CO2 Dynamics in Pure and Mixed-Metal MOFs with Open Metal Sites

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CO Dynamics in Pure and Mixed-Metal MOFs with Open Metal Sites Robert M. Marti, Joshua D. Howe, Cody R. Morelock, Mark S. Conradi, Krista S. Walton, David S. Sholl, and Sophia E. Hayes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07179 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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The Journal of Physical Chemistry

CO2 Dynamics in Pure and Mixed-Metal MOFs with Open Metal Sites Robert M. Marti1, Joshua D. Howe2, Cody R. Morelock2, Mark S. Conradi3,1,4, Krista S. Walton2, David S. Sholl2, Sophia E. Hayes1* 1

Department of Chemistry, Washington University, One Brookings Drive, Saint Louis, Missouri 63130 United States

2

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332 United States

3

Department of Physics, Washington University, One Brookings Drive, Saint Louis, Missouri 63130 United States

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ABQMR, 2301 Yale Blvd. SE, Albuquerque, New Mexico 87106 United States

ACS Paragon Plus Environment

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Abstract: Metal-organic frameworks (MOFs), such as MOF-74, can have open metal sites to which adsorbates such as CO2 preferentially bind. 13C NMR of 13CO2 is highly informative about the binding sites present in Mg-MOF-74. We used this technique to investigate loadings between ~0.88 and 1.15 molecules of CO2 per metal in Mg-MOF-74 at 295 K. 13C lineshapes recorded as a function of loading can be understood in terms of the dependence of the CO2 NMR frequency on the angle (θ) with respect to the CO2 axis and the channel of the MOF, reflected in the Legendre polynomial, P2. In the fast motion limit, the NMR spectra reveal the time-averaged value of P2, where θ is the angle between the instantaneous CO2 axis and the channel axis. DFT calculations were used to determine a weighted average of P2 in this regime and are in good agreement with experimental data. Static variable temperature (VT)

13

C NMR from cryogenic

temperatures to room temperature was used to investigate 13CO2 binding in Mg-MOF-74 loaded at two levels (~0.88 and 1.08 molecules of CO2 per metal), revealing temperature-dependent lineshapes. We have investigated the effect of partial substitution of Cd for Mg in Mg-MOF-74 on the 13CO2 variable temperature NMR spectra. The chemical shift anisotropy (CSA) that leads to characteristic lineshapes of

13

C indicates that incorporation of Cd leads to weaker binding

energies for adsorbed CO2.


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I. Introduction With increased attention on greenhouse gases such as CO2, many new materials have been proposed to capture CO2 from high concentration sources.1,2 Some of these materials include amine solutions,3–7 ionic liquids,8–19 zeolites,20–27 activated carbons,21,28–30 aminefunctionalized mesoporous silica,31–35 and metal-organic frameworks (MOFs).22,36–43 Although some liquid chemisorbents have high CO2 capture capacity, they are energetically expensive to regenerate, and in many instances they degrade after multiple capture-regeneration cycles.3,5–8,10– 12,14

Physisorbing solids, such as MOFs, are attractive as an alternative to chemisorbing liquids in sorption applications because of the potential for lower regeneration costs, high adsorption capacity, and chemical “tunability”44–55 of their properties. Among the many MOFs that exist, MOF-74 has garnered interest for adsorption applications due to its high gravimetric density of undercoordinated open metal sites.56–59 Figure 1 depicts the honeycomb-like structure of MOF-74. The five metal-oxygen bonds at the open metal site that form a nearly square-planar polyhedron are highlighted in Figure 1b. These polyhedra are connected together by linkers giving 1-dimensional hexagonal channels (perpendicular to the page, Figure 1a) lined with open metal sites. MOF-74 can be synthesized

a.)

b.)


Figure 1: a.) The hexagonal honeycomb structure of M-MOF-74 shown along the direction of the c-axis and b.) depiction of the handedness of the metal backbone (boxed in red in a.)) in addition to the four most likely binding positions of CO2 to the open 3 metal site, with position 4 being the most favorable.


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with a variety of metals.60–62 In this study, we focus on Mg-MOF-74 and Mg0.77—Cd0.23-MOF74, wherein Cd ions are substituted for some of the Mg ions in the MOF-74 topology. Interest in mixed-metal MOF-74 materials has been motivated by a previous study40 that showed the substitution of even 16% of the Mg sites in Mg-MOF-74 with Ni is enough to stabilize the structure under humid conditions. Mixed-metal MOFs are explored in an effort to evaluate how different metals are taken up in the final MOF structure in binary mixtures with Mg. The dynamics of CO2 interacting with Mg-MOF-74 monitored by

13

C NMR have been

reported previously.42,43 We extend these studies by a combination of different 13CO2 loadings in Mg-MOF-74 as well as variable temperature NMR of both Mg-MOF-74 and Mg0.77—Cd0.23MOF-74. DFT calculations were performed to determine the most preferred orientation of the CO2 molecule at the different binding sites with respect to the c-axis of the unit cell. The calculations provide insight into the highly sensitive dependence of the NMR lineshape on CO2 loadings, temperature, and MOF composition.

