Hydrogen Bonds in Disulfonic-Functionalized Acid Ionic Liquids for Efficient Biodiesel Synthesis

Regulating the states of hydrogen bonds in ionic liquids (ILs) is an effective way to improve their catalytic performance. In this paper, disulfonic-functionalized acidic ionic liquids (DSFAILs) were synthesized successfully, including novel SO3H-functionalized binuclear IL (bis[3-(CH2)(3)SO3H-1-(CH2)(2)-Im][HSO4](2)). For the biodiesel synthesis, compared with the traditional ILs catalysts, DSFAILs bis[(3-(CH2)(3)SO3H-1-(CH2)(2)-Im][HSO4](2), [Im(N (CH2)(3)SO3H)(2)][HSO4]) had higher catalytic activity even under mild reaction conditions. Using the density functional theory (DFT) method, the role of hydrogen bonds in different SO3H-functionalized acidic ionic liquids (SFAILs) was explored. The forms of hydrogen bonds existing in different ILs directly determine their acidity. It suggested that the forming status of the active sites (hydrogen bonds) were diverse in different SFAILs. Also, deep ionization of the hydrogen atoms from the cation-anion strong interaction could increase the acidity and catalytic performance of SFAILs. From this, the structure-activity relationship between the SFAILs structures and the catalytic activity of methyl oleate synthesis was proposed. Besides, the experimental results also showed that bis[3-(CH2)(3)SO3H-1-(CH2)(2)-Im][HSO4](2) catalyst had a high catalytic activity to obtain methyl oleate and the catalyst could be separated easily owing to its larger molecular weight. However, [Im(N(CH2)(3)SO3H)(2)][HSO4] had a stronger acidity and a lower steric hindrance and thus a higher catalytic activity and was the optimal catalyst for the methyl oleate synthesis. In the presence of a small amount of catalyst (6 wt %) and at low reaction temperature (353 K), the methyl oleate yield could reach up to 93%. After six recycles of the catalyst, the methyl oleate yield remained at 90%.