(234f) Ionic Liquids as Absorption Media for Pure and Mixed Gas Capture

Ionic liquids are being investigated for a variety of applications including reaction media, separation solvents, non-volatile electrolytes, heat transfer fluids, and gas capture. Our research interest focuses specifically on how ionic liquids (ILs) can be designed for specific and selective gas separations. One potential application for this research is the separation of industrial flue gases for removal of environmentally hazardous gases. The ILs' negligible vapor pressure and tunable properties make them ideal to replace volatile and/or corrosive solvents currently being used for these processes. To this end, gases of interest to our study include carbon dioxide, sulfur dioxide, oxygen, and nitrogen, as well as mixtures of these gases. Selection of different combinations of anions and cations influences the physical properties and functionality of an IL. Thus, one focus of this work is to understand the structure property relationship needed for enhanced gas solubility.

We are also interested in how the presence of CO₂ affects the solubility of other gases in ILs. CO₂ is quite soluble in ILs, while reactive gases such as O2 are only sparingly soluble. The presence of CO₂ may enhance the solubility of the sparingly soluble gases like O₂ in ILs but mixed gas solubility data are scarce. Consequently, another goal of this work is to measure the solubility of lower solubility gases in CO₂/IL mixtures.

Gases dissolve in ILs via a physical absorption mechanism, a chemical complexation mechanism, or a combination of the two. The measurements of the solubility of pure gases in ILs are carried out using accurate gravimetric microbalances; an Intelligent Gravimetric Analyzer (Hiden Analytical Limited, England) and a Magnetic Suspension Balance (Rubotherm, Germany). The gas absorption is determined by measuring the equilibrium mass uptake by the liquid for pressures up to 10 bar and temperatures from 10 to 60 ^oC. In all the cases studied, solubility increases with an increase in pressure and decrease in temperature. Results show that even common ILs like 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf₂N]) have excellent selectivity for CO₂ relative to N₂ and other components in flue gas based on pure physical absorption. In addition, ILs exhibit high absorption capacity for SO₂ without degradation of the IL. This may be beneficial as both SO₂ and CO₂ could be removed in a single separation step, thereby eliminating one step from current industrial processes.

The physical absorption of gas increased with increased IL fluorination, though chemical absorption by ILs containing amine, acetate and other functional groups was found to yield much higher capacities than physical absorption. These chemically absorbing compounds form weak chemical complexes that have high carrying capacity and keep the energy requirements to regenerate the solvent low. To date, chemically complexing ILs are capable of achieving a ten fold increase in the carrying capacity of ILs for CO₂ while maintaining good selectivity relative to other flue gas components. Desorption of CO₂ was much slower for the chemically absorbed gas than for the physically absorbed gas. This is a significant disadvantage even if the energy requirements for regeneration are less than conventional amines. Therefore, we will show that we are able to measure the gas solubility in various ILs using both physical and chemical absorption.

The mixed gas solubility of CO₂/O₂ was measured in [hmim][Tf₂N]. The solubility of O₂ increased substantially compared to its respective pure gas solubility at all pressures. Thus, we show that CO₂ does, indeed, enhance the solubility of O₂ even at low pressures in [hmim][Tf₂N]. We will also report preliminary simulation results of this mixed gas system.