Biodiesel production through esterification applying ionic liquids as catalysts

Abstract

Biodiesel is a liquid fuel obtained from several renewable sources by transesterification reaction of triglycerides. Its development is related to a new tendency: the search for alternatives to petroleum-based energy sources. Its utilization is associated with several environmental benefits, such as a reduction in pollutants emissions. However, due to the high cost associated to its usual feedstock, such as edible vegetable oils, biodiesel is not economically viable. Therefore, there’s a requisite to decrease the final price of this fuel. The logical way is by introducing cheaper feedstock into the industrial production, such as non-edible feedstock or waste cooking oil. The main characteristic of cheaper feedstock is the high content of free fatty acids (FFAs) and/or water when compared to edible feedstock. FFAs present on the feedstock must be converted into biodiesel, also referred to as fatty acid methyl esters (FAMEs), by an esterification reaction. The esterification reaction cannot be catalyzed by alkali catalyst, usually applied in the transesterification such as NaOH or KOH. Alkali catalysts in the presence of FFAs lead to the formation of soap, consuming the catalyst, decreasing its catalytic activity and turning the separation of the final products much more complex. Hence, only acidic catalysts are able to promote the esterification reaction of FFAs. Those acidic catalysts are able to catalyze both reactions, however, the rate of the transesterification reaction is about 4000 times slower than for the reactions promoted by alkali catalysts [1,2], leading to long reaction times and, again, high costs. In this way, there is an increasing need to find alternative catalysts that promote both the transesterification and the esterification reaction under adequate conditions. Thus, ionic liquids emerge as an alternative to conventional catalysts. Ionic liquids are molten salts composed of an organic or inorganic anion and an organic cation. The present study evaluated the use of the catalyst 1-methylimidazolium hydrogen sulfate ([HMIM][HSO4]) in the production of biodiesel through the esterification reaction of oleic acid. The influence of the main parameters (time, temperature, molar ratio methanol/oleic acid and catalyst dosage) on two responses (conversion of oleic acid and FAME content of the viii biodiesel samples) were studied through a response surface methodology (RSM) known as Box-Behnken Design (BBD). It was concluded that the most relevant parameters for both responses were the molar ratio between the reactants and the catalyst dosage. The optimum conditions for the conversion were determined as being 8 h, 110 °C, 15:1 molar ratio methanol/oleic acid and a catalyst dosage of 15 wt%, resulting in a 95% conversion and for the FAME content were 8 h, 110 °C, 14:1 molar ratio and a catalysts dosage of 13.5 wt%, leading to a content of 90%. The kinetics of the reaction were also studied. The activation energy was estimated as 6.8 kJ/mol and the pre-exponential factor as 0.0765 L2.mol-2.min-1.