Accelerating Biodiesel Catalytic Production by Confined Activation of Methanol over High-Concentration Ionic Liquid-Grafted UiO-66 Solid Superacids

Solid acids usually show lower catalytic activities than liquid acids because of their limited acid strength and presence of obvious diffusion limitation in the reactions. We report herein a variety of ionic liquid-grafted UiO-66 solid superacids synthesized from quaternization of UiO-66 with 1,3-propane sultone, ion-exchanging with H2SO4 or HSO3CF3. The developed UiO-66 solid superacids are highly efficient in catalyzing biodiesel production toward transesterification, which were better than the ionic liquid and neat H2SO4. The remarkable activities found in the UiO-66 solid superacids benefit from the synergistic effect of high acid concentrations (∼3.33 mmol/g), superacidity (determined by 31P solid-state NMR, Hammett indicators, and potentiometric titration), as well as unique shape-selective confinement for methanol into the ordered and uniform micropores, which targeted the activation of methanol to prevent the occurring of an inverse reaction in the reactions. The UiO-66 solid superacids show important application potentials in the area of biodiesel production in the industry.