An overview of ionic liquids as solvents in biodiesel synthesis

Transesterification is the most common method for producing biodiesel. Known as a suitable substitute to diesel fuel, the synthesis involves renewable sources as feedstock. Application of both organic and inorganic solvents in biodiesel production has been widely established. However, as the properties of conventional solvents are perpetually hazardous to human and environment, utilization of greener alternative is a better option. Among the various types of solvents available, ionic liquid seems prevalent. An ionic liquid is a combination of cations and anions, has low or negligible vapor pressure, and exists as liquid at temperature below 100 °C. The prospect of ionic liquids as green solvents in chemical processes is increasing in recent years, especially in biodiesel synthesis. This paper highlighted the properties of ionic liquids that emphasized their versatility as solvents, and the use of switchable ionic liquids as green solvents is also presented. The roles of ionic liquids in biodiesel synthesis are discussed, focusing on their pertinent capability as solvents, particularly as catalysts for transesterification reaction. Since the cost of ionic liquid may be an issue, a brief discussion about the recyclability of ionic liquids is also included.