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

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.