Computational Modeling of Adsorption of Xe and Kr in M-MOF-74

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Computational Modeling of Adsorption of Xe and Kr in MMOF-74 Metal Organic Frame Works with Different Metal Atoms Tijo Vazhappilly, Tapan K. Ghanty, and Bhagawantrao N. Jagatap J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02782 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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The Journal of Physical Chemistry

Computational Modeling of Adsorption of Xe and Kr in M-MOF-74 Metal Organic Frame Works with Different Metal Atoms Tijo Vazhappilly, Tapan K Ghanty*, B. N Jagatap Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400 085, India

Abstract Management of radioactive noble gases, i.e., Xe and Kr, needs special attention in the context of nuclear fuel reprocessing and postulated severe accident scenarios. Entrapment of these species in suitable matrix is generally difficult since Xe and Kr do not form bonds with substrates (physisorbed by van der Waals interactions) due to their chemical inertness. In recent years metal-organic frameworks (MOFs) are being actively discussed for adsorption / separation of various gases including noble gases. MOFs are considered superior in such applications owing to their high intake capacity, better selectivity and also the possibility of tuning the chemical properties by varying organic linkers or metal atoms. Theoretical modeling of MOFs and tailoring their properties appropriately for noble gas adsorption/separation are potentially useful to identify suitable candidates. In this regard, we investigate the binding energies of Xe and Kr in the well-known M-MOF-74 employing computer simulations. To understand the influence of different open metal sites on binding affinity, the central metal ion in M-MOF-74 are varied. Our results show that, Xe binds stronger than Kr and binding strength changes with the nature of the central metal atom in the MOF. The adsorption behavior obtained from our calculations is in good agreement with previous experimental and theoretical observations. The binding strength at different adsorption sites are favored by van der Waals interactions between noble gas atoms with MOF network and each other. Based on our studies, efficient MOFs for noble gas separation can be obtained within the MOF series by incorporating suitable metal centers.

Corresponding author: Email: [email protected], Phone: +91-22-25595089, Fax: +91-2225505151

1 ACS Paragon Plus Environment


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1. Introduction Radioactive noble gases (Ng) such as xenon and krypton, produced by nuclear fission, are released during processing of spent nuclear fuel and postulated nuclear reactor accidents. Entrapment of these species is necessary from the consideration of safety, storage and environmental issues.1 Of the radioactive Ngs produced aftermath of fission, 85Kr with a longer half-life of 10.8 years and

127

Xe with a half-life of 36.3 days are of particular concern in the

spent fuel processing.1 Moreover, the amount of Xe present in the reprocessing plant off-gas streams are much higher than Kr that roughly 10 mole of Xe is produced for every mole of Kr. Thus, the separation of Xe from Kr can considerably reduce the volume of nuclear waste to be stored for longer period. Apart from radioactive Xe and Kr, off gas streams from the spent nuclear fuel reprocessing plant also produces other radionuclides 3H2O,

14

CO2 and

129

I.

2

These

radionuclides are volatile, and efficient capture of these species is a demanding task. Normally, cryogenic distillation method is employed for separation (capture) of noble gases from the atmosphere. An alternative method to cryogenic distillation of noble gases is physisorption based separation by using porous materials. These porous materials include activated carbons, charcoal, zeolites and metal-organic frame works.3 . Due to their chemical inertness, Ng atoms in general do not make any conventional bonds with the substrate and poses difficult problem for gas separation. Noble gases are rather physisorbed (stabilized by van der Waals interactions) on these materials by charge and polarization effects. The physisorption processes are carried out at ambient conditions, which make them attractive due to lower production costs. Among various materials, MOFs have superior separation qualities due to high intake capacity, better selectivity and ability to tune chemical properties by varying organic linkers or metal atoms.1,

4

The

adsorbents such as zeolites and activated carbon have been tested for noble gas capture but have shown low capacity, selectivity, and lack of modularity.5-6 The zeolites have shown selectivity of 4-6 % with low uptake capacities of 20-30 % at ambient pressure and temperature.7 MOFs are composed of organic linkers connecting metal ion clusters8 with different pore diameters and pore topology. Metal organic frame works have large surface area (more than 6000 m2/g), high porosity (as high as 90% free volume) and excellent adsorption properties.9-11


