A Comprehensive Overview of the Stability of Amine-Functionalized Carbon Dioxide Adsorbents

This overview focuses on important stability issues facing amine-functionalized CO₂ adsorbents, including amine-grafted and amine-impregnated silicas, zeolites, metal-organic frameworks, and carbons. Adsorptive CO₂ capture, particularly by amine-functionalized materials, from typical gas streams such as flue gas, biogas, flare gas, syngas and ambient air has attracted tremendous attention from the scientific community and industry alike. Owing to the exceedingly large volumes of CO₂ emissions, large-scale adsorption processes involving large amounts of materials are required. To achieve economic viability, adsorbent materials should have (i) high CO₂ capacity within a desired range of temperature, (ii) high selectivity for CO₂, and (iii) fast adsorption kinetics while being readily regenerable. In addition, they should be cost-effective and stable over many thousands of adsorption-desorption cycles. Adsorbent stability is a multifaceted issue that includes long-term thermal, hydrothermal, chemical, and mechanical stability. Clearly, the stability of CO₂ adsorbents is of utmost importance since it controls the lifespan of the material. This is particularly crucial for CO₂ capture since the recovered CO₂ has often low commercial value, requiring low-cost separation technologies. Despite the importance of adsorbent stability, the great majority of CO₂ adsorption studies focused on adsorption equilibrium or near-equilibrium properties, kinetics, and selectivity, often using highly pure CO₂ and CO₂/N₂ or CO₂/CH₄ gas mixtures to simulate flue gas or biogas and natural gas.

Adsorbent stability issues may be associated with the feed gas composition and/or the adsorption process operational conditions. Ubiquitous impurities such as water vapor, sulfur-containing compounds such as sulfur oxides (SOx), hydrogen sulfide (H₂S) and carbonyl sulfide (COS), nitrogen oxides (NOx), and oxygen (O₂) are often present in actual feed gases. Therefore, the impact of such feed gas components on the adsorbent performance should be assessed. On a separate front, adsorbent materials may be exposed to harsh operational conditions during cyclic adsorption-desorption processes. This may include elevated desorption temperatures in temperature swing adsorption (TSA) processes, steam regeneration of CO₂-loaded adsorbents, and air exposure during adsorbent cooling after regeneration. This will require high thermal stability and high resistance to steam treatment and oxidative degradation. Herein, the impact of the adsorbent physical and chemical properties, the feed gas composition and characteristics, and the adsorption-desorption operational parameters on the long-term stability of amine-functionalized CO₂ adsorbents will be discussed. Moreover, important insights associated with the underlying deactivation pathways of the adsorbents upon exposure to high temperature, oxygen, dry CO₂, sulfur-containing compounds, nitrogen oxides, oxygen and steam will be provided. Specific measures to address outstanding stability issues will be discussed.