Enhanced O2 Selectivity versus N2 by Partial Metal Substitution in Cu

Mar 5, 2015 - Using a combination of density functional theory (DFT) calculations, postsynthetic metal substitutions, materials characterization, and ...
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Enhanced O2 Selectivity versus N2 by Partial Metal Substitution in CuBTC Dorina F. Sava Gallis,† Marie V. Parkes,‡ Jeffery A. Greathouse,‡ Xiaoyi Zhang,§ and Tina M. Nenoff*,† †

Nanoscale Sciences Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States § Argonne National Laboratory, Advanced Photon Source, X-ray Science Division, Argonne, Illinois 60439 United States ‡

S Supporting Information *

ABSTRACT: Here, we describe the homogeneous substitution of Mn, Fe, and Co at various levels into a prototypical metal−organic framework (MOF), namely Cu-BTC (HKUST-1), and the effect of that substitution on preferential gas sorption. Using a combination of density functional theory (DFT) calculations, postsynthetic metal substitutions, materials characterization, and gas sorption testing, we demonstrate that the identity of the metal ion has a quantifiable effect on their oxygen and nitrogen sorption properties at cryogenic temperatures. An excellent correlation is found between O2/N2 selectivities determined experimentally at 77 K and the difference in O2 and N2 binding energies calculated from DFT modeling data: Mn > Fe ≈ Co ≫ Cu. Room temperature gas sorption studies were also performed and correlated with metal substitution. The Fe-exchanged sample shows a significantly higher nitrogen isosteric heat of adsorption at temperatures close to ambient conditions (273−298 K) as compared to all other metals studied, indicative of favorable interactions between N2 and coordinatively unsaturated Fe metal centers. Interestingly, differences in gas adsorption results at cryogenic and room temperatures are evident; they are explained by comparing experimental results with DFT binding energies (0 K) and room temperature Grand Canonical Monte Carlo simulations.

1. INTRODUCTION In recent years, there has been increased interest in power generation via oxygen-enriched (oxy-fuel) combustion.1 Oxyfuel is a well-known approach to improve the heat transfer associated with stationary energy processes utilized by heavy industry or in power production.2 The addition of oxygen to the fuel stream results in higher thermal efficiency, improved fuel flexibility, and reduced exhaust gas volumes, all stemming from the reduction or elimination of the N2 component of air. In addition, NOx emissions can nearly be eliminated from the flue gas stream. Oxy-fuel combustion offers a lower-cost route to CO2 capture, for either subsequent utilization (such as in enhanced oil recovery) or geologic sequestration. While the glass melting industry, aluminum industry, and steel-making industry have adopted oxy-fuel combustion into some of their operations,3 its overall penetration into industrial and power markets is currently constrained by the high cost of existing air separation technologies for generating oxygen. Currently, feedstock oxygen is produced by air separation processes, primarily cryogenic air separation units, which result in approximately 99% oxygen purity.4a Unfortunately, it is a complex and expensive technology, in terms of both capital cost and energy consumption. Conversely, pressure swing adsorption (PSA) processes utilize porous materials such as zeolites for the separation of oxygen from air, with approximately 94% purity.4b Novel, robust, high surface area, and highly selective materials for O2 over competing air components (i.e., N2, ∼ © XXXX American Chemical Society

78% of air) are needed to increase the product (O2) purity of the PSA process to approximate that of the energy intensive cryogenic separation process. Metal−organic frameworks (MOFs)5 with their tunability for high selectivity and high surface area porosity are attractive candidate materials for oxygen separation materials in PSA processes.6 In particular, tunability of MOFs with unsaturated metal centers (UMCs) have the potential to play an important role in O2 selectivity, as it has been previously shown with other gases, such as H27a,b or CO2.7c While there have been a number of studies of MOFs for O2 separation from N2,8 there have only been a few studies that show significant O2/N2 selectivities.9 For example, Long and co-workers have presented single gas sorption studies at 298 K that show preferential O2 over N2 selectivity in two different MOF prototypical phases that have accessible UMCs: Cr2(BTC)39b and Fe2(DOBDC).9c Cu-BTC,10 also known as HKUST-1 or Cu2(BTC)3, is a high surface area MOF (1500−2000 m2/g) that consists of diatomic copper centers linked by 1,3,5-benzenetricarboxylate units. UMCs are accessible in the apical positions of the dimetal paddle wheel upon removal of coordinated water with heat and vacuum, Figure 1. Several metal substituted analogues of CuBTC obtained via conventional solvothermal techniques have Received: November 17, 2014 Revised: February 6, 2015

