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A Robust Squarate-Based Metal-Organic Framework Demonstrates Record High Affinity and Selectivity for Xenon over Krypton Liangying Li, Lidong Guo, Zhiguo Zhang, Qiwei Yang, Yiwen Yang, Zongbi Bao, Qilong Ren, and Jing Li J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03422 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019
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Journal of the American Chemical Society
A Robust Squarate-Based Metal-Organic Framework Demonstrates Record High Affinity and Selectivity for Xenon over Krypton Liangying Li,†,§ Lidong Guo,† Zhiguo Zhang,†,‡ Qiwei Yang,†,‡ Yiwen Yang,†,‡ Zongbi Bao,*,†,‡ Qilong Ren,†,‡ and Jing Li*,§ †Key
Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡Institute of Zhejiang University-Quzhou, Quzhou 324000, P. R. China § Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey, 08854, USA Supporting Information Placeholder ABSTRACT: The efficient separation of xenon (Xe) and krypton (Kr) is one of industrially important processes. While adsorptive separation of these two species is considered as an energy efficient process, developing highly selective adsorbent remains challenging. Herein, a rigid squarate-based MOF, having a perfect pore size (4.1 Å 4.3 Å) comparable with the kinetic diameter of Xe (4.047 Å) as well as pore surface decorated with very polar hydroxyl groups, is able to effectively discriminate Xe atoms, affording a record high Xe/Kr selectivity. Exceptionally high Xe uptake capacity of 58.4 cm3/cm3 and selectivity of 60.6 at low-pressure (0.2 bar) are achieved at ambient temperature. The MOF exhibits the highest Xe Henry coefficient (192.1 mmol/g/bar) and Xe/Kr Henry selectivity (54.1) among all stateof-the-art adsorbents reported so far. Direct breakthrough experiments further confirm the excellent separation performance. The density functional theory (DFT) calculations reveal that the strong interaction between Xe and framework is a result of the synergy between optimal pore size and polar porosity.
INTRODUCTION
Effective separation of xenon and krypton mixtures is a very important industrial process as both gases in high purity are needed in many applications, including medical imaging, commercial lighting, insulation, laser, illumination and spacecraft propellant.1-4 Xenon and krypton mixtures (20/80, v/v) are generally obtained as a by-product from air separations, from which pure streams of xenon and krypton are produced by cryogenic distillation.5 In addition, radioactive 127Xe and 85Kr are generated as fission product.6 They have a long half-life and therefore must be properly captured and stored to prevent them from contaminating the environment.7 Currently, cryogenic distillation is the most mature technology to separate Xe and Kr from air and from nuclear waste reprocessing.5 However, cryogenic distillation is excessively energy-intensive. Adsorption separation based on a highly selective adsorbent, is considered as an energy- and cost-effective alternative method to cryogenic distillation and has been widely used in the separation and purification of various gas mixtures. Metal-organic frameworks (MOFs) have shown great promise as adsorbents for gas separations due to their tailorable architectures, uniform pore sizes, and designable chemical functionalities.8-19 Given the fact that the atomic gases of Xe and Kr with no dipole or quadrupole moments,20 it is a grand challenge for efficient separation of Xe from other chemically inert gases with similar size, especially Kr. Many MOF materials have been investigated for Xe/Kr separation in the recent years. For example, MOFs with high surface area (e.g. IRMOFs, UiO-66) and polar functional groups (e.g. IRMOFX, UiO-66-X, X = Cl, F, Br, I, NH2, etc.) have shown high Xe
