Density Functional Theory Computational Study of Alkali Cation

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A Density Functional Theory Computational Study of Alkali Cation-Exchanged sod-ZMOF for CO, N, and CH Adsorption 2

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Jin Shang, Gang Li, Jiaye Li, Liangchun Li, Paul A. Webley, and Jefferson Zhe Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07833 • Publication Date (Web): 16 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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The Journal of Physical Chemistry

A Density Functional Theory Computational Study of Alkali Cation-Exchanged sod-ZMOF for CO2, N2, and CH4 Adsorption Jin Shang,†,‡ Gang Li,◊ Jiaye Li,§ Liangchun Li,# Paul A. Webley,†,‡* and Jefferson Z. Liu«* †

‡

Cooperative Research Center for Greenhouse Gas Technologies (CO2CRC), Australia

Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia

◊

Center for Energy, School of Mechanical & Chemical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia §

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School of Chemistry, Monash University, Clayton, Victoria 3800, Australia

Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, P. R. China

«

Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia

KEYWORDS: Adsorption, Carbon Capture, Density Functional Theory Calculations, sodZMOF

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Abstract

Porous adsorbents are promising for carbon capture and other industrially important gas separations, e.g., CO2/N2/CH4 separations. The zeolite-like Metal-Organic Frameworks (ZMOFs) as a new sub-class of MOFs have charged frameworks, similar to conventional zeolites, which endow them promising potentials such as a high adsorption capacity and different selective molecular admission schemes from those observed in zeolites. This paper presents a density functional theory computational study of alkali cation-exchanged sodalite-like ZMOF (sod-ZMOF) for CO2, N2, and CH4 adsorption. We found that large Cs+ cations favour sites close to the pore aperture so that three Cs+ cations form a positively charged gate, controlling the admission of gas molecules. These gases have an expected sequence of binding energy values: ∆Eads(CO2) > ∆Eads(CH4) > ∆Eads(N2). Interestingly, the energy barrier of gases passing through the gates shows an unusual sequence: ∆Ea(CO2) > ∆Ea(N2) > ∆Ea(CH4). This sequence can be largely attributed to their energy levels at the centers of the gates formed by Cs cations. The electrostatic interaction between the positively charged gate and CO2 leads to a much higher energy level at the gate center. This is in contrast to the corresponding zeolite structures where the apertures are enclosed by negatively charged oxygen atoms. In light of similar molecular structures at the apertures of all reported ZMOFs, our study suggests a new design route in which by appropriate selection of extraframework cations, a unique positively charged gate can be designed that can lead to different gas admission behaviour from conventional zeolite materials.

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1. Introduction Over the past few decades climate change has been a major concern and by consensus deemed to be mainly a consequence of human activities that cause the emission of greenhouse gases, in particular, carbon dioxide.1 One effective way of mitigating CO2 emissions is to capture CO2 from gas mixtures at “point sources”, i.e., flue gas (CO2 and N2 as main components) in power plants and natural gas (CO2, N2, and CH4 as main components) in processing plants. Apart from CO2 capture, recovering CH4 (a greenhouse gas 21 times more potent than CO2) from low grade natural gas not only benefits the environment but also boosts the methane economy. All these activities involve CO2/N2/CH4 separation. Adsorptive gas separation using porous materials is a promising technology, which requires adsorbents with high capacity and selectivity. Typical adsorbents include carbon, zeolites, and Metal-Organic Frameworks (MOFs). Among them, zeolites have advantages of exerting strong affinity on binding gas molecules owing to the polar structure formed by the anionic framework and extraframework cations, although they usually have limited adsorption capacity due to their inherently relatively low surface area and pore volume. In addition to providing binding sites, the extraframework cations can act as “door-keepers” to control guest admission and thus producing exceptionally high adsorption selectivity.2-10 MOFs, synthesized from metal cluster nodes and organic linkers, have a broad range of building blocks and thus virtually unlimited combinatorial possibilities, offering viable designability and flexibility in terms of structure and chemistry.11-15 Compared with zeolites, MOFs can have larger interior pore space, despite their weaker binding ability towards gas molecules due to the non-polar structure.16-17

