Acid Gas Adsorption on MetalâOrganic Framework Nanosheets as a

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Acid gas adsorption on metal-organic framework nanosheets as a model of an ‘all-surface’ material Joshua D. Howe, Yang Liu, Luis Flores, David A Dixon, and David S. Sholl J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/acs.jctc.7b00041 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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Journal of Chemical Theory and Computation

Acid Gas Adsorption on Metal-Organic Framework Nanosheets as a Model of an "All-Surface" Material Joshua D. Howe,† Yang Liu,† Luis Flores,‡ David A. Dixon,‡ and David S. Sholl∗,† †School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, USA ‡Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487, USA E-mail: [email protected]

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Abstract To establish a model of metal-organic framework (MOF) surfaces and build an understanding of surface-specific ligand adsorption phenomena in MOFs, we present a computational study exploring multiple models of a series of MOF-2 nanosheet materials, "M-BDCs", with M = Zn, Cu, and Co and BDC = benzene-1,4-dicarboxylate. We study and assess the appropriateness of a series of models ranging from small clusters (18 atoms) to fully periodic sheet models. We additionally study the interactions of these models with acid gases and energy-relevant small molecules (CO, CO2 , H2 O, SO2 , NO2 , and H2 S). We employ computational methods ranging from DFT with various exchange-correlation functionals to perturbative and coupled-cluster methods. For these systems, we present binding energies and enthalpies with the various ligands studied as well as IR frequency shifts for the normal modes of these ligands upon complexation with the open-metal sites of these materials. Our calculations lead to an understanding of phenomena unique to MOF surfaces and the importance of the periodicity in these materials in capturing surface-specific adsorption behaviors.

Introduction Metal-organic frameworks (MOFs) are hybrid organic-inorganic materials that have drawn interest for a variety of applications including catalysis, separations, and gas storage due to their high internal surface areas, outstanding crystallinity, and customizable structures. 1–3 Most work to date has focused on understanding phenomena within the bulk pores of MOFs, while the external surfaces of these materials remain largely unstudied. Despite this, when MOFs are made as small particles, the surface area contributed by the external surface of MOFs can be an appreciable percentage of the total adsorption surface area. Additionally, chemical and structural differences at surfaces as compared to the bulk can potentially affect catalytic activity by creating active sites; reduce accessibility of pore space; alter particle stability; 4 and determine the potential of MOF particles to interface with other materials, 2


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as in mixed-matrix membranes. 5,6 Therefore, developing an understanding of the nature and reactivity of MOF surfaces is of importance for understanding their potential for use in various applications. In order to develop an understanding of MOF external surfaces, it is desirable to constrain our study to MOFs that are as close to "all-external surface" as possible. Recently, MOF nanosheets that are two-dimensional analogs of the MOF-2 structure, 7 the bulk structure of which is shown in Fig. 1, have been synthesized. 5 This material is an effectively allsurface MOF with unique surface adsorption sites that are not accessible in the analogous bulk MOF. MOF-2-like nanosheet materials have been synthesized using Zn, Cu, and Co metals in place of the Zn ions of bulk MOF-2. The surfaces of these nanosheets include undercoordinated open-metal sites that do not exist in the bulk material as a result of coordination to other sheets in the layered MOF-2 structure, making the distinctions between metals particularly important in these nanosheet materials. Previous studies of MOF-2, like MOFs in general, have focused on bulk adsorption properties, neglecting the interactions available on the surfaces of these materials. 8–10 Interactions of acid gases and other strongly adsorbing species with the open-metal sites that exist uniquely on these surfaces are a balance of electrostatic, dispersion, Pauli repulsion, and orbital interactions. 11 Such interactions with comparable open-metal sites in bulk MOFs have been the subject of many studies in recent years. 9,12–19 Using first-principles electronic structure methods well suited to characterizing these interactions, we present a study of acid gas interactions with the open-metal sites of MOF-2-like nanosheets. The goal of this study is to provide a model of a simple MOF surface with active sites that are qualitatively distinct from the corresponding bulk material and to study the unique interactions of small acid gas molecules with these surface sites. There are two primary challenges to achieving this goal: creating a sufficient model of the MOF surface and accurately characterizing adsorbate interactions with these surfaces. As synthesized, MOF-2-like nanosheets are not single-layer materials, making full atom-

