Evaluation of heterogeneous catalysts for biodiesel production

Abstract

Biodiesel is a fuel derived from renewable sources suitable for use in conventional compressio n-ignition engines. The use of biodiesel is being promoted in many countries through government incentives and targets, and the production of biodiesel is increasing worldwide. As it is derived from renewable feedstocks, lifeCYCle C02 emissions are reduced when petrodiesel usage is displaced by biodiesel usage. Biodiesel is produced through the transesterification reaction of triglycerides with methanol. Typically, this reaction is carried out at 60 'C, in a batch reactor with an excess of methanol and a homogeneous sodium hydroxide catalyst. The products are biodiesel and glycerol. Following the reaction, the sodium hydroxide must be neutralized and the resulting salt is contained in the glycerol phase, making it difficult to purify the glycerol to phan-naceutical grade to sell as a by-product. Additionally, sodium hydroxide can cause unwanted side-reactions to form soap, through the saponification of triglycerides and the neutralization of free fatty acids present in cheaper feedstocks, resulting in a loss of yield. Therefore, it is desirable to replace sodium hydroxide with a heterogeneous catalyst. Potential heterogeneous catalysts were evaluated for their activity in the transesterification reaction, and their stability in the reaction environment. Alkali-doped metal oxides and silica-supported guanidines were found to be active, with conversions of >90% reached in 3 hours. This reaction time is feasible for use on an industrial scale. There was a strong correlation between base strength and activity: only catalysts with strong base sites were active. Unfortunately, the catalysts were found to leach and lost activity on repeated use. Thus, in their current form, these catalysts would not be suitable heterogeneous catalysts for biodiesel production because the advantages of using a heterogeneous catalyst are lost if the catalyst leaches. Amberlyst 26, a quaternary ammonium functionalized polystyrene resin was found to be stable, but only 8% conversion was obtained after 3 hours of reaction. This low activity was attributed to internal diffusion limitations.