Molecular Hydrogen Interactions Within Metal-Organic Frameworks Stephen FitzGerald and Jesse Rowsell Undergrad Students: Michael Friedman, Jesse Hopkins, Brian Burkholder, Ben Thompson, Jordan Gotdank, Jennifer Schloss Chris Pierce

Motivation: Hydrogen Storage for Fuel Cells High Pressure bar Liquid Hydrogen

Metal-Organic Frameworks Large complex unit cell H 2 binding dominated by van der Waals interactions Computation modeling challenge Metal ions linked by organic chains Very low density, voids of ~ 10 – 20 Å To date binding energy is to weak Vast number of possible structures

Experimental Techniques for Investigating H 2 Adsorption Loading Isotherms Easy but “low resolution” Neutron Diffraction Yields binding site locations but there are few facilities Infrared spectroscopy Yields dynamics but challenging for H 2 in MOFs

Diffuse Reflectance Spectroscopy Light bounces around within powder sample Very long path length enhances absorption signal Problem: requires large collecting optics

Diffuse Reflectance Spectroscopy: Cryostat Assembly Rev. Sci. Instr. 77, (2006)

Typical Spectra for H 2 in MOFs at 30 K

MOF-74 (M 2 C 8 H 2 O 6 ) where M can be Mg, Mn, Co, Ni, and Zn ~1 nm Neutron Diffraction Shows H 2 sites Coordinatively Unsaturated “Open-metal Site”

Spectra for H 2 in MOF-74 at 35 K Red spectrum low H 2 concentration Blue spectrum high H 2 concentration

Hydrogen-Hydrogen Interactions?

Spectra for H 2 in MOF-74 at 35 K Red spectrum low H 2 concentration Blue spectrum high H 2 concentration

Data from Chabal Group Spectra show low shift (secondary site) peak dominating Attribute 40 to 80 cm-1 to H 2 – H 2 interaction

Spectra as a function of H 2 concentration Spectra indicate site by site filling Concentrations match crystallographic assignments from neutron diffraction

Frequency Shift of pure Vibrational mode Highly shifted peaks (red) show major change across series Primary Site – Metal Distance = 2.6 Å Moderately shifted peaks show little change Blue Secondary Site – Metal Distance = 4.3 Å

Room Temperature Spectra Data consistent with low temp spectra Exposed-metal site fills first Secondary sites occupy before saturation of primary Exposure to air significantly alters spectrum Effect seems most pronounced for open- metal site

Room Temperature Spectra Spectra on air exposed sample virtually identical to Chabal spectra

H 2 – H 2 Interactions Shifts of at most 6 cm -1, most notably in S(0) bands

Conclusion Spectra show progressive site by site occupancy We see no evidence for large H 2 – H 2 induced shifts Air-exposure is a real concern when dealing with MOFs Van der Waals DFT models must be used cautiously