Solvent-Free Synthesis of Metal-Organic Frameworks Using Low-Melting Metal Salts

Abstract

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with myriad potential applications in chemical separations, catalysis, drug delivery, and beyond. However, a major barrier to the application of MOFs in industry is their scalable synthesis, as most frameworks are prepared under highly dilute (≤0.01 M) solvothermal conditions using toxic organic solvents. Herein, we demonstrate that directly combining a range of salicylate and azolate linkers with low-melting metal halide (hydrate) salts above the melting point of the metal salt leads directly to high-quality MOFs without added solvent. Frameworks prepared under these ionothermal condi-tions possess high 77 K N2 surface areas and crystallinities comparable to frameworks prepared under traditional sol-vothermal conditions, as confirmed by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). In ad-dition, we report the ionothermal syntheses of two frameworks that cannot be prepared directly under solvothermal con-ditions, namely, Fe2X2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate) and Fe2X2(m-dobdc) (m-dobdc4− = 4,6-dioxido-1,3-benzenedicarboxylate) (X = Cl, OH). These air-stable and porous Fe(III) members of the MOF-74 family pos-sess pressed-pellet conductivities comparable to or higher than those of highly air-sensitive Fe(II) congeners. Overall, the simple solvent-free method reported herein should be broadly applicable to the discovery and sustainable synthesis of metal-organic materials.