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) data show the presence of both Cr and Fe at the same metal nodes within the mixed-metal MOFs. Moreover, EXAFS results suggest the distortion of the trinuclear-oxo cluster within the mixed-metal MOFs, which possibly facilitates the facile phase transformation in the Cr/Fe system alternative to the single Cr system. This study provides a way to monitor the phase formation and/or transformation of the polymorphic Cr/Fe-terephthalate MOFs ascribable to the effect of the additional second metal. Moreover, it can be an example to understand the polymorphic phenomenon in metal-terephthalate MOFs and is believed to broaden the knowledge of MOF synthesis and design.