Many types of alkaline earth metal oxide catalysts can catalyze trans-esterification reactions. The
order of activity between alkaline earth metal oxide catalysts is BaO > SrO > CaO > MgO [14, 15]. SrO is
insoluble in vegetable oil, methanol, and fatty acid methylester. Despite the lower surface area of SrO as
reported elsewhere [16] there are other factors that cause the high activity of SrO. Besides the specific
surface area, alkalinities, and acid/base sites of alkaline earth metal oxides affect also the activity of the
catalyst [16].