O efeito de antioxidantes sintéticos, como BHT (butil-hidroxitolueno), BHA (butilhidroxianisol) e TBHQ (t-butil-hidroxiquinona), foi avaliado sobre o período de indução de uma amostra de ésteres etílicos de óleo de soja de baixa estabilidade à oxidação, empregando o método Rancimat (EN14112). Efeitos de interação também foram investigados empregando-se um planejamento fatorial, que teve como níveis mínimos e máximos as concentrações de zero e 1500 ppm de cada antioxidante. Dentre os antioxidantes sintéticos avaliados, o BHT apresentou maior eficácia nas concentrações compreendidas entre 200 e 7000 ppm. O TBHQ apresentou maior potencial estabilizador quando utilizado em concentrações mais elevadas, enquanto que o BHA mostrou-se pouco efetivo nesta amostra, sendo que em concentrações superiores a 2000 ppm, não proporcionou qualquer acréscimo do período de indução. O uso combinado destes antioxidantes, nos níveis testados neste estudo, não evidenciou qualquer efeito sinérgico positivo que justifique o seu uso em misturas binárias ou ternárias.The effect of synthetic antioxidants such as BHT (butyl-hydroxytoluene), BHA (butylhydroxyanisol) and TBHQ (t-butyl-hydroxyquinone) was evaluated on the induction time of soybean oil ethyl esters with low oxidation stability, employing the Rancimat method (EN14112). Interaction effects were also investigated by employing a 2 3 factorial design, which had as minimum and maximum levels the concentrations of zero and 1500 ppm of each antioxidant. Among the synthetic antioxidants evaluated, BHT displayed the highest effectiveness in the concentration range from 200 to 7000 ppm. TBHQ displayed a greater stabilizing potential when used at higher concentrations (8000 ppm), while BHA was found to provide no noticeable increase in the induction time at concentrations greater than 2000 ppm. The combined use of these antioxidants, at the levels tested in this study, yielded no evidence of any positive synergic effect that would justify their use in binary or ternary mixtures.
Biodiesel can be obtained from fatty acid raw materials through esterification. The reactivity of lauric, palmitic, stearic, oleic, and linoleic fatty acids with methanol using powdered niobic acid as a heterogeneous catalyst was investigated in this work, both experimentally (in a batch reactor) and theoretically. A 23 experimental design was used, with methanol/fatty acid molar ratio, catalyst concentration, and temperature as main factors. An empirical model demonstrated that temperature is the most important variable. Fourteen heterogeneous and 56 homogeneous‐like kinetic models were tested. A homogeneous‐like model considering zero order for all species and inhibition by water was the most adequate for experiments without catalyst. A homogeneous‐like model considering a second‐order reaction in relation to the fatty acid and no water inhibition was the most adequate for niobic acid catalyzed reaction. Molecular modeling confirmed the experimental results showing that the reactivity is directly related to the increase of unsaturated bonds and the reduction of carbon chain length. The polarity of the fatty acid is determinant in the reactivity. At the molecular level, reaction occurs between the HOMO orbital of methanol and LUMO orbitals of fatty acids and reactivity is higher when the energy difference between these orbitals is lower. Copyright © 2010 John Wiley & Sons, Ltd.
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