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All these attributes make MOFs interesting class of materials for storage of gases, separation processes, catalysis and biomedical imaging.4, 9 The most interesting properties relevant to gas adsorption in MOFs are open metal sites, ligand polarizability12, pore size13, surface area and accessible void volume.3 In general, MOFs are synthesized by solvothermal processes and are thermally stable. In MOFs, gas molecules do not make any metal−gas bonding and are adsorbed by van der Waals interactions with open metal sites and organic linker atoms. In the literature, only a few MOFs have been investigated for the separation of Xe/Kr.3, 14-21 These experiments are generally performed at 100 kPa pressure and near room temperature. The gas separation abilities of MOF primarily depend on properties like adsorption selectivity (preference for either Xe or Kr), uptake capacity and diffusion along the pore channels1 The separation ability and uptake capacities follow different order among MOFs used for Xe/Kr separation. For example, the Ag nanoparticle embedded Ni-MOF74 show highest uptake capacity followed by unmodified Ni-MOF-74 and Co3(HCOO)6 metal formate MOF. The selectivity is higher for Co3(HCOO)6 followed by Ag@Ni-MOF74 and Ni-MOF74.1 The MOFs with larger number of open metal sites or polarizable groups directly reachable with uniform pore sizes (comparable to diameter of Ng atoms) are more successful in Xe/Kr separation.1,12 Recently, Praveen and co-workers have reported some excellent results on Xe/Kr adsorption in Ni-MOF-74.5, 16, 22

23

Very recently, they investigated the binding mechanism in Ni-MOF-74 by

using XRD experiments and DFT simulations.24 The higher uptake capacity and selectivity of Ni-MOF-74 as compared to other MOFs, zeolites and activated carbon inspires us to investigate the Xe/Kr adsorption behavior on other metal substituted MOFs in the M-MOF-74 series.1, 5, 14, 23 In this regard, we investigate the noble gas separation properties of different metal centers in MMOF-74 where “M” corresponds to metal ion chosen. The different metals chosen are M= Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Since open metal sites are the strongest binding sites in MOFs for Ng adsorption, effect of different metal ions on adsorption are very interesting.25 We also consider different uptake capacities in each pore i.e. six Ng attached to open metal sites (6Ng + M-MOF-74) and an additional Ng atom at the center of the pore (7Ng + M-MOF-74). The periodic structures of bare M-MOF-74 and with six and seven Ng atoms in the pore are shown in Fig. 1. Different M-MOF-74s are reported experimentally in the literature by varying the central metal atoms such as Mg, Mn, Fe, Co, Ni, Cu and Zn.25 The Ni-MOF-74 shows higher



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uptake capacity and selectivity for Xe and Kr as compared to many other MOFs. Ni-MOF74 has a Xe adsorption capacity of 55 wt % and Kr adsorption capacity of 3 wt % at 298 K and 100 kPa.5 The Xe/Kr selectivity is 5-6 and the isosteric heats of adsorption from experiments are around -22 kJ/mol. The first MOF-74 synthesized was Zn-MOF-74 by Yaghi group using zinc nitrate tetrahydrate and dihydroxy-1,4-benzenedicarboxylic acid as linker.26 This organic linker is in the tetraanionic form 2,5-dioxido-1,4-benzene-dicarboxylate (DOBDC), where both the aryloxide and carboxylate moieties act as ligands to the metal.27 It has a hexagonal, one-dimensional pore with size of ~11 Å. The central metal ion is in MII state. The other names for Zn-MOF-74 are CPO27-Zn or Zn-DOBDC. Similar naming convention is used for other metal substituted M-MOF74s which are reported experimentally such as Co-MOF-7428 , Ni-MOF-7429 and Mg-MOF-7427. The MOF-74 series has high density of open metal sites, which will be very relevant for gas separation. The M-MOF-74 has a honey comb three dimensional structure with each M atom binding to five oxygen atoms leaving one apex site vacant i.e. one open metal site forming distorted octahedron. It has a trigonal structure with α=β=90 and γ=120.24 The structures obtained from experiments for different M-MOF-74s show same structural topology with R3 space group with similar lattice constants.25 There are two types of M-O bonds in M-MOF-74: one is in plane with the open metal site (total 4) and one opposite to open metal site of an quasi octahedral metal cation.25 The computational studies of noble gas separation widely employed Grand Canonical Monte Carlo (GCMC) methods using Universal Force Fields (FF).

