Post on 26-Sep-2020
Lawrence Livermore National Laboratory
Carbon Capture Using Small-Molecule Catalysts
That Mimic Carbonic Anhydrase
Roger D. Aines, Felice Lightstone, Joe Satcher, Bill Bourcier, and
Joshuah Stolaroff
2
Lawrence Livermore National Laboratory
Carbonic anhydrase is one of the most rapid enzymes known – it was
first discovered in human lungs, where it facilitates CO2 exhalation
Carbonic
anhydrase
appears to have
evolved
independently five
times, and has
hundreds of
structural variants
3
Lawrence Livermore National Laboratory
The problem for today – carbon dioxide separation is too slow
Separating pure CO2 from industrial sources, or from
the atmosphere, is a slow chemical reaction
This requires large process equipment and long times,
leading to high costs
• Separation from natural gas power is 3-4x slower
than coal
• Separation from air is 300x slower than from coal
flue gas
Water-based liquids separate CO2 from other gases
with very high efficiency because CO2 is very soluble in
water.
Lawrence Livermore National Laboratory
4
Lawrence Livermore National Laboratory
The transfer of CO2 into water or other liquids is almost always
dominated by chemical reactions at the liquid interface
Lawrence Livermore National Laboratory
The rate of CO2 uptake
directly depends on
the near-surface
conversion rate of
CO2(aq) to another
species
The liquid side “mass
transfer limitation” is
very large
aCO2
5
Lawrence Livermore National Laboratory
-2
-1
0
1
2
3
4
5
6
7
5 6 7 8 9 10 11 12 13 14 15
The reaction rate of CO2, and energy of that reaction, are strong
functions of pH (directly or indirectly)L
og
hyd
roly
sis
or
rea
cti
on
ra
te m
-1s
-1
pH
Water
Approximate Mass Transfer Limit Into Liquid
OH-
0.4 MJ/Kg CO2
2.5 MJ/Kg CO2Piperazine
DEA
MEA
1.9 MJ/Kg CO2
HCA II
Tetrazacyclododecane
34 small mimetics
Optimal Target
6
Lawrence Livermore National Laboratory
Masohan(2009) Ind J Sci
Tech 2:59-64
Solution enthalpy and entropy are a strong function of pH…
CO2(g) + H2O = H2CO3
CO2(g) + H2O = HCO3- + H+
CO2(g) + H2O = CO3-- + 2H+
-500.0
-450.0
-400.0
-350.0
-300.0
-250.0
-200.0
-150.0
-100.0
-50.0
0.0
0 2 4 6 8 10 12 14
S, J/m
ol
pH
135ºC
SUPCRT99
SUPCRT99
Enthalpy Entropy
…in major part because of the
changing carbon speciation.
H2CO3
HCO3- + H+
CO3-- + 2H+
7
Lawrence Livermore National Laboratory
Carbonic anhydrase is known to accelerate absorption into
carbonate solutions – but denatures at higher temperature
Y. Lu et al. / Energy Procedia 4 (2011) 1286–1293
25C
25C
300mg CA
25C
600 mg CA
50C
300mg CA
50C
Flux into K2CO3 solution
8
Lawrence Livermore National Laboratory
So what is needed for an effective
capture synthetic catalyst?
Rate: 104 to 105 enhancement (10 – 100x current)
Functional at moderate pH:
• A matched buffer system to absorb the carbonic acid
is needed
Robust under industrial conditions:
• Does not denature
• Metal ion not readily plucked by other chelating
agents
• Good structural integrity to maintain catalytic activity
on or near surfaces
9
Lawrence Livermore National Laboratory
First we want a general approach for mimicking enzymes
with small molecules - then to apply it to CO2 capture
CA Structure & Function:
His triad, axial –OH, coordinate Zn2+ center,
key amino acids bind CO2
Mimics:
optimize metal
and ligand identity to improve kinetics.
10
Lawrence Livermore National Laboratory
The enzyme creates a hydroxyl group for direct nucleophilic attack on
the carbon dioxide molecule - two steps are critical to the rate
+
bicarbonate
Lindskog
Product
Lipscomb
Product
Transition state
Product
release
carbonic acid
DEa
+
De
DErxn
11
Lawrence Livermore National Laboratory
We have focused on the first step of the reaction
Thr200
Thr199
Tyr7
His64
Asn62
Asn67
His119
His96
His94
12
Lawrence Livermore National Laboratory
Activation energy calculations are used to predict catalytic efficiency for
candidate molecules and environments
Reaction Coordinate
Po
ten
tia
l E
ne
rgy
+
Gaussian 03
Quantum Mechanics
DFT optimizations
B3LYP/6-311+G*
Transition
State
Reactants
Products
Activation
Energy
+
Reaction
Energy
13
Lawrence Livermore National Laboratory
Potential Energy Surface mapMethod: Plane-wave density functional theory
(DFT) with PBE-GGA xc + optional
self-consistent,Hubbard U technique.[1,2]
Approach: Choose metal and ligand
identity/number. Iterate over structural
variables (distance, angles) with constrained
relaxations or nudged elastic band for each
stationary point.
