In situ heterogeneous transesterification of microalgae using combined ultrasound and microwave irradiation

•KF/CaO as heterogeneous catalyst was first used to produce biodiesel from microalgae.•High biodiesel yield of 93.07 ± 2.39% was achieved under the combination of ultrasound and microwave irradiation.•Ultrasound or microwave alone was not efficient in microalgae heterogeneous conversion.

Heterogeneous KF/CaO catalyst, prepared by the wet impregnation method with different KF loadings (15–35 wt.%) and calcination temperatures (600–1000 °C), was used to assist biodiesel production from microalgae Chlorella vulgaris under the combination of ultrasound and microwave (US–MW) irradiation. It was found that the catalyst calcinated at 900 °C with 25 wt.% KF loading presented the optimum performance, when the test was conducted with 12 wt.% of catalyst and a methanol to biomass ratio of 8:1 at 60 °C for 45 min. The highest fatty acid methyl esters (FAME) yield was 93.07 ± 2.39%, which was significantly higher than that achieved by using US or MW irradiation individually. The catalysts were characterized by X-ray diffraction, scanning electron microscopy and Brunauer–Emmett–Teller.