Abstract

Increasing dependence and overconsumption of non-renewable, pollutant-emitting fossil fuels has been declared as a primary contributor to climate change, prompting the shift to renewable energy sources. Biodiesel has emerged as a promising, more sustainable alternative energy source, primarily formed through either the transesterification of triglycerides or the esterification of free fatty acids into fatty acid methyl esters (biodiesel). However, current biodiesel production uses refined vegetable oils as feedstock which has raised concerns due to the competition with food crops and contributes a staggering 90% to the overall cost of production. Additionally, a major hurdle in the sustainability of biodiesel production is the current basic homogeneous catalysts employed react with free fatty acids in oils to produce soap, significantly contaminating the product and requiring extensive purification steps, resulting in substantial wastewater. For this reason, heterogeneous catalysts have garnered interest as more suitable alternatives. As well as being able to perform the transesterification of triglycerides, acidic catalysts also catalyse the esterification of free fatty acids present in most non-edible and waste cooking oils into biodiesel. In this study, a lignin-derived heterogeneous catalyst converted over 97% of oleic acid - a representative of free fatty acids - into biodiesel under optimal conditions. A basic heterogeneous catalyst was also synthesised for the transesterification of canola oil - a representation of triglycerides - into biodiesel under optimal conditions. The combination of these catalysts can be integrated in a two-step conversion of waste-cooking oil into biodiesel. Lignin is currently considered a waste material, being a by-product of the paper/pulp industry, with 98% being burnt for energy. The upcycling of lignin into a value-added product to convert waste cooking oil offers increased sustainability as well as access to environments and economies that are limited in resources and financials. Overall, the replacement of the standard basic homogeneous catalysts with solid heterogeneous ones overcomes the difficulty of separation and the employment of acidic catalysts allows utilisation of non-edible and waste cooking oils, eliminating majority of production costs and competition with the agriculture market.