Phase-selective synthesis and phase-conversion of porous aluminum-benzenetricarboxylates with microwave irradiation

Three porous Al-BTCs (BTC: benzenetricarboxylate) have been synthesized in wide reaction conditions including heating methods (conventional electric and microwave heating) and reaction time. It has been found that an Al-BTC called MIL-100 converts into another phase (MIL-110) with increasing time and eventually a different phase (MIL-96) is observed. This phase-conversion is the first observation of the possibility to synthesize three MOFs from the very same reactant mixtures, and may be explained with the relative stability of the Al-BTCs (MIL-96 > MIL-110 > MIL-100) since the stability of a porous material may increase with decreasing porosity or pore size of the phases. The inter-conversion of a less stable Al-BTC into a more stable one is carried out via recombination of the degraded precursors. Moreover, due to the advantage of rapid crystallization of microwave heating, it can be found that microwave synthesis selectively leads to highly porous materials like MIL-100 or MIL-110. This phase-selective synthesis with microwaves is similar to the synthesis of another MOFs (Cr-BDCs) and aluminophosphates, and may lead to a new way to synthesize highly porous (unstable) materials.

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► Three different MOFs can be synthesized from the very same reactant composition.
► Microwave-syntheses have the advantage of phase-selectivity with high porosity.
► The interconversion of a phase into another one occurs after dissolution.