Exploration on the synergistic effect of antioxidants for coal-to-liquid base oil: From the perspective of oxidation resistance and thermal stability

•The compound antioxidant systems for coal-to-liquid (CTL) was firstly explored.•Test method was developed to evaluate the performance of antioxidants in CTL.•Compound antioxidants enhance the activation energy of thermal degradation of CTL.•The synergy mechanism of compound antioxidants on CTL was revealed. Coal-to-liquid (CTL) base oil is a high-quality lubricant base oil made from coal. However, its poor oxidation stability and short service life limit its application. Pressurized differential scanning calorimetry and Rotary bomb oxidation test are utilized for the evaluation of oxidation stability. Thermogravimetric analysis is performed for the thermal stability assessment. The results showed the reasonable combination of commercial antioxidants can significantly improve the oxidative and thermal stability of two typical CTL base oils, CTL3 and CTL6. The synergistic mechanism of antioxidants in CTL base oil was studied from the perspective of pyrolysis kinetics and pyrolysis mechanism. For CTL3, adding the compound antioxidant 2,6-di-tert-butyl-4-methylphenol (T501) and n-phenyl-1-naphthylamine (T531) results in an almost eightfold increase in oxidation induction time (OIT) and a 33.4 % increase in activation energy compared to pure CTL3. For CTL6, adding the compound antioxidant dialkyl dithiophosphate (T203) and diphenylamine (L57) results in an almost tenfold increase in OIT and a 10.9 % increase in activation energy compared to pure CTL6. The designed commercial antioxidant compound systems had excellent synergistic effects and the synergistic mechanisms were illustrated. [Display omitted]