Reversible Adsorption of Ammonia in the Crystalline Solid of a CO₂H-Functionalized Cyclic Oligophenylene

Ammonia (NH₃) is a viable candidate for the storage and distribution of hydrogen (H₂) due to its exceptional volumetric and gravimetric hydrogen energy density. Therefore, it is desirable to develop NH₃ storage materials that exhibit robust stability across numerous adsorption-desorption cycles. While porous materials with polymeric frameworks are often used for NH₃ capture, achieving reversible NH₃ uptake remains a formidable challenge, primarily due to the high reactivity of NH₃. Here, we advocate the use of CO₂H-functionalized cyclic oligophenylene 1a with high chemical stability as a novel single-molecule-based adsorbent for NH₃. Simple reprecipitation of 1a selectively yielded microporous crystalline solid 1a (N). Crystalline 1a (N) adsorbs up to 8.27 mmol/g of NH₃ at 100 kPa and 293 K. Adsorbed NH₃ in the pore of 1a (N) has a packing density of 0.533 g/cm³ at 293 K, which is close to the density of liquid NH₃ (0.681 g/cm³ at 240 K). Crystalline 1a (N) also exhibits reversible NH₃ adsorption over at least nine cycles, sustaining its storage capacity (1st cycle: 8.27 mmol/g; 9th cycle: 8.25 mmol/g at 100 kPa and 293 K) and crystallinity. During each desorption cycle, NH₃ was removed from 1a (N) under reduced pressure (∼65 Pa), leaving <3% of the total uptake, and 1a (N) was fully purged under dynamic vacuum conditions (∼5 × 10^-4 Pa at 293 K for 1 h) before the subsequent adsorption cycles. Thus, microporous crystalline 1a (N) can reliably adsorb and desorb NH₃ repeatedly, which avoids the need for heat-based activation between cycles.