Simplifying the Synthesis of Metal-Organic Frameworks.

Abstract

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes that have attracted widespread interest due to their permanent porosity and highly modular structures. However, the large volumes of organic solvents and additives, long reaction times, and specialized equipment typically required to synthesize MOFs hinder their widespread adoption in both academia and industry. Recently, our lab has developed several user-friendly methods for the gram-scale (1-100 g) preparation of MOFs. Herein, we summarize our progress in the development of high-concentration solvothermal, mechanochemical, and ionothermal syntheses of MOFs, as well as in minimizing the amount of modulators required to prepare highly crystalline Zr-MOFs. To begin, we detail our work elucidating key features of acid modulation in Zr-MOFs to improve upon current dilute solvothermal syntheses. Choosing an optimal modulator maximizes the crystallinity and porosity of Zr-MOFs while minimizing the quantity of modulator needed, reducing the waste associated with MOF synthesis. By evaluating a range of modulators, we identify the pKa, size, and structural similarity of the modulator to the linker as controlling factors in modulating ability. In the following section, we describe two high-concentration solvothermal methods for the synthesis of Zr-MOFs and demonstrate their generality among a range of frameworks. We also target the M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd; dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) family of MOFs for high-concentration synthesis and introduce a two-step preparation of several variants that proceeds through a novel kinetic phase. The high-concentration methods we discuss produce MOFs on multi-gram scale with comparable properties to those prepared under traditional dilute solvothermal conditions. Next, to further curtail solvent waste and accelerate reaction times, we discuss the mechanochemical preparation of M2(dobdc) MOFs utilizing liquid amine additives in a planetary ball mill, which we also apply to the synthesis of two related salicylate frameworks. These samples exhibit comparable porosities to traditional dilute solvothermal samples but can be synthesized in just minutes, as opposed to days, and require under 1 mL of liquid additive to prepare ~0.5 g of material. In the following section, we discuss our efforts to avoid specialized equipment and eliminate solvent use entirely by employing ionothermal conditions to prepare a variety of azolate- and salicylate-based MOFs. Simply combining metal chloride (hydrate) salts with organic linkers at temperatures above the melting points of the salts affords high-quality framework materials. Further, ionothermal conditions enable the syntheses of two new Fe(III) M2(dobdc) derivatives that cannot be synthesized under normal solvothermal conditions. Last, as a demonstrative example, we discuss our efforts to synthesize 100 g of high-quality Mg2(dobdc) in a single batch using a high-concentration (1.0 M) hydrothermal synthesis. Our Account will be of significant interest to researchers aiming to prepare gram-scale quantities of MOFs for further study.