CO2 Absorption Using Fluorine Functionalized Ionic Liquids: Interplay of Hydrogen and σ-Hole Interactions

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Chemistry

Background:

• Ionic liquids (ILs) are increasingly explored for carbon dioxide (CO2) capture due to their tunable properties and potential advantages over traditional methods.
• Investigating the fundamental interactions governing CO2 absorption in ILs is crucial for designing more efficient capture materials.

Purpose of the Study:

• To investigate the noncovalent interactions involved in CO2 capture by specific ionic liquids: S-ethyl-N,N,N ,N -tetramethylthiouronium ([ETT]) and 1-hexyl-3-methylimidazolium ([Hmim]) cations paired with tris(pentafluoroethyl)trifluorophosphate ([FEP]) anion.
• To elucidate the electronic structure of IL-CO2 complexes and understand the mechanisms driving CO2 absorption.

Main Methods:

• Computational derivation of electronic structures for ion pairs and their CO2-absorbed complexes ([ETT][FEP]·n(CO2) and [Hmim][FEP]·n(CO2), n up to 30).
• Analysis of molecular electrostatic potential anisotropy to identify key interaction sites.
• Examination of halogen bonding (O···F), electrostatic C···F interactions, and hydrogen bonding (O···H) contributions.

Main Results:

• CO2 binding is dictated by a combination of halogen bonding, electrostatic interactions between CO2 and fluorine atoms, and cation-mediated hydrogen bonding.
• Observed frequency shifts in infrared spectra correlate with hydrogen and halogen bonding, providing insights into the binding mechanisms.
• Binding energies indicate that [Hmim]-based ILs exhibit higher efficiency for CO2 capture compared to [ETT]-based ILs.

Conclusions:

• The study reveals the intricate noncovalent interactions governing CO2 capture by [ETT][FEP] and [Hmim][FEP] ionic liquids.
• [Hmim][FEP] demonstrates superior performance for CO2 capture applications due to its enhanced binding characteristics.
• Understanding these interactions is key to the rational design of advanced ionic liquids for effective carbon capture technologies.