Dissolved oxygen depletion catalyzed by amino acids in piperazine

Abstract

Dissolved oxygen (DO) significantly influences the oxidative degradation of amine solvents used for CO₂ capture from oxygen-containing gases. Amino acids are major degradation products in pilot testing of piperazine (PZ) solvent. This study used oxygen depletion to investigate the effect of metal ions and amino acids on amine oxidation in 5 m PZ. DO consumption was catalyzed by Fe³⁺ with a half-order dependency on Fe³⁺. A reaction order breakpoint occurred at 0.05 mM Fe³⁺, beyond which excess Fe³⁺ likely formed inactive clusters. Amino acids enhance the effective solubility and availability of monomeric Fe³⁺, amplifying its catalytic effect. Adding 0.05 mM ethylenediaminetetraacetic acid (EDTA) with 0.05 mM Fe³⁺ increased the rate constant for oxygen depletion from 0.53 h⁻¹ (clean 5 m PZ) to 6.5 h⁻¹. A plateau in DO consumption rate was seen at a critical Fe³⁺ concentration for each tested amino acid, beyond which additional Fe³⁺ became inactive. At lower Fe³⁺ (0.025 mM), amino acids such as nitrilotriacetic acid (NTA) sequestered Fe³⁺, reducing catalytic potential, while others enhanced it. This suggests that the role of amino acids is more complex than previously understood. Amino acids were ranked by their ability to increase DO consumption with 1 mM Fe³⁺ in 5 m PZ: 1 mM EDTA > 20 mM 1,2-di(piperazin-1-yl)ethan-1-one (PZAAPZ+) > 15 mM NTA > 15 mM N-(2-aminoethyl)glycine (AEG+) > 15 mM ethylenediaminediacetic acid (EDDA) > 15 mM glycine (GLY) > 15 mM iminodiacetic acid (IDA). In a degraded solvent sample from pilot plant testing, degradation products complex metal ions, enhancing their catalytic effects.