Ammonia Storage in Metal–Organic Framework Materials: Recent Developments in Design and Characterization

Since the advent of the Haber-Bosch process in 1910, the global demand for ammonia (NH₃) has surged, driven by its applications in agriculture, pharmaceuticals, and energy. Current methods of NH₃ storage, including high-pressure storage and transportation, present significant challenges due to their corrosive and toxic nature. Consequently, research has turned towards metal-organic framework (MOF) materials as potential candidates for NH₃ storage due to their potential high adsorption capacities and structural tunability. MOFs are coordination networks composed of metal nodes and organic linkers, offering unprecedented porosity and surface area, and allowing incorporation of various functional groups and metal sites that can enhance NH₃ adsorption. However, the stability of MOFs in the presence of NH₃ is a significant concern since many degrade upon exposure to NH₃, primarily due to ligand displacement and framework collapse. To address this, recent studies have focused on the synthesis and postsynthetic modification of MOFs to enhance both NH₃ uptake and stability. In this Account, we summarize recent developments in the design and characterization of MOFs for NH₃ storage. The choice of metal centers in MOFs is crucial for stability and performance. High-valence metals such as Al^III and Ti^IV form strong metal-linker bonds, enhancing the stability of the framework to NH₃. The MFM-300 series of materials composed of high-valence 3+ and 4+ metal ions and carboxylic linkers demonstrates high stability and high NH₃ uptake capacities. Ligand functionalization is another effective strategy for improving the NH₃ adsorption. Polar functional groups such as -NH₂, -OH, and -COOH enhance the interaction between the framework and NH₃, particularly at low partial pressures, while postsynthetic modification allows fine-tuning of these functionalities to optimize the framework for higher adsorption capacities and stability. For example, MFM-303(Al), incorporating free carboxylic acid groups, exhibits a high NH₃ packing density comparable to that of solid NH₃. Creating defect sites by removing linkers or adding metal ions increases the number of active sites available for NH₃ adsorption and shows promise for enhancing uptake. UiO-66, a stable MOF framework, can be modified to include defect sites, significantly enhancing the level of NH₃ uptake. The full characterization of MOFs and especially their interactions with NH₃ are vital for understanding and improving their performance. Techniques such as neutron powder diffraction (NPD), inelastic neutron scattering (INS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron paramagnetic resonance (EPR) spectroscopy, and solid-state nuclear magnetic resonance (ssNMR) spectroscopy can elucidate host-guest interactions and binding dynamics between NH₃ and the framework structure and afford crucial information for the future design and rational development of new sorbents. This Account highlights our current strategies for the synthesis and characterization of MOFs for NH₃ capture, providing an overview of this rapidly evolving field.