A

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2.1. Materials Synthesis. Synthesis of Mn-Exchanged Cu-BTC, Mn/Cu-BTC. MnCl2·4H2O (0.098 g, 0.5 mmol) was dissolved in 3 mL of N,N′-dimethylformamide (DMF). This solution was added to a solid sample of pristine Cu-BTC (0.045 g, 0.2 mmol) and was heated to 90 °C in a convection oven at a rate of 1.5 °C/min, held at 90 °C for 1 day, and then cooled to room temperature at a cooling rate of 1 °C/min. The solvent was decanted, and the solid was washed with 5 × 10 mL of DMF. Synthesis of Fe-Exchanged Cu-BTC, Fe/Cu-BTC. FeCl2·4H2O (0.045 g, 0.2 mmol) was dissolved in 3 mL of DMF. This solution was added to the pristine Cu-BTC (0.045 g, 0.2 mmol) and was heated to 90 °C in a convection oven at a rate of 1.5 °C/min, held at 90 °C for 1 day, and then cooled to room temperature at a cooling rate of 1 °C/min. The solvent was decanted and the solid was washed with 5 × 10 mL of DMF. Synthesis of Co-Exchanged Cu-BTC, Co/Cu-BTC. CoCl2·6H2O (0.117 g, 0.5 mmol) was dissolved in 3 mL of DMF. This solution was added to the pristine Cu-BTC (0.045 g, 0.2 mmol), heated to 90 °C in a convection oven at a rate of 1.5 °C/min, held at 90 °C for 1 day, and then cooled to room temperature at a cooling rate of 1 °C/min. The solvent was decanted, and the solid was washed with 5 × 10 mL of DMF. 2.2. Powder X-ray Diffraction (PXRD). Measurements were performed on a Siemens Kristalloflex D500 diffractometer, with Cu Kα radiation (λ = 1.54178 Å). 2.3. 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. 2.4. Sample Activation and Gas Adsorption Measurements. Prior to measuring the gas adsorption isotherms, all samples were soaked in 15 mL of ethanol for 3 days, with the solvent replenished every 24 h. Subsequently, the samples were activated under a vacuum on a Micromeritics ASAP 2020 surface area and porosity analyzer, at 473 K for 10 h. Gas adsorption isotherms were measured using a Micromeritics ASAP 2020 surface area and porosity analyzer. Nitrogen and oxygen gas of ultrahigh 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. Nitrogen and oxygen adsorption isotherms at 273, 283, and 298 K were measured in a dewar connected to a Polyscience circulating isothermal bath with an advanced digital temperature controller. Oxygen 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. 2.5. X-ray Absorption near Edge Structure (XANES) Analyses. The Fe−K edge X-ray absorption near edge structure (XANES) measurements were carried out at beamline 11-ID-D of the Advanced Photon Source, Argonne National Laboratory. The XANES spectra were measured in the transmission mode using two ion chambers. The sample (Fe/Cu-BTC) and two standards, Fe(II) oxalate and Fe(III) acetylacetonate, were mixed with boron nitride and pressed into pellets. 2.6. Molecular Modeling. 2.6.1. Initial Structures. Initial structures for known Fe-BTC15 and Cu-BTC10 materials were taken from published crystal structures, with solvent molecules removed and hydrogen atoms added, where necessary. Mn-BTC and Co-BTC frameworks were prepared by substituting manganese or cobalt, respectively, for iron in Fe-BTC. All structures were reduced to their primitive cells for calculations; M-BTC empty frameworks contained 156 atoms. O2- and N2-containing MOFs were prepared by adding one molecule of O2 or N2 into the fully optimized MOF framework, with the proximal gas atom approximately 2.0 Å from a metal center and O−O and N−N interatomic distances approximately 1.2 Å. Two initial structures, differing in the bonding geometry of the gas, were used for each gas-containing MOF. Preliminary structural models of MOFs containing O2 were prepared in both side-on bonded (M−O− O angle 67°) and bent (M−O-O angle 120°) geometries; initial

