Charred shrimp shells treated with potassium fluoride used as a catalyst for the transesterification of soybean oil with methanol

Abstract

A basic catalyst made of charred shrimp shells treated with potassium fluoride (KF) was employed in the transesterification reaction of soybean oil in the presence of methanol to produce methyl esters. The shells used here are wastes from freshwater shrimp farming in the western region of the state of Paraná, Brazil, which are normally discarded in dumps/landfills and are a rich source of the biopolymer chitin (poly-β-1,4-N-acetyl-D-glucosamine). Charring of the shells (at 450 °C for 2 h) is aimed at partially eliminating rests of proteins and fibers, as well as acetyl groups (-CH3CO), carbon dioxide (-CO2) and water, causing the rearrangement of the chitin structure. The subsequent treatment of the shells with KF (impregnation and activation at 250 °C for 2 h) potentiated the basic sites on the catalyst surface, rendering it active in the transesterification reaction. The catalysts and shrimp shells were characterized by thermal gravimetric analysis, differential thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, N2 physisorption (Braunauer, Emmet and Teller) (BET), and Hammett basicity. An experimental design was created to evaluate the effect of the following variables of the transesterification reaction:temperature, catalyst weight ratio, and molar ratio of oil:alcohol. The results indicate that the treatment with KF increased the basicity of the catalyst. The highest conversion into methyl esters obtained with the experimental design was 86.3%, using an oil:alcohol molar ratio of 1:9, 3.0% (w/w) of catalyst, 85 °C, and 3 h of reaction. Leaching tests indicated that, after 10 h of refluxing in methanol, a conversion rate of 52.4% was achieved using the catalyst leached in optimized conditions. Kinetic studies showed that chemical equilibrium was established after 8 h of reaction, reaching a conversion rate of 96.7%, which reveals the potential of the catalyst to produce methyl esters (biodiesel).