Thermodynamic, Thermal, and Structural Stability of Bimetallic MIL-53 (Al1–xCrx)

Abstract

Understanding the stability of porous materials, especially metal–organic frameworks (MOFs), is central to defining their applications in gas storage, separation, and catalysis. Herein, integrating high-temperature drop combustion calorimetry as well as simultaneous thermal and in situ structural analyses, we performed a comprehensive study on the thermodynamic, thermal, and structural stabilities of MOF in air. A family of MIL-53 (Al1–xCrx) with systematically tuned metal contents was intentionally chosen considering their unique property that H2BDC species serve as both coordinated linkers and guest species confined. The results suggest that as temperature increases, all samples underwent (1) a phase transition from the pore-filled Pnma to the Imma with empty pores, (2) structural degradation, and (3) complete oxidation (burning). At the same temperature, as the chromium (Cr) content increases, the thermal and structural stability of MIL-53 (Al1–xCrx) in air decreases. In contrast, interestingly, the intrinsic thermodynamic stability systematically increases as a function of Cr content, evidenced by the more exothermic enthalpies of formation, ranging from 92.8 ± 73.4 kJ/mol (slightly metastable) to −1593.2 ± 60.8 kJ/mol (stable). Such a phenomenon is likely due to enhanced H2BDC–MIL-53 guest–host interactions upon Cr substitution, which energetically neutralize the metastability of MIL-53 open frameworks. This study highlights that the thermal, structural, and energetic stabilities are different and have equal importance in governing the synthesis and applications of MOFs.