Reversible Adsorption of Ammonia in the Crystalline Solid of a CO 2 H-Functionalized Cyclic Oligophenylene

Ammonia (NH 3 ) is a viable candidate for the storage and distribution of hydrogen (H 2 ) due to its exceptional volumetric and gravimetric hydrogen energy density. Therefore, it is desirable to develop NH 3 storage materials that exhibit robust stability across numerous adsorption–desorption cycles. While porous materials with polymeric frameworks are often used for NH 3 capture, achieving reversible NH 3 uptake remains a formidable challenge, primarily due to the high reactivity of NH 3 . Here, we advocate the use of CO 2 H-functionalized cyclic oligophenylene 1a with high chemical stability as a novel single-molecule-based adsorbent for NH 3 . Simple reprecipitation of 1a selectively yielded microporous crystalline solid 1a ( N ). Crystalline 1a ( N ) adsorbs up to 8.27 mmol/g of NH 3 at 100 kPa and 293 K. Adsorbed NH 3 in the pore of 1a ( N ) has a packing density of 0.533 g/cm 3 at 293 K, which is close to the density of liquid NH 3 (0.681 g/cm 3 at 240 K). Crystalline 1a ( N ) also exhibits reversible NH 3 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, NH 3 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 NH 3 repeatedly, which avoids the need for heat-based activation between cycles.