Enhancement of biodiesel production via sequential esterification/transesterification over solid superacidic and superbasic catalysts

Recently, biodiesel can be produced from energy crops and has been recognized as an ideal candidate for satisfying the global energy demand. Owing to its fossil-fuel-similar physicochemical properties, biodiesel is also regarded as an alternative fuel for various applications. In present work, soybean oil with added fatty free acid (FFA; palmitic acid, 5.0 wt%) was converted to biodiesel via a two-step process in a catalytic fixed-bed batch reactor with SO42-/ZrO2/Al2O3, KF/CaO-Fe3O4, and Na/NaOH/Al2O3 catalysts. A solid superacidic (SO42-/ZrO2/Al2O3: 0.1-1.5M H2SO4) and two solid superbasic (KF/CaO-Fe3O4, 5-25 wt% KF; Na/NaOH/A(l)2O(3), 10-25 wt% NaOH) catalysts were fabricated using calcination at 300-700 degrees C and 200-700 degrees C, respectively. Notably, the FFA removal efficiency of SO42-/ZrO2/Al2O3 (80.0%) was much higher than that of commercial Amberlyst IR 120 (30.0%) in a 3-h reaction. Optimal biodiesel yields were obtained using KF/CaO-Fe3O4 (83.0%) and Na/NaOH/Al2O3 (100.0%). Catalytic reaction mechanisms of the conversion of soybean oil with FFA into biodiesel using solid superacidic and superbasic catalysts were also proposed. The FFA and soybean oil were converted to biodiesel over the superacidic and superbasic catalysts, respectively.