Theoretical study on the catalysis mechanism of sulfur-doped carbon nanotubes in CO₂ desorption from monoethanolamine solution

The CO₂ absorption by alkanolamine solution has been applied industrially because of its excellent efficiency. However, the energy consumption for CO₂ desorption is high. To reduce the energy consumption, the catalyst is introduced into the alkanolamine capture system. In this study, the catalysis mechanism of sulfur-doped carbon nanotubes (S-CNTs) in CO₂ desorption from monoethanolamine (MEA) solution is explored by simulation based on density-functional theory (DFT) calculations. It was found that compared with the single wall carbon nanotubes (CNTs), the adsorption performance of the key substances in the absorption-desorption process on S-CNTs was different, and the adsorption energy of the reactant MEA was reduced by 1.50 kcal/mol, and the adsorption energy of the absorption intermediates MEACOO⁻_MEAH⁺ was increased by 2.32 kcal/mol, and the adsorption energy of the absorption product carbamate (MEACOO⁻) increased significantly. By transition state searching, energy barrier for the rate-determining step was reduced by 1.15 kcal/mol in the desorption process with S-CNTs as the catalyst, suggesting that S-CNTs contributes to amine regeneration. By observing the Mulliken charge of C atoms in the vicinity of S atoms, it was found that the charge of C atoms changed from electroneutral (0.001 eV) to electronegative (−0.325 eV). The partial density of states (PDOS) of C, N and O atoms from the absorption intermediate MEA⁺COO⁻ and the absorption product MEACOO⁻_MEAH⁺ changes greatly when they adsorbed on CNTs and S-CNTs. In addition, compared with CNTs, the charge density on S-CNTs increases, and the C atoms near the doped S atoms attain obvious electronegativity. Compared with CNTs, the absorption intermediate MEA⁺COO⁻ and the absorption product MEACOO⁻_MEAH⁺ transfer more charges to S-CNTs. This paper is of guiding significance for protecting the environment, maintaining the sustainable development of the energy industry, improving the utilisation rate of raw materials, and reducing the production cost of desorbed CO₂. It aims to provide some theoretical basis for the design of catalysts through the study of the catalysis mechanism of S-CNTs in CO₂ desorption from monoethanolamine solution.