In this work, biodiesel has successfully prepared via ultrasonic method in a short time and low temperature by nano-solid-base catalyst (KF/γ-Al₂O₃). The catalyst was obtained by calcination of a mixture of KF and γ-Al₂O₃ m(KF)/m(γ-Al₂O₃) at 500 °C for 3 h. Nano-solid-base catalyst was characterized with scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermal gravimetry (TG) and the Hammett indicator methods. The TEM image depicted nanoparticles and uniform dispersion of active phase over alumina. The XRD analysis confirmed the formation of potassium aluminum fluoride (K₃AlF₆) and potassium oxide, active catalyst for transesterification. The transesterification of soybean oil with methanol was performed by using both low frequency ultrasonic reactor (20 kHz) and mechanical stirring in the presence of KF/γ-Al₂O₃. The influence of various parameters such as ultrasonic power, oil/methanol molar ratio, catalyst concentration, time, and temperature were studied on the biodiesel formation. The maximum yield (95%) was achieved by applying 45 W acoustic power, molar ratio of alcohol to oil at 12:1, catalyst concentration of 2.0 wt%, 40 min sonication, and temperature of 50 °C. The transesterification was performed in 360 min using mechanical stirring with 76% yield. The results confirm that ultrasound significantly accelerates the transesterification reaction in comparison with the mechanical stirring.

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