Ionic Liquids as CO2 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
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- hmimTf2N
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
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
Pure Gas Solubility Other gases

• Gas solubility measured in hmpyTf2N
• 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
Pure Gas Solubility SO2

• SO2 solubility in hmimTf2N
• Same as for CO2

Anderson, et al., J Phys Chem B, 110 (31) 2006
10
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
Pure Gas Solubilities CO2

• Chemical complexation
• High CO2 solubility
• High solubility may not last to high pressures

16
Enthalpies
Compound Enthalpy (kJ/mol)
hmimTf2N - 12
hmpyTf2N - 12
TMGNO3 hmimTf2N - 16
H2NC3H6mimTf2N - 49
MEA - 143
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
Mixed Gas Separation - Membrane
37oC
300oC
250oC
200oC
150oC
100oC
50oC
10
10000
1000
Selectivity
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
Mixed Gas Separation - Membrane
150oC
100oC
50oC
175oC
125oC
75oC
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