Amine-Functionalized Task-Specific Ionic Liquids: A Mechanistic Explanation for the Dramatic Increase in Viscosity upon Complexation with CO2 from Molecular Simulation

Abstract

The capture of CO2 from fossil fuel combustion, particularly in coal-fired power plants, represents a critical component of efforts aimed at stabilizing greenhouse gas levels in the atmosphere. Alkanolamines have traditionally been used to this end; however, drawbacks such as volatility, degradation, and regeneration costs have been drivers for the development of new, superior technologies. Recently, several seminal studies with ionic liquids (ILs), both experimental and computational, have demonstrated their potential as CO2 capture agents. In traditional ILs, experimental studies with CO2 have revealed its unusually high physical solubility in these media. Complementary simulation studies have provided evidence that this is attributable to CO2 occupying void space within the liquid and favorably interacting with the anion. Recently, a series of second-generation task-specific ionic liquids (TSILs) containing amine functional groups have been synthesized and demonstrated to have much higher capacities for CO2 due to their reactivity with CO2, as well unusually high viscosities in both the neat and complexed states. The current work extends the seminal studies of CO2 capture with ILs by providing insight from simulations into the mechanism responsible for the dramatic increase in viscosity upon complexation. Simulations conclusively demonstrate that the slow translational and rotational dynamics, which are manifest in the high viscosity, may be attributable to the formation of a strong, pervasive hydrogen-bonded network. Semiquantitative estimates of the cation and anion self-diffusion coefficients and rotational time constants, as well as detailed hydrogen bond analysis, are consistent with the experimentally observed formation of glassy or gel-like materials upon contact with CO2. This has significant implications for the design of new approaches or materials involving ILs that take advantage of these preconceived limitations, in the synthesis or manipulation of new TSIL frameworks for CO2 capture, and in novel experimental studies of chemistries and dynamics in persistent heterogeneous environments.