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. Here-in, we demonstrate that Zr- and Hf-based frameworks (eight examples) can self-assemble at much higher reaction concen-trations 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. Further, the use of well-defined pivalate-capped cluster pre-cursors avoids the formation of ordered defects and impurities that arise from standard metal chloride salts. These clus-ters also introduce pivalate defects that increase the exterior hydrophobicity of several MOFs, as confirmed by water con-tact 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.

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 sub-gram quantities. Recently, Matzger and coworkers, 137 along with our group, 138 have found that much higher reaction concentrations (up 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.…”

Trapping of Photoluminescent Kinetic Intermediates During High-Concentration Synthesis of Non-Emissive Metal-Organic Frameworks

“…We anticipate that these findings will guide the development of high-concentration syntheses of other MOFs. Two desirable outcomes can be envisaged: the formation of the same phase as the reaction concentration is increased, which allows for MOFs to be prepared with significantly reduced solvent use, [12][13][14] or the formation of new phases with distinct properties from known combinations of linkers and metal salts. As such, high-concentration solvothermal synthesis should be added to the growing lexicon of methods available for the synthesis of MOFs and other metal-organic materials.…”

“…8 Unfortunately, the large solvent volumes required for traditional solvothermal synthesis necessitate the use of liters of solvent to produce sub-gram quantities of MOF, contributing greatly to the cost of MOFs on scale and generating significant waste. 11 Solvothermal MOF syntheses at higher concentrations are rare, [12][13][14][15][16] as poorly crystalline or low surface area materials are typically isolated instead due to the rapid precipitation of products from solution. 13,[17][18][19][20][21] The MOF-74, CPO-27, or M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd; dobdc 4− = 2,5-dioxido-1,4-benzenedicarboxylate) family of frameworks embodies the aforementioned synthetic challenges (Figure 1).…”