High-yield biolubricant synthesis from waste cooking oil employing optimized dual-step transesterification

Owing to escalating ecological concerns and progressive depletion of fossil-based feedstocks, biolubricants have emerged as sustainable alternatives to mineral oil-based lubricants. In this study, a biolubricant was synthesized from waste cooking oil through a dual-stage transesterification process. In the first stage, waste cooking oil was transesterified with methanol using potassium hydroxide as the catalyst in a stirred-tank reactor to produce fatty acid methyl esters. In the second stage, these methyl esters were esterified with pentaerythritol using sodium methoxide as the catalyst to produce pentaerythritol triesters, the desired biolubricant. Pentaerythritol, a tetrafunctional alcohol, was selected to address the inherent limitations of unrefined vegetable oils, such as poor oxidative resistance and high pour points. Process optimization was performed using response surface methodology, investigating three key variables: reaction time, temperature, and molar ratio. Under optimized conditions (120 min, 140 °C, 6:1 molar ratio), the theoretical yield was 83.70%, while the experimental yield reached 84.00%. H NMR analysis confirmed the structure of the synthesized biolubricant, and its properties were benchmarked against ISO VG22. The biolubricant exhibited a viscosity index nearly double the reference (197.4 vs. 107.0), an acid number less than half the ISO limit (0.42 vs. 0.90 mg KOH·g⁻¹), and an oxidative stability of 12.62 h. The kinematic viscosity at 40 °C and 100 °C, as well as the density at 15 °C, were all within the acceptable ISO ranges. These findings confirm that the synthesized pentaerythritol triester is a high-performance, waste-derived alternative to petroleum-based lubricants.