Mechanistic insights into the oxidative degradation of amine-containing CO2 adsorbents

Abstract

One of the most challenging issues for large-scale implementation of amine-containing adsorbents for CO₂ capture, is their propensity to oxidative degradation via radical mechanisms. The nature of the early (primary) oxidation species depends on whether the deactivation took place under humid or dry, aerobic or anaerobic conditions. The current theoretical investigation provides new insights into the reaction mechanisms for such degradation products, specifically imine, aldehyde and CO₂, depending on the radical species involved, and the deactivation conditions. A common radical to all reactions referred to as αC^•, corresponds to the abstraction of a hydrogen atom from the α-position with respect to an amine group. In dry anaerobic environment, imine formation involving organic radicals R^• generated thermally, has an activation barrier of 13.54 kcal mol⁻¹. In humid anaerobic environment, the imine formation in the presence of hydroxyl radicals (HO^•) corresponded to much lower activation barriers than organic radicals. However, the generation of HO^• radicals would be difficult in the absence of oxygen. Hydroperoxyl radicals (HOO^•) occur only in the presence of oxygen, but their formation is facilitated in the presence of humidity. Oxidation of amine to aldehyde occurs in two stages, involving oxygen atom implantation on α-carbon, then the formation of aldehyde and ammonia. In dry aerobic conditions, oxygen implantation involving HOO^• has a high activation energy of 19.60 kcal mol⁻¹, while the subsequent reaction into aldehyde has a very low barrier of 2.38 kcal mol⁻¹. In contrast, in humid anaerobic environment, both steps occur in the presence of HO^• radicals, with a much lower activation barrier for the first step than the latter (1.52 vs. 22.34 kcal mol⁻¹). More importantly, under humid aerobic condition, amine oxidation is accelerated as HO^• and HOO^• play complementary roles, with the former facilitating oxygen implantation, while the latter is involved in the carbonyl formation.