The synthesis of polyethylene terephthalate PET from terephthalic acid TPA and ethylene glycol EG is typically carried out in two steps: esterification and polycondensation. The reaction is catalyzed by various metal compounds, with antimony trioxide Sb2O3 and titanium compounds e.g., titanium isopropoxide being the most common catalysts. The appropriate concentration and amount of catalyst depend on the specific process conditions and desired properties of the final product.1. Esterification: In the first step, TPA and EG react to form bis 2-hydroxyethyl terephthalate BHET . The esterification reaction is typically carried out at temperatures between 240-260C and atmospheric pressure. The molar ratio of EG to TPA is usually around 1.1-1.5:1 to ensure complete conversion of TPA to BHET. A catalyst is not always necessary for this step, but if used, a small amount of a titanium compound e.g., 50-200 ppm can be added to accelerate the reaction.2. Polycondensation: In the second step, BHET undergoes polycondensation to form PET. This reaction is typically carried out at temperatures between 270-290C and under vacuum 0.1-1.0 mmHg to remove the EG byproduct. The most common catalyst for this step is antimony trioxide Sb2O3 , with a typical concentration of 100-300 ppm based on the weight of TPA. Titanium compounds can also be used as a co-catalyst in combination with Sb2O3, with a concentration of 50-200 ppm.It is important to note that the optimal concentration and amount of catalyst may vary depending on the specific process conditions, equipment, and desired properties of the final PET product. Therefore, it is recommended to perform experiments and/or consult the literature to determine the best catalyst system for a given application. Additionally, the use of catalysts may have an impact on the color and stability of the final PET product, so it is essential to consider these factors when selecting a catalyst system.