3, 12, 30-33

The Xe/Kr selectivity has

been investigated using GCMC calculations over 137000 MOFs with respect to pore limiting diameter, largest cavity diameter, and accessible void volume.30 Keskin et.al34-35 have studied separation properties of noble gas mixtures on M-MOF-74 series where M=(Ni, Co). They have performed GCMC and equilibrium molecular dynamics simulations on both adsorption and diffusion of noble gases such as Xe, Ke and Ar.

Perry et. al investigated the noble gas

adsorption isotherms in M-MOF-74 with central metal atoms Mg, Co, Ni and Zn.3 The nonbonded interactions between the Ng atoms and MOF network are accounted by Lennard-Jones potential and MOF structures are considered rigid during the GCMC calculations. These studies revealed that adsorption strength increases with polarizability of noble gas and no clear trend has


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been seen by varying metal atoms. The M-MOF-74 results are underestimated as compared to experimental observations and this might come from the neglect of gas polarizability and rigid lattice approximation.3 GCMC calculations treat noble gases as neutral spheres even though actual interaction between Ng atoms and MOF are point charge-induced dipole interactions. The results for Xe/Kr in MOF-74 series clearly show that UFF force field based Lennard-Jones potentials do not include the effect of gas polarizability, high open metal site density, and consequently larger deviation from experiments are observed with increase in noble gas polarizability.3 Since it is difficult to take into account the subtle interaction between the Ng atoms and the MOFs at the microscopic level accurately, it is not expected to obtain a clear trend by varying the metal atom in a particular MOF. However, it is possible to understand the NgMOF interaction quite accurately at the molecular level through first-principle based computational technique(s). Density functional theory (DFT) is one such technique and is a viable alternative to Force field calculations since they give moderately accurate results for large systems such as materials. Electronic structure calculations can give valuable information about structural, mechanical and sorption properties. The modeling of M-MOF-74s using DFT is challenging due to failure of conventional exchange correlation functionals (both LDA and GGA) in describing localized d –electrons of transition metals and van der Waals (vdW) interactions.25 These two drawbacks of DFT functionals can be improved using Hubbard U corrections and vdW corrected functionals, respectively. Therefore, in this work our objective is to carry out a systematic investigation of the adsorption of Xe and Kr on various M-MOF-74 by using vdW corrected and Hubbard U corrected DFT. The previous experimental3,5,24,36 and GCMC simulations3,35 were restricted to M=(Ni, Co, Mg, Zn) in M-MOF-74 series. Here we have considered several M-MOF-74s (all the experimentally reported MOF-74s M=(Mn, Fe, Co, Ni, Cu, Zn, Mg) and additional M=(Ti, V, Cr)) to investigate their structures and properties by using first-principles based highly accurate van der Waals interaction corrected density functional theory, and subsequently studied their Xe/Kr adsorption behaviors. To the best of our knowledge, adsorption of Ng atoms on M-MOF-74 systems by first principles based DFT investigation has only been reported for the Ni-MOF-74 system.24

Here it also may be

mentioned that wave function based MP2 calculations are reported in the literature for MgMOF74 for adsorption of small molecule.37 In this context it may be noted that Snurr et. al have



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studied CO2 adsorrption in diifferent M--MOF-74s where w M=((Mg, Mn, F Fe, Co, Ni,, Cu, Zn) employiing DFT andd MP2 level of theory.338

Figure 1. The left panel depiccts the perioodic represeentation of tthe bare M-MOF-74. T The central panel shhows that six Ng atom ms are adsorrbed on the metal atom ms in the heexagonal poores of MMOF-744. The rightt panel show ws that seveen Ng atomss are adsorbbed on the m metal atoms and at the center oof the hexagonal pores oof M-MOF--74.

