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 first time the enhanced performance of nanoporous dicarboxylic acid-functionalized MOF-808 analogues toward NO2 removal under both dry and moist conditions. Based on a pKa-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-NH2IPA) exhibited a significant enhancement in NO2 capacities and the most remarkable reduction in the release of toxic NO byproduct as compared to pristine MOF-808. Detailed removal mechanisms for NO2 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 NOx, yielding nitramines, nitrosamines, and arenediazonium salts, were proposed to play a major role for the outstanding performance of 808-NH2IPA for dry NO2 removal. The acid–base reaction between HNO3 and the amino groups preponderated in the moist NO2 removal process, with the anilinium nitrate species turning into the principal product.