uptake but weak gas-adsorbent interactions in these cases have resulted in low gas selectivity.21-25 Additionally, MOFs with accessible, highly polarizable open metal sites (e.g. MOF-74 series, HKUST) have also been evaluated for noble gas separation because of their stronger affinity to Xe than Kr.4, 26-30 A number of reports suggest that optimal pore size comparable to the kinetic diameter of Xe is a key factor for high capacity and selectivity.3137 Among many MOF materials tested for Xe/Kr separation,30, 32, 35, 38-43 the highest Xe/Kr selectivity has been achieved recently by CROFOUR-1-Ni systems, reaching a value of 22 at 1.0 bar and 298 K.35 Considering the differences in the kinetic diameters (4.047 Å and 3.655 Å for Xe and Kr, respectively) and polarizability (40.4 1025 cm-3 and 24.8 1025 cm-3 for Xe and Kr, respcetively),20 high-throughput computational screening by Snnur etc44-46 suggests that porous materials with tube-like uniform pores and cavity diameters slightly larger than a single Xe atom, and/or high concentration of polarity on the pore surfaces would lead to the highest Xe/Kr selectivity, as a result of confinement effect. Along this line of thinking, we have developed an ultra-microporous and chemically stable MOF with available polar functionality (–OH groups) decorated on the one-dimensional pore wall and perfect pore size (4.1 Å 4.3 Å), [Co3(C4O4)2(OH)2]3H2O (C4O42- = squarate) (1), (dehydrated form, 1a), expected for selective recognition of Xe atoms. Adoption of precisely designed pore size as well as polar pore surface successfully customized the energy favorable binding sites for Xe atoms within this MOF. The exceptional performance of record high Xe/Kr selectivity and Xe uptake capacity at ambient temperature and low pressure can be attributed to the strong xenon-host interactions (XeO and XeH
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(a)
(b)
Figure 1. (a) The coordination environment of squaric ligand. (b) Perspective view of the structure showing the –OH groups decorated in the rhombohedral channels. (c) Experimental single-component adsorption isotherms of different gases at 313 K. (d) Comparison of the IAST selectivity of 1a versus those of previously reported best-performing materials for Xe/Kr (20/80, v/v) mixtures at varying pressures at room temperature. (e) Xe adsorption isotherms of 1a at low pressure (0-0.05 bar) at 298 K and comparison with other materials. (f) Survey of thermodynamic Xe/Kr separation performance in reported top-performing materials. Henry coefficients are extracted from single-component Xe and Kr adsorption isotherms at 298 K, except FMOF-Cu at 297 K, MOF-505 at 292 K.
interactions) that firmly confining single Xe atoms into the polar pores. Highly efficient separation performance for selective adsorption of Xe over Kr as well as other inert gases was also confirmed by experimental breakthrough tests.
EXPERIMENTAL SECTION
Materials and Physical Measurements. All chemicals (AR grade) were used as received from commercial suppliers without further purification. Optical images were performed on an Olympus SZ61 microscope. Power X-ray diffraction (PXRD) analysis of powder samples was carried out on a Rigaku UltimaIV automated diffraction system using Cu Ƙα emission radiation (λ = 1.5406 Å). The data were collected at room temperature in a range of 8-37° (2θ) with a scan speed of 2°/min. The operating power was 40 kV/44 mA. The single crystal X-ray diffraction of the as-synthesized and activated samples was performed at 173 K using a Bruker Smart Apex II diffractometer with graphitemonochromated Cu Ƙα radiation. Detailed crystallographic data are summarized in Table S1. Preparation of Squarate-Based Material. A mixture of organic linker squaric acid (2.9 mmol, 0.33 g) and CoCl26H2O (1.9 mmol, 0.46 g) was dissolved in 7 mL H2O, and then under stirring KOH (7.7 mmol, 0.43 g) was added. The mixture was finally transferred to a 25 mL Teflon-lined reactor. The reactor was heated to 220 oC for 48 h. After that, the reactor was cooled down to room temperature within several hours. The sample was filtrated and washed with deionized water several times, and then exchanged with deionized water for 3 days (2 times/day) prior to activation. Stability Tests of Squarate-Based Material. After being washed with deionized water, as-synthesized samples, about 0.2 g for each batch, were immersed in 10 mL of aqueous solution of pH = 1(HCl), pH = 4 (HCl), and pH = 10 (NaOH) at room temperature for 12 h, 24 h and 48 h, respectively. The treated
