Finding a more efficient method for the transesterification of triglycerides to biodiesel fuel (BD) is important in today's world. In this study, transesterification of trilaurin was carried out in a solution containing 4 wt% of the Lewis acid AlCl3 dissolved in a cosolvent of ethanol and 5 MPa CO2. A conversion rate of over 90% was achieved within 1 h at the low temperature of 180 degrees C. The process indicates a co-catalytic effect of the Lewis acid and CO2. We postulate several key steps for the mechanism. First, the CO2-ethanol mixture enhances the hydrogen bonding, increasing the concentration of C2H5O center dot. Second AlCl3 attacks the oxygen of C-O-C to weaken the bonds to form carbonyl carbon ORI, which is then easily attacked by C2H5O center dot to give the transesterified product (C(2)H(4)COOR1). Third, AlCl3 is finally replaced by H to form glycerin (GL) and intermediates, such as unmethyl esterified compounds (uME). AlCl3 was used as a flocculant and catalyst for converting waste cooking oil (WCO) to BD. The process achieved 97% free fatty acid (FFA) conversion at 120 degrees C in 90 min, making it one of the most efficient systems available for WCO recovery. AlCl3 was also successfully applied to microalgae, signaling the potential for a process that combines harvesting, lipid extraction, and transterification, leading to fully integrated, microalgae-based BD production. (C) 2015 Elsevier Ltd. All rights reserved.