LCS-FY2021-PP-02
Cost Evaluation of Direct Air Capture (DAC) Process (Vol. 3):
Evaluation of adsorption heat and capacity of DAC sorbent
Summary
For the purpose of performance evaluation of the sorbent to be used in DAC (Direct Air Capture), the structure of CO2 adsorbed on the alkyl amine tethered on the mesoporous silica surface and the heat of adsorption were estimated by the ONIOM (our own N-layered integrated molecular orbital and molecular mechanics) method.
APFD/6-311+G (2d, p) was used for the higher layer of the ONIOM method, and B3LYP/6-31G (d) or APFD/STO-3G was used for the lower layer of the same. The adsorption equilibrium constants and coverage of CO2 were calculated based on the normal vibrational frequencies and free energies of the optimized structures. If the interaction between the amine molecule and the silica surface is weak, the equilibrium constant near room temperature is small and it is not possible to capture low concentrations (400 ppm) of CO2 in the atmosphere. Adsorption structures applicable to DAC were found when the amine areal density on the silica surface is high and the interaction between adjacent amine molecules and the OH groups on the surface is strong. The 400 ppm of CO2 can be collected at an amine efficiency of 0.2 when the structure has 71 kJ/mol of heat of adsorption, and at an amine efficiency of 0.5 when the structure has 86 kJ/mol of heat of adsorption. In order to reduce the energy required for CO2 desorption and recovery by further decreasing the heat of adsorption, development and study of an adsorbent based on a novel mechanism instead of a simple Langmuir mechanism is necessary.