Understanding the Adsorption Mechanism of Xe and Kr in a Metal

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Understanding the Adsorption Mechanism of Xe and Kr in a Metal Organic Framework from X-Ray Structural Analysis and First-Principles Calculations Sanjit Ghose, Yan Li, Andrey A Yakovenko, Eric Dooryhee, Lars Ehm, Lynne E. Ecker, Ann-Christin Dippel, Gregory J Halder, Denis M. Strachan, and Praveen K. Thallapally J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00440 • Publication Date (Web): 16 Apr 2015 Downloaded from http://pubs.acs.org on April 22, 2015

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The Journal of Physical Chemistry Letters

Understanding the Adsorption Mechanism of Xe and Kr in a Metal Organic Framework from X-Ray Structural Analysis and First-Principles Calculations Sanjit K. Ghose1*, Yan Li2, Andrey Yakovenko3, Eric Dooryhee1, Lars Ehm1, 4, Lynne Ecker5, Ann-Christin Dippel6, Gregory J. Halder3, Denis M. Strachan7, and Praveen K. Thallapally8* 1

Photon Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973, USA.

2

Computational Science Center, Brookhaven National Laboratory, Upton, NY 11973, USA. 3X-

ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA, 4Mineral Physics Institute, Stony Brook University, NY 11794, USA. 5Nuclear Energy Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973, USA. 6

Deutsches Elektronen-Synchrotron DESY, D-22607, Hamburg, Germany. 7DM Strachan, LLC,

Bend, OR. 8Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Phone: (631) 344-3611 E-mail: [email protected] Phone: (509)-371-7183

ACS Paragon Plus Environment

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ABSTRACT: Enhancement of adsorption capacity and separation of radioactive Xe/Kr at room temperature and above is a challenging problem. Here, we report a detailed structural refinement and analysis of the synchrotron X-ray powder diffraction data of Ni-DODBC metal organic framework with in situ Xe and Kr adsorption at room temperature and above. Our results revealed that Xe and Kr adsorb at the open metal sites, with adsorption geometries well reproduced by DFT calculations. The measured temperature-dependent adsorption capacity of Xe is substantially larger than that for Kr, indicating the selectivity of Xe over Kr and is consistent with the larger adsorption energy (dominated by van der Waals dispersion interactions) predicted from DFT. Our results reveal critical structural and energetic information about host-guest interactions that dictate the selective adsorption mechanism of these two inert gases, providing guidance for the design and synthesis of new MOF materials for separation environmentally hazardous gases from nuclear re-processing applications.

TOC GRAPHICS

KEYWORDS: Metal-organic frameworks, X-ray diffraction, Density functional calculations, van der Waals interactions, Noble gases.


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The noble gas Xenon occurs naturally in the atmosphere at 0.087 ppmV, while Krypton occurs at 1.14 ppmV. Xenon is separated from air as a by-product in the fractional distillation of air to separate nitrogen and oxygen. Radioactive Xe enters the atmosphere from nuclear detonations, during the reprocessing of the irradiated nuclear fuel, in the production of medical isotopes, and from nuclear accidents such as the recent catastrophe at Fukushima Daiichi Nuclear Power Plant in Japan.1-2 Monitors for radioactive Xe isotopes are stationed around the globe to detect nuclear explosions banned under the Comprehensive Test Ban Treaty.3 Xe has several applications in industry, medicine and in day-to-day science. For example, Xe can be used in commercial lighting, imaging, anesthesia, and neuro-protection and nuclear magnetic resonance spectroscopy. Thus, capture and separation of Xe from air is important commercially and for atmospheric monitoring. Similarly, separating Kr from Xe is an important step in removing radioactive Kr during the reprocessing of irradiated nuclear fuel.4 The existing method uses cryogenic distillation to separate these gases at low temperatures, which is an energy intensive process. Even after such energy intensive separation processes, presence of any radioactive Kr in the pure Xe phase in not allowed to further distribution of Xe4. Therefore, discovery of new materials and processes operating at room temperature to reduce Kr to allowable (in Xe) levels could lead to a new source of Xe for nuclear re-processing industry.. Previous scientific reports suggest, zeolites and carbons were tested to separation Xe and Kr at near room temperature.4-5 For example, Na-based zeolites shown to selectively adsorb Xe over Kr with approximate selectivity of four to six.4-5 Unlike zeolites, metal−organic frameworks (MOFs) materials are composed of metal clusters connected with diverse set of organic building blocks via strong coordination bonds to produce three dimensional network structures with accessible porosity.6-13 One of the attractive features of the MOF materials includes the synthetic


