A metal-organic framework (MOF) compound can soak up large quantities of ammonia, packing the gas especially tightly in the pores of the new aluminum-based crystalline material (J. Am. Chem. Soc. 2021, DOI: 10.1021/jacs.1c01749). The finding may lead to new types of sorbents for storing large quantities of ammonia in a small volume, a key requirement for using the gas as a source of hydrogen fuel for transportation. The study may also result in advances in trapping trace quantities of the toxic gas, as needed for personal protection. Researchers have previously evaluated zeolites, silica, MOFs, and other materials for storing and capturing ammonia. Many of these materials were considered impractical because of their low storage capacity, irreversible uptake, or chemical instability after multiple adsorption-desorption cycles. A team led by Christopher Marsh, Sihai Yang, and Martin Schröder of the University of Manchester now reports that its new MOF, MFM-303(Al), reversibly stores ammonia with an exceptional room-temperature packing density (a measure of storage capacity by volume) of roughly 0.80 g/cm3. That value is only slightly lower than the density of solid ammonia—approximately 0.81 g/cm3. Analyses show that the gas packs efficiently because of the presence of OH and COOH binding sites, which line the pores of the new MOF.