Chemical Kinetics of the Autoxidation of Poly(ethylenimine) in CO₂ Sorbents

Abstract

The oxidative degradation rates of a CO 2 sorbent composed of a mesoporous alumina impregnated with poly-(ethylenimine) (PEI) are measured under systematically varied conditions and a reaction rate law is created. Good agreement is shown between the rate of oxidation obtained via in situ calorimetric heat measurement during oxidative degradation reactions and the loss of CO 2 capture performance presented as amine efficiency (mol CO 2 /mol amine). PEI mass loss and elemental composition are tracked over the course of the reaction and used in conjunction with the oxidation rate measurements to shed insight into the oxidation reaction(s). These data, in combination with measurements of the heat of reaction, suggest a common reaction set across the range of temperatures, oxygen concentrations, and sorbent compositions tested. The data are consistent with the basic autoxidation scheme (BAS), the accepted mechanism of autoxidation of aliphatic polymers. We propose a lumped kinetic model to describe the oxidation reaction set and estimate an activation energy of 105 kJ/mol and an oxygen reaction order of 0.5−0.7 from the data accordingly. These parameters can be incorporated into process cycle models to estimate the material lifetime, a critical uncertainty in the deployment of DAC technologies.

Direct air capture: process technology, techno-economic and socio-political challenges

“…The most recent work on sorbent development has focussed on understanding and mitigating oxidative degradation and urea formation in silica-supported amines, which have been found to occur during elevated-temperature sorbent regeneration and in CO 2rich atmospheres, respectively. [227][228][229][230][231] This has involved the use of functionalisation which modifies the distribution of primary, secondary and tertiary amines, 229 and to remove oxidation-catalysing metal ions, 232 which together results in an order of magnitude decrease in deactivation rate compared to the archetypal PEI on mesoporous silica. The use of hierarchical silica structures with meso-and macro-porosity, which may be more suitable for scale-up, results in increased CO 2 capacity under DAC conditions with PEI, 233 whilst replacing PEI with poly(propylenimine) (PPI) on mesoporous silicas results in increased capacity and higher resistance to oxidative degradation (Fig.…”

Recent advances in direct air capture by adsorption

“…105,[130][131][132][133][134] The chemical kinetics of the oxidation of PEI have also been well illustrated to quantitatively estimate the material lifetime. 135 Considering that water vapor is a ubiquitous component of air, understanding its impact on CO 2 adsorption is critical. In the presence of amine-containing adsorbents, water is an excellent promoter of CO 2 adsorption; it increases the rate of CO 2 capture and is even necessary for long-term stability.…”

“…Accelerated oxidative treatment by exposing sorbent samples to an oxygen atmosphere is usually deployed to simulate the oxidative processes in actual applications for amine-loaded adsorbents. 119,130,[132][133][134][135]254,[382][383][384][385][386]391 The variation in CO 2 capacity or amine efficiency before and after accelerated oxidative treatment is used to determine the oxidative stability of the samples. This variation commonly results from the O 2 -induced deactivation.…”

“…Therefore, the quantitative estimation of sorbent lifetimes requires an insightful investigation of oxidative degradation rates. Sakwa-Novak et al 135 recently proposed an experimental framework to identify the oxidative kinetics of PEI-impregnated γ-Al 2 O 3 in simulated ambient air. The extent of oxidation over time was expressed in terms of the oxidation reaction heat from DSC testing.…”

“…105,130–134 The chemical kinetics of the oxidation of PEI have also been well illustrated to quantitatively estimate the material lifetime. 135…”