Development and application of heterogeneous catalysts for direct cracking of triglycerides for biodiesel production

Abstract

Interest in biodiesel has been growing due to its potential role in moderating global climate change by lowering net CO2 emissions from fuels used for transportation. Most biodiesel fuels are currently synthesized by transesterification using alkaline catalysts and methanol. Heterogeneous transesterification catalysts have begun to be considered as alternatives, but many drawbacks remain. The costs of production and environmental concerns resulting from the ester washing step: neutralization of residual catalyst, removal of soap, glycerol, methanol and absorbent in some cases have prompted the search for more environmentally friendly processes and solid catalysts. Therefore, it is desirable to replace homogeneous or heterogeneous transesterification with the use of heterogeneous catalysts in direct thermocatalytic cracking. In principle, this could reduce the cost of biodiesel production, as it removes the need for alcohol and numerous downstream processing steps which add to the substantial running costs of transesterification. In addition the problem of glycerol in the product is eliminated. Four sulphated zirconia catalysts were synthesized via conventional wet-precipitation and solvent-free methods with different molar ratios of the sulphating agent. Their activity for direct thermocatalytic cracking of rapeseed oil was evaluated at a temperature of 270oC and atmospheric pressure. The nature and concentration of the active Brønsted and Lewis acid sites on the catalysts were examined. Brønsted acid sites were found to be important in the catalytic reaction. The catalysts at this temperature exhibited different selectivities towards formation of saturated and unsaturated methyl esters. The solvent-free catalysts were more active with a conversion of 78% in 21/2 hours, while the wet-precipitated catalysts had a maximum of 66% conversion after two hours. The catalysts prepared by the solvent-free method had 59% yield for methyl ester, with 75% of these being unsaturated. The wet-precipitated catalysts exhibited a lower yield for methyl esters (maximum: 32%), but within this a greater proportion (68%) were saturated. After regeneration, the solvent-free catalysts regained their catalytic properties, whereas the conventional catalysts did not. Three of the catalysts exhibited substantial leaching, with one of the conventional catalysts losing 100% of the sulphate responsible for its activity. Thus, to improve their properties the catalysts were supported with meta-kaolin which resulted in higher Brønsted acidity and better stability.