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 SOx or NOx in process streams can have detrimental effects. While we have recently developed a mechanistic picture of SOx-ZIF interactions, here we describe the remarkably different effects of NO2 on ZIFs. ZIFs with a representative range of SO stabilities are all unstable as defined by loss of crystallinity and porosity in dry and humid NO2, whereas most ZIFs are stable in dry SOx and some even in humid SOx. A detailed mechanism is developed based on Fourier transform infrared spectroscopy and density functional theory calculations. H-abstraction by free radical NO2 and subsequent acidic species formation are the major degradation pathways, while adsorbed HNO3 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 NO2 attack are required.