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

Abstract

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