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.

中文翻译：

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金属有机框架材料中的氨储存：设计和表征的最新发展


自 1910 年 Haber-Bosch 工艺问世以来，全球对氨 （NH₃） 的需求激增，这得益于氨在农业、制药和能源领域的应用。当前的 NH₃ 储存方法，包括高压储存和运输，由于其腐蚀性和毒性，带来了重大挑战。因此，研究已转向金属有机框架 （MOF） 材料作为 NH₃ 储存的潜在候选者，因为它们具有潜在的高吸附能力和结构可调性。MOF 是由金属节点和有机连接子组成的配位网络，提供前所未有的孔隙率和表面积，并允许掺入各种官能团和金属位点，从而增强 NH₃ 吸附。然而，MOF 在 NH₃ 存在下的稳定性是一个重大问题，因为许多 MOF 在暴露于 NH₃ 时会降解，主要是由于配体置换和框架崩溃。为了解决这个问题，最近的研究集中在 MOF 的合成和合成后修饰上，以增强 NH₃ 的摄取和稳定性。在本账户中，我们总结了用于 NH₃ 储存的 MOF 设计和表征的最新进展。MOF 中金属中心的选择对于稳定性和性能至关重要。Al^III 和 Ti^IV 等高价金属形成强金属接头键，增强了 NH₃ 框架的稳定性。MFM-300 系列材料由高价 3+ 和 4+ 金属离子和羧基接头组成，具有高稳定性和高 NH₃ 吸收能力。配体功能化是改善 NH₃ 吸附的另一种有效策略。 –NH₂、–OH 和 –COOH 等极性官能团增强了框架与 NH₃ 之间的相互作用，尤其是在低分压下，而合成后改性允许微调这些官能团，以优化框架以获得更高的吸附能力和稳定性。例如，MFM-303（Al） 包含游离羧酸基团，表现出与固体 NH₃ 相当的高 NH₃ 堆积密度。通过去除接头或添加金属离子来创建缺陷位点增加了可用于 NH₃ 吸附的活性位点的数量，并显示出增强摄取的前景。UiO-66 是一种稳定的 MOF 框架，可以修改以包括缺陷部位，从而显着提高 NH₃ 的摄取水平。MOF 的完整表征，尤其是它们与 NH₃ 的相互作用，对于理解和提高其性能至关重要。中子粉末衍射 （NPD）、非弹性中子散射 （INS）、漫反射红外傅里叶变换光谱 （DRIFTS）、电子顺磁共振 （EPR） 光谱和固态核磁共振 （ssNMR） 光谱等技术可以阐明 NH₃ 之间的主客体相互作用和结合动力学以及框架结构，并为未来设计和合理开发新吸附剂提供重要信息。本账户重点介绍了我们目前用于 NH₃ 捕获的 MOF 合成和表征策略，概述了这一快速发展的领域。