In Situ Investigation of CO2 Separation in Macrocyclic and Porous Ionic Liquids

Abstract

With the continued rise in atmospheric CO2 levels and the potential impact on climate, carbon capture and storage has gained prominence as a potential technological solution to reduce CO2 emissions, resulting in increased interest in advanced materials to facilitate the removal of CO2 from energy-related emissions. While a number of approaches have been explored to separate CO2 from mixed streams, ionic liquids are promising materials due to their negligible vapor pressure, high thermal stability, and the ability to tune physicochemical properties such as CO2 solubility. The incorporation of free volume into ionic liquids through appropriate development of bulky cation/anion pairs or through the creation of porous ionic liquids can impact both gas transport and solubility. Macrocyclic ionic liquids where the bulky cation consists of a nitrogen heterocycle connected by ether linkages can selectively remove CO2 from mixed gas streams and offer a potential avenue for CO2 capture. Porous ionic liquids are emerging materials that combine the attributes of porous materials within a nonvolatile liquid matrix. Permanent porosity is incorporated into the ionic liquid by adding a framework material (zeolitic imidazolate framework) where the cations and anions are too large to diffuse into the host. We investigated the selective absorption of CO2 in both macrocyclic and porous ionic liquids and examined the effect of porosity on CO2 sorption capacity and gas transport. Relevant physical and chemical properties of these unique materials such as solubility, density, viscosity and structure were measured. In addition, analysis of the vibrational modes of both the ionic liquid and the CO2 through in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements was used to better understand the mechanisms of CO2 interaction with the ionic liquids and the impact on solubility and transport. Moreover, we find that certain of these ionic liquids exhibit a unique binding and release of CO2 which could open new avenues for CO2 capture and storage.