Chemists in Egypt have transformed pomegranate peel waste into a copper-decorated carbon catalyst that converts oleic acid into biodiesel precursors with up to 97.4 percent conversion, retaining 97.1 percent of its activity after six consecutive reaction cycles. The study, published in BMC Chemistry, describes a one-pot hydrothermal route in which the fruit peel serves simultaneously as the carbon source and the reducing agent, eliminating the need for external reductants or synthetic surfactants.
The research team, led by Amal A. Alkahlawy, Delvin Aman and Tamer Zaki with Ahmed A. Galhoum, ground dried pomegranate peels to particles smaller than 53 micrometers and dispersed three grams of the powder in 75 milliliters of copper sulfate solutions at concentrations of 10, 25 and 50 millimoles per liter. After ultrasonication, the mixtures were sealed in a Teflon-lined stainless-steel autoclave and held at 175 degrees Celsius for 18 hours. The resulting brown powders, labeled Cu10/HC, Cu25/HC and Cu50/HC, consist of zero-valent copper nanoparticles embedded in a hydrochar matrix produced by hydrothermal carbonization of the peel itself.
Structural analysis confirmed the formation of highly pure, crystalline zero-valent copper with a face-centered cubic nanostructure and spherical morphology. Crystallite sizes estimated with Scherrer’s equation increased with copper loading, averaging 29.34, 39.62 and 54.79 nanometers for the three composites, values that closely matched particle size ranges observed by scanning electron microscopy.
X-ray photoelectron spectroscopy revealed that copper on the composite surfaces exists mainly as metallic Cu0 and cuprous Cu+, with little Cu2+ present, a conclusion reinforced by the absence of the satellite peaks typical of Cu2+ and by ammonia temperature-programmed desorption, which showed virtually none of the ammonia bound to Cu2+ ions. The spectroscopy also verified oxygen functionalities on the hydrochar, including carboxylate, carbonyl and hydroxyl groups, which the authors identify as Brønsted acid sites that cooperate with Lewis-acidic copper species during the reaction.
Textural measurements told a more nuanced story. Cu25/HC displayed the highest surface area at 29.64 square meters per gram and a pore volume of 0.099 cubic centimeters per gram, while Cu50/HC suffered a marked reduction in both, dropping to 14.94 square meters per gram and 0.066 cubic centimeters per gram, apparently because dense copper deposition blocked micro- and mesopores. Yet the copper-rich material compensated with acidity: NH3-TPD measured a total acid amount of 54.9 micromoles per gram for Cu50/HC, with desorption features indicating weak, medium and strong acid sites.
Catalytic testing used the esterification of oleic acid with ethanol as a model reaction for biodiesel production, monitored by titration of the acid value. After systematic single-factor optimization, the best conditions for Cu50/HC were 70 degrees Celsius, 5 hours, a catalyst loading of 1 weight percent relative to oleic acid, and an ethanol-to-oleic-acid molar ratio of 15:1, under which conversion reached 97.4 percent. Cu25/HC and Cu10/HC achieved 96.6 and 96.0 percent under comparable conditions, while pristine hydrochar without copper managed only 45.2 percent after 7 hours, demonstrating that the copper nanoparticles drive most of the activity. Raising the temperature to 80 degrees Celsius lowered conversion, most likely because ethanol, which boils at 78.4 degrees Celsius, evaporated from the reactor.
A correlation analysis identified surface copper density, rather than total copper content or surface area, as the primary descriptor of catalytic performance. When copper dispersion density, expressed as copper atoms per square nanometer of surface area, was plotted against conversion, the data fit a linear relationship with a correlation coefficient of 0.996. Bulk copper content showed a weaker correlation, and surface area actually correlated inversely with conversion, confirming that the material with the smallest surface area performed best because it exposed the most copper atoms, reaching 51.3 atoms per square nanometer for Cu50/HC.
Kinetic and thermodynamic analysis added quantitative depth. The reaction followed a pseudo-first-order model in oleic acid, with the rate constant rising from 0.205 per minute at 60 degrees Celsius to 0.9304 per minute at 80 degrees Celsius. The apparent activation energy was 73.6 kilojoules per mole, at the upper end of the typical 30 to 80 kilojoule range for esterifications and well above the threshold that would indicate diffusion control, meaning the reaction is governed by intrinsic chemical kinetics at the active sites. The activation enthalpy and entropy were 70.7 kilojoules per mole and minus 0.048 kilojoules per mole per kelvin, and the Gibbs free energy of activation ranged from 86.6 to 87.5 kilojoules per mole across the tested temperatures, indicating an endothermic, non-spontaneous activation process that requires continuous energy input.
Reusability testing under the optimized conditions showed only marginal decay. After six cycles, the catalyst retained 97.1 percent of its initial activity, and atomic absorption spectroscopy found that copper content fell only from 8.1 to 7.4 weight percent, a total leaching loss of 8.64 percent, or about 1.44 percent per cycle. X-ray diffraction and infrared analysis of the spent catalyst revealed partial surface oxidation of metallic copper to copper oxide species, along with adsorbed esterification intermediates, but the overall structure remained intact. The authors attribute the stability to strong anchoring of the copper nanoparticles by the oxygen-functionalized hydrochar framework.
The team proposes a bifunctional mechanism in which carbonyl oxygen of oleic acid coordinates to exposed Cu+ Lewis acid sites while hydrochar hydroxyl and carboxyl groups protonate the carbonyl, jointly polarizing the bond for nucleophilic attack by ethanol. The intermediate then dehydrates to release ethyl oleate and water, regenerating both types of acid sites. The authors caution that the catalyst has so far been evaluated only with model oleic acid, and that validation with real high-free-fatty-acid feedstocks, reduction of the relatively high 15:1 ethanol ratio, and techno-economic assessment for industrial scaling remain necessary before the peel-derived catalyst can move from laboratory to plant.
Subject of Research: Copper-decorated hydrochar nanocomposites from pomegranate peel waste as reusable catalysts for oleic acid esterification in biodiesel production
Article Title: Pomegranate peel-derived Cu/hydrochar nanocomposites: a sustainable catalyst for biodiesel production via oleic acid esterification
Article References: Alkahlawy, A. A., Aman, D., Galhoum, A. A., & Zaki, T. (2026). Pomegranate peel-derived Cu/hydrochar nanocomposites: a sustainable catalyst for biodiesel production via oleic acid esterification. BMC Chemistry, 20(1), Article 201. https://doi.org/10.1186/s13065-026-01885-z
Image Credits: AI Generated
DOI: 10.1186/s13065-026-01885-z
Keywords: biodiesel, oleic acid esterification, pomegranate peel waste, hydrochar, copper nanoparticles, heterogeneous catalysis, hydrothermal carbonization, nanocomposite, kinetics, thermodynamics, catalyst reusability, waste valorization
News Source: Bethany Barker. (October 30, 2026). Pomegranate Peel Waste Turned Into Copper Catalyst That Converts Oleic Acid to Biodiesel. Scienmag.
