Hydration–dehydration technique for property and activity improvement of calcined natural dolomite in heterogeneous biodiesel production: Structural transformation aspect Original Research Article

By means of a controlled hydration–dehydration process, the relatively low activity of calcined dolomite was modified into an active solid catalyst for the biodiesel production. This technique involves water treatment of the oxide phase under mild conditions followed by thermal decomposition at an elevated temperature. The results from various characterization techniques and the transesterification of palm olein showed that a structural transformation had taken place during the synthesis and structure–activity relation. Upon the calcination of dolomite, MgO grows on the surface of MgCa(CO3)2, which is progressively transformed into CaCO3 followed by the decomposition of calcite to CaO. The transformation of oxide to hydroxide phase and the reverse occurred simultaneously during the hydration–dehydration step with changes in the chemical and textural properties of the sample. The effectiveness of the hydration interaction appears to be due to a change in pore-size distribution, which is created by particle expansion in the formation of the hydroxide structure and the formation of more porosity and surface area during the dehydration and re-crystallization of oxide structure. Confirmed by TPD, in the hydration–dehydration step, the number of strong basic sites significantly increased while the chemical nature of these sites did not changed. The catalytic performance in transesterification is directly proportional to the number of strong surface base sites. This study provides an understanding regarding how this hydration–dehydration process influences the properties and activity of dolomite.