Study of thermal transformations in Na,Ca-based catalysts supported on activated carbon and their application in the synthesis of biodiesel from soybean oil

Catalysts containing Na and Ca compounds dispersed in a porous carbon support were prepared by a wet chemical route followed by heat treatments and their catalytic performance in the transesterification reaction of soybean oil with methanol was investigated. The chemical composition and the structural properties of the Na,Ca-based catalysts were modified by means of heat treatments at temperatures ranging from 500 to 800 degrees C. The chemical nature of the Na,Ca-containing phases was studied by X-ray diffraction and solid-state Na-23 nuclear magnetic resonance (NMR) spectroscopy, among other methods. The as-prepared material was found to be composed of a mixture of Ca(OH)(2) and Na2CO3; the heat treatment at 500 degrees C led to the formation of the double carbonate, Na2CO3 center dot CaCO3, whereas at high temperatures the dominant phases were CaO and Na2CO3. The best catalytic efficiency was achieved for the catalysts prepared at 700 and 800 degrees C. An important advantage of the prepared catalysts was the low concentration of Ca and Na leached into the produced biodiesel samples. It was found that the best heat treatment temperatures to prevent metal leaching were 700 and 800 degrees C. These results show that the simple method of wet impregnation followed by heat treatments is an efficient route for the production of carbon-supported Na,Ca-based catalysts with high porosity, good catalytic efficiency and that allow the synthesis of biodiesel samples with low metal contents.