Heterogeneous catalysts for biodiesel production from high free fatty acid vegetable oil

Abstract

A two-step catalyzed process consisting of esterification and transesterification reactions was employed for the efficient conversion of rice bran oil into fatty acid methyl ester (biodiesel) using acid and base catalysts. First, the heterogeneous base catalysts: MgAl hydrotalcites (Mg/Al molar ratio = 3 and 4), metal loaded on calcined MgAl hydrotalcites: M loaded MgAlO (M = K, Cs, Sr, Ba, La) as well as Mg(Al)La hydrotalcites, Mg(Al)LaO and rehydrated Mg(Al)La were synthesized and characterized by ICP, XRD, SEM, BET and FT-IR. Their basicity was measured by titration. To compare their catalytic activity, glyceryl tributyrate was used as a model compound for transesterification with methanol. The results indicated that among metal loaded mixed oxide, 1.5 wt% K loaded on MgAlO gave the highest conversion, however, metal leaching was also detected. Comparison between Mg(Al)LaO and rehydrated Mg(Al)La hydrotalcite, the results indicated that the rehydrated catalyst exhibited higher activity and less metal leaching. Crude rice bran oil having high free fatty acids (11.14%FFA) was chosen for the esterification and transesterification reactions with methanol. The series of the solid acid catalyst containing 15-80% of 12-tungstophosphoric acid (HPW) supported onto silica were synthesized and characterized by XRD, TPD and FT-IR. The activities of the acid catalysts on the conversion of FFA were determined. FFA can be reduced to less than 1.0% using 80 wt% HPW/SiO[subscript 2] catalyst when reaction was carried out at 70oC for 4 h using a 10:1 molar ratio of methanol to oil and a catalyst amount of 3 wt%. Then, the base catalyzed transesterification using the rehydrated MgAlLa hydrotalcite (La low) catalyst was performed at 100oC for 9 h with a 30:1 molar ratio of methanol to oil and a catalyst amount of 7.5 wt% to obtain biodiesel with 100% methyl ester content and 75% product yield.