Porosity Engineering of Eggshell-Derived Nanostructured Hydroxyapatite for Ethanol Upgrading: Structural, Textural and Surface Characterization

Abstract Hydroxyapatite (HAp) has attracted considerable attention as a catalyst owing to its tunable acid–base properties; however, the influence of porogen-assisted synthesis on its textural characteristics and catalytic behavior remains insufficiently understood. In this work, nanostructured HAp was synthesized via a sol–gel route using calcium precursors derived from chicken eggshell waste, combining waste valorization with sustainable catalyst design. The results demonstrate a relationship between porogen-assisted modification of eggshell-derived hydroxyapatite, the resulting textural and acid–base properties, and the relative n-butanol yield obtained during relative n-butanol yield under the investigated conditions. The incorporation of pore-forming agents modified the textural properties of HAp, increasing the specific surface area from approximately 28 to 155 m2 g–1 and enhancing the pore volume. Nitrogen adsorption–desorption analyses revealed mesoporous structures with pore sizes ranging from 5 to 35 nm, while the glucose-templated sample exhibited a bimodal pore-size distribution indicative of porosity. The increase in surface area and pore accessibility improved the availability of acid–base active sites, as confirmed by NH3-TPD and CO2-TPD measurements. A structure–activity relationship was observed, with the most porous catalysts achieving relative n-butanol yields above 87%, based on normalized GC peak areas at 180 °C. Product distribution analysis based on gas chromatographic peak areas revealed enhanced relative n-butanol yields and improved selectivity toward n-butanol formation for catalysts with higher surface areas and mesoporosity. These findings demonstrate that waste-derived eggshells can be effectively converted into porous hydroxyapatite catalysts with tunable textural and acid–base properties, offering a sustainable approach for ethanol upgrading toward n-butanol.