Eco-benign synthesis of spent vegetable oil-based biodiesel using thermally modified nanocrystalline snail shell...

The quest to mitigate the critical challenges in the global energy landscape and carbon emission crisis and promote sustainable energy practices calls for imminent and sustainable energy solutions. This study explores the eco-benign biofuel production from spent vegetable oil (SVO) using thermally modified snail shell (CalSS) catalysts. The catalyst was characterized using SEM, FTIR spectroscopy, EDX, and XRD. The transesterification process was optimized using the Box-Behnken Design (BBD). The biodiesel produced was characterized using ATR-FTIR spectroscopy and GC-MS analysis. Furthermore, this study incorporated artificial neural networks (ANNs), response surface methodology (RSM), and kinetic studies to optimize and elucidate the catalytic process. SEM and XRD analysis revealed an irregularly shaped, brittle, and porous highly crystalline nanocatalyst (65 ± 2.81 nm). EDX confirmed the formation of calcium oxide (CaO), Ca (49.80 wt%) and O (50.20 wt%), and FTIR spectroscopy showed an absorption band at 547 cm^-1. CalSS recorded an activation energy (E _a) of 42.2276 kJ mol^-1 and a pre-exponential factor (A) of 2.2 × 10⁴ L mol^-1 min^-1. The transesterification process recorded a fatty acid methyl ester (FAME) yield of 90.32% at an oil-to-alcohol ratio of 6 : 1, 10 wt% catalyst dose, 120 min reaction time and 65 °C temperature. The biodiesel produced had a low moisture content of 0.03% and a specific gravity of 0.800 kg m^-3. Gas chromatography-mass spectrometry (GC-MS) analysis of the biodiesel recorded 37.96% of 15-methyl-, methyl ester, hexadecanoic acid. A notable O-CH₃ stretching at 1114 cm^-1 was recorded by FTIR spectroscopy. DFT-derived descriptors indicated that the selected compound MAC enhanced electron donation, while NTA facilitated electron acceptance, synergistically promoting the transesterification process. The relatively low energy gap and complementary HOMO-LUMO characteristics confirmed their combined effectiveness in boosting catalytic reactivity. The ANN model outperformed the RSM model based on the reported R ² values of 0.5258 (RSM) and 0.8792 (ANN). The cost analysis showed that $3.30 is required to produce 1 kg of catalyst. The study presents thermally modified snail shells as sustainable and cost-effective catalysts for biodiesel production.