Optimizing the conditions for the microwave-assisted direct liquefaction of Ulva prolifera for bio-oil production using response surface methodology

•Ulva prolifera was converted to bio-oil through microwave-assisted direct liquefaction.•Response surface methodology was used to optimize the liquefaction technology.•A maximum liquefaction rate of 93.17 wt% bio-oil was obtained.•The bio-oil was composed of carboxylic acids and esters.

Microwave-assisted direct liquefaction (MADL) of Ulva prolifera was performed in ethylene glycol (EG) using sulfuric acid (H2SO4) as a catalyst. Response Surface Methodology (RSM) based on central composite rotatable design (CCRD) was employed to optimize the conditions of three independent variables (catalyst content, solvent-to-feedstock ratio and temperature) for the liquefaction yield. And the bio-oil was analyzed by elementary analysis, Fourier transform infrared spectroscopic analysis (FT-IR) and gas chromatography–mass spectrometry (GC–MS). The maximum liquefaction yield was 93.17%, which was obtained under a microwave power of 600 W for 30 min at 165 °C with a solvent-to-feedstock ratio of 18.87:1 and 4.93% sulfuric acid. The bio-oil was mainly composed of phthalic acid esters, alkenes and a fatty acid methyl ester with a long chain from C16 to C20.