We examine the diffusion of methane in the metal–organic frameworks M2(dobdc) (M = Mg, Ni, Zn; dobdc4– = 2,5-dioxido-1,4-benzenedicarboxylate) as a function of methane loading through a combination ...of nuclear magnetic resonance and molecular dynamics simulations. At low gas densities, our results suggest that favorable CH4–CH4 interactions lower the free energy barrier for methane hopping between coordinatively unsaturated metal sites and thus enhance the translational motion of methane down the c-axis. At higher gas densities, CH4–CH4 interactions become more significant, CH4–CH4 collisions become more frequent, and the gas self-diffusion begins to decrease. Finally, we observe that the self-diffusion coefficient of methane is inversely related to the binding energy at the coordinatively unsaturated metal sites, such that diffusion is most rapid in the Zn2(dobdc) framework.
(Sc2(OH)2(BPTC)) (H4BPTC = biphenyl-3,3′,5,5′-tetracarboxylic acid), MFM-400 (MFM = Manchester Framework Material, previously designated NOTT), and (Sc(OH)(TDA)) (H2TDA = thiophene-2,5-dicarboxylic ...acid), MFM-401, both show selective and reversible capture of CO2. In particular, MFM-400 exhibits a reasonably high CO2 uptake at low pressures and competitive CO2/N2 selectivity coupled to a moderate isosteric heat of adsorption (Q st) for CO2 (29.5 kJ mol–1) at zero coverage, thus affording a facile uptake–release process. Grand canonical Monte Carlo (GCMC) and density functional theory (DFT) computational analyses of CO2 uptake in both materials confirmed preferential adsorption sites consistent with the higher CO2 uptake observed experimentally for MFM-400 over MFM-401 at low pressures. For MFM-400, the Sc–OH group participates in moderate interactions with CO2 (Q st = 33.5 kJ mol–1), and these are complemented by weak hydrogen-bonding interactions (O···H–C = 3.10–3.22 Å) from four surrounding aromatic −CH groups. In the case of MFM-401, adsorption is provided by cooperative interactions of CO2 with the Sc–OH group and one C–H group. The binding energies obtained by DFT analysis for the adsorption sites for both materials correlate well with the observed moderate isosteric heats of adsorption for CO2. GCMC simulations for both materials confirmed higher uptake of EtOH compared with nonpolar vapors of toluene and cyclohexane. This is in good correlation with the experimental data, and DFT analysis confirmed the formation of a strong hydrogen bond between EtOH and the hydrogen atom of the hydroxyl group of the MFM-400 and MFM-401 framework (FW) with H–OEtOH···H–OFW distances of 1.77 and 1.75 Å, respectively. In addition, the accessible regeneration of MFM-400 and MFM-401 and release of CO2 potentially provide minimal economic and environmental penalties.
Classical molecular dynamics and Grand Canonical Monte Carlo simulations are carried out for sorbates (CO2 and N2) in zeolite NaKA using a universal type ab initio force field. By combining the ...results of these simulations, we reproduce the CO2 uptake as a function of the K+ content in the zeolite NaKA as measured experimentally by Liu et al. The experiment yielded an exceptionally high CO2-over-N2 selectivity of >200 at a specific K+/(K+ + Na+) ratio of 17 atom %. This high selectivity could be attributed to the nonlinear uptake dependency of the K+/(K+ + Na+) ratio measured for both CO2 and N2. Additionally, our simulations show a strong coupling between the self-diffusion of CO2 and the site-to-site jumping rate of the extra-framework cations. These results show that this nonlinear uptake dependency of CO2 is the result of molecular sieving. Following this, our simulations conclude that both thermodynamic and kinetic contributions must be taken into account when modeling the uptake of this and similar materials with the same functionalities.
