Biodiesel production from microalgal biomass by Lewis acidic deep eutectic solvent catalysed direct transesterification

Traditional two steps biodiesel production process generally involves corrosive sulfuric acid in the transesterification step, which is not environmentally benign. In this work, a novel procedure was presented for the direct production of biodiesel using Lewis acid deep eutectic solvent (LADES) to avoid using sulfuric acid. Synergy between the Lewis site of the LADES catalyst and the Brønsted site from biomass pretreated by Brønsted acidic deep eutectic (BADES) has resulted in the development of a bifunctional medium considered to be appropriate for high biodiesel formation and increase the FAMEs weight. The mechanism of biodiesel formation was anticipated through the experimental and computational studies of Lewis and/or Brönsted acid sites in the reaction process. The potential interference factors of the extraction process such as time of reaction, temperature, LADES catalyst concentration in methanol, and solvent volume were further optimized experimentally, followed by the Box–Behnken method. The highest amount (39.86 mg/g) of fatty acid methyl esters (FAMEs) was obtained from Chlorella pyrenoidosa biomass at 120 °C for 90 min, 5 mL solvent volume, and 5% concentration of LADES (ChCl-CrCl3.6 H2O) catalyst in methanol. The predominant FAMEs were C16:0 (9.47 mg/g), C18:2 (12.42 mg/g), and C18:3 (8.68 mg/g), and obtained biodiesel qualities meet international standards. The recyclability of LADES was tested, and satisfactory results of FAME weight were observed up to three cycles, and the FTIR peaks of LADES catalyst after four cycles of reusing have shown the structure of DES remain stable. Despite the existence of sustainable, efficient, and less toxic ways for microalgae transesterification, further investigation is required to assess the scalability of these methods for large-scale use.