Synthesis of Fatty Acid Methyl Esters from Pomace Oil Catalyzed by Zinc Stearate: A Kinetic Study of the Transesterification and Esterification Reactions

Abstract

In this work, the simultaneous transesterification and esterification reactions of olive pomace oil with methanol catalyzed by zinc stearate were studied. This catalyst is a crystalline solid at room temperature, but it is soluble in the reaction medium at reaction temperature. Zinc stearate has surfactant properties that cause the formation of an emulsion in the reaction system. The stability of the emulsion formed in the oil-methanol-catalyst system was compared to that in the FAME (fatty acid methyl esters)-methanol-catalyst system. It was observed that the emulsion formed in the presence of high amounts of FAME is much more unstable, which makes the catalyst easy to separate from the reaction products. The kinetics of esterification and transesterification were also studied. All the kinetic and equilibrium constants were determined with a complete model, considering the three stepwise reactions corresponding to the transesterification of triglycerides and the esterification of free fatty acids. The parameters obtained were used to model the operating conditions that would allow obtaining biodiesel that meets the quality standards.Catalysts 2019, 9, 978 2 of 15 not suitable for human consumption due to its high acidity, its color, and its odor, among other qualities [6,7]. In a previous work in our research group [8], raw pomace oil was studied as raw material in biodiesel production using zinc stearate as catalyst. At 140 • C, with 3% catalyst loading and a methanol/oil molar ratio of 30, 98% and 67% of triglyceride and free fatty acids (FFA) conversions and 84% of FAME yield were achieved.There are studies on the production of biodiesel from pomace oil using different catalysts. Yüsel [9] reported the use of Thermomyces lanuginosus lipase on olive pomace powder as a catalyst for biodiesel production. The maximum FAME yield obtained was 93% at 25 • C in 24 h. also used an immobilized lipase as catalyst to obtain biodiesel from alperujo oil with a high degree of acidity. They obtained a FAME yield of 29% after 6 h of reaction at 30 • C. Both studies added methanol in three stages to avoid the inhibition of lipase.As for homogeneous catalysis, Hernández et al.[11] studied the transesterification of pomace oil catalyzed by KOH. They achieved a FAME yield of 95% at 60 • C in 1 h of reaction time. On the other hand, Lama-Muños et al.[6] studied a two-stage process that included acid esterification with H 2 SO 4 followed by base transesterification using NaOH, obtaining a maximum FAME yield of 95% at 60 • C after 80 min of reaction.In the process of biodiesel production with acid raw materials, it is desirable to have an active catalyst in both the transesterification and esterification reactions, as well as an easy separation of the catalyst from the reaction medium.In a previous work carried out by our research group [12], the use of zinc carboxylate salts in the esterification of oleic acid and in the transesterification of refined soybean oil with methanol was studied. Specifically, zinc laureate (LaZn), zinc ...

Emerging catalysis in biomass valorisation: simple Zn(II) catalysts for fatty acids esterification and transesterification

“…Experimental data adhered to a kinetic model which considered the presence of emulsions instead of a pseudo‐homogeneous regime. A year later, a kinetic study by the same group, [84] on the transesterification of a raw pomace oil (acidity of 2.3 mgKOH/g) confirmed the hypothesis. The authors again observed that at 140 °C Zn(stearate) 2 partially transforms in ZnGly within 2 h. For shorter reaction times (30 min) the catalyst remains unchanged.…”

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