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Figure 1: The bulk structure of MOF-2. Zn atoms are silver, oxygen atoms are red, carbon atoms are brown, and hydrogen atoms are pink. The black box outlines the crystallographic unit cell. istic study of these materials computationally unfeasible due to the large number of atoms required and the scaling of these methods. Therefore, we propose and justify multiple models ranging from cluster models to fully periodic models in this study. We further test the limits of our models to support our conclusions from studying single adsorption events at single open-metal sites by examining the effects of loading of multiple nearby sites and the potential for coadsorption at a single primary adsorption site. To accurately describe the interactions of molecules with the open-metal sites in our models, we employ computational methods that account for the various interactions relevant in these systems. These methods include density functional theory (DFT), 20,21 which has some well-known challenges relevant to these materials. Because of the importance of d electrons in accurately representing the electronic structure of particularly the Cu- and Co-containing materials (in which the metal2+ ions have unpaired electron spins and unfilled 3d shells), we account for the self-interaction errors in our periodic DFT calculations by employing Hubbard U corrections 22 for the relevant d electrons of the transition metals. Additionally, to capture the dispersion interactions relevant for adsorption of acid gases, nonlocal electron correlation effects not included in conventional exchange-correlation approaches such as the local density approximation (LDA) or generalized gradient approximations (GGAs), we

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employ density functional methods that explicitly include these nonlocal interactions. Our periodic DFT calculations use the vdW-DF2 van der Waals-corrected density functional. 23 Additionally, we benchmark our DFT calculations using correlated molecular orbital theory at the MP2 24,25 and CCSD(T) 26–29 levels using the correlation-consistent basis sets. Previous work on first-principles study of MOF surfaces has been limited and mostly confined to cluster calculations, 30,31 which we demonstrate cannot always fully capture the complex environment of MOF surfaces imposed by the periodicity of the bulk structure.

Methods We investigate acid gas interaction with the undercoordinated open-metal sites of MOF-2like "M-BDC" (where "M" is the identity of the metal ion and "BDC" is the benzene-1,4dicarboxylate linker) nanosheets by developing models of these nanosheets based on the bulk MOF-2 crystal structure with which we then computationally study acid gas adsorption.

Computational Details We employ fully periodic DFT with a plane wave basis set and projector-augmented-wave pseudopotentials 32 within the Vienna Ab initio Simulation Package 33 (VASP) to study our periodic "sheet" model as well as molecular orbital basis set DFT calculations on large and small "cluster" models of the open-metal sites within the Gaussian 09 package. 34 The cluster calculations additionally allow us to employ correlated wavefunction methods to study these systems that are not currently computationally feasible for the fully periodic system. For our periodic DFT calculations we use vdW-DF2, while for the cluster calculations we use the B3LYP, 35–37 BP86, 38,39 PW91, 40,41 ωB97X, 42 and ωB97XD 43 exchange-correlation functionals. Our periodic DFT calculations use the DFT + U implementation of Dudarev et al. 22 to account for the self-interaction errors. U corrections are applied to d electrons; Uef f ective (U + J) values of 3.3 and 4.0 for Co and Cu, respectively, are taken from Ref. 44 5



