 and Sc-MIL-100 compounds derived from peak fitting of differential PDF data. FWHM stands for “full width at half maximum”. Bond

Distance (Å)

Relative Area

FWHM (Å)

Sc-O

2.11

1.5

0.19

O-O

2.81

0.3

0.22

Sc-C

3.08

0.8

0.26

Sc-Sc

3.53

0.5

0.24

Fe-O

2.01

1.7

0.22

O-O

2.70

0.1

0.16

Fe-C

2.96

0.9

0.22

Fe-Fe

3.34

1.2

0.23

The local geometry of the oxo-centered trimer at the nodes of the MIL-100 framework can be inferred from the M-O and M···M distances provided by the PDF analysis. In a recently published study, the local structural transitions of M6O8 nodes in NU-1000 and UiO-66 MOFs were probed.30 For ScMIL-100, the narrow width of the Sc-O peak suggests a narrow distribution of Sc-O bond lengths. Given a single M−O bond length (M−O(µ3) or M−O(carboxylate)), the peaks in the PDF suggests an M−O−M angle of 113° at the center of the trimer. Being less than the 120° associated with a planar configuration of the trimer, this implies a puckering of the trimer, with displacement of the µ3-O from the M plane by 0.5 Å. In contrast, in Fe-MIL-100, the broader Fe−O features suggests differing Fe−O lengths for Fe−O(µ3) and Fe−O(carboxylate) bonds. The lengths of these distinct bonds were estimated by fitting two Gaussian functions to this feature in the PDF, constraining the ratio of peak areas to 1:5 and the peak width to that observed for Sc-O. For Fe-MIL100, the shorter Fe-O(µ3) of 1.912 Å suggests a Fe−O(µ3)−Fe angle of near 120° and accordingly, a planar geometry of the trimeric node. A search of the Cambridge Crystallographic Structure Database,31 suggests that Fe tolerates a wider range of M−O bond lengths than seen in Sc−O, Figure S3. This may be the reason why Fe can accommodate a planar geometry, while Sc does not. Puckering or cupping of the trimeric framework nodes for Sc but not Fe, is consistent with the reduced crystallinity observed for Sc-MIL-100. Propagation of the local distortions induced by puckering of the Sc-trimer is expected to expand alternate super-tetrahedral cages in Sc-MIL-100 and to contract others. This allows and accommodates for the inclusion of larger Sc metal center into the crystallographic topology of the MIL-100 framework, as depicted below in Figure 4. (a)

(b)

Sc

Fe

Figure 4. Representative trimer geometries for (a) Sc-MIL-100 out of plane (puckered) and (b) Fe-MIL-100, planar. Hydrogen and terminal molecules in the metal cluster have been removed for clarity. The dashed line was included to visually aid the outof-plane/in-plane orientation of the O(µ3) atom per unique metal cluster. Atoms color scheme: Sc=blue, Fe= purple, C=gray, O=red.

The differences between the Sc and Fe compounds have been verified via structure refinement of their powder diffraction data. Employing the Fe-MIL-100 structure reported by Horcajada et al.17 as a starting model for the Rietveld refinements, the lattice was allowed to refine to obtain a good fit to the observed data. In the case of the FeMIL-100 structure the refined lattice parameter was a = 73.185(4) Å. The large lattice dimension for the Sc analogue


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compared to the Fe analogue (a = 74.518(31) Å cf. a = 73.185(4) Å, respectively) reflects the larger Sc cation size and Sc-O bond lengths. 3.2. Grand Canonical Monte Carlo (GCMC) simulated isotherms at 258 K, 298 K, and 313 K Complementary to the earlier DFT work,7 GCMC simulations were used here to help guide experiments. The results provided valuable information on: (i) the expected O2 vs. N2 uptake up to 10 bar; and (ii) the location of primary adsorption sites for both O2 and N2. GCMC isotherms show the preferential uptake of O2 over N2 at 1 bar over the entire temperature range studied (258 K, 298 K, and 313 K), Figures S4-S6. As the pressure gradually increases (between 5-10 bar), temperature effects are seen in the relative gas loadings, from N2 > O2 (258 K), Figure S4 to O2 > N2 (313 K), Figure S6.