samples were washed with deionized water for several times and dried at room temperature before PXRD measurements. Gas Adsorption Measurements. Single-component isotherms of Xe, Kr, Ar, N2, and O2 were collected between 298 and 313 K on Micromeritics ASAP 2460 adsorption apparatus. The assynthesized sample was exchanged with deionized water for 3 days prior to activation. The initial outgassing process was carried out under high vacuum ( 4 m Hg) at 100 °C for 12 h prior to adsorption measurements and about 100 mg of activated sample was used for gas adsorption studies. The free space of the system was determined by using helium gas. Column Breakthrough Experiments. In a typical experiment, about 5.78 g samples were activated before packed into a stainless steel HPLC column (4.6 mm I.D. 250 mm). The column packing was performed in glovebox filled with Ar. He gas was introduced into the column to further purge the samples before the breakthrough measurements. The experimental breakthrough data were collected under the flow of dry gas mixtures of Xe/Kr (20/80, v/v) with a flow rate of 1.0 mL/min at 298 K. The outlet gas from the column was monitored using GC-490 gas chromatography with the thermal conductivity detector (TCD). A dry gas mixture of Xe (400 ppm) and Kr (40 ppm) balanced with air was also measured under a flow rate of 20.0 mL/min at 298 K. The outlet gas passing through the column was analyzed using a Hiden HPR20 EGA mass spectrometer for continuous sampling gas analysis. After every breakthrough measurement, the column was regenerated with a He gas flow of 20.0 mL/min at 100 °C for 48 h, and then the reduplicated breakthrough experiments were conducted using the same column with the regenerated samples under the identical conditions. Density-Function Theory Calculations. First-principles density functional theory (DFT) calculations were performed using the Materials Studio’s CASTEP code.47 All calculations were conducted under the generalized gradient approximation
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(b)
(e)
(c)
(d)
(f)
Figure 2. Illustration of the adsorbed Xe atom (glaucous) at the primary binding sites in 1a as determined from simulation. The primary binding sites are located within the cage. (a) Top view of the Xe adsorbed packing diagram. (b) Side view of the Xe atoms in the onedimensional rhombohedral channels with framework omitted. (c, e) The Xe atom interacts with H (XeH, 2.825 Å) atoms from hydroxyl group (Site I) and O (XeO, 3.702 Å) atoms from organic ligands (Site II). (d, f) The Kr atom interacts with H (KrH, 2.876 Å) atoms from hydroxyl group (Site I) and O (KrO, 3.653 Å) atoms from organic ligands (Site II).
(GGA) with perdew-burke-ernzerhof (PBE). A semi-empirical addition of dispersive forces to conventional DFT was included in the calculation to account for van der Waals interactions.48 A cutoff energy of 544 eV and a 3 3 4 k-point mesh were found to be enough for the total energy to coverage within 0.01 meV atom-1. The structures of the synthesized materials were first optimized. The optimized structures matched well with the experimentally determined crystal structures. To obtain the binding energy, the pristine structure and an isolated gas molecule placed in a supercell (with the same cell dimensions as the pristine crystal structure) were optimized and relaxed as references. Xe and Kr gas molecules were then introduced to different locations of the channel pore, followed by a full structural relaxation. The static binding energy was calculated by the equation: 𝐸𝐵 = 𝐸 (𝑔𝑎𝑠) +𝐸 (𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑛𝑡) ―𝐸 (𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑛𝑡 + 𝑔𝑎𝑠).
