A review on microwave-assisted transesterification processes using various catalytic and non-catalytic systems

Biodiesel refers to be the future of fuel due to its distinctive properties, such as biodegradability, renewability, non-toxicity, and the net reduction in sulfur and carbon emission. The use of microwave as a heat source intensifies biodiesel synthesis process. The drastic reduction in reaction time for a homogeneous and heterogeneous catalyst is associated with the non-thermal and thermal effect of microwave on polar substance and microwave absorbing catalyst. This paper review the microwave-assisted transesterification involving various homogeneous and heterogeneous catalytic systems used for biodiesel synthesis. The microwave-assisted transesterification involving homogeneous acid or base catalyst is fast but associated with high purification cost. The use of heterogeneous catalyst under microwave requires a higher molar ratio of alcohol to oil, temperature, and time, but the production separation, catalyst recovery, and reuse are feasible. A special class of microwave-assisted heterogeneous system containing the enzyme and microwave-assisted supercritical transesterification reaction is reviewed. In the end, a continuous flow system reviewed to overcome the lower penetration depth of microwave in a batch system. The techno-economic analysis of microwave-assisted transesterification is energy efficient compared to the conventional method. The continuous flow system suggests technological up-gradation in biodiesel production. (C) 2019 Elsevier Ltd. All rights reserved.