II. Experimental Methods Materials All chemicals in this study were used as received from commercial sources without further purification: Mg(NO3)2·6H2O from Sigma-Aldrich; Cd(CH3COO)2·2H2O from ACROS Organics; 2,5-dihydroxy-1,4-benzenedicarboxylic acid (DOBDC) from TCI America; and N, Ndimethylformamide (DMF), ethanol, and methanol from VWR.

Preparation of Mg-MOF-74


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Synthesis of Mg-MOF-74 was based on the work from Glover et al.63 To a mixture of 360 mL DMF, 24 mL ethanol, and 24 mL deionized H2O in an Erlenmeyer flask, 14.81 mmol (Mg(NO3)2·6H2O and 4.8 mmol DOBDC were added under continuous stirring. Approximately 10 mL portions of the resulting mixture were transferred to 20-mL scintillation vials, which were tightly sealed and placed in a sand bath. The reaction mixture was heated in a pre-heated oven at 120 °C for 24 h. After cooling to room temperature, the mother liquor was decanted from each vial and replaced with fresh DMF, which was subsequently exchanged five times over two days with fresh methanol. The products in each vial were combined into one larger vessel and stored under methanol at room temperature until subsequent characterization. Preparation of “mixed metal” Mg0.77—Cd0.23-MM-MOF-74 A “mixed-metal” MOF-74 sample containing 23% Cd and 77% Mg was prepared in a similar manner as the parent Mg-MOF-74, using 10.39 mmol Mg(NO3)2 · 6H2O and 4.44 mmol Cd(CH3COO)2 · 2H2O, rather than Mg(NO3)2 · 6H2O alone. Powder X-ray diffraction The crystallinity and phase purity of as-synthesized samples were confirmed by comparing simulated powder X-ray diffraction (PXRD) patterns of the parent MOF-74 structure and those collected at room temperature on a PANalytic X’Pert X-ray diffractometer equipped with an X’Celerator detector module and using Cu Κα (λ = 1.5418 Å) radiation. Inductively coupled plasma optical emission spectrometry (ICP-OES) Elemental analysis for Cd and Mg in Mg0.77—Cd0.23- MOF-74 was performed using ICPOES on a PerkinElmer 7300DV ICP-OES instrument to determine the amount of each metal


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present. Prior to analysis, 15-20 mg of as-synthesized Mg0.77—Cd0.23-MOF-74 was activated at 250 °C for 12 h under vacuum. Computational methods Fully periodic density functional theory (DFT) with a planewave basis set within the Vienna Ab initio Simulation Package (VASP)64 was used to study Mg-MOF-74, Mg—Cd-MOF74, and adsorption of CO2 therein. Projector augmented wave pseudopotentials65 and the generalized gradient approximation of Perdew, Burke, and Ernzerhof66 (PBE) with the third version of the dispersion correction by Grimme et al.67 (DFT-D3) was used to study all systems in order to account for dispersion interactions important for intermolecular interactions. A planewave basis set cutoff energy of 600 eV with k-point sampling at the Γ-point were used to optimize unit cell lattice vectors and ionic positions until interionic forces were < 0.01 eV/Å. To study Mg-MOF-74 and CO2 loading therein, the structure of Cu-MOF-74 loaded with 0.5 CO2 per open metal site as determined by powder neutron diffraction data was used for high CO2 loadings.60 This structure indicates 13 unique sites for CO2 molecules within the primitive unit cell, which contains 6 metal ions.

We note that because all of these sites can be

simultaneously populated (even though they are not simultaneously all populated in the experiment), this allowed us to probe CO2 loadings up to 2.17 molecules per metal site. Mg ions were isostructurally substituted into the Cu-MOF-74 structure at the Cu sites, which was used as the starting point for optimizing the CO2-loaded Mg-MOF-74. This optimized structure was used as the starting point for all CO2 loadings, excepting calculations with only one CO2 per unit cell (0.17 CO2 per open metal site), which started from a previously reported structure of CO2 adsorbed in Mg-MOF-74 at the vdW-DF2 level of theory.68 For the reference energy of the isolated CO2 molecule, CO2 was optimized in a box of side length 20 Å.


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In order to describe Mg—Cd-MOF-74, a 2×1×1 supercell was used to probe orderings of metals not accessible in the primitive unit cell in our previous study of this material, where evidence was provided that disorder of metals in the mixed-metal material is not only entropically favorable but also energetically favorable.61 CO2 adsorption to the “disperse” model used in that work (where a single Cd ion is neighbored by five Mg ions in each metal-oxide backbone) as well as to Mg-MOF-74 and Cd-MOF-74 was studied at the PBE level in order to confirm that adsorption energetics are not different between the Mg and Cd ions in the homometallic and mixed-metal MOF-74 materials (Table S1). NMR studies Mg-MOF-74 and Mg0.77—Cd0.23-MOF-74 samples were stored in methanol after synthesis and activated under vacuum to avoid water adsorption. This detail is crucial because even though water can be desorbed from the MOF, it diminishes both the CO2 adsorption capacity69 and the BET surface area of the MOF without changing the crystal structure and unit cell dimensions (evidenced by its PXRD pattern).70 When activating a MOF sample for the first time, a small amount of sample was loaded into a 10 mm test tube with a visible layer of methanol just above the material. Activation of the MOF was achieved by continuously applying vacuum to the sample for 2 h, followed by heating the sample to 65 oC for 2 h, then to 250 oC for 12 h. Once activated, CO2 was loaded into the MOF-74 materials and allowed to equilibrate to a final pressure of 1 atm at room temperature (295 K). Subsequently the 10 mm test tube was flame-sealed to ensure a water-free environment. A second configuration with a valved sample space was used for variable CO2 loadings in MgMOF-74. To reactivate a previously activated and CO2-exposed MOF, the sample was heated to


