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Molecular Modeling and Design of Metal-Organic Frameworks
for CO2 Capture
Randy Snurr
Department of Chemical & Biological Engineering Northwestern University, Evanston, IL 60208
http://zeolites.cqe.northwestern.edu
Adsorption Separations
PSA, TSA, VSA
Adsorption separations• are widely used in processes such as
air separation• can be more energy efficient than
traditional distillation separations• can be based on differences in
adsorption thermodynamics (more common) or rates of diffusion (less common)
A key issue is the choice of the adsorbent
Novel adsorbent
“Nanotechnology for Carbon Dioxide Capture,” R.R. Willis, A.I. Benin, R.Q. Snurr, A.O. Yazaydin, in Nanotechnology for the Energy Challenge, J. Garcia-Martinez, Ed., Wiley-VCH, 2010.
Use specific transition-metal coordination chemistry:
Metal corners and organic linkers can be chosen to yield a wide variety of porous, crystalline structures� Pores or cavities of controlled sizes� Wide variety of chemical functionalities
A Building-block Approach to Materials Synthesis
+
Metal-Organic Frameworks
3D frameworks
• crystalline
• very open structures
• potential applications in energy storage, sensing,adsorption separations, and catalysis
Snurr, Hupp, Nguyen, 2004
Potential foradsorptionapplications
Example: IRMOFs IsoReticular Metal-Organic Frameworks
=
Zn4O
=
O O
OO
O O
OO
O
O
O O
OO
IRMOF-1 IRMOF-4 IRMOF-10
Omar Yaghi, Univ. California, Los Angeles
IRMOFs
Pyridine-only frameworks are generally unstable against channel collapse. However, a family of mixed-ligand MOFs shows permanent microporosity.
(1) Carboxylate paddle-wheel-type
coordination of Zn(II) pairs
2D sheets
(2) Pyridine/Zn linkages
open frameworks
[Ma, Mulfort, Hupp, Inorg. Chem. 2005
Mixed-Ligand MOFs
MOF-177
Diversity of MOFs
MOF-177
HKUST-1MIL-103
MIL-53
Molecular Tinker Toys
H. Furukawa, N. Ko, Y.B. Go, N. Aratani, S.B. Choi, E. Choi, A.Ö. Yazaydin, R.Q. Snurr, M. O’Keeffe, J. Kim, O.M. Yaghi, “Ultra-high porosity in metal-organic frameworks,” Science, in press.
Ultra-High Surface Area
• Extended linker lengths
• MOF-210 has the largest specific surface area reported to date: 6240 m2/g BET.
• These large-pore MOFs have very high capacity for CO2 at higher pressures.
Materials Design
?
Can tune material properties via synthesis• pore size• linker functionality• open-metal sites• extraframework cations or anions
Can also modify MOFs after their synthesis
• Screening of MOFs for CO2 Capture with Molecular Modeling– Model development– Test of model versus experiment– Screening results
• Identify candidate MOFs• What do we learn?
• Post-Synthesis Modification of MOFs for Improved CO2 Uptake
• Summary and Outlook
Outline
Simulation Model
• MOF atoms are held fixed at their crystallographic coordinates.
• Lennard-Jones parameters taken from the DREIDING force field.
• Charges on framework atoms from quantum chemical calculations.
� Atomistic representation of MOFs
� Atomistic representation of guest CO2 molecules• CO2/CO2 parameters taken from
TraPPE force field that matches bulk vapor/liquid equilibria*
• Lennard-Jones + Coulomb-0.35 -0.35+0.7
1.16 Å
* Potoff, Siepmann, AIChE J., 2001.
A phase equilibrium problem
At equilibrium:
TI = TII
PI = PII
µIi = µII
i for all species i
(or fiI = fiII for all species i)
Molecular Simulation of Adsorption
MOF phase, II
Fluid phase, I
Grand Canonical Monte Carlo (GCMC)
� adsorbed phase in equilibrium with bulk fluid
� µ, V, T constant as in adsorption experiments
� number of molecules fluctuates
� random moves
insertions
deletions of molecules
µ T
HKUST-1 ZIF-8
Yazaydin, Snurr, Park, Koh, Liu, LeVan, Benin, Jakubczak, Lanuza, Galloway, Low, Willis, J. Am. Chem. Soc., 2009.
CO2 Adsorption in MOFs
298 K IRMOF-1
Walton, Millward, Dubbeldam, Frost, Low, Yaghi, Snurr, J. Am. Chem. Soc., 2008.
CO2 Adsorption in MOFs
0 20 40 60 80 100 1200
200
400
600
800
1000
1200
1400
1600
1800
2000
CO
2 Loa
ding
, mg/
g
Pressure, kPa
195K 233K 208K 273K 218K GCMC
0 800 1600 2400 32000
200
400
600
800
1000
1200
1400
1600
CO
2 Loa
ding
, mg/
g
Pressure, kPa
MOF-177 IRMOF-3 GCMC
Systematic Comparison with Experiment
Farrusseng, Daniel, Gaudillere, Ravon, Schuurman, Mirodatos, Dubbeldam, Frost, Snurr, Langmuir, 2009.
