(392b) Gas Solubility and Selectivity Correlation to Free Volume in Ionic Liquids

Introduction

Research using ionic liquids (ILs) in gas separation membranes is now entering its second decade. The versatility and tunability of ILs has resulted in a number of unique materials being produced for membrane-based CO₂ separations. Approaches to IL-based membrane materials include supported liquid membranes, polymerized ILs or poly(ILs), block co-polymers interfaced with ILs, poly(IL)-IL composites and gels. Given the advancements over the past several years in the state of IL materials development, we now see a need to re-visit some fundamental aspects of ILs in order to better outline a path forward for IL-based membranes.

Experimental and Results

While fractional free volume (FFV) is known to play a significant role in the properties and performances of polymer gas separation membranes, little focus has been given to developing relationships between FFV and IL-based materials. We recently calculated FFV values for 165 combinations of 1-n-alkyl-3-methylimidazolium ILs ([C_nmim][X]) using the COSMOtherm package. FFV was found to be a function of IL molar volume, and as such, earlier models for gas solubility based on Regular Solution Theory can now be reinterpreted in terms of free volume. In concert with experimental data, trends and relationships of gas solubility for various cation/anion pairs were directly linked to computational results. For gases such as CH₄ and N₂, it was determined that as FFV increases, solubility increases in a manner that is directly proportional to free volume. However, within a given IL family (e.g. same anion with various cations), solubility of CO₂ was observed to decrease with increasing FFV. Using experimental data, the influence of free volume on gas solubilities has been determined, and suggests that CO₂ selectivity is a function of the free volume to the (-3/2) power.

Conclusions

Furthermore, we are now exploring new ways to control IL FFV as a means of improving CO₂ solubility, selectivity and permeability in IL-based membranes, using COSMOtherm as a means of guiding materials design.  Also, modeling of various functionalized positions of the imidazolium cation can aid in synthesizing high FFV polymer membranes.  Initial experimental and computational results obtained for these new generations of IL-based membranes will also be presented.