Short Alkyl-Imidazolium Ionic Liquids Enhanced In-Situ Transesterification of Microalgae

Ionic liquids (ILs) catalyzed in-situ transesterification (IST) of microalgae is a promising strategy to improve biodiesel yield through enhancing cellulose solubility and catalyzing transesterification. Effect of anions of ILs on biodiesel has been widely investigated previously. However, how do cations of ILs affect IST of microalgae is still unclear. On this basis, ILs composed of alkyl-imidazolium cations with different length, 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), and 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF4]) catalyzed IST of microalgae are experimentally and theoretically investigated. The results indicate that increasing alkyl chains of cations weakens interactions of ILs-cellulose, and reduces acidities of ILs, subsequently leads to decrease of cellulose extraction yield and biodiesel yield. Although cellulose extraction yield and biodiesel yield are affected by reaction temperature, dosage of methanol, and MILs: Algae, however, H-bonding interactions of ILs-cellulose and acidities of ILs are still dominate parameters affecting cellulose extraction yield and biodiesel yield, respectively, leading to their unchanged sort under variable reaction conditions as following: [Emim][BF4] > [Bmim][BF4] > [Hmim][BF4]. This work reveals that short alkyl chain of cation is favorable to both cellulose extraction and biodiesel yield, providing theoretical guides for screen and design of benign ILs to further improve biodiesel of in-situ transesterification of microalgae.