Synthesis of tributyl citrate catalyzed by solid superacid Tm-SO_4~(2-)/TiO_2

Solid superacid catalyst TmSO2-4/TiO2 was prepared and applied to esterification reaction of citric acid with butanol and compared with other catalysts such as sulfuric acid and SO2-4/TiO2. The reaction condition including initial citric acid/ butanol molar ratio, reaction time and catalyst amount were optimized. The results showed that TmSO2-4/TiO2 exhibited better catalytic properties than other catalysts, with citric acid conversion of 944%, selectivity to TBC of 992%, ester yield of 936% and product purity of over 99%. Citric acid conversion remained up to 931% after five runcycles. It was shown that the optimum synthesis condition for TBC over TmSO2-4/TiO2 were as follows: citric acid : butanol = 1∶4(molar), reaction time 35 h, catalyst amount = 15% (wt) of the amount of starting materials.