As mentioned above, the disadvantages of homogeneous base-catalyst transesterification are high
energy-consumption, costly separation of the catalyst from the reaction mixture and the purification of
crude BDF. Therefore, to reduce the cost of the purification process, heterogeneous solid catalysts such
as metal oxides, zeolites, hydrotalcites, and γ-alumina, have been used recently, because these catalysts
can be easily separated from the reaction mixture, and can be reused. Most of these catalysts are alkali
or alkaline oxides supported on materials with a large surface area. Similar to homogeneous catalyst,
solid base-catalysts are more active than solid acid-catalysts [47,48]. In this review, we focus on
popular solid base and acid catalysts.