Figure 1. Schematic ball-and-stick representation of the Cu-BTC framework and of accessible UMCs upon removal of axial coordinated water molecules. Atom color scheme: Cu = blue, C = gray, O = red, H = white.

been reported: Cr,9b Ru,11 Ni,12 Mo,13 Zn,14 and Fe.15 Of those, only the Cr and Ru samples exhibit expected surface areas, whereas the Ni and Mo samples have much lower than expected surface areas of approximately 1000−1100 m2/g. Furthermore, the Zn and Fe samples have no measurable accessible porosity. In particular, several attempts to dessolvate the Fe analogue were unsuccessful, presumably due to the presence of templating molecules which cannot be removed.15,7c Alternatively, in a distinct porphyrin template-directed synthesis, the Mg-, Mn-, Fe-, Co-, and Ni-BTC analogues were obtained; no porosity measurements were reported.16 Here, we detail a combined experimental and molecular modeling study of metal-substituted Cu-BTC MOFs, with a focus on both the (i) O2 versus N2 sorption capacities and (ii) binding affinities’ dependency on temperature. A postsynthetic approach to access the Mn, Fe, and Co porous analogues of Cu-BTC was implemented. This in-framework substitution, commonly referred to as metal ion postsynthetic exchange (PSE) or postsynthetic ion metathesis (PSIM), has been recently investigated17 as an alternative route to access materials that were proven challenging to synthesize via conventional routes. We present the structure−property relationship between the metal centers and the gas adsorption of O2 or N2, as determined experimentally with crystallography (powder Xray diffraction), microscopy (scanning electron microscopy with energy dispersive spectroscopy), and gas sorption at variable temperatures (77 K, 273−298 K). Comparisons are made with density functional theory (DFT) calculations and Grand Canonical Monte Carlo (GCMC) simulations.

2. EXPERIMENTAL SECTION All materials were purchased from commercially available sources and used without further purification. Cu-BTC powder was purchased from Sigma-Aldrich, under the commercial name of Basolite C300. B

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Chemistry of Materials structures of MOFs containing N2 were prepared in both bent (M− N−N angle 120°) and linear (M−N−N angle 179°) geometries. 2.6.2. Geometry Optimizations and Energy Calculations. Plane wave DFT calculations were performed on periodic structures in the Vienna Ab initio Simulation Package (VASP), version 5.2.12,18 with the Perdew−Burke−Ernzerhof (PBE) functional19,20 including dispersion corrections (DFT-D2).21 All calculations were spin-polarized and 2 × 2 × 2 k-point meshes centered at the Γ point were used to sample the Brillouin zone. For Mn-containing complexes, several initial magnetic moments were examined to ensure that the lowestenergy electronic structure was reached. Partial-occupancy smearing was done using the Methfessel−Paxton method of order 1, with a smearing width of 0.2 eV. Atomic positions were relaxed using a conjugate-gradient algorithm with scaling constant 0.5, and the planewave energy cutoff was set to 500 eV. Electronic convergence was met when the total energy difference between two consecutive steps was smaller than 10−4 eV; ionic convergence was met when all forces were smaller than 0.03 eV/Å. Core and valence electron interactions were described by the projector augmented-wave (PAW) formalism.22,23 Geometry optimizations were done in three successive steps, with final atomic positions, charge densities, and wave functions from each step used as input for subsequent steps: Step 1. Projected wave function evaluated in real space, real space operators optimized to an accuracy of 0.5 eV/atom Step 2. Projected wave function evaluated in real space, real space operators optimized to an accuracy of 0.25 eV/atom Step 3. Projected wave function evaluated in reciprocal space, real space operators optimized to an accuracy of 0.25 eV/atom 2.6.3. Calculation of Binding Energies. Static binding energies for MOF O2 and N2 at 0 K (ΔEMOF gas ) were calculated by ΔEgas = EMOF+gas − EMOF − Egas for gas = O2, N2, where E is the total energy of the system. The differences in binding energies (ΔΔE) for oxygen and nitrogen − ΔEMOF were calculated by ΔΔEMOF = −(ΔEMOF O2 N2 ). 2.6.4. Grand Canonical Monte Carlo (GCMC) Simulated Isotherms. Simulated adsorption isotherms in the pure MOFs CuBTC, Mn-BTC, Co-BTC, and Fe-BTC were generated for comparison with experimental uptake. Adsorption isotherm simulations were performed for both nitrogen and oxygen at 298 K at pressures up to 800 mm·Hg in the Grand Canonical Monte Carlo ensemble using the Music code.24 For each MOF, the unit cell was taken as the simulation box, and initial structures were taken from published crystal structures with the solvent removed. Rigid frameworks were simulated by keeping framework atoms fixed at their crystallographic coordinates, and periodic boundary conditions were applied in all three dimensions to account for crystalline periodicity. Parameters for O2, N2, and MOF atoms were taken from the literature (Table S9),25 and Lorentz− Berthelot combining rules were used to calculate the Lennard-Jones cross-parameters for all gas-framework interactions. The effect of electrostatic charges on N2 uptake in the parent MOF Cu-BTC was examined by simulating N2 uptake with and without atomic charges assigned. At the relatively low pressures of interest in this current study, we found that electrostatic charges did not significantly affect uptake, Figure S12. Additionally, since it has been previously shown26 that electrostatic interactions do not greatly affect O2 uptake in porous frameworks, Coulombic interactions were not taken into consideration. A total of 2 × 109 moves were performed in each simulation; the first half of these moves were used for equilibration and were not included when calculating gas loadings. Gas molecules underwent insertion, deletion, translation, and rotation with equal frequency. Simulated isotherms for metal-exchanged MOFs were prepared by weighting the pure MOF gas loadings at each pressure according to the percent substitution of the metal.