RESULTS AND DISCUSSION
The squarate-based MOF was prepared from the reaction of squaric acid (H2C4O4) and cobalt chloride hexahydrate (CoCl26H2O) in an aqueous potassium hydroxide solution at 220 oC for 48 h, affording purple block crystals of 1 proper for single crystal analysis. The crystal structure of 1 is illustrated in Figure 1, which shows that the framework contains one-dimensional (1D) rhombohedral channels along the crystallographic c-axis, with a pore size of 4.1 Å 4.3 Å (Figure S1). This size is precisely comparable to the van der Waals diameter of Xe atoms (4.1 Å). The framework is composed of infinite chains of edgesharing CoO6 octahedra connected through the squaric acid ligands (Figure 1a). The –OH groups partly serve as the template during the framework construction and point into the channel (Figure 1b), making it strongly polar. Single crystal analysis of the as-synthesized and activated samples showed that this MOF retains its structure after dehydration, with little changes in the
unit cell parameters (Table S1). The PXRD pattern also remained the same even after the sample was exposed to air for 1 week (Figure S2). Thermogavimetric analysis (TGA) indicated that the compound is stable up to 593 K (Figure S3). The permanent porosity of the dehydrated framework was confirmed by CO2 adsorption isotherm at 273 K, giving apparent surface area of 95 m2/g of 1a (Figure S4). The low porosity suggests the framework is relatively dense. Since the pore size of 1a is fairly close to the kinetic diameter of Xe atom, there might be a strong confinement effect for Xe adsorption, thus leading to high selectivity. Indeed, 1a is remarkable at separating Xe from other inert gases. Singlecomponent adsorption isotherms of Xe, Kr, N2, Ar and O2 were collected at 298 and 313 K (Figure 1c and S6). The results show exceptional performance of 1a for capturing Xe at low pressures, which is needed for actual industrial processes. The Xe adsorption isotherms present a typical type-I profile with a very steep slope, and approach saturation at very low pressure (~ 20 kPa at 298 K), indicative of high affinity for Xe. It is interesting to note that at 1.0 bar, 1a takes up 66.1 cm3/cm3 and 58.9 cm3/cm3 at 298 K and 313 K, respectively, corresponding to the gas occupancy of 1.18 and 1.05 Xe atoms per unit cell. At 298 K and 0.05 bar, the Xe uptake in 1a reaches 50.0 cm3/cm3, significantly higher than all previously reported state-of-the-art MOFs under the same conditions (Figure 1e and Table 1). The highest Xe uptake capacity of 1a under low pressure ( 0.05 bar) (Figure 1e) indicates that Xe atoms are packed much more efficiently within its pore landscape compared with other materials. In addition, the very high Xe/Kr selectivity over other materials (Figure 1d), signifies the true potential of 1a for use as a sorbent material in separating Xe and Kr at extremely dilute concentrations. The isosteric heats of adsorption (Qst) are calculated to evaluate the binding energies between the noble gases and the
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Table 1. Xe adsorption capacity and Xe/Kr selectivity for various materials at 298 K and 100 kPa. Adsorbents
Xe uptake (cm3/cm3) a
Material density (g/cm3)
IAST S(Xe/Kr)
Xe Qst (kJ/mol)
Xe coefficient (mmol/g/bar)
Henry’s selectivity
0.1 bar
1.0 bar
[Co3(HCOO)6]32
33.4
82.4
1.84
12
28
9.9
8.7
Ni-MOF-7428
23.0
110.9
1.19
5-6
22
8.4
5.8
SB-MOF-134
39.7
50.9
1.62
16
35
38.42
16.2
SB-MOF-233
22.0
74.2
1.19
10
26.4
10.5
8.6
MIL-101(Cr)23
4.4
24.1
0.78
-
21.4
-
5.3
MIL-100(Fe)23
2.3
17.6
0.69
-
20.9
-
5.6
UiO-66(Zr)23
11.0
49.9
1.41
-
25
-
7.2
Cu-BTC4, 49
11.8
41.0
0.88
2.6
17.5
12.2
8.5
1.62
2b
15
0.58
1.4
140.0
0.99
9-10c
-
10.26
6.8
24.1e
31.0
11.17
16.7
FMOF-Cu36, 50 MOF-50544
2.29 27.6
16.9
PAF-45S43
17.3
50.6
1.22d
MOF-Cu-H39
73.6
86.7
1.58
16.7
33.4
39.74
15.8
CROFOUR-1-Ni35
20.9
47.1
1.19
22
37.4
18.73
24.3
CROFOUR-2-Ni35
18.2
45.7
1.36
15.5
30.5
15.95
18.5
1a
53.6
66.1
2.19
69.7
43.6
192.06
51.4
The volumetric uptake = gravimetric uptake crystal density, b From breakthrough experiment (Xe/Kr 50/50), c Calculated from breakthrough experiment with Xe/Kr (20/80, v/v) mixture at 298 K. d Estimated from the porosity. e From breakthrough experiment (Xe 500 ppm, Kr 50 ppm).
