Development of novel API-based deep eutectic solvents for esterification of high free fatty acid

Low-value feedstocks containing high free fatty acid (FFA) content are incompatible with direct alkali-catalysed transesterification, and require a deacidification step through esterification to reduce its FFA level. Herein, innovative acid catalysts were developed based on deep eutectic solvents (DESs) to pretreat low-quality palm oil (12.43 % FFA). DESs were formed using Brønsted acids (5-sulfosalicyclic acid and benzenesulfonic acid) and an active pharmaceutical ingredient (paracetamol) at a 3:1 M ratio. The DESs were characterized using ATR-FTIR and Hammett acidity function (H₀). DES catalyst dosage, methanol requirement, reaction time and temperature parameters were optimized, and its recyclability was evaluated. The FFA contents were reduced to below the limit of < 2 % using acidic DESs at optimized conditions. Reaction kinetics revealed that DES-catalysed reactions followed the pseudo first order rate of reaction and required the lowest activation energy of 40.91 kJ/mol. Through the Eyring-Polanyi thermodynamic study, the DES-catalysed esterification reactions were endothermic (ΔH° > 0), non-spontaneous (ΔS° < 0 and ΔG° > 0) and endergonic. COSMO-RS computational calculations reveal the viable formation of the DESs based on its moieties, and supports the good solubility of the DESs in methanol. This study demonstrates the feasible valorisation of low-value feedstocks using innovative catalysts in enabling biodiesel production.