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

摘要

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), Zr (IV). As 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. Some metals, notably Mo and Re, showed an inhibitory effect on biodiesel oxidation. The metal oxides usually show similar effects on biodiesel oxidation 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.