Carbon Dioxide Chemisorption by Ammonium and Phosphonium Ionic Liquids: Quantum Chemistry Calculations

Abstract

Carbon capture and storage is an important technological endeavor aiming to improve the ecology by combating global warming. The present work investigates reaction paths that are responsible for CO2 chemisorption by the ammonium- and phosphonium-based ionic liquids containing an aprotic heterocyclic anion 2-cyanopyrrolide. We exemplify that 2 mol of CO2 per 1 mol of the gas scavenger can be theoretically fixed by such ionic liquids. Both the cation and anion participate in the chemisorption. The corresponding standard enthalpies and potential energies are moderately negative. The chemisorption reaction, as revealed by the simulations of competing pathways, is started by the donation of the proton from the cation to the anion. The double covalent bond in the cation’s structure emerges. The barriers to all reactions involving the phosphonium-based cation are relatively small and favor practical applications of the considered sorbents. The performance of the ammonium-based cation is less favorable due to the inherent instability of the tetraalkylammonium ylide. The role of phosphonium ylide in the mechanism of the reaction is carefully characterized. The performance of the aprotic anion as a CO2 scavenger is unaffected by the chemical identity of the counterion. The essential heights of the identified steric barriers underline the necessity to simulate the entire structures of the reacting species to obtain a reliable description of chemisorption. The reported results foster a fundamental understanding of the outstanding CO2 sorption performance of the quaternary ammonium- and phosphonium-based 2-cyanopyrrolides.

CO2 Chemisorption Behavior in Conjugated Carbanion-Derived Ionic Liquids via Carboxylic Acid Formation

“…We found that the −COOH formation mechanism is still the most favored (Δ H = −19.43 kJ mol –1 ), while the cation-based carboxylation at the α-C of phosphonium (Δ H = −2.01 kJ mol –1 ) is as unfavorable as the anion-based carboxylation formation (Δ H = −1.13 kJ mol –1 ). The main reason for the unfavorable cation reaction is that the proton transfer from phosphonium to the [MN] anion is highly uphill at 79.56 kJ mol –1 (Figure S18B) because it breaks the conjugation of the original carbanion; in contrast, it is more favorable for the [2-CNpyr] anion . Our calculation results are consistent with the spectroscopy results (Figure ), supporting the CO 2 chemisorption via the carboxylic acid formation pathway.…”

“…The main reason for the unfavorable cation reaction is that the proton transfer from phosphonium to the [MN] anion is highly uphill at 79.56 kJ mol −1 (Figure S18B) because it breaks the conjugation of the original carbanion; in contrast, it is more favorable for the [2-CNpyr] anion. 36 Our calculation results are consistent with the spectroscopy results (Figure 2), supporting the CO 2 chemisorption via the carboxylic acid formation pathway.…”

“…For STSILs in CO 2 chemisorption, besides the anion-involved reaction pathway, CO 2 capture could also be achieved via nucleophilic attack of the basic anion on the alkyl chains in the cation through the ylide formation pathway, which has been demonstrated both experimentally and theoretically. ,− We examined the possibility of this reaction mechanism in the [MN]-derived STSILs upon reacting with CO 2 by DFT (Figure S18). The cation (tetraethylphosphonium, [P 2222 ]) was included and an ion pair model was used to compare the three different reaction mechanisms of CO 2 chemisorption (Figure S18A).…”

Electrochemical Properties and Local Structure of the TEMPO/TEMPO+ Redox Pair in Ionic Liquids

“…In order to simplify the analysis, we will focus only on H CR atoms of imidazolium cations, and H 1A atoms of all N-methylene and P-methylene groups of pyrrolidinium and phosphonium cations. These hydrogen atoms demonstrate the highest RDF peaks in each case (Figure S5) and are known to have enhanced acidity and reactivity. − …”

Electrochemical Properties and Local Structure of the TEMPO/TEMPO+ Redox Pair in Ionic Liquids

“…S5) and are known to have enhanced acidity and reactivity. [49][50][51][52] In the series of imidazolium cations with different side chain lengths, the intensity of the first RDF peak at about 2.50 Å is the greatest for C 8 C 1 im + , while for C 2 C 1 im + and C 4 C 1 im + the probabilities are close (Fig. 7a).…”