Individual maps yield stability, electronic
structure. Map comparisons yield trends in
energetics with structures.
PES
mapData
search
Design
screenCharacterize
targets
Large-
scale
modeling
Expt.
synthesis
refine
redefine
Design screenMethod: Drug-design-like docking
calculations with molecular mechanics force
field relaxations.[3]
Approach: DFT PES acts as scoring function.
Candidates assembled from building blocks
in large library.
Data searchApproach: Use design criteria from DFT PES
map to search Cambridge Structural Database
for potential catalysts. Characterize activity
with DFT.
We use an iterative approach and several computational tools
and databases to search for, and optimize, new catalysts
14
Lawrence Livermore National Laboratory
We have calculated a two general types of scaffolds as basic
building blocks
[[12]aneN3Zn(OH)] [[12]aneN4Zn(OH)] [(nitrilotris(2-benzimidazolyl-methyl)Zn(OH)]
Zhang et al Inorg. Chem. (1995) 34, 5606-5614
15
Lawrence Livermore National Laboratory
We initially use small scale models to calculate reactivity
DERxn DEa De DERxn DEa De
S M(II)-N3 M(II)-N4
Co 3/2 -9 7 27 -11 5 20
Ni 1 -11 10 26 -18 12 24
Cu 1/2 -14 12 20 -17 7 22
Zn 0 -11 6 23 -13 5 23
Energies in kcal/mol
We optimize for 1) HCO3
- TS 2) Reaction energy 3) Product release
✓Best starting point:Co, Zn, four coordinate with N.
M(I
I):
meta
l id
en
tity
R:
lig
an
d id
en
tity
doublet singlet doublet singlet
quartet triplet quartet triplet
d7 d8 d9
d10
1 2
Small models, larger combinations
16
Lawrence Livermore National Laboratory
Potential energy surfaces are calculated using only the Zn and N atoms
+
CO2 hydration
Inc
rea
sin
g
q
55
0
Zn-N3 DErxn
17
Lawrence Livermore National Laboratory
Zn-N3 ΔEa
ΔEa
Incre
asin
g q
0
55
18
Lawrence Livermore National Laboratory
Class 1. Aza-
macrocycles
Class 2.
Benzimidazoles
Class 3.
Triazoles
Class 4. Triazole/PE
System
Cyclics Pendants-1
Pendants-2 Pendants-3
Initial synthetic targets are iterated among the organic chemists
and the theorists – bench chemistry is a huge filter
19
Lawrence Livermore National Laboratory
Tris_benzimidazole activation energy can be tuned through
substitutions
20
Lawrence Livermore National Laboratory
Calculated activation energy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
SO
3-
NH
2
CH
3
(NH
2)2
OH
OC
H3
C2H
5 H
SiH
3
OO
CH
CH
CH
2
C6
H5
OC
2H
5 F
CO
OH
CO
H
OO
CC
H3
CN
CO
OC
H3
CF
3
NO
2
SO
3H
Ac
tiva
tio
n E
ne
rgy (
eV
)
Electron
widthdrawing
group
Electron
donating
group
21
Lawrence Livermore National Laboratory
Optimizing the position of CO2 and HO- will provide a rate enhancement
– one reason to focus on benzimidazoles and triazoles
21OUO
Thr200
Thr199
Tyr7
His64
Asn62
Asn67
His119
His96
His94
Thr199 has been experimentally
shown to be catalytically important
• positions CO2
• positions hydroxide
• provides a hydrogen for the leaving
group
• 3000-fold rate enhancement
Hakansson et al, J. Mol. Biol. (1992)
227, 1192-1204
22
Lawrence Livermore National Laboratory
Now that our tools and methods are in place we can also focus on
the second step of the reaction – product release
22
Thr200
Thr199
Tyr7
His64
Asn62
Asn67
His119
His96
His94
Hydrogen bonding network
• Helps the bicarbonate leave
• Brings in a water
• Deprotonates the water
molecule to form hydroxide
23
Lawrence Livermore National Laboratory
A major benefit of these synthetic catalysts is that they are
stable at 100ºC, and their rate increases with temperature
Catalyst Buffer Temp (C) Kcat
Zn(BF4)2
ctrl
Hepes, phenol red, pH =
7.5
Tr 7
Cyclen-Zn Hepes, phenol red, pH =
7.5
Tr 540
Cyclen-Zn Hepes, phenol red, pH =
7.5
Post 18 h, 100
C
900
Cyclen-Zn AMPSO, thymol blue, pH =
