Optimization using Box-Behnken design of biodiesel synthesis: Microwave-assisted transesterification of coconut oil with isopropyl alcohol using K2O/γ-Al2O3 catalyst

Abstract

The production of isopropyl ester (biodiesel) from coconut oil using a K₂O/γ-Al₂O₃ catalyst derived from alum, combined with microwave-assisted transesterification, demonstrates strong potential for enhancing biodiesel production efficiency. This study aimed to optimize reaction parameters and evaluate the kinetic behavior, product composition, and physicochemical properties of the resulting biodiesel. Reaction temperature (60–80 °C), catalyst concentration (1–5 wt.%), and reaction time (5–30 min) were selected as independent variables, with ester yield as the response. Kinetic analysis showed that the transesterification reaction followed a second-order model with an activation energy of 21.26 kJ/mol, facilitated by microwave-induced molecular collisions. Optimization using the Box-Behnken design (BBD) identified optimal conditions at 68.48 °C, 3.22 wt% catalyst, and 20.96 min, achieving a maximum yield of 70.50 wt%. GC-FID analysis confirmed isopropyl laurate (39.47 wt%) as the major component. The produced biodiesel exhibited favorable fuel properties: a cetane number of 62.4, flash point of 164 °C, viscosity of 4.59 mm²/s, density of 878 kg/m³, and cloud and pour points of 11 °C and 8 °C, respectively. These properties meet ASTM D6751 and Indonesian National Standard (SNI 7182:2015), supporting its applicability as a high-quality, renewable diesel fuel. This study underscores the effectiveness of integrating heterogeneous catalysis with microwave technology for sustainable biodiesel production.