Transesterification of Tributyrin with Methanol Over Mg – Zr and Mg - Ti Mixed Oxide Catalysts

Biodiesel is an attractive biorenewable alternative to petroleum-based transportation fuels. Common feedstocks for the production of biodiesel include triglycerides found in vegetable oils and animal fats. These triglycerides undergo catalytic transesterification with methanol or ethanol to form monoalkyl esters, commonly known as biodiesel. Homogeneous base catalysts, such as alkali hydroxide or alkali methoxide, are often used to catalyze the transesterification reaction. However, neutralization of the base and separation of the resulting salt is required. Solid base catalysts can therefore improve the overall process by eliminating the need for neutralization and separation.

In this study, magnesium mixed oxides with zirconium or titanium were prepared using precipitation and sol-gel synthesis methods. These mixed metal oxides were subsequently used to catalyze the transesterification reaction of tributyrin, a model triglyceride, with methanol (tributyrin:methanol molar ratio 1:30) at 333 K to produce methyl butyrate and glycerol. The pseudo-first-order rate constants for a three-step reaction sequence for tributyrin transesterification were determined by fitting a kinetic model to the experimental results. The catalysts were also characterized by X-ray diffraction, nitrogen physisorption, carbon dioxide stepwise temperature programmed desorption, and FTIR spectroscopy of adsorbed carbon dioxide and methanol. Although sol-gel synthesis resulted in better atomic-level mixing of the magnesia with the zirconia or titania components, the catalysts prepared by precipitation were substantially more active. In particular, a mixed oxide of magnesium and zirconium, prepared by precipitation was about 5 times more active than pure magnesia based on exposed surface area, where as a mixed oxide of magnesium and titanium was similar to pure magnesia. Pure zirconia and titania were relatively inactive for the reaction. The relationship between catalyst structure and catalytic activity will be discussed.