Abstract
Solid acid catalysts consisting of zirconium sulfate (ZS) supported on a series of molecular sieves including HZSM-5, HMS and MCM-41 were prepared and used in the esterification of citric acid with n-butanol to produce tributyl citrate (TBC). Highly dispersed ZS was supported on 40wt.% ZS/HMS or ZS/MCM-41, whereas HZSM-5 loaded more bulk ZS species. The ZS loading and type of support showed obvious effect on thermal stability, acidity and type of acid sites. In the esterification of citric acid, 40wt.% ZS/HMS or ZS/MCM-41 exhibited higher catalytic activity than pure ZS or 40wt.% ZS/HZSM-5. An investigation focused on ZS/HMS showed that Brønsted and Brønsted+Lewis acid sites acted as the active center for the esterification reaction. 40wt.% ZS/HMS calcined at 180°C obtained the highest conversion of citric acid and TBC yield. The reaction was found to obey an Eley–Rideal mechanism.
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