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Recently, a new class of porous materials have emerged – zeolite-like Metal-Organic Frameworks (ZMOFs). These materials feature combined advantages of both zeolites (polar structure arising from anionic framework and cationic extraframework groups) and MOFs (high pore volume and tuneable surface chemistry).18-23 The superior hydrogen storage performance was demonstrated in several ZMOF candidates thanks to the polar structure.20, 23-24 Also, the potential gas separation ability of some cation-exchanged or amine-grafted ZMOFs has been examined by both experimental and computational studies and shown to have excellent adsorption capacity and adsorbent stability.25-31 However, previous studies neglected the potential of small-pore ZMOFs, which could achieve exceptionally high selective adsorption if appropriate cations are introduced to “keep” the pore aperture and control guest admission.2, 4 In particular, one type of small-pore sodalite-like ZMOF (sod-ZMOF) with potassium as extraframework cations was reported to show pore-blockage for N2 at 77 K in experiments.20 This implies that high adsorptive separation power can potentially be achieved if the poreblockage effect in ZMOF behaves in a similar way to the recently discovered “molecular trapdoor” mechanism where the “door-keeping” cations controls the admission of certain molecules at selected working temperature.4-5 However, no experimental or computational study explores this promising property. In this work, using density functional theory (DFT) calculations, we studied several alkali metal cation-exchanged sod-ZMOFs.20 The distributions of a series of alkali metal cations, including Li+, Na+, K+, Rb+, and Cs+, in the unit cell of the sod-ZMOF were examined. The cations that energetically favour the pore aperture sites are of particular interest in this paper since they directly affect gas admission. Using Cs+-exchanged sod-ZMOF as an illustrating example, we 4


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examined the adsorption and admission of CO2, N2, and CH4 in the sod-ZMOF structure. The most stable adsorption sites and the corresponding energy values were determined. The admission energy barrier for each guest molecule was calculated as well. Interestingly we found that the cations form positively charged gates in the sod-ZMOF in contrast to the negatively charged pore apertures in conventional zeolite structures. The electrostatic interaction between the gates and the CO2 molecule (quadrupole) leads to a higher energy barrier for CO2 admission than that for N2 or CH4. The CH4 has the lowest energy barrier despite its relatively large kinetic diameter. This is very different from that in zeolites. Our results imply an unusual gas separation regime and adsorbent materials design concept.

2. Computational Methodology All results were calculated using the Vienna Ab initio Simulation Package (VASP)32 with the projector augmented waves (PAW) approach.33 The cut-off energy of the plane wave basis-set was 500 eV. A k-point mesh of 3×3×3 was used for one unit cell of sod-ZMOF (including one six-membered ring (6MR) aperture). Such a cut-off energy and k-point mesh were tested to ensure the total energy value convergence within 1 meV/atom. The atomic positions were optimized with the conjugate gradient method until the forces acting on atoms were below 0.015 eV/Å, as suggested by Göltl and Hafner.34 We adopted the standard PBE exchange-correlation functional.35 To account for the van der Waals interactions, we also adopted the DFT-D3 functionals of Grimme with zero damping (IVDW=11) and the Becke-Jonson damping (IVDW= 12).36 Our previous experimental and DFT computation study for a similar system, adsorption of CO2, N2, and CH4 in Cs-exchanged chabazite, concluded that the imperative role of the van der 5



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Waals interaction correction in DFT calculations to predict the correct trend of adsorption energy values of the three different gas molecules, although it could overestimate the adsorption enthalpy values by approximately 10 kJ/mol compared with experiments.7 Owing to the potential overestimation, the PBE results were provided as a reference. For gas adsorption study, a single gas molecule was placed in the sod-ZMOF structure (one gas molecule per unit cell), and then the gas–sod-ZMOF complex was fully relaxed in DFT calculations. The adsorption energy/binding energy (at zero K) of different gas molecules is defined as the difference between the total energy of the gas–sod-ZMOF adsorption complex and the total energy of the sod-ZMOF augmented by the total energy of an isolated gas molecule: ∆Eads = Etot(sod-ZMOF+gas) – Etot(sod-ZMOF) – Etot(gas). In order to study the nature of bonding between adsorbate and adsorbent, we also performed analysis on (1) electron densities of the gas–sod-ZMOF complex using the DDEC (Density Derived Electrostatic Chemical net atomic charges)37-39 method, and (2) the difference-electron density, which is calculated as the difference between the electron density of the gas–sod-ZMOF complex and the electron density of the sod-ZMOF augmented by that of an isolated gas molecule, at PBE level. Our previous study showed that there were minor differences for both DDEC and difference-electron density analyses between the PBE and DFT-D levels.7

