Microwave-assisted continuous-flow synthesis of mixed-ligand UiO-66(Zr) frameworks and their application to toluene adsorption

Mixed-ligand metal–organic frameworks (MIXMOFs) containing two or more organic linkers that exhibit functionalities can show improved selective gas adsorption and separation properties with respect to those of the conventional MOFs. In this study, the mixed-ligand UiO–66(Zr) framework was rapidly prepared by employing a microwave-assisted continuous-flow method, which can be easily scaled up for efficient mass production. A microfluidic syringe pump system was used to introduce the precursor solutions of zirconium metal salt and ligand mixture (terephthalic acid and 2-amino terephthalic acid) into a polytetrafluoroethylene tubular reactor kept under microwave irradiation. MIXMOFs were obtained within 10 min, a much shorter time than the reaction time required by the conventional heating method. The prepared samples were characterized by N₂ porosimetry, field-emission scanning electron microscopy, Fourier transform infrared spectrometry, thermogravimetric, and powder X-ray diffraction analyses to elucidate the effects of the ligand ratios. Furthermore, the toluene adsorption–desorption tests were conducted using the synthesized mixed-ligand UiO–66(Zr) adsorbents at different temperatures.