Advances in synthesis of TiO2 nanoparticles and their application to biodiesel production: A review

Biodiesel is a strategic measure to mitigate greenhouse gas emissions and other negative environmental impacts in line with oxidation of fossil fuels. Alkali-catalysed transesterification is the conventional production protocols in biodiesel production due to its technical reliability in line with low maintenance cost. Nonetheless, alkalicatalysed biodiesel production required to use the refined lipid feedstock due to low tolerance against impurities. TiO2 NPs have been widely employed as heterogeneous catalysts. Also, the use of TiO2 NPs could be beneficial because TiO2 adaptation in production of biodiesel could combine transesterification and esterification into a single process. Nonetheless, the general protocol in TiO2 NPs synthesis could not circumvent to use hazardous/toxic chemicals leading to serious/acute health and environmental problems. Several studies have demonstrated that green synthesis of TiO2 NPs using microorganisms and plants as alternate approaches can eliminate the problems associated with conventional methods without sacrificing the exceptional features of these nano materials. Hence, the main objective of this review is to offer fundamental insights and recent advances in green methods for producing TiO2 NPs driven by various microorganisms and plants extract along with quick summary of physio-chemical methods. In particular, the TiO2 NPs with reduced sizes (<60 nm) are possible to obtain using cost effective and eco-friendly green synthesis. Moreover, activity of TiO2 NPs as catalysts and their effect on biodiesel selectivity is discussed using catalytic trans-esterification, esterification, and simultaneous trans-esterification and esterification processes. The combinations of TiO2 NPs with other metallic and non-metallic materials are very selective and efficient for biodiesel production upto 98 %. This study appeals to research communities for adopting green synthesized TiO2 NPs as nanocatalysts for eliminating economic and environmental issues.