Zeolitic imidazolate frameworks (ZIFs) are a set of nanoporous metal−organic frameworks (MOFs) with tunable porosity and functionality. Among MOFs, they also show relatively good stability with respect to temperature and humidity. These characteristics lead to their possible applications in separation processes. In many practical separation processes, adsorbents are exposed to a variety of molecular species including acid gases. However, there is little knowledge of the effects of such acid gas exposure on the adsorption and separation properties of ZIFs. Here, the stability of a model ZIF material (ZIF-8) under SO 2 exposure in dry, humid, and aqueous environments has been investigated in detail. Combined characterization by several techniques (PXRD, N 2 physisorption, EDX, XPS, and FTIR) allowed us to track the structural and compositional properties of ZIF-8 before and after SO 2 exposure. ZIF-8 is stable after prolonged exposure in dry SO 2 and in humid air without SO 2 . However, exposure to 10−20 ppm concentrations of SO 2 in the presence of high relative humidity led to its irreversible structural degradation over time as evidenced by substantial losses in crystallinity and textural properties. Exposure to similar concentrations of aqueous SO 2 did not lead to bulk degradation. Humid SO 2 exposed ZIF-8 showed a significant presence of sulfur (S) even after reactivation, with vibrational characteristics corresponding to (bi)sulfite and (bi)sulfate groups. A mechanism of ZIF-8 degradation combining the synergistic effects of SO 2 and humidity is proposed. Attack by sulfuric and sulfurous acid species (generated in humid SO 2 ) leads to protonation of nitrogen in the imidazole ring, resulting in cleavage of metal−linker (Zn−N) bonds. Our detailed experimental findings serve as a starting point for developing a generalized mechanism of acid gas interactions with ZIF materials.

Acid gases such as SO 2 and CO 2 are present in many environments in which the use of nanoporous metal− organic frameworks (MOFs) is envisaged. Among metal− organic frameworks, zeolitic imidazolate frameworks (ZIFs) have been extensively explored as membranes or adsorbents. However, there is little systematic knowledge of the effects of acid gas exposure on the structure of ZIFs, in particular the mechanistic aspects of ZIF degradation by acid gases as well as the effects of ZIF crystal topology and linker composition on their stability. Here we present a generalized and quantitative investigation of the kinetic and thermodynamic acid gas stability of a diverse range of ZIF materials. The stability of 16 ZIFs (of SOD, RHO, ANA, and GME topologies) under different environmentshumid air, liquid water, and acid gases CO 2 and SO 2 (dry, humid, and aqueous)is investigated by a suite of experimental and computational methods. The kinetics of ZIF degradation under exposure to humid SO 2 is studied in detail, and effective rate constants for acid gas degradation of ZIFs are reported for the first time. Remarkably, the kinetics of degradation of the diverse ZIFs correlate strongly with the linker pK a and ZIF water adsorption in a manner contrary to that expected from previous predictions in the literature. Furthermore, we find that the material ZIF-71 (RHO topology) shows much higher stability relative to the other ZIFs in humid SO 2 and CO 2 environments.

The Pechini synthesis was used to prepare nickel aluminate catalysts with the compositions NiAl 4 O 7 , NiAl 2 O 4 , and Ni 2 Al 2 O 5. The samples are characterized by N 2 physisorption, temperature programmed reduction (TPR), temperature programmed oxidation (TPO), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS). Characterization results indicate unique structural properties and excellent regeneration potential of nickel aluminates. Prepared samples are tested when unreduced and reduced prior to reaction for methane dry reforming and methane steam reforming reactivity. NiAl 2 O 4 in the reduced and unreduced state as well as NiAl 4 O 7 in the reduced state are active and stable for methane dry reforming due to the presence of four-fold coordinated oxidized nickel. The limited amount of metallic nickel in these samples minimizes carbon deposition. On the other hand, the presence of metallic nickel is required for methane steam reforming. Ni 2 Al 2 O 5 in the reduced and unreduced state and NiAl 2 O 4 in the reduced state are found to be active for methane steam reforming due to the presence of sufficiently small nickel nanoparticles that catalyze the reaction without accumulating carbonaceous deposits.

The behavior of metal−organic frameworks (MOFs) in the presence of acid gases may be decisive in their suitability for industrial applications. In this study, MIL-125 and MIL-125-NH 2 were investigated with SO 2 exposure in dry, humid, and aqueous environments. MIL-125 was found to be unstable in both humid and aqueous acidic environments, while MIL-125-NH 2 was stable under these exposure conditions, showing no change in textural properties or visual degradation, as observed through SEM. Both materials were stable in the presence of water and dry SO 2 , suggesting that the reaction of these molecules to form an acidic species is likely a key factor in the degradation of MIL-125. In situ IR experiments confirmed the presence of sulfite species, supporting the hypothesis that the presence of an acidic sulfur species likely leads to the degradation of the MIL-125 structure. Computational investigation of several potential reaction mechanisms in MIL-125 indicated reactions involving the bisulfite ion are favored over reactions with water or SO 2 . DFT simulations support the observation that MIL-125-NH 2 is stable in humid conditions, as all reactions are less favorable with the functionalized framework compared to the unfunctionalized framework. This combined experimental and computational study advances the fundamental understanding of MOF degradation mechanisms during acid gas exposure.

No abstract