Ionic Liquids as CO 2 Capture Media

1. Ionic Liquids as CO2 Capture Media Jessica L. Anderson, JaNeille K. Dixon, Mark J. Muldoon, Joan F. Brennecke, and Edward J. Maginn Chemrawn XVII Wednesday, 11 July, 2007

2. Motivation • Designing ILs for specific and selective gas separations – particularly CO2 capture • ILs as absorbent for separation of flue gas • CO2, N2, O2, H2O, NOx, SOx, etc. • Supported liquid membranes • Absorber/strippers • Packed beds • Replace volatile and/or corrosive solvents currently used for acid gas capture • Better understand the structure relationships for gas solubility in ILs

3. Cations Anions Imidazolium Pyridinium Cl-Br- Tf2N- NO3- PF6- TfO- BF4- DCA- Tetraalkylammonium Tetraalkylphosphonium Ionic Liquids • Salts with a melting point below 100 oC • Vast number of ILs can be made • 1018 possible room-temperature ILs!!

4. Ionic Liquids • Liquid over a large temperature range • Greater than 300 oC! • High thermal stability to 200 oC or higher Imidazolium Tf2N- Abbreviations: 1-hexyl-3-methyl-imidazolium [hmim]+ bis(trifluoromethylsulfonyl)imide [Tf2N]- [hmim][Tf2N]

5. Ionic Liquid Properties • ILs can be tailored by choice of cation and anion • Properties can be varied by choice of anion, cation and substituents • ILs have negligible vapor pressures (“green” potential) • No contamination of IL into gas stream • No loss of IL from evaporation • Good solvation properties • Has been shown to dissolve polars, non-polars, organic, inorganic, aromatics • Thermally stable • Reusable/recyclable

6. Research Equipment • Vapor-Liquid Equilibrium • IGA • Low pressure (0-20 bar) • Small sample (~75 mg) • Rubotherm • High pressure/high temperature • Larger sample (~1.5 g) Intelligent Gravimetric Analyzer (IGA) -Hiden Analytical, Inc. Rubotherm

7. solubility pressure solubility temperature [hmim][Tf2N] Pure Gas Solubility - CO2 • Gas solubility • Important for reusability of ILs • Absorb at low T • Remove at high T • Trend seen for CO2 solubility in all ILs measured Muldoon, et al., Manuscript Submitted

8. [hmpy][Tf2N] Pure Gas Solubility – Other gases • Gas solubility measured in [hmpy][Tf2N] • Similar trends are seen with other ILs • CO2 has the highest solubility of the gases measured • Good selectivity! Graph adapted from Anderson, et al., Manuscript in Preparation

9. solubility pressure solubility temperature [hmim][Tf2N] Pure Gas Solubility – SO2 • SO2 solubility in [hmim][Tf2N] • Same as for CO2 Anderson, et al., J Phys Chem B, 110 (31) 2006

10. [hmim][Tf2N] SO2 Pure Gas Solubility • SO2 has highest solubility in ILs measured • Possibility of simultaneous removal of both SO2 and CO2 Anderson, et al., J Phys Chem B, 110 (31) 2006

11. Pure Gas Solubility – CO2 • Increasing fluorination increases CO2 solubility • Anion effect greater than cation effect Muldoon, et al., Manuscript Accepted

12. Pure Gas Solubility – CO2 • Incorporation of ethers, esters has been shown to increase CO2 solubility • Known low toxicity (non-fluorous) ILs can have good CO2 solubility Muldoon, et al., Manuscript Accepted

13. Chemical Complexation - Literature • Chemical capture of CO2 by free amine • Stoichiometric capture of CO2 • 13C NMR evidence of carbamate formation • Reversible under vacuum with heating Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H., J. Am. Chem., 2002, 124, 926.

14. Possible Mechanisms • Amine interactions • Carbene interactions

15. [H2NC3H6mim][Tf2N] Pure Gas Solubilities – CO2 • Chemical complexation • High CO2 solubility • High solubility may not last to high pressures

16. Enthalpies

17. Mixed Gas Separation - Membrane • Collaboration with DOE NETL • CO2 Selective Membranes: Solution Diffusion • Ionic Liquids: • Negligible Vapor Pressure • Thermally Stable above 200oC • High CO2 Solubility Relative to H2, N2, and CH4 • Ionic liquids in porous polymer supports • Ionic liquid saturated with water before testing • Constant pressure flow system; Pressure slightly greater than 1 atm • Mixed gas permeabilities and selectivities

18. [hmim][Tf2N] Mixed Gas Separation - Membrane 300oC 250oC 200oC 150oC 100oC 50oC 37oC 10 10000 Selectivity 1000 Permeability, Barrer 100 1 1.5 1.9 2.3 2.7 3.1 Used with permission from D. Luebke 1000/T, K-1

19. Mixed Gas Separation - Membrane • Probable increase in solubility • Potential to optimize for higher temperature • New rate limiting step at low temperature CO2 CO2 CO2 CO2 Dissolution Evolution Decomplexing H2 Complexing CO2 H2 CO2 H2 CO2 CO2 *CO2 *CO2 H2 H2 Diffusion CO2 CO2 Used with permission from D. Luebke

20. [H2NC3H6mim][Tf2N] Mixed Gas Separation - Membrane 175oC 150oC 125oC 100oC 75oC 50oC 1000 100 100 10 Selectivity Permeability, Barrer 10 1 2.0 2.2 2.4 2.6 2.8 3.0 3.2 Used with permission from D. Luebke 1000/T, K-1

21. Mixed Gas Separation - Membrane FT-SILM (75oC) Lin et al., Science 311 (2006) 639. (R.T. and below) SILM (37oC) CO2/H2 Selectivity Polymer Literature Data Permeability, Barrer Used with permission from D. Luebke

22. Conclusions • ILs are possible ‘green’ solvents for current technologies • Tunable • Reusable • Able to absorb gas both physically and chemically • Physical Absorption • Good selectivities between gases • Anion effect larger than cation effect • Solubility of gas increases with increasing fluorination • Chemical Absorption • Chemical absorbents have the highest CO2 solubilities • Strength of the chemical CO2-IL bond can be “tuned” • Enthalpies are lower for ILs than for the current technology • Best selectivity/permeability reported for CO2/H2 membrane separation (Luebke et al.)

23. Acknowledgements • U.S. Department of Energy, National Energy Technology Laboratory, Award Nos. DE-FC26-04NT42122 and DE-FC26-07NT43091 • State of Indiana 21st Century Fund