Abstract

Large amounts of free fatty acids in oil used for the production of biodiesel cause the problems of soap formation and very low yield of biodiesel. To overcome these problems, free fatty acids must be esterified to their esters in the presence of an acid catalyst prior to alkaline-catalyzed transesterification. Sulfated metal oxides are an interesting group owing to their very high acidity. In this study, sulfated aluminium-tin mixed oxide (SO<inf>4</inf>^2-/Al<inf>2</inf>O<inf>3</inf>-SnO<inf>2</inf>) catalysts were synthesized and used for esterification of free fatty acids in crude palm oil for the first time. The SO<inf>4</inf>^2-/Al <inf>2</inf>O<inf>3</inf>-SnO<inf>2</inf> catalysts were prepared from different Al precursors, calcination temperatures, and sulfate concentrations. Characterization of the catalysts included X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques, nitrogen adsorption, Fourier transform infrared spectroscopy (FTIR), and potentiometric titration. The results show that the SO<inf>4</inf>^2-/Al<inf>2</inf>O<inf>3</inf>-SnO<inf>2</inf> catalyst has very strong acid sites. Different Al precursors lead to different activities of the SO<inf>4</inf>^2-/Al<inf>2</inf>O <inf>3</inf>-SnO<inf>2</inf> catalysts. Al<inf>2</inf>(SO<inf>4</inf>) <inf>3</inf> is the best precursor whereas Al(NO<inf>3</inf>)<inf>3</inf> is the worst one to prepare the SO<inf>4</inf>^2-/Al<inf>2</inf>O <inf>3</inf>-SnO<inf>2</inf> catalyst. The optimum calcination temperature is 450°C. The activity of the SO<inf>4</inf>^2-/Al<inf>2</inf>O <inf>3</inf>-SnO<inf>2</inf> catalyst increases with sulfate concentration. Introducing Al<inf>2</inf>O<inf>3</inf> to SO<inf>4</inf>^2-/SnO <inf>2</inf> increases its stability during the reusability process. Therefore, SO<inf>4</inf>^2-/Al<inf>2</inf>O<inf>3</inf>-SnO<inf>2</inf> is a better candidate than SO<inf>4</inf>^2-/SnO<inf>2</inf> for catalyzing the esterification of free fatty acids in crude palm oil. © Copyright Taylor & Francis Group, LLC.