(58a) Ionic Liquid Composite Membranes for CO2/Light Gas Separations

A novel approach for performing these separations is to use room temperature ionic liquids (RTILs) in various morphologies as membranes.  In comparison to conventional polymers, they perform gas separations due to solubility differences.  They can be converted to polymerizable molecules.  This allows promising RTILs to be prepared as membranes.  Thus, they can be converted to various morphologies as membranes while maintaining the inherent selectivity of the material.  They can be prepared as polymer films, composite structures with ionic liquid within the structure and gels.  Polymer films have been tested for gas separations up to 40 bar feed pressure.  The conditioning or morphology change due to the incorporation of gases such as CO₂ at these high pressures is reversible.  This is an important advantage in comparison to conventional polymers.  The wide range of morphologies and chemical structures allow for finding materials that not only have the desired physicochemical properties but also the mechanical properties needed to produce viable membranes.

Mixed matrix membranes (MMMs) look to be a very promising candidate for commercialization and large scale implementation. These MMMs consist of three individual components: polymerizable room temperature ionic liquids (poly(RTILs), normal RTILs, and zeolite particles. These components synergize to produce enhanced gas transport. The polymer matrix allows for facile and economical fabrication, while the zeolite provides excellent separation performance. The RTIL prevents defect formation within the membrane by acting as a wetting agent to help the polymer matrix interface aptly with the zeolite. We have investigated how varying the three components in our MMMs affects membrane permeability and selectivity. This initial study identified the optimal type of zeolite particles. More in depth analysis of the ionic liquid revealed the significance of molar volume in the RTIL and free volume in the poly(RTIL) on the membrane’s gas separation ability. Previous work established the correlation between increasing RTIL molar volume and decreasing CO₂ solubility selectivity. Current work shows how the polymerized analogues (poly(RTILs) display a similar trend with free volume. This resulted in the creation of MMMs made with smaller molar and free volume RTILs and poly(RTILs) that performed above Robeson’s 2008 upper bound for CO₂/CH₄. With this knowledge we aim to devise a systematic design approach for creating state of the art MMMs.