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tunability to adjust the pore size, shape, and structure to improve the sorbate−sorbent interactions.14 In this regard, MOFs have been explored for wide variety of gas separation applications over the past decade but very few scientific reports dealt with noble gas storage and separation.15-16

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For example, we have reported single component gas adsorption in several

microporous MOFs including MOF-5 and Ni-DOBDC (DOBDC = 2,5-dihydroxyterephthalic acid) which suggest Ni-DOBDC adsorbs significantly more Xe than MOF-5 and activated charcoal.18-20 Similarly, adsorption of Xe, Kr, and other lighter rare gases in various MOFs coupled with molecular modelling approaches were reported that exhibit preferential adsorption of Xe over other rare gases.

17, 21-22,

Further, we demonstrated that the adsorption capacity and

selectivity of Ni-DOBDC for Xe over Kr is far superior than any MOF tested using single and two-column breakthrough method at nuclear re-processing conditions.23 However, the mechanism of adsorption sites of these fission gases into Ni-DOBDC and its analogs were largely unknown. In this article for the first time we present results from a detailed study of these noble gas adsorption sites and mechanism in Ni-MOF-74 [Ni-DOBDC, DOBDC = 2,5-dioxido-1,4benzene-dicarboxylate; also known as CPO-27-Ni or Ni(dhtp)] at room temperature and above. We systematically investigate the Xe and Kr adsorption capacity and Xe/Kr selectivity with insitu X-ray diffraction (XRD) measurements. Our results provide a fundamental understanding of the adsorption structure and guest-host interactions by combining direct structural analyses from experimental XRD data and Density Functional Theory (DFT) calculations.


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(a)

(b)

(c)

Figure 1. Xe and Kr adsorption into the honeycomb network structure of Ni-DOBDC. Rietveld refinement of the structure with best fit to the XRD data for (a) empty Ni-DOBDC MOF after activation and Ni-DOBDC with (c) Xe adsorption and (e) Kr adsorption.


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The Ni-DOBDC MOF has a three-dimensional honeycomb-like network with helical chains of nickel oxygen octahedra running along the crystallographic axis. The detailed synthesis process and structure of the activated Ni-DOBDC MOF is described in the Supplementary Information. The evolution of XRD pattern during activation of the MOF is monitored to observe the desorption process in situ (Supplemental Figure S1). The structural model derived from the Rietveld refinement for the activated Ni-DOBDC structure with best fit of the XRD profile (Figure 1a). The XRD analysis (Supplementary Table S1) revealed that the long-range order of the activated framework remains intact under He gas flow at room temperature. The refined atomic structure for the empty MOF is shown in Figure S3a. The atomic distance between Ni-Ni in the hexagonal structure is 2.914(3) Å. Overall the effective honeycomb pore size remains close to ~11 Å. First principle DFT calculations reproduced the experimental lattice parameters within 1%. In situ XRD measurements were carried out during the sequential He-Xe-He loading and He-Kr-He loading cycles, and the obtained XRD patterns clearly show the differences upon Xe and Kr adsorption into the activated Ni-DOBDC at 298 K (Figure S1a and Figure S1c), accompanied by changes in the Residual Gas Analyzer (RGA) profiles for the exiting gas stream. Sequential Rietveld refinements were performed on the structure model to fit the XRD patterns from the bare framework and the MOF with different loadings of each gas. The starting structural model was derived from the difference Fourier analyses of the data from the fully gasloaded Ni-DOBDC data. Structural parameters of the best-fit structural models obtained with Rietveld refinement are listed in Supplementary Table S2 and S3. Two adsorption sites were revealed for Xe in Ni-DOBDC (Supplementary Figure S2). The Xe atoms at the primary sites (Xe1) were found to be decorated in the near-hexagonal symmetry in co-ordination with the Ni sites as viewed through the honeycomb channels. These findings are consistent with earlier