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in which U correction values for these metals are determined by reproducing the oxidation energy of the corresponding metal oxide to M2 O3 . We use a plane wave basis set with a cutoff energy of 1000 eV for optimization of our sheet model geometry and 600 eV for adsorption calculations wherein the shape of the unit cell is fixed. Interatomic forces are converged such that forces on all ions are less than 0.01 eV/Å. This provides convergence of total energies to roughly 1 kJ/mol in our calculations. We additionally study two cluster models: a large cluster and a small cluster. The large cluster (Fig. 2 left) has 70 atoms, consisting of two divalent metal centers (M2+ ) connected by 4 bridging BDC ligands. Terminal unbound carboxylate groups are capped with hydrogen atoms to preserve a neutral charge. The small cluster (Fig. 2 right) has 18 atoms and replaces the BDC ligand of the larger cluster with formate (O2 CH− ). Otherwise, its structure is analogous to the large cluster and to bulk MOF-2. This small cluster is necessary for higherlevels of correlated molecular orbital theory, for which calculations on the larger cluster or fully periodic model are computationally unfeasible. To study these clusters we employ the aforementioned list of methods in addition to testing the B97D3, 45,46 M06, 47 and M06X 47 exchange-correlation functionals. For H, C, N, O, and S, the DZVP2 basis set 48 was used, and for Zn, Cu, and Co Peterson’s aug-cc-pVDZ-PP basis set and effective core potential 49,50 were used. All systems were fully optimized to the standard optimization criteria in Gaussian 09. 34 To further validate our DFT results, we additionally used the 2nd order Møller-Plesset perturbation theory (MP2) 24,25,51 method to study the small cluster model. For our MP2 calculations, C, O, H, and N were treated using the aug-cc-pVDZ basis set; 52 S was treated using the aug-cc-pV(D+d)Z basis set; 53 and Zn, Cu, and Co were treated using the aug-ccpVDZ(PP) basis set. 49,50 Single-point energy calculations at the coupled cluster CCSD(T) level of theory 26–29 using the MP2-optimized geometry were computed using the same basis set as the MP2 calculations. The CCSD(T) calculations were performed using MOLPRO. 54,55 For the Zn-containing clusters, the optimized geometry differs from the geometry observed

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(vdW-DF2), constraining the unit cell shape to match the experimentally determined bulk geometry to avoid needing to converge the potentially soft interlayer degrees of freedom. A large plane wave basis set cutoff energy of 1000 eV was necessary in these calculations to converge the stress tensor to obtain even qualitatively correct intrasheet geometries; smaller basis sets resulted in a tendency of the pore shape to relax away from the bulk geometry in both the bulk and sheet model calculations. From the converged MOF-2 structure, we generate a model of a sheet by taking the geometry of a single layer of the material and placing a vacuum layer of 18 Å around it to minimize the effects of interactions with neighboring layer images. We allow the shape of the unit cell and the ionic coordinates to relax in optimizing our sheet model geometries to enforce that a vacuum layer is maintained. From the optimized Zn-BDC sheet model geometry we generate Cu-BDC and Co-BDC sheet models via isostructural substitution of the Zn ions with the appropriate metal ions, which we then optimize in the same way. For Cu-BDC and Co-BDC all reasonable spin states were considered for the appropriate oxidation state of each metal and the lowest-energy state was selected for further use in study of acid gas interactions. Our calculations predict that Hund’s rule is obeyed in both cases.

Figure 3: The periodic model for Zn-BDC nanosheets. Zinc atoms are silver, oxygen atoms are red, carbon atoms are brown, and hydrogen atoms are pink. The black box shows the unit cell; structure is shown outside of the box to illustrate the periodic boundary conditions with an emphasis on the 18 Å vacuum layer between periodic images of the Zn-BDC sheet.

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Adsorption and Spectroscopic Property Calculation Details We study adsorption of acid gases and relevant small molecules (CO, CO2 , H2 O, SO2 , NO2 , and H2 S) to M-BDC sheets (M = Co, Cu, and Zn) consistent with the synthesized materials reported by Rodenas et al. 5 Initial geometries of adsorbed molecules are based on previously determined binding motifs to bulk open-metal sites of the same metal identity, 11 with the exceptions of NO2 and SO2 , for which multiple candidate geometries are considered and only the lowest-energy relaxed structure is further studied. We compute binding energies of adsorbed species as -∆E = Emol+MOF -Emol -EMOF , where ∆E is the change in energy upon adsorption, Emol+MOF is the energy of the MOFplus-molecule adduct, Emol is the energy of the isolated molecule, and EMOF is the energy of the isolated MOF sheet. The sign is chosen such that a positive ∆E corresponds to an exothermic adsorption. To predict experimentally relevant heats of adsorption, we account for finite-temperature thermal and quantum nuclear zero-point energy corrections at the level of a harmonic approximation as described below. This enables us to compute binding enthalpies at a finite temperature analogous to our calculation of binding energies. In our periodic DFT calculations, we compute vibrational frequencies via a finite-difference method to obtain a Hessian matrix of second order derivatives of mode energies with a displacement of each atom of the adsorbed species symmetrically by 0.015 Å along each Cartesian axis. To obtain frequency shifts, we compare vibrational modes of molecules in the MOF-plus-molecule adducts with the vibrational modes of "free" molecules in 20 Å side-length cubic cells. In contrast to this, for our cluster calculations, we compute vibrational properties by using exact analytical derivatives.