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approximately 160 oC for 3 h while under vacuum (< 35 mTorr). The sample was cooled down to room temperature while under vacuum before loading with different amounts of CO2.) Static

13

C variable temperature (VT) NMR of Mg-MOF-74 and Mg0.77—Cd0.23-MOF-

74—each loaded with 1 atm

13

CO2 at room temperature, yielding loadings of 1.1 and 1.00 CO2

molecules per metal site, respectively (loading levels are determined using isotherm data from Queen et al.60 and isotherm data in Figure S1)—was performed from temperatures ranging from 8.5 to 295 K. Measurements were recorded on a 202 MHz (4.74 T) magnet at a Larmor frequency of 50.827 MHz using two different home-built static single-channel NMR probes. A combination of single-pulse (Bloch decay) and Hahn echo71 experiments was used to acquire static

13

C VT NMR data. All

13

C spectra were referenced to CO2 gas at 124.5 ppm. For low

temperatures from 8.5 to 100 K, a Kadel liquid helium dewar (Figure S2a) was utilized, which has a carbon-glass resistance temperature sensor (CGR) for temperature measurements. For temperatures from 124 K to just below room temperature, a home-built NMR probe coupled with a liquid nitrogen dewar (to create cold nitrogen gas to be delivered to the sample space, see Figure S2b) was used, and temperature control was maintained with a type T thermocouple. III. Results and Discussion Lineshapes observed in

13

C NMR spectroscopy of CO2 gas adsorbed by the MOF

structure are indicative of complex molecular motion, including hopping between metal sites and along the 1-dimensional channels of the Mg-MOF-74 structure.42,43 In this study, we use the 13C NMR lineshape, which changes dramatically as a function of loading, temperature, and MOF composition, as an indicator of the CO2 behavior. NMR spectroscopy coupled with DFT


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computations reveal the different types of binding sites of CO2 that are present under these conditions. Figure 2 shows the

13

C NMR spectra recorded for

13

CO2-filled Mg-MOF-74 with gas

loadings from 0.88 to 1.15 CO2 molecules per 0.88 CO2/metal

metal at 295 K. Low loadings of CO2 at room

0.91 CO2/metal

temperature (0.88 and 0.91 CO2 molecules per metal) show a

13

0.93 CO2/metal 1.01 CO2/metal

C lineshape that exhibits a

1.06 CO2/metal

positive chemical shift anisotropy (CSA) pattern,

1.15 CO2/metal

with the most intense feature on the right side. At intermediate loadings, a symmetric lineshape is found (~0.93 and 1.01 CO2 molecules per metal, respectively). At higher loadings, the

200

180

160

140

120

100

80

60

40

Chemical Shift (ppm) 13 Figure 2: Static C NMR of Mg-MOF-74 loaded with variable amounts of CO2 at 295 K; the number of molecules of CO2 per metal are given on the right side of the diagram. A vertical line is placed at 124.5 ppm to reference the isotropic chemical shift of free CO2.

lineshape exhibits a CSA with the most intense feature now on the left side (~1.06 and 1.15 CO2 molecules per metal, respectively). The change in the appearance or “flip” of the lineshape is referred to as a flip of the sign of the CSA from “positive” to “negative”, respectively (see Figure S3, for a schematic). This change with respect to loading level is notable, which we will address below. Related lineshapes have been observed previously for low loading of CO2. This regime was explored by an NMR study of Mg-MOF-7442 and found spectra similar to that shown in the top of Figure 2 (at 0.88 CO2/metal loading), whereas to the best of our knowledge, these are the first spectra of a highly-loaded Mg-MOF-74. In order to understand these NMR lineshapes, we must first define several factors related to the NMR chemical shift tensor of

13

C (in particular the chemical shift anisotropy or “CSA”)

that govern their appearance.72 The MOF-74 samples are powders with randomly oriented 1-


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dimensional channels possessing approximately 6-fold symmetry about the c-axis of the channel, and adsorbed CO2 molecules can undergo motion that consists of site-to-site hopping around the 6-fold-symmetry metal sites and along the 1-dimensional channel, leading to reorientation of the molecule. The CO2 stays in the channel for the duration of the acquisition of each NMR signal transient. On the timescale of an NMR experiment (approximately milliseconds), the motion of CO2 is not isotropic. As a result, the

