Vibrational Shift of Adsorbed CO2
within a Metal-Organic Framework
Outline
• Motivation: CO2 capture
• System: Metal-Organic Frameworks
• Data:
Unusual blue shift of adsorbed CO2 n3 mode
Room-temperature sidebands
Low-temperature bands reveal 2nd configuration
Motivation
• Carbon capture
– Separate carbon dioxide from exhaust gases
– Emissions reduction accompanying switch to clean energy sources
• Natural gas purification
– Separate CO2 from methane (CH4)
– Improve energy density of fuel, decrease pipe corrosion
• Current CO2 separation methods are costly
– Harmful materials
– High energy costs for regeneration
– A better way?
http://www.nma.org/ccs/carboncaptu
Metal-Organic Frameworks
Metal ions linked by organic chains
Very low density
Crystalline and “tunable”
Vast number of possible structures
Large voids, voids of ~ 10 – 20 Å for
molecular storage and separation
Complex unit cell makes computation
modeling challenging
Significant van der Waals interactions
MOF-74
Honeycomb structure
Metal-oxide clusters
linked by Benzene rings
Unsaturated metal ion
acts as primary binding
site for CO2
2+
2.4 Å
Diffraction indicates CO2 is nearly linear
H. Wu et al. J. Phys. Chem. Lett., 1(13):1946–1951, 2010.
MOF-74 Isostructural Series
Same structure, different metal
Mg-MOF-74, Mn-MOF-74, Fe-MOF-74 ,Co-MOF-74, Zn-MOF-74
http://legacy.owensboro.kctcs.edu/gcaplan/bio/Notes/BIO%20Notes%20C%20intro%20chem.htm
MOF-74 Selective Binding
Binding energy in Mg-MOF-74 ~ 40 - 50 kJ/mol
Binding energy in other MOF-74 at least 7 kJ/mol less
Difference is likely due to direct electrostatic
interaction via shorter Mg – O bond
Binding energy for CH4 in MOF-74 ~ 20 kJ/mol
Difference between CO2 and CH4 mainly attributed to
CO2 quadrupole moment
Caskey et al. J. Am. Chem. Soc., 130,10870, (2008).
H. Wu et al. J. Am. Chem. Soc., 131, 4995 (2009).
Park et al. Phys. Chem. Lett. 3, 826 (2012).
Yao et al. Phys. Rev. B. 85, 64302 (2012).
Diffuse Reflectance Spectroscopy
• Light bounces around
within powder sample
• Very long path length
enhances absorption signal
Diffuse Reflectance Spectroscopy: Cryostat Assembly
Rev. Sci. Instr. 77, 093110 (2006)
Absorbance (Arb. Units)
n3 mode of adsorbed CO2
Mg
Mn
Fe
Co
Zn
2300
2320
2340
2360
-1
Frequency (cm )
2380
2400
Vibration of adsorbed H2
Absorbance
Fe
Co
Mn
Mg
Zn
4050
4100
Frequency (cm )
-1
4150
Absorbance (Arb. Units)
n3 mode of adsorbed CO2
Mg
Mn
Fe
Co
Zn
2300
2320
2340
2360
-1
Frequency (cm )
2380
2400
Side Bands: Translational/Librational
000 →001
Absorbance (Arb. Units)
0.1
2250
010 →011
C13
2300
2350
2400
-1
Frequency (cm )
2450
Librational Motion
MOF-74
2+
H. Wu et al. J. Phys. Chem. Lett., 1(13):1946–1951, 2010.
Temperature Dependence
New band emerges below 150 K
251 K
1.0
163 K
0.8
104 K
14
ΔEB = 0.7 ± 0.1 kJ/mol
Degeneracy ratio of ~ 2
44 K
34 K
Frequency (cm )
12
16
-1
1000/T (K )
54 K
2360
-1
0.4
0.0
10
65 K
2350
0.6
0.2
75 K
2340
Room temperature peaks too broad to
resolve
Ln(A1/A2)
Absorbance (Arb. Units)
0.1
2370
18
vdW-DF2 Theory Calculations
Y. Yao et al. Phys. Rev. B, 85, 064302 ( 2012).
Predicts sites 2.96 and 3.09 Å away from
metal with 0.8 kJ/mol energy difference
Combination modes compared to Hitran Data
Absorbance (Arb. Units)
n3
000 → 001
n3 + Fermi resonance
000 → 101 and 000 → 021
2400
0.01
0.05
0.05
2300
n3 + 2 x Fermi resonance
000 → 201, 041, 121
3600
3800
-1
Frequency (cm )
4800
5000
5200
Conclusion CO2 in MOF-74
• Mg version n3 mode unique in showing blue shift
• All other modes show red shift
• Evidence for CO2 librational/translational motion
• Evidence for a 2nd “nearly degenerate” adsorbed CO2
configuration
Undergrad Students
Michael Friedman
Jordan Gotdank
Jesse Hopkins
Brian Burkholder
Ben Thompson
Chris Pierce
Jennifer Schloss