(122f) Ionic Liquids as Absorption Media for Co2 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. 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. The pure gas solubilities measured in various ILs can make use of both physical and chemical absorption. The solubility of pure gases in ILs is 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 13 bar and temperatures from 10-60 ^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(trifluoro-methylsulfonyl)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 show high absorption of SO₂ without degradation of the IL. This may be beneficial as both SO₂ and CO₂ may be removed in a single separation step removing a separation step from current industrial processes. The physical absorption of gas increases with increased fluorination of the IL. While the addition of non-fluorinate organic functional groups such as ethers, esters, sulfates, and N-acyl-N-sulfonyl imide show modest increases in the CO₂ solubility in the ILs, these functional groups substantially increase the melting points and viscosities of the ILs. Chemical absorption by ILs containing amine, acetate, or other functional groups is superior to physical absorption. These chemically absorbing compounds form weak chemical complexes that have high carrying capacity and keep the energy requirements to regenerate the sorbent 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. Preliminary calculations suggest that successful competition with amine-based scrubbing techniques requires a higher carrying capacity for CO₂ than currently obtained by ILs using only physical absorption. In order to determine the extent of improvement necessary, various ILs are compared to current CO₂ scrubbing solvents such as monoethanolamine (MEA) in a preliminary economic analysis.