Zirconium-based metal−organic frameworks (Zr-MOFs) have attracted tremendous attention as promising candidates for removing toxic chemicals under ambient conditions in virtue of their remarkable thermal, mechanical, and chemical stability. Here, we demonstrate for the f irst time the enhanced performance of nanoporous dicarboxylic acid-functionalized MOF-808 analogues toward NO 2 removal under both dry and moist conditions. Based on a pK a -directed solvent-assisted ligand exchange (SALE) strategy, a series of dicarboxylic acid ligands, including isophthalic acid, 5-hydroxyisophthalic acid, 5-aminoisophthalic acid, 5-nitroisophthalic acid, and pyridine-3,5-dicarboxylic acid, were precisely incorporated into the MOF-808 framework via substitution of formate ligands. The assembled MOF-808 derivatives demonstrated good crystallinity, well-defined morphology, high porosities, and tunable dicarboxylic acid contents, with the maximum molar ratios of dicarboxylic acid ligands to 1,3,5-benzenetricarboxylic acids in frameworks larger than 1:1. The microbreakthrough tests showed that the 5-aminoisophthalic acid-modified MOF-808 (808-NH 2 IPA) exhibited a significant enhancement in NO 2 capacities and the most remarkable reduction in the release of toxic NO byproduct as compared to pristine MOF-808. Detailed removal mechanisms for NO 2 were developed on the basis of multiple ex-situ characterization techniques and in-situ infrared Fourier transform spectroscopy. The effects of humidity on the mechanisms were also discussed. The reactions between the amino groups and NO x , yielding nitramines, nitrosamines, and arenediazonium salts, were proposed to play a major role for the outstanding performance of 808-NH 2 IPA for dry NO 2 removal. The acid−base reaction between HNO 3 and the amino groups preponderated in the moist NO 2 removal process, with the anilinium nitrate species turning into the principal product.

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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 f irst 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-NH 2 , and UiO-66-NH 2 xerogel monoliths, denoted as G808, G808-NH 2 , and G66-NH 2 , 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 NO 2 adsorption capabilities. The Zr-MOF xerogels showed considerably better NO 2 removal abilities than the precipitates, whether intrinsically or under simulated respirator canister/protection filter environment conditions. Multiple physicochemical characterizations were conducted to illuminate the NO 2 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-NH 2 xerogel as well as alleviate the surface resistance for the G808-NH 2 xerogel, which led to accelerated overall adsorption uptake and thus enhanced performance toward toxic chemical removal.

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