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 CO2(aq) with k (M–1 s–1) = 1.24 × 1012 exp[−5459/T (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 k (M–1 s–1) = 8.18 × 1012 exp[−8624/T (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.