According to Xie and Huang (2006) [41], high loading of Ca
content in the binary system will generally lead to the formation of
free CaO phase coated on the surface of mixed metal oxides that do
not bind with in mixed oxide system. This amorphous material is
not stable and, thus can easily be flushed out with reactants.
Therefore it is very likely that the minor leaching during the initial
period is caused by the dissolution of amorphous materials, which
are not considered active centers for transesterification. Therefore
we can assume that the catalyst started to show high reusability
after flushing of amorphous materials from the catalyst.