Powder X-ray diffraction (XRD) studies confirmed that the structural integrity of the original Cu-BTC structure is preserved upon metal substitution, Figure 2. Very similar

Figure 2. Powder X-ray diffraction patterns of the simulated (black) and experimental Cu-BTC (red), compared against experimental Mn(blue), Fe- (magenta), and Co- (green) substituted versions of the CuBTC.

peak profiles and intensities are noted for all structures. This demonstrates that the metal-exchange process does not disrupt the highly crystalline pristine Cu-based structure. Unit cell refinement was performed on all samples. Confirmation of inframework metal substitution is indicated by unit cell expansion, Table 1. The degree of the cell expansion is Table 1. Unit Cell Expansion in Co-, Fe-, and Mn/Cu-BTC As a Result of the Metal Exchange Process sample

expansion (Å)

M−O average bond length (Å)

Cu-BTC Co/Cu-BTC Fe/Cu-BTC Mn/Cu-BTC

0.043 0.019 0.030

1.70 2.08 2.00 2.17

correlated with the metal−oxygen average bond length, as extracted from the Crystallographic Structural Database (CSD).27 Thus, although the Mn−O bond is the longest among the metals studied, the Mn was the least exchanged in the Cu-BTC framework. Comparatively, Fe−O and Co−O have very similar bond lengths; however the cell expansion is more pronounced in the Co-substituted sample, as it reaches a higher degree of exchange as compared to the iron sample. The extent of this substitution is reflected in the larger cell parameter expansion. The actual values for the metal exchanges are discussed below. Further evidence of the in-framework metal exchange and the extent of the metal substitution levels in the Cu-BTC framework were provided by SEM-EDS analyses, Figures S1− S3. There is a gradual increase in the degree of the inframework exchange: 6 atomic % for Mn/Cu-BTC, Mn/Cu = 0.07; 13.5 atomic % for Fe/Cu-BTC, Fe/Cu = 0.16; and 25.5 atomic % for Co/Cu-BTC, with Co/Cu = 0.34. Although all substituted metals could adopt in principle the required

3. RESULTS 3.1. Materials Characterization. Solids resulting from the postsynthetic modification of Cu-BTC were repeatedly washed with DMF and dried in the air and the crystals isolated. A slight color change from blue to teal/light-green was noted for all crystalline samples. C

