Metal-organic framework-metal oxide composites for toxic gas adsorption and sensing

Abstract

Metal organic frameworks (MOFs) and metal oxide-MOF composites were investigated for adsorption and oxidation of carbon monoxide. Metal oxides were successfully included in MOFs via both impregnation and encapsulation. UiO-66, a zirconium-based MOF, was impregnated with magnesium or cobalt oxide. Cobalt oxide in UiO-66 increases the room temperature CO capacity and shows increased adsorption at 65°C due to strong cobalt-CO interactions. Titania and magnetic nanoparticles were encapsulated in HKUST-1, a copper-based MOF. Including titania in HKUST-1 lowers the CO oxidation onset temperature by over 100°C compared with HKUST-1, and the composite reaches complete conversion by 250°C. HKUST-1 with magnetic nanoparticles shows enhanced structural stability and increased room temperature adsorption of CO and hexane. MOF-74, an isostructural family with coordinatively unsaturated metal centers of cobalt, magnesium, nickel, or zinc, was investigated for the metal center’s impact on stability and adsorption. Pre-treatment conditions to optimize accessibility were found that maximize solvent removal while retaining structural integrity. The impact of air exposure on equilibrium CO capacity was investigated, and these predictions were compared to dynamic conditions, separating CO from nitrogen or air at room temperature. The cobalt analog loses only 25% of its CO capacity with air exposure, retaining higher capacity than the other analogs under ideal conditions. Unlike cobalt, the magnesium analog does not follow the predicted trends with air exposure, having higher dynamic capacities with pre-exposed samples. Under all dynamic conditions, the nickel analog oxidized a portion of the carbon monoxide feed.