Figure 5. Schematic ball-and-stick representation of the tetrahedral cage in the MIL-100 framework and an adsorbed O2 molecule (large spheres). Hydrogen and terminal molecules in the metal cluster have been removed for clarity. Atoms color scheme: metal=blue, C=gray, O=red; cage O2 molecule= brown.

According to the GCMC model, at lowest pressures, the gas molecules are preferentially located in the small tetrahedral cages, Figure 5. This cage has the highest density of metal centers and should assist with increased adsorbate affinity. Similar adsorption sites were found for both O2 and N2. A comparison of gas loadings from GCMC-equilibrated configurations at 298 K and 1 bar (Table S1) show the O2 cage occupancy is more than double that of N2. The slightly smaller O2 molecules are more energetically favored in these cages, leading to a higher overall adsorption capacity compared to N2. As expected, pore filling occurs as the pressure increases at 10 bar, and therefore additional O2 and N2 molecules start populating the larger cages. 3.3. Experimental gas sorption isotherms measured at 77 K and in the 258 K – 313 K range for Sc-MIL-100 Experimental O2 and N2 gas sorption were obtained for ScMIL-100 over a wide temperature range of 77 K, 258 K, 298 K and 313 K, Figure 6 (a)-(d). The adsorption isotherms were measured on dessolvated samples at 453 K for 12 hours.


Amount of gas adsorbed, mmol/g

(a)

(b) 20

15

10

5 O2@77K N2@77K

0 0.0

0.2

0.4

(c)

0.6

0.8

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O2@258K

0.5

N2@258K

0.4 0.3 0.2 0.1 0.0

1.0

0

100

(d)

P/P0 0.3

200

300

400

500

600

700

800

Absolute pressure, mmHg 0.20

O2@298K



N2@298K

0.2

0.1

0.0 0

100

200

300

400

500

600

700

O2@313K N2@313K

0.15

0.10

0.05

0.00 0

100

Absolute pressure, mmHg

(e)

Isosteric heat of adsorption, kJ/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



200

300

400

500

600

700


16 14 12 10 8 6 4 Sc MIL-100 O2

2

Sc MIL-100 N2 0 0.00

0.02

0.04

0.06

0.08

0.10

Amount of gas adsorbed, mmol/g Figure 6. O2 and N2 adsorption isotherms on Sc-MIL-100 measured at (a) 77 K, (b) 258 K, (c) 298 K, and (d) 313K. The dashed line represents the independent fit to the experimental data for each temperature using a modified virial equation. (e) O2 and N2 heats of adsorption derived from independently fitted virial isotherms at 258 K, 298 K, and 313 K, for Sc-MIL-100 sample.


The Langmuir surface area for the Sc-MIL-100 was calculated to be 1635 m2/g and is on par with previous reports.32 As predicted by GCMC simulations results, experiments show that O2 is more favorably adsorbed than N2 at all temperatures studied. At 77 K, the Sc metal center has enhanced interactions with O2 and results in higher loadings over N2. This finding is consistent with our previous results on pristine and metal-exchanged Cu-BTC materials, also consisting of uncoordinated metal centers.6 This trend is maintained at 258 K, 298 K, and 313 K, temperatures relevant to energy efficient oxygen purification processes, such as PSA. Importantly, the structural integrity of the sample is preserved after gas adsorption at these elevated temperatures, as evidenced by post-analyses XRD, Figure S8. Sc-MIL-100 is one of the few examples of MOF maintaining both its structural integrity and a higher O2 than N2 capacity at temperatures well above room temperature.33,8 Apparently, the uncoordinated-metal center is not directly accessible in the 258 K – 313 K range, but presumably it still plays a role in preferred O2 vs. N2 adsorption, possibly from the puckered geometry of the oxo-centered metal trimer. One explanation is expanded opening into the tetrahedral cage due to the cupping of the oxygen out of the plane. Isosteric heat of adsorption, Qst, provides important information regarding the energetics between the adsorbed gas (O2 or N2) and the Sc-MIL-100 framework. In order to calculate the Qst, each isotherm was independently fitted using a modified virial equation,34 represented as a dashed line in Figure 6 (b)-(d). Stronger binding affinities are calculated for O2 = 15.1 kJ/mol versus N2 = 14.7 kJ/mol. To assess the O2/N2 selectivity in conditions relevant to air separations, a 20:80 mixture of O2:N2 was analyzed for feed pressures of 0.1 – 1.0 bar, using Ideal Adsorbed Solution Theory (IAST)28 for both experimental Figure 7a and GCMC isotherms, Figure S7. Results indicate there is little variation in the O2/N2 selectivity with total pressure. Importantly, the highest separation values are observed at 313 K, O2/N2 = 1.5 suggesting great promise for increased energy efficiency via realistic separation processes that do not rely on cryogenic cooling. Further, we assessed the cycling performance of the ScMIL-100 sample at 298 K and 1 atm, Figure 7b. We tested 10 consecutive adsorption-desorption cycles, with no additional degassing in between events. No measureable loss to the O2 adsorption capacity is noted over the range studied. 3.4. Experimental gas sorption isotherms measured at 77 K and in the 258 K – 313 K range for Fe-MIL-100 As a comparison to the Sc-MIL-100 sample, the O2 and N2 adsorption isotherms were measured on a desolvated Fe-MIL100 sample at 423 K for 6 hours at 77 K, 258 K, 298 K and 313 K, Figure 8 (a)-(d). The Langmuir surface area was calculated from the nitrogen adsorption isotherm at 77 K, and was found to be 1900 m2/g. This value is comparable to the surface area reported in the technical note by the commercial source (2120 m2/g) though slightly lower than the one originally reported by Horcajada et al.17 These discrepancies might be related to distinct synthesis methods (the Fe-MIL-100 studied here was based on a fluorine-free synthesis), and/or incomplete pore evacuation.

