Type of Catalyst Merits Demerits Homogeneous catalysts Alkaline type (NaOH, KOH, CH3ONa, CH3OK) Less corrosive, high reaction rate Formation of saponified product, emulsion formation, high water and energy consumption, huge, high waste water discharges, high purification cost, feedstocks are limited to 0.5wt FFAs, not recycle Acid type (H2SO4) Zero soap formation, the catalyst can be used to catalyze both esterification and transesterification simultaneously More waste as a result of neutralization, recycling difficulty, high purification cost, energy consuming, low reaction rates Heterogeneous catalysts Solid alkaline (MgO, CaO, ZnO, KOH/NaY) Solid acid types (CaO/MgO, Al2O3-SnO, KOH/K2CO3, Al2O3-ZnO, Ca(NO3)2/Al2O3CaO/Al2O3, KOH/Al2O3, Al2O3/KI, Sr(NO3)2/ZnO, ZrO2/SO42-, TiO2/SO42-, ETS-10 zeolite,  Zeolite HY, Zeolite X) Environmentally friendly, easily recycle, less discharges, less separation difficulty, high purity glycerol, lower cost of separation, insignificant leaching of CaO/Al2O3 Leaching effects, catalysts preparation is complicated and expense relatively slow rates Enzymes types (Candida antarctica B lipase, Rhizomucormeihei lipase, candida rugosaPseuodonascepacia, M. meihei (Lypozyme), M. meihei(Lypozyme IM60),Aspergillus niger) P. Fluorescens, R. Oryzae Zero saponification products nonpolluting, easily separable, lesser reparation cost, high purity glycerol and biodiesel products, environmentally benign   Simple glycerol recovery Catalysts inhibition by water     High cost of enzymes

Table 2: Summarized catalytic report - homogeneous vs. heterogeneous. Adopted and modified from [26].