a
activated framework 1a. The Qst of Xe at nearly zero-loading for 1a is estimated to be 43.6 kJ/mol, significantly higher than those of Kr (35.9 kJ/mol), Ar (14.6 kJ/mol), N2 (21.0 kJ/mol) and O2 (19.3 kJ/mol) (Figure S7). These values are in trend with adsorbed amount of each gas, confirming the high selectivity of 1a for Xe atoms. Again, the highest Xe Qst value of 1a compared with reported materials (Table 1) demonstrates that the incorporation of polar functional groups as well as formulation of perfect pore size within this framework are the key factors for highly efficient adsorption of Xe atoms. The slight decrease in Qst with increasing Xe loading may be caused by the variability in the energetic heterogeneity.51, 52 The calculated IAST selectivity for 20:80 Xe/Kr binary gas mixture is ~69.7 at 298 K and 100 kPa, which outperforms all existing benchmark porous materials (Figure 1d). Furthermore, the IAST selectivity for Xe/N2, Xe/O2 and Xe/Ar binary mixtures are calculated to be 567, 1898 and 627, respectively (Figure 3c), based on the experimental adsorption isotherms and a concentration of 1% Xe. Additionally, the adsorption isotherm of Xe on 1a measured at low pressure range (0-0.05 bar) and 298 K was used for an accurate calculation of Henry’s constants, a useful measure of the intrinsic selectivity which represents the partition of the adsorbate between its bulk phase and adsorbed phase at very low pressures.53 As shown in Figure 1f, 1a clearly displays the largest Henry coefficient value of 192.1 mmol/g/bar for Xe and the highest Xe/Kr Henry’s selectivity of 51.4 compared with the best performing materials reported by far, including MOF-Cu-H (39.74 mmol/g/bar and 15.8)39, SBMOF-1 (38.42 mmol/g/bar and 16.2),34 and CROFOUR-1-Ni (18.73 mmol/g/bar and 24.3).35 To explore the nature of interaction of the Xe/Kr gas mixture within this material, modeling studies using first-principles DFTD (dispersion-corrected density functional theory) calculations
were performed. The simulations reveal that Xe or Kr is associated with two distinct adsorption sites, Site-I and Site-II, in channels (Figure 2) containing the polar –OH groups, resulting in higher polarization in Xe with respect to Kr which can effectively improve the binding energy for Xe. In addition to selective recognition of Xe due to the unique polar porosity, the optimal pore size is equally important. The calculated distance of XeH (2.825 Å) (Site I) is shorter than the average XeH contact distance of 3.71(4) Å38. Such a short distance insures close contact of gas-host so that Xe atoms are trapped firmly within the pore space of 1a (Figure 2c). On the other hand, Xe atoms also interact with the O atoms from ligand through multiple XeO interactions, with a XeO distance of 3.702 Å (Figure 2e), much shorter than the XeO distances reported in literature (3.86 Å and 4.26 Å).38 This arises from the negative nature of O atoms (Site II), suggesting that the small pore apertures increase the energy of framework-guest interactions by surrounding Xe with many O atoms in close proximity. The adsorption sites of Kr are similar to those found for Xe, with a much longer KrH distance of 2.876 Å, in Site-I (Figure 2d). Kr atoms are also surrounded with O atoms from the ligand linkers with a distance of 3.653 Å (Site-II) (Figure 2f). The observed longer distance between H and Kr atom is consistent with the high Xe/Kr selectivity in 1a being driven by the larger size and stronger polarizability of Xe atoms. The calculated binding energies of Xe and Kr are 44.1 kJ/mol and 33.7 kJ/mol, respectively, corresponding to adsorbate selection based on energetic favorability. From these results, it is clear that suitable pore size and polar –OH functionality contribute synergistically to the maximized interaction between Xe atoms and the 1a framework. The significantly high Xe Henry coefficient and Xe/Kr selectivity of 1a demonstrates its outstanding Xe/Kr separation
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Journal of the American Chemical Society performance which is further investigated through single-column breakthrough experiments at a concentration of a 20/80 molar mixture of Xe/Kr, a by-product in distillation of air separation. The Kr was first eluted after 30 min, whereas the retained time for Xe did not occur until 87 min (Figure 3a). The huge gap of residence time between Xe and Kr confirmed the great potential of 1a for Xe/Kr adsorption separation in industrial processes. A low Xe/Kr concentration under conditions expected for used nuclear fuel reprocessing (400 ppm Xe, 40 ppm Kr, balanced with air) was also tested. The mixture gas passed through the column packed with activated samples after initially purging with He (~10 min). As shown in Figure 3b, after injection of the gas mixture, N2, O2, Ar and Kr broke through the column after ~13 min, followed by Xe after ~43 min, suggesting preferential adsorption toward Xe over Kr from the off-gas mixture.
