Improving oxidative stability of biodiesel by cis-trans isomerization of carbon-carbon double bonds in unsaturated fatty acid methyl esters

Biodiesel is a renewable fuel source comprising of fatty acid methyl esters (FAMEs). However, biodiesel (FAMEs) is susceptible to autoxidation. Therefore, structural modification of FAMEs through decreasing the number of double bonds (e.g., hydrogenation) or trans isomerization of original double bonds in the cis configuration, could improve the oxidative stability of FAMEs. In this work, the trans isomerization of carbon-carbon double bonds in unsaturated FAMEs obtained from edible oils has been successfully achieved using simple and safe p-toluenesulfinic acid as catalyst. And the effects of reaction time, temperature, catalyst loading, amount of water and antioxidants on the trans isomerization were investigated systematically. The results indicated that the amount of water (0-20 wt%) and antioxidants (< 200 ppm) had no significant effect on the trans isomerization of unsaturated FAMEs. And the final product (trans FAMEs) was characterized by both GC and FT-IR. The conversion of trans double bonds of the isomerized lard FAMEs could reach 79.9% after heating at 100 degrees C for only 30 min without changing both the double bonds location and unsaturation degree. In addition, this convenient method was successfully applied in a variety of FAMEs prepared from different vegetable oils (e.g., safflower seed oil, camellia seed oil, high oleic sunflower oil and soybean oil) and approximately 80.0% selectivity of trans isomerization for all FAMEs were obtained. The present study showed a promising method to provide trans-FAMEs products for producing biodiesels with better oxidative stability (for high oleic FAMEs) and without adding of antioxidants.