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findings using non-inert gases that the open metal sites in MOFs play a crucial role in the hostguest interaction.24 Difference Fourier analyses also revealed a secondary adsorption site (Xe2) at the center of the hexagon, the inclusion of which improved the fit quality (Supplementary Figure S2). However, the Xe2 site occupancy (6.5%) is substantially lower than that of Xe1 (42%) (Supplementary Table S2). The honeycomb cage remains intact upon Xe adsorption with Ni-Ni distance 2.838(5) Å. The atomic distances are Xe1-Ni, 3.395(7) Å; Xe1-Xe1, 4.482(4) Å; and Xe1-Xe2, 4.481(4) Å. The final refined structure for the Xe adsorbed Ni-DOBDC MOF is shown in Figure S3b. The adsorption sites derived from analysis of X-ray diffraction measurements are further confirmed by DFT calculations (Table 1). For a single Xe atom in Ni-DOBDC, the lowest energy adsorption position was found close to the experimental Xe1 site with ݀Xe-Ni = 3.32 Å. The computed adsorption energy (Ead) of -31 kJ/mol is mainly dominated by van der Waals (vdW) dispersion interactions (~95%), with the largest contribution (21%) from the Xe-Ni pair. Table 1. Calculated structural parameters and adsorption energies of Xe and Kr with different loadings in Ni-DOBDC. Loading

Xe dXe-Ni

dXe-Xe

Kr Ead

dKr-Ni

-31

3.26

dKr-Kr

Ead

1

3.32

-22

6

3.33

4.57

-34

3.27

4.64

-24

7

3.36

4.55

E7th

3.28

4.62

E7th

=-22 Exp

3.41

4.48

=-10 3.26

4.59

E7th corresponds to the adsorption energy of the 7th atom at the center of MOF channel. All distances are in Å and energies in kJ/mol.


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Ead (a)

(b)

(c)

0 kJ/mol

-35 kJ/mol

Figure 2. Guest atoms positions inside the honeycomb network structure of Ni-DOBDC. Contour plots of Xe adsorption energy per atom (Ead) in Ni-DOBDC on the (001) half-plane, with a loading of (a) one (b) six and (c) seven Xe atoms per unit cell. In (c), six Xe atoms are fixed at equilibrium positions at the primary sites and Ead is taken as that of the 7th Xe atom. The MOF structure was fixed at the optimized geometry and only regions with exothermic adsorption (Ead ≤ 0) are shown in the contours. At a loading of six Xe atoms per unit cell, ݀Xe-Ni is slightly increased to 3.33 Å, whereas Ead is enhanced by about 3 kJ/mol due to the attractive Xe•••Xe interactions. The similar adsorption position and energy in the two cases can also be clearly seen from the contour plots of Ead on the (001) half-plane shown in Figure 2a and 2b. A distinctive difference is the sharp increase of Ead from negative (exothermic) to positive (endothermic) in the latter case due to unfavorable Xe•••Xe repulsion at close distances as the Xe atoms move toward the center, leading to a narrow region of exothermic adsorption. The insertion of a seventh Xe atom at the Xe2 site (Figure 2c) results in an adsorption energy of -22 kJ/mol, indicating the energetic preference for adsorbing additional Xe atoms in the MOF unit cell. In addition, the Ni−Xe distance is elongated by 0.03 Å while the Xe−Xe distance is shortened by 0.02 Å as a result of the competition between Xe•••MOF and Xe•••Xe interactions in the closely packed heptamer. More generally,