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Comparing the bulk M-MOF-2 structures to the sheet model structures, we observe a slight contraction of the M–M distances (thickness) of the sheet models due to a lack of binding to neighboring sheets relative to bulk MOF-2. The greater spread of the trans-linker distances (dM −M (trans-linker)) corresponds to a canting of the metal ion dimers and a flexing of the linker out of the plane of the sheet. Such a relaxation would be suppressed in the bulk structure, but still occurs somewhat as demonstrated by the spread of the M–O–C angles. This effect is most pronounced in the case of Zn-BDC, which will be discussed more later. Overall, the sheet model structures compare reasonably to the bulk, with deviations that can be rationalized by considering the lack of intersheet interactions present in the bulk crystal structure as well as the computational electronic structure method. Therefore, we proceed with this as a model of the M-BDC nanosheets and the surface of M-MOF-2 materials as it pertains to adsorption of small molecules. This decision will be assessed for validation later, considering effects of loading multiple adsorption sites as well as open-metal sites loaded with multiple molecules. Qualitatively, the structure of each M-BDC sheet is comparable to the others, with the greatest difference being in the M–O bond length as a function of metal identity. The CuBDC and Co-BDC sheets have the additional detail of ordering of the spins of the unpaired electrons. For Cu-BDC we studied all potential spin orderings that were accessible within a single unit cell (which has four unique metal ions) and found that metal ions whose interactions were mediated by a linker showed a negligible effect of spin alignment (energy differences of ∼1meV) and that metals near one another (coordinated to different oxygens of the same carboxylate group of a given BDC linker) preferred an antiferromagnetic state (their spins are anti-aligned with respect to one another) by roughly 70 meV (7 kJ/mol). This spin alignment preference allows for the pairing of the electrons in the formation of metal–metal bonds in the Cu-BDC and Co-BDC systems and was taken to be the ground state for systems with unpaired electrons and thus was used as the ground state spin ordering for Co-BDC. This is consistent with previous work involving ordering of unpaired electron

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spins in similar systems. 18 Table 2: Comparison of calculated M-BDC sheet and cluster model structures, M = Co, Cu, Zn. All sheet structures are predicted using periodic plane wave DFT with vdW-DF2, while clusters are using LCAO basis set DFT with ωB97X. Zn-BDC

Sheet Model Large Cluster, Optimized Large Cluster, Constrained Small Cluster, Optimized Small Cluster, Constrained

dM −M (thickness) [Å]

dM −O [Å]

dC−O [Å]

M–O–C angle [degrees]

2.63 2.76 2.67 2.76 2.67

2.05-2.06 1.91-2.12 2.00 1.91-2.11 2.01

1.29 1.26-1.28 1.27 1.26-1.29 1.26

113.8-131.5 108.7-140.8 123.4-123.5 107.2-139.0 122.5


dM −O [Å]

dC−O [Å]


2.51 2.51 2.53

1.99-2.00 1.95 1.94

1.29 1.27 1.27

118.4-122.7 121.0-121.1 119.8


dM −O [Å]

dC−O [Å]


2.74 2.18 2.19

2.00-2.06 1.89-1.90 1.90

1.29 1.27 1.27

114.5-134.9 117.3 116.4

Cu-BDC

Sheet Model Large Cluster, Optimized Small Cluster, Optimized

Co-BDC

Sheet Model Large Cluster, Optimized Small Cluster, Optimized

The structures of the optimized M-BDC cluster models, in contrast, show qualitative differences relative to both the sheet models and one another. We compare the sheet and cluster model geometries to one another in Table II. We observe a contraction of the M–M distance in the Co-BDC cluster relative to the sheet model, a possible indication that the cluster models better capture the formation of a metal–metal bond in these systems. The Co- and Cu-BDC clusters maintain a sheet-like geometry, while the Zn-BDC cluster shows a significant distortion toward a tetrahedral coordination environment, evidenced by the spread of the M–O–C angles. In the Cu-BDC and Co-BDC materials such a distortion is not observed; this is the result of pairing of the anti-aligned unpaired electrons in the Cu-BDC 12


