A Continuous-Wave Electron Paramagnetic Resonance Study of Carbon Dioxide Adsorption on the Metal-Organic Frame-Work MIL-53

作者：Mendt Matthias^*; Jee Bettina; Himsl Dieter; Moschkowitz Lutz; Ahnfeldt Tim; Stock Norbert; Hartmann Martin; Poeppl Andreas

来源：Applied Magnetic Resonance, 2014, 45(3): 269-285.

DOI：10.1007/s00723-014-0518-6

Continuous-wave electron paramagnetic resonance spectroscopy is applied to explore the adsorption of carbon dioxide (CO2) over the metal organic framework (MOF) MIL-53. Therefore, paramagnetic Cr3+ ions, which replace a small amount of the bulk Al3+ ions in MIL-53(Al/Cr), are used as magnetically active probes. CO2 was adsorbed on samples of MIL-53(Al/Cr) at equilibrium pressures between 0 and 2.5 bar. The transformation from the large pore phase to the narrow pore phase of MIL-53 was observed by electron paramagnetic resonance spectroscopy at small CO2 pressures between 0.2 and 0.4 bar, which is in accordance with adsorption results reported in literature. By analyzing the electron paramagnetic resonance signal intensities of the corresponding Cr3+ probes, the ratio between the amount of the narrow pore phase and the large pore phase before and after this phase transformation was quantified. A small fraction of the large pore phase remains even after this phase transition. CO2 adsorption at 77 K indicates the occurrence of the transformation of this MOF from a narrow pore phase to a large pore phase triggered by the adsorbed CO2. Similar observations were already made using powder X-ray diffraction or infrared spectroscopy. But in contrast to these methods electron paramagnetic resonance spectroscopy on Cr3+ seems to be very sensitive not only to large differences between crystallographic conformations like large pores and narrow pores but also to different amounts and configurations of CO2 molecules trapped in the same structural phase of MIL-53, taking advantage of the high sensitivity of the fine structure interaction of Cr3+.

• 出版日期2014-3