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the adsorption energy at the Xe2 site depends on the occupancy of the Xe1 sites and is nearly vanishing when the Xe1 sites are all unoccupied, consistent with the measured low occupancy of the Xe2 site. A recent XRD study25 on Xe in Ni-DOBDC below room temperatures reported a substantially shorter Xe-Ni distance (3.0 Å) at the primary site compared to our XRD measurements (3.4 Å) and DFT results (3.3 Å). We repeated the calculations using the vdW density functional26 and obtained a Xe-Ni distance of 3.5 Å. One notes that the sum of vdW radii of Ni and Xe is about 3.8 Å, although interactions between Xe and the rest of the MOF wall tend to shorten the Xe-Ni distance. Similar discrepancy exists for Kr adsorption sites but to a smaller extent. In addition, the proposed second type of adsorption site in Ref 25 was computed to be unstable at low loading from DFT, which effortlessly transitions to the primary site based on nudged elastic band calculations27. A further inspection of the quality of XRD structural analysis as well as a more comprehensive exploration of the adsorption energy map from theoretical calculations is needed to understand such discrepancy. The best fit and the structure model derived from Rietveld refinement obtained for the fully Kr adsorbed MOF are shown in Figures 1c and Figures S3c. For Kr, the location of the primary adsorption sites is similar to that of Xe1, but with a smaller distance to Ni (3.26(15) Å). Unlike Xe, there is negligible electron density at the center of the MOF channel from difference Fourier analyses, suggesting low or zero occupancy of such a site at room temperature. The honeycomb cage remains intact upon Kr adsorption with Ni−Ni distance 2.966(4) Å. The atomic distances are Kr-Ni, 3.26(15) Å; Kr−Kr 4.59(2) Å. The Kr occupancy is 15.5%, which is significantly lower than that of Xe1 occupancy. In comparison with Xe, the computed adsorption energy of Kr at the primary site is lower in magnitude by more than 30% (Table 1), despite the smaller Ni−Kr distance (3.3 Å) versus the Ni−Xe distance (3.4 Å). The insertion of the seventh Kr atom was


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Figure 3. Site Occupancy Factors obtained from sequential Rietveld refinements for different sites: Xe1 (red filled square), Xe2 (red open square), and Kr1 (blue filled circle) at 298 K. XRD data shown here are obtained using X-rays wavelength λ=0.72959 Å. computed to lower the total energy by about 10 kJ/mol, only about half of the energy gained compared to Xe. These results are consistent with the XRD results discussed above which do not support the existence of any additional Kr atom at the MOF center. In our earlier work we have demonstrated that the Ni-DOBDC has the highest dynamic Xe capacity found to date followed by activated carbon and HKUST-119. Considering the difference in total adsorption sites and the higher Xe occupancy factor in the MOF structure, we conclude that Xe has a higher adsorption capacity into the MOF than Kr. This is in complete agreement with previous breakthrough and


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isotherm experimental results.19, 23, and is also confirmed by the RGA profiles for the gas leaving the sorption bed, which shows a lower volume (partial pressure) for Xe (Figure S1b) than for Kr (Figure S1d). Sequential Rietveld analysis was carried out for Xe and Kr loading at 298 K (Figure 3). From the refined site occupancy factors (SOF), one can observe that Xe occupies both Xe1 and Xe2 sites while Kr sorbs only on the primary site. Further temperature-dependent sorption experiments were carried out by measuring XRD patterns at 323 K and 398 K for comparison of adsorption isotherms (Supplemental Figure S4). One can observe that at 323K, the trends are similar to that of 298K, but with lower SOF values. At higher temperatures the total SOF for Kr drops more significantly compared to Xe, indicating a higher absorption capacity for Xe at temperatures 298 K and above. At 398K, no Kr and very little Xe were found adsorbed to the metal sites (Supplemental Figure S4). In the whole temperature range of 298-398 K, a higher total SOF for Xe than Kr is always observed, indicating a higher heat of adsorption for Xe. These results are consistent with the DFT results showing that despite the slightly larger distance to Ni, Xe binds much more strongly to the MOF than Kr as a result of its larger atomic polarizability.16, 19