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and Co-BDC materials forming a metal–metal bond along the axis of the dz2 orbital to fill the electron shell of these metals, leading to a square-planar-like coordination geometry for these metals in contrast to the tetrahedral-like geometry that the Zn-BDC cluster takes as a result of its already-full electron shell. This observation is consistent with the known tendency of Zn2+ ions to coordinate in tetrahedral environments and Cu2+ and Co2+ to coordinate in square planar environments. 56

Adsorption of Acid Gases We study the interaction of a range of small molecules and acid gases with M-BDC sheets at the undercoordinated metal sites that exist on the surfaces of these materials. We select molecules relevant to energy-related applications that may also be of concern for material stability, such as trace acid gases. 57–62 Binding Energetics For each molecule, starting binding motifs are based on those predicted for adsorption at open-metal sites in bulk MOFs 11 or are constructed to sample reasonable interactions with the open-metal sites and nearby ligands. In the case of each M-BDC + molecule pair, the lowest-energy configuration from among those studied (after optimization) is taken as the ground-state configuration for that system for all further study. We report these binding energies in Table III, while binding geometries are given in the Supporting Information. To facilitate comparison between the periodic DFT calculations, the optimized cluster calculations, and the rigid cluster calculations, we include additionally in Table III rigid periodic DFT calculations. In our fully optimized calculations (all calculations that are not denoted "rigid" above, with the exception of the CCSD(T) calculation, which is rigid but using the fully optimized MP2 geometry due to calculation cost limitations), all ions of the MOF nanosheet model and the adsorbed acid gas molecule are optimized. In the rigid models, the MOF nanosheet model ions are held rigid in the symmetry of the bulk structure 13



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Table 3: Binding Energies of acid gases to M-BDC sheets, M = Co, Cu, Zn [kJ/mol]. Gases are ordered by increasing strength of adsorption to Zn-BDC as predicted by periodic DFT calculations. Metal identity

Model

Theory Level

NO2

CO2

Zn

periodic

vdW-DF2 vdW-DF2, rigid B3LYP B3LYP, constrained ωB97X ωB97X, constrained B3LYP B3LYP, constrained ωB97X ωB97X, constrained MP2 CCSD(T)

24 20 21 38 33 47 23 40 35 53 26 44

42 38 16 35 30 43 19 33 31 46 31 35

52 46 30 54 41 61 34 56 44 67 36 35

76 65 36 58 52 70 43 62 58 79 57 55

83 75 57 77 73 88 64 82 78 96 71 73

136 55 32 53 49 66 36 54 52 70 43 45

vdW-DF2 vdW-DF2, rigid B3LYP ωB97X B3LYP ωB97X MP2

18 17 13 20 11 19 22

26 25 19 33 19 32 36

27 25 33 45 35 47 45

39 29 33 50 36 45 61

46 42 46 63 50 51 66

38 34 31 49 34 37 44

vdW-DF2 vdW-DF2, rigid B3LYP ωB97X B3LYP ωB97X

57 34 65 120 32 48

43 31 21 40 23 39

47 33 57 68 60 71

34 23 41 62 47 66

79 64 58 78 63 83

64 47 38 60 42 62

large cluster

small cluster

Cu

periodic large cluster small cluster

Co

periodic large cluster small cluster

CO H2 S H2 O SO2

and only the adsorbed acid gas molecule is optimized in the presence of the bulk-like MOF nanosheet model. These differences are critical to understanding the differences in the calculated binding energies reported in Table III. In addition to the binding energies we report, we calculate binding enthalpies at the level of a harmonic approximation, which we report in the Supporting Information. The conclusions we draw from our binding energy calculations, are consistent with those we can draw from our binding enthalpy calculations, although we expect our binding enthalpy calculations to compare more directly with experimentally observed heats of adsorption. We focus our discussion on the binding energy calculations because we compute binding enthalpies only for our fully optimized calculations, while we

