Inverse molecular design of alkoxides and phenoxides for aqueous direct air capture of CO <sub>2</sub>
Zisheng Zhang, Amanda L. Kummeth, Jenny Y. Yang, Anastassia N. Alexandrova
Abstract
Aqueous direct air capture (DAC) is a key technology toward a carbon negative infrastructure. Developing sorbent molecules with water and oxygen tolerance and high CO 2 binding capacity is therefore highly desired. We analyze the CO 2 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 (lower p K a prevents protonation by water) and capture stoichiometry of 1:1 (2:1 for amines). All three molecular systems are found to generally obey the same linear scaling relationship (LSR) between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mrow> <mml:mtext mathvariant="bold">CO</mml:mtext> </mml:mrow> </mml:mrow> <mml:mn mathvariant="bold">2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mtext mathvariant="bold">a</mml:mtext> </mml:msub> </mml:mrow> </mml:math> , since both CO 2 and proton are bonded to the nucleophilic (alkoxy or amine) binding site through a majorly <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>σ</mml:mi> </mml:math> bonding orbital. Several high-performance alkoxides are proposed from the computational screening. Phenoxides have comparatively poorer correlation between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mrow> <mml:mtext mathvariant="bold">CO</mml:mtext> </mml:mrow> </mml:mrow> <mml:mn mathvariant="bold">2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mtext mathvariant="bold">a</mml:mtext> </mml:msub> </mml:mrow> </mml:math> , showing promise for optimization. We apply a genetic algorithm to search the chemical space of substituted phenoxides for the optimal sorbent. Several promising off-LSR candidates are discovered. The most promising one features bulky ortho substituents forcing the CO 2 adduct into a perpendicular configuration with respect to the aromatic ring. In this configuration, the phenoxide binds CO 2 and a proton using different molecular orbitals, thereby decoupling the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mrow> <mml:mtext mathvariant="bold">CO</mml:mtext> </mml:mrow> </mml:mrow> <mml:mn mathvariant="bold">2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mtext mathvariant="bold">a</mml:mtext> </mml:msub> </mml:mrow> </mml:math> . The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mrow> <mml:mtext mathvariant="bold">CO</mml:mtext> </mml:mrow> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>–</mml:mo> <mml:mtext mathvariant="bold">p</mml:mtext> <mml:msub> <mml:mrow> <mml:mi mathvariant="bold-italic">K</mml:mi> </mml:mrow> <mml:mtext mathvariant="bold">a</mml:mtext> </mml:msub> </mml:mrow> </mml:math> 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.