2. Com mputation nal Detailss Densityy functionall theory is of particullarly advanntageous forr studying large moleecular and periodicc systems ((e.g., MOFs) as well as systemss containing heavy ellements, beecause the computaational costt for solvinng the Kohnn-Sham equuations is low comparred to wavee function theory bbased methoods. Dependding on the particular pproperties of interest, pperiodic com mputations of the properties off a MOF cann be carriedd out on the experimenttally determ mined crystal structure or on a computatioonally optim mized structuure. In this study, bindding energyy of Xe and Kr in MMOF-744 is investiggated with DFT to undderstand thee adsorptionn mechanism m. DFT pllane wave calculattions are pperformed uusing VAS SP5.239 proggram with Projected Augmentedd Wave40 potentiaals. All calcuulations aree performedd with PBE41 exchange--correlation functional with D242 van der Waals corrrections andd Hubbard ccorrection pparameter, U for metal d-orbitals along a with spin-pollarization innto account . The U parrameters forr open-shelll 3d metals are taken frrom recent literaturre,25 except for Ni, whicch is taken from f Ref 24. The geomeetry optimizzations are pperformed with a pplane wave energy cuttoff of 5200 eV and 2× ×2×2 Brillouin zone saampling. Alll bare MMOF-744s are fully relaxed andd the latticee parameterss and atom positions arre allowed to t change. Once baare M-MOF F-74 structuures are opttimized theen lattice paarameters arre kept fixeed and all


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atoms are allowed to relax during Ng-M-MOF-74 structure optimizations. In all the geometry optimization calculations a force convergence criteria of 0.01 eV/ Å is used. The Adsorption energy (binding energy) is calculated as Eads = EMOF+Ng – EMOF – ENg where EMOF+Ng, EMOF and ENg denote the energies of the M-MOF-74 with adsorbed noble gas atoms, bare M-MOF-74 and the isolated noble gas atom, respectively. Negative adsorption energy means favorable binding i.e. exothermic reaction.

3. Results and Discussions For the crystal structures of different M-MOF-74s, we start from the optimized Ni-MOF-74 structure obtained from DFT simulations in recent literature24. The unit cell has a trigonal structure with 54 atoms. All the M-MOF-74s chosen here have similar topology with same R3 space group.43 Thus, in the present work, the initial crystal structures for each bare M-MOF-74 are obtained by replacing the central Ni atom with corresponding metal atom (M) in each MOF in the above mentioned Ni-MOF-74 structure. Subsequently, all the bare MOF structures are fully relaxed by optimizing the lattice parameters and atom positions. The lattice parameters obtained for each M-MOF-74 from our simulations and previous theoretical calculations25 are given in Table 1. In general, the lattice constants from the current study match very well with the computed values from recent work.25 For different metal atoms (M), the change in lattice parameters are found to be within 1 Å and 0.43 Å range for lattice constants a and c respectively. The lattice parameters do not show any clear trend in going from left to right in the 3d transition-metal series. This is in agreement with previous theoretical calculations on M-MOF-74s.44 The lattice constants for Ni-MOF-74 obtained from our calculations are also close to the values reported in recent literature24. The lattice parameters a and c from the current study are 25.73 Å and 6.72 Å as compared to the 25.82 Å and 6.76 Å from recent literature24 respectively. Furthermore, DFT calculations with a different van der Waals dispersion-corrected density functional (vdW-DF2) and Hubbard U correction (U =6.4) gave a = 25.97 Å and c = 6.84 Å.25 We also compare the lattice constants with parameters