13

C NMR spectra in nearly all cases exhibit an “axially-

symmetric” chemical shift anisotropy pattern—an asymmetric lineshape that is peaked on one side—found for molecules exhibiting a 3-fold (or higher) symmetry axis. These lineshapes are important because they add additional detail to the NMR spectroscopy of such gas-phase molecules, and components of the chemical shift tensor can be extracted from lineshapes. The lineshape permits assignment (using the Haeberlen convention73) of 3 elements of the diagonalized chemical shift tensor. (The familiar isotropic chemical shift, δiso, which is often reported in solid-state magic-angle spinning, and solution-phase NMR comes from the average of all 3 tensor elements, δxx, δyy, δzz.) Significant for our purposes here, the sign of the CSA lineshape is determined from the “reduced anisotropy” δ = δzz – δiso, where δzz is the principal component of the tensor furthest away from the isotropic chemical shift, δiso. As an aside, in NMR two values are often reported interchangeably74: σ, chemical shielding (a tensor) and δ, the chemical shift (which also can be expressed as a tensor). They are related to one another by using a chemical reference scale, where δ = (σref − σsample). We are using the IUPAC convention75,76 here: chemical shifts (δ) are positive to high frequency, whereas, absolute shieldings (σ) are positive to low frequency. Computational programs that predict NMR parameters (such as Gaussian,77 CASTEP78-NMR) typically give values for σ, whereas


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experimental data report δ values. Here, we use δ but refer to some aspects of theory that invoke shielding (σ). Central to this analysis for comparison to NMR lineshapes is the use of the angle θ between the long (C∞) axis of the CO2 molecule and the c-axis of the MOF channel. The NMR lineshape measured is an integral over all orientations of CO2 present, where the spectrum of each CO2 molecule also has an angular orientation with respect to the external magnetic field (B0) used for recording NMR spectra. A symmetric Lorentzian NMR resonance arises from isotropic motion among a random distribution of sites, but the symmetry of the MOF structure and CO2 molecules binding to it encodes the 13C spectra with reduced symmetry of motion along and around the 1-dimensional channels. This lineshape can best be expressed as a modulation of the chemical shielding arising from this θ angular dependence: − 1, ∆ = ∆ ∙ cos = ∆ ∙ 3

(1)

where P2 is the second-order Legendre polynomial. Equation 1 (Ref. 79) describes the angular dependence of the shielding that governs the spectrum’s linewidth and sign (positive or negative, see Figure S3). From Equation 1, it can be seen when θ is greater than 54.7°, the value of P2 is negative and when less than 54.7°, P2 is positive. Hence, computation of the multiple CO2 binding sites and their relative orientations can be compared via the NMR lineshapes. The linewidth of the axially-symmetric spectrum in the rigid lattice limit (|325| ppm,80 cf. Figure 7) is given by the range of chemical shielding ∆σ scaled by P2 to obtain the effective linewidth, ∆σeff. Here we express it as an absolute value in order to avoid confusion between its expression as shielding, ∆σ, or as chemical shift, ∆δ. Based on the IUPAC convention (Ref. 75, 76), these are linearly related and opposite in sign to one another, ∆δ = ‒∆σ (see Figure S3).


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As discussed above, the

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C lineshapes depend on CO2 loading (Figure 2) and can be

interpreted in terms of the site occupancy at each loading level. In the next section, we describe the use of DFT calculations to determine site occupancy the angle between the instantaneous CO2 axis and the channel axis, θ. We use these calculations to understand the Figure 2 13C NMR lineshapes. DFT calculations were performed to study the conformations and adsorption energetics of adsorbed CO2 molecules and the multiple types of adsorption sites that exist for CO2 as a function of loading in Mg-MOF-74. A multiple-site model of CO2 loading into MOF-74 has been previously reported by Queen et al., which found existence of primary, secondary, and even tertiary adsorption sites for CO2 in Cu-MOF-74.60 This model transferred to Mg-MOF-74 is the starting point for our analysis. In addition, a similar description of CO2 also based on the data from Queen et al.56 has recently been used by Sillar et al.76 to simulate CO2 adsorption isotherms in Mg-MOF-74 using first principles calculations; their results are consistent with our findings in this work. Previous studies42,43 have also addressed Monte Carlo simulations of low loading of CO2 at less than one gas molecule per metal to determine equilibrium configurations, and the resulting effect on 13C NMR resonances. Calculated binding energies with several CO2 loading levels from 0.17 to 2.17 CO2 molecules per open metal site are given in Table S2. Binding energies were computed using equation 2: − =

!"×

!$"%&×

!"×

−

!$"%&×

− .

(2)

is the energy of the system when N CO2 molecules are bound in the MOF, is the energy of the MOF with one fewer CO2 molecule bound within it, and


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ECO2 is the energy of the isolated CO2 molecule. We differentiate between CO2 molecules at primary, secondary, and tertiary sites and consider the energy per molecule, treating all molecules at each type of site as equivalent for purposes of deducing the binding energy of a single CO2 molecule.

