Tuning the Kinetic Water Stability and Adsorption Interactions of Mg-MOF-74 by Partial Substitution with Co or Ni

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 CO2 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 CO2 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 CO2 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 Ni2+ or Co2+ metal ions that are less vulnerable to hydrolysis than Mg2+. 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.