Biodiesel production directly from oils with high acid value by magnetic Na2SiO3@Fe3O4/C catalyst and ultrasound

•Low ultrasonic energy and magnetic Na2SiO3@Fe3O4/C for biodiesel production.•Soybean and Jatropha biodiesel yields reached 97.9% and 94.7%.•The catalyst resisted high acid value (AV) oils under ultrasonic.•93.2% and 82.5% Jatropha biodiesel yield at AV of 4.8 and 7.3 mg KOH/g.•The catalyst is stable for 5 cycles with biodiesel yield >70% at AV of 4.8.

Magnetic heterogeneous catalyst (Na2SiO3@Fe3O4/C) was prepared for the production of biodiesel from oils with high acid value (AV) under ultrasonic (US) irradiation and magnetic stirring (MS). With the catalyst and assisted by US, soybean biodiesel yield reached >90% in only 20 min (or at 318 K) and 97.9% under the optimal conditions. It was easily magnetically separated for 5 cycles with 94.9% recovery rate and biodiesel yield >80% with both US and MS. The catalyst transesterified Jatropha oil with biodiesel yields of 94.7%, 93.2% and 83.5% at AV of 1.3, 4.8 and 7.3 (mg KOH/g) with US, and was cycled 5 times with yield >70% at AV of 4.8 assisted by US and MS. High biodiesel yield (90.7%) was still achieved from high AV oil (4.8) at low US energy density (0.1 W/mL) and MS. The catalyst combined with US and MS can find practical application for direct production of biodiesel from oils with high AV.

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