Carbon Capture Using Small-Molecule Catalysts That Mimic ...Quantum Mechanics DFT optimizations B3LYP/6-311+G* Transition State Reactants Products Activation Energy + Reaction Energy

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


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


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.


4


The transfer of CO2 into water or other liquids is almost always

dominated by chemical reactions at the liquid 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

5


-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


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


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


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


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


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


We have focused on the first step of the reaction

Thr200

Thr199

Tyr7

His64

Asn62

Asn67

His119

His96

His94

12


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


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


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


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


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


Zn-N3 ΔEa

ΔEa

Incre

asin

g q

0

55

18


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


Tris_benzimidazole activation energy can be tuned through

substitutions

20


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


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


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


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


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


The trick is tethering the catalyst to the liquid/gas interface –

again nature is our example

26


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


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


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


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


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


Livermore, CA, USA 94550

925 423 7184 [email protected]

31


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


Y. Lu et al. / Energy Procedia 4 (2011) 1286–1293

33


Most fast chemistries come at a high energy cost – but

there are possible solutions


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


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

Documents

Carbon Capture Using Small-Molecule Catalysts That Mimic ...Quantum Mechanics DFT optimizations B3LYP/6-311+G* Transition State Reactants Products Activation Energy + Reaction Energy