Adsorption and Separation of Xe in MetalâOrganic Frameworks and

Subscriber access provided by UNIV DE VALENCIA

Article

Adsorption and Separation of Xe in Metal-Organic Frameworks and Covalent-Organic Materials Qian Wang, Hui Wang, Shuming Peng, Xuan Peng, and Dapeng Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp503255g • Publication Date (Web): 21 Apr 2014 Downloaded from http://pubs.acs.org on April 29, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Adsorption and Separation of Xe in Metal-Organic Frameworks and Covalent-Organic Materials

Qian Wang,1 Hui Wang,2 Shuming Peng,1 Xuan Peng3 and Dapeng Cao2*

1

2

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China 3

*

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, People’s Republic of China

Department of Information Science, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

Corresponding Authors. Email: [email protected]

Fax: +8610-64427616 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

Abstract We use grand canonical Monte Carlo simulations to investigate adsorption and separation of metal-organic frameworks and covalent-organic materials for the noble gas Xe. Results indicate that PAF-302 among these materials studied shows not only the highest gravimetric excess uptake of 4009 mg/g, but also largest volumetric uptake of 216 V(STP)/V, due to its large pore size and high accessible surface area. The gravimetric excess uptake of Xe at intermediate pressure follows the order of PAF-302 > UMCM-1 > IRMOF-1 > Cu-BTC > COP-4 > ZIF-8, which is entirely consistent with the accessible surface areas. Moreover, the maximum gravimetric excess uptakes of Xe in different materials exhibit an entirely linear correlation with the accessible surface area. However, at low pressure of p ZIF-8 > COP-4 > IRMOF-1 > UMCM-1 > PAF-302, which is actually the same with the order of difference of isosteric heats (DIH), and is in excellent agreement with our previous conclusion, i.e., the selectivity of a material is closely related to the DIH. In particular, the selectivity of Cu-BTC for Xe over N2 reaches 80, which is an excellent candidate for Xe separation. In short, this work indicates that PAF-302 is an excellent candidate for Xe storage at intermediate pressure, while Cu-BTC is an excellent material for Xe separation.

Keywords: Xe separation, Metal-organic frameworks, Covalent-organic frameworks, Grand canonical Monte Carlo simulation

2


Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The noble gas xenon (Xe) has been extensively applied to different fields, such as anesthetics,1 commercial lighting, imaging, ionic propulsion2 and neuro-protection.3-4 However, Xe concentration (0.086 ppmv) is very low in air and its chemical inertness makes it difficult to be separated from air. Industrially, Xe is concentrated from air primarily via cryogenic distillation as a byproduct in producing oxygen and nitrogen, but this process is expensive in energy. In addition, the radioactive Xe isotopes are also released into air as byproducts of nuclear fission and treatment of spent nuclear fuel.5 Thus, it is very significant to investigate separation of Xe from the atmosphere, especially for atmospheric monitoring.4 Selective adsorption by microporous adsorbents is an alternative method, besides cryogenic distillation. Conventional materials, such as zeolites, activated carbon

6-7

and

single-walled carbon nanotube bundles,8-9 have been applied to the Xe capture and purification. For example, Munakata et al. studied adsorption equilibrium of Kr, Xe and N2 in zeolite 5A and activated charcoal, 7 and found that both Xe and large amounts of N2 competitively inhibited adsorption of Kr from the mixture involving all three gases. Recently, a new class of metal-organic frameworks (MOFs) has received a lot of attention. Due to its tailoring chemical structure, high specific surface area, large pore volume and adjustable chemical functionalities, MOFs have been widely applied to gas adsorption and separation,10-11,12-14 catalysis15-16, luminescent probe

17-18

etc. To date, few reports on

MOFs were related to the capture and separation of Xe from atmosphere. Mueller et al. measured the volume-specific uptake of rare gases in IRMOF-1, and found that IRMOF-1 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

exhibits preferential adsorption of Xe over the lighter rare gases.19 Pawsey et al. reported adsorption isotherms of Xe in IRMOF-1, -2, -3, and -6 at 292 K by using

129

Xe NMR

spectroscopy to probe the pore environment, and found that Xe primarily binds to the carboxylate groups and tetrahedral Zn4O clusters in the corners of the cages in IRMOFs.20 Boutin et al. reported a Xe adsorption study in MIL-53(Al) in the temperature range 195-323 K. In this study, they clearly observed breathing transitions in the measured adsorption isotherms.21 Thallapally et al. examined properties of NiDOBDC adsorbent for capture of Xe at noncryogenic temperature, and found that the uptake is substantially higher than the typical prototype MOF-5.4 Liu et al. recently measured adsorption isotherms of Xe, Kr, the Xe/Kr mixture and the O2/N2/Ar/CO2/Xe/Kr mixture in HKUST-1, Ni/DOBDC and activated carbon, and found that Ni/DOBDC exhibits higher selectivity for Xe compared to Kr.22 Parks et al. used GCMC simulation to explore adsorption behavior of 16 MOFs for noble gases and nitrogen to identify key structural properties for selective adsorption of noble gases from air, and found that the heavier and more polarizable gases are easier to be separated from lighter and less polarizable gases (say, nitrogen) at low pressure.23 Recent results indicate that MOFs can effectively separate noble gases from air. More recently, as a subfamily of MOFs, covalent organic materials (COMs) (including covalent-organic frameworks (COFs), conjugated microporous polymers (CMPs), covalent-organic polymers (COPs) and porous aromatic frameworks (PAFs) etc.) have been synthesized

