[Objective

]

Direct air capture(DAC) technology based on solid amine adsorbents represents a promising negative carbon emission technology that offers the advantage of flexible deployment across diverse geographical and operational settings

which is of great significance for achieving China's “dual-carbon” goal. The primary objective of this study is to elucidate the underlying microscopic mechanisms governing the interaction between amine-based molecules and carbon dioxide(CO

2

)

thereby providing a theoretical foundation for the rational design and performance optimization of DAC adsorbents. [Methods

]

To achieve this goal

quantum mechanical simulations were employed to systematical

ly investigate the microscopic reaction mechanisms between amine molecules and CO

2

with a particular focus on identifying the detailed reaction pathways and intermediate species involved. The distribution of adsorption energy levels at the reactive sites of various amine modifiers interacting with CO

2

was computationally determined

enabling a quantitative characterization of the energy variations associated with the CO

2

adsorption reaction. Furthermore

the synergistic regulatory mechanisms by which the electronic properties of functional groups and steric hindrance effects influence CO

2

adsorption were thoroughly analyzed. In addition

the competitive adsorption behavior among CO

2

nitrogen(N

2

)

and water(H_2O) was examined

and the influence of multiple factors

including the chemical nature of the amine species

the presence of moisture

and the local steric environment

on adsorption selectivity was systematically assessed. [Results

]

The simulation results reveal that the reaction between amine molecules and CO

2

proceeds via a zwitterion mechanism. In this pathway

the amine group first reacts with CO

2

to form a zwitterionic intermediate that carries both positive and negative charges; this intermediate subsequently undergoes a deprotonation step to yield a carbamate or other stabilized products. When monoethanolamine(MEA) reacts with CO

2

in the absence of additional amine molecules or other auxiliary protontransfer agents

the calculated energy barrier is as high as 147.18 kJ/mol. However

when an additional MEA molecule participates in the proton transfer process

the reaction energy barrier is substantially reduced to 21.86 kJ/mol. In the presence of water molecules acting as proton-transfer mediators

the energy barrier decreases to 54.50 kJ/mol. Among the different amine structures investigated

the linear-chain configuration exhibits the most pronounced enhancement in CO

2

reaction performance

primarily because its reduced steric hindrance increases the availability of chemisorption sites. Notably

even though no chemical bond is formed at the interface between MEA and CO

2

a strong van der Waals interaction persists between them

contributing to the overall adsorption affinity. [Conclusion

]

The findings of this study provide a comprehensive mechanistic understanding of the amine-CO

2

reaction at the molecular level

highlighting the critical roles of proton transfer assistance

steric hindrance

and electronic effects in determining adsorption performance. These insights offer theoretical support for the amine modification and performance enhancement of DAC adsorbents

thereby facilitating the development of more efficient and robust direct air capture technologies.