The study of heterogeneous catalysts for biodiesel synthesis

A study in search of new heterogeneous catalysts that can be used in place of homogeneous catalysts for biodiesel synthesis has been carried out. The objective of this study is to investigate and evaluate the use of solid catalysts for transesterification of triglycerides and esterification of free fatty acid for biodiesel synthesis. Two types of heterogeneous catalyst have been studied that are solid acid and solid base. Three different solid acids and two solid bases were explored. The solid acids investigated were sulfonated hypercrosslinked polystyrene resin, sulfonated polyvinyl alcohol and sulfated zirconia. The solid bases were lithium zirconate and composite calcium oxide-magnetite.

Sulfonated hypercrosslinked polystyrene resin has been studied in the esterification reaction of oleic acid with methanol and rearrangement of α-pinene to camphene and limonenes. The activity of this catalyst was compared with conventional macroporous polystyrene sulfonic acids such as Amberlyst-15, Amberlyst-35 and Nafion SAC-13 which is a composite of Nafion and silica. The activity of this catalyst is superior to those of Amberlysts and SAC-13 in the esterification of oleic acid. This catalyst also has high reusability at elevated temperature.

Sulfonated polyvinyl alcohol catalyst has been studied in the same way the esterification of oleic acid and rearrangement of α-pinene. The activity of the catalyst was compared with sulfonated macroreticular polystyrene resin Amberlyst-35 and Nafion SAC-13. In the esterification reaction, sulfonated polyvinyl alcohol shows a better activity than SAC-13 and the Amberlyst-35 resin catalyst. In the rearrangement/isomerisation reaction, the sulfonated polyvinyl alcohol is the least active.

The activity of sulfated zirconia as a solid acid catalyst has been studied in the esterification of oleic acid and in simultaneous esterification- transesterification of a mixture oleic acid and triglycerides. In the simultaneous reaction, the activity of this catalyst was compared with sulfonated polystyrene catalysts (Amberlyst-15 Amberlyst-35 and Purolite D5081) and commercial sulfated zirconia XZO-1720. The result showed that in the transesterification reaction, the sulfated zirconias are more active but in the esterification reaction the sulfonated polystyrene catalysts are better than sulfated zirconia. The effect of calcination temperature on the catalytic activity of the catalyst was also studied. The study shows that 600 oC calcination gives the best catalytic activity.

The lithium zirconate is one of the solid base catalysts evaluated in this study. The catalyst was very active in the transesterification of tributyl glycerate with methanol. The effect of calcination temperature on its activity in the transesterification reaction of tributyl glycerate was studied. The study shows that 700 oC calcination yielded the highest catalytic activity which is associated with the formation of the tetragonal phase which gives the highest concentration of basic sites and basic strength.

A composite between calcium oxide and magnetite is the last catalyst reported in this study. The study of this catalyst is due to ease of separation by an external magnetic field. The catalytic performance in the transesterification of tributyl glycerate and methanol was studied. The catalytic activity of the catalyst was maximum at the calcination temperature of 700 oC. Loss of catalytic activity and magnetic properties were shown at higher calcination temperature.

Despite the relatively high activities found for the catalysts studied, further improvement is needed if the catalysts are to be applied for industrial use. However, sulfonated hypercrossliked polystyrene resin catalysts showed promising activity for the pre-esterification reaction and it is one of the best catalysts for reducing free fatty acid in low grade vegetable oil.