In–situ modification of UiO–66(Zr) organic ligand to synthesize highly recyclable solid acid for biodiesel production

Abstract

To transfer high free fatty acid oil into biodiesel in mild conditions is still facing challenge, thereinto, the key is efficient acid catalyst. In this study, 2,5–dimercaptoterephthalic acid was adopted to substitute terephthalic acid as ligand for UiO–66–(SH)2 synthesis, then the sulfydryl (–SH) was in-situ oxidized by hydrogen peroxide and acidified via sulfuric acid thus generating sulfonic catalyst UiO–66–(SO3H)2. To further reveal the relationship between physico–chemical property and catalytic activity, catalyst was characterized via thermogravimetry analysis (TG), X–ray diffraction (XRD), N2 absorption–desorption, scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and pyridine absorption–Fourier transform infrared spectroscopy (Py–FTIR). Results indicate the in–situ modification increases the quantity of acid sites for UiO–66(Zr), where the acidity is aggrandized from 0.02 mmol/g to 2.28 mmol/g. The maximum conversion of oleic acid to biodiesel was 86.52 % with catalyst amount of 10 wt% and molar ratio of methanol/oleic acid of 15 at 90 °C within 4 h. Moreover, UiO–66–(SO3H)2 exhibited favorable reusability and water resistance, which maintained an excellent esterification conversion after four cycles and no obvious impact was detected as the water content was 10 wt%. The quality of obtained biodiesel in this study satisfied the European Union standard of EN 14214, which could be used as transport fuel.