Identification of response surface patterns in ultrasound-assisted transesterification of castor oil using CaO-MgO catalysts derived from blood cockle shell and limestone

Abstract

The object of this study is the ultrasound-assisted transesterification of castor oil (Ricinus communis) into biodiesel using heterogeneous catalysts derived from natural sources: blood cockle shells (Anadara granosa) from Pekanbaru and limestone from Bangkalan, Indonesia. The problem addressed is that directly calcined limestone produces low biodiesel yield without complex post-treatment, and whether ultrasonic irradiation can compensate for this has not been tested. Three catalysts (shell-derived CaO, limestone-derived CaO-MgO, and their 1:1 physical mixture) were calcined at 900°C, 5 h, characterized by X-ray diffraction, Scanning electron microscopy with energy-dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy, and tested under ultrasonic irradiation at 28 kHz with 56.0 W effective power (calorimetric calibration, 100 W nominal). A central composite design (reaction time, catalyst loading) was employed for each catalyst. Mean yields were 95.4% (mixed), 93.2% (limestone), and 89.2% (shell). Response surface analysis revealed a maximum for limestone (97.9% at 93.9 min, 4.24 wt%), a saddle point for shell, and an inverted surface for the mixture. These distinct geometries arise from phase composition and active-site differences. Unlike prior work requiring calcination-hydration-dehydration and NiO impregnation, the limestone catalyst here achieved 93.2% yield by direct calcination and sonication alone, far exceeding the 16.45% reported without sonication. These findings apply to biodiesel production where shell and limestone are abundant, replacing complex catalyst post-treatment with direct calcination and sonication