Ionic Liquids as a Tunable CO Separations...

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

[email protected]

www.nd.edu/~wschnei1

GCEP Conference

Stanford University

October 10, 2012

mailto:[email protected]

http://www.nd.edu/~wschnei1

Separations 101

• Separations intrinsically require energy

• 500 MW coal-fired power plant ⇒ ~40 MW minimum

CO2 separation work


P, T P, T P, T

Separations 101

• Require selective phase separation “machine”

• Machine (energy/atom) efficiency always less than ideal


P, T

P, T

absorbent

Separations 101

• Require selective phase separation “machine”

• Machine (energy/atom) efficiency always less than ideal


P, T P, T P, T

P, T P, T

absorbent absorbent

Absorption isotherms


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


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


S

t

r

o

n

g

b

i

n

d

i

n

g

W

e

a

k

b

i

n

d

i

n

g

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-80 -70 -60 -50 -40 �

mo

le r

ati

o

�H

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% (!)


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


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


Predicting properties with computation


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


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


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


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


“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


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


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!


!

!

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


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


-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


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


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


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


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Ionic Liquids as a Tunable CO Separations...

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