Polymorphism of Mixed Metal Cr/Fe Terephthalate Metal–Organic Frameworks Utilizing a Microwave Synthetic Method

Polymorphic metal–organic frameworks (MOFs) possess distinct characteristics due to a variation of linking patterns and spatial orientations of similar chemical components. Herein, microwave synthesis is used to prepare polymorphic mixed-metal Cr/Fe-terephthalate MOFs, of which the formation is different from the single-metal Cr-terephthalate. By varying the reaction time at a fixed concentration of Fe at 33 mol %, three polymorphic Cr/Fe-MOFs, i.e., MIL-101, MIL-88B, and MIL-53, with remarkably different properties were obtained. Phase-pure Cr/Fe-MIL-101 is obtained in 5 min which shows a well-defined octahedral shape with a larger particle size, highlighting a kinetically preferable formation of Cr/Fe-MIL-101. Further prolongation of the reaction time conveys the formation of more thermodynamically stable phases, namely, Cr/Fe-MIL-88B and then Cr/Fe-MIL-53 at the reaction time longer than 2 h. Cr K-edge and Fe K-edge X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (EXAFS...