An investigation has been carried out to characterize and evaluate phenolic compounds of bio-oils produced by the microwave enhanced pyrolysis of spruce woodchips (picea abies) for their potential application in stabilizing biodiesel from autoxidation. Four extracts were isolated from the bio-oil through multi-fractionation steps using a liquid–liquid extraction method: water-soluble, neutral, phenolic and organic acids extracts. The crude bio-oil and the isolated extracts were characterized by GC-MS, GC-FID, total phenols by Folin–Ciocalteu assay, ATR-IR and 13C NMR. The antioxidative effect of the crude bio-oil, its isolated extracts and two significant phenolic components (eugenol and catechol) of the crude bio-oil were also investigated using methyl linoleate as a biodiesel model by means of a high temperature (120 °C) oxidation test. The results show that methyl linoleate induction time increased after blending small amounts (1.4–5.6% w/w) of either the crude bio-oil or the isolated extracts. However, the crude bio-oil showed higher induction times in comparison with its isolated extracts, which was significant because the crude bio-oil contained a lower concentration of phenolic species (23% w/w), especially in comparison to the phenolic concentration in the phenolic extract (49.6% w/w). Furthermore, catechol was found to be very effective and was similar to crude bio-oil in the inhibition of methyl linoleate autoxidation, unlike eugenol, which was less effective at equivalent molar concentrations. Also, the effect of catechol and the crude bio-oil on methyl linoleate induction time was approximately comparable with a commercial antioxidant (butylated hydroxytoluene) when treated at equivalent molar concentration of phenols.

•Methyl linoleate & squalane were used to model biodiesel & engine oil degradation.•Conditions of temperature used were comparable to those in engines.•At low temperatures (below 160 °C) methyl linoleate promotes lubricant oxidation.•At higher temperatures (above 160 °C) methyl linoleate shows antioxidant character.•This is due to O2 addition to doubly allylic radicals being reversible above 160 °C.The effect of low concentrations of methyl linoleate on the autoxidation of squalane (2, 6, 10, 15, 19, 23 hexamethyltetracosane) was investigated at temperatures 100 to 170 °C as a chemical model of the effect of biodiesel on the degradation of hydrocarbon lubricants during use. Below 158 ± 5 °C, methyl linoleate behaves as a pro-oxidant, whilst above 158 °C, it inhibits alkane autoxidation. This change of mechanism is consistent with the addition of oxygen molecules to doubly allylic carbon centred radicals formed by hydrogen abstraction from methyl linoleate becoming reversible above 158 °C.