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can get additional insights on the appropriateness of our models and methods by considering various geometric constraints, which we discuss below. The large and small cluster calculations agree well with one another, with the additional stabilization of the cluster by the larger ligand in the large cluster models generally correlating with a slight decrease in adsorption energy of the acid gas ligand to the more stable open-metal site. Similarly, the comparison between the optimized cluster and the rigid cluster models indicates a stronger adsorption of acid gases on the rigid cluster models. This is explained by the distortions stabilizing the open-metal site in the optimized cluster models that are not allowed in the rigid cluster calculations, which are constrained to bulk-like geometries. While the rigid calculations do not allow distortions to the open-metal site that stabilize the cluster and decrease the adsorption strength of acid gases, it also does not allow distortions to the open-metal site that would occur in the nanosheet materials that would increase acid gas binding strength. This is demonstrated by the comparison between the rigid and fully optimized periodic DFT calculations, which show systematically stronger binding energies for acid gases to the fully optimized structure as a result of distortions at the open-metal site from interaction with acid gas ligands. As a result of these competing distortions, the most appropriate comparison between our periodic DFT calculations and our cluster calculations is to compare the rigid calculations, which agree well across all theory levels represented. The correlated molecular orbital theory results support that the DFT results are providing good binding energies even though most of the binding energies are not that strong, with many binding energies less than 50 kJ/mol. Additionally, the periodic DFT results provide geometries more comparable to the macroscopic system as a result of the periodic boundary conditions. Overall, we find that ligands coordinate more strongly with Zn-BDC and Co-BDC than Cu-BDC, in agreement with previous understanding of coordination of ligands at openmetal sites in MOFs. 11 Generally, we find that H2 O, SO2 , and H2 S coordinate most strongly with open-metal sites, while CO, CO2 , and NO2 coordinate more weakly across the systems

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studied. As previously discussed, we observe the strongest binding in the fully optimized periodic sheet model calculations, while the cluster calculations allow for stabilization of the metal center and therefore coordinate more weakly to adsorbed ligands. These conclusions, however, are consistent regardless of the model employed. Below we discuss one particular case where this breaks down. A notable outlier highlighted by the ordering of the ligand species in Table III is the case of SO2 adsorption on Zn-BDC. Particularly, the fully optimized periodic sheet model indicates a much stronger coordination with the ligand than any of the constrained models or the optimized cluster models. This case illustrates the potential of the instability at the openmetal site created by the periodicity of the material to be important in facilitating strong ligand binding behaviors. In this particular case, a qualitative relaxation of the binding site away from a square planar or (accounting for the SO2 ligand) square pyramidal coordination environment toward an environment that more closely resembles a trigonal bipyramidal environment also enables a distortion of other nearby Zn ions toward the tetrahedral geometry observed in the optimized cluster calculations. For the same reasons related to metal–metal bond formation that we do not see such a distortion in the Cu-BDC and Co-BDC materials, we do not see such a strong difference in the fully optimized adsorption calculations with SO2 ligands on Cu-BDC or Co-BDC. The differences in the adsorption of SO2 between the rigid periodic DFT calculation and the optimized periodic DFT calculation are illustrated qualitatively in Fig. 5. This is illustrative of a case where both the rigid and optimized cluster models do not capture the same behavior as the optimized periodic DFT calculation as a result of substantial electronic interactions with the open-metal site being important in adsorption. Spectroscopic Properties For our periodic DFT calculations, we compute the vibrational frequencies of the normal modes of the adsorbed species studied. We compare these with the frequencies of the isolated

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Figure 5: Top: Constrained/rigid periodic DFT (vdW-DF2) calculation of SO2 adsorption on Zn-BDC. Bottom: Fully optimized periodic DFT calculation of SO2 adsorption on ZnBDC. Zn atoms are silver, oxygen atoms are red, carbon atoms are brown, and hydrogen atoms are pink. Images are of clusters for purposes of illustration only. molecules to compute frequency shifts upon adsorption, which we present in Table IV. For ease of comparison with experiment, the values of the free molecule frequencies are scaled to experimental frequencies 63–65 for these modes (see Supporting Information), with the corresponding scaling factor for each vibrational mode and method applied to the values for the vibrational frequency of the bound modes for computing frequency shifts. All frequency data and scaling factors are presenting in the Supporting Information. In general, we find that the magnitude of either red or blue shifts in adsorbed ligands’ vibrational mode frequencies are greater for ligands that are more strongly bound, in agreement with previously known behavior for ligands binding at open-metal sites. 11 The notable 17