In cases where

adsorption of N + 1 molecules is not considered in Table S2, the addition of multiple CO2 molecules is

Figure 3: CO2 binding at the most favorable primary binding site (position 4) in Mg-MOF-74 in the low loading limit.

accounted for by adjusting equation 2 to consider the average energy contribution by each of the multiple CO2 molecules added. These binding energies assume pairwise interactions between adsorbed CO2 molecules. As expected, the data indicate that once all primary adsorption sites are occupied successive occupation of secondary and tertiary sites show progressively decreasing strengths of adsorption as CO2 loads into the Mg-MOF-74 pore channel. We discuss more about the energetics of successive loading of binding sites with our discussion of the geometry of the adsorbed molecules in the following sections. At low loadings, CO2 is likely to spend the most time at the open metal binding site, referred to as the “primary” binding site and depicted in Figure 3, based on its calculated binding energy. Figure 3 shows the most favorable binding position at the open metal site at low loadings of CO2. The four most favorable binding positions at the open metal site with the corresponding binding energies can be found in the SI, Figure S4. Notably, a favorable lateral neighbor interaction of about 1 kJ/mol is observed between CO2 molecules in neighboring primary adsorption sites, meaning that neighboring sites are


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slightly favored to fill first. This is consistent with the observation of a favorable lateral CO2— CO2 interaction by Sillar et al.81 We note that the lower magnitude of our estimate can be attributed to our model implicitly including neighbors along the c-axis of the channel as a result of using the primitive unit cell in our simulations, making it impossible for us to fully eliminate interaction from nearest-neighbor CO2 molecules. At higher loadings, the sites sampled are similar to those shown in Figure 4. These include secondary sites at which CO2 coordinates predominantly to other molecules of alreadyadsorbed CO2 at primary sites, as well as to the organic DOBDC linker. In contrast to primary sites, secondary sites do not exhibit a significant nearest-neighbor interaction, the result of reduced intermolecular distance, and therefore secondary sites are favored to fill in a disperse fashion. At the highest loading levels, tertiary sites are found to exist at the pore center aligned with the channel axis. The energetics of binding at these sites are given in Table S2.


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a.) C

b.)

Primary

Secondary

Tertiary

θ = 68 P = -0.29

θ = 30 P = 0.62

θ=0 P = 1.00

o

2

o

2

o

2

Figure 4: a.) Cross section along the c-axis of Mg-MOF-74 loaded with CO2, showing primary (metal-associated), secondary (CO2- and linker-associated), and tertiary (channel-center CO2-associated) sites b.) Cross section looking down the c-axis of Mg-MOF-74 loaded with CO2. θ designates the orientation of the long axis of the CO2 species with respect to the c-axis of the channel. P2 is the calculated value from the orientation (that affect the measured 13C CSA) from the different sites. These individual molecule contributions to P2 are used to calculate weighted P2 values.

This is a dynamic situation where the CO2 molecules are moving from site to site on the NMR timescale, on the order of 1 ms. The P2 obtained from NMR reflects an averaging, separate spectra are not observed for each kind of site (primary, secondary, or tertiary), rather a single time-averaged value of P2 results. DFT is able to predict a weighted P2 value based on the occupancy and orientation of primary, secondary, and tertiary sites as a function of CO2 loading. This value is expressed in terms of the “number of CO2 molecules per metal”, which is graphed in Figure 5a. The manifestation of P2 in the NMR spectra in Figure 2 are in agreement with the trend for P2 predicted by DFT calculations shown in Figure 5a, where a weighted P2 was calculated based on the number of CO2 molecules at each site and their orientations relative to the c-axis of the unit cell. As CO2 loads into the MOF-74 unit cell, initially the open metal sites serve as the primary adsorption site–up to 1 CO2 molecules per metal occupy such sites, shown in Figure 5b (upper left image). These primary sites exhibit a negative P2 and are consistent with


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the low loading spectra in Figure 2 (θ’ = 67° to 77° is predicted). This change from 67° to 77° (a tilting of CO2 away from aligning with the c-axis) is a result of the introduction of a favorable lateral CO2—CO2 interaction which has also been noted by Sillar et al.81 Shown in Figure 5a, negative P2 values are predicted up through 7 CO2 molecules per unit cell. Once the primary adsorption sites (the open metal sites) are occupied, the secondary binding sites that exist through coordination to the DOBDC linker and the bound CO2 molecules will become occupied with increasing CO2 loading. These sites may break the symmetry of the primary sites when they are partially occupied as in Figure 5b (upper right image), resulting in different binding modes of neighboring CO2 molecules at primary adsorption sites (θ’ = 66°, 71°). In both cases, the CO2 molecules at primary adsorption sites are more aligned with the caxis of the MOF channel than when no secondary sites are occupied, in addition to the CO2 adsorbed at the secondary binding sites (θ’’ = 20°) being aligned with the pore channel relative to CO2 at primary binding sites. The change in orientation of already adsorbed CO2 molecules and the occupation of channel-aligned sites results in a transition from a negative to a positive weighted P2 value. With further loading, once all of the secondary adsorption sites become occupied (there are six such sites, shown in Figure 5b in the lower left image) all of the primary sites are again equivalent by symmetry (θ’ = 68°), as are the secondary sites (θ’ = 28°). The increased occupation of channel-aligned secondary sites further increases the weighted P2 value. The tertiary site at the channel center found at high loading is aligned with the pore channel (θ = 0°), giving the maximum (positive) P2 value (1.00), and shown schematically in Figure 5b, lower right.


