A review on biodiesel production using basic ionic liquids as catalysts

Scarcity of the unrenewable fossil fuel stocks and the escalating concerns about environmental issues has reinvigorated the need for the development and promotion of environmentally benign and sustainable energy resources. In this context, biodiesel has gained recognition as an optimal replacement option for fossil-fuel based diesel due to its reproducible and eco-friendly properties. Biodiesel is generally prepared through the transesterification of oils or fats with short-chain alcohol under the condition of a suitable catalyst. Among various catalysts available, ionic liquids have been regarded as the most promising viable substitute for conventional catalysts, including chemical and biological catalysts, in the production of biodiesel. In general, ionic liquids can be classified into two major categories according to their acidity/basicity: acidic and basic ionic liquids. The acidic ionic liquids have been well acknowledged and were widely applied as catalysts in preparing biodiesel. Whereas, there are rare reviews related to the construction and application of basic ionic liquids. Therefore, this paper provides a comprehensive review of the classification, synthetic schemes, catalytic mechanism of a variety of basic ionic liquids. In addition, the utilization of basic ionic liquids as catalysts for the preparation of biodiesel was discussed in detail, along with the current challenges and prospects for their future use.