NO₂ Removal under Ambient Conditions by Nanoporous Multivariate Zirconium-Based Metal–Organic Framework

Abstract

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.

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

“…The structural resistibility of UiO-66-NH 2 and MOF-808-NH 2 against NO 2 corrosion has been demonstrated previously. , As an illustration, the exhausted G808-NH 2 was characterized to examine the structural integrity of Zr-MOF xerogels upon NO 2 exposure. Neither additional peaks nor noticeable peak broadening is observed in the PXRD patterns of exhausted samples (Figure S9), indicating that the long-range order of G808-NH 2 remains largely intact.…”

“…Interestingly, the number of charge-balanced −OH groups connected to per Zr 6 node diminishes following NH 2 IPA integration, implying their involvement in the PSM reactions. − The thermogravimetry analysis sheds further light on the PSM process (Figures d and S4). The weight loss linked to coordinated NH 2 IPA decomposition (425–500 °C) increases from G808 to G808-NH 2 , while weight loss corresponding to dehydroxylation of the Z 6 clusters (315–410 °C) decreases concomitantly, , suggesting that the Zr–OH/Zr–OH 2 groups may be the main sites for NH 2 IPA substitution.…”

“…Furthermore, based on the reported crucial role of amino groups in NO 2 removal, an amino-functionalized dicarboxylic acid ligand, viz. NH 2 IPA, was incorporated into the MOF-808 framework via a PSM approach. , Finally, both the G808 and G808-NH 2 gels were transformed into the xerogel monoliths facilely through APD.…”

“…Zirconium (IV) oxychloride octahydrate (ZrOCl 2 ·8H 2 O), benzene-1,3,5-tricarboxylic acid (H 3 BTC), 2-aminoterephthalic acid (NH 2 BDC), 5-aminoisophthalic acid (NH 2 IPA), formic acid (HCOOH), glacial acetic acid (CH 3 COOH), hydrochloric acid, N , N -dimethylformamide (DMF), ethanol, n -hexane, acetone, cyclohexane, and acetaldehyde were supplied by Sinopharm Chemical Reagent Beijing Co., Ltd. Conventional MOF-808, MOF-808-NH 2 , and UiO-66-NH 2 microcrystalline precipitates were prepared based on the procedure described in the literature. − …”

“…Microbreakthrough tests were performed on a miniaturized system (Figure S1) described previously to evaluate the intrinsic NO 2 adsorption capacities of powder samples . Test conditions are summarized in Table S1.…”