Sustainable and Efficient Production of High-Density Liquid Fuel Precursors via the Aldol Condensation of Cycloalkanones Catalyzed by Modified Waste Eggshell-Derived CaO

Abstract To address rising energy demand and promote biomass-waste valorization, a biobased CaO catalyst was developed from waste eggshell modified with NaOH for producing high-density fuel precursors via aldol condensation. Characterization revealed that NaOH modification enlarged pore sizes and optimized electronic structure, significantly strengthening surface basic sites. The optimal 10 wt % NaOH-Eggshell achieved >90% cyclohexanone conversion at 180 °C, yielding dimeric and trimeric products with selectivities of 99.7% and 0.3%, respectively. The kinetic analysis confirmed a notably reduced activation energy. The catalyst also exhibited high activity in cross-condensation between cyclopentanone and methyl-substituted cyclohexanones, with cyclopentanone notably enhancing conversion and selectivity toward trimeric products. The resulting fuel derived from the cross-condensation of cyclohexanone and cyclopentanone exhibits a density of 0.933 g/cm3, a freezing point below −75 °C, and a calorific value of 38.9 MJ/kg, comparable to JP-10 aviation fuel. Furthermore, a “waste-to-wealth” strategy was proposed to upgrade real phenolic oil from coal tar into high-density fuel. By integrating selective hydrogenation, C–C coupling (catalyzed by 10 wt % NaOH-Eggshell), and hydrodeoxygenation, the black phenolic oil was efficiently converted into a transparent liquid fuel composed primarily of high-density polycycloalkanes. This work provides a green and cost-effective pathway for valorizing biomass waste while providing a solid foundation for the clean conversion of coal tar into high-density aviation fuels.