24

and widely applied to adsorption separation,25-28

catalysis16 and luminescent probe18 etc. The COM materials are formed by strong covalent bonds between light elements such as B, C, O, H, and N. They have lower density and 4


Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


higher stability than MOFs, moreover still retaining the unique characteristics of MOFs. Unfortunately, almost no report is related to Xe adsorption and separation on COMs. Therefore, it is very significant to explore adsorption and separation of Xe in MOFs and COMs, aiming at providing the useful guidance for adsorption and separation of Xe by examining the adsorption mechanism of Xe in MOFs and COMs. 2. Computational Methods and Details 2.1. Materials. In this work, we selected several representative materials, including MOFs, zeolitic imidazolate frameworks (ZIFs) and COMs. For MOFs, we considered IRMOF-1, UMCM-1 and Cu-BTC frameworks. IRMOF-1 is the prototype of the IRMOFs and it has the cubic topology with the octahedral Zn4O(CO2)6 clusters linked by BDC(1,4-benzenedicarboxylate) organic linkers. UMCM-1 consists of Zn4O clusters linked together by two BDC and four BTB linkers arranged in an octahedral geometry. Cu-BTC29 (also known as HKUST-1) is a reference material widely studied both experimentally and theoretically. HKUST-1 has paddle-wheel-type metal corners connected by BTC linkers. Each metal corner contains two copper atoms that are bonded to the oxygen atoms of four BTC linkers forming four-connected square-planar vertexes. These structures contain micropores and mesopores, which is a promising candidate for adsorption and separation of Xe. ZIFs, as a subclass of MOFs, have tetrahedral networks with transition metals (Zn, Co, etc.) linked by imidazolate ligands. Because of the strong bonding between the metal centers and imidazolate linkers, ZIFs exhibit high thermal and chemical stability in refluxing organic and aqueous media. In particular, ZIF-8 consists of zinc and 2-methylimidazolate (2-MeIM) linkers and is one of the most widely studied 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

prototypical ZIF compounds, which is also a good candidate for adsorption and separation of Xe. In addition, for COMs, we choose PAFs30 and COPs31 as representative examples to study adsorption and separation of Xe, because PAF-1 not only holds fantastic diamond-like structure and high physicochemical stability, but also exhibits ultrahigh Langmuir specific surface area of 7100m2/g and high uptakes of CO2 and H2. Moreover, the recently synthesized covalent-organic polymers (COPs) show a 2D porous structure with high hydrothermal stability and specific surface area, which is a good prototype of 2D porous materials.31 These selected materials were shown in Figure 1 and their structural properties were listed in Table 1. 2.2. Potential Models.

In this work, Xe was represented by a spherical Lennard-Jones

(LJ) potential model, and the potential parameters were taken from the literature.32 CO2 was modeled as a 3-site rigid linear molecule with three charged LJ interaction sites, in which the LJ potential parameters were taken from TraPPE force field developed by Potoff and Siepmann.33 A combination of the site-site LJ and Coulombic potentials was used to calculate the CO2-CO2 intermolecular interactions. N2 was represented by the 1-site model and the corresponding parameters were taken from Kaneko et al.34 The above potential models have been successfully used to predict gas adsorption in MOFs14 and COMs.35 Previous simulation studies have shown that Dreiding force field leads to accurate predictions of gas adsorption in MOFs36 and COMs.35, adsorbents, the Dreiding force field

38

37

Therefore, for the

was adopted to describe the interactions of

framework atoms. For Cu atom, the parameters were taken from the universal force field (UFF)39 because they are not available in the Dreiding force field. In all the simulations, 6


Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atomic partial charges on the MOF atoms were taken from the literature, while we ignored the atomic charges of the COMs, because the charge has no significant effect on adsorption of COMs.40 All the LJ cross interaction parameters were obtained by the Lorentz–Berthelot mixing rules ( ε ij = (ε iiε jj )

1

2

and σ ij = (σ ii + σ jj ) / 2 ). All Lennard-Jones

parameters are given in Table 2. 2.3. Computational methods. Grand canonical Monte Carlo (GCMC) simulations were used to study adsorption and separation of Xe in the selected porous materials. All the GCMC simulations were performed at room temperature T=298 K by using the MUSIC code.41 More detail description of the simulation methods can be found in the literature.35, 42

In order to compare with the experiment data, the absolute uptake Nabs obtained in the

GCMC simulation was converted into the excess uptake Nex by

N ex = N abs − ρ gVg

(1)

where ρg is the density of bulk gas calculated from the PR EOS, and Vg is the free volume of adsorbent accessible to the gas molecules and can be calculated by the method of Myers et al. from the ideal gas law:43

Vpore =

RNmT pmm

(2)

where R is the gas constant, T is the temperature, p is pressure, and Nm is the number of adsorbed probe molecules per molar mass mm of the adsorbents, which is obtained from the GCMC simulations of nonadsorbed helium ( ε He / Kb = 10.22K , σ He = 2.58 Å) gas in materials at low pressure and ambient temperature.44 The isosteric heat qst reflects the released heat for each molecule added to the adsorbed phase, given by45 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