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Chemistry of Materials coordination geometry within the dimetal paddle wheel secondary building unit, it is apparent that there are variable degrees of exchange attainable for the uniform synthesis conditions explored here. The ligand field strength, the polarity of the solvent, and the metal’s electronegativity are all factors that cooperatively contribute to the observed differences in degrees of substitutions.17 Additional information probing the distribution of the metal ions into the parent structure was gathered by high resolution SEM-EDS elemental mapping of individual single crystals, Figure 3. Homogeneous dispersions of the substituted metals in

Figure 4. XANES spectra for Fe/Cu-BTC (green), compared to Fe(II) oxalate (blue) and Fe(III) acetylacetonate (red) standards.

Figure 3. SEM-EDS mapping of the metal-exchanged series. Left: SEM image of a single-crystal of (a) Mn/Cu-BTC, (b) Fe/Cu-BTC, and (c) Co/Cu-BTC. Middle: corresponding elemental distribution of (a) Mn (red), (b) Fe (red), (c) Co (red). Right: elemental distribution of a−c Cu (green).

the Cu-BTC matrix are noted for all variants. There is a gradual change in the signal intensity in the maps corresponding to the Mn-, Fe-, and Co-substituted frameworks, respectively, as a function of their atomic percent doping within the structure. No secondary phases were noted in the SEM-EDS analyses, reinforcing the findings from the X-ray diffraction data. XANES studies were carried out in order to gather additional information regarding the oxidation state of the substituted Fe in the Fe/Cu-BTC sample. Two standards with Fe in a similar coordination environment were also investigated, Fe(II) oxalate and Fe(III) acetylacetonate, Figure 4. The XANES data show the similarity of Fe/CuBTC to that of the Fe(III) acetylacetonate, which indicates that the Fe in Fe/Cu-BTC is in a +3 oxidation state. 3.2. Experimental Gas Sorption Isotherms Measured at 77 K. In order to evaluate the effect of the in-framework metal-exchange on the overall gas adsorption, oxygen and nitrogen adsorption isotherms were measured on the pristine metal-substituted samples at 77 K, Figure 5 and Figure S4. All materials were activated using the same protocol. As expected, a noticeable color change was noted upon dehydration, inferring fully accessible coordinative UMCs. All samples are porous, and the Langmuir surface areas range from 1586 m2/g for the Co/ Cu-BTC to 1773 m2/g for Fe/CuBTC to 1791 m2/g for the

Figure 5. (a) O2 (red) and N2 (blue) adsorption isotherms measured at 77 K on the pristine Cu-BTC (square), Mn- (diamond), Fe(circle), and Co (pentagon)-substituted samples.

Mn/Cu-BTC, which are all slightly lower than the Langmuir surface area for pristine Cu-BTC, 2237 m2/g. The lower surface areas in the exchanged samples might be related to potential structural defects occurring in the framework during the substitution process.28 As a general trend, oxygen is more favorably adsorbed over nitrogen at 77 K, in all samples. An interesting crossover in the amount of nitrogen adsorbed by the Mn- and Fe-exchanged Cu-BTC is noted at ∼0.06 P/P0. The Mn sample has a slightly higher uptake at the lowest pressure, whereas the reverse is noted at the highest pressures, indicative of slightly stronger Mn− vs Fe−N2 interactions. 3.3. DFT Bonding Energy Simulations Conducted at 0 K. Previous studies have used DFT methods to accurately estimate binding energies of small molecules on UMCs in MOFs.29 DFT calculations of O2 and N2 binding energies (ΔEO2 and ΔEN2) were performed on homogeneous systems containing 100% of each of the targeted metals, Table 2. Additionally, in order to more accurately correlate these values D

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Chemistry of Materials Table 2. Calculated DFT Binding Energies ΔEO2 and ΔEN2 (kJ·mol−1) and the Difference in Binding Energies (ΔΔE) for M-BTC (100% Metal Substitution) and Corresponding Weighted Values Based on Metal Exchange (Cu/M) in Synthesized Samples ΔEO2