3. Results and Discussion 3.1. Crystal structure of sod-ZMOF

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The sod-ZMOF structure is shown in Figure 1. Each indium (In) metal atom coordinates four bis(bidentate) 4,6-pyrimidicarboxylate ligands through four In-O and four In-N bonds, forming the eight-coordinated molecular building block (MBB), with each ligand chelating two individual In atoms. The MBBs assemble in a fashion that gives rise to the formation of truncated octahedra (β-cages) that are further connected through shared square windows, forming a net with sodalite topology (Figure 1a).40 The sod-ZMOF structure contains two types of pore apertures – six-membered ring (6MR) aperture and four-membered ring (4MR) aperture, if the pseudo In-In bonds are used to represent the pore frame (Figure 1a). Clearly, the actual pore aperture boundary or effective pore aperture, which is relevant to guest admission, is formed by the carbonyl oxygen atoms of the ligands that intrude into the 6MR passage (Figure 1b). Specifically, three ligands present symmetrically on each side of the 6MR aperture, respectively, contributing six oxygen atoms (each ligand provides two) to form the effective 6MR aperture (Figure 1b) – we term it 6MR-gate. Concomitantly, the two oxygen atoms derive from two adjacent ligands (each ligand provides one) together with the other two equivalent oxygen atoms from another 6MR-gate form the 4MR-gate (Figure 1c). Accordingly, the approximate dimensions of these effective pore apertures are 4.8 Å for the 6MR-gate and ∆Ea(N2) > ∆Ea(CH4). CO2 has the highest energy barrier, because it has the strongest binding in front of the 6MR-gate and the most positive ∆Eads at the 6MR-gate. A deeper potential well of CO2 in front of the 6MR-gate certainly contributes to its higher ∆Ea value. This is similar to diffusion of ethane and ethylene in ZIF-7, in which the deeper energy well of ethane in front 19



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of the ZIF-7 aperture leads to a higher barrier.42-43 The high binding energy value of CO2 at the center of the 6MR-gate also plays a very important role. If we compare the CO2 and CH4 at the DFT-D3 level, the binding energy difference at the global minimal site is approximately 0.087 eV whereas the difference is 0.186 eV at the 6MR-gate. Overall the difference in energy barrier results is approximately 0.273 eV. The potential energy hill at the 6MR-gate center actually plays a more significant role. Table 2. The energy barrier of gas passing through the “partially closed” 6MR-gate in Cs-sodZMOFa

a

(eV)

∆Ea(CO2)

∆Ea(N2)

∆Ea(CH4)

PBE

0.358

0.159

0.103

IVDW=11

0.434

0.199

0.181

IVDW=12

0.440

0.211

0.167

The energy barrier was calculated by comparing the binding energy of a molecule at the center of the gate with that

at the global minimum.

The Cs-sod-ZMOF has an interesting feature in striking contrast to zeolite crystals: the small 6MR-gate is enclosed by three positive charged Cs cations, whereas the apertures in zeolite are enclosed by negative charged oxygen atoms. For a small molecule with a strong multipole moment passing through these two oppositely charged gates, its energy states can be very different. For example, it has been observed that the aperture center of Chabazite zeolite is a stable adsorption site for CO2.4,

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This is different from our Cs-sod-ZMOF case. Therein,

although the extraframework Cs cations cannot “completely” block the gate of sod-ZMOF to 20


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afford guest discrimination based on “molecular sieving” or “molecular trapdoor” mechanism,4 they do form a unique positively charged gate that can be used to discriminate certain molecules (with multipole moments), such as CO2, N2, and CH4. A careful study of all the ZMOF materials reported so far reveals that the molecular structures of apertures share similar feature to the sodZMOF. If designed properly (as shown in this paper), introducing appropriate extraframework cations could form the positive charged gates that could lead to different gas adsorption and separation behaviour in comparison with the open pore zeolites. The opposite sequence between the energy barrier for admission and the binding energy for these three gases would have an influence in real-world applications in CO2 capture. Given that the thermal energy of CO2 at an operating temperature is lower than its related energy barrier for diffusion, the kinetic separation mechanism would dominate and results in selective adsorbing N2 and CH4 from CO2, which is not good for CO2 capture, provided that CO2 is usually the major component in the gas mixture and selective adsorption of minor component is more energy costeffective. On the contrary, if the thermal energy of CO2 exceeds its energy barrier, equilibrium separation would take over giving rise to selective adsorption of CO2 from N2 and CH4 thanks to the larger binding energy for CO2, which is good for CO2 capture application. Further experimental evidence will confirm the real-world performance of this material for CO2 capture and separation.

