Preparation of Mg-Al Hydrotalcite by Urea Method and Its Catalytic Activity for Transesterification

Layered double hydroxides based on the structure (Mg(6)Al(2)(OH)(16)CO(3)center dot 4H(2)O) were synthesized by urea hydrolysis method and characterized by XRD, FTIR, SEM, and EDS. The results revealed that pH played a crucial role in the Mg-Al hydrotalcite precipitation by controlling [urea]/[NO(3)(-)] molar ratio in reaction solution at 378 K and the optimized [urea]/[NO(3)(-)] molar ratio was 4.0. The sample calcined at 773 K was used as a solid catalyst for biodiesel synthesis. The catalyst was found to have a high catalytic activity in transesterification of rape oil to methanol with about 94% oil conversion at 338 K for 3 h. The water content of the oil could be kept below 2.0 wt % and free fatly acid content of the oil could be kept below 3.0 mg KOH center dot g[oil](-1) in order to get the best conversion. So, the solid catalyst was more tolerant to free fatty acid and water in rape oil than homogeneous basic-catalysts. Moreover, the catalyst could be reused, but catalytic activity decreased on reuse of the catalyst although it remained highly active for the five uses. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1229-1235, 2009