Transesterification of rapeseed oil with ethanol. I. Catalysis with homogeneous Keggin heteropolyacids
Résumé
Heteropolyacids (HPA) with Keggin structure were evaluated as homogeneous Brnsted acid catalysts in the reaction of rapeseed oil transesterification with methanol and ethanol at 358 K and atmospheric pressure. Rapeseed oil transesterification with ethanol over anhydrous Keggin HPAs leads to higher conversion level than H2SO4 compared at equivalent H+ concentration and H2O/H+ molar ratio. By contrast, Phosphoric acid, a quite weaker mineral acid, structurally close to H3PW12O40, is not operative under similar conditions. This demonstrates the advantages of strong Brnsted acids in vegetable oil transesterification with ethanol in mild conditions. It is worth noticing that differences in acid strength between anhydrous Keggin heteropolyacids, H3PW12O40, H4SiW12O40, H3PMo12O40, H4SiMo12O40, measured by calorimetry, did not correlate simply the catalytic activity. Indeed, the proton solvation with water molecule was shown to be a crucial parameter. Then, Mo samples exhibited higher activities due to their ability to lose crystallization water at lower temperatures compared to W samples. Finally, it was shown that higher transesterification rates were obtained with ethanol than methanol in presence of HPA.