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obtained from XRD measurements for Mn, Fe, Co, Ni, Cu, Zn and Mg. Surprisingly, in most of the cases the deviation is around 1% except of Mn where it is close to 2%. The computed magnetic moment for each metal atom in the M-MOF-74 shows a high-spin ground state.25 However, varying the metal ion in the MOF structure also changes the M-O bond length between metal atom and the ligand O atoms. This could change the ionic character of metal and linker atoms.3 For the starting structures of Ng atoms adsorbed on M-MOF-74s, the initial positions of Ng atoms are taken from recent work24 dealing with Ni-MOF-74 where Xe/Kr atoms are attached to the open metal sites (uptake capacity of six) and also to the center of the pore (uptake capacity of seven). These are the energetically favorable sites from both DFT simulations and XRD measurements.24 During the adsorbate-MOF structure optimization, the lattice is kept fixed at the optimized bare MOF geometry obtained in the present work and all the atoms are allowed to relax. The energies for isolated Ng atoms are calculated at optimized adsorption positions using the same super cell. For the calculation of ENg in 6Ng+M-MOF-74 systems, only one Ng atom is taken in the super cell and total energy for six Ng atoms are obtained by multiplication with 6. Subsequently, the binding energies per Ng atom, Eads(Ng) are calculated for each M-MOF-74 with different uptake capacities i.e. 6 Xe + M-MOF-74, 7 Xe+M-MOF-74, 6 Kr + M-MOF-74 and 7 Kr + M-MOF-74. For the uptake capacity of seven, the binding energy Eads(7th Ng) is calculated for accommodating 7th Ng atom at the center of the hexagonal pore with six Ng atoms. Table 1: Optimized Lattice Constants of M-MOF-74s from Current Study and Previous Theoretical Calculations Metal (M)

Recent work25 (PBE-vdW-DF2+U) Å

Current study (PBE-D2+U) Å a

c

a

c

Ti

26.166

7.043

26.46

7.17

V

26.240

6.976

26.47

7.15

Cr

26.363

6.833

26.62

6.70

Mn

26.736

6.888

26.55

7.16


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a

Fe

26.273

6.874

26.47

6.97

Co

26.048

6.733

26.18

6.90

Ni

25.726

6.723

25.97(25.82a)

6.84(6.76a)

Cu

26.189

6.611

26.17

6.36

Zn

26.249

6.928

26.20

6.92

Mg

26.050

6.733

26.05

6.91

24

Lattice constants from recent literature

The adsorption energy of Xe for different M-MOF-74s with uptake capacities of six and seven is given in Table 2. In Ni-MOF-74, we have obtained Eads of -33.87 kJ/mol for Xe with a loading of six Xe atoms inside the pore. This is in good agreement with -34 kJ/mol from similar DFT simulations in recent literature24. The interatomic distances for M-Ng and Ng-Ng (Ng atoms attached to metal atoms) from the optimized adsorbate-MOF structures are shown in Fig. 3. The interatomic distances from our relaxed MOF structure are 3.32 Å and 4.60 Å for Ni-Xe and XeXe as compared to 3.33 Å and 4.57 Å, respectively, from recent literature.24 From the XRD measurements, Ni-Xe and Xe-Xe distances are 3.395 Å and 4.482 Å, respectively. This shows that our optimized MOF geometry has very close structural properties as compared to previous experimental and theoretical results. For the 7Xe+Ni-MOF-74, the binding energy of 7th Xe is 17.89 kJ/mol, which is lower than the -22 kJ/mol from recent work24. For this system, we observe the same trend as in recent work24 that Ni-Xe (3.38 Å) length is increased and Xe-Xe (4.52 Å) distance is reduced compared to 6Xe+Ni-MOF-74. In recent work24, they are 3.36 Å and 4.55 Å respectively. Thus, the incorporation of 7th Xe atom at the center of the pore changes the interatomic distances slightly as compared to loading of six atoms in the pore. Apart from that, Eads(7th Xe) is much smaller than Eads(Xe) in the current study and previous theoretical investigations for Ni-MOF-74.24 The reason for lower Eads(7th Xe) as compared to Eads(6 Xe) can be attributed to the increased steric repulsion between the 7th Xe atom and the neighboring six Xe atoms, thereby reducing the preference of adsorption. The optimized unit cell of Ni-MOF-74 with seven Xe atoms is shown in Figure 2. Analyzing the binding energies in 6Xe+M-MOF-74 systems, Ti and Ni are associated with with highest binding energies while Mn and Cu have the least binding energies. The Eads(Xe) follows this trend Ti > Ni > Co > Zn > Cr > Fe > V > Mg > Mn > Cu among different metal atoms in M-MOF-74s. The Zn-MOF-74 has lower heats of



