A water-based synthetic route to the metal–organic framework UiO-66 starting from PET-derived terephthalate esters

Abstract

The benchmark Zr-terephthalate MOF UiO-66 is typically prepared by solvothermal synthesis in N,N-dimethylformamide (DMF), one of the few solvents able to dissolve terephthalic acid. The use of DMF is one of the major drawbacks for the transfer of UiO-66 to large-scale applications, since DMF is an expensive and toxic organic solvent. In this work, we propose a water-based route to synthesise UiO-66 using either dimethyl terephthalate or bis(2-hydroxyethyl) terephthalate, which can be obtained from chemical recycling of waste polyethylene terephthalate, as a source of the linker. Hydrochloric acid and acetic acid were used as modulators during the synthesis to control the kinetics of ester hydrolysis and MOF crystallisation, aiming to avoid the collateral precipitation of terephthalic acid. A chemometric design of experiments was employed to optimise the reaction parameters, showing that 15 molar equivalents of hydrochloric acid enable hydrolysis without inhibiting crystallisation, while acetic acid controls which phase is obtained, favouring the desired face centred cubic topology at 15 molar equivalents. The optimised conditions afford UiO-66 with high crystallinity and porosity in just 2 hours at 90 °C. A crucial role in the process is played by the monoester, which is more soluble than both the diester and the diacid in the reaction environment and can be involved in the formation of secondary building units. We also developed a DMF-free workup protocol based on the use of ethanol/dimethylsulfoxide mixtures and water. A 50-fold scale up (500 mL) was demonstrated using a round bottom flask, producing UiO-66 with properties comparable to the product of the small-scale synthesis and with a space-time yield >200 kg m⁻³ d⁻¹.