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.

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.

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Metal-organic frameworks (MOFs) often display promising performance in ideal, one-or two-component systems; however, industrial adsorption and catalytic applications are almost always in the presence of acid gases that degrade the adsorbent or catalyst. Therefore, it is necessary to understand the interaction of acid gases with MOFs to drive future material design. Acid gas adsorption has been widely investigated on metal oxides, while few fundamental studies exist for MOFs. Therefore, MOF-derived oxides were prepared to give insight into adsorbed species on MOFs by connecting to the understanding that exists for adsorbed species on metal oxides. These MOF-derived oxides retain the overall morphology of the parent MOF, allowing direct comparison of the effect of morphology and the metal coordination environment on adsorbed species and acid gas stability of MOF, MOF-derived oxide, and traditionally synthesized metal oxide. A cerium-based MOF with open-metal sites, CeBTC, and the Ti-based MIL-125 were chosen to prepare MOF-derived oxides. IR studies show that adsorbed species during SO 2 and CO 2 adsorption on the MOF materials could be directly correlated to species observed on the MOF-derived and traditional oxides. In addition, the adsorbed species on the MOF-derived oxides differed from traditional oxides due to their different morphology and retained porosity. SEM and TEM images taken before and after CO 2 /SO 2 adsorption experiments revealed degradation of all materials giving visual insight into the degradation mechanism after acid gas exposure. This study advances the understanding of acid gas adsorption on MOFs by correlation of adsorbed species with MOFderived and traditional metal oxides.

The effect of surface structure of TiO2 nanocrystals on the structure, amount, and strength of adsorbed CO2 and resistance to SO2 was investigated using in situ IR spectroscopy and mass spectrometric techniques along with first-principles density functional theory (DFT) calculations. TiO2 nanoshapes, including rods {(010) + (101) + (001)}, disks {(001) + (101)}, and truncated octahedra {(101) + (001)}, were used to represent different TiO2 structures. Upon CO2 adsorption, carboxylates and carbonates (bridged, monodentate) are formed on TiO2 rods and disks, whereas only bidentate and monodentate carbonates are formed on TiO2 truncated octahedra. In general, the order of thermal stability of the adsorbed CO2 species is carboxylates ≈ monodentate carbonates > bridged carbonates > bidentate carbonates ≈ bicarbonates. TiO2 rods and disks adsorb CO2 more strongly than TiO2 truncated octahedra, which is explained by the larger number of low coordinated surface oxygen and oxygen vacancies on the rods and disks than the truncated octahedra. Further IR studies showed that the structure and binding strength of the adsorbed CO2 species are affected by the presence of SO2. Among the three TiO2 nanoshapes, CO2 binding strength for truncated octahedra shows the most decrease due to accumulation of sulfates formed during the SO2 adsorption cycle. The fundamental understanding obtained here on the effects of the surface structure, oxygen vacancies, and SO2 on the interaction of CO2 with TiO2 may provide insights for the design of more efficient and sulfur-resistant TiO2-based catalysts involved in CO2 capture and conversion.