Efficient heterogeneous acid synthesis and stability enhancement of UiO-66 impregnated with ammonium sulfate for biodiesel production

[Display omitted] •UiO-66 and ammonium sulfate are adopted to synthesize high efficient acid catalyst.•High acid amount is obtained as the catalyst precursor calcined under nitrogen.•Second calcination is creatively used to enhance the catalytic stability.•Interaction between zirconium and sulfate is evidently strengthened after second calcination. Sulfated zirconia is a potential heterogeneous acid in catalyzing esterification for biodiesel production. While the catalytic stability is still a challenge during successive batch experiment due to the serious leaching of active site. To address this defect, UiO-66 and ammonium sulfate were employed to synthesize the high efficient acid catalyst for biodiesel production. Catalyst preparation factors and esterification parameters were further investigated to obtain the optimal conditions. Based on these, this study creatively adopted ‘two-stage calcination’ to enhance the catalytic stability. In order to elucidate impact of the second calcination, catalysts were characterized by X-ray diffraction (XRD), thermogravimetry-differential thermogravimetry (TG-DTG), N2 absorption-desorption, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), pyridine absorption-Fourier transform infrared spectroscopy (Py-FTIR), Boehm-titration method, scanning electron microscope (SEM), and energy disperse spectroscopy (EDS). Results indicated that the catalytic activity of catalyst calcined under nitrogen atmosphere (UiO-66/SFN) is higher than that calcined under air atmosphere (UiO-66/SAN). The satisfying oleic acid conversion to biodiesel of 96.2% was achieved by UiO-66/SFN with catalyst amount of 8 wt%, molar ratio of methanol/oleic acid of 8 at 70 °C for 2 h. After being secondly calcined at 500 °C (UiO-66/SSN), the interaction between sulfate and zirconium was evidently improved and the conversion decrement is reduced by 66.25% compared with UiO-66/SFN within five cycles.