Motivation: CO 2 capture System: Metal-Organic Frameworks Data: Unusual blue shift of adsorbed CO 2 3 mode Room-temperature sidebands Low-temperature bands reveal 2 nd configuration Vibrational Shift of Adsorbed CO 2 within a Metal-Organic Framework Outline

Motivation Carbon capture –Separate carbon dioxide from exhaust gases –Emissions reduction accompanying switch to clean energy sources Natural gas purification –Separate CO 2 from methane (CH 4 ) –Improve energy density of fuel, decrease pipe corrosion Current CO 2 separation methods are costly –Harmful materials –High energy costs for regeneration –A better way? p

Metal-Organic Frameworks Large voids, voids of ~ 10 – 20 Å for molecular storage and separation Complex unit cell makes computation modeling challenging Significant van der Waals interactions Metal ions linked by organic chains Very low density Crystalline and “tunable” Vast number of possible structures

Honeycomb structure Metal-oxide clusters linked by Benzene rings MOF-74 H. Wu et al. J. Phys. Chem. Lett., 1(13):1946–1951, Diffraction indicates CO 2 is nearly linear 2+ Unsaturated metal ion acts as primary binding site for CO Å

MOF-74 Isostructural Series Same structure, different metal Mg-MOF-74, Mn-MOF-74, Fe-MOF-74,Co-MOF-74, Zn-MOF-74

MOF-74 Selective Binding Binding energy in Mg-MOF-74 ~ 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 CH 4 in MOF-74 ~ 20 kJ/mol Difference between CO 2 and CH 4 mainly attributed to CO 2 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, (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, (2006)

3 mode of adsorbed CO 2

Vibration of adsorbed H 2

3 mode of adsorbed CO 2

Side Bands: Translational/Librational 000 → →011 C 13

Librational Motion

MOF-74 H. Wu et al. J. Phys. Chem. Lett., 1(13):1946–1951,

Temperature Dependence ΔE B = 0.7 ± 0.1 kJ/mol New band emerges below 150 K Degeneracy ratio of ~ 2 Room temperature peaks too broad to resolve

vdW-DF2 Theory Calculations Y. Yao et al. Phys. Rev. B, 85, ( 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 → Fermi resonance 000 → 101 and 000 → x Fermi resonance 000 → 201, 041, 121

Conclusion CO 2 in MOF-74 Mg version 3 mode unique in showing blue shift All other modes show red shift Evidence for CO 2 librational/translational motion Evidence for a 2 nd “nearly degenerate” adsorbed CO 2 configuration

Michael Friedman Jordan Gotdank Jesse Hopkins Brian Burkholder Ben Thompson Chris Pierce Jennifer Schloss Undergrad Students