In this study, four kinds of Brønsted acid ionic liquids ([PrMIM][HSO4]) , ([PrSO3HMIM][HSO4]), ([BMIM][HSO4])and ([BSO3HMIM][HSO4]) were synthesized by two-step method. Their structures were characterized by FTIR and TGA. And then, Brønsted acid ionic liquids were used as solid base catalyst to catalyze oil conversion and synthesis of biodiesel, the effects of molar ratio, reaction temperature, reaction time, catalyst amount and the deactivation of water on catalytic activity were investigated. The detection and analysis showed that their structures are in accordance with their theoretical structure. Their decomposition temperatures were higher than 200 ℃, so they were enough stable as the catalyst in synthesis of biodiesel. Brønsted acid ionic liquid with long alkyl chains and sulfonated groups exhibited the best catalytic activities due to their strong Brønsted acidity. Under the conditions of molar ratio of methanol to oil at 12∶1, mass fraction of ionic liquid([BSO3HMIM][HSO4]) 2%(w), reaction temperature 130 ℃ and reaction time 3 h, the conversion rate of biodiesel can reach more than 99%. Water molecules competed with the anion to bind with the protons of the imidazolium cation, which will destroy the structure of the ionic liquid and lead to inactivation. While increasing the reaction temperature could alleviate this negative effect of water. The conversion rate of biodiesel remained constant 85% at 130 ℃, even when the water content reaches 5%(w).
Key words
biodiesel /
ionic liquid /
transesterification /
sulfonate
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