Stability of Zeolitic Imidazolate Frameworks in NO₂

Abstract

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 NO2 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 NO2, 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 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.

Transistor‐Based Work‐Function Measurement of Metal–Organic Frameworks for Ultra‐Low‐Power, Rationally Designed Chemical Sensors

“…Our sensing results are well complemented by the recent characterization of ZIF‐8 in NO 2 by Bhattacharyya et al In their spectroscopic work, they find that the first defects from NO 2 in dry air come from H‐abstraction from the linker and functionalize the linker with NO 2 . The reaction forms HNO 2 as a byproduct.…”

“…These acids can immediatelyf orm inorganic nitrates. [33] We observe greaterw ork-function shifts from NO 2 exposures that occur in higherr elative humidity environments. These adsorptione ventsa re primarilyt he NO X À at zinc metal sites after the linker becomes protonatedr ather than bond-making with the linker.…”

“…The ZIF-8 sensors respond most strongly and recover most when exposed to NO 2 in humid rather than dry air.T hese two findings are complementary to as eparate literature spectroscopice xperiments that showed that humid NO 2 attacks metal sites and dry NO 2 attacks the linker. [33] Lastly,t he MOF MFM-300(In) was used as af inal example of how work-function responses can be predicted based on chemical resistance rules arising from the metal chemistry and hard-soft acid-base chemistry.…”

“…These adsorptione ventsa re primarilyt he NO X À at zinc metal sites after the linker becomes protonatedr ather than bond-making with the linker. [33] This is strong evidence that targeting adsorption at metals rather than linkersi sauseful heuristic for designing MOF chemical sensors.…”

“…There is ar ecovery in work function after the NO 2 exposure in humid air.G iven the ease with which zinc-based MOFs exchange ligands, it is plausible that the frameworki sr ecovering from the inorganic nitrate exposure. X-ray diffractionp attern of the filmexposed to NO 2 is given in Figure S2 (Supporting Information)a nd shows only as light decrease in diffraction intensity.M ore advanced characterization methods, such as in [33] The ZIF-8 NO 2 sensing experiments show that there are other uses for work-function measurements of MOFs. Those studying MOF stabilityc ould benefitf rom the sensitivity of the technique to measuret he extent of-and rate of-reaction of the framework when exposed to harsh gases,s ince traditional methods to do so are quite laborious.…”