3 research outputs found

    Separation of Carbon Isotopes in Methane with Nanoporous Materials

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    Traditional methods for carbon isotope separation are mostly based on macroscopic procedures such as cryogenic distillation and thermal diffusion of various gaseous compounds through porous membranes. Recent development in nanoporous materials renders opportunities for more effective fractionation of carbon isotopes by tailoring the pore size and the local chemical composition at the atomic scale. Herein we report a theoretical analysis of metal–organic frameworks (MOFs) for separation of carbon isotopes in methane over a broad range of conditions. Using the classical density functional theory in combination with the excess-entropy scaling method and the transition-state theory, we predict the adsorption isotherms, gas diffusivities, and isotopic selectivity corresponding to both adsorption- and membrane-based separation processes for a number of MOFs with large methane adsorption capacity. We find that nanoporous materials enable much more efficient separation of isotopic methanes than conventional methods and allow for operation at ambient thermodynamic conditions. MOFs promising for adsorption- and membrane-based separation processes have also been identified according to their theoretical selectivity for different pairs of carbon-isotopic methanes

    Distributions of Hydrochloric Acid between Water and Organic Solutions of Tri‑<i>n</i>‑octylphosphine Oxide: Thermodynamic Modeling

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    Tri<i>n</i>-octylphosphine oxide (TOPO) is a widely used extractant because of its high extractive ability. However, there is no systematic research on the thermodynamics of TOPO/<i>n</i>-dodecane in the separation of hydrochloric acid (HCl) from aqueous solution. In this study, the liquid–liquid equilibrium (LLE) system (water + <i>n</i>-dodecane + TOPO + HCl) was investigated. Both the equimolar series and slope methods were used to determine the composition of the complex formed in the equilibrated organic phase. The form of the water molecules in the equilibrated organic phase was first investigated by the thermodynamic method. The thermodynamic model was established with the Pitzer equation for aqueous phase and both Margules and organic Pitzer equations for the organic phase. Two chemical equilibrium constants and their corresponding interaction parameters were regressed from experimental LLE data. The correlated results were in good agreement with the experimental data. Furthermore, this model can also be used to predict the organic phase composition for this system. This confirmed that the thermodynamic model chosen was suitable for the extraction system

    Amino Acids as Carbon Capture Solvents: Chemical Kinetics and Mechanism of the Glycine + CO<sub>2</sub> Reaction

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    Amino acids are potential solvents for carbon dioxide separation processes, but the kinetics and mechanism of amino acid–CO<sub>2</sub> reactions are not well-described. In this paper, we present a study of the reaction of glycine with CO<sub>2</sub> in aqueous media using stopped-flow ultraviolet/visible spectrophotometry as well as gas/liquid absorption into a wetted-wall column. With the combination of these two techniques, we have observed the direct reaction of dissolved CO<sub>2</sub> with glycine under dilute, idealized conditions, as well as the reactive absorption of gaseous CO<sub>2</sub> into alkaline glycinate solvents under industrially relevant temperatures and concentrations. From stopped-flow experiments between 25 and 40 °C, we find that the glycine anion NH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub><sup>–</sup> reacts with CO<sub>2(aq)</sub> with <i>k</i> (M<sup>–1</sup> s<sup>–1</sup>) = 1.24 × 10<sup>12</sup> exp­[−5459/<i>T</i> (K)], with an activation energy of 45.4 ± 2.2 kJ mol<sup>–1</sup>. Rate constants derived from wetted-wall column measurements between 50 and 60 °C are in good agreement with an extrapolation of this Arrhenius expression. Stopped-flow studies at low pH also identify a much slower reaction between neutral glycine and CO<sub>2</sub>, with <i>k</i> (M<sup>–1</sup> s<sup>–1</sup>) = 8.18 × 10<sup>12</sup> exp­[−8624/<i>T</i> (K)] and activation energy of 71.7 ± 9.6 kJ mol<sup>–1</sup>. Similar results are observed for the related amino acid alanine, where rate constants for the respective neutral and base forms are 1.02 ± 0.40 and 6250 ± 540 M<sup>–1</sup> s<sup>–1</sup> at 25 °C (versus 2.08 ± 0.18 and 13 900 ± 750 M<sup>–1</sup> s<sup>–1</sup> for glycine). This work has implications for the operation of carbon capture systems with amino acid solvents and also provides insight into how functional groups affect amine reactivity toward CO<sub>2</sub>
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