Inhibitory effect of antioxidants on biodiesel crystallization: Revealing the role of antioxidants

Abstract Effect of six antioxidants (butylated hydroxytoluene, butyl hydroxyanisole, di-tert-butylhydroquinone, tert-butyl hydroquinone, propyl gallate, and pyrogallic acid) on crystallization properties of biodiesel was systematically investigated. Results showed that all the six antioxidants effectively inhibited biodiesel crystallization at low temperatures as reflected by the decrease of cold filter plugging point by 1 ~ 3℃, solid fat content by 0.01 ~ 2.30%, crystallization rate from 10-4 to 10-5 ~ 10-6 k/min, crystal size from 214 × 14 ~ 357 × 43 to 71 × 14 ~ 286 × 29 (length × width, μm × μm), the absolute values of crystallization enthalpy changes by 1.5 ~ 5.5 J/g. The minimum concentrations of antioxidants for biodiesel to reach the induction period (8 h) regulated by the European Union standard were 500 mg/kg for TBHQ, PG, and PY, 1000 mg/kg for BHA and DBHQ, and 2000 mg/kg for BHT, respectively. Antioxidants were inserted between initial crystal nucleuses through hydrogen bonds. The large steric hindrance of antioxidants inhibited the clustering of initial crystal nucleuses, thus inhibiting the growth of methyl ester crystals. Biodiesel with the six antioxidants all had the same crystalline form (two-dimensional flake crystals). The weaker inhibitory effect of BHA than the other five antioxidants which didn’t present significant difference in inhibiting biodiesel crystallization was because the methoxy group of BHA cannot form a hydrogen bond well with the carbonyl group of methyl ester molecule.