ΔEN2

3.4. Experimental O2 and N2 Sorption Isotherms in the 273−298 K Range. Oxygen and nitrogen adsorption isotherms were measured in the room temperature range, at 273, 283, and 298 K, Figure 6 and Figures S5 and S6. This was attempted for two main purposes: (i) to directly compare the calculated metal-gas binding affinities with experimental heats of adsorption and (ii) to identify any temperature dependency trends in the adsorption of O2 and N2. The isotherms were independently fitted using a modified virial equation, see the SI for details.30 Here, a distinct trend is noted compared to the low temperature data: as the temperature increases, oxygen loadings decrease with respect to nitrogen. The overall differences in the total oxygen and nitrogen uptake at 298 K are very small compared to the 77 K measurements. A slightly larger volume of nitrogen is adsorbed over oxygen in pristine Cu-BTC for the 298 K isotherm, Figure 6a and b. A similar performance is noted for the Co- and Mn/Cu-BTC samples, Figures S5 and S6, respectively. A unique behavior is shown by the Fe/CuBTC sample. In this case, a very similar nitrogen and oxygen uptake is noted, Figure 6c and d. Also, a slightly larger amount

ΔΔE

metal

100%

Cu/M

100%

Cu/M

100%

Cu/M

Cu Mn Fe Co

−103 −264 −45 −81

−103 −113 −95 −97

−100 −188 −12 −62

−100 −105 −88 −90

3 76 33 19

3 8 7 7

with the low temperature experimental data, the corresponding weighted binding energies were calculated based on the equivalent percentage of the substituted metal in each system. Last, the difference in O2 and N2 binding energies (ΔΔE) was calculated as −(ΔEO2 − ΔEN2). In this convention, positive ΔΔE indicates stronger O2 binding versus N2.

Figure 6. N2 and O2 adsorption isotherms measured at 273, 283, and 298 K on (a and b) the pristine CuBTC and (c and d) the Fe/Cu-BTC samples. The dashed line represents the independent fit to the experimental data for each temperature using a modified virial equation. E

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measurable porosity. Hence, an alternative postsynthetic metal ion exchange route was sought in order to avoid the limitations associated with pore guest removal. Our strategy was primarily attempted in order to (i) take advantage of the anticipated inherent porosity in these analogs and (ii) evaluate the binding affinity of presumably favorable charge transfer interactions between O2 and accessible metal sites. This relatively new synthesis technique has mainly emerged as a useful approach to access targeted compounds where conventional methods failed.33 Possible mechanisms for the metal exchange process were proposed, and it is believed that the electronegativity of the metal, along with its coordination geometry are instrumental in successful in-framework metal substitution.17 In the case of Cu-BTC, the apical positions of the dimetal paddle wheel cluster are occupied by labile water molecules, which should in principle aid a facile metal exchange process. Using the postsynthetic modification method, we obtained various degrees of substitution: 6 atomic % for Mn/ Cu-BTC, 13.5 atomic % for Fe/Cu-BTC, and 25.5 atomic % for Co/Cu-BTC, as evidenced by SEM-EDS analyses, Figures S1− S3. XRD experiments confirmed the in-framework substitution, Figure 2, based on the degree of unit cell expansion, which correlates well with the expected metal−oxygen average bond length, Table 1. First, O2 and N2 gas sorption isotherms were measured at 77 K. Consistent for all samples studied, O2 was more favorably adsorbed over N2 (Figure 5 and Figure S4), with a O2/N2 selectivity at 0.2 P/P0 trend as follows: Mn/Cu-BTC (1.32), Fe/Cu-BTC (1.27), Co/Cu-BTC (1.27), and Cu-BTC (1.13). These data correlate well with the trend established when considering the difference in O2 and N2 binding energies (ΔΔE) calculated from DFT. This relationship is accurate both for initial 100% metal substitution [Mn-BTC (76 kJ/mol), FeBTC (33 kJ/mol), Co-BTC (19 kJ/mol), Cu-BTC (3 kJ/mol)] as well as when considering weighted binding energies derived from the actual equivalent percentage of the substituted metal in each system [Mn/Cu-BTC (8 kJ/mol), Fe/Cu-BTC (7 kJ/ mol), Co/Cu-BTC (7 kJ/mol), Cu-BTC (3 kJ/mol)]. DFT calculations also provided additional insights regarding the geometry of the M···N2 and M···O2 interactions, Figures S10 and S11, respectively. Fe···N2 exhibits a bent configuration, while a linear N2 binding mode is observed for Cu, Mn, and Co. For M···O2, a predominant bent orientation is noted for Cu, Fe, and Co; in contrast, the early transition metal Mn, with its empty d orbitals, is able to accommodate a stronger side-on bond with O2. Further, experimental O2 and N2 isotherms were measured in the room temperature range, at 273, 283, and 298 K. The latter data set was compared against GCMC simulations, conducted at the same temperature, Figure S9. There is a good correlation between the experimental and simulated data for both O2 and N2 isotherms. When comparing the results over the entire 273−298 K range, it was noted that as the temperature increases, the amount of O2 loading decreases with respect to N2 in three of the four samples studied: Cu-BTC, Mn/CuBTC, and Co/Cu-BTC. The exception is the Fe/Cu-BTC sample, which exhibits similar O2 and N2 gas uptake in this temperature range. In a previous study, Long et al. demonstrated distinct preferential O2 adsorption at 298 K vs N2 in another Cu-BTC analog, Cr-BTC.9b This is presumably due to enhanced charge transfer interactions of the Cr2+ metal centers with O2, but not with N2. Therefore, it is apparent that the identity or electronic configuration of the metal center plays

