A one-pot route to tunable sugar-derived sulfonated carbon catalysts for sustainable production of biodiesel by fatty acid esterification

Carbon-based solid acid catalysts possessing up to 1.29 mmol g−1 –SO3H active centers were synthesized from glucose via an efficient one-pot hydrothermal carbonization-sulfonation without the need for high temperatures. Catalysts combined aromatic structure with hydrophilic –OH and –COOH groups and a high density of –SO3H centers (up to a total acid density of 5.31 mmol g−1). The level of –SO3H (0.81–1.29 mmol g−1) proved synthetically tunable. The relevance of the catalyst to the production of more sustainable fuels was tested using oleic acid (a free fatty acid whose esterification can be employed as a model for biodiesel production). Optimizing catalyst and conditions (20:1 MeOH:oleic acid molar ratio, 5 wt % catalyst loading wrt oleic acid, 80 °C, 120 min) enabled oleic acid esterification to the corresponding methyl oleate (a biodiesel component) with 97.5 ± 0.4% conversion and a low 37.6 kJ mol−1 activation energy. Activity loss upon re-use of the catalyst was proved to be by de-sulfonation and could be completely reversed. Hence, re-sulfonation of spent catalyst by a one-step process again delivered a 97.4 ± 0.5% conversion of oleic acid.