Stability of Zeolitic Imidazolate Frameworks in NO₂

The use of nanoporous zeolitic imidazolate frameworks (ZIFs) in separation processes is attractive, but the presence of acid gases such as SO x or NO x in process streams can have detrimental effects. While we have recently developed a mechanistic picture of SO x –ZIF interactions, here we describe the remarkably different effects of NO 2 on ZIFs. ZIFs with a representative range of SO x stabilities are all unstable—as defined by loss of crystallinity and porosity—in dry and humid NO 2, whereas most ZIFs are stable in dry SO x and some even in humid SO x . A detailed mechanism is developed based on Fourier transform infrared spectroscopy and density functional theory calculations. H-abstraction by free radical NO 2 and subsequent acidic species formation are the major degradation pathways, while adsorbed HNO 3 formation in humid conditions is an additional pathway in hydrophilic ZIFs. These findings strongly suggest that new strategies to stabilize ZIFs/metal–organic frameworks toward NO 2 attack are required.