(394a) Catalytic Effects of Transition Metals on the Oxidative Stability of Various Biodiesels

In the vegetable oils, oxidation can lead to the formation of high molecular weight species. Biodiesel is produced by transesterification of these same oils and has similar susceptibility to oxidation. Thus the high molecular weight species produced by oxidation may cause problems in automotive fuel delivery systems. We report the effects of Group IV transition metals on the oxidative stability of biodiesel and biodiesel blends. Transition metals have the ability to catalyze free radical autoxidation of unsaturated fatty acid methyl esters. These molecules contain highly reactive bis-allylic hydrogen which can form free radicals. These free radicals then react with oxygen to form hydroperoxides. Catalytic metals function by accelerating the decomposition of hydroperoxides, thus accelerating the rate of autoxidation. Potential sources of catalytic metals are: 1) the raw oils, 2) metals introduced during manufacture / storage, and 3) metals present in the ultra low sulfur diesel (ULSD).

The objectives of this study are to determine the catalytic effects of group IV transition metals on the oxidative stability of biodiesel. Group IV transition metals were selected because they are common industrial metals likely to be present in biodiesel and biodiesel blends. Soybean oil biodiesel (SBO) and cottonseed oil biodiesel (CBO) were selected to determine how changes in feedstock affect the sensitivity to metals. Two different biodiesel concentrations were investigated: 100% biodiesel (B100) and 20% biodiesel mixed with ultra low sulfur diesel (B20). These were selected to determine how changes in blend level affect the sensitivity to metals. The group IV transition metals selected were: V, Cr, Mn, Fe, Co, Ni, Cu and Zn. All metals were added to biodiesel as nitrate or chloride salts dissolved in anhydrous methanol. The oxidative stability of biodiesel was measured as induction period (IP) and determined by Rancimat® according to EN 14214. Given the variables of metals, concentration and feedstock, Δ % IP was selected as the means to report the reduction of induction period determined.

All metals tested showed catalytic activity, but the level varied greatly. Ni is a low activity metal with Δ %IP = -34% at 100 ppm in soy biodiesel, in contrast to Cr with Δ%IP = -83% at 1 ppm in soy biodiesel. The differences due to feedstock can be seen in V which is less active in soybean than cottonseed biodiesel. This may be the results of different antioxidants present in soybean oil versus cottonseed oil. Catalytic activity can be broken down into different concentration ranges. Highly active metals, such as Cu or Cr can substantially reduce IP at concentrations of ≤ 2 ppm. Less activity metals, such as Fe or Ni require concentrations above 10 ppm to significantly affect the induction period of biodiesel. It should also be noted that metals with low activity (Ni and Fe) show an approximately linear relationship between concentration and Δ %IP. While the metals with high activity (Cr and Cu) show a clearly nonlinear (logarithmic) relationship. This difference could be explained by metal salt solubility. During the Rancimat® test, low activity metals must be added in high concentration. This forms visible precipitates in the reaction vessel. High activity metals form no observable precipitates. Thus the difference between linear and nonlinear response could be the difference between an insoluble catalytic surface (low activity) and a soluble catalytic ion (high activity).

In the foods industry, chelators have been used to counteract the effects of catalytic metals. The capacity of transition metals to promote oxidation and decrease the quality of foods has been known for many decades. Organic chelators function by binding to the coordination sphere of transition metals thus reducing their ability to participate in hydroperoxide decomposition. The effect of chelators to counteract the catalytic effects of group IV transition metals will also be discussed.

From this study it was concluded that all group IV transition metals tested showed catalytic activity. The catalytic activity varies with metal species, metal concentration, feedstock and blend level. Neither feedstock is clearly superior in its ability to resist the catalytic effect of these metals. Chelators can be effective in ameliorating the negative effects of catalytic metals.