(201e) First-Principles Modeling of Thermal Degradation of CO2-Loaded Aqueous Amine Solvents

Chemical absorption using an aqueous amine-based solvent appears to be the most promising near-term solution for post-combustion capture of CO₂ from flue gas.^1,2 Monoethanolamine (MEA) has been widely studied and is considered the benchmark solvent. Although it exhibits several advantages including a relatively high absorption rate, high solubility, low volatility, and low viscosity, widespread commercial implementation of MEA-based CO₂ capture systems remains hindered by costs associated with problems arising from solvent degradation and corrosion, in addition to the high parasitic energy consumption during regeneration.^1–8 Makeup of MEA loss from the solvent contributes about (~10 %) to the total cost of CO₂ capture, including the energy requirements for the reclamation process.^9–11 Moreover, some products of degradation may be corrosive and hazardous if released to the environment.^12,13

Thermal degradation has typically been defined as degradation occurring due to elevated temperatures in the stripper, and tends to increase with CO₂ loading.^14,15 Thermal degradation of primary amines including MEA is thought to be initiated by cyclization of carbamates, which may result in the formation of 2-oxazolidinone (OZD) that is a minor product and proposed intermediate.¹⁶ The OZD may undergo further reaction with amines to form diamines, imidazolidinones and urea, which are typically reported to be major degradation products.^14–16 Thermal degradation of CO₂-loaded aqueous amines could be mitigated by reducing the stripper operating temperature. However, lowering the temperature also typically increases the overall energy cost required for solvent regeneration.^17,18 The molecular mechanisms underlying thermal degradation of aqueous amines remain unclear, while an improved mechanistic understanding may provide valuable hints on how to improve the performance of existing solvents and in the rational design and synthesis of more degradation-resistant solvent materials.

In this talk, we will present our recent computational studies on the molecular mechanisms of thermal degradation of aqueous MEA in the CO₂ capture process. We have evaluated key elementary reactions using various computational methods. Static quantum mechanical calculations with an implicit solvent model were used to assess the relative stabilities of possible intermediates and products, and ab initio molecular dynamics (AIMD) simulations combined with metadynamics sampling were employed to predict free-energy barriers for key reaction steps, with particular focus on the effect of direct interaction between reaction intermediates and surrounding water molecules. Our work highlights that the thermal degradation mechanisms are largely governed by the competition of intramolecular and intermolecular interactions, besides steric constraints, which is a strong function of the arrangement and dynamics of water surrounding the intermediates.

References

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