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The relationship between the structure characteristics and performances of coal-based hydrogenation isomeric (CTL) base oil and metallocene-catalyzed coal-based poly-alpha-olefin (mPAO) base oil is clarified in this paper. CTL and mPAO were compared with typical petroleum-based and natural gas-based commercial API III and IV base oils. Pressurized differential scanning calorimetry (PDSC), the rotary bomb oxidation test (RBOT), and a four-ball friction tester were used to evaluate the oxidation stability and lubrication performance of base oils under different working conditions. The sensitivity of different base oils to typical antioxidants and extreme-pressure antiwear agents was compared. In particular, the composition and structure of CTL base oil are clearly different from GTL and mineral base oil. The coal-based CTL and mPAO base oils exhibit commendable viscosity–temperature properties, coupled with low-temperature fluidity, fire safety, and minimal evaporation loss. The lubricating properties, oxidation stability, and sensitivity to extreme-pressure antiwear agents of CTL are close to those of similar base oils. However, the sensitivity of CTL to typical antioxidants is relatively poor. In addition, compared with commercial PAO base oil, mPAO has a lower isomerization degree and fewer isomerization types. The oxidation stability and sensitivity to typical antioxidants of mPAO base oil are comparable with those of commercial PAO base oil, while its lubrication performance and sensitivity to typical extreme-pressure antiwear agents are significantly better than those of commercial PAO base oil. [ABSTRACT FROM AUTHOR]

The relationship between the structure characteristics and performances of coal-based hydrogenation isomeric (CTL) base oil and metallocene-catalyzed coal-based poly-alpha-olefin (mPAO) base oil is clarified in this paper. CTL and mPAO were compared with typical petroleum-based and natural gas-based commercial API III and IV base oils. Pressurized differential scanning calorimetry (PDSC), the rotary bomb oxidation test (RBOT), and a four-ball friction tester were used to evaluate the oxidation stability and lubrication performance of base oils under different working conditions. The sensitivity of different base oils to typical antioxidants and extreme-pressure antiwear agents was compared. In particular, the composition and structure of CTL base oil are clearly different from GTL and mineral base oil. The coal-based CTL and mPAO base oils exhibit commendable viscosity–temperature properties, coupled with low-temperature fluidity, fire safety, and minimal evaporation loss. The lubricating properties, oxidation stability, and sensitivity to extreme-pressure antiwear agents of CTL are close to those of similar base oils. However, the sensitivity of CTL to typical antioxidants is relatively poor. In addition, compared with commercial PAO base oil, mPAO has a lower isomerization degree and fewer isomerization types. The oxidation stability and sensitivity to typical antioxidants of mPAO base oil are comparable with those of commercial PAO base oil, while its lubrication performance and sensitivity to typical extreme-pressure antiwear agents are significantly better than those of commercial PAO base oil.

Coal-to-liquid (CTL) base oil, synthesized by Fischer–Tropsch process, has a good prospect for synthetic high-grade lube oil. However, the oxidation induction period of CTL base oil is short, and the oxidation stability is slightly worse. Oxidation stability determines whether the lubricating oil deteriorates during use, which directly affects the service life and high temperature performance of lubricating oil. In this study, the influence of antioxidant additives on the thermal oxidative degradation of CTL base oil was examined. The addition of diphenylamine (L57) and 2,6-di-tert-butyl-4-methylphenol (T501) can inhibit the formation of carbonyl group (C=O), and the oxidation stability of CTL base oil is significantly improved. The results of thermodynamic analysis show that L57 can obviously improve the thermal decomposition performance of CTL base oil. The molecular structure plays an important role in the activation energy for the oil samples. CTL base oil model was established by the experiment result of 13C NMR. Molecular dynamics simulation studies show that the effect of antioxidant can be quantified and compared by the diffusion of antioxidants in CTL base oil. The smaller the diffusion coefficient of the antioxidant in CTL base oil is, the smaller the free volume of O2 in CTL base oil and antioxidation system is, and the better the antioxidation performance is. This study hopes to provide some guidance for the application of CTL base oil in high-grade lube oil market. [ABSTRACT FROM AUTHOR]

• 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] [ABSTRACT FROM AUTHOR]