of nitrogen is adsorbed at the lowest loadings in this sample at 273 K, indicative of more favorable gas-framework interactions compared to all other samples. Last, experimental heats of adsorptions (Qst) were calculated based on the independently virial-fitted isotherms for each temperature, using the Clausius−Clapeyron equation. In the case of O2, the heat of adsorption varies with loading levels for all samples studied, Figure S8a. A general trend shown by the data indicate unexpectedly low Qst values at zero loadings, where presumably the stronger interactions are anticipated to occurwhereas the values at higher loadings level off. The highest Qst at the highest loadings is observed for the Co/Cu-BTC, ∼15.7 kJ/mol, while the lowest is noted for pristine Cu-BTC, ∼10.7 kJ/mol. The N2 heats of adsorption are very similar for pristine Cu-BTC and the Mn- and Coexchanged sample, ∼13−15 kJ/mol (Figure S8b). These results are in good agreement with GCMC simulated isosteric heats for O2 and N2 adsorption onto Cu-BTC, previously reported elsewhere: O2/Cu-BTC, 14.7 kJ/mol26 and N2/Cu-BTC, 14.2 kJ/mol.31 A significantly higher Qst value for N2 at lowest loadings is noted for the Fe/Cu-BTC sample, ∼30 kJ/mol (Figure 7), which drops with increased loading, indicative of sample heterogeneity, consistent with the nature of the mixed system studied.

Figure 7. N2 and O2 heats of adsorption derived from independently fitted virial isotherms at 273, 283, and 298 K, for pristine Cu-BTC and Fe/Cu-BTC.

4. DISCUSSION Similar to reports of our earlier work,32 we used molecular modeling to verify and complement the experimental findings in this study. The focus was placed on identifying the O2 and N2 gas affinity dependency with temperature in MOFs with UMCs. As a result, experiments were first directed at the synthesis of Mn, Fe, and Co analogs of Cu-BTC, a known MOF with UMCs.10 These metals were particularly chosen as they (i) are known to accommodate the necessary paddle wheel coordination geometry, as evidenced in the CSD,26 and (ii) are redox-active and could favor the separation of O2 over N2.9b,c Nevertheless, when synthesized using templating molecules such as triethylenediamine15 and porphyrin16 via conventional synthesis techniques, the analogs did not exhibit any F

