Post-Synthetic Metalation of MOFs: Alternative Adsorption Sites for Sulfur Gas Uptake

Natural gas and biogas have both gained popularity in recent years as lower-carbon sources of energy. While the main component of interest in these gas streams is methane (CH₄), they also contain small amounts of hydrogen sulfide (H₂S) that must be removed in order to maximize heat content and protect process equipment. Additionally, SO₂ present in flue gas streams must be removed to prevent the formation of acid rain. Selective adsorption of these gases from process streams has the potential to be an energy-efficient method of purification, with metal-organic frameworks (MOFs) being one of the most promising class of adsorbents. Unfortunately, MOF structures often degrade upon adsorption of acid gases; enhancing MOF stability would increase their industrial viability. One strategy for improving structural stability is the introduction of alternative adsorption sites that are separate from the MOF backbone. Specifically, this work focuses on the post-synthetic coordination of metal ions to MOF functional groups. Additional metal sites serve the dual function of providing a stable adsorption site as well as increasing guest-site interactions and thus adsorption capacity. These modified materials were tested in mixtures of CH4/CO2/H2S and CO2/SO2 to represent real-world conditions. This work can be applied to the development of strategies to further stabilize MOFs in acid gas mixtures.