Inverse molecular design of alkoxides and phenoxides for aqueous direct air capture of CO2

Significance Direct air capture (DAC) of CO2 is essential for carbon net negative technologies. The sorbent properties of amines, alkoxides, and phenoxides are computationally screened for their suitability for DAC. The alkoxides and phenoxides are found to be more suitable for aqueous DAC than amines because of their greater water tolerance and capture stoichiometry of 1:1, in contrast to 2:1 for amines. Higher CO2 binding constants broadly correlate with higher pKa, which can lead to an undesirable acid–base reaction with water. Structural properties that lead to greater CO2 binding at milder pKas were identified and experimentally validated. This study identifies promising sorbent candidates for aqueous DAC, as well as presents a computational workflow to optimize other classes of functional molecules.

Scheme S1. Schematics showing the conversion among molecular structure, gene representation for GA search, and one-hot encoded vector representation for predictive ML model. Figure S1. The LFER fitting with experimental pKa using three computational methods for !"#$%& : DLPNO-CCST(T)/def2-QZVPP, PWPB95-D3/def2-QZVPP, and CBS-QB3, with their R 2 coefficients labeled next to the dashed fitting lines. The computational method and its corresponding LFER parameters adopted in this work are labeled on the lower right corner. Figure S2. Scatter plots of (a) p '( ! versus p )* , (b) p '( ! versus p )+ , and (c) p )+ versus p )* for investigated primary and secondar amines, with the fitting parameters and R 2 labeled above each plot. Figure S3. The LFER fitting with experimental pKa using six SQM computational methods for !"#$%& with their R 2 coefficients labeled next to the dashed fitting lines. The computational method and its corresponding LFER parameters adopted in this work are labeled on the upper right corner. Figure S4. The evolution diagram of the most favorable Δ ,-.! in the current population as a function of the number of sampled candidates during the ten independent GA searches. Figure S5. The electrostatic potential-mapped van der Waals surface of (a) CO2, (b) phenoxide, and (c) 2,6-xylenoxide. Red and blue regions represent positive or negative electrostatic potential. Minima and maxima points are plotted as cyan and orange spheres, respectively. Unit of all labeled electrostatic potentials are kcal/mol. Figure S6. Non-covalent interaction map of (a) phenoxide-CO2 adduct, (b) 2,6-xylenoxide-CO2 adduct, and (c) 2,6-di-tert-butylphenoxide-CO2 adduct. Red and green regions on isosurfaces correspond to repulsive and attractive non-covalent interactions, respectively.   Note S1. Discussion of the molecular characteristics of the top 10 candidate from GA search, beside the bulky ortho substituents.
The meta groups are not in vicinity of the O site, so they should not have any direct interactions with the bound adsorbates as the ortho groups do. However, the prevalence of bulky meta groups in the top 10 suggests a secondary steric effect: bulky meta groups can crowd ortho groups into configurations that maximizes their steric interactions with the oxo site, and that indirectly promotes the perpendicular adsorption configuration of CO2. In addition, the meta groups, of course, influence the reactivity of the oxo via electronic effects.
The para groups are in an even farther position from the O site. We expect them to also have the aforementioned secondary steric effects, but with a much smaller magnitude. Their influence on the O site should be mostly from electronic effects.
Alkoxy groups have a moderate volume near the CO2-binding site, which enables them to promote the formation perpendicular adsorption configuration of CO2 at any positions on the ring. However, due to the flexible O bridge, they are not as bulky as alkyl groups, which is the reason why they rarely appear on the ortho position in the GA search. Besides, alkoxy groups are also strong electron donating groups, especially at the para position, and that can increase the nucleophilicity of the O site (Hirshfeld charge on O in phenoxide has a negative shift by 0.02 |e| after introduction of para -OMe) and strengthen CO2 binding.
The effect of vinyl groups should be primarily steric. Due to the Pi conjugation, vinyl groups tend to adopt an in-plane configuration, which maximizes its effective bulkiness and promotes the perpendicular adsorption configuration of CO2. They are also expected to have electronic effects through resonance, but we expect such influence to be minor and unfavorable (Hirshfeld charge on O in phenoxide has a positive shift by 0.02/0.05 |e| after introduction of meta/ortho -OMe). This is likely the reason why vinyl-containing molecules did not make it into the top 5.