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adsorption as compared to Co and Ni MOF-74s due to screening from completely filled dorbitals, which would screen metal ion interaction with incoming adsorbate atoms.3 We see similar trend from our DFT simulations that binding energy for Xe/Kr is higher for Zn than for both Ni and Co MOF-74s. For the adsorption of 7th Xe atom at the center of the pore, all MMOF-74s show very close binding energies, except V-MOF-74. The binding affinity for VMOF-74 is almost double than the next largest binding affinity obtained. This could be due to larger variation in Xe-Xe bond lengths induced by 7th Xe adsorption from the equilibrium position in 6 Xe+V-MOF-74. The reason for close Eads(7th Xe) among M-MOF-74s could be due to same local environment. Here, the 7th Xe atom is surrounded by six Xe atoms in all the MOFs, which are arranged in a hexagonal shape. The interactions of the 7th Xe atom with the open metal sites and other MOF atoms are weak due to large interatomic distances from the 7th Xe atom. In both systems, the M-Xe and Xe-Xe distances are changed very little among different metal atoms. This could be due to same vdW radii and dispersion (C6) coefficients for 3d transition metals in the Grimme’s D2 corrections.45 Apart from that, closely packed hexagonal arrangement of Ng atoms in the pore restricts larger deviation in M-Ng and Ng-Ng distances in different M-MOF-74s. Table 2: The Adsorption Energies for Xe Binding (per Xe atom) in Various M-MOF-74s with Different Uptake Capacities Metal (M)

6Xe + M-MOF-74 7Xe + M-MOF-74 Eads(Xe) (kJ/mol)

Eads(7th Xe) (kJ/mol)

Ti

-35.66

-20.35

V

-14.73

-42.38

Cr

-21.02

-20.22

Mn

-6.47

-19.23

Fe

-17.69

-19.75

Co

-23.05

-21.60

Ni

-33.87

-17.89

Cu

-6.01

-21.44


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Zn

-22.87

-211.06

Mg

-14.07

-211.34

Figure 2. The optimized unitt cell of Ni-M MOF-74 wiith seven X Xe adsorbed on the Ni metal m sites and alsoo at the cennter of the ppore is show wn. Here X Xe atoms arre in blue, N Ni atoms inn green, O atoms inn red, C atom ms in grey aand H atoms in white color.

In Tablee 3, the binnding energiies for Kr aatoms from 6Kr+M-MO OF-74 and 7Kr+M-MO OF-74 are given. A Among 6Krr+M-MOF-774 systems, the calculaated Eads(Kr)) is smaller than Eads(X Xe) for the respective metal attoms. This clearly show ws that Xe atoms are adsorbed a m more stronglyy than Kr. In a clooser inspecttion, the binding energgies of Xe are generaally 8-10 kJJ/mol higheer than Kr indicatinng that MO OF-74 can efficiently e s separate Xee from Kr. The strongger binding of Xe as compareed to Kr is due to highher atomic polarizabiliity of Xe. The atomicc polarizability of Xe (27.34 aau) is rougghly 10 au higher thann Kr (16.7440 au).46 O Our calculatted Eads(Kr)) (-23.72 kJ/mol) for Ni-MO OF-74 is veery close too the -24 kkJ/mol repoorted in recent work24. The Kr binding energies arre generallyy favorable (negative) for most off the metalss except Cuu and Mn. The binnding affinityy for Kr shoows similar trend in M--MOF-74 thhat is observved in the case of Xe. The Eads(Kr) follow w this order among diffferent MOF--74s, Ti > N Ni > Co > C Cr > Zn > F Fe > Mg > V > Cu > Mn. Thee Eads(7th Krr) is much sm maller than the Eads(Krr). This reduuced bindinng strength



