Optimization of a two-step biodiesel production from waste cooking oil: Comparative evaluation of n-hexane and CPME as transesterification cosolvents

Biodiesel derived from waste cooking oil (WCO) represents a promising strategy for meeting energy demands, mitigating environmental impact, and supporting carbon neutrality goals in countries such as Japan and India. However, conventional alkaline transesterification faces challenges, including soap formation from free fatty acids (FFAs) in WCO and poor miscibility of oil and alcohol phases, both of which limit efficiency. Although one-step acidic esterification and traditional organic co-solvents have been explored to overcome these drawbacks, such approaches raise economic and environmental concerns. Thus, this study investigated a two-step acid-base catalysis process employing the low-toxicity cosolvent cyclopentyl methyl ether (CPME) for the production of biodiesel from WCO, which was collected from local restaurants in Tokyo, Japan. The process was carried out through initial acidic esterification to convert FFAs to esters, followed by alkaline transesterification optimized at a 1:6 oil-to-cosolvent ratio. CPME, with low toxicity and excellent miscibility, facilitated a high biodiesel yield of 97.5%, outperforming n-hexane (96%) and reactions without cosolvent (89%). Gas chromatography-mass spectrometry (GC-MS) analysis confirmed that the synthesized biodiesel met the EN14214 European quality standard (≥96.5% FAME). The process operated at a mild temperature of 40°C, enhancing yield with lower energy input. Overall, the CPME-assisted two-step process offers an efficient and viable route for biodiesel production from waste feedstocks.