Toward reactive extraction processes for synthesizing long-chain esters: A general approach by tuning bifunctional deep eutectic solvent

• Reactive extraction for different long chain ester synthesis is explored by tuning DES. • Halogen-free Brønsted acidic DESs triazole-BSA/PTSA are for the first time prepared. • Triazole-BSA/PTSA DESs are tunable from weak acid to strong acid. • σ -Potential gives consistent prediction on phase splitting with experiment. • Optimal DES for longer ester is less polar from the perspective of ester phase purity. The synthesis of long-chain esters in reactive extraction using bifunctional deep eutectic solvent (DES) demands a wide solvent selection range and optimal solvent choice. In this work, a general approach towards rational DES selection is presented. First, novel halogen-free DESs based on triazole and p -toluenesulfonic acid (PTSA) or benzenesulfonic acid (BSA) are prepared to expand the selection range of Brønsted acidic DESs, whose binary solid–liquid phase diagrams are measured to confirm the DES formation. Then, the triazole-PTSA/BSA DESs together with the imidazole (Im)-PTSA/BSA DESs are characterized by pH measurement, 1 H-NMR characterization, and COSMO-RS interaction energy analysis, which reflect a two-step formation mechanism. The Im-PTSA/BSA DESs are acid-base tunable while the triazole-PTSA/BSA DESs are tunable from weak acid to strong acid. Afterwards, four strong acidic DESs at the eutectic compositions namely [Im:2BSA], [Im:2PTSA], [triazole:4BSA] and [triazole:4PTSA] are determined as dual solvent-catalyst for the intensified esterification with ester carbon chain length changing from C6 to C11. Combining σ -potential thermodynamic analysis with experimental results, the applicable boundary and change rule of carbon chain length versus different DESs are clarified. On this basis, a rational DES selection strategy is formulated aiming to the development of reactive extraction for synthesizing different long-chain esters.