Post on 25-Feb-2021
W. F. Schneider Carbon capture @ ND
Ionic Liquids as a Tunable CO2 Separations
Platform
Bill Schneider Dept. of Chemical and Biomolecular Engineering
Dept. of Chemistry and Biochemistry
University of Notre Dame
wschneider@nd.edu
www.nd.edu/~wschnei1
GCEP Conference
Stanford University
October 10, 2012
Separations 101
• Separations intrinsically require energy
• 500 MW coal-fired power plant ⇒ ~40 MW minimum
CO2 separation work
W. F. Schneider Carbon capture @ ND
P, T P, T P, T
Separations 101
• Require selective phase separation “machine”
• Machine (energy/atom) efficiency always less than ideal
W. F. Schneider Carbon capture @ ND
P, T
P, T
absorbent
Separations 101
• Require selective phase separation “machine”
• Machine (energy/atom) efficiency always less than ideal
W. F. Schneider Carbon capture @ ND
P, T P, T P, T
P, T P, T
absorbent absorbent
Absorption isotherms
W. F. Schneider Carbon capture @ ND
Langmuir (single site) absorption
A + CO2 (g) ↔ A⋅CO2 Keq(T)
CO2
CO2
A-CO2
A-CO2 A
PCO2
cCO2
T
O2
N2
H2O
absorbent
mixture
Temperature-swing carrying capacity
W. F. Schneider Carbon capture @ ND
Langmuir (single site) absorption
A + CO2 (g) ↔ A⋅CO2 Keq(T)
Absorption
Low T, low P
Desorption
High T, high P
Carrying capacity
Mol CO2/mol absorbent
Absorption optimization
W. F. Schneider Carbon capture @ ND
S
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b
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b
i
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d
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-80 -70 -60 -50 -40 �
mo
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Leading existing CO2 absorbent “machines”
• 1˚, 2˚, 3˚ aqueous amine chemistry – MEA, Fluor Econamine, Mitsubishi KS-1, …
• Chilled NH3 – Alstom, Powerspan
– (NH4)2CO3 + CO2 + H2O ↔ 2 (NH4+)(HCO3
–)
• Catalyzed carbonate – Carbozyme
• Short-comings include complex chemistry, slow rates, energy and
infrastructure (space) cost, corrosivity, NOx/SOx cross reactivity,
decomposition, … – Deratings unknown but projected to be >30% (!)
W. F. Schneider Carbon capture @ ND
Why RTIL absorbents for CO2 separations?
• IL intrinsic properties favorable for CO2 capture – Non-aqueous
– Negligible volatility
– High intrinsic physical selectivity for CO2
– Modest physical capacity
• Chemically functionalize to increase CO2 capacity – Primary amine functional
groups build off of well-known aqueous amine chemistry
– “Task-specific” Ils
W. F. Schneider Carbon capture @ ND
Henry’s Law
Pa = Hxa
Bates, E.D., et. al., J. Am. Chem. Soc. Comm. 2001, 124, 926.
Room Temperature Ionic Liquids
• RTILs – Salts that are liquid at
ambient temperatures
• Huge diversity of potential compounds – Mix and match cations
and anions
– Easily prepared
• ND leading experts in RTIL synthesis and characterization
W. F. Schneider Carbon capture @ ND
Predicting properties with computation
W. F. Schneider Carbon capture @ ND
1:1 stoichiometry for anion-tethered 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
• Vibrational spectroscopy supports 1:1 assignment
• Calorimetry in exceptionally good agreement with calculations
W. F. Schneider Carbon capture @ ND
Gurkan et al., JACS 2010, 132, 2116-2117
!