9.0
Tr 2500
Cyclen-Zn AMPSO, thymol blue, pH =
9.0
Post 18h, 100
C
2260
Cyclen-Zn AMPSO, thymol blue 50C 11,500
Demonstrated stability and enhanced kinetics for cyclen at elevated temperature and pH
conditions
24
Lawrence Livermore National Laboratory
We still have to deal with transfer across the gas-water interface
The rate of CO2 uptake
directly depends on
the near-surface
conversion rate of
CO2(aq) to another
species
The liquid side “mass
transfer limitation” is
very large
aCO2
25
Lawrence Livermore National Laboratory
The trick is tethering the catalyst to the liquid/gas interface –
again nature is our example
26
Lawrence Livermore National Laboratory
Polyethyleneglycol (PEG) linkers do not deform the catalysts and
appear to be appropriate for tethering to hydrophobic groups
~50 PEG units keep the
catalyst removed from
the immediate surface
PEG
27
Lawrence Livermore National Laboratory
Finally, catalysts permit use of less expensive
and more abundant inorganic solvents
Essential characteristics:• Must be basic –
Have a pKa > 6.5 (pKa of bicarbonate) But no longer need very high pH for improved
reaction kinetics• Water soluble
Partial pressure of water compatible with flue gas (5%)
• High loading capacity for CO2
Comparable with amines • Amenable to rate enhancement with our catalyst • Must be abundant on the planet
Available locally in tens of million ton amounts
Advantages over amines:• Improved lifecycle impacts• Reduced energy use by avoiding high heats of
solvation of CO2
Carbonates
Borates
Phosphates
28
Lawrence Livermore National Laboratory
Inorganic solvents may have advantages –
millions of tons will be needed
Inorganic salt
buffers
• Carbonate
• Borate
• Phosphate
Loading capacities
-Solid precipitants
Kinetics
Costs
Blue – 30 wt % solvent
Red – with solids
Black – air saturation
29
Lawrence Livermore National Laboratory
Capacities can be increased by allowing
solids to precipitate
Na2CO3
To
tal carb
on
Path shows reaction
of 30 wt % sodium
carbonate solution
with flue gas
Na2CO3 + CO2(g)
= 2Na+ + 2HCO3-
(NaHCO3)
(Na2CO3.10H2O)
30
Lawrence Livermore National Laboratory
Catalysts will hopefully open many new pathways for capture
Faster reactions with existing systems (e.g. K2CO3, perhaps MEA)
Lower energy solvents and inorganic systems now fast enough for
use
New ways to create surface area and recover CO2 at much lower
or much higher temperatures can be used
Roger Aines
Carbon Fuel Cycle Program Leader
Lawrence Livermore National Laboratory
Livermore, CA, USA 94550
925 423 7184 aines1@llnl.gov
31
Lawrence Livermore National Laboratory
Conventional sorption is optimized for amine
solvents
Amines have -
High CO2 carrying capacity
10 wt % CO2 in 30 wt % MEA
Fast kinetics
Higher than pH vs. rate curve
But are -
Corrosive
High energy use and environmental impact
32
Lawrence Livermore National Laboratory
Y. Lu et al. / Energy Procedia 4 (2011) 1286–1293
33
Lawrence Livermore National Laboratory
Most fast chemistries come at a high energy cost – but
there are possible solutions
Lawrence Livermore National Laboratory
Approximate Mass
Transfer (Gas-Side) LimitOptimal
Capture Target
MEA
DEA
Current small
mimetics
Tetrazacyclododecane
Trizacyclododecane
NaOH
Minimum
energy
to
recover
CO2
Energy
CO2 in
Flue Gas
CO2 In
Solvent
Pure CO2
0.4 MJ/kg
1.9 MJ/kg
2.5 MJ/kg
Piperazine
Lo
g H
yd
roly
sis
ra
te
34
Lawrence Livermore National Laboratory
Additional compounds have been calculated and compared
34
[(imidazole)3Zn(OH)] [(tris(4,5-dimethyl-2-imidazolyl)phosphine)Zn(OH)]
[(tris(4,5-diethyl-2-imidazolyl)phosphine)Zn(OH)]
35 different compounds have been calculated
3 new candidates have been synthesized