Atomistic Simulations of CO2 During “Trapdoor” Adsorption onto Na-Rho Zeolite

Bamberger, Nathan D.; Kohen, Daniela

doi:10.1007/978-981-10-1128-3_10

Cited by 3 publications

(10 citation statements)

References 23 publications

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“…This would opportunistically allow CO2 to partly enter the channel as the cation gate undergoes thermal fluctuations -although no actual crossing of the window was observed. 11 Figure 2 shows a rare event observed during classical simulation as the carbon dioxide molecule enters a D8R blocked by a cation (in blue). Presumably the fact that other guest molecules lack such strong interaction with the zeolite, result in their incapability to squeeze by the gating cation.…”

Section: Introductionmentioning

confidence: 99%

Molecular Insight into CO₂ “Trapdoor” Adsorption in Zeolite Na-RHO

Coudert

Kohen

2017

Chem. Mater.

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Among other zeolites, sodium-substituted zeolite Na-RHO exhibits a phenomenon known as "trapdoor" adsorption, with extra-framework cations acting as gates to small windows in the zeolite structures. While carbon dioxide can diffuse through these gates, methane and other guests cannot, leading to strong potential for gas separation. This effect has been attributed in the literature to specific cation-guest interactions which would allow to CO2 to pull the trapdoor open. We investigate here the gating phenomenon using ab initio molecular dynamics simulations combined with free energy methods. Our findings invalidate this previously-proposed mechanism, showing that presence of CO2 does not significantly affect the motion of the gating cation. We put forward and demonstrate an alternative mechanism, showing that the thermal motion of the cations are of large amplitude and that CO2 is able to squeeze while the gate is open, while nonpolar guests such as methane cannot. This brings an image of the mechanism that is closer to swinging doors than trapdoors. ACKNOWLEDGMENTWe thank Fabien Trousselet, Rodolphe Vuilleumier, Anne Boutin, and Marie-Laure Bocquet for stimulating discussions. We acknowledge access to HPC platforms provided by a GENCI grant (x2016087069).

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Section: Introductionmentioning

confidence: 99%

Molecular Insight into CO₂ “Trapdoor” Adsorption in Zeolite Na-RHO

Coudert

Kohen

2017

Chem. Mater.

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“…At low pressures, the steep rise of CO 2 absorption may be a consequence of pore filling through incompletely blocked 8R in the dehydrated phase . The CO 2 prefers to move closer to the Cd 2+ sitting slightly outside the 8R aperture, and the dynamic motion of cations Cd 2+ allows an open access of partially blocked windows and CO 2 absorption to occur . It shows the most negative enthalpies of absorption −46.45 ± 0.44 kJ/mol from near-zero coverage to ∼0.1 mol/mol TO 2 and a gradual increase until 0.17 mol/mol TO 2 , indicating similar interactions with the cations throughout this stage in Figure B.…”

Section: Resultsmentioning

confidence: 99%

“…The CO 2 molecules probably occupy two sites, one coordinated to Na + cations in the 8R sites or Cs + in the D8R (small fraction) and the other coordinated to 6R cations in the neighboring 6R within the alpha cages. 38 At low pressures, CO 2 molecules appear to first absorb preferentially to 6R Na + , showing the most negative enthalpies of absorption near-zero coverage in Figure 7B. With increasing pressure, the momentary "trapdoor" motion of Na + cations in the 8Rs occurs to permit CO 2 molecules to enter the alpha cage through window regions (8R or D8R), and the corresponding enthalpy reaches a plateau at 0.03 mol/mol TO 2 (0.2 P/P°).…”

Section: ■ Experimental Methodsmentioning

confidence: 99%

“…7 The CO 2 prefers to move closer to the Cd 2+ sitting slightly outside the 8R aperture, and the dynamic motion of cations Cd 2+ allows an open access of partially blocked windows and CO 2 absorption to occur. 38 It shows the most negative enthalpies of absorption −46.45 ± 0.44 kJ/mol from near-zero coverage to ∼0.1 mol/mol TO 2 and a gradual increase until 0.17 mol/mol TO 2 , indicating similar interactions with the cations throughout this stage in Figure 8B. After passing through the 8R windows, it is possible that more CO 2 molecules enter the alpha cage and form closer coordination to Cd 2+ in the S6R and 8R.…”

Section: ■ Experimental Methodsmentioning

confidence: 99%

“…The further entropy increase is relatively flat through 0.22 mol/mol TO 2 , suggesting a well-coordinated ordering of absorbed CO 2 molecules, distributed around the crystallographic sites of extra-framework cations. 38 In the final stage, entropy increases with increasing CO 2 loading, representing weak ordering for the additional absorption of CO 2 molecules. CO 2 molecules prefer to occupy two sites in Li,H-RHO, one in the middle of each 8R within the window region and the other between two extra-framework cations in the neighboring 6R near the walls of the alpha cages.…”

Section: ■ Experimental Methodsmentioning

confidence: 99%

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Thermodynamics of H₂O and CO₂ Absorption and Guest-Induced Phase Transitions in Zeolite RHO

2018

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Gas absorption calorimetry at low pressures has been employed to probe the interaction of water and carbon dioxide with several ion-exchanged zeolite RHO samples. It reveals that guest-induced framework flexibility transitions from a less hydrated (initially anhydrous) acentric form to the more hydrated centric phase during water absorption. The differential enthalpy of absorption as a function of water loading directly identifies the strengths of different interactions along with possible water binding mechanisms. Interactions with CO 2 are weaker, and no phase transition is seen except in Li,H-RHO. The most negative initial enthalpies have been obtained in both water and CO 2 absorption for Cd and Cs-RHO. However, Li,H-RHO shows the strongest capture ability for CO 2 despite a less exothermic initial enthalpy of absorption. The derived differential enthalpy, chemical potential, and entropy elucidate the thermodynamic behavior of multiple interactions of small guest molecules (H 2 O and CO 2 ) and ion-exchanged flexible zeolite frameworks.

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