 ∂ (U total − U intra )  qst = RT −   ∂N total  T ,V

Page 8 of 28

(3)

where Utotal is the adsorption energy including the adsorbate-adsorbate interaction energy and the adsorbate-adsorbent interaction energy, and U int ra is the intramolecular energy of the adsorbate molecules in the bulk phase and is equal to zero for the rigid molecules. In a separation process, the adsorption selectivity is a good indication to evaluate the separation ability of a porous material. The adsorption selectivity of component i over component

j in a mixture is defined as Sads (i / j ) =

xi / x j yi / y j

(4)

where x and y are the molar fractions of the two components in the adsorbed and bulk phases, respectively. 3. Results and discussion In order to test the accuracy of our simulation method, we also compared the simulated results with the experimental data. Figure 2 shows the comparison of experimental data and simulation results of Xe in IRMOF-1 at 292 K. Actually, experiment data and simulation results are in a good agreement, especially for low pressure region of p UMCM-1 > IRMOF-1 > Cu-BTC > COP-4 > ZIF-8, which is entirely consistent with the accessible surface areas (ASA) (Table 1). This observation indicates that the uptake of Xe in MOFs and COMs at intermediate pressure is mainly related to the material surface area, which is in agreement with the conclusion of Frost et al.47 Forst et al. showed that hydrogen uptake in MOFs falls into three regimes. At low pressures, the uptake in different MOFs correlates with the heat of adsorption; at intermediate pressures, the uptake correlates with the MOF surface area, while at the high pressures, the uptake correlates with the free volume available within the MOFs. Interestingly, we also found that the maximum uptakes of Xe in different materials exhibit a linear correlation with the calculated ASA, as shown in inset of Figure 3a. Actually, the calculated ASAs of materials are also in agreement with experimental data. For example, the ASA of PAF-302 from experiment 48 is 5600 m2/g and the calculated ASA is 5735 m2/g. They are in good agreement. In addition, the calculated adsorption capacities of Xe in UMCM-1, IRMOF-1, Cu-BTC and ZIF-8 at 273 K and 3MPa are 26, 18.3, 10.5 and 7.7 mmol/g, which are also in excellent agreement with the results from Ryan et al. (They are 25, 17, 10 and 8 mmol/g for UMCM-1, IRMOF-1, Cu-BTC and ZIF-8 respectively at 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

273K). 49 At low pressure, adsorption trends of the selected materials changes significantly. Definitely, at low pressure Cu-BTC shows a higher adsorption uptake (500mg/g at 1bar) than the other five adsorbents (Figure 3b), because Cu-BTC has the exposed metal sites and small side pockets in the framework. ZIF-8 has a smaller pore size which increases the interaction with gas, and therefore leads to a higher uptake at low pressure. PAF-302 and COP-4 have lower uptake due to their weaker affinity for gases. Actually, the Xe uptake at low pressure follows the order of Cu-BTC > ZIF-8 > PAF-302 > IRMOF-1 > UMCM-1 > COP-4 (see Figure 3b), while the isosteric heats of Xe follow the order of Cu-BTC > ZIF-8 > COP-4 > IRMOF-1 > UMCM-1 > PAF-302 (see Figure 3c). In general, the gravimetric uptakes of MOFs at low pressure correlate with the isosteric heats at zero pressure. However, COP-4 and PAF-302 do not follow the rule, which may be caused by their different structures. COP-4 is a 2D structure and therefore shows a relatively high isosteric heat, while PAF-302 is a 3D structure without metal atoms, which leads to a relatively low isosteric heat, compared to MOFs and COP-4. Figure 4 displays the volumetric excess isotherms of Xe at 298 K in six selected materials at intermediate and low pressures. We can see from Figure 4a that the volumetric uptakes also exhibit a maximum at special pressure for all six materials. The maximum volumetric uptakes of Xe in IRMOF-1 and PAF-302 are almost the same (about 216 V(STP)/V ) at 37 bar because of canceling the influence of the framework density. The maximum volumetric uptakes of Xe in UMCM-1 and Cu-BTC are 190 V(STP)/V at 37 bar and 27 bar, respectively, while they are about 120 V(STP)/V for ZIF-8 and COP-4, 10


Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


which is smallest among six materials. At low pressure of p ZIF-8 > IRMOF-1 > UMCM-1 > PAF-302, except for the 2D structure COP-4, which means that the volumetric uptake of Xe at low pressure of p< 1.0 bar is closely related to the isosteric heats. All the calculated results indicate that PAF-302 is a promising candidate for Xe adsorption at intermediate pressure, while Cu-BTC is a better material for Xe adsorption at low pressure. Figure 5 shows the adsorption isotherms of CO2 and N2 in six selected porous materials. At p< 10 bar, the CO2 uptake of Cu-BTC is the largest and reaches adsorption saturation at p=15 bar because Cu-BTC contains the exposed metal sites and small side pockets in the framework, which exhibit large affinity for CO2 with quadrupole moment. However, at intermediate pressure (10 bar < p UMCM-1 > IRMOF-1 > Cu-BTC > COP-4 > ZIF-8, which is entirely same as the order of ASA of materials. In particular, the CO2 uptakes of PAF-302 and UMCM-1 reach 1846 mg/g and 1417mg/g. The N2 uptake mainly follows the similar trends like the CO2 uptakes. The different is that N2 uptake of COP-4 is smallest. The isosteric heats of N2 follow the order of Cu-BTC > ZIF-8 > IRMOF-1 > COP-4 > UMCM-1 > PAF-302 (see Figure 5c), which is nearly the same order with the one of Xe in the materials, except for the order of COP-4 and IRMOF-1. To further understand the adsorption mechanism of Xe molecules on adsorbents with different topologies, preferential adsorption sites of Xe on Cu-BTC and ZIF-8 were also 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

