High-Concentration Self-Assembly of Zirconium- and Hafnium-Based Metal–Organic Materials

Abstract

Metal–organic frameworks (MOFs) are crystalline, porous solids constructed from organic linkers and inorganic nodes that are promising for applications in chemical separations, gas storage, and catalysis, among many others. However, a major roadblock to the widespread implementation of MOFs, including highly tunable and hydrolytically stable Zr- and Hf-based frameworks, is their benchtop-scalable synthesis, as MOFs are typically prepared under highly dilute (≤0.01 M) solvothermal conditions. This necessitates the use of liters of organic solvent to prepare only a few grams of MOF. Herein, we demonstrate that Zr- and Hf-based frameworks (eight examples) can self-assemble at much higher reaction concentrations than are typically utilized, up to 1.00 M in many cases. Combining stoichiometric amounts of Zr or Hf precursors with organic linkers at high concentrations yields highly crystalline and porous MOFs, as confirmed by powder X-ray diffraction (PXRD) and 77 K N2 surface area measurements. Furthermore, the use of well-defined pivalate-capped cluster precursors avoids the formation of ordered defects and impurities that arise from standard metal chloride salts. These clusters also introduce pivalate defects that increase the exterior hydrophobicity of several MOFs, as confirmed by water contact angle measurements. Overall, our findings challenge the standard assumption that MOFs must be prepared under highly dilute solvothermal conditions for optimal results, paving the way for their scalable and user-friendly synthesis in the laboratory.

Simplifying the Synthesis of Metal–Organic Frameworks

“…Using either ZrCl 4 or ZrPiv as the node precursor at concentrations up to 1.0 M in DMF with stirring yields crystalline UiO-66 that is comparable in quality to the material prepared under the standard dilute conditions (Figure b). In particular, these high-concentration samples exhibit high Brunauer–Emmett–Teller (BET) surface areas of 1327 ± 8 and 1232 ± 7 m 2 /g for UiO-66–1.0 M (ZrCl 4 ) and UiO-66–1.0 M (ZrPiv), respectively …”

“…An interesting question is why Zr-MOFs tend to form the same phase at low and high linker concentrations, , whereas M 2 (dobdc) materials undergo a phase transition to a new kinetic product at high reaction concentrations. , We propose that Zr- and Hf-MOFs are especially well-suited for high-concentration synthesis because their M 6 nodes are robust and well-defined and, thus, strongly direct linker coordination into specific structures, even at high concentrations. In contrast, the metal centers that form the backbone of M 2 (dobdc) materials can engage in many different coordination environments with the linkers during MOF self-assembly.…”

“…Right: UiO-66–1.0 M (ZrCl 4 ) (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.…”

“…As MOFs transition from academic curiosities to industrially relevant materials, their scalable synthesis has emerged as a major area of study . Several sustainable and scalable alternatives to traditional solvothermal MOF syntheses have been developed. ,, User-friendly approaches to reduce the volume of organic solvents required for MOF synthesis include hydrothermal, high-concentration solvothermal, ionothermal, and continuous flow methods . Using more specialized equipment (e.g., ball mills or extruders), MOFs can also be synthesized under mechanochemical conditions with minimal solvent use while cutting synthesis times down to the order of minutes …”

MOFganic Chemistry: Challenges and Opportunities for Metal–Organic Frameworks in Synthetic Organic Chemistry

“…In particular, the need for liters of solvent(s) and disproportionate quantities of acid modulators often limit MOF synthesis to subgram quantities. Recently, Matzger and co-workers, along with our group, found that much higher reaction concentrations (up to 1 M in some cases) can be used to produce high-quality MOFs with significantly reduced solvent waste. In a similar vein, we carried out a structure–property study to identify the optimal modulator for Zr-MOF synthesis, with the goal of reducing the amount of modulator required to produce highly crystalline material .…”