(673f) Multi-Objective Inverse Molecular Design of CO2 Capturing Agents

Aqueous direct air capture (DAC) is a key technology toward a carbon negative infrastructure. Developing sorbent molecules with water- and oxygen-tolerance and high CO2 binding capacity is therefore highly desired.

Herein, we analyze the CO2 absorption chemistries on amines, alkoxides, and phenoxides with density functional theory calculations, and perform inverse molecular design of the optimal sorbent. The alkoxides and phenoxides are found to be more suitable for aqueous DAC than amines thanks to their water-tolerance (overall lower pKa makes alkoxides less prone to protonation by water) and capture stoichiometry of 1:1 (2:1 for amines). All three molecular systems are found to obey the same linear scaling relationship (LSR) between pK_(CO_2 ) and pK_a, since both CO2 and proton are bonded to a nucleophilic site (alkoxy or amine) through a σ-type bonding orbital. Several high-performance alkoxides are proposed from the computational screening. In contrast, phenoxides have relatively poor correlation between pK_(CO_2 ) and pK_a, showing promise for optimization.

We apply genetic algorithm to search the chemical space of substituted phenoxides for the optimal sorbent that "escape" the LSR. The most promising off-LSR candidate phenoxides feature bulky ortho substituents forcing the CO2 adduct into a perpendicular configuration with respect to the aromatic ring. In this configuration, CO2 and a proton from water bind to the phenoxide using different molecular orbitals, thereby decoupling the pK_(CO_2 ) and pK_a. The pK_(CO_2 )-pK_a trend and off-LSR behaviors are then confirmed by experiments, validating the inverse molecular design framework. This work not only extensively studies the chemistry of the aqueous DAC, but also presents a transferrable computational workflow for understanding and optimization of other functional molecules.