(a)1.5

1.4 O2/N2 Selectivity

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1.3

1.2 Sc-MIL-100 258 K Sc-MIL-100 298 K Sc-MIL-100 313 K

1.1 0.0

0.2

0.4

0.6

0.8

1.0

Total pressure, bar

(b) 3.0 Amount of O2 adsorbed, cc/g

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2.5 2.0 1.5 1.0 0.5 0.0 0

2

4

6

8

10

Cycle

Figure 7. (a) Calculated (IAST) O2/N2 selectivity for a 20:80 O2/N2 mixture from Sc-MIL-100 experimental isotherms measured at 258 K, 298 K, and 313 K. (b) O2 adsorption and desorption in Sc-MIL-100 over 10 cycles at 298 K and 1 atm.

At 77 K, O2 is more favorably adsorbed over N2. At 258 K, Fe-MIL-100 adsorbs slightly more N2 than O2, Figure 8b. However, this is reversed again as the temperature increases closer to ambient conditions, with O2 being adsorbed ~ 20 % more than N2 at 298 K, Figure 8c. At 313 K, the amount of adsorbed O2 increases even more, ~ 35% more than N2, Figure 8d. In the Fe-MIL-100 sample, binding affinities for N2 are stronger than those for O2, Qst for N2 = 11.5 kJ/mol versus O2= 8.5 kJ/mol, Figure 8e. The explanation for this is a different adsorption mechanism and/or binding sites as compared to the Sc-MIL-100 sample. Similar to the Sc analogue, the Fe sample maintains structural integrity post testing at 313 K, as shown by powder XRD, Figure S9. The calculated IAST O2/N2 selectivity for the 20:80 O2:N2 mixture is constant along the entire pressure range up to 1 bar, Figure 8f. Unique to this framework, the O2/N2 selectivity increases as the temperature increases: O2/N2 ~1 at 258 K, O2/N2= 1.25 at 298 K and reaching up to O2/N2= 1.7 at 313 K.



(a) 20

15

10

5 O2@77K N2@77K

0 0.0

0.2

0.4

0.6

0.8

0.40



(b)

O2@258K

0.35

N2@258K

0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

1.0

100

(c)

(d) O2@298K



0.25 N2@298K

0.20

0.15

0.10

0.05

300

400

500

600

700

800

0.20 O2@313K N2@313K

0.15

0.10

0.05

0.00

0.00 0

100

200

300

400

500

600

700

0

800

100


(f)

12

8 6 4

800

1.8

0.02

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Fe MIL-100 O2

Fe-MIL-100 258 K Fe-MIL-100 298 K Fe-MIL-100 313 K

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Fe MIL-100 N2 0 0.00

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Figure 8. O2 and N2 adsorption isotherms on Fe-MIL-100 measured at (a) 77 K, (b) 258 K, (c) 298 K, and (d) 313K (the dashed line represents the independent fit to the experimental data for each temperature using a modified virial equation); (e) O2 and N2 heats of adsorption derived from independently fitted virial isotherms at 258 K, 298 K, and 313 K, for Fe-MIL-100 sample; (f) Calculated (IAST) O2/N2 selectivity for a 20:80 O2/N2 mixture from Fe-MIL-100 experimental isotherms measured at 258 K, 298 K, and 313 K.