investigated, because number density of Xe in Cu-BTC and ZIF-8 are largest at low pressure, which is favorable for our observation. Figure 6 shows the contour plot of the probability density of Xe mass center in Cu-BTC and ZIF-8 at 0.8 bar and 11 bar. At low pressure of 0.8 bar, we can see from Figure 6a that most of Xe are adsorbed in the smaller tetrahedron-shaped side pockets in Cu-BTC, while the organic chain and tetrahedral pore adsorbed less molecules. With the pressure increasing to 11 bar, tetrahedron-shaped side pockets are basically filled, and the square-shaped channels and organic linkers have also adsorbed more molecules, which is in agreement with the results from Gutiérrez-Sevillano et al., i.e. nonpolar molecules adsorb preferentially in the small retrahedral cages.50 At low pressure of 0.8 bar, Xe molecules occupy the MeIM organic linker and close to the C=C bond in ZIF-8 (Figure 5c), which agrees with the results of H2 and CH4 in ZIF-8 reported by Wu et al.51-52 With pressure increasing to 11 bar, Xe molecules start to fill up the center of the sodalite (SOD) cage, as shown in Figure 5d. The adsorption behavior of Xe in ZIF-8 is very similar to the results of H253 and CH454 in ZIF-8. Any promising materials for Xe capture require not only high Xe uptake but also excellent selectivity for Xe. The above results show that these materials possess high Xe uptakes, which motivates us to further investigate the gas selectivity of these materials. Because Xe concentration is very low in air, only 0.086 ppmv, we considered the Xe-N2 and Xe-CO2 mixtures with three molar fractions, i.e., CO2 or N2 concentrations are 0.99, 0.999 and 0.9999. Figure 7 shows adsorption selectivities of the selected materials for Xe over N2. For the Xe/N2 mixture with yN 2 = 0.99, with the increase of pressure, the adsorption selectivity of Cu-BTC decreases from 80 to 32 significantly, while the pressure 12


Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


almost has no influence on the selectivity of Xe/N2 in other five materials. The selectivity follows the order of Cu-BTC > ZIF-8 > COP-4 > IRMOF-1 > UMCM-1 > PAF-302, which is exactly the same with the order of difference of isosteric heats (DIH) at zero pressure (see Figure 7e). Actually, this is in excellent agreement with our previous conclusion, i.e., the selectivity of a material is closely related to the DIH.40 Parks et al. studied the selectivities of MOFs for Xe over N2 at infinite dilution,

23

and found that the

selectivities of PCN-14, ZIF-8 and IRMOF-1 is 26, 18.3 and 5.2, respectively, which are in agreement with our simulation results that the selectivities of ZIF-8 and IRMOF-1 are 18.5 and 7. As discussed earlier, Cu-BTC has exposed metal sites, which shows strong affinity to Xe and is favorable for separation of Xe and N2. ZIF-8 has smaller pore size, so it is also good candidate for separating Xe from Xe/N2 mixtures. For the Xe/N2 mixture with yN 2 = 0.999 (Figure 7b), the order of the selectivity basically remains unchanged and selectivities in IRMOF-1 and COP-4 have some fluctuations, while for the Xe/N2 mixture with yN2 = 0.9999 (Figure 7c), the selectivities fluctuate more significantly than those in Figure 7b. The fluctuation is more apparent at low pressure than high pressure. In order to clearly show the effect of N2 concentration on the separation performance of materials, we presented in Figure 7d the selectivity of Cu-BTC for Xe over N2 at different N2 concentrations due to the best performance of Cu-BTC for Xe separation. In general, the selectivity of Cu-BTC for Xe over N2 increases with the increase of concentration of N2 (i.e. the decrease of Xe concentration), while the selectivity trend basically remains the same for different concentrations of N2. Figure 8 displays the adsorption selectivities of the selected materials for Xe over CO2. 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

For the Xe/CO2 mixture with yco2 =0.99, the selectivity of Cu-BTC for Xe quickly reduces from 4.3 to 1 with the increase of pressure. Because the considered system contains only the trace amount Xe, most adsorption sites are occupied by CO2 with the increase of pressure. The selectivity of COP-4 is about 2.6, while the one of ZIF-8 decreases from 2.7 to 1.9 with increasing pressure. The selectivities of PAF-302, IRMOF-1 and UMCM-1 are about 1.8, 1.5 and 1.2, respectively. The COMs with smaller pore size may be the good candidates for the separation of Xe/CO2. For the Xe/CO2 mixture with yco2 =0.999 (Figure 8b), the order of the selectivity basically remains unchanged, but compared with y(CO2)=0.99, the selectivity has more fluctuation, while in the case of yco2 = 0.9999 (Figure 8c), the selectivities fluctuate more violently. The possible reason is that with the concentration increase of CO2, the concentration of Xe is very small, which largely affects the selectivity evaluated by eq(4). Figure 8d shows the selectivities of Cu-BTC at different CO2 concentrations, which basically remain unchanged with the concentration. 4. Conclusions We have used the GCMC simulations to study adsorption and separation of Xe in MOF and COM materials at 298K. Results indicate that PAF-302 shows the highest gravimetric excess uptake of 4009 mg/g at 27 bar due to its large pore size and high accessible surface area, and UMCM-1 shows the excess uptake of 2888 mg/g at 37 bar. The Xe uptake at intermediate pressure follows the order of PAF-302 > UMCM-1 > IRMOF-1 > Cu-BTC > COP-4 > ZIF-8, which is entirely consistent with accessible surface areas. Moreover, the maximum gravimetric excess uptakes of Xe in different 14


Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials exhibit a linear correlation with the accessible surface area. Both maximum volumetric uptakes of Xe in IRMOF-1 and PAF-302 are 216 V(STP)/V) at 37 bar. Simultaneously considering the gravimetric and volumetric uptakes, PAF-302 is an excellent candidate for Xe storage at intermediate pressure. However, at low pressure of p ZIF-8 > COP-4 > IRMOF-1 > UMCM-1 > PAF-302, which is exactly the same with the order of DIH. Actually, this is in excellent agreement with our previous conclusion, i.e., the selectivity of a material is closely related to the DIH. In particular, the selectivity of Cu-BTC for Xe over N2 reaches 80, which is an excellent candidate for Xe separation. For the Xe/CO2 mixtures, these materials have strong affinity to both Xe and CO2, so it is hard to separate Xe and CO2.. Acknowledgements This work is supported by NSF foundation (No. 91334203, 21274011, 21121064), National 863 Program (2013AA031901), National 973 Program (2011CB706900), National Scientific Research Funding (ZZ1304) and Outstanding Talent Funding (RC1301) from BUCT. References 1. 2. 3.

4.

Liu, L. T.; Xu, Y.; Tang, P., Mechanistic Insights into Xenon Inhibition of NMDA Receptors from MD Simulations. J. Phys. Chem. B 2010, 114, 9010-9016. Ooms, K. J.; Wasylishen, R. E., 129Xe NMR Study of Xenon in Isoreticular Metal-Organic Frameworks. Micropor. Mesopor. Mater. 2007, 103, 341-351. Fernandez, C. A.; Liu, J.; Thallapally, P. K.; Strachan, D. M., Switching Kr/Xe Selectivity with Temperature in a Metal-Organic Framework. J. Am. Chem. Soc. 2012, 134, 9046-9049. Thallapally, P. K.; Grate, J. W.; Motkuri, R. K., Facile Xenon Capture and Release at Room Temperature Using a Metal-Organic Framework: A Comparison with Activated 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5. 6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16. 17.

18.

Page 16 of 28

Charcoal. Chem. Commun. 2012, 48, 347-349. Fontaine, J. P.; Pointurier, F.; Blanchard, X.; Taffary, T., Atmospheric Xenon Radioactive Isotope Monitoring. J. Environ. Radioact. 2004, 72, 129-135. Bazan, R. E.; Bastos-Neto, M.; Moeller, A.; Dreisbach, F.; Staudt, R., Adsorption Equilibria of O2, Ar, Kr and Xe on Activated Carbon and Zeolites: Single Component and Mixture Data. Adsorption 2011, 17, 371-383. Munakata, K.; Fukumatsu, T.; Yamatsuki, S.; Tanaka, K.; Nishikawa, M., Adsorption Equilibria of Krypton, Xenon, Nitrogen and Their Mixtures on Molecular Sieve 5A and Activated Charcoal. J. Nucl. Sci. Technol. 1999, 36, 818-829. Foroutan, M.; Nasrabadi, A. T., Adsorption Behavior of Ternary Mixtures of Noble Gases Inside Single-Walled Carbon Nanotube Bundles. Chem. Phys. Lett. 2010, 497, 213-217. Foroutan, M.; Nasrabadi, A. T., Adsorption and Separation of Binary Mixtures of Noble Gases on Single-Walled Carbon Nanotube Bundles. Physica E 2011, 43, 851-856. Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe, M.; Yaghi, O. M., Systematic Design of Pore Size and Functionality in Isoreticular MOFs and Their Application in Methane Storage. Science 2002, 295, 469-472. Millward, A. R.; Yaghi, O. M., Metal-Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature. J. Am. Chem. Soc. 2005, 127, 17998-17999. Xiang, Z.; Cao, D.; Lan, J.; Wang, W.; Broom, D., Multiscale Simulation and Modelling of Adsorptive Processes for Energy Gas Storage and Carbon Dioxide Capture in Porous Coordination Frameworks. Energy Environ. Sci. 2010, 3, 1469-1487. Xiang, Z.; Hu, Z.; Cao, D.; Yang, W.; Lu, J.; Han, B.; Wang, W., Metal–Organic Frameworks with Incorporated Carbon Nanotubes: Improving Carbon Dioxide and Methane Storage Capacities by Lithium Doping. Angew. Chem. Int. Ed. 2011, 50, 491-494. Xiang, Z.; Lan, J.; Shao, X.; Wang, W.; Broom, D. Hydrogen Storage in Mesoporous Coordination Frameworks: Experiment and Molecular Simulation J. Phys. Chem. C 2009, 113, 15106-15109. Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T., Metal-Organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450-1459. Zhou, Y.; Xiang, Z.; Cao, D.; Liu, C., Covalent Organic Polymer Supported Palladium Catalysts for CO Oxidation. Chem. Comm. 2013, 49, 5633-5635. Liu, S.; Xiang, Z.; Cao, D., Zeolitic Imidazolate Framework-8 as a Luminescent Material for the Sensing of Metal Ions and Small Molecules. J. Mater. Chem. 2011, 21, 6649-6653. Xiang, Z.; Cao, D., Synthesis of Luminescent Covalent–Organic Polymers for Detecting Nitroaromatic Explosives and Small Organic Molecules. Macromol. Rapid Comm. 2012, 33, 1184-1190. 16


Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19. Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre, J., Metal-Organic Frameworks - Prospective Industrial Applications. J. Mater. Chem. 2006, 16, 626-636. 20. Pawsey, S.; Moudrakovski, I.; Ripmeester, J.; Wang, L.-Q.; Exarhos, G. J.; Rowsell, J. L. C.; Yaghi, O. M., Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks. J. Phys. Chem. C 2007, 111, 6060-6067. 21. Boutin, A.; Springuel-Huet, M.-A.; Nossov, A.; Gedeon, A.; Loiseau, T.; Volkringer, C.; Ferey, G.; Coudert, F.-X.; Fuchs, A. H., Breathing Transitions in MIL-53(Al) Metal-Organic Framework Upon Xenon Adsorption. Angew. Chem. Int. Edit. 2009, 48, 8314-8317. 22. Liu, J.; Thallapally, P. K.; Strachan, D., Metal-Organic Frameworks for Removal of Xe and Kr from Nuclear Fuel Reprocessing Plants. Langmuir 2012, 28, 11584-11589. 23. Parkes, M. V.; Staiger, C. L.; Perry, J. J.; Allendorf, M. D.; Greathouse, J. A., Screening Metal-Organic Frameworks for Selective Noble Gas Adsorption in Air: Effect of Pore Size and Framework Topology. Phys. Chem. Chem. Phys. 2013, 15, 9093-9106. 24. Xiang, Z.; Cao, D., Porous Covalent-Organic Materials: Synthesis, Clean Energy Application and Design. J. Mater. Chem. A 2013, 1, 2691-2718. 25. Xiang, Z.; Cao, D.; Wang, W.; Yang, W.; Han, B.; Lu, J. Postsynthetic Lithium Modification of Covalent-Organic Polymers for Enhancing Hydrogen and Carbon Dioxide Storage. J. Phys. Chem. C 2012, 116, 5974-5980. 26. Xiang, Z.; Zeng, X.; Cao, D., Tetrahedral Node Diamondyne Frameworks for CO2 Adsorption and Separation. J. Mater. Chem. A 2014, 2, 4899-4902. 27. Cao, D.; Lan, J.; Wang, W.; Smit, B., Lithium-Doped 3D Covalent Organic Frameworks: High-Capacity Hydrogen Storage Materials. Angew. Chem. Int. Ed. 2009, 48, 4730-4733. 28. Lan, J.; Cao, D.; Wang, W.; Smit, B., Doping of Alkali, Alkaline-Earth, and Transition Metals in Covalent-Organic Frameworks for Enhancing CO2 Capture by First-Principles Calculations and Molecular Simulations. ACS Nano 2010, 4, 4225-4237. 29. Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D., A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n. Science 1999, 283, 1148-1150. 30. Lan, J.; Cao, D.; Wang, W.; Ben, T.; Zhu, G., High Capacity Hydrogen Storage in Porous Aromatic Frameworks with Diamond-like Structure. J. Phys. Chem. Lett. 2010, 1, 978-981. 31. Xiang, Z.; Zhou, X.; Zhou, C.; Zhong, S.; He, X.; Qin, C.; Cao, D., Covalent-Organic Polymers for Carbon Dioxide Capture. J. Mater. Chem. 2012, 22, 22663-22669. 32. Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B., Molecular Theory of Gases and Liquids. Wiley: New York 1965. 33. Potoff, J. J.; Siepmann, J. I., Vapor–liquid Equilibria of Mixtures Containing Alkanes, Carbon Dioxide and Nitrogen. AIChE. J 2001, 47, 1676-1682. 34. Ohkubo, T.; Miyawaki, J.; Kaneko, K.; Ryoo, R.; Seaton, N. A., Adsorption Properties 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

35.

36.

37. 38. 39.

40.

41. 42.

43. 44.

45.

46.

47.

48.

49.

Page 18 of 28

of Templated Mesoporous Carbon (CMK-1) for Nitrogen and Supercritical Methane Experiment and GCMC Simulation. J. Phys. Chem. B 2002, 106, 6523-6528. Yang, Z.; Cao, D., Effect of Li Doping on Diffusion and Separation of Hydrogen and Methane in Covalent Organic Frameworks. J. Phys. Chem. C 2012, 116, 12591-12598. Martín-Calvo, A.; García-Pérez, E.; Manuel Castillo, J.; Calero, S., Molecular Simulations for Adsorption and Separation of Natural Gas in IRMOF-1 and Cu-BTC Metal-Organic Frameworks. Phys. Chem. Chem. Phys. 2008, 10, 7085. Garberoglio, G.; Vallauri, R., Adsorption and Diffusion of Hydrogen and Methane in 2D Covalent Organic Frameworks. Micropor. Mesopor. Mater. 2008, 116, 540-547. Mayo, S. L.; Olafson, B. D.; Goddard, W. A. DREIDING: A Generic Force Field for Molecular Simulations. J. Phys. Chem. 1990, 94, 8897-8909. Rappe, A. K.; Casemit, K. S.; Goddard, W. A.; Skiff, W. M., UFF, a Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamic Simulations. J. Am. Chem. Soc. 1992, 114, 10024-10035. Yang, Z.; Peng, X.; Cao, D., Carbon Dioxide Capture by PAFs and an Efficient Strategy To Fast Screen Porous Materials for Gas Separation. J. Phys. Chem. C 2013, 117, 8353-8364. Gupta, A.; Chempath, S.; Sanborn, M. J.; Clark, L. A.; Snurr, R. Q., Object-Oriented Programming Paradigms for Molecular Modeling. Mol. Simul. 2003, 29, 29-46. Peng, X.; Cheng, X.; Cao, D. P., Computer Simulations for the Adsorption and Separation of CO2/CH4/H2/N2 Gases by UMCM-1 and UMCM-2 Metal Organic Frameworks. J. Mater. Chem. 2011, 21, 11259-11270. Talu, O.; Myers, A. L., Molecular Simulation of Adsorption: Gibbs Dividing Surface and Comparison with Experiment. AIChE. J 2001, 47, 1160-1168. Wenzel, S. E.; Fischer, M.; Hoffmann, F.; Froሷba, M., Highly Porous Metal-Organic Framework Containing a Novel Organosilicon Linker − A Promising Material for Hydrogen Storage. Inorg. Chem. 2009, 48, 6559-6565. Jiang, J.; Sandler, S. I., Separation of CO2 and N2 by Adsorption in C168 Schwarzite: A Combination of Quantum Mechanics and Molecular Simulation Study. J. Am. Chem. Soc. 2005, 127, 11989-11997. Greathouse, J. A.; Kinnibrugh, T. L.; Allendorf, M. D., Adsorption and Separation of Noble Gases by IRMOF-1: Grand Canonical Monte Carlo Simulations. Ind. Eng. Chem. Res. 2009, 48, 3425-3431. Frost, H.; Duren, T.; Snurr, R. Q., Effects of Surface Area, Free Volume, and Heat of Adsorption on Hydrogen Uptake in Metal-Organic Frameworks. J. Phys. Chem. B 2006, 110, 9565-9570. Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W.; Xu, J.; Deng, F.; Simmons, J. M.; et. al. Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area. Angew. Chem. Int. Ed. 2009, 48, 9457-9460. Ryan, P.; Farha, O. K.; Broadbelt, L. J.; Snurr, R. Q., Computational Screening of Metal-Organic Frameworks for Xenon/Krypton Separation. AIChE J. 2011, 57, 18


Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1759-1766. 50. Jose Gutierrez-Sevillano, J.; Manuel Vicent-Luna, J.; Dubbeldam, D.; Calero, S., Molecular Mechanisms for Adsorption in Cu-BTC Metal Organic Framework. J. Phys. Chem. C 2013, 117, 11357-11366. 51. Wu, H.; Zhou, W.; Yildirim, T., Hydrogen Storage in a Prototypical Zeolitic Imidazolate Framework-8. J. Am. Chem. Soc. 2007, 129, 5314-5315. 52. Wu, H.; Zhou, W.; Yildirim, T., Methane Sorption in Nanoporous Metal-Organic Frameworks and First-Order Phase Transition of Confined Methane. J. Phys. Chem. C 2009, 113, 3029-3035. 53. Zhou, M.; Wang, Q.; Zhang, L.; Liu, Y.-C.; Kang, Y., Adsorption Sites of Hydrogen in Zeolitic Imidazolate Frameworks. J. Phys. Chem. B 2009, 113, 11049-11053. 54. Guo, H.C.; Shi, F.; Ma, Z.F.; Liu, X.Q., Molecular Simulation for Adsorption and Separation of CH4/H2 in Zeolitic Imidazolate Frameworks. J. Phys. Chem. C 2010, 114, 12158-12165. 55. Materials Studio; Accelrys: San Diego, C., 2008. 56. Krishna, R.; van Baten, J. A., Investigating Cluster Formation in Adsorption of CO2, CH4, and Ar in Zeolites and Metal Organic Frameworks at Suberitical Temperatures. Langmuir 2010, 26, 3981-3992. 57. Amrouche, H.; Aguado, S.; Perez-Pellitero, J.; Chizallet, C.; Siperstein, F.; Farrusseng, D.; Bats, N.; Nieto-Draghi, C., Experimental and Computational Study of Functionality Impact on Sodalite-Zeolitic Imidazolate Frameworks for CO2 Separation. J. Phys. Chem. C 2011, 115, 16425-16432.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

Table 1. Structure data of porous materials

materials

Unit cell Cell angle Sacc Vpore Vfree ρ 2 3 Å º m /g cm /g g/cm3 Cu-BTC a=b=c=26.343 0.80 0.71 0.88 α=β=γ=90 2088.6 ZIF-8 a=b=c=16.991 0.57 0.52 0.92 α=β=γ=90 1278.2 IRMOF-1 a=b=c=25.832 1.33 0.79 0.59 α=β=γ=90 3468.7 UMCM-1 a=b=41.526 α=β=90 4568.5 2.30 0.85 0.39 c=17.492 γ=120 PAF-302 a=b=c=23.7195 2.59 0.82 0.32 α=β=γ=90 5735.0 COP-4 a=38.332,b=22.131 0.86 0.57 0.67 α=β=γ=90 1540.1 c=3.600 The accessible surface areas (Sacc) of materials studied were estimated using the “Atoms Volume & Surfaces” calculation within the Material Studio 4.1 package55 and it was calculated by a probe molecule with diameter equal to the kinetic diameter of N2 (0.368 nm). Vpore was the pore volume of the adsorbent accessible to the gas molecules. Vfree was obtained from the ratio of free volume Vpore to the total volume per unit cell.

Table 2. Force field parameters for adsorbates and adsorbents

molecules Xe N2 (1-site) CO2

atom

ε/kb (K) σ (Å) q (e) 216.9 4.1 0 94.95 3.549 0 C 27.0 2.80 +0.70 O 79.0 3.05 -0.35 MOFs C 47.9 3.47 a H 7.66 2.85 a O 48.158 3.033 a N 39.007 3.263 a Zn 27.718 4.045 a Cu 2.518 3.114 a a The partial charges are taken from different references, for example, IRMOF-1, 56 Cu-BTC, 56 ZIF-8, 57 UMCM-1. 42 PAF-302 and COP-4: no charge

20


Page 21 of 28

Figure 1 Structure illustrations of the selected framework materials (a) IRMOF-1, (b) UMCM-1, (c) Cu-BTC, (d) ZIF-8, (e) COP-4, (f) PAF-302.

