Time-Resolved in Situ Polymorphic Transformation from One 12-Connected Zr-MOF to Another

Understanding polymorphism in metal-organic frameworks (MOFs) provides opportunities to unravel the process of MOF crystallization, and it enables the elucidation of structure-property relationships of compositionally identical crystals. Here, we present the modulator- and temperature-mediated polymorphic transformation of the kinetic product from Zr-6-based MOF synthesis, EHU-30, to the thermodynamic product, UiO-66. The partial dissolution-recrystallization process was demonstrated by a combination of in situ powder X-ray diffraction (PXRD) and in situ H-1 NMR spectroscopy where EHU-30 was heated in the presence of a monotopic acid modulator, acetic acid. Density functional theory (DFT) calculations show that the EHU-30 polymorph is less stable because the bent linkers have higher Gibbs free energy compared to linear linkers in the thermodynamic product, UiO-66.