Advances in CaO-based catalysts for sustainable biodiesel synthesis

It is no longer unfamiliar to the world that usage of fossil-derived fuel is accompanied by several downsides, including environmental fatality associated with toxic gas emissions from diesel engines and the recurring surging price of diesel fuel. The concern regarding the exploitation of natural resources and subsequent environmental threats has caught the attention of all nations. Therefore, scientists have stepped forward to come up with the most suitable alternative to conventional diesel fuel. They utilized abundance of oil feedstocks and performed transesterification reactions with short-chain alcohols to produce less toxic, eco-friendly diesel fuel termed biodiesel which was considered to be a viable substitute. In order to carry out the transesterification reaction feasibly, certain homogeneous and heterogeneous catalysts were employed. Heterogeneous catalysts got more recognition among researchers; however, many of these catalysts did not attain large-scale production. Among many heterogeneous catalysts, calcium oxide (CaO) appeared to be one of the most suitable heterogeneous solid base catalysts for biodiesel synthesis. CaO can be derived chemically as well as from natural sources such as chicken eggshells, snail shells, biont shells and many more. CaO exhibits high catalytic activity to perform transesterification reactions. Additionally, CaO is a cheap and eco-friendly catalyst that can also be used as support material for various catalyst systems. Constructing effective CaO-based catalysts for industrial biodiesel production unlock the scope for process advancements in this area. This paper aims to review the catalytic performances of recently reported CaO as heterogeneous solid-base catalytic systems for the production of biodiesel. Moreover, the catalyst preparation conditions, various reaction parameters, properties of biodiesel obtained, advantages and disadvantages, life cycle assessment, and circular economy are discussed.