nitrogen and oxygen adsorption in metal- organic ... · nitrogen and oxygen adsorption in metal-...
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Nitrogen and oxygen adsorption in metal-organic frameworks for gas separations
Stephanie Gnewuch, Otterbein University
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Overview
Industrial N2/O2 gas separation
Metal-Organic Frameworks
Neutron Powder Diffraction
Preliminary gas mixture studies
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Cryogenic Distillation
O2
O2O2
O2 O2
O2
Oxygen OUTAir IN
Cool
Boiling point N2:
-195°C Boiling point O2:
-183°C
Expensive,
energy-intensive process
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Pressure Swing Adsorption (N2/O2 example)
Nitrogen OUT
Depressurized Chamber
ADSORB O21
2 RELEASE O2
COLLECT N2Pressurized Chamber1
Oxygen Adsorbed
Oxygen OUT
AIR IN
Looking for better adsorption material:
• Porous, crystalline powder
• Higher selectivity for O2 over N2 for more ambient operating conditions
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Metal-Organic Frameworks (MOFs)
+ =Metal Organic Linker MOF
Metal atoms
Organic LinkerMetal atoms
C
OH
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+ =Metal atoms
Organic LinkerMetal atoms
C
OH
Metal= Mg, Ni, Mn, Fe, Co, Cu, Zn
dobdc = 2,5-dioxido-
1,4-benzenedicarboxylate M2-2,5-dioxido-1,4-benzenedicarboxylate
MOF-74
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Open metal sites in MOF-74
solvent
solvent
solvent
solvent
solvent
solvent
Open metal sites for gas to adsorb
solvent
MOF-74 materials have a high concentration of open metal sites = adsorption enhancement
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Gas molecules adsorb at the open metal site
Example: N2
adsorbed to the framework
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sample
θ
2θ
detector
d
Bragg’s Law: nλ = 2dsinθ
Powder diffraction
2θ
cou
nts
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Powder neutron diffractometer (BT-1)
sample
detectors
Neutron beam from reactor
Monochromatic beam
Collimated beam
Ge(311) monochromator = 2.078 Å
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Rietveld Refinement
Difference between data and fit
Refinement model× data
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Powder diffraction pattern with and without gas
2theta
20 40 60
scale
d c
ounts
1000
2000
— Evacuated material— Evacuated material dosed with gas
cou
nts
2θ
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Rietveld Refinement
Model with no adsorbed gas
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N2?
Fourier Difference Techniques - to find the gas molecules
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N2
Fourier Difference Techniques - to find the gas molecules
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Rietveld Refinement
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Our Project
Extensive experience using this method on single-component adsorption, but…
Can we extend this to mixtures to learn more about selectivity in these materials?
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MOF-74 Synthesis
M(NO3)2 • 6H2O + H4dobdc100℃, DMF
M2dobdc•xDMF synthesis
M2dobdc • xDMF60℃, MeOH
M2dobdc • xMeOH solvent exchange
M2dobdc • xMeOH180℃, vacuum
M2dobdc activation
M = Ni,Mg
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Preparing the sample for data collection
Crystalline, powder product
Samplecan
Gas lineCCR
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Gas dosing Gas dosing cart
Dosing gas
Pressure
gauge
To sample
To vacuum
manifold
valve
valve valve
valvePV = n RTP = ?V = known volume of manifoldn = e.g. 1 gas molecule: metal siteRT = constants
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Nitrogen
Dosing Level:0.75 N2 : Mg site
Refined occupancy:0.7505
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Oxygen
Dosing Level:0.75 O2 : Mg site
Refined occupancy:0.7718
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Can we find both molecules if dosed with a mixture of the two?
N2 and O2?
+ =
N2 O2
79%N2/ 21% O2
mixture(to simulate air)
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Mixture (79% N2/21% O2)
Dosing Level:1 molecule: Mg siteEffectively 0.79 N2:Mg
0.21 O2:Mg
Refined occupancy:N2 0.6026O2 0.2773(occupancy of N2 at a secondary site is 0.1975)Total: 1.0774
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Mixture experiments using Ni-MOF-74…
Objective—Directly observe selectivity for O2 over N2indicated by higher occupancies of O2 at the metal site
• Higher dosing—3 molecules: Ni site
• Range of temperatures—90K to 300K
Data analysis in progress
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Conclusions
Determined N2 and O2 binding orientations at metal site
Were able to refine dosings of mixture with both molecules at the same adsorption site
Future work
• Continue refinement work on Ni-mixture data
• Extend technique to other mixtures and MOFs
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Acknowledgements Dr. Zeric Hulvey (Advisor)
Dr. Matt Hudson and Dr. Craig Brown
Dr. Julie Borchers, Dr. Terrell Vanderah, Dr. Bob Shull (MML/NCNR Materials SURF directors)
Dr. Robert Dimeo (NCNR director)