300

Excess uptake (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

100 expriment simulation 0

0.0

0.2

0.4 0.6 0.8 Pressure(bar)

1.0

1.2

Figure 2. Adsorption isotherms of Xe in IRMOF-1 at 292K. The open square symbols are experimental data, and the filled cycle symbols are the simulation results.

21



1500

Cu-BTC ZIF-8 IRMOF-1 UMCM-1 PAF-302 COP-4

4000

500

3000 2000 1000 1000

2000

3000

4000

5000 2

6000

Surface area (m /g)

0 0.1

（ a） 1 10 Pressure (bar)

35

400 300


200 100 0 0.0

（ b） 0.2

0.4 0.6 0.8 Pressure (bar)

1.0


30 qst(KJ/mol)


3000


4500 Excess uptake (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

25 20 15

(C)

10

0

5

10 15 20 25 30 uptake (mmol/g)

35

Figure 3. Excess gravimetric isotherms of Xe at 298K in metal-organic frameworks and covalent-organic materials (a) at intermediate pressure, (b) at low pressure. (c) the isosteric heats of Xe. The inset in (a) is maximum uptakes of Xe in different materials changing with the calculated accessible surface area.

22


Page 23 of 28

3

75

0 0.1

60 40


3

3 3

150

80 Cu-BTC ZIF-8 IRMOF-1 UMCM-1 PAF-302 COP-4

Nex(cm (STP)/cm )

225 Nex(cm (STP)/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) 1 10 Pressure (bar)

20 0 0.0

(b) 0.2

0.4 0.6 0.8 Pressure (bar)

1.0

Figure 4. Excess volumetric isotherms of Xe at 298K in metal-organic frameworks and covalent-organic materials. (a) at intermediate pressure, (b) at low pressure.

23



Excess Uptake(mg/g)

2000 Excess Uptake (mg/g)


1500 1000 500 0

(a) 0

10

20 30 40 Pressure (bar)

50


200 150 100 50 0

(b) 0

10

20 30 40 Pressure(bar)

50

CuBTC ZIF-8 IRMOF-1 UMCM-1 PAF-302 COP-4

16

qst (KJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

12

8 (C) 0

3

6 9 Uptake (mmol/g)

12

15

Figure 5. Gravimetric isotherms of (a) CO2 and (b) N2 in metal-organic frameworks and covalent-organic materials at 298K. (c) The isosteric heats of N2 in metal-organic frameworks and covalent-organic materials.

(a) 24


Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c) (d) Figure 6. Contour plots of the probability densities of mass center of Xe in the Cu-BTC and ZIF-8 at different pressures. (a) Cu-BTC at 0.8 bar. (b) Cu-BTC at 11 bar. (c) ZIF-8 at 0.8 bar. (d) ZIF-8 at 11 bar. Dark red, O; brown, Cu; Yellow, C; blue, H; Purple, Zn; lightskyblue, N.

25



100

100

y=0.999

Sads (Xe/N2)

Sads (Xe/N2)

y=0.99

10

1


(a) 0

5 Pressure(bar)

10

1

10

100


(b) 0

5 Pressure(bar)


(c) 0

5 Pressure(bar)

Sads (Xe/N2)

10

1

10

y=0.9999

CuBTC

80

Sads (Xe/N2)

60

40

20

10

0.99 0.999 0.9999

(d) 0

5 Pressure(bar)

10

100 Cu-BTC

ZIF-8

Sads

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

COP-4

10

IRMOF-1 UMCM-1 PAF-302

1

(e) 6

9

12

∆q0st(KJ/mol)

15

Figure 7. Adsorption selectivities of MOFs and COMs for Xe/N2 at different molar fractions. (a) yN 2 =0.99, (b) yN 2 =0.999, (c) yN 2 =0.9999,（d）Adsorption selectivities of Cu-BTC for Xe over N2 at different CO2 concentrations. (e) Adsorption selectivities of MOFs and COMs for Xe/N2 versus the difference of isosteric heats at zero pressure.

26


Page 27 of 28


4 3

Sads (Xe/CO2)

Sads (Xe/CO2)

5

2 1


4 3 2 1

(a) 0

5 Pressure (bar)

5

10


4 3

0

2

(b) 0

5 Pressure(bar)

Cu-BTC Sads (Xe/CO2)

0

Sads (Xe/CO2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

10

0.99 0.999 0.9999

2

1

(c)

0 0

5 Pressure(bar)

10

0

(d) 0

5 Pressure(bar)

10

Figure 8. Adsorption selectivities of MOFs and COMs for Xe/CO2 at different molar fractions. (a) yCO2 =0.99, (b) yCO2 =0.999, (c) yCO2 =0.9999

(d) Adsorption selectivities

of Cu-BTC for Xe over CO2 at different CO2 concentrations.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

Table of Contents Graphics

Adsorption and separation of Xe in metal-organic frameworks and covalent-organic materials Qian Wang, Hui Wang, Shuming Peng, Xuan Peng and Dapeng Cao

28


Adsorption and Separation of Xe in MetalâOrganic Frameworks and

Recommend Documents

Adsorption and Separation of Xe in MetalâOrganic Frameworks and