Molecular Crystal Sponge for Extraordinary NH3 Capture.

Ammonia is a chemical commodity in high demand that is produced primarily via the Haber-Bosch process. The low equilibrium single-pass conversion of this over 100-year-old process necessitates the refrigeration condensation removal of ammonia to increase the overall hydrogen yield, which has consumed a massive amount of energy. Developing highly efficient ammonia adsorbents to replace the condensation process is the key to realizing sustainable ammonia synthesis. Here, we report a soft hydrogen-bonded molecular crystal built solely from a very simple, easily available, and economical molecule of benzene-1,2,4,5-tetracarboxylic acid. This material exhibits an unusual sponge-like stoichiometric sorption behavior to capture ammonia with extraordinary performances with respect to low concentration uptake capacity (15.1 mmol g-1 at 10 mbar and 298 K), long-term stability, and industrial practicability, outperforming those of reported materials for this purpose. Breakthrough experiments confirm the excellence of this material for recovering captured NH3 with ultrahigh purity (≥99.9998%) under both dry and wet conditions. Detailed single-crystal and in situ powder X-ray diffraction (PXRD) analyses, coupled with modeling, unequivocally decipher the charge-assisted hydrogen bonding between ammonia and carboxylic acid moieties in driving stepwise pore expansion. This work thus provides a conceptual blueprint of engineering soft molecular crystals for driving selective ammonia capture and energy-efficient ammonia production.