Novel self-solidifying double-site acidic ionic liquid as efficient and reusable catalyst for green biodiesel synthesis

In this work, two double-site acidic ionic liquids were synthesized by a two-step method via the reaction of 1,2-bis(diphenylphosphino) ethane (DPhPE) with 1,3-propanesulfonate (PS) to form a DPhPE-PS zwitterion followed by acidification with different acids. The obtained ionic liquids were characterized and further used as catalyst for the esterification of various free fatty acids with methanol to produce biodiesel. The suitable self-solidifying double-site acidic ionic liquid [DPhPE-PS][HSO4]2 is homogeneous catalytic due to its solubility during the esterification, which is precipitated after reaction. The evaporation of the residual methanol and generated water combines the advantages of homogeneous and heterogeneous catalysts, including their excellent catalytic activity and convenient recovery. [DPhPE-PS][HSO4]2 exhibits 98.65% of oleic acid conversion under operating appropriate conditions optimized by response surface methodology (catalyst dosage of 6.0 wt%, molar ratio of methanol to oleic acid of 10.0:1, reaction temperature of 60.0 °C, and reaction time of 4.0 h) and keeps outstanding catalytic performance after 9 cycles. Moreover, [DPhPE-PS][HSO4]2 also displays excellent catalytic activity and good versatility in the esterification of various free fatty acids to biodiesel. Hence, the self-solidifying double-site acidic ionic liquid [DPhPE-PS][HSO4]2 can be used as an environmentally friendly and efficient catalyst for synthesis of biodiesel via esterification.