In this study, biodiesel,
also known as fatty acid methyl ester
(FAME), was synthesized from multi-stage frying waste soybean oil
using chicken eggshell-derived CaO and potassium-impregnated K
+
-CaO heterogeneous catalysts. Potassium-impregnated catalysts
(1.25% K
+
-CaO, 2.5% K
+
-CaO, and 5% K
+
-CaO) were developed by treating the calcined waste eggshell powder
with KOH in different wt % ratios. The catalysts were characterized
using FTIR, XRD, FESEM, EDS, BET, and particle size analysis techniques.
Box–Behnken design-based optimization was exploited to optimize
the reaction parameters. A maximum yield of 98.46%, calculated via
1
H NMR, was achieved following a 5% K
+
doping, 12:1
methanol to oil molar ratio, 3% catalyst amount, 180 min reaction
time, and 65 °C reaction temperature. The catalyst (5% K
+
-CaO) responsible for maximum biodiesel production was found
to be highly reusable, with a 30.42% conversion decrease in activity
after eight cycles of reuse. Gas chromatography was used to determine
the composition of FAME produced from different cycles of waste soybean
oil. Physicochemical parameters of the synthesized biodiesel were
found to be compatible with EN and ASTM standards. This study has
shown that the waste eggshell-derived heterogeneous catalysts have
significant catalytic activity at relatively low K
+
doping
and catalyst loading leading to high biodiesel conversion.