3 research outputs found
Separation of Carbon Isotopes in Methane with Nanoporous Materials
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
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
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> expÂ[−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> expÂ[−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>