Synthesis and Characterization of a Deep Eutectic Solvent Catalyst Using Oxalic Acid Extracted from Averrhoa Bilimbi L. for Biodiesel Production

Abstract

Deep eutectic solvents (DESs) have drawn attention as alternative catalytic systems for biodiesel production. This study synthesized and characterized a choline chloride–oxalic acid-based DES using Averrhoa bilimbi L. as a natural source of oxalic acid. Oxalic acid was extracted at different temperatures and combined with choline chloride at molar ratios of 1:1, 1:2, and 1:3. Oxalic acid content was determined by titration, while the extracted material and DESs were characterized using FTIR (Fourier-transform infrared spectroscopy), SEM-EDX (scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy), and pH measurement. The highest measured oxalic acid content was 33.8% at 90 °C. SEM revealed a relatively ordered solid morphology, while EDX showed carbon and oxygen contents of 15.22% and 50.69%, respectively. The synthesized DESs remained liquid and exhibited pH values of 1.22–2.05, with the lowest pH obtained at a 1:3 molar ratio and 90 °C. FTIR confirmed characteristic functional groups associated with choline chloride and oxalic acid and indicated changes in the hydrogen-bonding environment. These results demonstrate the feasibility of developing choline chloride–oxalic acid-based DESs from A. bilimbi L. for further investigation in biodiesel-related esterification applications.