Effect of Cation on Physical Properties and CO<sub>2</sub> Solubility
for Phosphonium-Based Ionic Liquids with 2‑Cyanopyrrolide
Anions
- Publication date
- Publisher
Abstract
A series of tetraalkylphosphonium
2-cyanopyrrolide ([P<sub><i>nnnn</i></sub>][2-CNPyr]) ionic
liquids (ILs) were prepared
to investigate the effect of cation size on physical properties and
CO<sub>2</sub> solubility. Each IL was synthesized in our laboratory
and characterized by NMR spectroscopy. Their physical properties,
including density, viscosity, and ionic conductivity, were determined
as a function of temperature and fit to empirical equations. The density
gradually increased with decreasing cation size, while the viscosity
decreased noticeably. In addition, the [P<sub><i>nnnn</i></sub>][2-CNPyr] ILs with large cations exhibited relatively low
degrees of ionicity based on analysis of the Walden plots. This implies
the presence of extensive ion pairing or formation of aggregates resulting
from van der Waals interactions between the long hydrocarbon substituents.
The CO<sub>2</sub> solubility in each IL was measured at 22 °C
using a volumetric method. While the anion is typically known to be
predominantly responsible for the CO<sub>2</sub> capture reaction,
the [P<sub><i>nnnn</i></sub>][2-CNPyr] ILs with shorter
alkyl chains on the cations exhibited slightly stronger CO<sub>2</sub> binding ability than the ILs with longer alkyl chains. We attribute
this to the difference in entropy of reaction, as well as the variation
in the relative degree of ionicity