High-efficiency capture of ammonia using hierarchically porous Zr-MOF nanoarchitectures under ambient conditions: Thermodynamics, kinetics, and mechanisms

Zirconium-based metal–organic frameworks (Zr-MOFs) have emerged as a promising candidate for efficiently eliminating toxic chemicals under ambient conditions. Herein, we demonstrate the remarkable ammonia capture capabilities of the hierarchically porous MOF-808 xerogel (G808) macrostructure and its amino-functionalized analogue (G808-NH2), which were constructed through a sol–gel-based strategy combined with convenient ambient pressure drying. Static isotherm measurements and dynamic breakthrough tests using engineered Zr-MOF xerogel granules were conducted to illuminate the thermodynamic and kinetic characteristics for ammonia sorption. The G808 and G808-NH2 xerogels manifest substantially higher ammonia uptake, intensified interfacial interactions, and enhanced irreversible retention toward ammonia compared with the state-of-the-art activated carbon-based materials, particularly at the low-concentration region. Moreover, these hierarchical Zr-MOF nanoarchitectures exhibit extraordinary ammonia removal performance as well as excellent environmental robustness under simulated respirator canister/protection filter circumstances, with the breakthrough capacities of G808-NH2 being individually more than two and one orders of magnitude greater than those for the pristine and metal-amine impregnated carbons. The kinetic ammonia adsorption behaviors can be well delineated by the pseudo-first-order Wheeler-Jonas model. An in-depth discussion on the ammonia sorption regime, detailing five binding patterns besides reversible physisorption with a special focus on the impacts of polar amino groups and humidity, was proposed based on multiple characterizations and Grand Canonical Monte Carlo simulation.