Microwave synthesis and phase transition in UiO-66/MIL-140A system

Abstract

Abstract We report an accurate study of phase formation in the Zr4+/terephthalic acid system under MW heating at 220 °C. We have focused on water and/or acetic acid additives on the phase composition of the product. In our recent work, we have claimed that water plays a crucial role in UiO-66 phase formation during solvothermal synthesis. This trend was observed during MW synthesis as well. However, microwaves provide fast heat transfer, and it obstructs the formation of the high-symmetric UiO-66 phase and shifts the balance to MIL-140A formation even in the presence of water. Moreover, during solvothermal synthesis, acetic acid additive acts as a modulator and leads to the formation of a highly crystalline UiO-66 phase. Conversely, in MW conditions, acetic acid additive shifts the balance to the MIL-140A phase because it improves the capability of the reaction mixture to absorb microwaves and, therefore, to reach higher temperatures faster. In other words, we have observed two opposite trends in phase formation under MW irradiation – water additive provides ZrO2 nuclei and shifts balance toward the UiO-66 phase, while fast MW heating results in MIL-140A.