Due to increasing concerns about global warming and dwindling oil supplies, the world's attention is turning to green processes that use sustainable and environmentally friendly feedstock to produce renewable energy such as biofuels. Among them, biodiesel, which is made from nontoxic, biodegradable, renewable sources such as refined and used vegetable oils and animal fats, is a renewable substitute fuel for petroleum diesel fuel. Biodiesel is produced by transesterification in which oil or fat is reacted with short chain alcohol in the presence of a catalyst. The process of transesterification is affected by the mode of reaction, molar ratio of alcohol to oil, type of alcohol, nature and amount of catalysts, reaction time, and temperature. Various studies have been carried out using different oils as the raw material; different alcohols (methanol, ethanol, butanol); different catalysts; notably homogeneous catalysts such as sodium hydroxide, potassium hydroxide, sulfuric acid, and supercritical fluids; or, in some cases, enzymes such as lipases. This article focuses on the application of heterogeneous catalysts for biodiesel production because of their environmental and economic advantages. This review contains a detailed discussion on the advantages and feasibility of catalysts for biodiesel production, which are both environmentally and economically viable as compared to conventional homogeneous catalysts. The classification of catalysts into different categories based on a catalyst's activity, feasibility, and lifetime is also briefly discussed. Furthermore, recommendations have been made for the most suitable catalyst (bifunctional catalyst) for low-cost oils to valuable biodiesel and the challenges faced by the biodiesel industry with some possible solutions.
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