|An Inside Look at a MOF in Action|
A unique inside look at how the electronic structure of a highly touted metal–organic framework (MOF) material changes as it adsorbs carbon dioxide gas should help in the design of new and improved MOFs for carbon capture and storage. Mg-MOF-74, which features as its metal component magnesium atoms with open sites for bonding, has emerged as one of the most promising materials for capturing and storing greenhouse gases. Working at ALS Beamline 6.3.1, researchers applied near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy to Mg-MOF-74 samples and obtained what are believed to be the first-ever measurements of chemical and electronic signatures inside of a MOF during gas adsorption.
MOFs are molecular systems consisting of metal oxide centers connected by organic "linker" molecules that form a highly porous three-dimensional crystal framework. This microporous crystal structure enables MOFs to serve as storage vessels with a sponge-like capacity for capturing and containing greenhouse gases. When a solvent molecule that was applied during the formation of the MOF is subsequently removed, the result is a MOF with "unsaturated" (open) metal sites that have a strong affinity for carbon dioxide. Such open metal sites preferentially adsorb carbon dioxide over nitrogen or methane due to carbon dioxide's larger quadrupole moment and greater polarizability. The molecular geometry of Mg-MOF-74, in which the (magnesium) metal atoms are centered in the base of a square pyramid with the apex site vacant, is especially selective for carbon dioxide over other greenhouse gases and has an exceptionally large storage capacity.
To better understand these binding mechanisms, NEXAFS measurements of Mg-MOF-74 were made at ALS Beamline 6.3.1. NEXAFS spectroscopy is an element-specific technique, probing unoccupied electronic states associated with an excited atom. For systems with specific chemically distinct binding sites, such as the magnesium sites in Mg-MOF-74, NEXAFS spectra provide high sensitivity to changes in the local electronic structure and coordination at the binding sites upon adsorption of gas molecules.
A special gas cell designed by the team enabled NEXAFS measurements to be made as the CO2 pressure was varied from vacuum up to 100 Torr at ambient temperature. This capability allowed direct comparisons between empty and bound sites of the same Mg-MOF-74 sample. In addition, the researchers collected spectra in the presence of the solvent used during MOF synthesis (DMF) and also of an expanded analogue to Mg-MOF-74, with longer organic linkers, to explore how electronic interactions differ for different adsorbed species and for binding sites in a larger framework.
In general, the researchers observed a distinct pre-edge peak when the metal site was open. When CO2 (or DMF) was introduced, the peak was suppressed and the main-edge features were blue-shifted. These spectral signatures evolved as a function of CO2 gas pressure and reverted as the gas was pumped out. Theoretical spectra, based on first-principles calculations of electronic structure with and without CO2, reproduced the findings and provided insight into the origin of the pre-edge peak: when the metal site is open, it breaks the site's octahedral symmetry and induces orbital mixing. This activates otherwise forbidden transitions in the NEXAFS spectrum, producing the pre-edge peak. When CO2 or DMF is adsorbed, symmetry is restored and the peak is suppressed.
Having established NEXAFS spectroscopy as an effective experimental tool for the study of MOFs and gas adsorption, the researchers expect to see many more studies of fundamental adsorption interactions inside of MOFs. Regarding open-metal-site MOFs, similar studies in which the metal species are transition metals will be interesting, as will systematic studies of different metal sites in the same MOF structure. Such studies should provide fundamental insights and help explain why some MOFs work better than others. This, in turn, should help us to predict which are the best metals to consider as MOF design evolves.
Research conducted by: W.S. Drisdell, J.B. Neaton, D. Prendergast, and J.B. Kortright (Berkeley Lab); R. Poloni (Berkeley Lab, UC Berkeley, and the French National Center for Scientific Research); T.M. McDonald and J.R. Long (Berkeley Lab and UC Berkeley); and B. Smit (UC Berkeley).
Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Support for this work was provided by the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center (EFRC) funded by the DOE Office of Science. Operation of the ALS is supported by DOE BES.
W.S. Drisdell, R. Poloni, T.M. McDonald, J.R. Long, B. Smit, J.B. Neaton, D. Prendergast, and J.B. Kortright, "Probing Adsorption Interactions in Metal−Organic Frameworks using X‑ray Spectroscopy," J. Am. Chem. Soc. 135, 18183 (2013).
ALS Science Highlight #292