Cation Effects of Phosphate Additives for Enhancing the Oxidative Stability of Amine-Containing CO2 Adsorbents

Abstract

Amine-containing adsorbents have been widely investigated for capturing CO₂ but suffer from rapid oxidative degradation. We recently reported that ppm-level metal impurities (Fe and Cu) in commercial amine polymers can accelerate the oxidative degradation of these amine-containing adsorbents, which can be suppressed by adding chelators such as phosphate. In the present study, we investigated the amine-stabilization effects of monocationic phosphate salts containing cations of different charge densities, such as Li⁺, Na⁺, tetramethylammonium (TMA⁺), and tetraethylammonium (TEA⁺). The results showed that phosphate salts containing organophilic cations of low charge densities can effectively stabilize amine polymers. Such organophilic phosphate salts might be better dispersed within the organic amine polymers and have a loosely associated ion-pair form, which is beneficial for binding with metal impurities. Consequently, the adsorbent stabilized with the most organophilic TEAH₂PO₄ exhibited unprecedentedly high stability; specifically, it showed a very minor decrease in CO₂ capacity (<3.5%) even after 1 month of aging in O₂-containing flue gas at 110 °C.