Modulator Effects on the Water-Based Synthesis of Zr/Hf Metal–Organic Frameworks: Quantitative Relationship Studies between Modulator, Synthetic Condition, and Performance

Abstract

The modulated synthesis of metal−organic frameworks (MOFs) remains empirical and challenging. Modulators are often applied and assumed capable of facilitating crystal growth by adjusting the reaction kinetics. However, most of the current studies are based on qualitative analysis and performance-leading synthesis, while no quantitative insights between modulator feature and MOF performance have been offered. In this work, we carried out a comprehensive study of the effects of three modulators (acetic acid, formic acid, trifluoroacetic acid) on the water-based modulated synthesis of UiO-66-type MOFs by using Zr or Hf as the building block and fumarate as the ligand. The modulator effects on crystallinity, yield, morphology, pore size, defects, porosity, stability, and gas separation performance of resultant MOFs have been discussed. A relationship between optimal molar ratio y and pK a value of modulator x is modeled as y = 12.72 + 0.193 × exp(1.276x). For MOF synthesis using ligands of different acidity, it tends to follow the equations of y = −40.78 + 39.1x and y = −21.7 + 25.58x for acetic acid and formic acid, respectively. Our results have thus provided pioneering quantitative analysis and synthetic guidelines on the further synthesis of water-stable MOFs that require modulators.

A highly stable metal‐organic framework with optimum aperture size for CO2 capture

“…31,37 In our previous study, we have demonstrated that the synthetic condition can play a determining role in the quality of resultant MOFs, which cannot be solely evaluated by powder x-ray diffraction (PXRD). 38 Herein, we report the optimal synthesis of this MOF with superb quality. By carrying out systematic studies via previously developed MHT approach, we found that highly crystalline (Supporting Information Figure S2) and octahedral shaped opt-UiO-66(Zr)-(OH) 2 nanocrystals ($100 nm, Figure 1d & Supporting Information Figure S2) could be easily obtained using an optimal molar ratio (105) of acetic acid (AA) to DOBDC in aqueous solutions with ZrOCl 2 Á8H 2 O as the metal source.…”

Synthesis, Structure, and Characterization of Defect‐free [Hf6(μ3‐O)4(μ3‐OH)4(C4H2O4)6] (Hf‐UiO‐66‐Fum)

“…The synthesis of Hf‐UiO‐66‐Fum has been previously reported using water as the solvent . In combination with different modulators only products containing linker defects were obtained . As shown in our investigation, the use of a solvent mixture composed of DMF and HCOOH and a threefold excess of linker leads to the defect‐free Hf‐UiO‐66‐Fum.…”

“…The synthesis of Hf‐UiO‐66‐Fum has been previously reported using water as the solvent . In combination with different modulators only products containing linker defects were obtained .…”

“…In comparison to these results only very limited information on the synthesis of Hf‐MOFs is available in the literature , , . Thus, for only Hf‐UiO‐66‐Fum only compounds containing linker defects have been reported and the defect chemistry of Hf‐UiO‐66‐Fum was shown to depends strongly on the modulator and the modulator to water ratio employed in the synthesis …”

“…The synthesis of Zr‐UiO‐66‐Fum has been described in detail in several studies and the effects of various synthesis parameters such as the use of different modulators and their concentration on the crystallinity and defect chemistry was investigated . In comparison to these results only very limited information on the synthesis of Hf‐MOFs is available in the literature , , . Thus, for only Hf‐UiO‐66‐Fum only compounds containing linker defects have been reported and the defect chemistry of Hf‐UiO‐66‐Fum was shown to depends strongly on the modulator and the modulator to water ratio employed in the synthesis …”