In situ vibrational spectroscopy
• IR distinguishes physically and
chemically absorbed CO2
• Confirms 1:1 reaction
stoichiometry
• Carbamate peak in good
agreement with calculation
W. F. Schneider Carbon capture @ ND
N2
Vacuum
Thermocouple P
I
R
Silicon
probe
CO2
P-controller
trap
vent
vent
Problems with primary amines and CO2
• Complex protic chemistry – Stoichiometry, enthalpy unpredictable
– Potentially slow kinetics
• Adverse consequences for physical properties – Viscosity increases dramatically in face of extensive H-bonding oppor
W. F. Schneider Carbon capture @ ND
A-C hydrogen
bond
Gutowski and Maginn, JACS 2008, 130, 14690
Puxty, JES&T 2009, 43, 6427
TSIL design targets
Design targets
• Anion-functionalized IL – 1:1 reaction stoichiometry
• Disrupt H-bonding network – Aprotic base
• Tunable absorption energy
• Clean, reversible kinetics
• Aprotic heterocyclic anions (“AHA”s) – Simple, tunable Lewis bases
– Takes advantage of intrinsic nucleophilicity of anions
W. F. Schneider Carbon capture @ ND
“Pyrrolide”
“Imidazolide”
“”Pyrazolide”
TSIL design targets
Design targets
• Anion-functionalized IL – 1:1 reaction stoichiometry
• Disrupt H-bonding network – Aprotic base
• Tunable absorption energy
• Clean, reversible kinetics
• Aprotic heterocyclic anions (“AHA”s) – Simple, tunable Lewis bases
– Takes advantage of intrinsic nucleophilicity of anions
W. F. Schneider Carbon capture @ ND
B3LYP/6-311+G(d,p) calculations
Pyrrolide reactions with CO2?
• CO2 predicted to bind strongly at pyrrolide nitrogen
• π-bonding reflected in planar conformation and 40 kJ
mol-1 rotational barrier
• Ring substitutions influence binding energy
– Induction, conjugation, and steric contributions
W. F. Schneider Carbon capture @ ND
1.38 Å
1.37 Å 1.37 Å
1.38 Å
1.43 Å
1.53 Å
134!
N
O OC
C
C C
C
1.40 Å
1.38 Å 1.36Å
1.39 Å
1.41 Å
171!
137!
1.58 Å
C
C C
CC
CO O
NN
1.39Å
1.35Å 1.37Å
1.37 Å
1.44 Å 179!
1.56Å
136!
N
N
O OC
C
C C
C
C
-109 kJ mol–1 -70 kJ mol–1 -49 kJ mol–1
Gaussian G3 calculations
Gurkan et al., J. Phys. Chem. Lett. 2010, 1, 3494
[P66614][2-CNpyrollide] experiments
• AHAs readily form room-temperature ionic liquids
• CO2 isotherms consistent with 1:1 reaction
• Isotherms fit with Langmuir + Henry’s Law model – ∆Hrxn = –43 kJ mol–1
– ∆Srxn = –130 J mol–1 K–1
• Virtually no change in viscosity!
W. F. Schneider Carbon capture @ ND
!
!
mo
l C
O2/m
ol a
nio
n
Pressure (bar)
22˚C
40˚C
60˚C
80˚C
100˚C
CO2 absorption isotherms (stirred reactor)
Viscosity
Vis
co
sity (
cP
)
Temperature (˚C) Gurkan et al., J. Phys. Chem.
Lett. 2010, 1, 3494
AHA reaction energy trends
• Wide variety of AHAs predicted to combine with CO2
• Binding energies diminished with increasing N substitution and conjugation
• Substituents offer wide range of reaction tunability
W. F. Schneider Carbon capture @ ND
0
10
20
30
40
50
60
70
80
1 2 3 4 5 6
Re
ac
tio
n E
ne
rgy (
kJ/m
ol)
Indole substitution position
CN CF3
F C(=O)OCH3
C(=O)H Parent
Parent indole
B3LYP/6-311++G(d,p)
+ CO2
+ CO2
Generation 3 “Aprotic Heterocylic Anion” ILs
• New class of ionic liquids
designed specifically to
have tunable CO2 uptake
properties
• US patent filed by Notre
Dame Nov. 2010
• AHA platform ideally suited
for CO2 separations
W. F. Schneider Carbon capture @ ND
-109 kJ mol–1 -70 kJ mol–1 -49 kJ mol–1
Gaussian G3 calculations
Funded under DOE NETL DE-FC26-07NT43091
Computational design Laboratory development
ND Research activity surrounding AHA ILs
NETL CO2 Capture (2004-2012)
ARPAe PCIL
(2010-2013)
Exploit ability of some AHAs to phase-change
with CO2
ND SEI CO2
(2010-2012)
Extend AHA concept to new platforms
NSF PFI
(2012-2014)
Develop applications in new energy spaces
Stanford GCEP
(2012-2015)
Develop AHAs for pre-combustion
ARPAe VC
(2010-2013)
Develop AHA-CO2 refrigeration cycle
Ionic Research Technologies
Innovation Park spin-off
LANL Grand Challenge
(2010-2013)
Develop ILs for actinide separations
W. F. Schneider Carbon capture @ ND
GCEP: Pre-combustion CO2 separations
• Pre-combustion CO2 separations present challenges distinct from post-combustion – Separate CO2 primarily from H2
– Higher nominal temperatures and pressures, e.g. from WGS
– Lower gas volumes
– Low water, low sulfur
• Proposed work
• Model-driven materials development
– Weak-specific binding ionic liquids
– Exploit “physical” cooperativity
– Exploit “structural” cooperativity
• Synthesis and characterization
• Systems and life cycle assessment
W. F. Schneider Carbon capture @ ND
3
University of Notre Dame Confidential Information
networks.10,15
Examples of effective AHA ILs include the
phosphonium salts of pyrrolides and pyrazolides. Their viscosities
before and after CO2 complexation are comparable (Figure 2).
