Acidic ionic liquid-catalyzed esterification of oleic acid for biodiesel synthesis

Abstract

Acidic ionic liquids (ILs) are used as environmentally-friendly and promising acid catalysts for biodiesel synthesis owing to their beneficial characteristics such as high catalytic activity, high selectivity, and ease of recycling. In this paper, seven different acidic ILs were examined as catalysts in the synthesis of biodiesel from the esterification of oleic acid with methanol. It was found that the stronger the acidity of the IL, the higher its esterification activity. The introduction of a SO3H group into the IL greatly increases its Brönsted acidity and results in a bifunctional nature of the ILs for use as either a catalyst or environmentally-friendly solution in the esterification reaction. All of these effects contribute to product formation. Of all the tested acidic ILs, 1-sulfobutyl-3-methylimidazoliumhydrosulfate ([BHSO3MIM]HSO4) exhibited the best catalytic performance. The [BHSO3MIM]HSO4-catalyzed esterification of oleic acid with methanol was systematically explored, and the reaction conditions were optimized using a response surface methodology. The optimum molar ratio of methanol to oleic acid, catalyst amount, reaction temperature, and reaction time were 4:1, 10% (based on the mass of oleic acid), 130 °C, and 4 h, respectively, under these conditions, and a yield of methyl oleate (biodiesel) of 97.7% was achieved. Furthermore, [BHSO3MIM]HSO4 retained around 95.6% of its original catalytic activity after 10 successive reuses (4 h per period of use), showing excellent operational stability. In addition, the use of [BHSO3MIM]HSO4 for biodiesel synthesis from waste oils containing 72% of free fatty acids was examined, and yields as high as 94.9% after 6 h were obtained. Clearly, [BHSO3MIM]HSO4 shows considerable potential for the synthesis of biodiesel.