Guest-Driven Flexibility in a Hydrogen-Bonded Organic Framework for NH3 Capture.

Ammonia (NH3) capture and low-energy release remain a critical challenge in adsorptive separation. Herein, we report a guest-driven flexible hydrogen-bonded organic framework (HOF), designated FDU-HOF-7, constructed from 1,3,5-benzenetricarboxylic acid. The as-synthesized material is nonporous and undergoes a reversible crystalline phase transition upon NH3 adsorption and desorption. During this process, the original COOH···COOH hydrogen-bonding network is replaced by a set of energetically favorable charge-assisted hydrogen bonds (CAHBs), including COO-···HOOC, COO-···NH3, COO-···NH4+, and NH4+···NH3, leading to the formation of a well-defined cocrystal. Combined ex-situ powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction analyses reveal that the structural transformation is reversible upon NH3 desorption. This adaptive behavior enables 100% utilization of adsorption sites within the bulk crystals. Consequently, FDU-HOF-7 delivers high NH3 uptake capacities of 21 mmol g-1 at 298 K and 15 mmol g-1 at 343 K, outperforming benchmark metal-organic frameworks (MOFs) and zeolites under identical conditions. This work unveils a reversible guest-driven phase transition mechanism in a nonporous HOF, offering an avenue for efficient gas capture through adaptive molecular packing.