Modulating biodiesel yield and purification with plant-derived hydrophobic iron oxide nanocatalysts

This study investigates the impact of the hydrophobicity of iron oxide (FeNP) nanocatalysts on biodiesel production and post-reaction purification. FeNPs were green synthesized using distinct hydrophobic extracts of <i>Rosmarinus officinalis</i> (ROS), <i>Matricaria pubescens</i> (MAT), <i>Juniperus phoenicea</i> (JUN), and <i>Artemisia herba-alba</i> (ARM), whose phytochemical contents showed large variations in hydrophobic polyphenols (flavonoids (TCF): 209.50-353.75 mg AGE per g; condensed tannins (TCCT): 853.04-871.45 mg CE per g). Biodiesel production was performed under optimized conditions (ethanol-to-oil volume ratio, 3 : 1; catalyst loading, 0.20 wt%; 65 °C), and the biodiesel/purification performance was evaluated using FTIR and UV-vis analysis of retained (Gly_Bio) and free glycerol (Gly_free). The results show a strong positive correlation between extract hydrophobicity and catalytic efficiency. The most hydrophobic extract (ROS: TCF = 353.75 ± 1.02 mg AGE per g; TCCT = 871.45 ± 0.89 mg CE per g) produced FeNPs that achieved the highest biodiesel yield (92.60 ± 1.12%), glycerol separation efficiency (98.30 ± 0.01%), and ester content (98.25%), with minimal glycerol contamination (1.46 ± 0.21 mM; 152.70 µg g^-1). Conversely, FeNPs synthesized from the least hydrophobic extract (ARM: TCF = 209.50 ± 0.89 mg AGE per g; TCCT = 853.04 ± 0.83 mg CE per g) exhibited significantly lower biodiesel yield (81.42 ± 2.03%), purification efficiency (88.60 ± 0.63%), and ester content (89.09%), with higher glycerol contamination (8.69 ± 0.32 mM; 909.40 µg g^-1). ANOVA (<i>p</i> < 0.0001) and Tukey's HSD confirmed statistically significant differences between the four green nanocatalysts. Spectroscopic analysis further supported these findings, showing reduced OH bands from glycerol and enhanced 3,5-diacetyl-1,4-dihydrolutidine (DDL) signals in samples purified with more hydrophobic catalysts, demonstrating effective oxidation and removal of glycerol. Overall, nanocatalysts derived from hydrophobic extracts retained less glycerol and promoted cleaner phase separation, while less hydrophobic extracts favored stronger glycerol surface interactions, reducing biodiesel purity. This work highlights the novel link between extract hydrophobicity, nanoparticle surface chemistry, and biodiesel quality, providing a green strategy for designing plant-based nanocatalysts capable of producing EN 14214-compliant biodiesel (≤1.91 mM glycerol). The economic assessment underscores the commercial promise of this method. The production cost for biodiesel was calculated to be $1.12 per kg, a figure that is highly competitive and partly attributable to the use of a hydrophobic ROS-FeNP catalyst. This property significantly reduces downstream purification costs by facilitating the effortless separation of glycerol. Coupled with a low catalyst cost of $6.538 per kg and compliance with international EN 14214 standards, this methodology highlights significant potential for large-scale industrial implementation.