Structure Evolution of Synthetic Amino Acids-Derived Basic Ionic Liquids for Catalytic Production of Biodiesel

A two-step strategy was attempted to develop the best amino acid-based basic ionic liquids for catalytic production of biodiesel via transesterification. Cholinium with various amino acids as paired anions were first synthesized to screen anionic moiety. Arginine and histidine were selected for further structural evolution by varying the substituents of tetraammonium cation. Tetrabutylammonium arginine ([TBA][Arg]) was found to be the most effective catalyst to obtain 98.0%-99.8% yield of biodiesel at 80 °C within 15 min with catalyst loading of 28.84 mmol/100 g high oleic sunflower oil. ¹³C NMR spectra of reactants and products certified the progress of transesterification structurally. Biodiesel yield of 98.80% was obtained under the optimal conditions: catalyst loading 6% (oil basis, w/w), temperature 90 °C, methanol to oil mole ratio 9:1, and 15 min reaction. The catalytic transesterification by [TBA][Arg] was applicable for different alkyl alcohols, but the activity decreased with increasing alkyl chain length. The catalyst did not show specificity and preference to different glycerides and different fatty acids. The strong protonizability of the guanidine moiety in [Arg] ^- and stability of [TBA] ⁺, [TMA] ⁺ and [Ch] ⁺ in methanol are suggested to be responsible for the high catalytic activity of the ILs. The developed catalyst significantly reduced the reaction time and might be greener and more sustainable due to the properties of the substrates and the preparation in water.