A Toolkit for Phase Identification and Reproducible Synthesis of Fe-Terephthalate Metal-Organic Frameworks: MIL-88B, MIL-101, MIL-53, and MIL-68

Iron-terephthalate metal-organic frameworks (Fe-BDC MOFs) are an important and abundant class of materials that exhibit rich polymorphism, making reproducible phase-pure synthesis and phase identification challenging. Here, we systematically examine how precursor type, solvent, temperature, and pH influence the formation of the four known Fe-BDC phases: MIL-88B, MIL-53, MIL-101, and MIL-68. We provide a wide range of complementary analyses of phase pure samples including powder Xray diffraction (PXRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Mössbauer spectroscopy, elemental (CHN) and ICP-OES analysis, nitrogen and water sorption analysis. We identify that solvent-depended PXRD measurements reveal fingerprints for respective Fe-BDC phases due to the different breathing behaviors, providing a rapid and reliable method for phase discrimination even in phase mixtures. This work, establishes robust synthesis protocols and experimental reference data for navigating the phase identification in 2 the Fe-BDC system and clarifies synthesis-structure-property relationships, with a focus on structural flexibility.