Current progress and perspective of heterogeneous nanocatalytic transesterification towards biodiesel production from edible and inedible feedstock: A review

Ever-increasing pollution, high surge in energy demand and depletion of fossil fuel reserves has triggered the issue of non-uniform supply and demand of conventional fuels. This issue has diverted the focus towards exploration of a promising renewable substitute for petro-diesel fuel. Biodiesel could be an energy-efficient, environmentally benign and cost-competitive potential contender to mitigate the energy need. Catalytic transesterification has emerged as the most adopted process of biodiesel production due to its simple operation, easy maintenance, process efficacy and better yield. In contrast to homogeneous catalysts, heterogeneous catalysts are much favoured owing to its simple separation from the final product and retention of catalytic action in reuse for consecutive cycles. Nonetheless, the nanocatalysts as heterogeneous catalysts are widely utilized in biodiesel production due to their efficacious catalytic productivity. This paper comprehensively reviews the diverse edible and inedible feedstocks along with their merits and demerits towards fruitful application in large-scale industries, various categories of nanocatalysts utilized in biodiesel production and the different methodologies of biodiesel production. The current technical literature survey reveals that waste/used cooking oil (WCO/UCO) seems to be a viable feedstock in contrast to other feedstocks owing to their easy and copious availabilities with low cost. Various modern techniques of nanocatalysts synthesis have been outlined with critical insights. This review aims to compare the efficiencies of various nanocatalysts with a motivation to perceive a technical and scientific essence for their recommendation in commercial practice. Physicochemical properties of different feedstocks and biodiesel produced from assorted precursors utilizing nanocatalysts have been summarised and compared. Nanocatalysts like CaO-TiO2, SO3H-aryl-SiO2, ZnMgFe2O4, Fe3O4@SiO2-SO3H, Cs-Ca/SiO2-TiO2, Ca/Fe3O4@SiO2, biochar/CaO/K2CO3, Na-K/CaO, Na-SiO2@TiO2, MgO/clinoptilolite, and K+ trapped clay nanotubes have reportedly shown the biodiesel yields > 97 %. Hence, it is imperative to design, adopt and promote the development of efficient nanocatalyst to enhance the biodiesel productivity.