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5. DISCUSSION Directed by our earlier DFT simulations study,7 experimental work in this study was focused on probing the higher affinity of early transition metals for O2 over N2 and building appropriate MOFs to exploit that trait. Based on exceptional predictions of enhanced O2 binding affinities for early transitional metals, we chose to study a Sc-based MOF, Sc-MIL-100, with potential for high porosity and density of unsaturated metal centers. Herein a combination of predictive modeling (GCMC), synthesis, crystallography, and sorption experiments were performed. For comparative purposes, we also studied the Fe-analogue in this isostructural series. GCMC simulations were used to identify O2 vs. N2 uptake trends, modeled at 258 K, 298 K and 313 K and up to 10 bar. Results indicate that O2 is preferentially adsorbed versus N2 over the entire temperature range studied and 1 bar. This trend is not maintained at higher pressures up to 10 bar, but values equalize, presumably due to pore filling, rather than enhanced O2-metal centers interactions. GCMC results also show that the O2 molecules are primarily adsorbed in the small tetrahedral cages, which have the highest density of metal centers and should assist with increased adsorbate affinity. The windows accessing the tetrahedral cages have a free opening of ~ 2.3x 0.8 Å, when considering the van der Waals radii of the nearest atoms and are smaller than the kinetic diameters of both gases, 3.46 Å for O2 and 3.64 Å for N2. However, it is well documented that MOFs structures have dynamic flexibility and allow the diffusion of larger guests through small apertures.35,36,37,38 Additionally, it has been reported that reversible linker dissociation plays a role in the inclusion of large molecules inside MOF pores.39 Therefore, it is possible that O2 is adsorbed preferentially in the small tetrahedral cage of the MIL-100 structure. Experiments correlate very well and validate the trends established by the DFT and GCMC simulations. Experimental gas adsorption studies at 77K reveal that O2 is preferentially adsorbed over N2 in both Sc- and Fe-MIL-100. However, equimolar amounts of O2 are adsorbed in these two samples, despite a lower surface area measured for Sc-MIL-100 sample as compared to the Fe-MIL-100. This indicates more favorable interactions between O2 and open metal centers in Sc-MIL100.7 Specifically, this can be directly correlated with a sideon binding of the O2 to the Sc metal center, which was shown by DFT calculations to lead to stronger binding energies than O2 binding in a bent configuration to Fe metal center.7 Further, distinct adsorption profiles for the Sc and Fe-MIL100 are noted in the higher temperature ranges studied: 258 K, 298 K, and 313 K. In the Sc-MIL-100 sample, O2 is preferentially adsorbed over N2 at all temperatures. A different adsorption mechanism is observed for the Fe-MIL-100 sample, where less O2 gas is adsorbed than N2 at 258K. A switchover in the gas adsorption is noted when the temperature increases to 298 K and 313 K. The different gas sorption behaviors can be attributed to the subtle differences in the structures of the Fe and Sc analogues. PDF analyses show different local geometry around the oxocentered trimer: while in Fe-MIL-100 a planar trimer geometry is evident, in Sc-MIL-100 the corner-shared µ3-O is displaced from the Sc plane by 0.5 Å. This distortion is likely coupled with a rotation of the individual ScO6 octahedra and displacements of the btc ligands, which not only reduces the