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Table 4: Scaled frequency shifts of normal vibrational modes of ligand species upon complexation with M-BDC open metal sites. Calculations are based on adsorption to periodic sheet model compared to isolated gas molecules in 20 Å side-length box computed using vdW-DF2 and are reported in [cm−1 ]. Molecule/mode

Zn-BDC Cu-BDC Co-BDC

CO stretch

60

27

3

-1 -4 -11 -15

-5 -2 -7 -10

-4 -4 -12 -14

-66 -54 3

-40 -31 1

-61 -73 -4

-21 -19 -9

-10 -10 -4

-11 -13 -10

-117 -105 -52

-25 -28 -14

-99 -63 4

-45 -41 3

-29 -17 1

-49 -51 17

CO2 asymmetric stretch symmetric stretch bend bend H2 O asymmetric stretch symmetric stretch bend H2 S asymmetric stretch symmetric stretch bend NO2 asymmetric stretch symmetric stretch bend SO2 asymmetric stretch symmetric stretch bend

exception here is the case of NO2 , where orbital interactions with the unpaired electron of the NO2 ligand allow for a greater change in the electronic structure of the ligand and therefore a greater shift to the frequencies of the normal modes of the ligand. We additionally highlight that we observe a notable blue-shift of the CO vibrational mode, consistent with previous work on CO adsorption at open-metal sites in MOFs. 66 The frequencies for all modes for

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all ligands in both isolated and adsorbed cases are provided in the Supporting Information. Additionally, we have computed the frequencies for our cluster calculations at a variety of levels of theory. We have not provided those data here for brevity, but all data are provided in the Supporting Information. Coadsorption of Gases and Loading Effects All of the calculations presented thus far have focused on adsorption of a single acid gas molecule at an open-metal site on a model of a M-BDC nanosheet, while adsorption on actual nanosheets will also include possibilities of coadsorption or competitive adsorption at a single site or loading of multiple sites. Additionally, all of our models have included a single layer or part of a layer of a MOF-2-like structure as a representation of the M-BDC nanosheet, while actual nanosheets will not be entirely one layer thick. We have shown that our single-layer periodic model is qualitatively bulk-like and discussed how our cluster models can qualitatively differ from the bulk and periodic sheet structures. Here we further support the validity of such a model by demonstrating that adsorption energetics at a single primary binding site are not strongly changed when considering loading or coadsorption phenomena. To study loading effects, we consider NO2 , CO2 , CO, H2 S, and H2 O adsorbed to a primary site of Zn-BDC or Cu-BDC as well as loading two sites either of the same metal dimer or two sites on the same side of a sheet mediated by a BDC linker. We denote these as trans-sheet and trans-linker, respectively, and present our results in Table V. As shown in Table V, we find very little effect of loading on the strength of adsorption of acid gases to Zn-BDC and Cu-BDC nanosheets. We find no significant change in adsorption strength for trans-linker sites (roughly 11 Å distance between molecules) or trans-sheet sites (roughly 7 Å distance between molecules) as a result of simultaneously occupying a pair of sites with a single adsorbate. This also demonstrates that the MOF sheet effectively screens the interaction between gas molecules and that interactions between the adsorbates and the open-metal sites is more important than any intermolecular interaction between adsorbates. 19



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Table 5: Adsorption strengths of acid gases to Zn-BDC and Cu-BDC calculated using periodic DFT on a sheet model with vdW-DF2 to compare effects of loading multiple nearby primary adsorption sites on opposite sides of the same sheet (trans-sheet) or on the same side of a nanosheet mediated by a single organic linker (trans-linker). Energies are reported in kJ/(mol·molecule). Metal identity or binding motif

