Synthesis, Characterization, and Ammonia Adsorption Properties of Mesoporous Metal-Organic Framework (MIL(Fe))-Graphite Oxide Composites: Exploring the Limits of Materials Fabrication

Composites of MIL-100(Fe) and graphite oxide (GO) were prepared with various ratios of the two components and tested for ammonia removal in dynamic conditions. The initial and exhausted samples were characterized by X-ray diffraction, nitrogen adsorption, thermal analysis, Fourier Transform infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy. The results indicate that the formation of well-defined MIL-100(Fe)/GO composites is not favored. This is linked to the specific geometry of MIL-100(Fe). The attachment of the GO carbon layers to the spherical cages of MIL-100(Fe) (via coordination between the oxygen groups of GO and the metallic sites of the metal-organic framework) prevents the proper formation of the MIL-100(Fe) structure. Therefore, the composite with the highest GO content has a lower porosity and smaller ammonia adsorption capacity than those calculated for the physical mixture of MIL-100(Fe) and GO. The main mechanism of ammonia retention is via Bronsted interactions between ammonia and the water molecules present in MIL-100(Fe). Nevertheless, the presence of excess water in the system lowers the acidity of the MIL material, and consequently causes a decrease in the ammonia adsorption. The Lewis interactions between ammonia and the metal centers in MIL also take place during the adsorption process.