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 Zr6-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 1H 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.