(67a) On the Role of Fe2+ and O2 in Oxidative Degradation of Aqueous Monoethanolamine (MEA)

The most suitable technology for post-combustion CO₂ capture appears to be chemical absorption using an aqueous amine-based solvent.^1,2 Monoethanolamine (MEA) has been widely studied and is typically considered the benchmark solvent, and it exhibits several advantages such as relatively fast absorption rates, low viscosity and volatility, and high solubility.^1–8 However, its widespread implementation remains hindered by costs associated with the parasitic energy consumption during regeneration, as well as problems associated with corrosion and solvent degradation.^1–8 Costs incurred from degradation include loss of solvent, the energy requirements for the reclamation process, and the corrosivity and toxicity of the degradation products.^9,10

Oxidative degradation is thought to occur due to the presence of O₂ from the flue gas (~ 3-4%) and metal ions from stainless steel corrosion (Fe²⁺, Cr³⁺, Ni²⁺, Mn²⁺) in the absorber, but the underlying molecular mechanisms remain unclear.^11,12 It has been proposed that oxidative degradation is initiated by an electron or hydrogen abstraction mechanism, whereby a radical abstracts either an electron from the lone pair of N or hydrogen from the amine, respectively.^11,13,14 The radical could be a transition metal complex, such as solvated Fe²⁺, or reactive oxygen species (ROS) formed from dissolved O₂.^15–17 Reported oxidative degradation products in aqueous MEA include ammonia/ammonium, aldehydes, formate/formic acid, glycolate/glycolic acid, oxalate/oxalic acid, acetone, formamides, and imidazolidones.^{11,13,14,18,19} Few theoretical studies have been undertaken to elucidate the mechanisms underlying oxidative degradation,^20,21 while an improved fundamental understanding could aid in the development of inhibitors.

In this work, we elucidate the role of Fe²⁺ and O₂ in the oxidative degradation of MEA in aqueous solution using a theoretical approach. First, we evaluate the energetics of redox reactions and relative stabilities of intermediates from Fe²⁺ and O₂ using static QM calculations. Thereafter, AIMD simulations are used to demonstrate elementary reaction steps and involved during oxidative degradation of MEA assuming Fe²⁺ and O₂ are in aqueous solution.

Our analysis clearly demonstrates that Fe²⁺ can transfer 2 electrons to O₂ in aqueous amine solution to form ferryl (Fe⁴⁺) hydroxide complexes and reactive oxygen species such as superoxide (O₂^-•) and hydroxyl radical (^•OH), which are short-lived intermediates that easily abstract hydrogen from the C-H, N-H and O-H bonds in MEA. These results clearly demonstrate that oxidative degradation by MEA is initiated by hydrogen abstraction, rather than electron abstraction. After hydrogen abstraction, MEA can dissociate to formaldehyde and imine radicals, which can further participate in reactions with H₂O and hydroxyl radical to form acetic acid, ammonia, and possible precursors to glycolic and oxalic acids. The basicity of aqueous amines enhances the electron transfer between Fe²⁺ and O₂ due to the electron donation to Fe²⁺/Fe³⁺ from OH^- ligands, as described in previous studies^22,23 and predicted by our static QM calculations. This study highlights how first-principles studies can play a significant role in assessing the reaction mechanisms and intermediates underlying degradation, while it would be difficult to experimentally isolate and characterize short-lived reactive species involved.

References

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