NO2

CO2

CO H2 S H2 O

Zn single adsorbate trans-sheet trans-linker

20 20 20

38 38 38

46 46 46

65 65 65

75 74 75

Cu single adsorbate trans-sheet trans-linker

17 17 17

25 25 25

25 25 25

36 36 36

42 42 42

This result additionally supports the validity of our use of a single-layer model for these multi-layered materials, demonstrating that the effects of coodination to the open-metal site on the opposite side of the sheet (trans-sheet) does not affect binding at a particular primary adsorption site. This is an important result, because in multi-layered materials, these transsheet sites coordinate with neighboring layers of this material in a network wherein a metal ion interacts electrostatically with the oxygen of the BDC linker of a neighboring layer, and vice-versa, leading to the bulk MOF-2 structure. These results therefore support the idea that our calculations are appropriate to describe surface sites in both single-sheet and multi-sheet materials. Table 6: Binding strengths of CO on Zn-BDC calculated using periodic DFT on a sheet model with vdW-DF2 to study possible coadsorption at a single openmetal site on M-BDC nanosheets. Energies are reported in kJ/(mol·molecule). System

Binding energy

Zn-BDC + CO Zn-BDC + 2 CO (equidistant) Zn-BDC + 2 CO (primary site-dominant)

48 24 31

To study coadsorption, we consider the case of two CO molecules competing for adsorp-

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tion at a single open-metal site on Zn-BDC, comparing the energetics on a per-molecule basis for binding motifs favoring a primary and secondary adsorption site as well as equidistant coadsorption. Our data, presented in Table VI, demonstrate that adsorption of a single molecule at a primary open-metal adsorption site is energetically the most favored configuration among those considered, and that loading of additional CO molecules onto the Zn-BDC material is likely to result in the occupation of secondary binding sites rather than fundamentally changing the nature of the primary binding site. This supports our study of a single molecule at a single adsorption site for purposes of understanding acid gas interactions with the surfaces of M-BDC nanosheet materials. While it is likely that in real systems coadsorption can also happen between different molecules, such a study is beyond the scope of this manuscript and will be investigated in future works. Here, we use this basic test to support our focus on the primary adsorption site.

Conclusions We have presented here a computational study of acid gas adsorption on models of the surface of M-BDC nanosheets or MOF-2 surfaces. These nanosheets are model materials for studying an "all-surface" MOF for purposes of gaining insight into external surface-specific phenomena related to MOFs in general. This area has not received nearly the attention that bulk phenomena have in MOFs, despite indications that MOF external surfaces are of importance for material stability, catalysis, and interfacing with other materials. Through our study of various models of MOF nanosheets, we have examined the qualitative and quantitative differences between multiple cluster models as well as a fully periodic model of Zn-BDC, Cu-BDC, and Co-BDC nanosheets. Through a comparison with bulk MOF-2 geometries and study of loading and coadsorption effects, we have supported the use of a single layer model as sufficient for studying adsorption of ligands on exposed reactive surface open-metal sites, and we have also demonstrated that focusing on a single molecule adsorbed

21



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at a primary adsorption site can be a reasonable model for ligand interactions at open-metal sites on these surfaces. We have used our models of MOF nanosheet surfaces to study the interactions of CO, CO2 , H2 O, SO2 , NO2 , and H2 S with the open-metal sites on Zn-, Cu-, and Co-BDC. We have found Zn-BDC and Co-BDC to coordinate most strongly with ligands, while Cu-BDC tends toward a weaker coordination across all methods employed in this study. Additionally, we predict that SO2 , H2 O, and H2 S tend to coordinate with open-metal sites more strongly than other ligands studied. Particularly, we have predicted that SO2 should coordinate very strongly (130 kJ/mol) with the open-metal site on Zn-BDC, a consequence of strong electrostatic and orbital interactions enabling an energetically favorable distortion of the Zn-BDC open-metal site toward a preferred tetrahedral or trigonal bipyramidal geometry from its square planar geometry in MOF-2 and M-BDC nanosheets.

Acknowledgement This work was supported as a 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. Some of the computational work was performed at the Molecular Science Computing Facility, William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for DOE by Battelle Memorial Institute under Contract No. DE-AC0676RLO-1830. We thank the Alabama Supercomputing Center for providing computational resources. D.A.D. thanks the Robert Ramsay Chair Fund of The University of Alabama for support.

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Supporting Information The Supporting Information for this manuscript includes computed binding enthalpies of the studied molecules to M-BDC nanosheets, vibrational properties of the studied molecules in free as well as bound environments, and structure files for periodic and cluster M-BDC materials. This information is available free of charge via the Internet at http://pubs.acs.org.

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