N2 CH4
Kr CO2
Xe
0 -5 -10 -15 -20 -25 -300
-5
-10
-15
-20
-25
-30
�H s
imul
atio
n / K
J.m
ol-1
�H experimental / KJ.mol-10 -5 -10 -15 -20 -25 -30
0
-5
-10
-15
-20
-25
-30
�H s
imul
atio
n / K
J.m
ol-1
�H experimental / KJ.mol-1
N2
CH4
KrCO2
Xe
n-C4H10
i-C4H10
IRMOF-1 IRMOF-3
Screening MOFs for CO2 Capture
� Given the large number of possible MOF topologies, linkers, and metal nodes, there are an almost unlimited number of MOFs that could be synthesized.
� Screening and understanding of the fundamental structure/function relationships are, thus, very important for developing new processes based on MOFs.
� Choose a diversity of materials for screening to help improve our understanding of CO2 capture in MOFs.
CO2/MOF Screening Collaboration
� Team Approach– Synthesis– Characterization– Testing– Modeling
� Team Members– Richard Willis, Annabelle Benin, Syed Faheem, John Low at
UOP LLC, Des Plaines, IL– Adam Matzger at University of Michigan, Ann Arbor– Douglas LeVan at Vanderbilt University– Stefano Brandani at University of Edinburgh– Ozgur Yazaydin at Northwestern University
Screening MOFs for CO2 Capture
14 MOFs
� Mg\DOBDC� Ni\DOBDC� Co\DOBDC� Zn\DOBDC� Pd(2-pymo)2
� HKUST-1� UMCM-150(N)2
� UMCM-150� MIL-47� ZIF-8� IRMOF-1� IRMOF-3� UMCM-1� MOF-177
M\DOBDC (1D Channels)
Pd(2-pymo)2
(Narrow pores)
UMCM-1(High surface area)
HKUST-1(Side pockets)
Experimental CO2 uptake at 0.1 bar and 298 K
M\DOBDC MOFs perform particularly well.� MOFs with large free volume
perform the worst at low pressure.
� MOFs having coordinatively unsaturated metal sites (open-metal sites) demonstrate the best performance.
Yazaydin, Snurr, Park, Koh, Liu, LeVan, Benin, Jakubczak, Lanuza, Galloway, Low, Willis, J. Am. Chem. Soc., 2009.
Screening MOFs for CO2 Capture
Screening MOFs for CO2 Capture
Yazaydin et al., J. Am. Chem. Soc., 2009.
No correlation with SA
No correlation with free volume
There is a strong correlation between CO2 uptake and heat of adsorption at low pressure.
Screening MOFs for CO2 Capture
CO2 density profile in Mg\DOBDC at 0.1 bar from simulations.
Location of adsorbed CO2 molecules in Ni\DOBDC from X-ray diffraction data and IR spectroscopy.
Dietzel, Johnsen, Fjellvag, Bordiga, Groppo, Chavan, Blom, Chem. Comm. 2008.
Why do M\DOBDCs perform better than other MOFs which also have open-metal sites?
Screening MOFs for CO2 Capture
Why do M\DOBDCs perform better than other MOFs which also have open-metal sites?
MOF Sites / nm2 Sites / nm3
Mg\DOBDC 3.031 7.074
Ni\DOBDC 2.655 7.298
Co\DOBDC 2.559 7.109
Zn\DOBDC 2.798 7.564
HKUST-1 1.400 3.730
UMCM-150 0.738 2.015
UMCM-150(N)2 0.764 2.036
Surface and free volume density of metal atoms in MOFs with open-metal sites
Simulation versus Experiment
Experiment GCMC
Mg-MOF-74 1 2
Ni-MOF-74 2 3
Co-MOF-74 3 5
Zn-MOF-74 4 4
Pd(2-pymo)2 5 1
HKUST-1 6 6
UMCM-150(N2) 7 9
UMCM-150 8 8
MIL-47 9 7
ZIF-8 10 11
IRMOF-3 11 10
UMCM-1 12 12
MOF-177 13 13
IRMOF-1 14 14
This diverse set of MOFs is a stringent test of simulation.
• Ranking from simulation is very close to that from experiment.
• The top 5 MOFs are correctly identified by the simulations.
Simulation versus Experiment
Simulation vs. experiments at room temperature
� There is generally good agreement between predicted and measured adsorption, R2 = 0.67.
� One exception is the M\DOBDC samples, particularly at 0.1 bar (open blue circles). If the M\DOBDC data at 0.1 bar are excluded, R2 = 0.79.
� The simulations perform well, with a level of agreement that is satisfactory for screening purposes.
Yazaydin et al., J. Am. Chem. Soc., 2009.
