Use of Vanadium(V) Oxide as a Catalyst for CO₂ Hydration in Potassium Carbonate Systems

The kinetics of CO₂ absorption into 30% w/w K₂CO₃ solutions containing 0.1–0.5 M K₄V₂O₇ was investigated at temperatures of 40, 60, and 75 °C using a wetted wall column. Vanadium­(V) speciation diagrams were developed under these conditions as a function of CO₂ loading using ⁵¹V NMR spectroscopy. From these studies it was determined that there are two oxyvanadate ions that promote the absorption of CO₂, HVO₄^2–, and HV₂O₇^3–. The Arrhenius expressions for the rate constants of these two species were found to be <i>k</i>_HVO₄ = 2 × 10¹¹ exp­(−4992/<i>T</i>) and <i>k</i>_HV₂O₇ = 5 × 10¹⁸ exp­(−10218/<i>T</i>), respectively. Comparison of the observed rate constants with other promoters revealed that both active vanadium species showed performances comparable with that of MEA and vastly superior performances over those of other inorganic promoters. Due to speciation changes as the vanadium concentration is increased, the relative performance of vanadium diminished with increasing total vanadium concentration. As such, vanadium may be more suitable as a secondary component and corrosion inhibitor in a promoted carbonate system

Amino Acids as Carbon Capture Solvents: Chemical Kinetics and Mechanism of the Glycine + CO₂ Reaction

Amino acids are potential solvents for carbon dioxide separation processes, but the kinetics and mechanism of amino acid–CO₂ reactions are not well-described. In this paper, we present a study of the reaction of glycine with CO₂ 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 CO₂ with glycine under dilute, idealized conditions, as well as the reactive absorption of gaseous CO₂ 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 NH₂CH₂CO₂^– reacts with CO_2(aq) with <i>k</i> (M^–1 s^–1) = 1.24 × 10¹² exp­[−5459/<i>T</i> (K)], with an activation energy of 45.4 ± 2.2 kJ mol^–1. 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 CO₂, with <i>k</i> (M^–1 s^–1) = 8.18 × 10¹² exp­[−8624/<i>T</i> (K)] and activation energy of 71.7 ± 9.6 kJ mol^–1. 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^–1 s^–1 at 25 °C (versus 2.08 ± 0.18 and 13 900 ± 750 M^–1 s^–1 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 CO₂