Nonporous Molecular Crystals for Phase‐Transition‐Driven Ammonia Capture and Water‐Assisted Release

Ammonia sorbent design has been predominantly confined to porous materials, where uptake is governed by interactions within permanent voids. Here, we demonstrate a fundamentally distinct mechanism based on nonporous molecular crystals, in which ammonia capture proceeds via a proton‐coupled phase transformation. The nitrate salt of hexakis(aminomethyl)benzene, (( HAB H 6 ) 6 + ·6NO 3 − ), delivers an exceptional NH 3 uptake of 13.1 mmol g − 1 under ambient conditions despite the absence of intrinsic porosity. Structural analyses reveal a rapid crystal‐to‐crystal transformation to NH 4 NO 3 and a partially deprotonated HAB ‐derived phase mediated by ammonia–ammonium exchange, establishing a stoichiometric, solid‐state capture pathway that contrasts sharply with conventional adsorption processes. Strikingly, although the lattice resists water uptake, trace moisture enables efficient NH 3 release at 40 °C through a water‐assisted reverse proton transfer, affording highly reversible cycling. The material further demonstrates effective NH 3 removal in both gas and aqueous phases, underscoring its operational robustness. Collectively, these results establish phase‐transition‐driven sorption in nonporous molecular crystals as a new chemical paradigm, decoupling sorption performance from permanent porosity and redefining design principles for gas capture materials.