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We note that the transition from a

a.) 0.3

negative to a positive P2 value that is indicated by

both

the

experimentally

0.2

measured

0.1 0.0

lineshape and the DFT-predicted structures

P2

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


occurs around a loading of one CO2 molecule per metal site in both cases, although in

-0.1 -0.2 -0.3 -0.4

experiment this transition occurs at slightly lower loading than we predict using DFT. A P2 value of zero results in a narrower

-0.5 0.00

0.33

0.67

1.00

1.33

1.67

2.00

2.33

Number of CO2 Per Metal

b.)

symmetric resonance, observed at intermediate loadings in Figure 2. The loading levels over which we observe a transition from negative to positive are also broader (a less sharp transition) in theory than experiment.

We

attribute these differences to thermal disorder of bound CO2 molecules and the dynamic sampling of some secondary sites at loadings below one CO2 per metal. We additionally note that we have determined our estimates of experimental experimentally isotherms,

so

CO2

loading

measured

from adsorption

any uncertainty in

those

isotherms will directly translate to uncertainty

Figure 5: a.) Weighted average of CO2 loading dependence on the second order Legendre polynomial, P2. The shaded region shows the CO2 loading levels measured in Figure 2. b.) CO2 loadings in Mg-MOF-74 unit cell, corresponding to the calculated P2 values in part a.). θ’ is the Primary site, θ’’ is the Secondary site, θ’’’ is the Tertiary site, and θ’1and θ’2 are to distinguish two orientations observed at the Primary site when some Secondary sites occupied.


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in our estimates of loading. Variable temperature 13C NMR was measured to investigate changes to the lineshape on a sample with a high loading of 13CO2 (~1.1 molecules of CO2/metal). There are three regimes to consider: a low temperature regime where the CO2 is immobile, an intermediate regime where the CO2 motion is on the order of the NMR timescale (during the pulse sequence acquisition), and the high temperature (fast motion) regime where the gas molecule is able to dynamically sample available binding sites. These regimes all exhibit different lineshapes, and the fast motion regime will be discussed first. The fast motion regime (at higher temperatures) has been introduced above for multiple loading levels. At 295 K there is evidence of unbound CO2, seen as an isotropic narrow Lorentzian CO2 gas peak at 124.5 ppm overlapping another lineshape with a characteristic axially-symmetric CSA pattern (see Figure 6, bottom right spectrum). The CSA pattern “flipping” (where the CSA changes sign from negative to positive) reported previously in related studies42,43 is not observed in this highly CO2-loaded Mg-MOF-74 sample as we change temperatures. We have confirmed that this finding is sensitive to the CO2 loading by reproducing the findings in the earlier work, at a lower loading of CO2 in a separate set of experiments (see Figure S5), which finds the expected effects. Here, in this analysis of high loadings of CO2, the molecules are more crowded in the channels and adopt an orientation resulting in a positive P2 value, in agreement with the schematic picture provided by DFT calculations shown in Figure 4. As we cool the sample, the persistence of the axially-symmetric pattern shows that the origin of broadening is dominantly chemical shift anisotropy.


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At this level of (high) loading, an isotropically-averaged narrow resonance is seen at the center of the room temperature spectrum, likely from CO2 gas that is outside of the 1dimensional channels of the MOF structure, such as the interparticle spaces in the powder where it is not in exchange with other sites at the metal centers filling the channel. The spectral lineshapes were modelled using the Dmfit program,82 and an asymmetry parameter of zero (η = 0) consistent with an axially-symmetric CSA was used for all lineshapes (unless indicated otherwise). Notably, all isotropic chemical shifts (extracted from fits to each of the lineshapes) are approximately 124.5 ± 2 ppm, which is the isotropic chemical shift of free CO2 gas.83 CO2 motion over a wide range of temperatures is evident from the narrowing of the CSA pattern from 143 to 295 K. If the CO2 hopping was arrested at any point in this range from oversaturation of CO2 on the most favorable sites, lacking any exchange at these higher temperatures we would expect to see broad lineshapes like those observed at and below 60 K (shown in Figure 6).


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8.5 K

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124 K

40 K

143 K

60 K

165 K

84 K

238 K

124 K

295 K

Figure 6: Static variable temperature 13C NMR spectra of Mg-MOF-74 loaded with 1.1 molecules of 13CO2 per metal site. Spectra were recorded at the indicated temperatures ranging from 8.5 to 295 K. Individual fits are in blue and magenta, and the sum of the fits are shown in red. Note the difference in chemical shift scale, between left and right sides.