crystallinity but can contribute to dilation of some of the framework apertures. The cupped trimer creates a cavity that may be well suited to binding O2. The concave surface is likely electron deficient while the convex surface is likely electron-rich. This may contribute to the higher O2 binding affinity found for Sc compared to Fe. At higher temperatures, a dynamic-inversion mode of the cupped trimer may become activated. This would contribute to substantial changes in the sorption characteristics at higher temperatures. While no DFT calculations have been performed on the trimer metal cluster for comparison, DFT simulations of O2 binding to UMCs in M2(dobdc) indicate that side-on binding of O2 molecules causes significant distortion of the MO5 square pyramid.40 Specifically, the metal atom moves out of the square plane to bind O2, resulting in a distorted octahedron. Importantly, the Sc-MIL-100 sample maintains its structural integrity after O2 sorption at 298 K, and no noticeable change to the O2 storage capacity is noted after ten consecutive adsorption-desorption cycles at the same temperature. Consistent with previous results on a different isostructural MOFs sample series,6 the metal centers in the MIL-100 samples studied here do not seem accessible to directly interact with either of the two gases at 258 K, 298 K and 313 K. In a recent study,41 the direct O2 interaction with V3+ metal centers at 298K in the V-MIL-100 analog was reported. However, in this case V3+ oxidizes to V4+, leading to stronger binding than desirable for a reversible process, and precluding industrial separation applications. Lastly, it is interesting to note that a much higher O2 binding affinity is observed in the Sc sample as compared to Fe, 15.1 kJ/mol vs. 8.5 kJ/mol, which compares qualitatively with binding energy trends obtained from our previous DFT study.7

6. CONCLUSIONS The deliberate enhancement of O2 adsorption over N2 to achieve efficient air separations is a challenging task. In this modeling-directed experimental study, we investigated the O2 and N2 adsorption properties at both cryogenic and at ambient temperatures. GCMC simulations indicated that O2 is more favorably adsorbed than N2 at 258 K, 298 K, and 313 K and 1 atm and provided valuable insights regarding the location of the preferential adsorption sites for the gases in the smaller tetrahedral cages. Excellent correlations with the predicted trends from both GCMC and DFT modeling studies were found experimentally. For Sc-MIL-100, O2 is more favorably adsorbed than N2 over a wide temperature range, 77 K, 258 K, 298 K, and 313 K. Notably, a distinct nonlinear behavior is observed in Fe-MIL100, where O2 > N2 at 77 K, 298 K and 313 K; O2 < N2 at 258 K. These trends might be explained by the gas-binding energies; the Qst in the Sc sample (15 kJ/mol) is significantly higher than for Fe-MIL-100 (8.5 kJ/mol) indicating a higher O2 affinity for early transitional metals, as predicted by DFT calculations. This finding is correlated with a stronger side-on binding of O2 to the Sc metal center versus weaker bent configuration for Fe-O2. Structure-property relationship established via dPDF analyses revealed different geometries of the M−O(µ3)−M angle in the trimeric building block: planar, for Fe-MIL-100



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versus out of plane Sc-MIL-100, further supporting enhanced O2-framework in the Sc analogue. This distortion in the ScMIL-100 probably allows closer proximity of the O2 to the Sc metal sites in the cluster. Overall, the comprehensive experimental gas sorption testing in this study confirms that MOFs constructed from early transition metals have more favorable interactions with O2 as compared to late transitional metals, and are more suitable to be used in gas separation processes. Concurrent to this work, computational efforts in our group are focused on introducing for the first time ab initio Molecular Dynamics (AIMD) simulations to monitor subtle gas-metal interactions and temperature effects on competitive gas adsorption.40

ASSOCIATED CONTENT Supporting Information. XRD, TGA-MS, SEM-EDS analyses, GCMC modeling results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT The authors would like to thank Dr. David Fairen-Jimenez for helpful discussions and for providing the atomic charges for MIL100(Cr) and to Kenneth Croes for help with synthesis and TGAMS experiments. This work is supported by the Laboratory Directed Research and Development Program at Sandia National Laboratories. Sandia National Laboratories is a multi-program lab managed and operated by Sandia Corp., a wholly owned subsidiary of Lockheed Martin Corporation, for the US DOE’s NNSA under contract DE-AC04-94AL85000). Work done at Argonne and use of the Advanced Photon Source, an Office of Science User Facility operated for the US DOE/Office of Science by Argonne National Laboratory, was supported by the US DOE, Contract No. DE-AC02-06CH11357.

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Sc-MIL-100

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O2>N2 O2@ Sc-MIL-100

∢ Sc−O−Sc ~ 113°


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Selective O2 Sorption at Ambient Temperatures via Node Distortions

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