Kinetic Parameters for KOH-Catalyzed Transesterification of Neem Oil (Azadirachta indica) with Ethanol to Produce Fatty Acid Ethyl Ester

ABSTRACT

Biodiesel derived from non-edible neem oil (Azadirachta indica) is a promising alternative to first-generation biodiesel produced from edible vegetable oils. This study investigated the kinetics of the transesterification of pretreated neem oil using potassium hydroxide (KOH) as a catalyst and anhydrous ethanol at temperatures between 45°C and 65°C. The crude neem oil initially had an acid value of 20.93 mg KOH/g (free fatty acid, FFA = 10.46%). To reduce the FFA content, the oil was subjected to a two-step acid esterification pretreatment, which lowered the acid value to 0.84 mg KOH/g (FFA = 0.42%). After pretreatment, base-catalyzed transesterification was performed. A pseudo-first-order kinetic model was used to analyze time-dependent biodiesel yield at reaction times of 15, 30, 60, and 90 minutes under four experimental conditions. Linear plots of −ln(1 − X) versus reaction time produced good fits (R² = 0.967–0.982) at the three temperature levels, confirming the pseudo-first-order assumption under excess ethanol conditions. The rate constants increased with temperature, from 0.0030 min⁻¹ at 45°C to 0.0079 min⁻¹ at 55°C and 0.0367 min⁻¹ at 65°C. An Arrhenius plot of ln(k) versus 1/T yielded an activation energy of 111.97 kJ/mol (R² = 0.998) and a pre-exponential factor of 2.06 × 10⁹ min⁻¹. These results indicate that the reaction is chemically controlled with negligible mass-transfer resistance. The derived rate law provides essential data for reactor design and supports scaling up neem oil biodiesel production in Nigeria and other parts of sub-Saharan Africa.

Key words: Biodiesel, Neem oil, Azadirachta indica, Transesterification, Kinetics, Pseudo-first-order, Activation energy, Arrhenius equation, Nigeria