(297a) Activated Mg-Al Hydrotalcite Catalysts for Transesterification: Effect of Rehydration

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 can undergo catalytic transesterification with methanol or ethanol to form monoalkyl esters commonly known as biodiesel. Homogeneous base catalysts such as alkali hydroxide or methoxide are often used to catalyze the transesterification reaction, however separation of the base is difficult. Solid base catalysts can make the separation process much easier.

In this study, Mg-Al hydrotalcite with Mg/Al molar ratio of 4 was synthesized by a coprecipitation method. Calcined and calcined-rehydrated Mg-Al hydrotalcite materials were 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 calcined-rehydrated Mg-Al hydrotalcite exhibited one order of magnitude higher catalytic activity than calcined Mg-Al hydrotalcite, presumably because of the Brønsted base sites present on the surface of the rehydrated sample. Differential thermal analysis results showed two weight losses from calcined-rehydrated Mg-Al hydrotalcite at 309 K and 363 K, which were attributed to the removal of physisorbed and interlayer water. The catalytic activity of calcined-rehydrated Mg-Al hydrotalcite decreased as the interlayer water was removed by heating. Results from adsorption of CO2 on calcined-rehydrated hydrotalcite after different thermal treatments were consistent with a loss of Brønsted base sites as interlayer water was removed. X-ray diffraction experiments confirmed that removing the interlayer water of calcined-rehydrated Mg-Al hydrotalcite decreased the crystalline order of the sample. The pseudo first order rate constants for a three step reaction mechanism for tributyrin transesterification were determined by fitting the model to the experimental data.