Phase-transition and phase-selective synthesis of porous chromium-benzenedicarboxylates

A study of the crystallization of two topical porous chromium- benzenedicarboxylates, denoted as MIL-53 and MIL-101, has been reported. Both conventional electric (CE) and microwave (MW) heating have been explored in order to understand phase-selectivity and phase-transition between these solids. One chromium benzenedicarboxylate, the kinetically favorable MIL-101 (lower density phase), is the phase obtained at the early stage of the reaction, while the thermodynamically favorable MIL-53 phase (higher density phase) is obtained at the expense of MIL-101 at longer synthesis time. Phase-transition from MIL-101 to MIL-53 does not occur by direct conversion. Instead, at the longer synthesis time the MIL-101 is degraded, and subsequently, MIL-53 is observed via the reorganization of the decomposed species. Moreover, it is suggested that MW irradiation provides a phase-selective synthesis of MIL-101 due to rapid synthesis, preventing the conversion into the thermodynamically favorable phase (MIL-53). Therefore, the MW synthesis may lead to a new way to find new metal-organic frameworks (MOFs) especially those that are hard to synthesize due to interconversion into a more stable phase or dense phase.