Reports: DNI552148-DNI5: Investigation of Host-Guest Interactions in Metal-Organic Framework Materials Using Spectroscopic Methods

Jenny Lockard, PhD, Rutgers, the State University of New Jersey (Newark)

Metal organic frameworks (MOFs) constitute a relatively new class of hybrid porous materials comprised of metal-containing inorganic clusters connected by organic linker groups. Their microporous nature, ability to self-assemble and versatile chemical composition and structure make them ideal for potential applications in gas separation and storage and heterogeneous catalysis. Most studies of MOF materials that focus on these functionalities give only cursory explanations of the electronic and structural properties behind them. In particular, a molecular level understanding of the interactions between the host framework and guest molecules contained within the pores is rarely known, yet is crucial for the rational design of MOF materials for their intended applications. The objective of this research project is to investigate the molecular level host-guest interactions and local coordination environment around the metal sites in MOF materials using a combination of vibrational and X-ray absorption spectroscopy (XAS) methods.

In the first year of this ACS-PRF grant, our lab has successfully completed the initial phase of the project, which involved establishing the feasibility of extracting new structural information on MOF materials using these combined spectroscopy techniques. In collaboration with Prof. Jing Li’s group at Rutgers University (New Brunswick), we focused our attention on a flexible MOF comprised of Co(II) ions and 4,4’-oxybis(benzoic acid) linkers that, in its activated form, exhibits catalytic behavior for olefin epoxidation reactions. While the overall crystallinity of the material survives the vacuum-assisted activation process, which involves removal of the metal coordinated water molecules, the resulting microcrystalline powder leaves limited options for characterization by traditional XRD methods. The framework however was suspected to undergo important local structure rearrangements that may influence its ultimate catalytic performance. We used in situ X-ray absorption and Raman spectroscopy to build a composite picture of the MOF activation process by revealing both the substantial local structural changes about the Co sites and the more subtle rearrangements of the organic linkers that are not apparent through standard powder X-ray diffraction studies. Briefly, analysis of both the XANES and EXAFS data revealed a metal coordination environment change from 6 coordinate octahedral to 4 coordinate tetrahedral geometry accompanied by an overall reduction in the first shell coordination sphere.  The Raman studies supported these findings and also revealed subtle structural changes on the organic linker components, in particular the dihedral angle about the oxygen bridge. A manuscript we prepared describing these results was recently accepted for publication as an article in the Journal of Physical Chemistry.

Encouraged by these first spectroscopic studies on MOF material activation processes, our group has embarked on the second phase of the project, which is essentially geared toward elucidating the structural aspects of the host-guest interactions of gas adsorption processes in MOF materials.  While the X-ray beamline we use at the National Synchrotron Light Source (NSLS) at Brookhaven National Lab to perform our XAS experiments is equipped with controlled environment chambers for in situ studies of MOF gas adsorption, the Raman instrumentation in our own lab did not have this capability. Over the past several months, our group has worked to design and implement a custom-made sample chamber for in situ Raman measurements at variable temperature and under controlled gas and/or vacuum environment. We are now collecting preliminary Raman data and anticipate that this instrumentation will become a work horse for the Raman spectroscopic studies of gas adsorption processes in MOFs.  Carbon dioxide adsorption in MOF materials containing open metal sites and/or pendant amine groups for carbon sequestration applications is the initial target of this part of the project, but we anticipate expanding to other adsorption based applications such as gas phase heterogeneous catalysis.  And, just as before, we will combine these measurements with in situ XAS studies carried out at NSLS.

This PRF grant has provided direct partial support for two post-docs in the group, Raghabendra Samantaray and Justin Rhinehart, and indirect support for my one graduate student, Yuan Chen all of whom have played an integral role in carrying out the work associated with this project.  Three trips to Brookhaven for XAS experiments have been partially funded by the grant. The soon to be published XAS results obtained from those experiments were included in a successful general user proposal and were therefore instrumental in securing additional beamtime for the next 2 year proposal cycle at NSLS.  The progress we have made on this project thanks to this grant has also put us in a better position to apply for funding through other agencies such as NSF.