Theoretical calculations are performed on a model zeolite A where sodium ions have successively been exchanged with potassium. Using both isolated cluster and periodic DFT calculations, we made an ...attempt to explain how the chemisorbed carbonate species in the material contribute to the exceptionally high CO2 over N-2 selectivity of nearly 200 found in recent experiments Liu et al. 1 with the zeolite NaKA as adsorbent to capture and separate carbon dioxide from a gas mixture containing nitrogen. We have shown that the high carbonate forming at the potassium positions in the 8R windows (KII) results in a larger 8R window diameter potentially enhancing the CO2 uptake if adsorption is measured for individual gases.
Ab initioMolecular Dynamics (AIMD) is used with spatial constraints to estimate the free energy barriers of diffusion for CO sub(2) and N sub(2) gas molecules in zeolite NaA and KA. We investigate ...the extent to which the diffusion of these gas molecules is hindered, in the two separate cases of a smaller Na super(+) ion or a larger K super(+) ion blocking the 8-ring pore window. In contrast to classical Molecular Dynamics, AIMD performs these computations accurately and unbiased in the absence of empirical parameterization. Our work has resulted in stable and reliable force profiles. The profiles show that the larger K super(+) ion effectively blocks the passage of both CO sub(2) and N sub(2) molecules while the smaller Na super(+) ion will allow both molecules to pass. These results are a quantitative demonstration of the concept of pore blocking where we compute the effect, which the size of the respective cation occupying the pore window has on diffusive properties of each gas molecule. Hence, this effect can be altered through ion exchange to fine-tune the functionality of a specific zeolite as a molecular sieve.
Adsorbents with high capacity and selectivity for adsorption of CO2 are currently being investigated for applications in adsorption-driven separation of CO2 from flue gas. An adsorbent with a ...particularly high CO2-over-N2 selectivity and high capacity was tested here. Zeolite ZK-4 (Si:Al ∼ 1.3:1), which had the same structure as zeolite A (LTA), showed a high CO2 capacity of 4.85 mmol/g (273 K, 101 kPa) in its Na+ form. When approximately 26 at. % of the extraframework cations were exchanged for K+ (NaK-ZK-4), the material still adsorbed a large amount of CO2 (4.35 mmol/g, 273 K, 101 kPa), but the N2 uptake became negligible (<0.03 mmol/g, 273 K, 101 kPa). The majority of the CO2 was physisorbed on zeolite ZK-4 as quantified by consecutive volumetric adsorption measurements. The rate of physisorption of CO2 was fast, even for the highly selective sample. The molecular details of the sorption of CO2 were revealed as well. Computer modeling (Monte Carlo, molecular dynamics simulations, and quantum chemical calculations) allowed us to partly predict the behavior of fully K+ exchanged zeolite K-ZK-4 upon adsorption of CO2 and N2 for Si:Al ratios up to 4:1. Zeolite K-ZK-4 with Si:Al ratios below 2.5:1 restricted the diffusion of CO2 and N2 across the cages. These simulations could not probe the delicate details of the molecular sieving of CO2 over N2. Still, this study indicates that zeolites NaK-ZK-4 and K-ZK-4 could be appealing adsorbents with high CO2 uptake (∼4 mmol/g, 101 kPa, 273 K) and a kinetically enhanced CO2-over-N2 selectivity.
There is currently a rare opportunity to inform emerging efforts to implement coastal and marine spatial planning (CMSP) in the United States, Europe and elsewhere around the world. In particular, ...the newly formed US National Ocean Council is developing a strategic action plan for CMSP over the next 18–24 months. In order to identify priority needs for significantly advancing CMSP, a group of experts in the science, policy and practice of CMSP developed recommendations for (1) process development, (2) communication and engagement efforts, (3) tradeoff and valuation analyses, and (4) decision support. Some of these priorities are supported by existing activities in the United States and elsewhere. Others have yet to be addressed and merit immediate attention.
► An international panel of experts developed a short list of top-priorities for advancing CMSP. ► Priorities fell into four categories: process, communication, tradeoff assessment, and decision support. ► Many existing research and management activities can be used to inform and advance these priorities.
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