Recyclable methanesulfonic acid-based deep eutectic solvents for efficient biodiesel production via transesterification

This study explores, for the first time, the catalytic application of two previously developed methanesulfonic acid-based deep eutectic solvents (MSA-based DESs), namely MSA/choline chloride (MSA/ChCl) and MSA/tetraoctylammonium bromide (MSA/TOAB), for the transesterification of triglyceride-based oils into biodiesel. The aim was to develop recyclable acid catalysts and integrated processing strategies for biodiesel production, leveraging the thermal stability and recovery potential of MSA-based DESs. While these DESs have exhibited efficiency in esterification, their performance in transesterification remains unexamined. Both DESs were assessed under atmospheric and pressurized conditions to evaluate catalytic activity, methanol retention, and process efficiency. Fatty acid methyl ester (FAME) yields were quantified by gas chromatography, revealing that under mild conditions, the methanol retention system enabled slow but sustained conversion, with MSA/ChCl reaching 45.34 % yield after 30 d. Transitioning to a pressurized reactor significantly enhanced reaction kinetics, with MSA/ChCl reaching a FAME yield of 97 % within 3 h under optimized conditions (120 °C, 2 wt% catalyst, 60 wt% methanol). Further increases in DES concentration enabled yields exceeding 99 %. Key parameters, including methanol dosage, reaction time, temperature, and DES concentration, were optimized, and catalyst reusability was validated over five cycles, with yields remaining above 83 %. A closed-loop process was proposed for DES and methanol recovery to enhance scalability and minimize waste. This work extends the application of MSA-based DESs to transesterification, demonstrating a recyclable, high-activity Brønsted acid catalyst capable of achieving high biodiesel yields at low dosage, thereby addressing key limitations of conventional acid systems and supporting the development of sustainable industrial biodiesel processes.