Search

This study presents the impact of the side chains in 1‐n‐alkylimidazolium ionomers with varying side chain lengths (CnH2n+1 where n = 1, 4, 10, 16) on Cu‐catalyzed electrochemical CO2 reduction reaction (CO2RR). Longer side chains suppress the H2 and CH4 formation, with the n‐hexadecyl ionomer (n = 16) showing the greatest reduction in kinetics by up to 56.5% and 60.0%, respectively. On the other hand, C2H4 production demonstrates optimal Faradaic efficiency with the n‐decyl ionomer (n = 10), a substantial increase of 59.9% compared to its methyl analog (n = 1). Through a combination of density functional theory calculations and material characterization, it is revealed that the engineering of the side chains effectively modulates the thermodynamic stability of key intermediates, thus influencing the selectivity of both CO2RR and hydrogen evolution reaction. Moreover, ionomer engineering enables industrially relevant partial current density of –209.5 mA cm−2 and a Faradaic efficiency of 52.4% for C2H4 production at 3.95 V, even with a moderately active Cu catalyst, outperforming previous benchmarks and allowing for further improvement through catalyst engineering. This study underscores the critical role of ionomers in CO2RR, providing insights into their optimal design for sustainable chemical synthesis. [ABSTRACT FROM AUTHOR]

The reaction kinetics and molecular mechanisms of CO2 absorption using nonaqueous and aqueous monoethanolamine (MEA)/methyldiethanolamine (MDEA)/2‐amino‐2‐methy‐1‐propanol (AMP) solutions were analyzed by the stopped‐flow technique and ab initio molecular dynamics (AIMD) simulations. Pseudo first‐order rate constants (k0) of reactions between CO2 and amines were measured. A kinetic model was proposed to correlate the k0 to the amine concentration, and was proved to perform well for predicting the relationship between k0 and the amine concentration. The experimental results showed that AMP/MDEA only took part in the deprotonation of MEA‐zwitterion in nonaqueous MEA + AMP/MEA + MDEA solutions. In aqueous solutions, AMP can also react with CO2 through base‐catalyzed hydration mechanism beside the zwitterion mechanism. Molecular mechanisms of CO2 absorption were also explored by AIMD simulations coupled with metadynamics sampling. The predicted free‐energy barriers of key elementary reactions verified the kinetic model and demonstrated the different molecular mechanisms for the reaction between CO2 and AMP. [ABSTRACT FROM AUTHOR]

This study attempts to explain the well-known experimental observation that 1,3-bis(2-aminoethyl)urea (urea) is preferentially formed over the other major product, 2-imidazolidone (IZD), from thermal degradation of aqueous ethylenediamine (EDA) during the CO2 capture process. This is in direct contrast to the case of monoethanolamine (MEA), preferentially forming oxazolidinone (OZD), rather than urea, which undergoes further reactions leading to more stable products. Given their similar molecular structures, the different preferred degradation pathways of EDA and MEA impose an intriguing question regarding the underlying mechanism responsible for the distinct difference. Thermal degradation of both EDA and MEA tends to proceed mainly via formation of an isocyanate intermediate that may further undergo either cyclization to IZD (or OZD) or a reaction with EDA (or MEA) forming urea. For the EDA case, our first-principles simulations clearly demonstrate that the urea formation reaction is kinetically more, but thermodynamically less, favorable than the cyclization reaction; the opposite is true for the MEA case. Our further analysis shows that EDA–isocyanate is less prone to cyclization than MEA–isocyanate, which in turn allows for more facile urea formation. The reconfiguration dynamics of isocyanate is found to be governed by the dynamic nature of the interaction between its terminal group and surrounding solvent molecules. Our work highlights the importance of kinetic effects associated with the local structure and dynamics of solvent molecules around the intermediates that may significantly alter the degradation process of amine solvents. [ABSTRACT FROM AUTHOR]

Thermal degradation of aqueous monoethanolamine (MEA), a benchmark solvent, in CO2 capture processes still remains a challenge. Here, we present molecular mechanisms underlying thermal degradation of MEA based on ab initio molecular dynamics simulations coupled with metadynamics sampling. Isocyanate formation via dehydration of carbamic acid (MEACOOH) is predicted to be highly probable and more kinetically favorable than the competing cyclization–dehydration reaction to 2-oxazolidinone (OZD), albeit not substantially. Isocyanate may undergo cyclization to form OZD, which is found to be more facile in aqueous MEA solution than reaction with MEA to form urea, although the latter is thermodynamically more favorable than the former. Our simulations also clearly demonstrate that OZD is a long-lived intermediate that plays a key role in MEA thermal degradation to experimentally observed products. Overall, this work highlights the importance of entropic contributions associated with the local structure and dynamics of solvent molecules around the intermediates, which cannot be solely explained by thermodynamics, in predicting the mechanism and kinetics of thermal degradation of CO2-loaded aqueous amine solutions. [ABSTRACT FROM AUTHOR]

