Reactive Adsorption of Humid SO2 on Metal-Organic Framework Nanosheets

We investigate the interactions of copper, zinc, and cobalt metal-organic framework (MOF) nanosheets with sulfur dioxide (40 ppm in air of 85% relative humidity at 25 degrees C), with the aim of understanding chemically induced structural changes of open-metal MOFs when exposed to wet acid gases such as flue gas. The structural evolution of the frameworks and the sulfur dioxide adsorption characteristics were examined by vibrational spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The stabilities of the materials under the experimental conditions were established as a function of the metal center, with zinc benzenedicarboxylate (ZnBDC) being the most stable followed by CuBDC and CoBDC. After 80 ppm-days of exposure to humid SO2 atmosphere, each MOF was found to contain roughly one sulfur atom per metal site. Adsorbed sulfur was found in the +6 and +2 oxidation states with the distribution across these states varying according to the metal in the material. The implication of this result is that the aqueous chemistry of adsorbed sulfur dioxide is strongly influenced by the metal center. The ability to direct electron transfer to or from adsorbed sulfur while maintaining comparable particle and crystal structure is a powerful potential tool for desulfurization technologies.