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for 7th Kr atom at the center of the pore is similar to the reduced 7th Xe atom adsorption affinity calculated for different M-MOF-74s. The calculated Eads(7th Kr) (11.08 kJ/mol) for Ni-MOF-74 matches very well with the -10 kJ/mol reported previously in recent work.24

The interatomic

distances for Ni-Kr and Kr-Kr are also found to be in the same range as reported by Praveen et. al.24 Apart from that, in all M-MOF-74s, Ng-M distances are larger for Xe while Ng-Ng distances are shorter for Xe as compared to that in Kr cases. The current predictions about adsorption sites in MOFs are in good agreement with the literature that open metal sites are the strongest binding sites in MOFs. Table 3: The Adsorption Energies for Kr Binding (per Kr atom) in Various M-MOF-74s with Different Uptake Capacities

Metal(M)

6Kr + M-MOF-74 7Kr + M-MOF-74 Eads(Kr) (kJ/mol)

Eads(7th Kr) (kJ/mol)

Ti

-24.66

-6.01

V

-3.45

-10.60

Cr

-12.83

-10.42

Mn

+3.44

-8.69

Fe

-9.40

-3.60

Co

-13.34

-9.33

Ni

-23.61

-11.75

Cu

+2.51

-10.84

Zn

-11.95

-13.71

Mg

-4.53

-10.52


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Figure 3. The inteeratomic distances betw ween hexagoonally arrangged Ng atom ms (Ng-Ng)) and to M atoms (N Ng-M) in M-MOF-74 M aare shown fo for different uptake capaacities.

To furthher understaand the nobble gas adsoorption mecchanism in different M M-MOF-74s, we have perform med Bader chharge analyysis47 on barre M-MOF--74 and 6 X Xe + M-MOF F-74. The ccharges on M and llinker O atooms before aand after thee Xe adsorpption are listted in Tablee 4 along w with charge on adsorbed Xe atooms. We nottice that all the metal atoms a are in M2+ oxidattion state evven though a strict judgment oof oxidationn state from m charge annalysis is noot feasible. The chargee on each metal attom is in thhe range of +1e to +2e,, which is inn good agreeement withh previous ttheoretical calculattions in M-M MOF-74.44 The O atooms (5 of tthem) connnected to m metal atoms possess a partial negative n chaarge of ~-00.1e to -0.3ee. The otherr atoms, C aand H in thee MOF netw work show little chaarge before and after thhe adsorptioon. The charrges on M and a O atomss in the MOF F are only slightly changed aft fter the Xe aadsorption. T The Xe atom m show +0.002e to +0.044 e charge, indicating the pressence of weak van der W Waals interractions. Thus the Badeer charge annalysis show ws that the charge ttransfer from m the adsorbbed Xe atom m to the MO OF network atoms is verry small.



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Table 4: Bader Charge Analysis of 6Xe+M-MOF-74 and M-MOF-74 Optimized Structures Metal (M-MOF-74)

bare M-MOF-74 (e)

6Xe+ M-MOF-74 (e)

M

O (6)

M

O

Xe

Ti

+1.60

-1.19

+1.59

-1.19

+0.03

V

+1.46

-1.19

+1.44

-1.2

+0.03

Cr

+1.40

-1.16

+1.40

-1.14

+0.02

Mn

+1.48

-1.21

+1.47

-1.20

+0.03

Fe

+1.41

-1.19

+1.40

-1.18

+0.03

Co

+1.32

-1.15

+1.31

-1.15

+0.03

Ni

+1.26

-1.13

+1.25

-1.14

+0.04

Cu

+1.15

-1.10

+1.14

-1.10

+0.03

Zn

+1.36

-1.16

+1.34

-1.16

+0.03

Mg

+1.68

-1.31

+1.68

-1.31

+0.02

The heats of adsorption for Xe are greater than Kr from experimental measurements.3, 24, 36 This trend is consistent for different uptake capacities of Ng atoms in the pore (one, six and seven Xe/Kr atoms in the pore). The open metal sites are the strongest binding sites (primary sites) observed in experiments. The second occupation site for noble gas atoms in M-MOF-74 is at the center of the pore, which has weaker binding as compared to open metal sites. The experimental investigation in Ref