At 143 K in Figure 6, the start of a transitional regime is evident as the sample is cooled further. As the temperature continues to decrease, the sample enters a regime where CO2 hopping is a rare event on the NMR timescale.43 At 124 K, the signal-to-noise ratio is noticeably worse than that of other spectra, despite the fact that more scans (number of transients) were acquired at this temperature. Such attenuation of the signal occurs because the correlation time of the CO2 motion is on the order of the NMR pulse sequence spin echo delay time—here 180 µs. Thus, at the start of the acquisition, the CO2 molecule is likely to be positioned on one site, and at the end of the acquisition on a different site, thereby changing both its orientation and its frequency, resulting in imperfect refocusing of the magnetization. Cooling the sample further, the CSA lineshape becomes much broader, approximately 320 ppm wide. At these temperatures (≤ 84 K), most of the CO2 molecules remain on a single site during NMR acquisition. At very low temperatures (in Figure 6, 84 K and lower), the linewidth of the axially-symmetric powder pattern is unchanging, even though the spectral features are slightly broadened. These low-temperature conditions lead to the powder pattern


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observed here, similar in linewidth and appearance to that of fixed CO2 in the solid (325 ppm), the rigid lattice linewidth.80 Notably, at 8.5 K (the lowest temperature studied) the carbon resonance becomes broad and featureless. The origin of the broadening is due to paramagnetic sites in the host that become more aligned by the magnetic field at low temperature and therefore interact more strongly with the nearby

13

CO2 molecules, leading to such a broadened, distorted resonance. An electron

paramagnetic resonance (EPR) measurement of Mg-MOF-74 at 10 K confirms that paramagnetism is present at low temperatures (Figure S6). Paramagnetism is important to note because it can cause NMR signals to broaden or shift—so acknowledgement of an unsaturated electron in Mg-MOF-74 is chemically interesting to note (and will be the object of future study). Similar VT

13

C NMR experiments were performed on the mixed-metal MOF-74

(Mg0.77—Cd0.23-MOF-74). Static VT 13C NMR spectra loaded with 1 atm (at 295 K) of

13

CO2

were recorded, from 9.5–295 K (shown in Figure 7). A loading of 1 atm for Mg0.77—Cd0.23MOF-74 corresponds to approximately 1.00 CO2 per metal site, based on isotherm data (Figure S1). Figure 7 shows several

13

C spectra with axially-symmetric CSA lineshapes that change

from a positive to a negative CSA pattern when going from high to low temperatures, respectively, even with high loading of CO2, in contrast to the Mg-MOF-74 findings. The reversal (“flipping”) of the most intense portion of the CSA powder pattern from one side to another is due to the change in favored orientation(s) of the CO2 molecules relative to the c-axis of the MOF-74 host, similar to that found in low-loading Mg-MOF-74. The Mg sites have a stronger affinity for CO2 than the Cd sites, evidenced by the isotherm data (Supporting


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Information, Figure S1) and density functional theory (DFT) calculations (Table S1). With nearly 1 in 4 metal sites being Cd, this composition with multiple metal sites gives CO2 the opportunity on average to align with a negative P2, tending toward a direction orthogonal to the c-axis. The results from Mg-MOF-74 under different loading permit us to intuit the behavior. The Cd sites, with their weaker binding of CO2, would appear to permit the primary binding at Mg metal sites to dominate the behavior, lessening the influence of secondary sites.

126 K 9.5 K 58 K

137 K 154 K 169 K

80 K 126 K

243 K 295 K

Figure 7: Static variable temperature 13C NMR spectra of Mg0.77-Cd0.23-MOF-74 loaded with 1.00 molecules of 13CO2 per metal site. Spectra were recorded at temperatures ranging from 9.5 to 295 K. Individual fits are in blue and magenta, and the sum of the fits are shown in red. Note the difference in chemical shift scale between left and right sides.

At 295 K the lineshape can be deconvoluted into two resonances: one with a CSA lineshape and one Lorentzian. In Figure 7, we can see that there is a significant increase in the breadth of the CSA lineshape when cooling the sample from approximately 140 to 100 K. This is the transitional regime as seen previously, where CO2 exits the fast motion regime of hopping from site-to-site (> 140 K), and enters a regime of hindered motion (< 100 K), on the NMR timescale. At these lower temperatures in Mg0.77—Cd0.23-MOF-74, there is evidence at 80 K for two types of CO2. There are CO2 molecules contributing to the CSA-broadened powder pattern, as well as CO2 with a Gaussian-shaped resonance, likely due to motion, which would match the reduced binding affinity offered by Cd sites. As temperature is decreased from 80 K, the CSA


22

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line width continues to broaden, and the broad Gaussian is no longer evident by 9.5 K. The span of the lineshape at 9.5 K exceeds 300 ppm, which is similar to that of solid CO2,80 and no evidence of broadening from interaction with paramagnetic sites was found. In the higher temperature range, both types of MOF structures have similar linenarrowing changes with respect to temperature, even if one set (Mg0.77—Cd0.23-MOF-74) has a narrower linewidth (see Figure S7). At the lowest temperatures, the linewidth for CO2 in MgMOF-74 is unchanging, indicating CO2 is not moving (on the NMR timescale). In contrast, the mixed-metal Mg0.77—Cd0.23-MOF-74 is still undergoing line narrowing even in this low temperature regime, noting that the

13

CO2 linewidth converges to the same value at the lowest

temperature recorded for the mixed-metal MOF. Conclusions The combination of NMR with DFT confirms the sensitivity of the NMR lineshape with respect to small changes in molecular orientations. CO2 adsorbed in the MOF-74 structure is in exchange between multiple sites and has an orientational dependence dictated by the number of CO2 molecules per metal site. The molecules undergo a transition from aligning (on average) normal to the pore channel axis at low loading to aligning with the pore channel axis upon higher loading. This phenomenon manifests in the 13C NMR lineshape, wherein the CSA pattern “flips” sign from negative to positive between low and high loadings of CO2, undergoing this transition roughly at 1 CO2 per open metal site. DFT calculations show that the orientation of the long axis of these molecules can be calculated as a function of loading, expressed as a value of P2 that depends upon the angle θ with respect to the channel axis. The weighted P2 value agrees with experimental data in the fast motion regime,


