Synthesis of biodiesel from the methanolysis of sunflower oil using PURAL® Mg–Al hydrotalcites as catalyst precursors

Abstract

A series of commercial Mg–Al hydrotalcites have been used as catalyst precursors for the methanolysis of sunflower oil. The solids were characterized by low Mg/Al molar ratios in the 0.5–2.3 range. Additionally, a solid consisting mainly of Mg(OH)2 but containing some Al (4.2wt.%) was also employed. The as-received materials were inactive for biodiesel synthesis so calcination and rehydration in boiling water were considered as activation treatments. Among the calcined solids, only the material consisting of MgO was significantly active, achieving about 50% oil conversion after 24h at 60°C, methanol/oil molar ratio of 12 and 2wt.% of catalyst. The effects of the calcination temperature in the 350–700°C range have been investigated; calcination at 500°C led to the best catalytic performance. In the case of the rehydrated materials, only the solids with the highest Mg/Al molar ratios recovered a well-ordered layered double hydroxide structure. These samples showed an improved catalytic activity compared with their calcined counterparts. A good correlation between catalytic activity and the basic properties determined using Hammett indicators and benzoic acid titration has been found. Rehydrated hydrotalcites were slightly more selective to biodiesel (75%) at intermediate levels of oil conversion than the calcined counterparts (66%). It has been verified that no Mg or Al leaching in the reaction mixture took place; however, the rehydrated samples deactivated significantly. In the case of MgO, after washing and calcination, the recycled catalyst gave 68% of the original oil conversion.