59th Annual Report on Research 2014

Metal organic frameworks (MOFs) are an emerging class of hybrid materials composed of metal ions or clusters connected by organic linker molecules to form crystalline coordination polymer networks. The intrinsic porosity, high surface areas and ordered arrangement of organic and inorganic components within these self-assembled systems make them well-matched for adsorption-based applications like gas separation and storage and heterogeneous catalysis. MOF function is ultimately dictated by the specific arrangement of the framework components and their cooperative interaction with guest species contained within the cavities. In most cases, these fundamental molecular level underpinnings remain largely unexplored, leaving significant ambiguities in MOF structure/function relationships that give rise to their observed behavior and utility. This PRF-funded project targeted these unknown and underexplored characteristics by utilizing electronic and structurally sensitive spectroscopy techniques, namely optical, vibrational and X-ray absorption spectroscopy.

In the first grant year, the PI’s lab successfully completed the initial phase of the research, which involved establishing the feasibility of extracting new structural information on MOF materials using these combined spectroscopy techniques. One of the main projects involved elucidating the activation process in a flexible MOF comprised of Co(II) ions and 4,4’-oxybis(benzoic acid) linkers using spectroscopy methods. The results of this work, which was done in collaboration with Prof. Jing Li’s group at Rutgers University (New Brunswick), were published last year. The second phase of the project, carried out over the final year of the grant, involved expanding the application of these spectroscopy techniques to explore the structural aspects of the host-guest interactions of gas adsorption processes in MOF materials. This work involved the continued use of the National Synchrotron Light Source (NSLS) at Brookhaven National Lab and the custom controlled environment reaction chambers available to users for in situ XAS studies. The design and construction of a custom-made sample chamber for in situ Raman measurements at variable temperature and under controlled gas and/or vacuum environment was successfully completed. Additional laser excitation sources have been implemented to greatly expand the versatility of the Raman measurements. This unique instrument has become a work horse for the Raman spectroscopic studies of gas adsorption processes in MOFs. These expanded experimental capabilities have enabled the exploration of more challenging adsorption based processes in MOF materials. One successful study that was completed in the last year focused on carbon dioxide adsorption in a Cu-based framework containing both open metal sites and linker localized amine groups for carbon sequestration applications. This material was obtained through continued collaboration with Prof. Jing Li. In that study MOF activation and CO₂ gas adsorption were probed using XAS, UV-Vis diffuse reflectance and Raman spectroscopy to reveal the electronic and structural changes of both the framework and the CO₂ adsorbate that occur in the process. A manuscript based on this work is currently in preparation and submission for publication is imminent. Other frameworks with CO₂ adsorption sites are currently being investigated using these complementary in situ spectroscopy techniques. Expansion to other adsorption based applications such as gas phase heterogeneous catalysis as well as liquid phase catalytic reactions is anticipated. Other collaborative work involving characterization of the latter is currently underway. The first XAS studies on these types of materials used to confirm catalyst coordination environment were published this year. Reactor cells for in situ spectroscopic monitoring of these condensed phase process are current being designed.

The results of these fundamental studies tie into the long range goals of the research program, which include correlating the specific structure/function relationships described above with observed MOF performance in terms of gas separation and catalytic activity. Identifying these connections will ultimately enable the rational design of new materials with improved functionality.

This PRF grant has provided direct partial support for two post-docs in the group, Raghabendra Samantaray and Justin Rhinehart, and indirect support for one graduate student, Yuan Chen all of whom have played an integral role in carrying out the work associated with this project. The in situ sample chambers for Raman and diffuse reflectance spectroscopy were constructed by both Ragahabendra and Justin. This instrument building experience helped Justin secure a permanent position in industry following his tenure in the PI’s lab. Three trips to NSLS at Brookhaven National Lab for XAS experiments over the last year have been partially funded by the grant. These X-ray experiments have served as invaluable educational experiences particularly for Yuan, who gained extensive synchrotron instrumentation skills. The most recent XAS results obtained from those experiments, in addition to being submitted for publication were included in a successful general user proposal to the Advanced Photon Source at Argonne National Lab and were therefore instrumental in securing a new round of X-ray spectroscopy beamtime. The progress made on this project thanks to the ACS-PRF-DNI grant has also put the PI in a better position to apply for funding through other agencies such as NSF. An NSF CAREER Award proposal was recently submitted by the PI and is now pending the support decision.