This study presents the impact of the side chains in 1-n-alkylimidazolium ionomers with varying side chain lengths (C n H 2n+1 where n = 1, 4, 10, 16) on Cu-catalyzed electrochemical CO 2 reduction reaction (CO 2 RR). Longer side chains suppress the H 2 and CH 4 formation, with the n-hexadecyl ionomer (n = 16) showing the greatest reduction in kinetics by up to 56.5% and 60.0%, respectively. On the other hand, C 2 H 4 production demonstrates optimal Faradaic efficiency with the n-decyl ionomer (n = 10), a substantial increase of 59.9% compared to its methyl analog (n = 1). Through a combination of density functional theory calculations and material characterization, it is revealed that the engineering of the side chains effectively modulates the thermodynamic stability of key intermediates, thus influencing the selectivity of both CO 2 RR and hydrogen evolution reaction. Moreover, ionomer engineering enables industrially relevant partial current density of -209.5 mA cm -2 and a Faradaic efficiency of 52.4% for C 2 H 4 production at 3.95 V, even with a moderately active Cu catalyst, outperforming previous benchmarks and allowing for further improvement through catalyst engineering. This study underscores the critical role of ionomers in CO 2 RR, providing insights into their optimal design for sustainable chemical synthesis., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

Amino acid ionic liquids (AAILs) are promising green materials for CO 2 capture and conversion due to their large chemical structural tunability. However, the structural understanding of the AAILs underlying the CO 2 reaction dynamics remains uncertain. Herein, we examine the steric effects of AAIL anions with various chemical structures on CO 2 capture behavior. Based on ab initio free-energy sampling, we assess reaction mechanisms for carbamate formation via a two-step reaction pathway with a zwitterion intermediate undergoing dynamic proton transfer. Our results show that free-energy barriers for carbamate formation can be significantly reduced as the degree of steric hindrance of the anions decreases. Further analyses reveal that reduced steric hindrance of anions causes markedly stronger intermolecular interactions between zwitterion and anions, leading to an increased kinetically favorable intermolecular proton transfer for carbamate production. We also describe the correlation strength between intramolecular interactions within the zwitterion and intermolecular interactions between the zwitterion and anions. We conclude that the favored structural flexibility due to the less steric hindrance of the zwitterion leads to enhanced intermolecular interactions, facilitating proton transfer to nearby AAIL anions for carbamate formation. Our study provides invaluable insight into the influence of various degrees of steric hindrance of the AAIL anions governing CO 2 chemisorption. These findings may aid in the design of optimal AAIL solvents for the CO 2 capture process.

Amino acid ionic liquids have received increasing attention as ideal candidates for the CO 2 chemisorption process. However, the underlying molecular mechanisms, especially those involving proton transfer, remain unclear. In this work, we elucidate the atomistic-level reaction mechanisms responsible for carbamate formation during CO 2 capture by amino acid ionic liquids through explicit ab initio molecular dynamics augmented by well-tempered metadynamics. The resulting ab initio free-energy sampling reveals a two-step reaction pathway in which a zwitterion, initially formed from the reaction between the anion of serine and CO 2 , undergoes a kinetically facile intermolecular proton transfer to the O atom of the COO - moiety in the nearby serine. Further analysis reveals that the significantly reduced free-energy barriers are attributed to enhanced intermolecular interaction between the zwitterion and serine, thus facilitating the kinetic favorability of the proton transfer, which governs the overall CO 2 capture mechanism. This work provides valuable insight into the important mechanistic and kinetic features of these reactions from explicit condensed phase ab initio MD free-energy sampling of the CO 2 capture process.

This study attempts to explain the well-known experimental observation that 1,3-bis(2-aminoethyl)urea (urea) is preferentially formed over the other major product, 2-imidazolidone (IZD), from thermal degradation of aqueous ethylenediamine (EDA) during the CO 2 capture process. This is in direct contrast to the case of monoethanolamine (MEA), preferentially forming oxazolidinone (OZD), rather than urea, which undergoes further reactions leading to more stable products. Given their similar molecular structures, the different preferred degradation pathways of EDA and MEA impose an intriguing question regarding the underlying mechanism responsible for the distinct difference. Thermal degradation of both EDA and MEA tends to proceed mainly via formation of an isocyanate intermediate that may further undergo either cyclization to IZD (or OZD) or a reaction with EDA (or MEA) forming urea. For the EDA case, our first-principles simulations clearly demonstrate that the urea formation reaction is kinetically more, but thermodynamically less, favorable than the cyclization reaction; the opposite is true for the MEA case. Our further analysis shows that EDA-isocyanate is less prone to cyclization than MEA-isocyanate, which in turn allows for more facile urea formation. The reconfiguration dynamics of isocyanate is found to be governed by the dynamic nature of the interaction between its terminal group and surrounding solvent molecules. Our work highlights the importance of kinetic effects associated with the local structure and dynamics of solvent molecules around the intermediates that may significantly alter the degradation process of amine solvents.

Thermal degradation of aqueous monoethanolamine (MEA), a benchmark solvent, in CO2 capture processes still remains a challenge. Here, we present molecular mechanisms underlying thermal degradation of MEA based on ab initio molecular dynamics simulations coupled with metadynamics sampling. Isocyanate formation via dehydration of carbamic acid (MEACOOH) is predicted to be highly probable and more kinetically favorable than the competing cyclization-dehydration reaction to 2-oxazolidinone (OZD), albeit not substantially. Isocyanate may undergo cyclization to form OZD, which is found to be more facile in aqueous MEA solution than reaction with MEA to form urea, although the latter is thermodynamically more favorable than the former. Our simulations also clearly demonstrate that OZD is a long-lived intermediate that plays a key role in MEA thermal degradation to experimentally observed products. Overall, this work highlights the importance of entropic contributions associated with the local structure and dynamics of solvent molecules around the intermediates, which cannot be solely explained by thermodynamics, in predicting the mechanism and kinetics of thermal degradation of CO2-loaded aqueous amine solutions.