Molecular Engineering of New Ionic Liquid Sorbents for CO2 Capture
Edward Maginn, Joan Brennecke, William Schneider and Mark McCready
Dept. of Chemical and Biomolecular EngineeringUniversity of Notre Dame
9th
Annual Carbon Capture and Sequestration MeetingPittsburgh, PA
May, 2010
9th CCS May, 2010Maginn, Brennecke, McCready and Schneider
Ionic Liquids and Their Potential as CO2 Sorbents
•
Pure salts that are liquid around ambient temperature–
Not simple salts like alkali halides•
Many favorable properties–
Nonvolatile–
Anhydrous–
High thermal stability–
Huge chemical diversity–
High intrinsic CO2
solubility and selectivity
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
Examples of cations Examples of anions
SO2 and CO2 solubility high…
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
Anderson, Dixon and Brennecke, Acc. Chem. Res., 40, 12081208
(2007)
•
Process modeling indicates physical CO2
solubility still not high enough to be practical for post-combustion capture–
Need to add chemical functionality to increase capacity
SO2
CO2 C2 H4
C2 H6 CH4 O2
N2
Adding reactive groups to ionic liquids increases capacity
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
•
Build on aqueous amine chemistry:
NH22 +O
CO
NH
CO- + NH3+
1:2O
1 atm CO2Room temp.
Problems with conventional TSILs
•
Liquid becomes extremely viscous upon CO2 contact •
2:1 mechanism is inefficient…1:1 mechanism possible?
•
How to tune the physical and chemical properties?
No CO2 17 mbar CO2
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
Use molecular modeling + experiments to design ILs for CO2 capture
Explanation for viscosity increase
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
A-C salt bridge
B-C salt bridge
Water A-C
Gutowski and Maginn, JACS 2008, 130, 14690
Atomistic MD simulations show an increase in viscosity upon reaction with CO2 just like experiments.
Why? Formation of a pervasive network of salt bridges between carbamate and ammonium / unreacted species.
Solution: design aprotic molecules that do not form salt bridges
Achieving 1:1 binding stoichiometry?
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
NH22 +O
CO
NH
CO- + NH3+
1:2O
NH
COH
O
1:1
Conventional primary amine chemistry requires two active groups for every one molecule of CO2
Is it possible to design molecules that bind CO2 1:1?
Optimal binding energy from process modeling
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
Results from Trimeric Corp.
Can ILs be designed with optimal CO2 reaction enthalpy?
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
Approach: electronic structure calculationsApproach: electronic structure calculations•
Evaluate structural and energetic properties of potential absorbents
•
Examine relative energies for different binding mechanisms
•
Investigate how substituent groups affect reaction enthalpy–
Possible to computationally screen a huge number of candidates
•
Hybrid-DFT calculations implemented in Gaussian–
B3LYP/6-311++G(d,p)–
Harmonic frequency analysis–
Standard ZPE and gas-phase free energy analysis –
Boltzmann averaged reaction energies
+ +
N
O
MEA vs. MEA vs. cationcation-- vs. anionvs. anion--tethered amines tethered amines
•
Local cation tethering favors 2:1 binding•
Local anion tethering disfavors 2:1 binding•
Tethering ion and tethering point as important as functional groups in controlling CO2
reactions–
DOF unique to ILs!!
+
-
MEA:
Pyridinium aminecation:
Amino acetateanion:
+
1:1
-
0
+2
-4
0 -
+ …NH3+
-17
2:1
+17-
-
+-
+ …NH3+
+ …NH3+
Reaction energies in kcal/mol relative to MEA
+CO2
+CO2
+CO2
-H+
-H+
-H+
Mindrup and Schneider, ACS Symp. Series 2010
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
1:1 stoichiometry for anion functionalized IL
•
Simulations predict prolinate
(–71 kJ mol–1)
stronger 1:1 absorber then methionate
(–55)
•
Experimental RT isotherms consistent with this ranking and with ~1:1 reaction stoichiometry
•
Excellent agreement with calorimetric experiments
Gurkan et al., JACS 2010, 132, 2116-2117
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
In situ vibrational spectroscopy
•
IR distinguishes physically and chemically absorbed CO2
•
Confirms 1:1 reaction stoichiometry–
Ammonium peaks absent–
Carbamic acid peaks appear
N2
Vacuum
ThermocoupleP
I R
Silicon probe
CO2
P-controller
trap
vent
vent
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
•
Calculations predict reversible carbamate formation–
Tunable reaction energy
1.40 Å
1.36 Å
1.42 Å
–0.25
1.38 Å
1.37 Å
1.43 Å
1.53 Å
134°
CO2
1.41 Å
1.37 Å1.35 Å
1.41 Å
1.40 Å
–0.17
1.40 Å
1.38 Å1.36 Å
1.39 Å
1.41 Å
171°
137°
1.58 Å
∆Eo = –96 kJ mol–1
N
C
CC
C–
–C
CC
C
COO
N
N
C
CC
C –C
N
CO2
C
CC
C
COO
NC N
∆Eo = –35 kJ mol–1
B3LYP/6-311++G(d,p)
–
Tuning reaction enthalpy using substituent groups
Putting it all together: solvent design targets
Design targets•
Disrupt salt bridge network–
Aprotic
base
•
Anion-functionalized IL–
1:1 reaction stoichiometry
•
Tunable absorption energy–
Choice of substituent groups
•
Clean, reversible kinetics
•
Aprotic
heterocyclic anions (“AHA”s)–
Simple, tunable Lewis bases
NR1
R2R3
R4
N
NR1
R3
R4
“Pyrrolide”
“Imidazolide”
N
N
R2R3
R4
“Pyrazolide”US patent pending, University of Notre Dame
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
Computed reaction enthalpy tracks experimental uptake
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
N
O C H 3
O
NN
CF 3
NCH3
O
NCCl3
O
N C N22 °C
increasing
computed enthalpy
Approaches 1:1 binding: atom efficient
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
As predicted, little viscosity increase upon reaction
•
Modest viscosity change upon reaction!•
Process simulations suggest substantial COE reduction relative to amines (see poster by Trimeric, #1291)
Molecular modeling shows that only cation- anion interactions are present; no salt bridges formed!
•
Ionic liquids a promising platform for chemically tailored gas separations
•
Molecular simulations have helped design a new class of reactive
ionic liquids that–
Do not increase in viscosity upon reaction–
Have desired 1:1 binding mechanism–
Have tunable enthalpies of reaction for process optimization•
In-house synthesis, characterization and testing capability•
Many technical hurdles remain to be overcome–
Run a lab scale demonstration unit–
Additional testing, optimization and characterization
•
Students and postdocs–
Elaine Mindrup, Mandelle
Danser, Thomas Sentfle, Tom Rosch, Hao
Wu, Jindal
Shah, Marcos Perez-Blanco, Matthew LeMay, Burcu
Gurkan, Juan de la Fuente, Erica Price, Devan
Kestel, Samuel Seo, Marjorie Massel, Levi Wilson, Michael Glaser, David Zadigan, Brett Goodrich, Lindsay Ficke
ConclusionsConclusions
NETL Project 43091NETL Project 43091
Financial support
Maginn, Brennecke, Schneider and McCready 9th CCS May, 2010
