Preparation of heterogeneous catalysts from limestone for transesterification of vegetable oils—Effects of binder addition

A limestone from the South of Thailand was utilized as a source for preparation of heterogeneous base catalysts for transesterification of various vegetable oils with methanol and ethanol has been investigated. Series of CMA catalysts were prepared through a wet mixing of ground limestone with different divalent and trivalent metal precursors, as binders, followed by drying and calcination. Effects of preparation conditions, including types of wetting media and metal precursors, metal compositions, and thermal treatment temperatures for the parent limestone and the mixed catalyst precursors, on the structural properties and the transesterification activity of the resultant catalysts, and on the catalyst formulation were studied. The catalytic test under batch conditions at 60 °C indicated an enhancement of the methyl ester formation over the catalysts prepared due to the presence of new active phase generated in situ upon the calcination of the mixed catalyst precursors at high temperatures. The metal precursors also acted as good binders in the catalyst shaping. Using the sodium salt of Al as a precursor remarkably improved the mechanical strength of the catalyst extrudates. The CMA catalysts can be regenerated and reused without a severe drop of the methyl ester yield. Moreover, the transesterification in a continuous-flow fixed bed reactor indicated a good activity and stability of the catalyst extrudates prepared.