Efficient synthesis of tributyl citrate plasticizer <i>via</i> esterification reaction using SO₄²⁻/ZrO₂–TiO₂ as catalysts

Tributyl citrate, an environment-friendly and nontoxic plasticizer, is commonly synthesized <i>via</i> the esterification of citric acid and <i>n</i>-butanol. Highly effective SO₄ ^2-/ZrO₂-TiO₂ catalysts were prepared by coprecipitation and impregnation for tributyl citrate production. The excellent conversion of 95.1% for citric acid and selectivity of 98.8% for tributyl citrate were achieved over a 3SO₄ ^2-/ZrO₂-TiO₂ (4 : 1) catalyst with Zr/Ti mole ratio ∼4/1 under optimal reaction conditions of 120 °C and acid-to-alcohol ratio of 1/5. The textural and surface properties of the catalysts were characterized by nitrogen adsorption-desorption, XRD, pyridine-FTIR, <i>etc.</i> The characterization results confirmed that the addition of TiO₂ promoted the formation of ZrO₂ tetragonal phase to a certain extent, increased the specific surface area of the catalyst, stabilized the combination of SO₄ ^2- and ZrO₂, and reduced the loss of active sites. At the same time, the 3SO₄ ^2-/ZrO₂-TiO₂ (4 : 1) catalyst has stronger acidity and higher acid content, which greatly improves the activity and stability of the catalyst. No significant decrease in the catalytic activity and selectivity were found during 10 runs. A promising solid acid catalyst with an excellent performance was developed for esterification reactions.