Homogenous catalysts are very effective catalysts which are frequently used but the major problem associated with the use of these
catalysts is their removal from methyl ester which needs excessive
washing. A lot of energy, water, and time are consumed; moreover
these catalysts cannot be reused. In contrast to that, the heterogeneous (solid) base catalysts being insoluble, are separated simply
with filtration and can be reused many times. A lot of research has
been done in this direction. Commonly used solid base catalysts
are alkaline earth metal oxides, zeolite, KNO3 loaded on Al2O3,
KNO3/Al2O3,BaO, SrO, CaO, MgO etc. [109,110].