Green synthesis of ester base oil with high viscosity – Part Ⅰ: Catalyst preparation, characterization, evaluation, and mechanism analysis

Abstract

High viscosity ester base oil, as an eco-friendly lubricant base oil with high performance, can significantly reduce the friction, wear, and energy consumption of large machinery. However, there is less research on the preparation of heterogeneous catalysts for synthesis of high viscosity ester oil. In this work, Sn/Zr mixed oxide catalysts were prepared for the green synthesis of ester base oil with high viscosity. This approach gets rid of additional template, surfactant, organic solvent, sulfur, nitrogen, phosphorus, and halogen, thereby avoiding the potential environmental pollution. A superior powdered catalyst MO-Sn8Zr-9.0-160-18 was developed with optimized Sn/Zr molar ratio, pH, hydrothermal temperature, and hydrothermal time of 8:1, 9.0, 160 °C, and 18 h, respectively. Results of catalyst characterization indicated that the developed catalysts were mesoporous materials with hydrophobic properties, allowing for free diffusion of macromolecules, adsorption of reactants, and desorption of water. Additionally, the specific surface area, pore volume, pore diameter, and surface hydrophobicity of catalyst were enhanced by doping Zr, so the catalysts with higher [Sn/Zr]ICP/[Sn/Zr]XPS exhibited better catalytic activity. Preformed catalyst was prepared by simple mechanical mixing method to achieve better catalyst separation. The specific surface area, acid amount, and acid strength of catalyst were indeed improved significantly after catalyst molding. By programmed heating, the preformed catalyst achieved a maximum carboxyl conversion of 97.2%. Moreover, spent catalyst could be reactivated via solvent washing, drying, calcination, and hydrogen reduction, exhibiting good reusability.