23


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Page 24 of 45

showing that the occupation of secondary CO2 adsorption sites plays a key role in affecting the structure of adsorbed CO2 and the flip of the CSA lineshape. This finding, coupled with previous work on low CO2 loadings, has enabled newly emergent insight into the importance of CO2 loading in understanding qualitatively the evolution of the CSA lineshape with temperature in Mg-MOF-74. In Mg—Cd-MOF-74, we observe different behavior at high loading and in the fast motion regime. Due to differences in CO2 loading, the CSA lineshape for Mg—Cd-MOF-74 loaded with CO2 is flipped in sign relative to that of pure Mg-MOF-74. This inversion in sign from a lower loading causes CO2 to adopt an orientation, on average, where the CO2 molecules align orthogonally to the channel axis. What is of particular significance is the demonstration in this work that NMR coupled with DFT is able to reveal, through lineshape analysis, the binding sites of CO2, not only to the metals but also secondary and tertiary sites that are present. NMR can therefore provide such unique insights into the behavior of adsorbates in porous materials. Supporting Information Available (a) CO2 isotherm for the mixed-metal MOF-74 sample at 25 °C; (b) schematics of the apparatuses used for variable temperature measurements; (c) conventions for shielding and chemical shift; (d) depictions of the four most likely binding positions on the primary site; (e) variable temperature 13C NMR of a lower CO2 loading in Mg-MOF-74; (f) EPR of Mg-MOF74 at 10 K; (g) graph of the linewidth versus temperature for both MOFs; (h) tables of adsorption energies; and (i) atomic positions of all atoms for structures used in calculations. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author


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*(S.E.H.) Phone: 314-935-4624. E-mail: [email protected]. Acknowledgements This work was supported as part of the Center for Understanding and Control of Acid GasInduced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic energy Sciences under Award # DE-SC0012577. The purchase of the Bruker EMX-PLUS EPR spectrometer was supported by the National Science Foundation (MRI, CHE-1429711).


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Table of Contents (TOC) Graphic


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Figure 1: a.) The hexagonal honeycomb structure of M-MOF-74 shown along the direction of the c-axis and b.) depiction of the handedness of the metal backbone (boxed in red in a.)) in addition to the four most likely binding positions of CO2 to the open metal site, with position 4 being the most favorable. 78x29mm (200 x 200 DPI)



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Figure 2: Static 13C NMR of Mg-MOF-74 loaded with variable amounts of CO2 at 295 K; the number of molecules of CO2 per metal are given on the right side of the diagram. A vertical line is placed at 124.5 ppm to reference the isotropic chemical shift of free CO2. 46x34mm (200 x 200 DPI)


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Figure 3: CO2 binding at the most favorable primary binding site (position 4) in Mg-MOF-74 in the low loading limit. 39x45mm (200 x 200 DPI)



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Figure 4: a.) Cross section along the c-axis of Mg-MOF-74 loaded with CO2, showing primary (metalassociated), secondary (CO2- and linker-associated), and tertiary (channel-center CO2-associated) sites b.) Cross section looking down the c-axis of Mg-MOF-74 loaded with CO2. θ designates the orientation of the long axis of the CO2 species with respect to the c-axis of the channel. P2 is the calculated value from the orientation (that affect the measured 13C CSA) from the different sites. These individual molecule contributions to P2 are used to calculate weighted P2 values. 94x52mm (200 x 200 DPI)


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Figure 5: a.) Weighted average of CO2 loading dependence on the second order Legendre polynomial, P2. The shaded region shows the CO2 loading levels measured in Figure 2. b.) CO2 loadings in Mg-MOF-74 unit cell, corresponding to the calculated P2 values in part a.). θ’ is the primary site, θ’’ is the secondary site, θ’’’ is the tertiary site, and θ’1and θ’2 are to distinguish two orientations observed at the primary site when some secondary sites occupied. 60x133mm (200 x 200 DPI)



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Figure 6: Static variable temperature 13C NMR spectra of Mg-MOF-74 loaded with 1.1 molecules of 13CO2 per metal site. Spectra were recorded at the indicated temperatures ranging from 8.5 to 295 K. Individual fits are in blue and magenta, and the sum of the fits are shown in red. Note the difference in chemical shift scale, between left and right sides. 89x39mm (200 x 200 DPI)


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Figure 7: Static variable temperature 13C NMR spectra of Mg0.77-Cd0.23-MOF-74 loaded with 1.00 molecules of CO2 per metal site. Spectra were recorded at temperatures ranging from 9.5 to 295 K. Individual fits are in blue and magenta, and the sum of the fits are shown in red. Note the difference in chemical shift scale between left and right sides.

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88x38mm (200 x 200 DPI)


CO2 Dynamics in Pure and Mixed-Metal MOFs with Open Metal Sites

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