Adding Yttrium to SO₄ ^2-/ZrO₂ Catalysts To Improve the Synthesis Performance of Esters for Electrical Insulating Oils

Synthetic ester insulating oils exhibit excellent electrical insulating properties, biodegradability, and oxidative stability, rendering them attractive alternatives to conventional mineral- and vegetable-based insulating oils. Typically, synthetic ester insulating oils are prepared via esterification reactions between fatty acids and alcohols using various catalysts. In particular, the solid acid catalyst SO₄ ^2-/ZrO₂ is commonly used in this process. In this study, a small amount of yttrium was substituted into a ZrO₂-catalyst support to form oxygen vacancy structures in the framework. This modification was intended to alter the interactions with SO₄ ^2- ions and improve both the distribution of acid sites and catalytic activity for esterification. Using the sol-gel method, the synthesized catalyst was calcined at 600 °C for 3 h to promote crystallization. The structural and surface properties of the catalyst, as well as its chemical states, thermal stability, electronic structure, and acidity, were investigated using various advanced characterization techniques, including XRD, XPS, and NH₃-TPD. The catalytic performance was evaluated through the esterification of fatty acids (decanoic acid (capric acid) and 2-ethylhexanoic acid) and alcohol at 220 °C. Among the fabricated catalysts, the SO₄ ^2-/ZrO_2(0.98)-Y₂O_3(0.02) catalyst demonstrated the highest conversion of 95.5%. Thus, our developed catalyst could contribute to improving the efficiency of synthetic ester production processes for insulating oil applications.