Microwave-assisted synthesis of zirconium-based metal organic frameworks (MOFs): Optimization and gas adsorption

Microwave-assisted synthesis of zirconium (Zr) based metal organic frameworks (MOFs) were performed and the yield and porous property of UiO-67 was optimized by varying the quantity of the modulator (benzoic acid, BenAc and hydrochloric acid, HCl), reaction time and temperature. It was found that (i) an increase in the amount of modulator enhanced the specific surface area and pore volume of UiO-67 due to the promotion of linker deficiency; and (ii) the presence of modulators influenced the number of nuclei (and hence the crystal size) and nucleation time (and hence the yield). Optimum amounts of BenAc and HCl for synthesizing UiO-67 under microwave irradiation were determined as 40 mol equivalent and 185 mol equivalent (to Zr salt), respectively. In comparison to conventional solvothermal synthesis, which normally takes 24 h, microwave methods promoted faster syntheses with a reaction time of 2–2.5 h (at similar temperatures of 120 °C and 80 °C for BenAc and HCl, respectively). The thermal effect of microwave is believed to contribute to the fast synthesis of UiO-67 in the microwave-assisted synthesis. The reaction mass efficiency and space-time yield show that microwave heating promoted the simple yet highly efficient preparation of Zr-based MOFs. In addition, UiO-67 MOFs from different synthesis methods (i.e. the microwave-assisted and solvothermal method) were evaluated using single-component (CO₂ and CH₄) adsorption, showing comparable gas uptakes.