Carbon dioxide (CO2) capture via the phosphonium ylide mechanism represents an interesting alternative to alkylamine-based carboxamidation. The tertiary cation-based CO2 sorption supplements the nitrogen-containing anion-based CO2 sorption which ultimately results in very competitive aggregate capacities of such ionic compounds. In the present work, we reveal the impact of the cation and anion sizes on the deprotonation and subsequent carboxylation of the α-methylene carbon of the phosphonium-based cations. In particular, we computationally investigate the role of the alkyl chains and rationalize the previously published experimental results. Since the anions strongly influence the kinetics and thermodynamics of the processes involving their counterions, we herein supplemented the triethyl-butyl phosphonium [P2224] and trihexyl-tetradecyl phosphonium [P66614] cations in our models with the benzimidazolide [BENZIM], and 2–methylthio-benzimidazolide aprotic anions [2MTBENZIM]. By having determined the energetic effects, activation barriers, and steric hindrances faced by the above-referenced processes through potential energy surface scans empowered by electronic-structure simulations, we formulated lucid guidelines for the molecular design of the CO2 scavengers of this class. We found that the alkyl chain length plays a marginal role in the reaction profiles for CO2 capture. In turn, an identity of the anion was found to impact the phosphonium ylide formation stages by 20 % by adjusting local intermolecular interactions. By analyzing molecular geometries in their initial, transition, and product states along with the atomic hydrophobicities, we explained the stepwise energetics of the CO2 chemisorption by the phosphonium-based sorbents of complicated structure and two-stage performance. As we did not detect differences of commensurate magnitude between the [P2224] and [P66614] cations paired with both [BENZIM] and [2MTBENZIM], we linked a somewhat deteriorated experimentally detected performance (∼10 %) of [P66614] to differing shear viscosities and, thus, slowed down the diffusion of CO2. For the sake of practical applications, the [P2224]-based ionic liquids look preferable as long as their physicochemical properties meet the criteria of a specific technological task and the costs of their synthesis are comparable to those of the [P66614]-based ones.
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