Pomegranate peel-derived Cu/hydrochar nanocomposites: a sustainable catalyst for biodiesel production via oleic acid esterification

Abstract Transforming pomegranate peel waste into a biodiesel catalyst advances waste-to-resource circularity. A facile hydrothermal carbothermal reduction method was used to synthesize copper-decorated hydrochar nanocomposites derived from peel bio-waste and CuSO 4 , with Cu(II) concentrations of 10, 25, and 50 mmol/L (denoted as Cu10/HC, Cu25/HC, and Cu50/HC, respectively). The physicochemical properties of the nanocomposites were characterized by FTIR, XRD, BET, XPS, SEM-EDX, and NH₃-TPD, and their catalytic performance was evaluated for biodiesel production via the esterification of oleic acid (OA) with ethanol. Structural analysis confirmed highly pure, crystalline zero-valent copper (Cu⁰) with an FCC nanostructure and spherical morphology (average size ~ 25–63 nm), well-dispersed throughout the hydrochar-derived carbon matrix. XPS verified oxygen functionalities (–COO⁻, > C = O, C–OH) alongside Cu⁰ on the nanocomposite surfaces. Moreover, textural analysis revealed that Cu25/HC exhibited the highest surface area (29.6 m²/g) and pore volume (0.099 cm³/g), whereas Cu50/HC showed reduced porosity but introduced significant acidity (54.9 µmol/g, according to NH 3 -TPD). Catalytic efficiency increased with higher Cu dispersion density (Cu atoms/nm 2 ; R² =0.996): Cu50/HC achieved the best performance (97.4% conversion), followed by Cu25/HC (96.6%) and Cu10/HC (96.0%), outperforming bulk Cu content and BET surface area. Under optimal conditions (70 °C, 5 h, 1.0 wt% catalyst relative to OA, and an ethanol/OA molar ratio of 15:1) the Cu50/HC catalyst attained complete OA conversion. Thermodynamic parameters were determined and indicated an endergonic and thermodynamically non-spontaneous reaction. Notably, the catalyst retained 97.1% of its initial activity after six consecutive cycles, demonstrating excellent operational stability, as confirmed by the FTIR, XRD, and Cu content. These findings position the Cu50/HC nanocomposite as a cost-effective, sustainable, and efficient catalyst for biodiesel production.