Mixed metal oxide catalysts derived from natural sources for production of fatty acid methyl esters

Abstract

The objectives of this research are to study active sites of Ca-based catalysts derived from calcite (CaCO3) and dolomite (CaMg(CO3)2), and to clarify catalytic effect of MgO in the calcined dolomite as catalyst in transesterification of palm oil with methanol. Firstly, the effects of calcination temperature on evolution of MgO in dolomite and its catalytic activity were studied. The MgO containing in the calcined dolomite was not active, while the main active site was CaO. However, Ca and Mg mixed oxides obtained from calcined dolomite had higher activity than that prepared by physical mixing in which calcite and magnesite (MgCO3) were mixed at the same metal composition as dolomite. The results indicated that the presence of MgO in the calcined dolomite reduced the CaO crystallite size, and stabilized the catalytic activity. The direct reaction of Ca-based oxides with methanol indicated that the calcined dolomite with smaller CaO crystallites gave higher formation rate of calcium methoxide (Ca(OCH3)2) than the pure CaO. In the case of direct reaction with glycerol, the formation of calcium glyceroxide (Ca(C3H7O3)2), related with leaching of calcium active phase, was delayed over the mixed oxide catalysts. Subsequently, the Ca-based oxides were prepared in form of Ca-based methoxides and glyceroxides and tested in the transesterification. Since the Ca-based oxides had a higher initial rate than the others, they were easily changed to other calcium compounds, such as Ca(OH)2, CaCO3 and Ca(C3H7O2)2 during the transesterification. The Ca-based methoxides were more stable than the Ca-based oxides although their activity was the least. On the other hand, the Ca-based glyceroxides promoted the reaction via homogenous catalysis routes. The structural stability of catalysts exposed to ambient air was investigated since CO2 and moisture were strongly chemisorbed onto the basic sites, leading to catalyst deactivation. The Ca-based oxides were the most stable catalysts because the Ca(OH)2 generated was somewhat active. Finally, the Ca-based catalysts in extrudate form were used in biodiesel production using continuous-flow fixed-bed reactor. The effects of hydroxyethyl cellulose addition on the extrusion and the characteristics of the extrudates obtained were studied. Premixing the catalyst extrudates with different types of commercial methyl esters (CMEs) resulted in a stable FAME yield of ≈96.5% throughout the operation. This beneficial effect arises from high solubilities of both reactants in the CME layer covering the catalyst extrudates.