Limitations of the Model
Model underpredicts adsorption on the open-metal sites.
Since classical model does not include orbital interactions, this is expected.
• Screening of MOFs for CO2 Capture with Molecular Modeling– Model development– Test of model versus experiment– Screening results
• Identify candidate MOFs• What do we learn?
• Post-Synthesis Modification of MOFs for Improved CO2 Uptake
• Summary and Outlook
Outline
Post-Synthesis Modification of MOFs
Hypothesis• Fluorine groups
will increase CO2uptake
• Change in pore size may also play a role in CO2selectivity over N2
1 3
4
+
Zn(NO3)·H2O
5
100oC
150oC
1) Soak in CHCl3/4-(trifluoromethyl)pyridine
2) Heating at 100 C
Pyridine-CF3
Dimethylformamide
(DMF)
Bae, Farha, Hupp, Snurr, J. Mater. Chem., 2009.
Enhancement of CO2 / N2 Selectivity
Pressure [bar]
0 2 4 6 8
Se
lec
tivit
y [
-]
0
10
20
30
40
50
3 (with coordinated solvents)
4 (with open metal sites)
5 (with Py-CF3 ligands)
CO2 / N2 SelectivityCavity modification can be used to enhanceselectivity.
These are among the highestselectivities reported.
Need to increase the capacities.
Selectivity
Selectivities predicted from ideal adsorbed solution theory
BB
AA
y/x
y/x��
Bae, Farha, Hupp, Snurr, J. Mater. Chem., 2009.
HKUST-1 (Cu-BTC)*
� Cubic unit cell
� 0.5/0.9 nm pores
� Cu2 corners
� Benzene-1,3,5-tricarboxylate linker
� As synthesized HKUST-1 has one coordinated water molecule per Cu
� HKUST-1 has been the subject of numerous experimental and modeling studies.
*Chui, Lo, Charmant, Orpen, Williams, Science, 1999.
Effect of Coordinated Water Molecules
CO2 Simulations CO2 Experiments
Yazaydin, Benin, Faheem, Jakubczak, Low, Willis, Snurr, Chem. Mater., 2009.
Effect of Coordinated Water Molecules
298 K
HKUST-1 ZIF-8
Yazaydin, Snurr, Park, Koh, Liu, LeVan, Benin, Jakubczak, Lanuza, Galloway, Low, Willis, J. Am. Chem. Soc., 2009.
CO2 Adsorption in MOFs
Selectivity for CO2 over N2 from mixture GCMC simulations
� Significant increase in selectivity if water molecules are present.
� Axial ligation of coordinatively unsaturated metal sites by various molecules could open up new possibilities for tuning the adsorption behavior of MOFs for CO2 capture and other applications.
Effect of Coordinated Water Molecules
Yazaydin, Benin, Faheem, Jakubczak, Low, Willis, Snurr, Chem. Mater., 2009.
SelectivityBB
AA
y/x
y/x��
� We have screened a diverse set of 14 metal-organic frameworks for low-pressure CO2 uptake using a consistent, predictive molecular modeling approach.
� The model was validated against experiments. Given this validation, the molecular model can aid in selection of MOFs for flue gas separation by screening a large number of materials and providing insight into the mechanism of CO2 adsorption.
� Parameters from generalized force fields are usually good enough to predict experimental data. However, the model can be further improved to account for the strong interactions between open-metal sites and CO2.
� MOFs with a high density of open-metal sites are good candidates for CO2 capture from flue gas.
Summary
• Strengths of MOFs– Huge variety of potential structures– Ready functionalization– Heats of adsorption are lower than zeolites– Molecular modeling can be used for screening
• Challenges for MOFs– MOFs with high CO2 uptake tend to adsorb water– Stability in flue gas environment– Cost uncertainties
Outlook
Acknowledgments
• Post-docs– A. Özgür Yazaydin – Krista Walton (Georgia Tech)– David Dubbeldam (U. Amsterdam)
• Screening Collaborators– Rich Willis (UOP) – John Low (UOP)– Annabelle Benin (UOP) – M. Doug LeVan (Vanderbilt U.)– Stefano Bandani (U. Edinburgh) – Adam Matzger (U. Michigan)
• Other Collaborators– Omar Yaghi (UCLA) – David Farrusseng (CNRS)
• Northwestern Collaborators– Joseph Hupp – SonBinh Nguyen
• Funding– Department of Energy, NETL– Department of Energy, Basic Energy Sciences– TeraGrid Computing Resources
298 K
Walton, Millward, Dubbeldam, Frost, Low, Yaghi, Snurr, J. Am. Chem. Soc., 2008.
CO2 Adsorption in MOFs
0.1 1 10 100 10000.0
0.2
0.4
0.6
0.8
1.0
1.2
CO
2 Den
sity
, g/c
m3
Pressure, bar
IRMOF-1 IRMOF-10 IRMOF-16�b, 298K