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binding energy calculations here and in our recent work.37 The following trend is established when considering the O2/N2 selectivities determined experimentally, in conjunction with the difference in O2 and N2 binding energies calculated from DFT modeling data: Mn > Fe ≈ Co ≫ Cu. This finding is not extended to the room temperature regime (273−298 K). In this case, it appears that the unsaturated metal centers are not strong binding sites, and the gas sorption uptake is primarily dominated by framework features: surface area, pore size, and pore volume. Under these conditions, O2 loadings are slightly lower than N2. The exception is the Fe/ Cu-BTC sample, in which O2 loadings are lower than N2 at the lowest loading levels relative to the other metal-substituted samples, presumably due to the higher heats of adsorption and favorable N2···Fe interactions. Current research in our group is focused on implementing more advanced calculations which will help to better address the thermal effects which contribute to preferred O2- or N2unsaturated metal centers interactions. Future studies are also geared toward gaining a better understanding of the cation exchange thermodynamics in these systems, while varying the experimental conditions to access higher substitution levels. At the same time, ongoing studies at intermediate temperatures as compared to those studied here will offer valuable information concerning the most suitable temperature range to target both increased capacity and favorable O2 binding energies.

a very important role in the adsorption process at room temperature. Additional insights regarding the strength of the gasframework interactions in these materials were gathered from experimental isosteric heats of adsorption, calculated from the independently fitted isotherms at 273, 283, and 298 K using a modified virial equation.30 In contrast to previous findings where unsaturated Cr and Fe metal centers contribute to high O2 loadings at 298 K and low pressures,9b,c the relatively modest Qst values for O2 at the lowest loadings in all samples studied, in addition to low O2 uptake in the 273−298 K range, indicate that the UMCs studied here do not directly interact with the gas in this temperature range. Higher Qst values are noted for N2 gas, indicative of slightly more favorable interactions between the framework and N2 over O2 in these systems. The exception is the Fe/Cu-BTC sample, which has a high N2 binding affinity of ∼30 kJ/mol at the lowest loadings, as compared to ∼15 kJ/mol for all other samples. Additionally, it was noted that binding energy calculations were no longer well correlated with the experimental heats of adsorption obtained from the near ambient data. This indicates that gas interactions at metal sites in these MOFs are highly temperature dependent, a property that is missing from standard force fields used here. Such thermal factors need to be accounted for in current models widely implemented in simulations of MOF adsorption properties; this will enable predictions over a wide temperature range.34 The results for Fe/Cu-BTC gas framework interactions prompted the investigation of the oxidation state of the Fe in this material. XANES studies showed that although the synthesis was based on a Fe2+ starting reagent, the sample oxidized to Fe3+ during the synthesis. This result helps further explain the lack of O2 affinity for the Fe/Cu-BTC sample, as no charge transfer is anticipated between Fe3+ and oxygen gas. This was previously demonstrated to be the case in another Febased MOFs, where enhanced Fe−O2 interactions were observed, when Fe was in a +2 oxidation state.9c As a general remark, it was found that the metal sites studied in this platform do not seem to play a significant role in favorable O2 or N2 adsorption at near-ambient temperatures. Rather, the sorption is controlled by distinct structural features in this porous platform. These findings correlate well with previous GCMC simulations which show that both N2 and O2 adsorb preferentially in the octahedral cages in Cu-BTC, not at the coordinatively unsaturated metal sites.31,35 A similar observation was recently reported by Farha et al. for Cu-BTC and NU-125 materials,36 both containing accessible Cu-metal sites. Although that study was conducted at higher pressures, no direct interactions between the O2 gas and metal centers were observed in that case either.

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ASSOCIATED CONTENT

S Supporting Information *

SEM-EDS analyses, additional gas adsorption measurements, and modeling results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Mark A. Rodriguez for his help with PXRD unit cell refinement and helpful discussions. This work is supported by the Laboratory Directed Research and Development Program at Sandia National Laboratories. Sandia National Laboratories is a multiprogram lab managed and operated by Sandia Corp., a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. 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 U.S. DOE/Office of Science by Argonne National Laboratory, was supported by the U.S. DOE, Contract No. DE-AC02-06CH11357.

5. CONCLUSIONS Here, we demonstrated the viability to substitute Mn-, Fe-, and Co- metal centers into Cu-BTC to various degrees, via a postsynthetic metal exchange process. Importantly, porosity was retained as a result of this exchange process, a feature previously not possible when using conventional synthesis methods. Significant insights into the relationship between the identity of the metal ion and its temperature-dependent sorption affinities for O2 and N2 gas were gathered. All coordinatively unsaturated metal centers studied result in relatively higher O2 over N2 adsorption capacities at cryogenic temperatures. This finding correlates very well with DFT

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