(a)
(b)
(c)
Figure 3. (a) The experimental breakthrough curves for a binary Xe/Kr (20:80, v/v) mixture with a flow-rate of 1.2 mL/min at 298 K and 1.0 bar. (b) The experimental breakthrough curves for Xe/Kr mixture at dilute concentration at 298 K and 1.0 bar. Inlet is a gas-mixture with 400 p. p.m. Xe and 40 p. p. m. Kr balanced with dry air. Ci,0 is the total concentration of each singlecomponent gas at the outlet. The total flow-rate of He and gasmixture is 20 mL/min. (c) Calculated IAST selectivity for different gas components of 1a at 298 K. (a)
(b)
Figure 4. Microscope images of as-synthesized 1a (a) and single crystals treated in the various solutions of pH =1 (b) and pH = 10 (c). (d) PXRD patterns for 1a samples treated in the various pH solutions for 48 hours. As many MOF materials are easily oxidized under oxygen, regeneration capability under ambient conditions is must be considered for a real-world application. Our experiments show that under ambient conditions, 1a adsorbed with Xe, Kr, N2, O2, and Ar can be fully regenerated and its performance does not decline with noticeable decrease in the mean retention time for Xe and Kr in three continuous cycles (Figure S9). These results indicate that 1a is stable in the oxygen atmosphere, which is further confirmed by PXRD patterns that remained unchanged after the sample was exposed to air for 1 week (Figure S2). Moreover, chemical stability of 1a is evaluated by treating the samples at harsh conditions. The optical images (Figure 4 and S10) of the single crystals of 1a show that there is no apparent change in their morphology after being treated in various pH solutions. Their PXRD patterns also remained intact, demonstrating that there is no phase transition or structure collapse (Figure 4d and Figure S11-12). According to crystal field theory, the coordination between Co2+ and C4O42- offers higher crystal field stabilization energy than that between Co2+ and OH/H2O, which may be the reason, at least partially, that attributes to the strong resistance of 1a to the harsh chemical environment.54-56 The extraordinary robustness of 1a adds another unique advantage for its use in gas separation. In summary, the special pore structure decorated with polar – OH groups and optimized pore size within the framework of 1a is a crucial factor that affords adequate strength of host-guest interaction for selective binding of Xe atoms. Exceptionally high uptake of Xe at very low pressure and recorded high IAST selectivity of Xe/Kr have been achieved for this material, making it a new benchmark adsorbent for recognition of Xe. The exceptional separation performance, excellent framework stability, as well as full recyclability renders this material a truly promising candidate for adsorptive separation of inert gases.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. XRD, BET, TGA, dual sites Langmuir-Freundlich isotherm model fitting, Henry coefficient, and isosteric heat of adsorption calculation of squarate-based MOF (PDF) Crystallographic file of squarate-based MOF (CIF)
AUTHOR INFORMATION Corresponding Author *
[email protected] *
[email protected] Notes (c)
(d)
The authors declare no competing financial interests.
ACKNOWLEDGMENT This work is supported by the National Key R&D Program of China (No.2016YFB0301500), National Natural Science Foundation of China (No.21722609 and No.21436010), Zhejiang Provincial Natural Science Foundation of China (No.LR17B060001), and the Fundamental Research Funds for the Central Universities (No.2018XZZX002-14). We would also like to acknowledge the partial support from the Office of Basic
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Research, Energy Sciences of the U.S. Department of Energy through Grant No. DE-FG02-08ER-46491.
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