Influence of water on the activity and stability of activated MgAl hydrotalcites for the transesterification of tributyrin with methanol

Magnesium–aluminum hydrotalcite with a Mg/Al molar ratio of 4 was synthesized by a coprecipitation method. Thermally-decomposed and rehydrated Mg–Al hydrotalcites were used to catalyze the transesterification of tributyrin, a model triglyceride, with methanol (tributyrin:methanol molar ratio 1:30) at 333 K to produce methyl butyrate, monobutyrin, dibutyrin, and glycerol. The pseudo first order rate constants of a three step reaction sequence for tributyrin transesterification were determined by fitting a kinetic model to the experimental data. Although decomposed and rehydrated Mg–Al hydrotalcite was one order of magnitude more active than decomposed Mg–Al hydrotalcite based on surface area measured by N₂ adsorption, the activity correlated well to the CO₂ adsorption capacity. The most active rehydrated samples also deactivated faster, presumably because butyric acid produced by hydrolysis of ester with adsorbed water reacted with the base sites. The areal rate and CO₂ adsorption capacity of decomposed-rehydrated Mg–Al hydrotalcite decreased as the interlayer water was removed by heating.