Glycerol-Vanillin Acetals as Green and Efficient Antioxidants

Acetals were synthesized upon the acid-catalyzed reaction of glycerol with vanillin, ethyl vanillin, cinnamaldehyde, and 1-napthaldehyde using Amberlyst-15 as a solid acid catalyst. The acetals were successfully isolated, characterized, and evaluated for their antioxidant activity using the DPPH radical scavenging assay. The antioxidant efficiency, measured as the effective concentration (EC₅₀), was 2.1 mol per mol of DPPH for the ethyl vanillin-glycerol acetal (EVGA) and 5.5 mol per mol of DPPH for VGA, significantly superior to the values found for the other glycerol acetals. These values are close to BHT, a fossil-derived commercial antioxidant, which showed an EC₅₀ of 1.0 mol per mol of DPPH under the same test conditions. Vanillin and ethyl vanillin presented EC₅₀ values of 21.5 and 32.6, respectively. EVGA and VGA were tested as antioxidants in soybean biodiesel, presenting an increased induction period in the racimat test relative to the neat biodiesel. DFT calculations showed a decrease in the bond dissociation energy (BDE) of the phenolic OH of EVGA and VGA with respect to vanillin and ethyl vanillin, explaining their superior antioxidant performance. In addition, the methine C-H bond of the acetal group could potentially act as an antioxidant center according to DFT calculations. EVGA and VGA acetals are promising green antioxidants, obtained from biomass-derived raw materials, that could be used in food, fuel, and materials applications, as substitutes for BHT.