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 pK_a, 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; dobdc^4- = 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.

Figure 1.

a) Structure of M₂Cl₂(btdd) (M = V, Mn, Fe, Co, Ni, Cu, btdd²⁻ = bis(1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin), the secondary building unit, and the organic linker. Black, gray, white, blue, red and green spheres correspond to nickel, carbon, hydrogen, nitrogen, oxygen, and chlorine, respectively. b) Representative dilute synthesis of M₂Cl₂(btdd) (M = V, Mn, Fe, Co, Ni, Cu). c) Synthesis of Cu₂Cl₂(btdd) on 1.0 g of linker scale.

Figure 2.

a) Acid-modulated synthesis of UiO-66. b) Volume-weighted average crystalline domain sizes (LVol-IB) vs. pK_a of UiO-66 samples prepared using 10 or 50 equiv. of acid modulators. MsOH = methanesulfonic acid, HCl = hydrochloric acid, TFA = trifluoroacetic acid, 2-TP = 2-thiophenecarboxylic acid, FA = formic acid, 3-TP = 3-thiophenecarboxylic acid, BA = benzoic acid, AA = acetic acid, PA = pivalic acid, PhOH = phenol. The gray box indicates the range between the pK_a1 and pK_a2 values of terephthalic acid. ^★ indicates modulators for which impurities were observed at 50 equiv. No MOF was formed when 50 equiv. of MsOH was used. Adapted with permission from ref. Copyright 2022 American Society of Chemistry.

Figure 3.

a) Synthesis of UiO-66 from either ZrCl₄ or ZrPiv. b) PXRD patterns of UiO-66 samples prepared using ZrCl₄ or ZrPiv and a [H₂bdc] of either 0.01 or 1.0 M. The simulated patterns based on the single-crystal X-ray diffraction (SCXRD) structures of ZrPiv and UiO-66 are included for reference. Ordered defect domains with a reo topology are indicated (*). c) SCXRD structure of ZrPiv and UiO-66. The gray, red, and light blue spheres represent carbon, oxygen, and zirconium, respectively. Hydrogens are omitted for clarity. d) Left: UiO-66–1.0 M (ZrCl₄) (top) and UiO-66–1.0 M (ZrPiv) (bottom). Right: UiO-66–1.0 M (ZrCl₄) (left) sinking in water, in contrast to UiO-66–1.0 M (ZrPiv) (right) floating on water. Adapted with permission from ref. Copyright 2023 American Society of Chemistry.

Figure 4.

a) Synthesis of M₂(dobdc) and Mg₂(dobpdc) under mechanochemical conditions. b) Synthesis of Mg₂(m-dobdc) under solvothermal (left) and mechanochemical (right) conditions. c) PXRD patterns of Mg₂(m-dobdc) under mechanochemical conditions (blue) and solvothermal conditions (red). The simulated pattern based on the SCXRD structure of the isostructural Co₂(m-dobdc) is included for reference. d) 30 °C CO₂ and adsorption isotherms in Mg₂(m-dobdc)-ST (red) and Mg₂(m-dobdc)-MC (blue). The lines correspond to individual fits to the dual-site Langmuir model. A data point was considered equilibrated when < 0.01% pressure change occurred over a 30 s interval. Adapted with permissions from ref.^, Copyright 2020, 2022 Royal Society of Chemistry.

Figure 5.

a) Ionothermal synthesis of Fe₂X₂(dobdc) and Fe₂X₂(m-dobdc) (X=Cl, OH). Structures of Fe₂X₂(dobdc)and Fe₂X₂(m-dobdc) (X=Cl, OH). Gray, white, red, orange, and green spheres correspond to carbon, hydrogen, oxygen, iron, and chlorine, respectively. b) SEM images of Fe₂X₂(dobdc) and Fe₂X₂(m-dobdc) (X=Cl, OH) prepared under ionothermal conditions. c) Mössbauer profile of the Fe(III) MOFs Fe₂X₂(m-dobdc) (X=Cl, OH). The Mössbauer profile of the Fe(II) MOF Fe₂(dobdc) is included for comparison. Adapted under terms of the CC-BY license. Copyright 2023, published by Wiley-VCH GmbH, Angewandte Chemie International Edition.