High-concentration aqueous synthesis of salicylate-based metal–organic frameworks

A current barrier to the practical applications of metal-organic frameworks (MOFs) is the vast quantity of organic solvents required for their preparation under dilute solvothermal conditions. Herein, we report the rapid, ambient-temperature, and high-concentration (up to 1.0 M) aqueous syntheses of three families of salicylate-based MOFs: M₂(dobdc) (M = Mg, Co, Ni, Zn; dobdc^4- = 2,5-dioxido-1,4-terephthalate), M₂(dobpdc) (M = Mg, Co, Ni, Zn; dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), and M₂(<i>m</i>-dobdc) (M = Mg, Co, Ni; <i>m</i>-dobdc^4- = 4,6-dioxido-1,3-benzenedicarboxylate). High-concentration MOF formation is accomplished by incorporating NaOH to deprotonate the linker molecules <i>in situ</i>, generally avoiding the crystallization of phases with partially protonated linkers favored under high-concentration solvothermal conditions. 77 K N₂ surface area measurements confirm that the MOFs (especially Zn-based frameworks) demonstrate comparable or enhanced surface areas relative to traditionally prepared materials. Furthermore, this method enables the first synthesis of Zn₂(<i>m</i>-dobdc), which does not form under standard solvothermal conditions. This material exhibits higher CO₂ uptake and ideal adsorbed solution theory (IAST) CO₂/N₂ selectivities compared to the canonical framework Zn₂(dobdc), highlighting the utility of aqueous high-concentration methods to facilitate the discovery of porous materials with improved gas sorption properties. Overall, our findings offer a practical and general alternative to dilute solvothermal syntheses of salicylate-based MOFs, paving the way for their production and implementation in industrial settings.