Biodiesel production over sodium carbonate and bicarbonate catalysts

Abstract

Biodiesel, a mixture of alkyl esters (FAME), is viewed as a feasible renewable fuel to substitute fossil diesel. The first-generation biodiesel raises several sustainability issues and new production processes using low-grade fats and heterogeneous catalysts are envisaged. Biodiesel from alimentary grade oil and waste frying oil was produced by methanolysis (oil/methanol = 12 M ratio), at 67 °C, using anhydrous sodium carbonate and bicarbonate, commercial materials as catalysts (5% by oil weight) in a batch reactor at atmospheric pressure. Sodium carbonate had a higher catalytic activity for the methanolysis of the fresh oil (97% of FAME after 90 min) than for the frying oil (12% of FAME after 90 min), which can be attributed to the greater acidity of the fried oil, which promotes the partial neutralization of the catalyst. Sodium bicarbonate, less active than sodium carbonate, showed similar catalytic activities for both fats (84–85% of FAME after 180 min), appearing to be less sensitive to the acidity of the raw material, which may derive from its lower basicity. Both catalysts suffer crystallographic changes during reaction being both converted into similar mixtures of hydrated carbonate/bicarbonate slats, like trona. The methanol solubility of sodium carbonate seems to have a null homogeneous contribution to the catalytic process since the co-produced glycerin presented a pale-yellow color which is characteristic of heterogeneous catalyzed methanolysis processes. Such result was corroborated by thermogravimetric analysis of glycerin which showed small residual masses due to minor solubilization of catalyst. FTIR spectra of glycerin showed low contamination with MONG (Matter Organic Non-Glycerin), thus the contribution of homogeneous catalysis was not meaningful.