Abstract
The transesterification of vegetable oil and/or animal fats in homogeneous alkaline medium is still the most widely used method for the production of biodiesel. However, this process requires raw materials with low acidity and moisture content to prevent undesirable side reactions such as saponification, which leads to emulsification and promotes losses in the reaction yield. Many solid compounds can be used in catalytic processes to reduce these limitations. Heterogeneous catalysts allow easy separation of the reaction media and have the possibility of reuse in several cycles. In this work, [CTA+]MCM-41 molecular sieves were synthesized and characterized by several methods (XRD, SEM, TGA and BET) to be applied in the methanolysis of soybean oil. The resulting materials were characterized as mesoporous solids of type IV with similar textural properties and thermal stability. The catalytic activity of [CTA+]MCM-41 in soybean oil methanolysis was analyzed by gel permeation chromatography (GPC) and the best solid catalyst was applied in a factorial design that was validated by Analysis of Variance (ANOVA). The oil:methanol molar ratio and the catalyst concentration were the variables with the highest statistical effects, with the latter showing a quadratic profile in relation to the response function. The best conversion was achieved at 343 K, 30 min and 3.75 wt % catalyst, which corresponded to a product with 99.2% in fatty acid methyl esters. Calcination caused a total loss in catalytic activity due to the removal of CTA+ cations from the mesoporous solids. Hence, such activity was associated with the formation of (SiO-)(CTA+) ion pairs at the surface of the solid catalyst
Highlights
The production of diesel fuels from renewable resources has increased considerably over the last decades because of environmental issues related to greenhouse gas emissions and to the dependence of developing countries on the use of fossil fuels in their energy matrix (Demirbas, 2009)
All [CTA+]MCM-41 syntheses resulted in the recovery of around 2 g of the solid catalyst after drying and this was in agreement with the amount of reactants used in the experimental procedure
The peaks related to the 100, 110 and 200 diffraction planes were indexed between 1.5° < 2θ < 5° and these seem to be due to the formation of a hexagonal phase of ordered mesopores (Grün et al, 1999; Martins and Cardoso, 2007)
Summary
The production of diesel fuels from renewable resources has increased considerably over the last decades because of environmental issues related to greenhouse gas emissions and to the dependence of developing countries on the use of fossil fuels in their energy matrix (Demirbas, 2009). Biodiesel is produced by alkaline transesterification of triacylglycerides in homogeneous media using low molecular mass primary alcohols, usually methanol or ethanol, releasing glycerol and the corresponding fatty acid alkyl esters as the reaction coproducts (Choedkiatsakul et al, 2015). This process is catalyzed by alkaline metal alkoxides or by alkali metal hydroxides that are able to generate alkaline metal alkoxides in situ (Silva et al, 2013). Heterogeneous catalytic processes usually facilitate or even eliminate such purification steps by allowing an easy removal of the solid catalyst and offering the possibility of its reuse in several reaction cycles, reducing the environmental impact of handling process streams and improving the overall economics and/or sustainability of the entire production process
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