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.

Varying amounts of Co and Ni were substituted into the metal−organic framework Mg-MOF-74 via a one-pot solvothermal reaction, and the effects of these substitutions on CO 2 adsorption and kinetic water stability properties were examined. Based on elemental analyses, Co and Ni are more favorably incorporated into the MOF-74 framework from solution than Mg. In addition, reaction temperature more strongly impacts the final metal composition in these mixed-metal (MM) MOF-74 structures than does the reaction solvent composition. Single-component CO 2 adsorption isotherms were measured for the MM-MOF-74 systems at 5, 25, and 45 °C, and isosteric heats of adsorption were calculated. These results suggest that CO 2 adsorption properties can be adjusted by partial metal substitution. Water adsorption isotherms were also measured for the MM-MOF-74 samples, with powder X-ray diffraction patterns and Brunauer−Emmett−Teller surface areas measured both before and after water exposure. Results show that Mg-MOF-74 can gain partial kinetic water stability by the incorporation of Ni 2+ or Co 2+ metal ions that are less vulnerable to hydrolysis than Mg 2+ . Of particular note, Mg−Ni-MM-MOF-74 shows a significant increase in water stability when incorporating as little as 16 mol % Ni into the Mg-MOF-74 structure.

Defect engineering in metal−organic frameworks (MOFs) is an emerging strategy that can be used to control physical or chemical characteristics of MOFs, including adsorption behavior and textural, mechanical, and conductive properties. Understanding the impact of defects on textural properties and chemical stability of MOFs is imperative to the development of MOFs with tunable defect sites. In this work, systematic adsorption measurements were performed with three adsorbate molecules (SO 2 , benzene, and cyclohexane) to investigate changes in the pore size of defective UiO-66. Compared to the parent UiO-66, the defective UiO-66 shows significant changes in adsorption capacities among the selected adsorbate molecules, demonstrating that pore size is significantly enlarged by the missing cluster defects. BET surface area analysis and DFT calculations were also performed to interrogate the chemical stability of the defective MOFs after exposure to water and acidic environments. This work shows that pore size can be tuned as a function of defect concentration. Further, it is shown that the structural incorporation of trifluoroacetate groups in defective UiO-66 leads to an increase in average pore size without sacrificing chemical stability toward water and acidic species. The results of this work advance the understanding of textural properties and chemical stability of defect-engineered MOFs and also suggest a preparation method for synthesizing defective but stable MOFs.

The mixed-ligand metal–organic framework (MOF) DMOF-1 has been successfully synthesized via ligand: 2,3,5,6-tetramethylterephthalic acid (TM), 9,10-anthracenedicarboxylic acid (ADC), 1,4-naphthalenedicarboxylic acid (NDC), 2,5-dimethyl terephthalic acid (DM), and metal (Zn, Cu, Ni, Co) substitutions to develop structure–property relationships for adsorption of water and SO2, an acid gas commonly found in flue gas streams at ppm-level concentrations. The substitution of bulky ligands, TM and ADC, resulted in not only improved water stability but also stability toward dry SO2, whereas substitution with NDC and DM did not provide water or SO2 stability. ADC-functionalized Zn–DMOF exhibited the greatest stability under humid SO2 exposure of the ligand-functionalized materials that were tested. Metal substitution of Cu into the DMOF–TM structure resulted in the greatest stability of the M–DMOF–TM samples upon exposure to humid SO2, and Ni–DMOF–TM provided the second most stable material. These results follow the prediction of the Irving–Williams series of metal node/ligand bond strengths.

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