Effect of spruce-derived phenolics extracted using microwave enhanced pyrolysis on the oxidative stability of biodiesel

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.