3. Tuning reaction enthalpy. Using first principles calculations,
we designed AHA ILs tuned to have CO2 reaction enthalpies ranging
from –31 kJ/mol (relatively weak) to –80 kJ/mol (strong bonding).
Subsequently, we synthesized these ILs and experimentally verified
the predictions.16
In general, the experimental results match the
calculations to within 5-10 kJ/mol.
II. Proposed Research Overview
To date, our work has focused on post-combustion streams, where the flue gas is saturated with water
at about 40-50 ºC (coming from the gas desulfurization unit), is at a total pressure of 1 bar (predominately
N2), and has a partial pressure of CO2 of about 0.15 bar. In contrast, absorbent materials for pre-
combustion CO2 capture must (a) separate CO2 primarily from H2 not N2, (b) take advantage of much
higher total pressures (e.g., as high as 60 bar) and partial pressures of CO2 (e.g., 15 bar) and (c) operate at
higher temperatures. For example, in an IGCC process, the water gas shift reactor effluent (mostly H2
and CO2) is about 200 ºC. This stream must be cooled to 35 ºC or below for CO2 capture with SelexolTM
or Rectisol® and then reheated before combustion in the gas turbine; however, separation with ILs at
higher temperatures could eliminate this cooling and reheating. We propose here to develop novel ILs
tailored for these pre-combustion separation
conditions.
Specifically, we will first investigate
modifications to the AHA IL platform that will
lead to materials appropriate for pre-combustion
capture. Our fundamental approach is to develop,
synthesize, and test ILs based on both (a)
computational property predictions and (b)
systems analysis and LCA modeling, as indicated
in Figure 3. We have used this approach very
effectively in previous research to drastically
narrow potential IL options and speed progress
towards specific applications. We believe this
model-driven development framework will lead to
materials with the desired selectivity and will
enable tuning of the absorption capacity and
enthalpy for pre-combustion CO2 separation. Our team is unique in capabilities that span through first
principles quantum mechanical (QM) and classical molecular simulations, synthesis of entirely new IL
compounds, testing of all pertinent thermodynamic and transport properties, and system and LCA
analysis. This expertise places us in an ideal position to make significant and rapid progress.
We propose two complementary paths for this research project. First, we will develop new AHA ILs
that complete the separation in the desired performance range by varying both the anion and cation
components. We anticipate that the ideal absorbents for pre-combustion CO2 capture will form relatively
weak complexes (compared to post-combustion capture) with CO2; i.e., weak specific binding. Even
though CO2 partial pressures are relatively high in pre-combustion gases, the desire to absorb at
somewhat higher temperatures means that chemical complexation will probably be necessary. Our
experience suggests that this will probably be in the weak binding range of -20 to -40 kJ/mol of CO2. In
figure 8 below, we show preliminary results for some new AHA ILs that indicate that weakening the
chemical complexation with CO2 is indeed possible. A key variable that we have not explored in previous
work is the possibility of performing the separation at higher temperatures (up to 200 ºC), rather than at
40-50 ºC for post-combustion capture. Therefore, part of this research path will be the systems analysis
Figure 2: Viscosity of AHA ILs before and after complexation with CO2.
Figure 3: Research approach for this project.
ND GCEP Team
W. F. Schneider Carbon capture @ ND
Prof. Brandon Ashfeld Group
Prof. Joan Brennecke Group
Prof. Edward Maginn Group
Prof. Mark Stadtherr Group
Prof. Bill Schneider Group
Cohorts in carbon capture:
Stanford + ND = 2 Great Teams
W. F. Schneider Carbon capture @ ND