Influence of Biodiesel on Base Oil Oxidation as Measured by FTICR Mass Spectrometry

Internal combustion engine lubricants are subject to thermo-oxidative degradation during use and must be designed to withstand oxidation in order to extend their useful life. Understanding the complex chemical process of thermo-oxidative degradation is essential to designing higher performing engine lubricants. In this study base oil samples composed of a Group II base oil, doped with three different levels of biodiesel (B0, B15, and B100), were subjected to benchtop oxidation testing of up to 168 h, which mimics the conditions experienced in an internal combustion engine. The resulting samples were analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) for ultrahigh-resolution characterization to monitor oxidation as a function of time and biofuel content. Both negative-ion nanoelectrospray ionization and positive-ion atmospheric pressure photoionization were utilized. Most of the oxidation products were found to be polyoxygenated species containing 1-8 oxygen atoms, with the number of detected species increasing with oxidation time. Assessment of the maximum carbon number of protonated classes indicated the involvement of oligomerization reactions; additionally, modeling of mean double bond equivalents (DBE) for each protonated class suggests increasing carbonyl content for each particular class with increasing oxidation time. The oxidations of B15 and B100 doped samples were compared to that of B0. BIS samples were found to correspond closely to B0 samples, with a similar number of species detected. B100 samples showed a significant increase in number of species generated at 24-72 h relative to B0 and B15; however, a similar number of species were observed at 168 h for all samples, indicating a similar level of base oil oxidation at the final oxidation point. FTICR MS is shown to afford new insights into base oil oxidation as a function of time and biofuel content.