Theoretical and Computational Chemistry

Inverse Molecular Design of Alkoxides and Phenoxides for Aqueous Direct Air Capture of CO2



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. In this work, we analyze the CO2 absorption chemistries on amines, alkoxides, and phenoxides with density functional theory (DFT) calculations and search for the optimal sorbent using an inverse molecular design strategy. The alkoxides and phenoxides are found to be more suitable for aqueous DAC than amines thanks to their water-tolerance 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 the nucleophilic binding site through a majorly σ 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 (GA) to search the chemical space of substituted phenoxides for the optimal sorbent. Several promising candidates that break the LSR are discovered. 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 utilizes a different molecular orbital for binding than does the proton, and the pK_(CO_2 ) and pK_a are thus decoupled. 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.


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Supplementary material

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Supplementary Information for Inverse Molecular Design of Alkoxides and Phenoxides for Aqueous Direct Air Capture of CO2
Schematics of the conversion between data representations; LFER fitting plots of DFT and SQM calculations; Scatter plots showing correlation among pK_(CO_2 ), pK_a1, and pK_a2 of amines; the evolution diagram of the ΔG_bind during the GA searches; the ESP and NCI map of the investigated substituted phenoxides and CO2 adducts; validation of the trained NN model; experimental set-up for CO2 absorption capacity measurement; the H-bond geometry and bond order between water and CO2-bound phenoxides.