4. Conclusions In this paper, we have examined the sod-ZMOF exchanged with alkali (i.e., Li, Na, K, Rb, and Cs) cations and the gas (CO2, N2, CH4) adsorption and admission using DFT calculations at the 21



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PBE and the DFT-D3 levels. We found that for this system, the results obtained at both levels show an overall similar trend. We found that larger cations including Cs and Rb prefer to occupy sites that are close to the 6MR-gate and directly affects the admission of gas molecules in the sod-ZMOF, while small cations including Li, Na, and K do not. A further evaluation of the stability of a cation residing at a certain site shows that Cs cation tends to block the pore aperture (6MR-gate) most stably. Regarding gas adsorption, an unusual trend of energy barriers for passing through the Cs-blocked pore aperture is obtained: ∆Ea(CO2) > ∆Ea(N2) > ∆Ea(CH4). Our analysis shows that this sequence can be largely attributed to the energy level of different gases at the center of the positively charged gate formed by the Cs cations. Our study provides a new idea to design advanced absorbent materials with admission schemes different from those of zeolites.

5. Author Information Corresponding Authors *Email: [email protected]; Tel: +61-3-99053627 *Email: [email protected]; Tel: +61-3-90357873

6. Acknowledgement

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This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government. J.S., P.A.W., and J.Z.L acknowledge the Australian Research Council for providing the funding (DP2013000024).

7. Supporting Information Additional details including results calculated at DFT-D3 with zero damping (IVDW = 11) and tables listing distances between atoms can be found in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org

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(37) Manz, T. A.; Sholl, D. S., Chemically Meaningful Atomic Charges That Reproduce the Electrostatic Potential in Periodic and Nonperiodic Materials. J. Chem. Theory Comput. 2010, 6, 2455-2468. (38) Manz, T. A.; Sholl, D. S., Methods for Computing Accurate Atomic Spin Moments for Collinear and Noncollinear Magnetism in Periodic and Nonperiodic Materials. J. Chem. Theory Comput. 2011, 7, 4146-4164. (39) Manz, T. A.; Sholl, D. S., Improved Atoms-in-Molecule Charge Partitioning Functional for Simultaneously Reproducing the Electrostatic Potential and Chemical States in Periodic and Nonperiodic Materials. J. Chem. Theory Comput. 2012, 8, 2844-2867. (40) Navarro, J. A. R.; Barea, E.; Salas, J. M.; Masciocchi, N.; Galli, S.; Sironi, A.; Ania, C. O.; Parra, J. B., H2, N2, Co, and Co2 Sorption Properties of a Series of Robust Sodalite-Type Microporous Coordination Polymers. Inorg. Chem. 2006, 45, 2397-2399. (41) Hudson, M. R.; Queen, W. L.; Mason, J. A.; Fickel, D. W.; Lobo, R. F.; Brown, C. M., Unconventional, Highly Selective Co2 Adsorption in Zeolite Ssz-13. J. Am. Chem. Soc. 2012, 134, 1970-1973. (42) Gücüyener, C.; van den Bergh, J.; Gascon, J.; Kapteijn, F., Ethane/Ethene Separation Turned on Its Head: Selective Ethane Adsorption on the Metal−Organic Framework Zif-7 through a Gate-Opening Mechanism. J. Am. Chem. Soc. 2010, 132, 17704-17706. (43) van den Bergh, J.; Gücüyener, C.; Pidko, E. A.; Hensen, E. J. M.; Gascon, J.; Kapteijn, F., Understanding the Anomalous Alkane Selectivity of Zif-7 in the Separation of Light Alkane/Alkene Mixtures. Chemistry – A European Journal 2011, 17, 8832-8840.

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TOC IMAGE


Density Functional Theory Computational Study of Alkali Cation

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