Heteropolyanion-based ionic liquids catalyze esterification of ammonium succinate in aqueous solution

Propane sulfonate (PS) functionalized organic cations were combined with Keggin heteropolyanions to synthesize a series of sulfonated organic heteropolyacid salts (HPA-ILs). Their structures were characterized by FT-IR and elemental analyses (C, H, N). These catalysts were applied to directly catalyze the esterification of ammonium succinate with 1-butanol in aqueous solution. Compared with HPAs, H₂SO₄, [MIMPS]HSO₄ and Nafion^®NR-50 resins, HPA-ILs exhibited excellent catalytic activities. Finally, the excellent catalyst [MIMPS]₃PMo₁₂O₄₀ was selected to investigate systemically the reaction conditions. The optimization of the reaction conditions was ammonium succinate concentration 50% (mass), catalyst dosage 1.0% (based on ammonium succinate), n(1-butanol):n(ammonium succinate)=5:1, reaction time 12 h, reaction temperature 130℃ and vacuum degree 0.02 MPa. The highest yield of esters could reach 89.7%. After reusing 6 times the catalyst still showed the superior catalytic performance with yield of above 85%.