Amber Mace

NaKA sorbents with high CO2-over-N2 selectivity and high capacity to adsorb CO2

The uptake of carbon dioxide and nitrogen gas by zeolite NaKA was studied. A very high ideal CO(2)-over-N(2) selectivity and a high CO(2) capacity were observed at an optimal K(+) content of 17 at.%. NaKA is a very promising adsorbent for CO(2) separation from water-free flue gases.

Oxygen-oxygen correlations in liquid water: Addressing the discrepancy between diffraction and extended x-ray absorption fine-structure using a novel multiple-data set fitting technique

The first peak of the oxygen-oxygen pair-correlation function (O-O PCF) is a critical measure of the first coordination-shell distances in liquid water. Recently, a discrepancy has been uncovered between diffraction and extended x-ray absorption fine-structure (EXAFS) regarding the height and position of this peak, where EXAFS gives a considerably more well-defined peak at a shorter distance compared to the diffraction results. This discrepancy is here investigated through a novel multiple-data set structure modeling technique, SpecSwap-RMC, based on the reverse Monte Carlo (RMC) method. Fitting simultaneously to both EXAFS and a diffraction-based O-O PCF shows that even though the reported EXAFS results disagree with diffraction, the two techniques can be reconciled by taking into account a strong contribution from the photoelectron scattering focusing effect in EXAFS originating from nearly linear hydrogen bonds. This many-body contribution, which is usually neglected in RMC modeling of EXAFS data, is included in the fits by precomputing and storing EXAFS signals from real-space multiple-scattering calculations on a large number of unique water clusters. On the other hand, fitting also the O-O PCF from diffraction is seen to enhance the amount of structural disorder in the joint fit. Thus, both structures containing nearly linear hydrogen bonds and local structural disorder are important to reproduce diffraction and EXAFS simultaneously. This work also illustrates a few of many possible uses of the SpecSwap-RMC method in modeling disordered materials, particularly for fitting computationally demanding techniques and combining multiple data sets.

Free energy barriers for CO2 and N2 in zeolite NaKA: an ab initio molecular dynamics approach

Ab initio Molecular Dynamics (AIMD) is used with spatial constraints to estimate the free energy barriers of diffusion for CO2 and N2 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(+) ion or a larger K(+) 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(+) ion effectively blocks the passage of both CO2 and N2 molecules while the smaller Na(+) 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.

K+ Exchanged Zeolite ZK-4 as a Highly Selective Sorbent for CO2

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.

Crystal structure and magnetic properties of the S = 1/2 quantum spin system Cu7(TeO3)6F2 with mixed dimensionality.

The new oxofluoride Cu7(TeO3)6F2 has been synthesized by hydrothermal synthesis. It crystallizes in the triclinic system, space group P1. The crystal structure constitutes a Cu-O framework with channels extending along [001] where the F(-) ions and the stereochemically active lone-pairs on Te(4+) are located. From magnetic susceptibility, specific heat, and Raman scattering measurements we find evidence that the magnetic degrees of freedom of the Cu-O-Cu segments in Cu7(TeO3)6F2 lead to a mixed dimensionality with single Cu S = (1)/2 moments weakly coupled to spin-chain fragments. Due to the weaker coupling of the single moments, strong fluctuations exist at elevated temperatures, and long-range magnetic ordering evolves at comparably low temperatures (TN = 15 K).

Temporal coarse graining of CO2 and N2 diffusion in zeolite NaKA: from the quantum scale to the macroscopic.

The kinetic CO2-over-N2 sieving capabilities in narrow pore zeolites are dependent on the free-energy barriers of diffusion between the zeolite pores, which can be fine-tuned by altering the framework composition. An ab initio level of theory is necessary to accurately compute the energy barriers, whereas it is desirable to predict the macroscopic scale diffusion for industrial applications. Using ab initio molecular dynamics on the picosecond time scale, the free-energy barriers of diffusion can be predicted for different local pore properties in order to identify those that are rate-determining for the pore-to-pore diffusion. Specifically, we investigate the effects of the Na(+)-to-K(+) exchange at the different cation sites and the CO2 loading in Zeolite NaKA. These computed energy barriers are then used as input for the Kinetic Monte Carlo method, coarse graining the dynamic simulation steps to the pore-to-pore diffusion. With this approach, we simulate how the identified rate-determining properties as well as the application of skin-layer surface defects affect the diffusion driven uptake in a realistic Zeolite NaKA powder particle model on a macroscopic time scale. Lastly, we suggest a model by combining these effects, which provides an excellent agreement with the experimental CO2 and N2 uptake behaviors presented by Liu et al. (Chem. Commun. 2010, 46, 4502-4504).