Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity

Abstract

The deployment of transformational nonaqueous CO2-capture solvent systems is encumbered by high viscosities even at intermediate uptakes. Using single-molecule CO2 binding organic liquids as a prototypical example, we present key molecular features that control bulk viscosity. Fast CO2-uptake kinetics arise from close proximity of the alcohol and amine sites involved in CO2 binding in a concerted fashion, resulting in a Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of internal hydrogen bonds between the two functional groups determines the solution viscosity. Unlike the ion pair interactions in ionic liquids, these observations are novel and specific to a hydrogen-bonding network that can be controlled by chemically tuning single molecule CO2 capture solvents. We present a molecular design strategy to reduce viscosity by shifting the proton transfer equilibrium toward a neutral acid/amine species, as opposed to the ubiquitously accepted zwitterionic state. The molecular design concepts proposed here are readily extensible to other CO2 capture technologies.

Two coexisting liquid phases in switchable ionic liquids

“…Similarly, negative characteristic ionic peaks, e.g., Interestingly, the observed solvent peaks (i.e., m/z + 152 and 153) and CO2-bonded molecules (i.e., m/z + 349, 367, 451) coexist in the CO2-loaded SWILs, even at 100% CO2 complexation, where one would expect the only species to be the CO2-bonded ion pair, represented by m/z + 367 and 451. This observation confirms the prediction from a recent DFT study, 13 which hypothesized that SWILs should have two phases, a polar ionic (CO2-bonded) phase and a non-ionic phase. The heterogeneous liquid structure observed here differs from ILs, where the solvent remains 100% ionic regardless of the presence of CO2 complexation in solution.…”

“…12 Density Functional Theory (DFT) calculations were used to study the solvent structure of [1-((1,3-dimethylimidazolidin-2-ylidene)amino)propan-2-ol]. 13 The simulations predicted clusters of ions that change their size and shape as a function of gas loading. Saunders et al observed similar (5-10 nm) cluster sizes (albeit on a switchable carbamate) using two-dimensional (2D) Nuclear Magnetic Resonance (NMR).…”

“…It is likely that the non-linear viscosity increase with CO2 loading observed in our earlier work [29][30][31] is linked to the shape and size of the ionic clusters growing in solution. 13 Also, non-ionic (low polarity) regions may provide voids with high CO2 solubility and unreacted solvent available to complex with CO2, accounting for the higher than expected CO2 mass transfer in and out of SWILs. 32 This may explain how a non-polar additive dissolves into solution to change the solvation free energies of the ions in solution, facilitating CO2 release.…”

Reinventing Design Principles for Developing Low‐Viscosity Carbon Dioxide‐Binding Organic Liquids for Flue Gas Clean Up

“…In our recent computational studies, we showed proof‐of‐principle that the molecular structure can be modified to increase intramolecular hydrogen bonding in CO 2 ‐bound molecules that will in turn induce viscosity reductions . Here, two guanidine‐based CO 2 BOL variants (1‐PADM‐2‐BOL and 1‐IPADM‐2‐BOL) bearing linear and branched alkyl alcohol, respectively, in the side chain, were chosen as case study to validate the theory‐generated hypothesis.…”

“…We previously showed that a critical indicator of the high viscosity of CO 2 BOLs is the hydrogen bonding network . Hydrogen bonding and viscosity correlations in ionic liquids (ILs) have also been discussed at length .…”