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

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