Efficient production of biodiesel from ionic liquid catalyzed esterification using ultrasonic-microwave combined intensification

•Biodiesel was produced by esterification of oleic acid with methanol.•Ultrasonic-microwave assistance was first applied to biodiesel production.•The reaction time is dramatically reduced.•The production was optimized by Box–Behnken response surface methodology.•The conversion of oleic acid was 97.85 % close to conventional methods.