The effect of metals and metal oxides on biodiesel oxidative stability from promotion to inhibition

Authors: Knothe, Gerhard; Steidley, Kevin

Submitted to: Fuel Processing Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/11/2018
Publication Date: 4/26/2018
Citation: Knothe, G., Steidley, K.R. 2018. The effect of metals and metal oxides on biodiesel oxidative stability from promotion to inhibition. Fuel Processing Technology. 177:75-80.

Interpretive Summary: Fuels are sensitive to the conditions they are exposed to during storage and transportation. One of these conditions affecting biodiesel, a biogenic alternative to conventional diesel fuel derived from petroleum, is exposure to air. The oxygen in the air can react with some components of the biodiesel, the so-called unsaturated fatty acid methyl esters, to give undesired products. This eventually causes fuel quality deterioration, thus being a significant issue affecting the commercial use of biodiesel. A major factor potentially accelerating this so-called oxidation reaction is the exposure of the biodiesel to metals, for example, in metal containers used for storage. While it has been known that some metals accelerate oxidation, a comprehensive study on the effect of numerous metals had not been carried out. This work reports such an investigation and also includes so-called metal oxides, as these are common forms in which metals are found in nature and they have been used as so-called catalysts in the production of biodiesel. Besides copper, other metals accelerating oxidation are some metals, such as molybdenum and rhenium, which retard oxidation. Metal oxides accelerating oxidation include chromium and copper, as well as several other oxides including molybdenum inhibiting oxidation. These results are important to assess the possible effects of biodiesel storage as well as the effect of possible remaining catalysts on biodiesel used in its production.

Technical Abstract: Biodiesel, usually the methyl esters of plant oils or other triacylglycerol-containing materials, has become an established alternative to conventional, petroleum-derived diesel fuel. Several technical problems persist when using biodiesel, one of which is oxidation stability upon exposure to oxygen in air. Factors affecting oxidation stability are not only the composition of the fuel with polyunsaturated fatty acid methyl esters being more susceptible to oxidation but also temperature, presence of light, storage headspace, and presence of extraneous materials such as pro- and antioxidants, metals, and other substances. In case of metals, it has been stated/shown that some metals are especially effective in promoting oxidation of biodiesel, especially copper. In this work, studies on the effect of metals are expanded to include a total of 26 metals for their effect on biodiesel in form of soybean oil methyl esters. The metals included in this study are Ag, Al, Au, Co, Cr, Cu, Fe, In, Mg, Mn, Mo, Nb, Ni, Pd, Re, Rh, Ru, Sc, Si, Sn, Ta, Ti, V, W, Zn and Zr. Besides these neat metals, the oxides of 29 metals were studied. These oxides are those of Ag (I), Ag (II), Al (III), Au (III), Ba (II), Ca (II), Co, Cr (III), Cr (VI), Cu (II), Eu (III), Fe (II), Fe (III), In (III), La (III), Mg (II), Mn (III), Mn (IV), Mo (IV), Mo (VI), Nb (II), Nb (IV), Nb (V), Ni (II), Sc (III), Si (IV), Sn (IV), Sr (II), Pd (II), Re (VI), Re (VII), Ru (IV), Ta (V), Ti II), Ti (III), V (V), W (IV), W (VI), Zn (II), and Zr (IV). An experimental procedure, the Rancimat method, which is included in biodiesel standards, was used. The results confirm that CU indeed is the metal most strongly promoting oxidation, while most other metals gave results in which the induction time in the Rancimat test did not differ widely from that of the used biodiesel. The metal oxides usually show similar effects on biodiesel oxidation from that of the widely used biodiesel. Some metals, notably Mo and Re, showed an inhibitory effect on biodiesel oxidation. The metal oxides usually show similar effects on biodiesel oxitation from promotion to inhibition with oxides of Ag, Au, Cr, Cu, Re and Ru showing the greatest promoting effect and oxides of Mo the greatest inhibiting effect.