36

on Ni-MOF-74 and Mg-MOF-74 finds another adsorption site close to

carboxylate and phenolic oxygen atoms. This adsorption site is found to be unstable from DFT calculations reported in recent literature.24 Thus in the current study, we investigated the adsorption affinity only at open metal sites and center of the pore. From the experiments, the NiMOF binds Xe/Kr stronger than Mg-MOF and this might be due to larger polarizability of Ni as compared to Mg.36 The heats of adsorption for Xe and Kr in Mg-MOF-74 are -22 kJ/mol and -18 kJ/mol, respectively. In Ni-MOF-74, these values are -30 kJ/mol and -19 kJ/mol for Xe and Kr, respectively. The experimental isosteric heats of adsorption for Xe in Ni-MOF-74 is -22 kJ/mol in Ref5. Regarding the bond length between Ng and metal sites, it falls in the range of sum of vdW radii of metal and noble gas atoms. In fact, the attractive interactions with other MOF


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atoms decrease the Ng-M distance.24 The experimental studies on M-MOF-74s(M=Ni, Co, Mg and Zn) at different temperatures and pressures predicted distinct adsorption characteristics for Xe and Kr.3 The adsorption isotherms are linear for Kr and non-linear for Xe at different pressures among these MOFs.3 The heats of adsorption for Ng atoms show distinct values but no clear trend with varying metal ions. Based on the current study and comparison with previous experimental observations, the following assumptions are valid for noble gas adsorption in MMOF-74 a) Xe binds stronger than Kr b) Open metal atoms are the strongest binding sites c) The variation of metal atom (M) in M-MOF-74 influences the binding affinity 4. Conclusions In this paper, we show that one can control the binding strength of Xe/Kr in M-MOF-74 by varying the central metal ion in the MOF network. The screening of metal electrons in 3d orbitals is different for early and late transition metals and can influence the interactions with the Ng atom due to their orbital filling. Due to the larger polarizability of Xe as compared to Kr, XeXe interactions are more favorable than Kr-Kr interactions. The DFT simulations show that Xe has higher binding energy than Kr due to larger polarizability of Xe as compared to Kr. Also, the binding affinity is highest at the open metal sites as compared to the center of the pore. These two predictions are in excellent agreement with the reported experiments and previous theoretical calculations. Among different M-MOF-74s investigated here, the favourable negative binding energies are found to be the highest for the Ti-MOF-74 system for both Xe and Kr adsorption. For the adsorption of 7th Ng atom at the center of the channel, all of the metal atoms gave negative binding energy and most of them have very close energies irrespective of the metal atom. The binding energies obtained from DFT calculations should be corrected for temperature effects to get the heats of adsorption obtained from experiments.38

Previous

theoretical studies of small molecule adsorption on M-MOF-74 showed that zero-point energy and thermal corrections are expected to be few kJ/mol (~4 kJ/mol) and might not change the binding energy behavior considerably.43-44



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Finally, the present study based on computer simulations provides valuable information in designing superior materials for entrapment of Xe and Kr. There are so many porous materials reported in the literature and experimental (synthesize and sorption) analysis of all these materials for noble gas separation is a very time consuming task. The computational modeling of these materials will be much easier task to choose the best materials for noble gas separation. Similar modeling strategies can also be employed for finding the efficient materials to trap other nuclear fission off-gas components, for example, radioactive iodine. ACKNOWLEDGMENT TV thanks K S Krishnan Research Associate fellowship from BRNS for funding. BARC Anupam supercomputing facility is gratefully acknowledged for computing support.

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Table off Content Graphicss

M-MOF-74

Ng

Ng+M-MOF F-74


Page 20 of 20

Computational Modeling of Adsorption of Xe and Kr in M-MOF-74

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