In addition, a decrease in the SOF for the Xe2 site was observed with increasing temperature

for Xe, whereas the site is unoccupied for Kr at or above room temperature, consistent with fact that the computed adsorption energy is smaller at Xe2 site than that at the open metal sites and is strongly dependent on the occupancy of the latter. From the SOF profiles at all temperatures (Figure S4), it was apparent that not only Xe occupancy rose faster than that of Kr, it retains Xe at and above room temperature. Sequential XRD patterns for Xe-Kr, Kr-Xe and mixed (50/50) Xe/Kr adsorption into the empty Ni-DOBDC MOF at 298K were taken to further compare the adsorption affinity between Xe and


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Kr (Figure 4). For the Xe-Kr cycle, the peaks sensitive to the metal site arise from the Xe gas loading (curve #2) and remain unchanged even after Kr loading (curve #3). For the reverse sequence, i.e. first Kr loading, there are minimal structural changes (curve #4). However, significant changes to the pattern are noted as Xe is loaded (curve #5). It is observed that Xe

Figure 4. Sequential Xe-Kr, Kr-Xe and mixed Xe/Kr adsorption in Ni-DOBDC. XRD patterns showing the intensity variation with 2θ after the gas flow for 20 minutes at rate 0.17 mL/s. (1) with empty MOF after activation, (2) with first Xe gas flow through the empty MOF, (3) followed by Kr gas flow, (4) with first Kr gas flow through the empty MOF, (5) followed by Xe gas flow, (6) with mixed Xe and Kr (50:50) gas flow through the empty MOF. Regions with the most distinct variations are marked by red ovals. XRD data showed here are obtained using Xrays wavelength λ=0.71956 Å


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sorbs preferentially and could be displacing Kr that might have been previously sorbed into the empty cage as observed by our previous measurements19. Similar features as in the Xe-only loading is observed even in the Xe/Kr mixed loading patterns (curve #6). The observed competitive adsorption of Xe and Kr indicates a stronger affinity of Xe to the Ni-DOBDC, consistent with its higher adsorption capacity of Xe from X-ray measurement and the more negative adsorption energy from DFT calculations. These findings are consistent with the previous findings on Xe/Kr selectivity in Ni-DOBDC MOF.19, 23 In addition, the effective pore size of Ni-DOBDC (~11 Å) enhances the adsorption energy with increasing loading through the neighboring Xe-Xe vdW interactions. The measured Xe-Xe distance in Xe-loaded Ni-DOBDC (4.48 Å) is similar to the equilibrium dimer distance (4.36 Å) while that for Kr (4.59 Å) is nearly15% larger than the corresponding dimer distance (4.01Å). In conclusion, for the first time we provided a fundamental understanding of the sorption mechanism and guest-host interaction in Ni-DOBDC based on detailed analysis of XRD data and DFT calculations. We have described the competitive capacity and selectivity of Xe over Kr adsorption in the Ni-DOBDC MOF at room temperature and above. Our work suggests that the adsorption of a noble gas into a MOF depends not only on the open metal-site density and pore morphology but also the interaction energetics of the guest-atom with metals and the atomic radius of the guest atom. The high separation capacity of the Ni-DOBDC MOF suggests high potential for application in removing Xe from Kr in the off-gas streams in nuclear spent fuel reprocessing as well as filtering Xe at low concentration from other gas mixtures. The methods used here allowed us to obtain accurate structural and energetic information that could be used to develop an effective strategy for the synthesis of MOFs that strongly retain one gas in preference to another.


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ASSOCIATED CONTENT Supporting Information. Acknowledgment and funding sources, as well as further material about procedure, method, etc., are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interests. (1)

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Understanding the Adsorption Mechanism of Xe and Kr in a Metal

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