Enhanced Adsorption and Mass Transfer of Hierarchically Porous Zr-MOF Nanoarchitectures toward Toxic Chemical Removal

Zirconium-based metal-organic frameworks (Zr-MOFs) have shown tremendous prospects as highly efficient adsorbents against toxic chemicals under ambient conditions. Here, we report for the first time the enhanced toxic chemical adsorption and mass transfer properties of hierarchically porous Zr-MOF nanoarchitectures. A general and scalable sol-gel-based strategy combined with facile ambient pressure drying (APD) was utilized to construct MOF-808, MOF-808-NH2, and UiO-66-NH2 xerogel monoliths, denoted as G808, G808-NH2, and G66-NH2, respectively. The resulting Zr-MOF xerogels demonstrated 3D porous networks assembled by nanocrystal aggregates, with substantially higher mesoporosities than the precipitate analogues. Microbreakthrough tests on powders and tube breakthrough experiments on engineered granules were conducted at different relative humidities to comprehensively evaluate the NO2 adsorption capabilities. The Zr-MOF xerogels showed considerably better NO2 removal abilities than the precipitates, whether intrinsically or under simulated respirator canister/protection filter environment conditions. Multiple physicochemical characterizations were conducted to illuminate the NO2 filtration mechanisms. Analysis on adsorption kinetics and mass transfer patterns in Zr-MOF xerogels was further performed to visualize the underlying structure-activity relationship using the gravimetric uptake and zero length column methods with cyclohexane and acetaldehyde as probes. The results revealed that the synergy of hierarchical porosities and nanosized crystals could effectively expedite the intracrystalline diffusion for the G66-NH2 xerogel as well as alleviate the surface resistance for the G808-NH2 xerogel, which led to accelerated overall adsorption uptake and thus enhanced performance toward toxic chemical removal.